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Radiation Dose In Computed Tomography.

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Radiation exposure of patients in computed tomography (CT) is receiving attention in the literature, and the general conclusion is that CT now is considered a high-dose procedure. For example, Gray[1] identifies CT as an examination with the potential for unlimited exposures. More importantly, he states, "Most imaging professionals are not aware of the high doses of radiation that CT produces. It is therefore essential to educate the imaging community and referring physicians about CT's high exposures. Although the use of this modality should not be discouraged where its benefits are clear, referring physicians should be aware that CT is one of the highest radiation exposure examinations in diagnostic radiology."[1]

With this in mind, there are g central questions that should be of interest to all those working in CT:

* Why are the doses in CT so high?

* What is the dose to the patient at my institution?

* What can be done to reduce the high exposures in CT to protect both patients and personnel?

There are 2 reasons why these questions are important First, CT images can be improved if the dose is increased (dose-limited technique). Because of the digital nature of CT, a higher dose does not render the image dark and useless, as in conventional radiography.. Such images can be improved by digital image processing. Second, and perhaps most important, is the radiation risk to the patient having the examination. In this regard, the dose for a CT examination may have to be estimated to make decisions regarding the benefits vs risks of the procedure. CT manufacturers are now required by law to provide a dose table that shows the doses delivered to patients from their CT scanners.

The purpose of this article is to describe the fundamental concepts of dose in CT that are important to the CT technologist. Major topics covered arc radiation quantities and their units; factors affecting dose in CT; CT dosimetry, including dose phantoms and measurement concepts; dose in spiral/helical CT; dose reduction techniques and radiation protection considerations.

Relevant Radiation Quantities And Their Units: An Overview

Three radiation-related concepts have been mentioned so far: exposure, dose (radiation quantities) and the risks to the patient from a CT examination. A cursory glance at the literature on dose in CT indicates that the doses are reported in various units, such as roentgens, rads and rems (old units) or coulombs per kilogram, grays and sieverts (International System or SI units), depending on what is being measured and described. These units are associated with several radiation quantities and a brief review is in order. The radiation quantities relevant to this topic are exposure, absorbed dose, dose equivalent (equivalent dose) and effective dose.

Exposure

Exposure is a quantity that can be measured easily. It refers to the concentration of radiation at a particular point on the patient. Exposure can be measured by an ionization chamber positioned at the point of measurement. Radiation falling on the chamber ionizes the air in the chamber to produce ion pairs (charges). The exposure now can be defined as a measure of the amount of ionization produced in a specific mass of air by x-rays or gamma radiation. The ionization indicates the amount of radiation to which a patient is exposed.

The conventional unit of exposure is the roentgen (R); the SI unit is the coulomb per kilogram (C/kg). One roentgen produces 2.58 x [10.sup.-4] C/kg of air at standard temperature and pressure. Exposure is reported in the literature in milliroentgens (mR), a much smaller unit (1 R = 1000 mR) or in microcoulombs/kilogram ([micro] C/kg) where 1 R = 258 [micro] C/kg.

Absorbed Dose

In terms of radiation protection, the significant radiation quantity is the absorbed dose. This is the unit of energy absorbed per unit mass of material (ie, tissue). Any risk associated with radiation is related to the amount of energy absorbed.

The conventional unit of absorbed dose is the rad (r), which is equal to an energy absorption of 100 ergs per gram of absorber. The SI unit of absorbed dose is the gray (Gy), named to honor Louis Harold Gray, a British radiobiologist who devised ways to measure the absorbed dose. The gray is defined as 1 joule (J) of energy deposited in 1 gram of material. One Gy is equal to 1 J/kg; 1 rad is equal to 0.01 Gy; 100 rad is equal to 1 Gy. Relevant submultiples of the gray are the centigray (cGy), which equals 1 rad, and the milligray (mGy), which equals 100 millirads (mrads). For the sake of simplicity, 1 rad is approximately equal to 0.01 Gy.

Dose Equivalent

The bioeffects of radiation are linked not only to the absorbed dose but also depend on the type and energy of the radiation. For example, radiations with a high rate of energy transfer will produce more biologic damage than radiations with a low rate of energy transfer. These differences in the biological effectiveness of the radiations are dealt with by a quantity referred to as the dose equivalent, H, which can be calculated as follows:

H=DxQ

where D is the absorbed dose and Q is a quality factor of the radiation. The Q for x-rays is 1, while the Q for neutrons is 10. This means that 1 rad of neutrons produces about the same biological damage as 10 rads of x-rays.

While the old unit of dose equivalent is the rem, the SI unit is the sievert (Sv), named to honor the Swedish physicist Rolf Maximillian Sievert. Dose equivalent is related to the absorbed dose as follows:

Dose Equivalent (Sv) = Absorbed Dose (Gy) x [W.sub.R]

where [W.sub.R] is a radiation weighting factor.

In radiology (including CT), technologists wear radiation badges to record their occupational exposures. These badges are evaluated in terms of sieverts (rems). While 1 Sv = 100 rem, 1 mSv = 100 mrem.

Equivalent Dose

In 1990, the International Commission on Radiological Protection (ICRP) recommended that the term dose equivalent (weighted absorbed dose at a point) be superseded by the term equivalent dose, [H.sub.T], which is a weighted absorbed dose in a tissue or organ rather than at a point.[2] The equivalent dose is therefore equal to the sum of the weighted absorbed doses.

Another point to note is that bioeffects also depend on the type of tissue or organ being irradiated, apart from the energy of the radiation. Consequently, the ICRP introduced yet another factor to account for this. This factor is the tissue weighting factor ([W.sub.T]), which can be found in tables established by the ICRP.

Effective Dose

When the equivalent dose ([H.sub.T]) is weighted by the tissue weighting factor, .another quantity emerges. This quantity is called the effective dose, E, which was previously referred to as the effective dose equivalent ([H.sub.E]), and is conceptually similar. E is used to quantify the risk from partial-body exposure as opposed to that from an equivalent whole-body dose. E is "the sum of the weighted equivalent doses in all tissues and organs of the body."[2]

Patient doses from diagnostic exams "are now being reported in terms of the effective dose equivalent or effective dose, by most national and international organizations."[3] For example, the National Council on Radiation Protection and Measurements (NCRP) lists the effective dose from a barium enema as 4.05 mSv.[4]

This simply means that the risk from a barium enema is equivalent to the risk of an exposure of 4.05 mSv to the whole body.

Why use effective dose equivalent or the effective dose? Because background radiation always is present in the environment, it may be useful to compare patient effective doses from various examinations with that of natural background radiation. Additionally, as Castronovo[5] points out, these doses may be used for informed consent for humans who volunteer to participate in research programs involving additional radiation exposure. A more comprehensive review of these dosimetric quantities and their units is provided in Radiation Protection.[6]

Typical Patient Doses From Radiological Procedures

As stated earlier, CT is one of the radiologic examinations that delivers the highest radiation exposures to patients. Various studies on dose to patients in radiology have substantiated this claim. Typical doses are shown in Table 1 in terms of entrance and gonadal exposures in mrads. Table 2 lists the effective doses (mSv) from various examinations.

Table 1 Typical Patient Doses From Various Radiologic Examinations(7)
Examination      Entrance Dose    Gonadal Dose
                   (mrad)            (mrad)

Skull                200              <1
Chest                 10              <1
Lumbar spine         300             225
Abdomen              400             125
Pelvis               150             150
CT head             3000              50
CT pelvis           4000            3000


Table 2 Effective Doses From Various Examinations(8)
Examination        Effective Dose
                       (mSv)

Skull series          0.05
Chest                 0.05
Lumbar spine          1
Abdomen               1
Pelvis                1.6
CT head               2
CT chest              7
CT abdomen            9
CT pelvis             9


To put these numbers in perspective, one can compare doses from medical exposures, airplane flights and living in Denver. The dose from a roundtrip flight from San Francisco to Boston is 0.05 mSv. Natural background exposure for someone who lives in Denver (the "Mile High City") for 1 year is 0.5 mSv. Therefore, compared to background levels from these 2 activities, CT delivers very high doses.

