Radiation safety in fluoroscopy. (Directed Reading).
Basic Principles of Radiation
The German physicist Wilhelm Conrad Roentgen discovered x-rays in 1895, when fortunate coincidences set the stage for his discovery. These coincidences involved the complex interaction of the behavior of cathode rays (electrons) in cathode ray tubes enclosed in black cardboard. Roentgen noticed that light escaping the end of the tube glowed brightly on a small piece of paper located nearby that was coated with barium platinocyanide.
Roentgen also noticed that various objects he placed between the tube and the fluorescent screen affected the bright ness, meaning the ray penetrated some objects more easily than others. His own hand surprisingly showed as a skeletal outline. In 1901, he received the first Nobel Prize for physics for his contribution to science in discovering x-rays. (5,6)
X-rays fall under the spectrum of electromagnetic radiation. This radiation category can be likened to bundles and waves of energy, much like the ripples caused by dropping a rock in a still pond. A combination of electric and magnetic energy transports electromagnetic radiation through space (hence the name electromagnetic). Radio waves, visible light, radiant heat and the short wavelengths that characterize x-rays are all forms of electromagnetic radiation. (4,5)
X-ray wavelengths are much like packets or bundles of energy, each containing an amount of energy directly proportional to its frequency. X-rays characterized by short wavelengths have high frequencies and correspondingly large amounts of energy per photon. (4,6) Just as an atom is the smallest quantity of an element, a photon is the smallest quantity of any electromagnetic radiation. Photons move through space at the speed of light and have no mass. Their electric and magnetic fields continuously move in a wavelike fashion, called a sine wave.
Frequency represents the rise and fall of the sine wave, while wavelength is the distance from one crest to another, one valley to another, or from any sine wave point to the next corresponding point. (7)
The relationship of photons to x-rays is critical to an understanding of radiobiology. The transfer of radiation energy to matter occurs as the result of photons interacting with atoms. (6,8) The energy is deposited unevenly in discrete packets in the tissues and cells. Large individual bundles of ionizing radiation (ie, those with enough propagation of energy to displace one or more orbital electrons) can break the bond of an electron to its parent nucleus.
Breaking the chemical bond initiates a chain of events that culminates in biologic change to matter. (4,6) Non-ionizing radiation (ie, energy that does not displace orbital electrons) like that used in microwaves, shortwave radio, diagnostic ultrasound and magnetic resonance imaging, differs in the amount of energy the waves contain. (4,7)
Matter absorbs heat or mechanical energy evenly and uniformly, and much higher quantities must enter living things to cause damage. X-rays are more potent to living matter because they enter tissues and cells in such large individual packets of energy, rather than because of a larger overall total absorbed. (6)
Electromagnetic radiation--in fact, all radiations--are swiftly moving particles that traverse a radiation field. (8) Within this field, various quantities and measures help determine the amount of radiation dose delivered to both patient and operator. The oldest known unit of measure is the roentgen (R), a measure of the charge produced when gamma and x-rays ionize a given volume of air. Roentgen measures radiation exposure, but not absorption. Its expression (at least for intensity) requires a time frame (eg, R/min). (4)
Basic Principles of Fluoroscopy
In effect, Roentgen's discovery of x-rays related directly to fluoroscopy, because fluorescence on the material in the room caused him to take note of the x-ray's properties. (5) In 1896 inventor Thomas A. Edison created the fluoroscope, consisting of a zinc-cadmium sulfide screen that was placed above the patient's body in the x-ray beam. (7) (See Fig. 1.) In first-generation units, the radiologist stared directly at a faint yellow-green fluorescent image through a sheet of lead to prevent the x-ray beam from striking his or her eyes. The exam room required complete darkness and the radiologist had to first wear red goggles for up to 20 or 30 minutes to adapt the eyes to darkness to see a faint fluorescent image. (5,7)
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The dynamic design of fluoroscopy enables real-time radiographic imaging of moving anatomic structures. (7,9) Generally, fluoroscopy procedures require a radiologist to perform and monitor the examination, and to view the study "live." (10) A continuous beam from an x-ray tube placed beneath the table passes through the patient and falls onto a fluorescent screen. (7,9) (See Fig. 2.)
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Projections taken during fluoroscopy are named based on the patient's position relative to the fluoroscopic table. For the right posterior oblique (RPO) projection, the patient lies with the right side of his or her back against the table and the left side elevated away from the table, for example. The x-ray beam from the tube beneath the table passes through the patient to the film located above the patient. (9) Most fluoroscopic rooms also have an x-ray tube located overhead so that they can be used for conventional radiography as well. These often are referred to as "R and F" rooms. (11)
Early generation fluoroscopic images presented particularly difficult viewing challenges for radiologists. The human retina contains 2 types of image receptors--rods and cones. Cones, or central vision, operate more effectively in bright light while rods, or peripheral vision, are more sensitive to low light. Because rods are also more sensitive to blue-green light, the radiologists' red goggles filtered out blue-green wavelengths to allow the rods to recover peak sensitivity before viewing fluoroscopic images.
The dimness of the early images and required use of rod vision resulted in poor visual acuity and poor contrast distinction because rod vision does not lend itself as well to distinguishing between shades of gray. To allow radiologists to use cone vision but minimize patient exposure, the industry developed the image-intensifier tube in the 1950s. (5,7)
Today, the image intensifier amplifies the faint light pattern emitted by the fluorescing screen and the radiologist views the image on a monitor. Fluoroscopic imaging is very useful in evaluating dynamic anatomic structures, such as gastrointestinal functions, diaphragmatic movement of respiration and cardiac functions. Fluoroscopy also is valuable in performing studies that require continuous imaging and monitoring, such as barium studies or catheter placements. (9)
Fluoroscopy Systems and Equipment
While the x-ray tube rests under the patient table, the fluoroscopic image intensifier and spot film cassette generally are located above the table (or opposite the x-ray tube). (7,12) Typical fluoroscopic x-ray tube currents are 1 to 5 mA, compared to 50 to 500 mA for conventional radiographic tubes. The table supporting the patient usually rotates to an upright position for certain examinations and can be lowered to horizontal position for other procedures. (11) In some instances, the unit is operated from outside the room. A number of technologies are available to record images created during fluoroscopic procedures. (7,13)
The image intensifier plays a key role in increasing image brightness without forcing increased radiation exposure by converting the transmitted x-rays into a brightened, visible light image. (7,12) The intensifier's input phosphor converts x-ray photons to light photons, which then are converted to photoelectrons within a photocathode. (See Fig. 3.) A series of electrodes strikes the output phosphor, accelerating and focusing the electrons into light photons that can be captured by a number of different imaging devices.
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Early-generation image intensifiers had small input sizes and a glass vacuum case. Today's models come with large input field sizes (up to 57 cm in diameter) with little image distortion. The tubes now are encased in metal to protect the intensifier's glass vacuum envelope. (7,12)
The outermost component of the intensifier is the input window, which is convex and made to minimize x-ray absorption, scatter and cost. Just behind the window lies the layer of input phosphor, cesium iodide (CsI), that converts the x-ray energy into visible light. CsI crystals are grown as tiny, tightly packed needles that offer excellent spatial resolution and little dispersion. Input layer thickness is a compromise of spatial resolution vs x-ray absorption efficiency. Thicker phosphor layers offer higher x-ray absorption efficiency.
Bound to the input phosphor is the photocathode, the layer made up of antimony-cesium (Sb[Cs.sub.3]). These compounds emit electrons when stimulated by light, a process called photoemission. To maximize efficiency of conversion from light photon to photoelectron, the light emitted from the input phosphor should match the photocathode's sensitivity spectrum.
The photocathode accelerates photoelectrons to the anode at the back of the intensifier. They are focused down to the size of the output phosphor by a series of electrostatic focusing electrodes that are very sensitive to external electrical and magnetic fields. Therefore, possible external influences of nearby magnetic resonance imaging units or unstable high voltages must be monitored to prevent distortion of fluoroscopic image quality.
When the high-energy electrons reach and interact with the output phosphor, they produce a considerable amount of light. Typically, the output phosphor consists of a compound made of silver-activated zinc-cadmium sulfide (ZnCdS:Ag). The ratio of the number of light photons at the output phosphor compared to the number of x-rays at the input phosphor is called the flux gain and is the reason for the image's increased illumination. (7,12)
Image monitoring can help control brightness and depends primarily on the anatomic structure under study. As with other imaging techniques, operators can control kVp and mA, and these factors affect both image quality and patient exposure. Generally, high kVp and low mA are preferred for fluoroscopy. (7)
Automatic Brightness Control
Automatic brightness control helps maintain the image intensifier exposure rates based on the subject's thickness. (5) Automatic brightness control monitors the light output from an area of the face of the image intensifier, usually near its center. This feature tries to maintain the signal level within an approximate range of that output and adjust tube potential (kVp) and tube current (mA) according to a predefined algorithm. Different image intensifier manufacturers set different algorithms for their respective equipment. (14)
Continual comparison against a reference signal is the hallmark of modern automatic brightness control (or automatic brightness stabilization) systems. Several different combinations of mA and kVp adjustments can automatically control fluoroscopic brightness. (See Fig. 4.) The figure's table shows these combinations. The last 2 approaches listed in the table generally apply to vascular applications.
