Radiation Dose Issues in Longitudinal Studies Involving Computed

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					Radiation Dose Issues in Longitudinal Studies Involving
Computed Tomography
John R. Mayo1,2
    Vancouver General Hospital, Vancouver, British Columbia, Canada; and 2University of British Columbia, Vancouver, British Columbia, Canada

Computed tomography (CT) examinations are increasingly used for                            These capacities have made CT an invaluable diagnostic and
clinical diagnostic and research purposes, as they provide in vivo                     research tool, accounting for the explosive growth of CT exami-
anatomic information similar to that provided by gross anatomy. In                     nations in the last 20 years. It is estimated that in 2006 more than
conjunction with physiologic maneuvers or contrast media, CT                           62 million CT scans were obtained in the United States, as
may also provide in vivo physiologic information. Using calibrated                     compared with about 3 million in 1980 (1). Similar increased
acquisition protocols, accurate noninvasive measurements of tissue                     utilization has been reported in studies from the United Kingdom
density, air volume, blood volume, and capillary perfusion can be
                                                                                       (2) and Canada (3). Research studies are also increasingly using
performed. Serial CT scans can provide longitudinal measurements
                                                                                       serial CT examinations for the noninvasive evaluation of disease
indicative of disease progression or regression, allowing noninvasive
assessment of treatment effects. However, the X-ray radiation
                                                                                       progression and treatment effects.
associated with CT has been associated with a small but significant                         However, the increased utilization of CT comes with a price:
increased risk of malignancy, which may be fatal. Large studies have                   increased population radiation exposure. In most cases, adding CT
detected this small risk, which appears to be related to the cumula-                   to diagnostic imaging algorithms substantially increases patient
tive radiation dose of all previous exposures in a linear fashion. It has              X-ray radiation exposure. For example, a chest CT examination
been shown that the risk from a given radiation exposure is greater in                 (3–6 mSv) delivers 60 to 120 times more radiation dose compared
young people and females compared with older males. The combi-                         with a postero-anterior (PA) chest radiograph acquired using film
nation of these two risk-enhancing factors, found in pregnant                          (z0.05 mSv) or 90 to 180 times that of the same view obtained
females, provides the greatest risk. Radiation risk decreases with                     using digital radiography (z0.03 mSv). In addition, since CT is so
increasing age for both men and women, asymptotically approach-                        available and easy to perform, it is liberally applied to exclude
ing zero. Radiation risk can be calculated using dose metrics pro-                     potentially serious but statistically unlikely diagnoses, often solely
vided on current CT scanners as outlined in this article. Ethically,                   to reassure anxious patients and clinicians. In the chest, serial CT
given that radiation is associated with measurable risk, clinically                    studies are widely employed to assess disease progression in
indicated and research CT examinations must provide an increase in
                                                                                       chronic obstructive pulmonary disease (COPD), interstitial lung
knowledge that has substantial benefit to the subject. This benefit
                                                                                       disease, and cystic fibrosis. Repeated CT scans are often employed
should be related to the potential of saving of life or to the pre-
                                                                                       to follow suspicious lung nodules in patients at risk for lung cancer.
vention or mitigation of serious disease.
                                                                                       The net result of these actions is greatly increased CT utilization
Keywords: radiation dose; computed tomography; longitudinal studies;                   and population radiation dose. It is noted that in some cases CT
cancer risk                                                                            replaces examinations with higher dose (e.g., bronchography for
                                                                                       bronchiectasis, nuclear medicine ventilation perfusion scintigra-
The invention and rapid development of computerized tomo-                              phy followed by pulmonary angiography for suspected pulmonary
graphy (CT) is one of the major medical advances of our time.                          embolism) while providing equivalent or superior diagnostic
Current multidetector CT scanners can image the entire chest in                        information, but these situations are in the minority.