Biological Effects of Radiation

The most important reason for having a clear understanding of the dose in CT relates to the biological effects of radiation. These effects are classified as stochastic and deterministic (nonstochastic). It is not within the scope of this paper to outline the details of these effects; however, for the sake of relating these effects to dose in CT, a brief review is in order.

Stochastic Effects

Stochastic effects are effects for which the probability (rather than the severity) of the effect occurring depends on the dose. Probability increases linearly with increasing dose, and there is no threshold dose for these effects. This is demonstrated using the linear dose-response model without a threshold. (See Fig. 1.) This is the radiation risk model most favored by radiobiologists in estimating the risk of exposure in radiology, instead of supra linear or linear-quadratic models with no threshold. All of our radiation protection standards, guidelines and recommendations are based on this model.

[Figure 1 ILLUSTRATION OMITTED]

The dose-response model states that radiation risk increases as dose increases and that there is no threshold dose. Even a small dose has the potential to cause a biological effect. There is no risk-free dose. Examples of stochastic effects include cancer, leukemia and hereditary effects. Stochastic effects are considered late effects, because they occur years after the exposure. As Specht and Russo[9] note, "The NCRP has estimated the risk of developing a fatal cancer at 4 in 10000 (0.04%) per rem of radiation exposure."[9]

Deterministic Effects

Deterministic (nonstochastic) effects are those for which the severity of the effect (rather than the probability) increases with increasing dose and for which there is a threshold dose. Below the threshold dose, these effects are not observed. Threshold doses are considered to be relatively high doses that can kill cells and cause degenerative changes in tissues exposed to radiation. Examples of deterministic effects include skin erythema, epilation, pericarditis and cataracts. The threshold dose for cataracts, for example, is about 2 Gy (200 rads).

Exposure to the Conceptus (Embryo and Fetus)

Finally, in terms of bioeffects of radiation exposure, it is important to consider the situation of a woman who undergoes a CT examination and later finds out that she was pregnant at the time of the exam. In this case, it is necessary to estimate the dose to the conceptus. Knowing how to measure the dose in CT can provide a solution to this problem.

Dose Distribution in Radiography and CT

The dose distribution pattern in a patient is different in radiography than CT because of the geometric aspects of data gathering and the attenuation of radiation as it passes through the patient. The dose distributions or typical exposure patterns (depth dose curves) for radiography and CT are illustrated in Fig. 2.

[Figure 2 ILLUSTRATION OMITTED]

The entrance exposure is 100% and is sometimes used to represent risk. However, this is an overestimate because the dose decreases as it passes through the patient. This decrease is due to attenuation and the inverse square law. The dose distribution pattern for CT is somewhat different from in radiography and it is clear that the dose fall-off in the middle of the patient is much less than in radiography. However, the dose is greater in the middle of the patient for CT. Additionally, the dose distribution is more uniform in CT because the x-ray beam is rotating 360 [degrees] around the patient's body. (The dose will be less uniform for rotation less than 360 [degrees].) Fig. 2 shows the distribution for a single slice. Distribution will be somewhat different when multiple slices are imaged. This will be described later.

CT Beam Geometry

The term beam geometry refers to the size and shape of the x-ray beam emanating from the x-ray tube and passing through the patient to strike a set of detectors that collect radiation attenuation data. A typical beam geometry for CT is shown in Fig. 3. The important characteristics for dose in CT are:

[Figure 3 ILLUSTRATION OMITTED]

* A thin, fan-shaped beam is used along the z-axis (longitudinal axis) of the patient.

* The z-axis is perpendicular to the transverse axial slice.

* The thin beam of radiation is also perpendicular to the z-axis.

If the radiation intensity along the z-axis is plotted, a bell-shaped intensity profile is obtained. This profile also is referred to as the dose profile (see inset diagram in Fig. 3). It is extremely important to dose in CT, because it is this profile that is being measured. Ideally, a rectangular dose profile across the slice should be obtained (all radiation is confined to the slice), but in reality a bell-shaped dose distribution curve is obtained due to scatter and penumbra. The dose profile differs among CT scanners.

Factors Affecting Dose in CT

The size and shape of the dose profile depend (m the design of the CT scanner as well as on the selectable scan operating parameters.[10]

Design of the CT Scanner

Factors affecting the dose in CT that relate to the construction characteristics of a CT scanner include the scanner type and beam geometry, detector type and the beam quality.

* Scanner type/beam geometry. CT scanners have evolved into 2 types: single-slice conventional scanners and spiral/helical volume scanners. While single slice scanners acquire data one slice at a time, spiral/helical scanners acquire a volume of data and subsequently use computer processing to divide the volume into single slices. In this section, the discussion will focus on single slice CT scanners only. Spiral/helical CT dose will be discussed later.

Single slice scanners fall into 2 categories based oil their data collection method. Both methods are illustrated in Fig. 4. While both methods employ fan beam geometries and 360 [degrees] rotation of the x-ray tube around the patient, the fundamental differences are the detector configuration and the distance of the x-ray tube from the patient. In third-generation CT scanners, the x-ray tube and detectors are coupled and rotate synchronously around the patient. In fourth-generation scanners, the x-ray tube rotates 360 [degrees] around the patient but it is positioned within a stationary ring of detectors. This arrangement places the x-ray tube closer to the patient compared to third-generation scanners. With smaller source-to-object distances, the concentration of photons per unit area (skin dose) is somewhat greater, due to the inverse square law. Third-generation systems have a larger source-to-object distance and hence there is less concentration of photons per unit area for the same technique.

[Figure 4 ILLUSTRATION OMITTED]

Apart from the source-to-object distance, the beam geometry also influences the dose to the patient in CT. Such geometry depends on the size of the focal spot and the width of the prepatient collimation. Specifically, it is the penumbra from the focal spot that contributes to the dose. Larger focal spot sizes produce greater penumbra compared to smaller focal spot sizes. However, focal spot size is not considered a major contributing factor to the dose in CT, because smaller focal spot sizes are used in an attempt to reduce the dose.[10]

* Collimation. Collimation plays an important role in determining the dose to the patient. In radiography, collimation is intended to protect the patient by limiting the beam to the area of interest. If the collimation is made smaller, however, then exposure factors have to be increased (thus increasing dose) to maintain the same signal-to-noise on the film because scatter production is reduced.

In CT, collimation determines the width of the beam striking the patient and the detector. It also ensures a uniform beam width from the x-ray tube to the detector. The beam width should be such that it matches the size of the detector element. In general, as beam width decreases, the dose to the patient increases because more photons are required to maintain the same signal-to-noise ratio at the detectors. (In other words, noise is reduced because more photons produce a greater detector signal.)

* X-ray spectrum. The x-ray spectrum refers to the quality and quantity of the photons needed for CT imaging. While quality refers to the effective energy or penetrating power of the beam, quantity refers to the number of photons per unit energy in the x-ray beam. If the number of photons per unit energy is plotted as a function of energy, an x-ray spectrum is obtained.

In CT, the spectrum depends on the kVp, the voltage waveform and the tube filtration. All 3 of these factors affect the dose to the patient. It is not within the scope of this article to discuss the influence of voltage waveform on dose; however, filtration and kVp will be reviewed.