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Automatic brightness control is critical to patient dose and image quality as demonstrated by the shape of the automatic brightness control curve. This ensures that the most penetrating radiation dose is used, reducing patient dose when possible. (13)
Image Monitoring and Recording
Once imaging is accomplished, the fluoroscopy system provides for image viewing or recording. A number of technologies offer today's radiologist real-time or film recording options, including spot film devices, television monitoring systems, photofluorography, digital fluorography and cinefluorography.
Conventional films can be obtained during fluoroscopic procedures by exposing a screen-film cassette during fluoroscopic viewing. The technologist loads the cassette between the patient and the image intensifier. However, during filming, the cassette remains parked out of the way and shrouded in a lead-lined case until triggered by the radiologist for exposure. Usually, spot film devices allow more than one image on a film.
The photo-spot camera works much like a motion picture camera, but for a spot film it freezes only one frame when activated. At trigger time, a positioner centers the film. Collimators automatically reduce x-ray field size at the time of exposure to minimize scatter dose and patient dose.
Screen-film systems used in vascular imaging are automatic film changers. They consist of' supply magazines to hold unexposed film, a receiving magazine, a pair of radiographic screens and a mechanism for transferring film. The system mechanically separates screens when an exposure is required, pulling the film into place. Operators preprogram the number of films and filming rates.
Television monitoring couples the image intensifier's output phosphor directly with a television camera tube. The camera tube converts the light image into an electrical signal that then is sent to the television monitor and reconstructed as an image on the screen. Television viewing allows several observers to see the image at one time and offers electronic control of brightness level and contrast.
Many facilities have replaced conventional image recording systems with digital technology and the television monitoring system lends itself to easier digital conversion. Digital charged couple device (CCD) television cameras have begun replacing conventional camera tubes in fluoroscopy systems. CCD systems allow operators to digitally increase display contrast, reduce noise and enhance the edges of images. In addition, spot images captured with digital technology do not require film processing and images can be viewed immediately. (7,13)
For recording motion, cinefluorography remains the standard for imaging movement of contrast agent through vessels. This process involves recording fluoroscopic images on movie (cine) film through a cine camera that records from the image intensifier's output phosphor. As in spot film recording, a beam-splitting mirror allows for simultaneous television monitor viewing and cine recording.
Cine film is smaller than spot film (35 mm wide) and longer, capable of operating at up to 90 images (or frames) per second. Lower frame rates also are available. The higher rate typically is used in cardiac procedures, especially pediatric imaging. Cine imaging is limited to the horizontal rectangular format of the film industry and once was plagued by blurring. The technique has improved and today exact framing can reduce patient dose in many, but not all procedures.
Videotape recording also is used to record moving fluoroscopic images using systems only slightly more sophisticated than home videotape units. High-resolution television cameras can record as a replacement for cinefluorography. Though not as sharp as cine recording, this alternative does not require as much radiation to achieve the same quantum noise level. (5,13)
Common Fluoroscopic Procedures
The increase in fluoroscopic procedures results in part from the growth of managed care in the United States. Managed care's emphasis on less invasive, less costly procedures favors techniques that replace surgery. Therefore, many fluoroscopically guided procedures like coronary angioplasty, neuroembolization and placement of transjugular intrahepatic portosystemic shunts have grown in frequency, while incidence of traditional surgical solutions has declined.
Patient doses in fluoroscopy depend on a number of factors, such as machine type; patient age, size and body composition; user setup; and others. Dose also varies according to procedure. Mean fluoroscopic times for diagnostic and interventional procedures are reported in Tables 1 and 2. Fluoroscopically controlled diagnostic procedures have significantly shorter exposure times and lower entrance skin doses than interventional procedures performed under fluoroscopic guidance. Some common diagnostic and interventional fluoroscopic procedures are described below. (1)
Several studies use a barium sulfate preparation to help evaluate patients' ability to swallow. Esophageal indications include painful or difficult swallowing, sensation of a lump in the throat, trauma, suspected aspiration and tumor. The esophagram may reveal tumors, constrictions or spasms and is performed with the patient in the RAO (right anterior oblique) or LPO (left posterior oblique) position.
Upper gastrointestinal (GI) series commonly are used to evaluate stomach symptoms, such as nausea, vomiting, gastrointestinal bleeding, suspected hiatal hernia, gastroesophageal reflux, symptoms of peptic ulcer disease and masses or strictures. Patients must drink a barium sulfate solution, then the radiologist obtains spot films. If the physician also is interested in evaluating the small intestine, the series will involve films of the abdomen every hour to image the progress of the barium solution through the small intestine. Addition of the small intestine to the study is indicated by suspicion of tumors or obstructions and malabsorption of nutrients.
The barium enema is probably the most commonly performed fluoroscopic examination of the colon. Usually indicated to evaluate masses, polyps, obstructions or diverticulitis, this study is often uncomfortable for the patient and requires efficiency on the part of the technologist and radiologist. The patient receives extensive dietary instructions for preparation beginning the day before the exam. Barium solution is introduced into the colon, which can cause cramping and discomfort. The colon also can be inflated with air, which creates further contrast in the image. During barium enemas, patients are placed in a number of different positions.
Cystograms assess bladder anatomy and function. The voiding cystogram evaluates urination. In cystograms, the patient's bladder is filled with a contrast agent through a Foley catheter in the urethra. Spot films are taken to study the full bladder. In a voiding cystogram, the tabletop is placed in the vertical position. The patient empties his or her bladder while under fluoroscopic study. Radiologists obtain a postvoid film of the bladder as well.
The endoscopic retrograde cholangiopancreatogram is indicated for suspected malignancies or obstructions in the pancreas or biliary system. The common bile duct is located using an endoscope and then a contrast medium is injected into the duct. The tabletop is in the horizontal position and the patient is placed in various positions, depending on the particular filling and imaging needs.
The hysterosalpingogram examines the uterus and fallopian tubes to evaluate suspected infertility or details of uterine anatomy. The structures are filled with an iodinated contrast medium through the cervical canal. (10,15)
Some cardiac and vascular procedures involve multiple views and long, real-time fluoroscopic exposures because of their interventional nature. (1) Most fluoroscopic skin injuries occur as a result of interventional procedures. (16) In addition to long exposure times, these patients often undergo repeat procedures. (3,16)
An arteriogram shows the arteries in a particular region of the patient's body. After an iodine-based contrast medium is injected through a catheter, a rapid sequence of images is made so that blood flow through the artery can be observed. (7,10)
Coronary angiography refers to imaging of heart vessels injected with contrast media. (7) This procedure uses various projections, usually left or right anterior oblique, with differing degrees of craniocaudal angulation to view the entire coronary system. Increased angulation also increases the distance the x-ray travels through tissue and puts the skin closer to the x-ray source. Angioplasty, a therapeutic procedure to enlarge blood vessels, can average 20 minutes of fluoroscopy time.
Cardiac catheter radiofrequency ablation procedures are used to treat supraventricular and some ventricular tachyarrhythmias. Fluoroscopy helps physicians position intracardiac catheters. Fluoroscopic times for this procedure average up to 45 minutes. (16) Other cardiac interventional procedures performed with fluoroscopic guidance include placement of vascular stents and percutaneous transluminal angioplasty (ie, enlarging vessels with a balloon catheter). (7,16)
To evaluate the structures in and around a joint space, physicians perform arthrograms. Most commonly, arthrograms involve the knee or shoulder. These fluoroscopic procedures are becoming less common, however, as magnetic resonance (MR) imaging has virtually replaced fluoroscopic joint imaging. (10)
Interestingly, a recent British study reported that use of fluoroscopy for imaging of certain pediatric pelvic conditions actually lowered dose to the gonadal area in children while still providing useful follow-up images for orthopedic surgeons. (17) Children with chronic hip conditions, such as Perthes disease and developmental dysplasia of the hip (DDH), require repeated pelvic x-rays around highly radiosensitive organs. Investigators in the British study used digital grid-controlled pulsed fluoroscopy with proper gonad protection. (17) These fluoroscopic techniques delivered lower radiation doses than conventional radiographs. Clinical image quality may not have been sufficient for large children, but overall fluoroscopic image quality was deemed adequate for the required follow-up studies. (17)
Radiation Sources In Fluoroscopic Imaging
During fluoroscopy, patients and operators are susceptible to radiation from primary, leakage and scatter radiation sources. (5,7) In addition to distance and time factors controlled by technique and equipment features such as automatic brightness control, physical barriers to radiation sources are critical to controlling radiation exposure.