2 to 5 seconds, producing up to 1,000 slices, each composed of                             The increased radiation dose of chest CT compared with the
sub-millimeter isometric voxels. These high signal to noise ratio,                     PA chest radiograph arises from two properties of the CT
large field of view images provide noninvasive anatomic evalua-                         technique (4). First, unlike analog film radiography, in which
tion of the chest with information content similar to that achiev-                     the image acquisition and display are both reliant on the film, CT
able at autopsy. In vivo physiologic functional information                            is a digital technique in which image acquisition and display can
regarding pulmonary and systemic perfusion or gas transport                            be independently manipulated. Therefore, when CT dose is
can be obtained by acquiring CT images while administering                             excessive, the image does not become too dark (as it does in film
intravenous contrast media or performing breathing maneuvers,                          radiography), but instead improves because of decreased image
respectively. Expert radiologic interpretation of these images can                     noise (5). Second, visualization of image noise is enhanced by the
differentiate diseases that are indistinguishable on history and                       ability to map the entire visible gray scale onto a selected segment
physical examination but demonstrate unique changes at the                             of the CT number scale. As a result, image degradation due to
gross anatomic level. Using standardized and calibrated acquisi-                       quantum noise (mottle) is easily visible and interferes with image
tion protocols; accurate noninvasive measurements of tissue                            interpretation. At high noise levels, images may be clearly non-
density, air volume, blood volume, and capillary perfusion can
                                                                                       diagnostic. However, at lower noise levels more subtle image
be performed. After validation of surrogate outcome measures,
                                                                                       degradation occurs which may lead to diagnostic inaccuracies or
serial CT scans can provide longitudinal measurements of disease
                                                                                       a lack of confidence in image interpretation, effects that are
progression and treatment effects.
                                                                                       difficult to detect and measure. Image noise also affects the
                                                                                       accuracy of measurements made on chest CT images (e.g.,
                                                                                       assessment of the extent of emphysema). Failure to standardize
(Received in original form August 4, 2008; accepted in final form September 23, 2008)
                                                                                       acquisition protocols and account for equipment specific differ-
Correspondence and requests for reprints should be addressed to John R. Mayo,          ences in noise levels can lead to systematic errors in measure-
M.D., Department of Radiology, Vancouver General Hospital, 899 West 12th
                                                                                       ments that may introduce errors in surrogate measures of disease
Avenue, Vancouver, BC, V5Z 1M9 Canada. E-mail: John.Mayo@vch.ca
                                                                                       activity and progression.
Proc Am Thorac Soc Vol 5. pp 934–939, 2008
DOI: 10.1513/pats.200808-079QC                                                             In the absence of standardized validated protocols, radiolog-
Internet address: www.atsjournals.org                                                  ists often obtain CT images using high radiation exposure levels to
Mayo: Radiation Dose in CT                                                                                                                 935

minimize image noise and maximize image quality. Studies in            stochastic risk (10). Stochastic risks are believed to be cumulative,
multiple jurisdictions have shown that the lack of standardization     with increasing risk seen over successive exposures.
leads to wide variation in the level of radiation administered for         Subjects exposed to the atomic bomb explosions in 1945 have
the same CT examination between institutions (2, 3), with no           been extensively studied in the last 60 years. This group is unique
detectable difference in patient outcomes. In addition, studies        since it is large, covers all ages, and was not selected on the basis of
have shown that radiologists, referring clinicians, and patients       underlying disease. A substantial portion of the survivors re-
may be unaware of the high level of radiation exposure associated      ceived less than 50 mSv, a low level of exposure that approximates
with CT examinations (6). This knowledge gap undoubtedly               the dose range delivered by multiple chest CT exams. The major
contributes to the overuse of CT in low-yield diagnostic situations    negative effect seen in this group is an increase in the number of
and its overuse in following disease progression or treatment          cancers over that found in a nonexposed population. An earlier
effects.                                                               presentation of cancers has not been observed. However, to
    In the early 1990s, concern was raised regarding radiation dose    assess the risk from a single chest CT scan requires extrapolation
in chest CT (7–9). The authors of these early studies suggested        of these results to even lower doses, and the nature of this
that greater consideration needed to be given to optimizing CT         extrapolation has proven to be highly controversial.