Beam quality (energy) depends not only on the kVp, but also on filtration. High kVp is needed for CT imaging "to provide adequate transmission through the patient while maintaining suitable differentiation between soft tissues. Increasing kVp while maintaining mAs results in higher dose and better image statistics. Increasing kVp and reducing mas can produce the same statistics with lower dose."[10]

In radiography, a filter is used to remove the low energy (long wavelength) x-rays from the beam, as these rays increase patient dose and do not play a role in image formation. The filter is therefore placed between the x-ray tube and the patient to absorb low-energy photons and protect the patient from unnecessary x-rays. In CT, a specially shaped aluminum, carbon or plastic filter is used to produce a uniform beam at the detector and to prevent beam hardening artifacts. Additionally, the filter reduces the dose to the patient's skin surface; however, the patient dose in this case is largely due to the size of the area irradiated and the size and positioning of the patient in the field of view. "Because different shaped filters may be used for different field sizes, patient dose is often more dependent on field size, patient size and position in the field when the shaped filters are used."[10]

* Detector technology. The CT detector system is a significant component of the CT scanner and is yet another design factor that affects patient dose. Specifically, the efficiency of the detectors affects dose. Detector efficiency refers to the amount of radiation absorbed by the detectors. Such efficiency can range from 50% to 99%.

Two types of detectors are used in CT systems: solid state detectors and gas-ionization detectors. While solid state detectors (scintillation crystals coupled to photodiodes) have efficiencies from 90% to 99%, gas ionization detectors (xenon, for example), have efficiencies ranging from about 50% to 60%. In general, the dose to the patient increases as the detector efficiency decreases.

Selectable Scan Operating Factors

The selectable scan operating factors are those that can be changed by the operator. Several operating factors have been identified and discussed in the literature.10,11 These include exposure technique factors, collimation, filtration, field size, placement of the patient within the scan field of view (FOV), patient orientation, slice thickness, repeat scans, factors that affect the appearance of the image, rotation angle for data collection and techniques such as dynamic examinations, dual-energy examinations and spiral/helical CT techniques.

* Exposure technique factors. These refer to kVp, mA and scan time used for the examination. Remember that mA and scan time are proportional to the dose. An increase in either of these factors increases the dose proportionally. If the kVp is increased without a corresponding change in mA and time, the dose will increase because kVp also plays a role in increasing the number of photons per unit energy.

* Filtration. as mentioned earlier, a filter removes the low energy photons and decreases the number of photons per unit energy. Therefore, the beam becomes harder (more penetrating, because the mean energy of the beam increases) and the dose to the patient decreases. A 4-mm aluminum added filter will decrease the quantity of photons by a factor of 2, compared to a 2-mm aluminum filter.

* Collimation. Collimation in CT is intended to ensure a constant beam width at the detector. It also determines the slice thickness. If narrower collimator widths are used, thin slices will be imaged. However, in order to maintain the same image statistics at wider widths, the dose will have to be increased tot CT imaging because more x-ray photons are required to maintain the image quality.

* Slice thickness. Thinner slices require more dose to the patient to maintain image quality. X-ray exposure factors are increased to maintain the same signal-to-noise ratio (ie, noise is kept constant).

* Slice spacing and the number of adjacent slices. These 2 parameters are determined by the radiologist and radiologic technologist. A large number of adjacent slices increases the volume of the patient that will be exposed to the beam. In this case, the dose increases because the dose profiles overlap. In addition, when the spacing between slices is reduced, the dose increases due to the contribution of scatter radiation from adjacent slices.

* Patient positioning and orientation. In CT, the patient should be centered precisely in the scan FOV. Off centering changes the dose distribution across the volume of tissue irradiated and distribution may be skewed to one side. Additionally, "patient orientation (eg, supine or prone positions) may significantly affect the dose to critical organs (eg, eyes) for scanners or scanning modes with asymmetric dose distributions."[10]

* Repeat scans. As pointed out by Romans,[12] "areas of the patient that are rescanned for contrast studies or other technical or clinical reasons receive additional radiation. The effects are cumulative."

* Factors affecting the appearance of the CT image. The appearance of the CT image on the monitor depends on the pixel size and the reconstruction filters used to enhance details in the image. These parameters affect the dose to the patient only if the exposure technique factors are increased to improve image quality. For example, when pixel size decreases to improve spatial resolution, the dose has to be increased to decrease the noise in each pixel.

* Rotation angle. The rotation angle refers to the angle through which data is collected. In the first- and second-generation CT scanners (now obsolete), data was collected through 180 [degrees]. Third- and fourth-generation scanners collect data through 360 [degrees] rotation or more (overscan). This means that an asymmetric dose results from rotation angles other than 360 [degrees].

* Other imaging techniques. These techniques include dynamic studies, dual-energy scans and spiral/helical data acquisition. Because spiral/helical CT has become current state-of-the-art CT technique, it will be discussed subsequently.

* Patient characteristics. Finally, dose in CT depends on patient characteristics such as size, shape and tissue density. Thicker patients and tissues that are more dense will result in greater dose due to an increase in radiation attenuation. According to Rothenberg and Pentlow,[10] "For 360 [degrees] scans, the scatter-to-primary ratio at the center of the patient will be higher, on average, than at the surface. For 180 [degrees] scans, the surface opposite the x-ray tube will have an even higher scatter-to-primary ratio. This effect broadens the dose profiles and increases the ratio of the multi-scan dose to the single scan dose."[10]

Image Quality and Dose Considerations

It is clearly apparent that image quality and dose are closely related. Image quality includes spatial resolution, contrast resolution and noise. While spatial resolution depends on geometric factors such as focal spot size, slice thickness and pixel size, contrast resolution and noise depend on both the quality (beam energy) and quantity (number of x-ray photons) of the radiation beam. Several mathematical equations have been derived to express the relationship between dose and image quality. For technologists, the following expression is important:

Dose: (Intensity x Beam Energy)/ ([Noise.sup.2] x Pixel [Size.sup.3] x Slice Thickness)

where intensity and beam energy depend on mA and kVp respectively and noise depends on the number of photons detected. This expression is read as follows: dose is directly proportional to the product of mA and kVp and inversely proportional to the product of noise squared, pixel size cubed and slice thickness.

The expression also implies the following about dose and image quality:

* To reduce the noise in an image by a factor of 2 requires an increase in the dose by a factor of 4.

* To improve the spatial resolution (pixel size) by a factor of 2 while keeping the noise constant requires an increase in the dose by a factor of 8.

* To decrease the slice thickness by a factor of 2 requires an increase in the dose by a factor of 2 (keeping the noise constant).

* To decrease both the slice thickness and the pixel size by a factor of 2 requires an increase in the dose by a factor of 16 ([2.sup.3] x 2 = 2 x 2 x 2 x 2).

* Increasing mA and kVp increases the dose proportionally. A 2-fold increase in mA increases the dose by a factor of 2.

The considerations explained above simply imply that there must be a trade-off between dose and image quality. The examination should be optimized to produce the best possible image quality with as low as reasonably achievable doses.

CT Dosimetry

The term dosimetry refers to the instrumentation and methods used to measure patient dose from a CT scanner. The measurement of CT doses is an important concept for CT technologists because such an understanding will help them to:

* Assist the CT physicist with dose measurements.

* Conduct the measurement when a physicist is not available.

* Compare their measured doses with the national average.

* Be a more integral part of CT acceptance testing and ongoing quality control programs.

* Estimate the dose from CT examinations that involve several scans.

* Participate actively in providing dose information to other hospital personnel.

This section describes early dose studies, dosimeters used to measure CT doses, the dose specification requirement for CT scanners, CT dosimetry phantoms and dose parameters specific to CT.