Primary radiation for fluoroscopy consists of the useful beam. The unique setup of the fluoroscopic table requires attention to design to protect the patient and operators inside the room as well as those in adjoining rooms. For combination radiographic/fluoroscopic rooms, designers base barrier calculations on radiographic workload rather than fluoroscopic workload because the fluoroscopic screen acts as a primary barrier when the unit's x-ray tube is energized. In effect, fluoroscopy has lower primary barrier requirements for room design. (7)
The operator controls the output and direction of the primary beam, thus affecting the amount of stray radiation and risk to staff in the fluoroscopy room. The equipment should be designed and maintained so that the primary beam is confined to the image receptor. (18)
Scatter radiation and leakage radiation constitute the 2 types of secondary radiation from fluoroscopy. Scatter radiation occurs when the useful beam intercepts any object and scatters x-rays. During fluoroscopy, the patient generally is the most important intercepting object. As a rule, the intensity of the scatter radiation 1 meter from the patient equals 0.1% of the useful beam's intensity. (7)
Distribution of scatter is asymmetric and changes depending on beam orientation. Many interventional systems have a cradle that allows angling the patient to direct backscatter away from the operator's torso. (18)
Leakage radiation passes through the x-ray tube's housing when the beam is activated. (5) Regulatory requirements set limits for leakage radiation at 100 mR/hour (26 [micro]C [microcuries] /kilogram-hour) at 1 meter. (7) Some leakage radiation occurs as long as x-rays are being produced within the tube; in practice, however, leakage radiation falls well below the regulatory level. (5,7) The fluoroscopic image intensifier is enclosed in a metal housing with lead to absorb scattered radiation. (12)
Dose Exposure and Calculation
Absorbed dose represents the energy (or amount of ionizing radiation) absorbed per unit per mass at a certain point within irradiated matter. (8) Some of the largest doses to patients in diagnostic radiology occur in fluoroscopy. The greatest dose rate occurs at the skin, where the x-ray beam enters the patient. (6) Table 3 compares bone marrow and gonadal radiation doses for some common radiographic exams, including fluoroscopy.
Absorbed Dose Units
The radiation absorbed dose (rad or Gy) is based on absorbed dose and represents the quantity of radiation the patient receives. On exposure to 1 R of x-radiation, 1 g of most tissue absorbs about 1 rad. In recent years, the movement toward use of System Internationale (SI) units in radiobiology and nuclear medicine has spread to the United States. The gray (Gy) is the SI dose measurement unit.
SI units are based on kilograms, meters and seconds. Therefore, 1 Gy is the dose of radiation that results in absorption of 1 joule (unit of work) of energy per kilogram of material absorbing radiation. One Gy equals 100 rad and 1 cGy corresponds to 1 rad.
In measuring radiation through occupational exposure, the rem or seivert is used to account for differences in various types of radiation. One rad of particulate radiation can cause more damage to a biological system than an equivalent rad of x-ray. The rem generally is defined as a dose of any type of radiation that produces biological effects equal to those of 1 rad of gamma or x-rays. (4,7) Relatively speaking, the amount of radiation delivered from a single diagnostic radiology exam--even fluoroscopy--is small in terms of its ability to cause biological damage. However, radiation therapy doses tend to measure in the gray range. (18)
The accepted SI unit for dose equivalence that measures occupational exposure for health care workers and nuclear power plant personnel is the sievert (Sv). The sievert corresponds to a dose producing a biologic effect equal to 1 Gy of x-ray or gamma rays, and thus equals 100 rem. (4,7)
Entrance Exposure Rates
Fluoroscopes operate in a number of different modes, including normal, high-dose, conventional and digital cine fluoroscopy. They also can record analog or digital spot images throughout procedures. Although many factors related to dose come into play, the patient's fluoroscopic radiation dose is best characterized by measuring receptor entrance exposure rates and skin entrance exposure rates.
The receptor entrance exposure rate measures exposure at the surface of the image receptor (with the grid removed) required to produce a single image for a given x-ray spectrum. This exposure rate is critical to the patient's skin dose because skin dose increases with increasing receptor entrance exposure. Also, the level of image noise and resulting low-contrast detail depend on receptor entrance exposure. (1) (See Table 4.)
To measure the receptor entrance exposure rate, the ionization chamber is placed 20 to 30 cm from the surface of the image intensifier with a fixed source-to-image intensifier distance of 100 cm. This reduces backscatter contribution to the exposure rate. (See Fig. 5.)
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Skin entrance exposure rates are critical to measuring the likelihood of a radiation skin injury from fluoroscopy. Also referred to as the entrance skin exposure rate (ESER), this universal measurement refers to the exposure rate at the center of the radiation field where the x-ray beam enters the patient's body. (1,19)
Accurate skin exposure measurement is elusive because variables such as kVp, the patient's body thickness, use of a grid, source-to-image distance and other factors can affect dose. The typical skin entrance exposure dose rate for an adult of medium build is about 30 mGy/min (3 rad/min). However, dose generally runs higher during image recording. (1) (See Tables 1 and 2.)
Legal and Regulatory Fluoroscopic Dose Rates
Various regulatory agencies have set national dose limits for exposure to ionizing radiation for workers and the general public, but hesitate to set maximum legal doses for patients because each patient and condition differs. However, regulatory and clinical guidelines Expect health care personnel to limit the volume of exposed tissue and total dose to the minimum necessary to achieve diagnostic or therapeutic goals.
Likewise, while maximum permissible doses for health care workers have been set and redefined, most organizations emphasize the ALARA principle ("as low as reasonably achievable") in handling occupational exposure to radiation. (4)
Despite the critical importance of receptor entrance exposure rates to patient skin dose, no regulations limit these rates. Fluoroscopy system manufacturers set receptor exposure values to predetermined acceptable levels. Regulatory bodies set the maximum skin entrance exposure rate at 2.58 mC [millicurie]/kg per minute (10 R/min) for normal fluoroscopy and 5.16 mC/kg per minute (20 R/min) for high-dose fluoroscopy. Cineradiography is exempt from the regulations and no maximum limits exist for other fluoroscopic imaging modes, such as digital subtraction angiography, (1,19)
Likewise, although performance measures help optimize fluoroscopic equipment function by measuring image intensifier entrance exposure rate and other constancy checks, at present there are no internationally accepted constancy checks guiding quality control of fluoroscopy units. (19,20)
The FDA recommends measuring radiation exposure rates and developing techniques to minimize radiation dose. (21) The American College of Radiology (ACR) offers guidelines for medical physics performance monitoring of fluoroscopic equipment in its standards, most recently revised in 2001. (22)
Radiation's Biological Effects
Early in the history of medical x-rays, radiation injuries occurred frequently. Skin injuries (erythema), hair loss and low red-cell count (anemia) were most often reported. By about 1910, improved tube design and the Snook transformer decreased exposure times and radiation in the x-ray beam, thus reducing injury to both patients and operators. (7)
When ionizing radiation (either particulate or electromagnetic) is absorbed in cells, it interacts with a biologically important target molecule. One type of interaction is direct effect, or direct absorption of the ionizing radiation's energy by the target molecule. Direct effect accounts for about 20% of the target molecule's radiation damage.
The second type of interaction, the indirect effect, accounts for the other 80% of damage. Indirect effect occurs in water near the target molecule and is mediated by free radicals. (23) These are highly reactive fragments of atoms or molecules containing unpaired electrons. (6) The free radicals migrate to the target molecule and transfer their energy, resulting in damage to the target molecule.
Responses to radiation that occur within minutes or days after exposure are called early or acute effects. These occur when a large number of cells die. Late or delayed effects occur after months or even years. (See Table 5.) At each stage of radiation damage, the process is reversible and ionized atoms can become neutral again by attracting a free electron. Even cells and tissues can repair injuries. (7) Acute damage is repaired more rapidly and may reverse completely; late damage takes longer to repair and may never reverse completely. (6) Late effects may involve any organ or tissue.
It should be noted that dividing injuries into stages or phases of recovery simplifies morphologic events that are unique and therefore cannot be classified. Both early and late radiation effects are expressed in terms of alterations of blood vessels, stroma and parenchymal cells. But the degree of injury and how the damage is expressed vary considerably between early and late effects. (4) (See Table 6.)
When the probability of effect, rather than the severity of effect, increases with dose, it is termed a stochastic effect. Examples include radiation-induced cancer and genetic effects. The likelihood of developing radiation-induced leukemia increases after exposure to 1 Gy but no difference in the severity of disease occurs at this exposure level. With stochastic effects, injury to a few cells (or a single cell) could produce the effect and therefore no true threshold level is believed to exist.
