Am I in danger? How far away should you stand during an x-ray exam?
We have all been there. We help a patient sit upright, place a cassette behind his or her back, walk 6 feet away, simply utter a word and pandemonium occurs. People start diving behind walls or racing across the hallway simply because we said the word "x-ray."
After witnessing this a few times, a couple of us R.T.s started to wonder if we were in danger. Did these folks know something we didn't? We harkened back to our training days for the resolution. A refresher course in safety definitely was in order. So we dusted off our books and grabbed a few additional ones to investigate the issue of proximity to an x-ray source. We broke our task into three subtopics: defining radiation, studying how radiation loses energy and determining a relatively safe distance to stand from the x-ray source.
Radiation and Its Dangers
Electromagnetic radiation is photons of various energy levels. X-radiation, specifically, is an ionizing type of radiation, that is, radiation that potentially is harmful to living tissue because the energy level is high enough to ionize atoms. Energy levels above 13.6 eV have enough power to disrupt or ionize matter. In contrast to microwave or radiofrequency wavelengths, x-radiation is strong enough to penetrate and disrupt cells because it is capable of producing photons well above 13.6 eV. X-radiation is one of many sources of ionizing radiation.
Radiation is in our atmosphere, in our soil and in a few consumer products, namely cathode-ray televisions and smoke detectors. We even produce ionizing radiation ourselves. Our exposure ranges from 200 mRem inhaled annually from radon in the atmosphere to 10mRem annually from TVs and smoke detectors, according to Christensen's Physics of Diagnostic Radiology.
The danger from ionizing radiation varies. The more you receive, the more harmful it can be. Additionally, the longer you are exposed to ionizing radiation, the more danger there is.
At 25,000 mRem, the human body shows signs of impairment. If a person receives more than 300,000 mRem of ionizing radiation in one occurrence, there is a 50 percent chance of mortality.
It's a Matter of Energy
Since medical x-rays are created by the radiographic unit, one important factor in determining their energy level is the original technical exposure factors used to create the image. All photons are not equal; they have different energy levels. R.T.s adjust the amount or quantity of photons created primarily through choosing mA and time settings. The more mA applied, the more electrons are available for the creation of photons; the more time you have to create photons, the more of them there will be. Therefore, mA and time primarily determine quantity.
The strength or quality of photons is associated with kVp, which determines the penetrating ability of the photons. However, because the x-ray beam is heterogeneous, there always will be relatively weak photons no matter what kVp setting is used. When kVp is escalated, more powerful photons are added, creating a beam with a mixture of weak, medium and strong photons. Maximum energy is determined by the kVp used to create the primary x-ray beam.
After these diverse photons depart from their source of production, they encounter a number of impediments that significantly reduce their overall effectiveness. Photons diverge as they leave the x-ray tube, dispersing in random straight lines, and are absorbed by anything in their path. As a result, the number of photons in a given area (photon fluence) significantly decreases the farther away they are from the source.
As photons pass through an object, including room air, they lose power as they interact with that object's atoms. This interaction, or attenuation, depends on the thickness and density of the object. Relatively dense items such as bones attenuate more photons than less dense tissues. Similarly, a thicker anatomical structure such as a femur attenuates more photons than a thinner structure such as a finger.
The combination of a heterogeneous primary beam and diverging and attenuated photons results in intensity loss, which is the amount of energy per area and per unit of time. The farther we stand from the beam's source, the less powerful the beam. Using the inverse square law, intensity is reduced by one quarter every time the distance is doubled. For example, if you are 1 foot from the source in an area with an intensity level of 10 R, moving an additional foot away reduces the intensity to 2.5 R.
Exposure Levels Count
Determining how far from the source R.T.s should stand when conducting a radiographic exam is complicated. The primary beam, leakage from the x-ray tube and scatter radiation from the patient must be taken into consideration. Additionally, what type of dose amount is being considered--gonadal, organ, marrow? To keep this as simple as possible, I will only reference the radiation that reaches the topical skin and leave the other types of measurements for another time.