Early Dose Studies

Early dose studies provided the foundation for dose measurement in CT. Such a foundation later led to the introduction and development of concepts and parameters that reflect a more precise and accurate methodology for measuring and estimating the doses from a series of scans. In reviewing these studies, only reported doses will be listed, because it is not within the scope of this article to reflect on the methodological details of the studies.

The first dose study in CT was done by Perry and Bridges,[13] who investigated the radiation doses from the first EMI head scanner, invented by Nobel Prize winner Godfrey Hounsfield. They found the doses to the skull to be on the order of 2 R, received by the right side of the head. Their results showed that the dose to the head was comparable to that of a conventional skull series. This study paved the way and provided the motivation for further dose studies.

From 1974 to 1977, other researchers investigated the CT doses for other body parts. Their results are summarized in Table 3. In 1977, a study by Shrivastava, Lynn and Ting[15] reported an interesting finding. They stated, "The fact that the radiation exposure for a single section is only about half that due to 4 sections seems to be significant and to us a somewhat unexpected finding. This suggested that scattered and leakage radiations contribute about half the total exposure in spite of the common belief that because of the narrow beam utilized in this unit, the scatter is negligible."[15]

Table 3 CT Dose to Body Parts[14]
Body part   Dose range
Head        0.6-1.5 R
Right eye   200-2800 mR
Left eye    200-1300 mR
Orbit       2.5-7.0 R
Body        0.4-1.9 R
Thyroid     107 mR
Chest       112 mR
Gonads      7 mR


Types of Dosimeters

CT dose studies continued from 1977 to 1979, with the major goal of developing a methodology for measuring dose that was accurate and easy to implement. Various measurement schemes were developed using film dosimetry, thermoluminescent dosimetry (TLD) and specially designed ionization chambers.[11] In 1981, the Bureau of Radiological Health (BRH), now the Center for Devices and Radiological Health (CDRH), introduced a significant step toward CT dose measurement that was not only accurate but also easy to perform compared with earlier measurement techniques.

The BRH suggested a measurement procedure based on 2 concepts, the Computed Tomography Dose Index (CTDI) and the Multiple Scan Average Dose (MSAD), using a single ionization chamber measurement. Because the CTDI/MSAD concept is state-of-the-art methodology for providing CT dose estimates, the use of the TLD and film dosimetry techniques will not be described here. While TLDs can provide "detailed quantitative measurements of dose profiles," it is time consuming, costly and requires personnel trained ill TLD techniques and handling.[16] Film dosimetry, on the other hand, "has been used primarily to obtain qualitative evaluations of the dose from different section thicknesses available on a scanner, particularly the thinnest sections."[10]

The CTDI/MSAD method suggested by the CDRH uses a pencil ionization chamber shown in Fig. 5. The chamber is the measuring instrument and it consists of a chamber filled with air. Radiation falling on the chamber ionizes the air and the resulting charge is collected and measured in coulombs (1 C = 1.6 x [10.sup.19] electrons). The amount of charge collected (Q) is directly proportional to the amount of radiation passing through the chamber. It is critical to note at this point that the ionization chamber is measuring the exposure and not dose. Therefore, a factor is used to convert exposure to dose.

[Figure 5 ILLUSTRATION OMITTED]

Phantoms for CT Dose Measurement

In addition to the concepts of CTDI and MSAD, the CDRH also suggested the use of 2 differently sized phantoms made of acrylic (polymethylmethacrylate) to standardize CT dose measurements. The acrylic is not only homogeneous but also tissue equivalent.

While 1 phantom simulates the size of a patient's head, the other, somewhat larger, phantom is intended to simulate a patient's body. The shape and dimensions of both phantoms are illustrated in Fig. 6. Both phantoms have holes drilled at specific locations to accommodate the ionization chamber during dose measurement. The chamber is positioned in 1 hole at a time while the other holes are filled with acrylic plugs. An exposure is subsequently made and recorded. This is done for all holes so that dose measurements can be obtained for a number of positions in the phantom. Fig. 7 is a photograph of a head phantom together with a pencil ionization chamber attached to the instrument that measures the charge from the chamber.

[Figures 6-7 ILLUSTRATION OMITTED]

CT Dose Parameters: The CTDI and the MSAD

Earlier dose studies reported CT doses using various terms to describe the absorbed dose in a CT examination. These included single-scan peak dose, multiple-scan peak dose, dose profile and others such as multiple scan average dose.[17] These results led the CDRH to introduce and recommend the use of CTDI and MSAD in the Federal Performance Standard as the dose parameters specific to CT, because they are "related to the clinical conditions of use under which the dose is delivered."[17] For example, because most CT examinations consist of a series of scans (slices), the MSAD would be the dose descriptor for use in a clinical situation.

* The MSAD. The MSAD is the average dose at the center of a series of scans.17 The concept is illustrated in Fig. 8. The following points help to explain the concept:

1. The diagram in C represents the dose distribution or dose profile for a single scan (recall Fig. 3). The dose distribution can be described by the function D(z). In general, D(z) differs from 1 CT scanner to another. The area under the single scan dose profile (C, the shaded portion) is numerically equal to the integral in the equation shown below in D.

2. In A, the dose distribution for 7 scans is shown with each scan separated by some distance BI, the bed index distance. Notice that the profiles overlap.

3. The doses from all 7 scans (total dose) are added together and the total dose looks like the curve shown in B. Observe that where the profiles overlap in A, the result is a higher dose than from only one scan.

4. When the peaks and valleys of the curve shown in B are sampled, an average dose can be calculated (the dotted line running through the curve shown in B). This average dose is the MSAD. Mathematically, this can be written as:

MSAD = CTDI x (SW/BI)

where SW = slice width (in millimeters), BI = bed index or slice spacing and CTDI = Computed Tomography Dose Index.

[Figure 8 ILLUSTRATION OMITTED]

* The CTDI. CTDI is the MSAD at the center of a series of 14 contiguous scans[17] and is the dose descriptor used in the Federal Performance Standard for CT scanners. Manufacturers of CT scanners must provide the CTDI value as a standard index for each CT scanner they market.

The CTDI concept is illustrated in Fig. 9. Note the following points:

1. The dose distribution (profile) for a single scan is shown and is described by the function D (z).

2. The area under the single curve D(z) (the shaded region) is numerically equal to the integral in the equation.

3. The CTDI is obtained by dividing this area by the slice width (SW), as shown in the equation.

4. The CTDI then is used to calculate the MSAD. When the SW equals the BI, the MSAD equals CTDI.

[Figure 9 ILLUSTRATION OMITTED]

The CTDI values for 2 CT scanners are shown in Table 4. While the CTDI for the head imaged with a scanner that uses gas-ionization detectors is 50 mGy, it is 40 mGy for a CT scanner that uses solid-state detectors. On the other hand, the CTDI values for the body are listed as 14 mGy and 11 mGy for scanners using gas-ionization and solid-state detectors respectively.

Table 4 Published CTDI Values From 2 Manufacturers
Unit Type       kVp   MA    Time      Slice     Head    Body
                            (sec)   Thickness   CTDI    CTDI
                                      (mm)      (mGy)   (mGy)

Gas detectors   120   170     2        10        50      14

Solid-state     120   170     2        10        40      11
detectors


* Measuring the CTDI and calculating the MSAD. Measurement of the CTDI involves the following steps:

1. The pencil ionization chamber is placed into 1 of the holes in the phantom, while the other holes are filled with acrylic plugs.