Nonstochastic or deterministic effects do not cause harm at small doses of radiation but damage becomes apparent at a certain threshold level. (1) Severity of damage increases as dose increases above the threshold. (1,24) Cataracts and skin injuries like erythema and epilation are examples of deterministic effects that can occur as a result of exposure to high radiation levels. Recent increases in the use of interventional fluoroscopic procedures have raised concern about deterministic effects, primarily skin damage and hair loss. (1)
Fluoroscopically Induced Skin Injuries
Radiation injury from fluoroscopy results in a deterministic effect, most often to the patient's skin. (21,24) The dermatological effects of radiation injury increase in severity, beginning with erythema and hair loss and progressing to chronic skin ulceration that requires excision and skin grafting. Patients who receive extremely high radiation exposure from interventional procedures may have a 5% greater risk of developing skin cancer. (21)
* Skin structure. The skin makes up one of the largest organs in the body, weighing about 2 kg and boasting a surface area of 1.2 to 2.3 m. (4) Normal skin consists of 3 layers: epidermis, dermis and subcutaneous fat and connective tissue. These layers help protect the body from environmental injury, regulate internal temperature, balance water and electrolytes, assist with immunologic responses and provide sensory functions.
The outer layer (epidermis) varies in thickness from 40 to 50 [micro]m (micrometers) around the face and trunk to 150 [micro]m on the backs of the hands and 300 to 500 [micro]m on the fingertips. The epidermis is further divided into layers. The lowest of these are basal cells. Basal cells are stein cells that mature as they migrate to the upper surface of the epidermis. The skin acts as a continuing cell-renewal system, and skin cells are replaced at the rate of about 2% per day. The basal cells that rise to the surface of the epidermis are slowly lost and replaced by new basal cells. Damaged basal cells are the earliest manifestation of skin radiation injury. (4,7)
Below the epidermis lies the intermediate layer--the dermis. It is made up of connective tissue and is divided into 2 layers. Below the dermis is a subcutaneous layer of fat and connective tissue. In addition to the skin's layers, sweat glands, hair follicles and sensory receptors are accessory structures that may react to radiation.
Different tissues are more sensitive to x-rays than others and individual patients vary in sensitivity as well. For example, skin on the back of the hand is more sensitive than the skin of the palm. Patients with certain diseases, such as diabetes mellitus or collagen vascular disease, have more sensitive skin in relation to x-rays. Younger patients generally recover from skin injury more efficiently because they have greater reserves of healthy tissue. (24)
* Erythema. Probably the first observed biologic response to radiation exposure, erythema activates histamine-like substances. This results in permeability and dilation of small vessels. (7,24) Erythema can develop within hours of fluoroscopy and likely fades 1 to 2 weeks following the procedure. (25) In fact, erythema may peak at 24 hours and recede quickly. Each case's course and severity depends on dose and dose rate. Erythema caused by doses near the threshold (about 2 Gy) appear similar to a blush. At high doses (closer to 6 Gy), erythema may blend with reactions at the basal cell level. (24) At these higher doses, erythema may present with more apparent edema and inflammation and even small hemorrhages. (4)
* Epilation. At a single dose threshold of 3 Gy, epilation (hair depletion) can begin. The first documented case of radiation-induced (but recoverable) hair loss occurred in 1896. Onset of epilation usually occurs at about 3 weeks following radiation. (21,24) When x-rays destroy the replicative function of sensitive cells at the germinal layer of the hair follicle, the number of cells available for hair growth is diminished. As a result, hair at the follicle is thinner. If sufficiently deprived of cells, the hair becomes weak and will break off at the skin line, resulting in hair loss. Recovery occurs in about 6 weeks, unless exposure exceeds 7 Gy, in which case hair loss may be irreversible.
* Desquamation. At thresholds of about 14 Gy, more basal cells are affected, seriously compromising the skin's ability to regenerate. The first stage, dry desquamation, can occur at doses as low as 10 Gy and appears much like a flaking sunburn. At higher dose levels, depopulation of cells in the epidermis becomes more serious and vesicles (blisters) and moist desquamation can occur. (4,6,24) Onset of dry or moist desquamation generally occurs at about 4 weeks following radiation exposure. (24) The process of healing moist desquamation involves repopulation of surviving clonogens (colony-forming cells) from the edges of the irradiated area. This process can cause a great deal of discomfort to the patient. (6)
* Dermal necrosis. Dermal necrosis is a late effect of radiation injury, occurring months to years after higher doses of radiation. (6) It occurs at doses of 18 Gy and begins at 10 to 16 weeks after exposure to x-rays. Dermal necrosis is characterized by increasing loss of endothelial cells and reduced capillary density. The insufficient vascular supply leads to dermal ischemia and necrosis. (24) (See Fig. 6C-D.)
[FIGURE 6 OMITTED]
* Secondary ulceration. Occurring at a single-dose threshold of 20 to 24 Gy, skin ulcers become apparent at about 6 weeks or more after exposure. (21,24) The overlying epidermis weakens from the injury, and the skin sloughs, leaving the underlying tissue susceptible to minor injury and infection. Therefore, ulcers may heal and recur over a period of months to years due to minor trauma or exposure to ultraviolet light. (24) (See Fig. 6B.)
* Other skin manifestations. Some doses (around 10 Gy) may not result in ulceration but still cause dermal atrophy, especially if moist desquamation occurs. Atrophy can begin 3 months after radiation exposure and progress at a gradual pace for several years. (24) Telangiectasia (dilation of small or terminal blood vessels) also occurs at a 10 Gy threshold at about 4 months after fluoroscopic interventional procedures in some patients. However, onset usually becomes noticeable only after 1 year and progresses for up to 10 years. (24) Telangiectasia results in reduced blood flow to the irradiated skin, leading to ischemic necrosis and eventual infection. (4)
Induration (invasive fibrosis) is a healing process often observed after ulceration and dermal necrosis. Milder forms of induration also are apparent after doses of about 10 Gy and commonly progress with time. The skin and subcutaneous fat feel woodlike, while the area feels tender to the patient. Induration can seriously limit range of movement if it occurs near a joint. (24)
Factors that Increase Sensitivity
In addition to characteristics that make some patients more sensitive to radiation than others--including greater genetic susceptibility to radiation--previous high doses from earlier procedures increase the risk of radiation injury from an interventional fluoroscopic procedure. (24,26) Even though time between procedures permits skin to recover to some extent, it may not prove sufficient for full recovery. This results in reduced tolerance of the skin to future interventional procedures. Large patients with compromising connective tissue disease, diabetes mellitus, homozygosity for ataxia telangiectasia and previous high-dose fluoroscopies are at greater risk for skin injuries because such patients require high fluoroscopic outputs. The geometry of the procedure puts the large patient's skin in closer proximity to the x-ray source. (26)
Radiation Injury Cases
From about 1960 to 1990, so few reports of radiation injury in diagnostic radiology occurred that the profession in effect "let down its guard." Their, in the 1990s, the increasing frequency of fluoroscopic procedures brought increasing reports of injury from lengthy, high-dose interventional procedures. (24) In fact, in 1994 alone, at least 40 injuries were reported to the FDA. Of all reports from 1992 to 1995, most related to cardiac catheter ablation and other cardiac procedures. (1,26) In 1994 an estimated 300 000 coronary angioplasty procedures were performed in the United States. The current number of procedures is at least double that. (1)
Table 7 presents examples of skin injuries reported to the FDA in the early 1990s. In many cases, the agency's follow-up failed to provide adequate information to estimate absorbed dose and exposure times. This contributed to the FDA's 1995 recommendation that facilities record absorbed dose to the skin for any procedure exceeding a suggested threshold of 1 Gy. The procedures most likely to exceed this threshold include radiofrequency cardiac catheter ablation, vascular embolization, transjugular intrahepatic portosystemic shunt (TIPS) procedures and percutaneous endovascular reconstruction. (27)
Examples of Fluoroscopic Radiation Skin Injury
* A 61-year-old man with TIPS procedure. This patient was an obese white man who presented with hematemesis and melena. He had a complicated previous medical history, including noninsulin-dependent diabetes mellitus, pancytopenia secondary to aplastic anemia, polycystic kidney disease and atherosclerotic coronary artery disease. His history also included lupus erythematosus with associated Sjogren syndrome; however, diagnosis of these diseases was unclear.
After an initial angiographic procedure (and diagnostic biopsies and venography), the patient developed an intensely itchy area on his back just right of the midline. At about the T4 to T12 area, physicians noted a clustered area of papules, slightly abrasive in appearance but with no vesicles or pustules. They determined the area to be contact dermatitis.
Six months later, the patient returned with hematemesis. Because he was a poor candidate for surgery, a transjugular intrahepatic portosystemic shunt (TIPS) procedure was recommended. The procedure proved difficult, requiring multiple wire exchanges and sequential angioplasties. However, it was clinically successful after nearly 5 hours. High-dose fluoroscopy was used only during the initial puncture of the portal vein and deployment of the stent.