There are very few practices that potentially expose R.T.s and others in the room to the primary beam. This danger could come from holding the patient or improperly collimating and misdirecting a horizontal tube. The three most common exams that require a horizontal beam from a portable machine are the AP chest, lateral cross-table C-spine and surgical cross-table lateral hip. R.T.s should adjust the mobile equipment to ensure that the primary beam is never directed toward the operator. The following examples list patient entrance skin exposure doses at the range of recommended SID, from Hendee and Ritenour's Medical Imaging Physics.
For an AP chest exam, with 90 kVP at 4 mAs, the exposure is 19.5 mR at 6 feet SID and 4.8 mR at 12 feet SID.
For a lateral cross-table C-spine exam with 80 kVp at 32 mAs, the exposure is 124.6 mR at 6 feet SID and 31.15 mR at 12 feet SID.
I used my medical facility's ER exposure chart to calculate the exposure of a surgical cross-table lateral hip exam using 90 kVp at 80 mAs with an SID of 3 feet 6 inches. The exposure is 1,113 mR. At an SID of 9 feet 6 inches, it's 142 mR.
Scatter radiation, particularly Compton scatter, causes great alarm because it is the major source of exposure to the R.T. Compton scatter occurs when a photon from the primary beam deflected from its primary path.
The amount of scatter depends on the exposure technique used to produce the image, field size, patient thickness, object density and even room design. Important variables include the composition of the room walls and how close they are because the electron continues to bounce off objects until it is completely absorbed loses its energy. Scattered photons are of different strengths, depending on their angle of dispersion. The most energy is lost when a photon is scattered at a 180-degree angle and lessens it nears 0 degrees.
The following examples are based on scatter produced from a phantom that is 9 inches thick. The loss of energy is from kVp only, not mA, assuming all the photons are deflected at the same angle and none are absorbed or otherwise alleviated. The numbers result in significantly higher dose calculations than would occur in reality, and go beyond worst-case scenario.
The data include the same three exams that were used for the primary beam exposure and two more: the supine abdomen exam, which is frequently ordered for inpatients, and the lateral spot L-spine exam, which requires the most radiation to produce an image.
Using a 6 foot distance, the scatter from an AP chest exam at 90 degrees, according to Stoker's Introduction to Chemical Principles, is 3.6 mR. For a lateral cross-table C-spine exam, it's 3.4 mR; 234.5 mR for a surgical cross-table lateral hip exam; 76 mR for a supine abdomen exam and 351.8 mR for a lateral spot L-spine exam.
Now to the Question
Putting everything in perspective, keep in mind that there is no escape from ionizing radiation. It occurs naturally from the sky and the rocks and soil we walk on. We welcome it into our homes with open arms when we use smoke detectors to protect our loved ones. Since we can't avoid exposure to natural, background radiation, we must try to limit, whenever possible, our occupational exposure.
In a real-life scenario, adhering to the ALARA (as low as reasonably achievable) concept and practicing sound radiation safety principles will lessen our exposure. If the R.T. stands at a 180-degree angle from the patient, is 6 feet away from the x-ray source, wears the appropriate protective apparel and is not in the beam's primary path, the occupational exposure should be minimal.
The old adage that knowledge is power applies well here. Most institutions have policies for staff technologists on radiation protection, and are regulated by agencies such as the National Council on Radiation Protection & Measurements. There is a need to follow the cardinal principles of radiation safety, which include minimizing the time you are exposed to ionizing radiation, maximizing the distance from the source and maximizing shielding. When those practices are followed, no one should feel the need to run for cover.
George Tolekidis, B.A., R.T.(R), works part time at Sutter Solano Medical Center in Vallejo, Calif. He works full time at nearby Travis Air Force Base where he is on active duty.
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By George Tolekidis, B.A., R.T.(R), Contributing Writer
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|Date:||Feb 1, 2007|
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