2. An exposure is made for a single scan, and the chamber measures the exposure (not dose).

3. The chamber converts the x-rays into charge. The total charge, Q, is multiplied by a factor that converts exposure to dose. This charge represents the integral in the CTDI equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Recall that the integral is the area under a single curve, D (z). The CTDI is obtained by dividing this area by the slice width, SW. Now the MSAD can be calculated. This is accomplished using the following expression:

MSAD = CTDI (SW/BI)

In summary, the important points to grasp from the discussion so far are:

1. If the dose for a single slice is 3 rads, the dose for a 10-slice examination is not 30 rads (3 x 10) but rather about 3 rads.

2. To explain the above, the concepts of CTDI and MSAD are needed.

3. While the MSAD is the average dose to the center of a series of scans, the CTDI is the dose to a single slice in the middle of a series of contiguous slices.

4. The CTDI can be measured by a single pencil ionization chamber measurement, and this measurement is used to calculate the MSAD.

5. When the SW equals the BI, the MSAD numerically equals the CTDI.

The most recent survey of the MSAD from 250 CT scanners in 26 states was published in 1992. The results of this survey indicate that "typical MSADs are in the range of 40 to 60 mGy for head scans and 10 to 40 mGy for body scans."[10]

Rules for estimating dose (MSAD) in CT are provided in Table 5.

Table 5 Rules for Estimating Dose To the Patient (MSAD) in CT(*)

1. Dose is directly proportional to mA.

2. Dose is directly proportional to scan time of the mAs for 360 [degrees] (except for pulsed beam scanners whose dose is proportional to exposure time, which may be significantly less than the set scan time).

3. Dose increases with increasing kVp. Compared with a scan obtained at 120 kVp at a given mAs, the dose at 80 kVp typically will be 0.2 to 0.4 times less, and the dose at 140 kVp typically will be 1.2 to 1.4 times greater.

4. Central axis dose is similar to surface dose for head scans, but significantly less than surface dose for body scans (about 0.5 times).

5. Nonuniform surface doses are produced by partial scans and overscans. The position of the maximum surface dose may vary from scan to scan (eg, slip ring systems). A "half scan" (220 [degrees] to 230 [degrees]) will lead to a reduction of about 0.6 times in dose at the axis, compared with a full scan.

6. Dose varies with section thickness. For a well-collimated system, MSAD increases only slightly as slice thickness decreases.

7. The ratio of the multiscan dose to the singlescan dose depends on the slice thickness, the slice separation, the number of scans taken, and the shape of the single-scan dose distribution. For 360 [degrees] scans, the ratio near the surface of the head or body should be less than 2 times on a well-designed scanner. For 180 [degrees] scans the ratio may be significantly higher near the center of the patient or at the exit surface.

8. Scout view entrance dose is normally no more than 1 mGy.

(*) Reprinted with permission from: Rothenberg L, Pentlow K. CT dosimetry and radiation safety. In: Goldman LW, Fowlkes JB, eds. Medical CT and Ultrasound: Current Technology and Applications. College Park, Md: American Association of Physicists in Medicine; 1995:550.

Dose in Spiral/Helical CT

The introduction of spiral/helical CT scanning has raised a number of questions regarding the dose to patients. For example:

* Are the factors that affect the dose in conventional CT the same for spiral/helical CT scanning?

* Are the dose descriptors, the CTDI and the MSAD used to quantify the dose in conventional CT adequate for spiral/helical CT?

* How does the dose in conventional CT compare with the dose delivered to the patient in spiral/helical CT?

To answer these questions, it is necessary to review the data acquisition geometry used in spiral/helical CT.

Data Acquisition Geometry for Spiral/Helical CT

The technical details of data acquisition in spiral/helical CT scanners have been described by Kalender[18] and Seeram.[19] During data collection, the patient moves continuously through the gantry while the x-ray tube rotates continuously around the patient until the desired volume of the patient has been scanned. The beam geometry is a spiral or helical path traced by the rotation of the x-ray about the patient. In general, the patient travels at a rate of 1 to 10 mm/sec.

Factors Affecting the Dose in Spiral/Helical CT The equation:

Dose = (Intensity x Beam Energy)/

([Noise.sup.2] x Pixel [Size.sup.3] x Slice Thickness)

shows several factors affecting the dose in conventional CT. For spiral/helical CT, the same factors affect dose because most of the scan parameters are very similar. As the mAs and kVp increase, dose increases proportionally. As noise, pixel size and slice thickness decrease, the dose to the patient increases.

The nature of the spiral/helical CT beam geometry and data collection scheme introduces a new factor referred to as pitch. Pitch is the ratio of the distance the table moves per 360 [degrees] rotation to the slice thickness. For example, when the table moves 5 mm per 360 [degrees] rotation and the slice thickness is 5 mm, the pitch is 1. The dose in spiral/helical CT decreases with increasing pitch.

Dose Descriptors for Spiral/Helical CT

Are the 2 standard dose descriptors, the MSAD and the CTDI, valid for assessing the dose delivered to a patient during spiral/helical CT scanning? In 1994, McGhee and Humphreys[20] examined this question by measuring the dose for both conventional and spiral/ helical CT using thermoluminescent dosimeters positioned in standard dosimetry phantoms. The doses were then compared to determine validity. The results of their study indicate that: "The concepts of CTDI and MSAD can be applied directly to dose estimation for spiral CT. In particular, evaluation of MSAD values for spiral distributions provides values in excellent agreement with the multiple contiguous slice distribution that would have been used to image the same volume. Maintenance of these dose indicators for spiral is of obvious advantage."[20]

Dose in Conventional vs Spiral/Helical CT: A Comparison

Initially, it was thought that the dose in spiral/helical CT would be higher simply because of volume scanning. This prompted comparisons, and in general the dose is found to be about equal. A dose comparison for the CTDI for one commercial CT scanner is given in Table 6. The CTDI measurements were made in the middle of both the head and body phantoms. It is apparent that the CTDI values for both standard or conventional CT and spiral/helical CT are the same.

Table 6 CTDI Dose Chart for a Commercial CT Scanner
Scanner Type                   CTDI (mGy/100 mAs)
(kVp)                16-cm Phantom             32-cm Phantom
               10 mm slice   5 mm slice   10 mm slice   5 mm slice

Standard CT        8.5          6.9           3.4          2.4
(120 kVp)

Spiral CT          8.5          7.0           3.4          2.4
(120 kVp)


Additionally, Kalender[18] points out that "there are several practical reasons why the dose imparted to the patient is less in spiral CT than in conventional CT:

* Tube currents in spiral CT are set to lower values than in conventional CT.

* The need to retake single scans that sometimes results from a lack of patient cooperation is largely eliminated in spiral CT.

* The practice of taking overlapping scans in conventional CT for high-quality 3-D displays is replaced by the ability to calculate arbitrarily many overlapping images from 1 spiral scan without renewed exposure.

* The possibility of using pitch values [is greater than] 1 leads to an immediate reduction in dose compared with contiguous scanning.

Thus spiral CT may help to limit the patient dose in CT examinations."[18]

Effective Dose

What about the effective doses from spiral/helical CT? Recall that the effective dose allows one to quantify the risk from partial body exposure based on that from an equal whole-body dose. In this regard, Kalender[18] reported effective doses to the head, chest, abdomen and pelvis to be 1.1 mSv, 6.7 mSv, 4.g mSv and 2.7 mSv respectively.

In a recent study by Huda et al[21] the effective dose to both pediatric and adult patients was estimated. The investigators showed that if the energy imparted to a patient who is having a CT examination is known, then the effective dose can be computed easily. The authors found that the effective dose ranged from 1.5 to 6 mSv for head CT studies and 3.1 to 5.3 mSv for abdominal CT examinations. To put this in perspective, the effective dose from natural background radiation is about 2.4 mSv per year.[18]

Effective Dose Computer Program

A computer software program is available from Nuclear Associates (Carle Place, NY) to calculate the effective dose to patients undergoing CT examinations. The program is called "eXoDOSE" and it can generate effective doses for patients of any age and size undergoing CT scans of the head, chest, abdomen, pelvis or extremity on any CT scanner in North America.