The next day, the dermatology department evaluated the patient for a warm, itchy, erythematous area about 10 x 15 cm located right of the midline on his back. Physicians noted that the erythema had been present for several weeks. The patient was treated and released from the hospital the next day.
Two weeks later, he returned to the hospital for the persistent rash, which progressed with time. By 5 months, the affected area was 15 x 12 cm with an ulceration of 10 x 7 cm. Physicians repeatedly but unsuccessfully attempted grafting the lesion. At 6 months, the ribs were exposed. (See Fig. 7.) Eventually, the wound required medical and surgical attention, including use of hyperbaric oxygen and resection of at least one rib. Follow-up care took about 5 years total. (26)
[FIGURE 7 OMITTED]
* A 57-year-old man with repeat catheterizations. The patient, a 6-foot, 2-inch man weighing 220 pounds, developed angina pectoris and exertional dyspnea. He underwent cardiac catheterization, during which physicians noted 3 areas of marked stenosis in the left circumflex artery. A cardiologist proceeded with balloon dilation and adjunctive rotational atherectomy. The procedure involved 173 minutes of fluoroscopy time and multiple cinefluorographic images.
Five months following the cardiac procedure, the patient experienced chest discomfort again. A diagnostic cardiac catheterization revealed good flow in the left circumflex artery, but marked stenosis in the left anterior descending artery. Another lengthy procedure with 74 minutes of fluoroscopy and more than 2700 cinefluorographic images was performed.
Within 24 hours of the second procedure, the patient developed pain and skin erythema below his right scapula. The area ulcerated and necrosed over the next 5 months. The patient eventually required extensive skin grafting.
In this case, the patient filed suit against the cardiologist for medical malpractice, alleging that the physician used excessive amounts of fluoroscopy during the angioplasty procedures. Although the cardiologist testified that the extreme nature of the patient's cardiac disease necessitated the lengthy procedures, another cardiologist testifying on behalf of the patient stated that the defendant cardiologist should have examined the patient's back for evidence of exposure from the first procedure before proceeding with the second procedure only 6 months later. Furthermore, he said the defendant cardiologist should have informed the patient of the risk of additional radiation injury to the skin. The jury found that the defendant cardiologist had been negligent and awarded the plaintiff $1 million in compensation. (3)
* Repeated hepatic and biliary procedures. This patient received a number of diagnostic and interventional procedures including percutaneous cholangiography, mesenteric angiography and multiple embolizations over a 4-week period. In addition, an unknown amount of exposure occurred from some procedures performed before the patient arrived at the tertiary care facility, so staff estimated skin exposure from data on system technique factors, total fluoroscopic exposure times, number of frames and other factors recorded by the facility performing the earlier procedures. (See Table 8.)
Size and location of the irradiated area varied with the different procedures. However, most skin entrance exposure occurred in the patient's lower right back with significant likelihood of overlapping fields. As a result, the potential for skin injury was significant. Actual injury could not be discerned because the patient died 2 days after the last procedure.
* A 40-year-old man with repeat coronary procedures. The patient with progressive skin injury shown in Figure 6 received coronary angiography, coronary angioplasty, a second angiography due to complications and a coronary artery bypass graft, all in one day. The figure shows his injury at 6 to 8 weeks postprocedure. The area first looked similar to a small second-degree burn, then like a healed burn except for a small ulcerated area near the center. Over the following months, skin continued to break down with progressive necrosis (see Fig. 6c-d), eventually requiring a graft. The exact dose is not known but probably exceeded 20 Gy. (27)
The FDA Health Advisory
In 1992 the FDA began investigating such reports of radiation-induced skin injuries from fluoroscopically guided interventional procedures. In the same year, the American College of Radiology's bulletin published an article by its Commission on Physics and Radiation Safety warning the radiology community that new fluoroscopy equipment with high-level control modes could generate unacceptable radiation exposure rates. (3,27)
Most reports to the FDA came from equipment manufacturers as a result of the Safe Medical Devices Act of 1990. The FDA then followed up on required information with telephone calls, personal visits and letters, asking for additional voluntary information. (27)
The FDA advisory concluded that a number of interventional procedures could potentially cause skin injury even with fluoroscopic times of less than 1 hour at normal dose rates. (1) Specifically, the report listed the following procedures involving extended fluoroscopic times:
* Percutaneous transluminal angioplasty (coronary and other vessels).
* Radiofrequency cardiac catheter ablation.
* Vascular embolization.
* Stent and filter placement.
* Thrombolytic and fibrinolytic procedures.
* Percutaneous transhepatic cholangiography.
* Endoscopic retrograde cholangiopancreatography.
* Transjugular intrahepatic portosystemic shunt placement.
* Percutaneous nephrostomy.
* Biliary drainage.
* Urinary/biliary stone removal.
The FDA warning suggested that facilities performing fluoroscopically guided procedures observe the following principles:
* Establish standard operating procedures and clinical protocols for each specific type of procedure performed. The protocols should address all aspects of the procedure, such as patient selection, normal conduct of the procedure, actions in response to complications and consideration of limits on fluoroscopy exposure time.
* Know the radiation dose rates for the specific fluoroscopic system and for each mode of operation used during the clinical protocol. These dose rates should be derived from measurements performed at the facility.
* Assess the impact of each procedure's protocol on the potential for radiation injury to the patient.
* Modify the protocol, as appropriate, to limit the cumulative absorbed dose to any irradiated area of the skin to the minimum necessary for the clinical tasks, and particularly to avoid approaching cumulative doses that would induce unacceptable adverse effects. Use equipment that aids in minimizing absorbed dose.
* Enlist a qualified medical physicist to assist in implementing these principles in such a manner so as not to adversely affect the clinical objectives of the procedure. (28)
Finally, the FDA found that these radiation-induced skin injuries were further complicated by their delayed onset--often not evident until weeks after the procedure. (1,28) Facilities and physicians performing interventional fluoroscopic procedures should monitor delivered patient doses and identify in the medical record those areas of the patient's skin that received an absorbed dose that may approach or exceed the selected threshold. Furthermore, the medical record should contain an estimate of the cumulative absorbed dose (or enough data to permit estimation) to each irradiated area. (27)
Minimizing Fluoroscopic Radiation Exposure
In addition to the measures the FDA recommended, investigators continue to advocate new techniques to reduce exposure to both the patient and operator. The following factors should be considered to help minimize fluorographic exposure.
Size of Patient
Fluoroscopically guided procedures use low-energy x-rays that rapidly attenuate as the beam penetrates tissue. Therefore, absorption is most intense at the surface where the beam enters the patient, and dose decreases with soft tissue depth. This low penetrability requires higher entrance skin dose rates. Data is lacking on the precise relation of patient weight to injury prevalence, but reviews suggest a higher incidence of injury among heavyset individuals. (16)
In a recent study measuring skin dose in percutaneous transluminal coronary angiography (PTCA), investigators noted that during fluoroscopy, the skin dose rate can vary by a factor or 50 or more (eg, from 0.01 to more than 0.5 Gy/min), depending on variables such as thoracic circumference, beam incidence, source-to-image intensifier distance, source-to-skin distance, image intensifer-to-patient distance, pulsed vs continuous fluoroscopy and field size.
When measuring entrance dose rates in a phantom 20 to 25 cm thick, tissue thickness (a parameter related to thoracic circumference) and field dimensions most strongly influenced the entrance dose delivered to the skin surface. The study's authors recommended that under unfavorable physical conditions, interventional radiologists and cardiologists should minimize risk of fluoroscopically induced skin injury by distributing the skin dose over 4 areas instead of 2. (29)
In modern fluoroscopy systems, the tube current is set automatically, along with the kVp, by the automatic brightness control feature. (29,30) Decreasing milliampereseconds (mAs) helps control patient exposure. (21) Pulsed fluoroscopy helps achieve this goal by emitting the x-ray beam as a series of short, strobe-like pulses rather than continuously. (1,30) Studies continue to show that dose-saving pulsed fluoroscopy reduces the dose rate to skin. (16) However, if the tube current is set too high to achieve better quality images, the advantage of pulsed fluoroscopy is defeated. (1)
In fact, low mA (typically 1 to 5 mA vs 50 to 500 mA for conventional radiography tube currents) is preferred in fluoroscopic imaging because of longer operating/exposure times. (11)
Early-generation pulsed fluoroscopy units often provided uneven current and a significant percentage of low-energy beams, not really improving images. However, recent modifications in design originate pulses in the x-ray tube rather than the generator. One study compared the diagnostic accuracy of grid-controlled fluoroscopy with conventional continuous fluoroscopy for a number of abdominal and pelvic procedures. A negatively-charged grid is placed between the cathode and the x-ray producing anode, preventing the ramping and trailing effect that occurs with the uneven current of conventional pulsed fluoroscopy. (See Fig. 8.) This technique offers advantages, including reduction of overall radiation dose by changing the selected frame rate. The study authors suggested that continuous fluoroscopy should be used only rarely for abdominal and pelvic fluoroscopic procedures. (31)
[FIGURE 8 OMITTED]
KVp and Distance
Kilovolt peak (kVp) acts as the primary control of beam quality and penetrability. Higher kVp, or higher quality beams, can better penetrate the anatomy under study. (7) This is particularly important in fluoroscopy, as a low kVp inversely requires a higher entrance dose rate. By selecting higher kVp, the operator increases the average beam energy of the x-rays (known as "beam hardening") and therefore increases the fraction of the entrance beam that passes through to the image receptor.