Specifically, the program computes dosimetric parameters in CT including the energy imparted (mJ), the mean dose (mGy), the effective dose (mSv), and the average dose to individual organs, ranging from red bone marrow, bone surfaces, breasts, lungs, thyroid and gonads, to bladder, colon, liver, esophagus, skin and stomach. Additionally, the surface and center values of the CTDI for the head and body (10 mm slice thickness or maximum thickness available) at kVps ranging from 80 to 140 can be obtained in either SI units (Sv) or in the old system of units (rems).

The program also has an "analysis database" that allows the user to enter data on the CT scanner type, patient data (name, age, gender, patient record number), referral information (referring physician, radiologist), patient thickness (AP and lateral diameters in centimeters or inches), weight (in kilograms or pounds) and examination parameters (date, type, number of sequences, kVp, mA, rotation time, slice thickness and number of slices).

Dose Reduction Techniques

To reduce the dose to the patient, one should consider ways to reduce the MSAD. There are 2 possibilities:

1. Increase the bed index. From the relationship:

MSAD = CTDI (SW/BI)

it can be seen that if the BI quantity is increased, the MSAD decreases; however, there are limits to increasing the BI (too much of an increase results in gaps between slices that lead to image degradation).

2. Reduce the area under the dose profile curve. Such reduction can be accomplished by reducing not only the width of the curve, but also the height of the curve as well.

The major factor that controls the width of the curve is the collimator width. Close collimation will result in narrower dose profile widths. On the other hand, reducing the height of the curve can be accomplished by reducing the mAs.

It is also important to remember the relationship between image quality and dose [Dose = (Intensity x Beam Energy)/([Noise.sup.2] x Pixel [Size.sup.3] x Slice Thickness)] when contemplating dose reduction. Dose is affected by beam intensity (mAs), beam energy (kVp and filtration), noise, pixel size and slice thickness. Exposure factors can be adjusted to reduce the dose to the patient. For example, increasing the kVp from 120 to 130 or 140 means that the mAs can be reduced by a factor of 2 or more, with the same noise level (calibration at higher kVp will be required).[1] Studies conducted by Mayo et al[22] and Naidich et al[23] demonstrate that CT doses to the chest can be reduced significantly without loss of image quality when minimum tube currents are used.

More recently, Hupke, Hahn and Tschammler[24] examined the optimum dose without loss of image quality using ultra fast ceramics (UFC) detectors in spiral/helical CT. Images of the chest, neck, abdomen and cerebrum were obtained with mA values that were about half that used in conventional imaging. Image quality was assessed by 3 independent observers. Their results indicate that the UFC detectors offer significant dose reduction without loss of image quality. The mean dose reductions for the chest, neck, abdomen and cerebrum were 65%, 46%, 35% and 15% respectively.

The other factor affecting beam energy as well as intensity is filtration. Filtration reduces intensity and increases the mean energy of the beam. When filtration is increased, patient dose can be reduced by 25% to 35% or more.[1]

In the equation Dose = (Intensity x Beam Energy)/([Noise.sup.2] x Pixel [Size.sup.3] x Slice Thickness) because noise, pixel size and slice thickness are in the denominator, increasing any one of these factors will reduce the dose to the patient. It is important to realize, however, that there must be a balance between dose and image quality. Factors must be chosen not only to optimize image quality but also to reduce dose to the patient.

Last but not least, there are other techniques to reduce dose in CT that are essentially directed toward referring physicians and radiologists. For example, Gray[1] suggested that appropriate referrals and avoiding examinations with and without contrast media are 2 techniques that can reduce dose.

Radiation Protection Considerations

The dose reduction factors described so far focus on methods to reduce the MSAD, that is, the dose to the patient. What about the protection of personnel in CT scanning?

Radiation protection of both patients and personnel in medicine is guided by 2 triads shown in Fig. 10. While 1 triad deals with radiation protection actions and involves time, shielding and distance, the other triad addresses the radiation protection principles of justification, optimization and dose limitation.

[Figure 10 ILLUSTRATION OMITTED]

Radiation Protection Actions: Time, Shielding and Distance

Time, shielding and distance are intended to protect both patients and personnel in radiology. For example, to minimize the dose to patients, one would use shorter exposure times, because the dose is proportional to the exposure time. To protect personnel in CT, it is essential to minimize the time spent in the CT scan room during the exposure.

Distance, on the other hand, is a major dose reduction action because dose is inversely proportional to the square of the distance. This means that the further one is away from the radiation source, the less the dose received. For CT, because the patient is the main source of scatter, technologists should be positioned as far away from the patient as possible if there is a need to be present in the scan room during scanning. This implies that the use of a power injector that can be controlled from outside the scan room is recommended. If a hand injector is used, then a long tubing should be used.

Shielding is intended to protect not only patients (gonadal, breast, eye and thyroid shielding) but also personnel and members of the public. Patients often are concerned about the exposure of their gonads during a CT examination. Since most of the gonadal exposure will come from internal scatter, and not from the primary beam (unless the gonadal region is being examined), there is no need for this concern. However, technologists should place gonadal shielding on the patient because it may alleviate fears about the risks of being exposed to radiation. Shields are now commercially available to protect the eyes, breasts and thyroid of patients undergoing CT examinations. These shields are made of flexible, nonlead (bismuth) material that offers protection of critical organs by as much as 60% without compromising image quality (personal communication, F and L Products Inc, 1999).

When personnel are expected to be present in the CT scan room during the exposure, lead aprons should be worn, due to the presence of scatter. Manufacturers always provide the distribution of scatter (isoexposure profiles) around a CT scanner. One such distribution in both the horizontal (plan view) and vertical (elevation) is illustrated in Fig. 11. The profiles usually are reported in mR/scan. Given this information, it is possible to calculate the occupational exposure of a technologist who stands in a CT room when the tube is energized and who is not wearing a lead apron.

[Figure 11 ILLUSTRATION OMITTED]

Problem:

If a technologist stands in the CT room without a lead apron at the position where the isoexposure profile is 0.50 mR/scan, what is the occupational exposure for a 15-scan examination?

Solution:

Occupational exposure

= mR/scan x number of scans

= 0.50 mR/scan x 15

= 7.50 mR

This reading would be recorded by the technologist's personnel dosimeter.

In view of the fact that scatter radiation is present during a CT examination and strikes the walls of the CT room, should these walls be shielded to protect members of the public and employees outside the CT room? The answer is yes.

Shielding depends not only on the intervening absorber (the patient), but also on the volume of tissue irradiated and the workload (mA/min). The introduction of spiral/helical CT scanners has resulted in greater weekly workloads, and in this regard, Rothenberg and Pentlow[10] point out that "even with a very high workload, the required shielding for the scanner room barrier walls will not normally exceed 1.5 mm of lead. In some older CT installations, adequate shielding may be present even though walls are lined with lead and viewing windows do not contain leaded glass. While the use of minimum shielding in the form of thick plate glass control room viewing windows and gypsum wall board with no lead content can sometimes provide the minimum required shielding, the choice of the more conventional absorbing barrier materials as recommended by the NCRP should be considered in view of current ALARA philosophy."[10]

Radiation Protection Principles: Justification, Optimization and Dose Limitation

To be effective, a radiation protection program for CT (or any other radiologic modality) always should ensure justification, optimization and dose limitation -- principles that are vital to radiation protection regulation.

Justification involves the concept of net benefit, that is, there must be a benefit associated with every exposure. This requirement is intended for referring physicians and is one effort to reduce dose to patients undergoing x-ray examinations. As Wolbarst indicates, "justification provides an essential moral stance for the intelligent use of radiation."[25]

Optimization is a principle intended to ensure that doses delivered to patients are kept as low as is reasonably achievable (ALARA), taking into account economic and social factors. In implementing ALARA, radiologic technologists always should apply all relevant technical radiation protection practices to ensure that the patient receives an exposure as low as is reasonably achievable without sacrificing image quality.

The concept of dose limitation is a major, integral component of regulatory guidance on radiation protection. This concept addresses the maximum permissible dose that an individual may receive annually or accumulate over a working lifetime. These doses should be within the limits established by international organizations such as the ICRP and national bodies such as the NCRP. These recommended limits are intended to reduce the probability of stochastic effects and prevent detrimental deterministic (nonstochastic) effects.

The 1993 NCRP annual effective dose limit for radiologic technologists is 50 mSv. For cumulative occupational exposure, the dose accumulated in N years is equal to 10 x N mSv. For members of the public (recall that shielding is intended to protect people outside the CT room), the annual effective dose limit for continuous or frequent exposure is 1 mSv. This implies that the shielding of the CT room walls would have to ensure that no member of the public in close proximity to the CT room would be exposed to 1 mSv/year.

Conclusion

CT is one of the highest radiation-exposure examinations in radiology. Therefore it is mandatory and critical to pay close attention not only to the dose factors but also to the methods used to reduce the dose to the patient because of the risks associated with exposure to radiation. In this regard, Gray emphasizes that: "The risk of developing a cancer as a result of CT of the liver is 12.5 per 10,000, but only 1.06 per 10,000 AP lumber spine films and 0.009 per 10,000 for a PA chest. In other words, the CT carries a risk of 208 times higher than that of an AP lumbar spine and 1388 times higher than for a PA chest.[1] This article offers one small step toward understanding the factors affecting the dose in CT and how to reduce the dose to both patients and personnel.

References

[1.] Gray JE. Lower radiation exposure improves patient safety. Diagn Imaging. September 1998:61-64.

[2.] International Commission on Radiological Protection (ICRP). 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication No. 60. Annals of the ICRP 21 (1-3). Elmsford, NY: Pergamon Press; 1991.

[3.] Huda W, Gkanatsos NA. Radiation dosimetry for extremity radiographs. Health Phys. 1998;75:492-499.

[4.] National Council on Radiation Protection and Measurements (NCRP). Exposure of the U.S. Population from Medical Radiation. NCRP Report No. 100. Bethesda, Md. 1989.

[5.] Castronovo FP. An attempt to standardize the radiodiagnostic risk statement in an institutional review board consent form. Invest Radiol. 1993;28:533-538.

[6.] Seeram E. Radiation Protection. Philadelphia, Pa: Lippincott; 1997.

[7.] Bushong S. Radiologic Science for Technologists. St. Louis, Mo: Mosby-Year Book Inc; 1997.

[8.] Aldrich J. Internet site available at: http://web. ucs.ubc.ca/aldrich/radpublic.htm.

[9.] Specht NT, Russo RD. Practical Guide to Diagnostic Imaging. St Louis, Mo: Mosby; 1998.

[10.] Rothenberg L, Pentlow K. CT dosimetry and radiation safety. In: Goldman LW, Fowlkes JB, eds. Medical CT and Ultrasound. College Park, Md: American Association of Physicists in Medicine; 1995:519-553.

[11.] Cacak R. Measuring patient dose from CT scanners. In: Seeram E. Computed Tomography: Physical Principles, Clinical Applications, and Quality Control. Philadelphia, Pa: WB Saunders Co; 1994.

[12.] Romans L. Introduction to Computed Tomography. Baltimore, Md: Williams & Wilkins; 1995.

[13.] Perry BJ, Bridges C. Computerized transverse axial scanning (tomography) III. Radiation dose considerations. Br J Radiol. 1973;46:1048-1051.

[14.] Seeram E. Computed tomography: physical basis and technology. X-ray Focus. 1979;2:34-39.

[15.] Shrivastava PN, Lynn SL, Ting JY. Exposures to patient and personnel in computed tomography. Radiology. 1977;125:411-415.

[16.] Yoshizumi TT, Suneja SK, Teal JS. Practical CT dosimetry. Radiol Technol. 1989:60:505-509.

[17.] Knox HH, Gagne RM. Alternative method of obtaining the computed tomography dose index. Health Phys. 1996;71:219-224.

[18.] Kalender WA. Technical foundations of spiral CT. Seminars in Ultrasound, CT and MRI. 1994;15:81-89.

[19.] Seeram E. Computed Tomography: Physical Principles, Clinical Applications, and Quality Control. Philadelphia, Pa: WB Saunders Co; 1994.

[20.] McGhee PL, Humphreys S. Radiation dose associated with spiral computed tomography. J Can Assoc Radiol. 1994;45:124-129.

[21.] Huda W, Antherton JV, Ware DE, Cumming WA. An approach for the estimation of effective radiation dose at CT in pediatric patients. Radiology. 1997;203:417-422.

[22.] Mayo JR, Hartman TE, Lee KS, et al. CT of the chest: minimal tube current required for good image quality with least radiation dose. Am J Roentgenol. 1995;164:603-607.

[23.] Naidich DP, Marshall CH, Gribbin C, et al. Low dose CT of the lungs: preliminary observations. Radiology. 1990;175:729-731.

[24.] Hupke R, Hahn D, Tschammler A. Low-dose CT imaging with the new UFC detector. Electromedica. 1997;66(2):56-57.

[25.] Wolbarst AB. Physics and Radiology. Norwalk, Conn: Appleton and Lange; 1993.

Euclid Seeram, B. Sc., M. Sc., R. T. (R), graduated with the highest honors in medical radiography from the Ottawa General Hospital School of Radiography in 1970. After working as a technologist in both routine and special procedures radiography, he obtained a Bachelor of Science degree from Carleton University in Ottawa in 1975. In 1990 Mr. Seeram completed his Master of Science degree in Instructional Computing from Simon Fraser University in British Columbia.

Mr. Seeram began his teaching career in radiography in 1971 at the Ottawa General Hospital School of Radiography, specializing in radiographic instrumentation. Presently, he is a full-time faculty member of the British Columbia Institute of Technology and teaches in the medical radiography diploma program as well as the bachelor of technology degree program in medical imaging. Mr. Seeram has published 7 books and contributed 27 technical papers to professional radiologic technology journals. He is a member of the Canadian Association of Medical Radiation Technologists (CAMRT) and the Canadian Radiation Protection Association. In 1995 he received the Lamp of Knowledge Award from the CAMRT for significant contributions to the field of education. He also received the WQ Stirling Award in 1997 from the British Columbia Association of Medical Radiation Technologists (BCAMRT), for scientific and professional advancement of the members of the BCAMRT.

Radiation Dose In Computed Tomography

To receive Category A continuing education credit for this Directed Reading, read the preceding article and circle the correct response to each statement. Transfer your responses to the answer sheet on page 559 and then follow the directions for submitting the answer sheet to the American Society of Radiologic Technologists.

(*) Your answer sheet for this Directed Reading must be received by the ASRT on or before this date.

1. Recent evidence supports the fact that CT is a:

a. high-dose procedure.

b. medium-dose procedure.

c. low-dose procedure.

d. low- or medium-dose procedure, depending on the examination.