Imaging a thick patient with a low kVp is a worst case scenario in fluoroscopy. Operators must balance some loss of image contrast that occurs with use of a high-energy beam. Studies have shown, however, that the loss in image contrast from increasing the kVp from 60 to 70 is not significant, but the resulting decrease in skin entrance dose from the same action is quite significant--about 30%. Therefore, maintaining the highest possible kVp that provides acceptable image contrast helps reduce entrance skin dose. (1)
Distance factors into both patient and staff exposure and maintaining a safe distance from the x-ray source is important. Doubling the distance from the source reduces the intensity by a factor of 4. Most of the scattered radiation comes from the patient's entrance surface. Color coding of the floor for safe distances (several steps from the table for staff members who do not need to be near the patient while the beam is on) can help prevent some exposure. Also critical is avoiding excessive distance from the patient to the image intensifier. Greater distance requires increased x-ray output. (18)
Image intensifiers provide magnification of certain images, allowing for better imaging of small structures. (11) However, magnification almost always results in higher patient doses. Geometric magnification uses the diverging x-ray beam to project a smaller region in the patient onto a larger area in the image intensifier. If source-to-image receptor distance is fixed, image magnification requires increased skin dose as the patient is moved closer to the x-ray source.
To magnify the image, the operator must move the source closer to the patient or the receptor further away. (1) To maintain the same noise level, dose quadruples when magnification doubles. Most modern fluoroscopic systems perform multiple steps of magnification. (12)
Modern systems also can magnify the image electronically. With electronic magnification, dose generally increases by the square of the ratio of the image intensifier diameter. (1) Digital zoom also is available in digital fluoroscopy, which can take place at the electronic monitor level rather than in the image intensifier. The magnification then becomes a matter of manipulating the pixels, or spatial domains that make up digital images. (18)
Several authors have reported on magnification contributing to high doses of radiation to patients. In one angiogram, a 4.5-inch field size was used during a 100-minute procedure. The patient received an estimated 13 to 22 Gy in that time. (16)
Grids improve contrast and image quality but increase dose to both patients and staff. Studies have shown that use of grids can increase dose by a factor of 2 or more and that removing them, particularly in pediatric procedures, causes little or no reduction in contrast or image quality. (1) The grid's purpose is to remove scattered x-rays from the image, but in exchange, x-ray output is increased, thus increasing patient dose.
Studies have shown that procedures with a large air gap between the image intensifier and the patient in particular do not need the grid because the air gap allows for escape of a good portion of the scattered x-rays before interaction with the image receptor. (16)
A 1997 report noted that cautious use of grids can help minimize patient dose in barium enema procedures. By removing the grid during the filling phase of the procedures, when image detail was not required, investigators reported a 46.5% reduction in dose for the filling phase. (32) It often is recommended that operators remove the grid before magnification of images to help reduce skin dose. (18)
Field of View and Collimation
The radiation field size contributes to the severity of radiation wounds. Larger lesions are less well tolerated and when dose is large enough to completely deplete basal cells, healing occurs mainly from the edges of the lesion. Therefore, regeneration of larger lesions is less effective and takes longer, exposing tissues to a higher risk of secondary ulceration.
Collimators can be manually adjusted to reduce the field of view. (16) Adjustable collimators limit the x-ray beam to the size of the active image receptor and ideally to the region of interest. An aperture plate limits the size of the emitted beam. Variable collimators should be used in all interventional systems, as they adapt to changes in the image receptor's field of view, as well as changes in the source-to-image-receptor distance. (18)
Using collimators in fluoroscopy reduces the risk of stochastic effects by reducing the amount of tissue exposed, reducing scattered radiation in the examination area, allowing for better recovery if tissue is injured and contributing to lower accumulated skin entrance dose by eliminating overlap of fields. (16)
Last Image Hold
Similar to "freeze framing," last image hold displays the last captured image continuously on the monitor. The technique works by digitizing the output image and continuously displaying it on the output monitor. (11) With last image hold and instant replay, physicians can study the last depiction of the procedure and make clinical decisions based on the image without activating the fluoroscopic x-ray beam. Most current systems are equipped with these features. (18)
Combining last image hold with electronic collimation provides additional dose savings because the operator can adjust field dimensions with the overlaid collimator blade on the last image hold without exposing the patient. (1)
Beam On Time
Radiologists are trained to depress a foot switch to sequence the fluoroscopic beam on and off, thus avoiding having the x-ray beam on continuously during the examination. A timer records the amount of time the x-ray beam is on. A 5-minute reset timer reminds the radiologist how much time has elapsed. (7) An audible 5-minute timer is required by law on all fluoroscopic systems. (11)
Other Methods to Minimize Exposure
In the majority of interventional procedures, a great deal of time is spent in one anatomic region. Operators can achieve some reduction in skin dose by periodically rotating the fluoroscope around a center within the anatomy of interest. By doing so, the maximum dose spreads over a broader area of the patient's skin so that no single region of the skin receives the entire dose. (1)
Care should be taken in longer interventional procedures to ensure that extraneous body parts are not exposed to the beam. Injuries to the arms and breasts have occurred in many angiography procedures and keeping patients' arms out of the field of view decreases overall dose by not forcing the machine to penetrate the extra tissue. If the extraneous tissues are located on the port side of the x-ray system, they can accumulate dose rapidly. Arm rests and effort on the part of operators to avoid direct irradiation of breasts can reduce dose. (16)
Radiologists and physicists should ensure that equipment operates at optimal levels to ensure acceptable image quality with the lowest possible radiation dose. (3) Departments should implement a quality improvement program that includes regular equipment maintenance and inspection and establish procedure protocols that are reviewed and updated periodically.
Logs should be maintained that track trends in patient, physician and staff exposure, and any problems should be analyzed. A radiation safety officer or committee should analyze fluoroscopic exposure on a quarterly basis. (21) In addition, studies suggest that increasing staff awareness of radiation dose and continuous audit of dose parameters can lead to a reduction in patient dose. In fact, one study found that continuous dose monitoring during examination led to a reduction in dose and that most radiologists would check the dose-area product meter on the equipment after fluoroscopic procedures when aware that their department was continuously monitoring dose readings of every case. (33)
Training in fluoroscopic technique and dose reduction techniques is critical to reducing patient and operator dose. The FDA emphasized training in its 1994 health advisory, including understanding of system operation and implications of each operational mode on radiation exposure. (1,16) Physicians from a variety of disciplines now use fluoroscopy in interventional work and most of these specialties provide little or no training in the use of radiation or the biologic effects of entrance skin dose. (16)
A study reported in 2000 (34) noted that dedicated gastrointestinal radiologic technologists can be trained to perform certain GI fluoroscopy procedures, namely esophagrams and double-contrast barium enemas. The study compared performance of these exams by technologists with a minimum of 2 years of experience in gastrointestinal fluoroscopy plus additional training to performance by radiology residents. The mean fluoroscopy times for the technologists were shorter than those of the residents and the technologists eventually became comfortable performing the examinations with little assistance or guidance from the attending radiologists. (34)
Personnel Exposure and Monitoring
Portable radiography and fluoroscopy produce the highest occupational exposures of any radiographic techniques. The long fluoroscopic times of interventional procedures, as well as extensive use of cineradiography, further expose operators to the x-ray beam and scatter radiation. Also, many interventional systems lack an intensifier tower protective curtain. (7)
Occupational Dose Limits and Monitoring
The dose limit for imaging personnel is 50 mSv per year (5000 mrem per year). However, experience shows that conventional radiography departments aim for much lower exposures--closer to 5 mSv per year. (7) Experience with ALARA has helped to keep overall exposures low. However, original dose limits were set on "tolerance dose," or the level at which a worker could tolerate a dose with no risk of injury.
By the 1950s, however, radiation protection began focusing on both types of radiation injury--stochastic and deterministic. With stochastic risk, cumulative dose becomes more important, and inheritable genetic damage or malignant changes in cells can take place over time. No threshold dose dictates when these effects occur. (2,18)
The National Council on Radiation Protection and Measurements (NCRP) published maximum permissible doses for occupational exposure in addition to the overall limit of 50 mSv per year. Important in fluoroscopy is the limit for skin, hands and feet, frequently irradiated areas for fluoroscopic operators. The NCRP maximum permissible dose is 500 mSv per year for the skin, hands and feet. In addition, the NCRP sets a cumulative dose limit of 10 mSv x age in years. (18)
To monitor personal exposure, most fluoroscopic staff wear monitors at all times. Those involved in fluoroscopy should place their monitors on the collar above the protective apron. (7) In interventional situations, personnel should wear monitors midline at the waist, under the lead apron; midline at the thyroid; and above the lead thyroid collar. Personnel who might place their hands in the x-ray beam should wear dosimeter rings. (18)
Fluoroscopic Personnel Radiation Protection
Aside from the ALARA principle and compliance with state and departmental monitoring and exposure reduction policies or regulations, fluoroscopic personnel can use several protective devices and techniques to minimize occupational exposure. (2,18)
The most effective methods for decreasing radiation exposure to personnel are those that also decrease patient exposure. Scatter radiation to staff is directly influenced by safe, effective procedure and technique, such as low framing speed and self-surveillance with electronic radiation dosimeters. (35)
A recent report studied the use of a shield placed on the patient to attenuate scatter radiation. The device used was a commercially available lead-free, disposable drape that was sterile and provided substantial radiation protection to the primary operator. Authors also found that the device could provide protection to support personnel in the cardiac catheterization laboratory during fluoroscopy. (See Fig. 9.)
[FIGURE 9 OMITTED]
Although some operators object to the time and expense, the authors found that opening the package and positioning the drape took less than 1 minute per procedure. Cost for the drape was reportedly $34. One noted drawback was that the drape's material is radiopaque, which can lead to the need to reposition the drape throughout a procedure. However, the authors found that the material is flexible enough for easy adjustment and that if the largest area of the drape is initially placed nearest the operator, little adjustment should be needed. (2)
Interventional protection typically includes movable shielding that provides shadows for the operators and support staff. Table-mounted lead flaps and ceiling-mounted face shields, as well as floor-mounted mobile devices assist interventional personnel. The movable shields are most effective if placed close to the patient where they block the line of sight to the x-ray beam's entrance point. (18)
All personnel involved in fluoroscopic procedures should wear appropriate protective garments. Although lead aprons offer excellent protection, their weight at typical 0.5 mm thickness (up to 22 pounds) can be cumbersome and can cause considerable fatigue by the end of a lengthy fluoroscopic procedure. Some aprons contain tin or similar elements. Protective aprons for angiography should be of wrap-around design. If the armholes are too large, much of the protective effect is compromised. (7,18)
Separate thyroid collars extend from lead aprons to shield the thyroid with little discomfort to the wearer. Most are approximately 0.5 mm in thickness. Lead gloves also are commercially available and offer limited protection from scatter radiation. A misconception persists that these gloves offer adequate protection for the operator's hands placed in the primary x-ray beam. On the contrary, interventional systems' automatic dose-rate control features produce enough output to penetrate the lead glove when it is present in the measuring field. Leaded eyeglasses often are recommended to prevent deterministic effects from exposure. Radiogenic cataracts can occur with a single fraction threshold of 2 Gy.
An additional method to reduce operator exposure involves good communication and choreography of patient care. Staff involved in the procedure can be carefully orchestrated to move in and out efficiently when the x-ray beam is off and to keep staff distance to a maximum level. When possible, staff should approach from the image intensifier side of the fluoroscopy system. (18)
Despite the FDA's health advisory, reports of radiation burns from fluoroscopic procedures continue. A group made up of federal and state regulators, medical physicists and industry representatives is asking the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) to require training and credentialing in radiation safety for any physician who operates fluoroscopy equipment.
Meanwhile, the FDA is working on proposed amendments to its fluoroscopy equipment safety standards, calling for improved beam filtration, real-time displays of current and cumulative exposure and tighter beam collimation. (36)
Improved awareness of radiation-induced skin injury and both stochastic and deterministic effects of radiation have spurred advances in the design of fluoroscopic equipment and protective devices as well as in procedural techniques. With appropriate education and attention, operators can reduce radiation dose to patients and personnel substantially. (21)
Approaches to Automatic Exposure Control Milliamperage Pulse Width Kilovolt Peak Control Control Control Automatic Manual Fixed Automatic with manual threshold Manual Fixed Manual Automatic Fixed Automatic Automatic Fixed Automatic Manual Automatic Automatic with manual filter selection Automatic Fixed or automatic Table 1 Radiation Exposures During Diagnostic Procedures (1) Procedure Mean Fluoroscopic Mean Entrance Exposure Time Skin Dose In Minutes (Range) In Milligrays * (Range) Barium enema 3.3 (<1 -5) 44 (23-59) Barium swallow 3.8 (2.5-6.1) 66 (41-150) Renal angiography 5.1 (2.9-7.6) 100 (80-220) Cerebral angiography 12.1 (2.9-36) 22 (60-590) Hepatic angiography 12.1 (3.6-42) 340 (100-580) Percutaneous transhepatic cholangiography 14.6 (2.9-44) 210 (30-520) * 10 mGy = 1 rad Table 2 Radiation Exposures During Interventional Procedures (1) Procedure Mean Fluoroscopic Mean Entrance Exposure Time Skin Dose In Minutes (Range) In Milligrays * (Range) Nephrostomy 7.0 (1.3-21) 110 (<1-410) Insertion or removal 7.1 (0.6-26) 110 (10-370) of biliary stent Endoscopic retrograde cholangiopancreatography 5.1 (2.9-7.6) 100 (80-220) Cerebral embolization 34.1 (15-56) 340 (190-660) * 10 mGy = 1 rad Table 3 Doses of X-radiation In Some Diagnostic Imaging Procedures (4) Procedure Bone Marrow Gonadal Dose (b) Dose (a) (mGy) (mGy) Plain chest film (PA and lateral) 0.03-0.1 <0.001 Plain abdominal film 0.2-1.3 0.06, 0.85 Lumbosacral spine 0.2 0.04, 0.5 Dental film 0.02 negligible Descending urography (intravenous pyelography) 0.22-5.8 0.09, 4 Upper GI (with barium) 5.1-8 variable Colon (barium enema) variable 0.5, 16 Head CT variable 0.07 Body CT 0.7-2.52 0.0004, 0.0145 Film mammography 0.4 mGy to breast Fluoroscopy 2-5 cGy (2-5 rad) per minute to the skin. Doses to marrow and gonads generally are higher than in other procedures, depending on time and the site imaged. (a) Refers to hemopoietic bone marrow (red marrow). (b) The first gonadal dose refers to the testes, the second to the ovaries. Note: These figures are based on data from various sources and are only approximations. Body CT values assume 20 slices, each 5 to 10 mm thick. In general, the skin (entrance) dose is higher than the marrow or gonadal dose (eg, 4.5-8 mGy to the skin for the lumbosacral spine and 20-50 mGy to the skin for a head CT). Table 4 Suggested Receptor Entrance Exposures For Fluoroscopic Imaging Modes (1) Operational Mode REE * REE Range Range ([micro]R) (nC/kg) ([double ([dagger]) dagger]) Normal fluoroscopy (a) 0.39-0.65 1.5-2.5 High-dose fluoroscopy (a) 0.77-1.55 3-6 Cine film fluoroscopy (b) 2.58-3.87 10-15 Digital angiography (b) 12.9-25.8 50-100 Digital subtraction angiography (b) 129-258 500-1000 Screen-film imaging (400 speed) (b) 77.4 300 Note: Values are for an image intensifier with a 23 to 25 cm field of view without a grid at 80kVp. * REE indicates receptor entrance exposure, ([dagger]) nanocuries per kilogram ([double dagger]) microroentgens (a) per video frame. (b) per image. Table 5 Human Responses to Ionizing Radiation (7) Early Effects Acute radiation syndrome Hematologic syndrome Gastrointestinal syndrome Central nervous system syndrome Local tissue damage Skin Gonads Extremities Hematologic depression Cytogenic damage Late Effects Leukemia Other malignant disease Bone cancer Lung cancer Breast cancer Local tissue damage Skin Gonads Eyes Life span shortening Genetically significant dose Effects of Fetal Irradiation Prenatal death Neonatal death Congenital malformation Childhood malignancy Diminished growth and development Table 6 Early and Delayed Manifestations of Radiation Injury (4) Tissue/Cell Type Early Delayed Radiosensitive Necrosis, Ischemic atrophy; epithelial and primarily ulceration; impaired parenchymal of stem cells stem cell reserves; cells atypia, dysplasia and neoplasia Vessels Dilatation; Sclerosis of small arteries necrosis and arterioles; capillary of endothelial telangiectasia; hypertrophy cells; of endothelial cells increased vascular permeability Stroma Edema Fibrosis; collagen deposition; abnormal fibroblasts Table 7 Examples of Skin Injuries From Fluoroscopy (27) Nature of Exposure Patient Sex/Age Procedure Injury Time A M/40 Coronary Skin necrosis Unknown-- estimated angiography requiring to have and PTCA * a 12 x 10-cm exceeded followed by skin graft 120 minutes a second coronary angiography B F/? RF cardiac 7.5 x 12.5-cm Unknown catheter second-degree ablation skin burn C F/25 RF cardiac Skin breakdown Unknown -- procedure catheter 3 weeks time of ablation postprocedure 325 minutes D F/34 RF cardiac Draining skin Unknown -- procedure catheter lesion on time of ablation back 5 weeks 190 minutes postprocedure E F/62 Balloon Burn-like Unknown dilation of injury on duct back requiring anastomosis skin graft F F/61 Renal Skin necrosis Unknown -- procedure angioplasty requiring time of skin graft 165 minutes * percutaneous transluminal coronary angiography Table 8 Breakdown of Estimated Total Skin Exposure To 1 Patient From a Series of Biliary Procedures (27) Procedure Fluoroscopy Estimated Skin Exposure Time Exposure From (Minutes) Fluoroscopy (R) Percutaneous 21 184 cholangiogram Mesenteric angiogram 187 1536 and multiple embolizations (2) Hepatic embolizations 58 419 Total skin exposure 266 2139 from mode (3) Procedure Total No. of DSA Estimated Skin Frames Exposure From DSA Frames (R) Percutaneous 16 2 cholangiogram Mesenteric angiogram 325 258 and multiple embolizations (2) Hepatic embolizations 149 90 Total skin exposure 490 350 from mode (3) (1) Procedures were performed during a 4-week period. Estimates are of entrance skin exposure and do not include backscatter. (2) Two different fluoroscopy systems were used, due to equipment failure; multiple dose rates (magnification modes) were used. (3) Total exposure may not have been delivered to a single area of skin due to movement of the x-ray beam. Location of the beam was not monitored during various procedures.