2. Which of the following measures the amount of ionization in air produced by x-rays?

a. exposure.

b. absorbed dose.

c. equivalent dose.

d. effective dose.

3. An energy absorption of 100 ergs per gram of absorber (tissue) is the:

a. exposure.

b. absorbed dose.

c. equivalent dose.

d. effective dose.

4. The weighted absorbed dose in a tissue or organ (rather than at a point) is the:

a. exposure.

b. effective dose.

c. equivalent dose.

d. dose equivalent.

5. The International System (SI) unit of exposure is the:

a. gray (Gy).

b. sievert (Sv).

c. coulomb per kilogram (C/kg).

d. milligray (mGy).

6. The SI unit of absorbed dose is the:

a. gray (Gy).

b. coulomb per kilogram (C/kg).

c. millisievert (mSv).

d sievert (Sv).

7. Because it is useful to compare doses from radiologic procedures with that from natural background radiation, patient doses are now reported by various international and national organizations in terms of:

a. absorbed dose.

b. effective dose.

c. exposure.

d. entrance dose.

8. Which of the following examinations delivers the highest effective dose to the patient?

a. lumbar spine.

b. CT chest.

c. CT head.

d. CT pelvis.

9. Which of the following is a risk-free dose?

a. 0 mSv.

b. 0.02 mSv.

c. 0.2 mSv.

d. 2 mSv.

10. Which of the following is not a stochastic effect?

a. cancer.

b. leukemia.

c. hereditary effects.

d. skin erythema.

11. Dose distribution in a patient is more uniform in:

a. conventional radiography.

b. conventional tomography.

c. CT with a tube rotation of 360o around the patient.

d. CT with a tube rotation of 180o around the patient.

12. In considering the geometry of data collection in CT, the following characteristics are important to dose:

1. the thin, fan-shaped x-ray beam.

2. the z-axis of the patient, which is perpendicular to the axial slice.

3. the perpendicular relationship between the thin narrow beam and the z-axis.

a. 1 and 2.

b. 1 and 3.

c. 2 and 3.

d. 1, 2 and 3.

13. Which of the following affect the size and shape of the dose profile in CT?

1. scanner type and beam geometry.

2. detector technology.

3. collimation.

a. 1 and 2.

b. 1 and 3.

c. 2 and 3.

d. 1, 2 and 3.

14. In which generation of CT scanner does the x-ray tube rotate within a stationary ring of detectors?

a. first.

b. second.

c. third.

d. fourth.

15. In which generation of scanner is the x-ray tube coupled with an array of detectors that rotate around the patient for at least 360 [degrees]?

a. first.

b. second.

c. third.

d. fourth.

16. The purpose of collimation in CT is to:

a. increase the signal-to-noise ratio.

b. ensure a uniform beam width at the detector.

c. ensure a uniform density of the image for objects of uneven thickness.

d. increase the image contrast.

17. All of the following affect the x-ray spectrum (beam quality) except:

a. kVp.

b. filtration.

c. mAs.

d. voltage waveform.

18. Because shaped filters are used in CT, the dose to the patient is often more dependent on:

1. slice thickness.

2. patient size.

3. field size.

a. 1 and 2.

b. 1 and 3.

c. 2 and 3.

d. 1, 2 and 3.

19. Which of the following decrease dose to the patient during a CT examination?

1. thicker filtration.

2. thicker slices.

3. reducing the spacing between slices.

a. 1 and 2.

b. 1 and 3.

c. 2 and 3.

d. 1, 2 and 3.

20. Which of the following CT image quality parameters depends on geometric factors?

a. spatial resolution.

b. low-contrast resolution.

c. noise.

d. all of the above.

21. Dose in CT is:

a. equal to the product of beam energy (E) and intensity (I).

b. directly proportional to I x E.

c. directly proportional to I x E and inversely proportional to the noise squared x pixel size cubed x slice thickness.

d. inversely proportional to noise x pixel size x slice thickness.

22. Dose in CT decreases if:

1. beam energy and quantity increase.

2. noise increases.

3. slice thickness increases.

a. 1 and 2.

b. 1 and 3.

c. 2 and 3.

d. 1, 2 and 3.

23. If the noise in CT is reduced by a factor of 2, then the dose to the patient increases by a factor off

a. 2.

b. 4.

c. 8.

d. 16.

24. All of the following are used to measure dose to the patient in CT except:

a. radiation survey meters.

b. thermoluminescent dosimetry.

c. film dosimetry.

d. ionization chambers.

25. Compared to earlier dose measurement techniques, which of the following is accurate and easy to perform?

a. single ionization chamber measurement.

b. film dosimetry.

c. thermoluminescent dosimetry (TLD).

d. inserting TLD chips into holes strategically drilled in a phantom.

26. The ionization chamber used in the measurement of the CTDI records:

a. exposure.

b. absorbed dose.

c. effective dose.

d. occupational exposure.

27. The average dose at the center of a series of scans in CT is the:

a. CTDI.

b. MSAD.

c. D(z).

d. absorbed dose from that series of scans.

28. Which is the correct expression for the MSAD?

a. MSAD = CTDI x BI.

b. MSAD = CTDI/(BI x SW).

c. MSAD = CTDI x (SW/BI).

d. MSAD = (SW x BI)/CTDI.

29. The most recent survey of dose from CT scanners across the United States indicates that typical MSADs for body scans range from:

a. 10 to 40 mGy.

b. 40 to 60 mGy.

c. 10 to 40 Gy.

d. 40 to 60 Gy.

30. As pitch increases, dose to the patient:

a. increases.

b. decreases.

c. remains the same.

31. The dose to the patient in spiral/helical CT is less than in conventional CT because:

1. mA values are lower in spiral/helical CT.

2. repeat single scans largely are eliminated in spiral/helical CT.

3. it is possible to use pitch values [is greater than] 1.

a. 1 and 2.

b. 1 and 3.

c. 2 and 3.

d. 1, 2 and 3.

32. Dose to the patient in CT can be reduced through:

1. increasing the bed index and increasing the filtration.

2. reducing the width of the collimator,

3. reducing the mAs and increasing the kVp.

a. 1 and 2.

b. 1 and 3.

c. 2 and 3.

d. 1, 2 and 3.

33. If a technologist stands in a CT room without a lead apron where the isoexposure profile is 0.7 mR/scan, her occupational exposure for a 20-scan examination is:

a. 0.14 mR.

b. 1.4 mR.

c. 14 mR.

d. 140 mR.

34. The cumulative effective dose for a CT technologist who has worked for 20 years in CT is:

a. 0.2 mSv.

b. 2.0 mSv.

c. 20 mSv.

d. 200 mSv.

35. The risk of developing cancer from a CT liver scan is:

a. 12.5 per 1000.

b. 12.5 per 10000.

c. 1.06 per 10000.

d. 0.009 per 10000.

Reference No. DRI0009009

The 2 most important treasures in Mr. Seeram's life are his wife Trish and son David. When not engaged in academic and professional pursuits, he listens to classical guitar music, as well as New Age Celtic music.

Mr. Seeram is grateful to all investigators who have done original experiments describing and reporting dose measurement concepts in CT. In particular, he acknowledges Lawrence Rothenberg, Ph.D., Keith Pentlow, M.S. and Robert Cacak, Ph.D.

Reprint requests may be sent to the American Society of Radiologic Technologists, Communications Department, 15000 Central Ave. SE, Albuquerque, NM 87123-3917.

[C] 1999 by the American Society of Radiologic Technologies
COPYRIGHT 1999 American Society of Radiologic Technologists
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1999 Gale, Cengage Learning. All rights reserved.

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Author:SEERAM, EUCLID
Publication:Radiologic Technology
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Jul 1, 1999
Words:11543
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