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(11.) Fosbinder RA, Kelsey CA. Essentials of Radiologic Science. New York, NY: The McGraw-Hill Co; 2002.
(12.) Wang J, Blackburn TJ. The AAPM/RSNA physics tutorial for residents. X-ray image intensifiers for fluoroscopy. Radiographics. 2000;10:1471-1477.
(13.) Geise RA. The AAPM/RSNA physics tutorial for residents. Fluoroscopy: recording of fluoroscopic images and automatic exposure control. Radiographics. 2001;21:227-236.
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Directed Reading Continuing Education Quiz
Radiation Safety In Fluoroscopy
DRI0002009 Expiration Date: Aug. 31, 2004 * Approved for 2.5 Cat. A CE credits
To receive Category A continuing education credit for this Directed Reading, read the preceding article and circle the correct response to each statement. Choose the answer that is most correct based on the text. Transfer your responses to the answer sheet on Page 566 and then follow the directions for submitting the answer sheet to the American Society of Radiologic Technologists. You also may take Directed Reading quizzes online at www.asrt.org. Effective October 1, 2002, new and reinstated members will be ineligible to take Directed Readings from journals published prior to their most recent join date.
* Your answer sheet for this Directed Reading must be received in the ASRT office on or before this date.
1. The -- is the smallest quantity of any electromagnetic radiation. a. sine wave b. atom c. photon d. ion 2. Wavelength is defined as the: a. rise and fall of the sine wave b. distance from one crest or valley of the sine wave to another c. transfer of radiation energy to matter d. speed at which ionizing radiation travels 3. X-rays are more potent to living matter than heat or mechanical energy because they: a. enter tissues and cells in large individual packets of energy b. represent a larger overall total of absorbed energy c. move swiftly throughout the tissue d. have a long half-life 4. In fluoroscopy, projections are named based on the patient's position relative to the: a. image intensifier b. spot-film cassette c. fluoroscopic operator d. fluoroscopic table 5. Which of the following is true of rod vision? a. it operates better in bright light b. it is central rather than peripheral c. it is more sensitive to blue-green light d. it is not used in viewing fluoroscopic images 6. Generally, x-ray tube currents for fluoroscopy units are: a. 1 to 5 mA b. 5 to 50 mA c. 50 to 100 mA d. 100 to 500 mA 7. The part of the image intensifier that converts x-ray photons to light photons is the: a. photocathode b. input phosphor c. output phosphor d. input window 8. The ratio of the number of light photons at the image intensifier's output phosphor to the number of x-rays at the input phosphor is termed: a. flux gain b. automatic brightness control c. photoelectron count d. image distortion 9. High kVp and low mA generally are preferred for fluoroscopy. a. true b. false 10. Which of the following fluorographic recording systems features smaller, rectangular film? a. videotape recording b. digital charged couple device cameras c. photo-spot cameras d. cinefluorography 11. According to the text, one explanation for the increased number of fluoroscopically guided interventional procedures is: a. increased preference by managed care b. increased marketing efforts by manufacturers c. improved digital fluoroscope technology d. improved radiation safety and dose reduction efforts 12. According to the article, a barium swallow study provides a mean entrance skin dose of -- mGy. a. 44 b. 66 c. 100 d. 220 13. According to the article, cerebral embolization has a mean fluoroscopic exposure time of -- minutes. a. 7.0 b. 13.6 c. 34.1 d. 45.8 14. Imaging of the heart's blood vessels after injection of contrast media is called: a. angioplasty b. coronary angiography c. radiofrequency ablation d. thrombolysis 15. At a distance of 1 meter from the patient, the scatter radiation in fluoroscopy equals about --% of the useful beam's intensity. a. 0.1 b. 0.5 c. 10 d. 15 16. The sievert corresponds to a dose producing a biologic effect equal to: a. 1 cGy of x-ray or gamma ray b. 1 Gy of x-ray or gamma ray c. 10 rem d. 1000 rem 17. The skin entrance exposure rate measures the exposure rate at the: a. surface of the image receptor required to produce a single image b. surface of the patient required to produce a single image c. center of the radiation field where the x-ray beam enters the patient's body d. center of the image receptor where the beam exits the x-ray tube 18. What is the typical dose skin entrance exposure rate for an adult of medium build? a. 0.3 mGy/min b. 3 mGy/min c. 30 mGy/min d. 30 Gy/min 19. The indirect effect of ionizing radiation is mediated by: a. the target molecule b. free radicals c. water d. ionized atoms 20. According to the article, which of the following is an example of a late effect of radiation on humans? a. breast cancer b. hematologic depression c. gastrointestinal syndrome d. congenital malformation 21. An example of a radiation-induced stochastic, effect is: a. erythema b. epilation c. telangiectasia d. leukemia 22. -- is a radiation-induced skin injury that occurs about 4 weeks after exposure at doses as low as 10 Gy but normally about 14 Gy. In its first stage, it appears like a flaking sunburn. a. Induration b. Desquamation c. Dermal necrosis d. Telangiectasia 23. -- is a radiation-induced skin injury that occurs at doses of about 10 Gy and causes the skin and subcutaneous fat to feel woodlike upon palpation. a. Induration b. Dermal necrosis c. Erythema d. Desquamation 24. The primary control of beam quality and penetrability is: a. mA b. automatic brightness control c. image magnification d. kVp 25. Doubling the distance from the source reduces the intensity of x-rays by a factor of: a. 1 b. 2 c. 4 d. 6 26. According to the text, which of the following fluoroscopic features almost always results in higher patient doses? a. collimation b. last image hold c. image magnification d. digital zoom 27. According to the text, which of the following has been shown to contribute to the severity of radiation wounds? a. size of the radiation field b. grid use c. image magnification d. lack of proper shielding 28. The NCRP set a maximum permissible annual dose rate of -- for occupational exposure to the skin, hands and feet. a. 5 mSv per year b. 50 mSv per year c. 500 mSv per year d. 10 mSv x age in years 29. Gloves offer adequate protection for operators' hands when placed in the primary beam. a. true b. false
Teresa Norris, B.A., is a freelance writer living in Albuquerque, New Mexico. She has worked in health care communications for more than 15 years and is a member of the American Medical Writers Association. In addition to continuing education manuscripts, Ms. Norris has written and edited medical essays for consumers and edits a national newsletter on medical practice management. She is a former business development director for a large radiology practice and the recipient of health care writing awards.
Reprint requests may be sent to the American Society of Radiologic Technologists, Communications Department, 15000 Central Ave. SE, Albuquerque, NM 87123-3917.
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|Author:||Norris, Teresa G.|
|Date:||Jul 1, 2002|
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