exposures and ensuring appropriate clinical use guidelines on the          Disagreement regarding the extrapolation of nuclear explo-
single detector row CT scanners in use at that time. These early       sion data is based on three nonresolvable issues: uncertainty in
warnings were not heeded, and the last 15 years have been char-        the actual radiation exposure received, since on-site radiation
acterized by ever-increasing CT utilization. The increased utili-      dose measurements were not obtained; differences in the natural
zation of CT has been hastened by the development of multi-            cancer risk of the Japanese population compared with other
detector row CT scanners, leading to expanded clinical indications     populations; and the different quality of the radiation imparted by
for CT examinations (e.g., pulmonary embolism, cardiac gated           atomic bombs compared with X-ray–based medical imaging. As
CT angiograms [CCTA], trauma CT). This has fueled an increase          a result of differences in interpretation, learned societies have
in both the number of installed scanners and the number of             come to varying conclusions on the risk attributable to radiation
patients scanned per shift. These advances have served to further      exposure at the levels found in chest CT. The International
increase population CT radiation exposure. The development of          Commission on Radiological Protection, or ICRP, used a linear
evidence-based guidelines governing the use and technical              no-threshold extrapolation of nuclear explosion data and esti-
parameters for CT is required to responsibly use this valuable         mated 50 additional fatal cancers induced per million people
diagnostic test.                                                       exposed to 1 mSv of medical radiation (11). In contrast, the
    The purpose of this review is to outline (1) evidence indicating   French Academy of Science concluded that there was not
the detrimental effect of radiation dose at the level administered     sufficient evidence to support an increased cancer risk associated
in chest CT examinations, (2) parameters that affect CT radiation      with radiation exposures less than 20 mSv (12), a level above that
dose, (3) advances in dose reduction in the chest CT, and (4) the      delivered in chest CT examinations (, 6–11 mSv). Further con-
interaction between CT radiation dose and diagnostic accuracy.         flicting evidence on the impact of low-level radiation exposure is
A complete review of radiation dosimetry and bioeffects is             found in tissue culture experimental studies that have shown
beyond the scope of this review.                                       induction of free radical detoxification mechanisms with low-
                                                                       level radiation exposure (13). This has led some to suggest that
                                                                       low-level radiation exposure may be beneficial, an effect known
                                                                       as radiation hermesis. Finally, the long-term study of the mortal-
                                                                       ity of British radiologists showed lower cancer mortality than
There has been considerable debate within the medical commu-           predicted by the atomic bomb data (14). It is postulated that this
nity regarding the risk of low-level radiation exposure from CT.       may be accounted for by the healthy worker effect, the beneficial
The reason for this debate arises from an incomplete knowledge         effects of dose fractionation, or overestimation of the dose
of the complex link between ionizing radiation and future neg-         received by these physicians.
ative outcomes in humans. In broad overview, the negative                  In 2007, additional important data were added to this debate
outcomes of ionizing radiation in humans can be divided into           (15) when the 15-country study reported the cancer induction
two major categories that can be separated on the basis of time        effect of low-level radiation exposure studied in 407,000 radi-
and exposure: deterministic effects seen immediately after large       ation workers followed for over 20 years providing 5.2 million
exposures, and stochastic effects seen after a long latent period      person-years of follow-up. This study is unique as it reports on
(6–25 yr) and associated with low exposures.                           the largest cohort to date, has accurate dosimetry, and in-
    Deterministic effects, skin erythema, skin necrosis, and hair      vestigated multiethnic workers. Ninety percent of the subjects
loss only occur above a threshold dose that lies well above those      received a dose less than 50 mSv and on average each worker
administered in diagnostic chest CT examinations. In medical           received a dose of 19 mSv. Therefore this study is focused on
imaging, deterministic dose levels are only seen in complex            low-level doses, close to that received during a single chest CT
interventional cases using large quantities of fluoroscopy time.        examination (6–11 mSv). The authors reported an excess rela-
These effects will not be discussed further in this review.            tive risk (ERR) for all-cause mortality of 0.42 per Sievert
    By comparison, stochastic effects are believed to have no          (0.00042 per mSv), with a statistically significant increasing
radiation dose threshold, and therefore are associated with the        excess relative risk with increasing radiation dose (P , 0.02)
low radiation doses delivered during chest CT. Mechanistically,        indicating a dose–response effect. The increased risk in all-
stochastic effects are believed to be mediated by chemical dam-        cause mortality was mainly due to an increase in mortality from
age to the DNA molecule and clinically manifest as an increased        all cancers.
risk of cancer and genetic defects. Stochastic effects occur               A subanalysis stratified by dose categories (less than: 400,
randomly and the risk of their occurrence depends on the type          200, 150, and 100 mSv) showed that cancers in the highest dose
of ionizing radiation administered, the tissue receiving the           categories did not drive the risk estimates. Therefore, this study
radiation, and the age of the subject. It is believed that dose        supports the concept that there is a small cancer risk from low-
fractionation, a substantial modifier of detrimental effect for         dose radiation delivered in CT examinations. These new data
deterministic radiation doses, does not substantially modify the       add supportive evidence to the concern over radiation dose
936                                                                        PROCEEDINGS OF THE AMERICAN THORACIC SOCIETY VOL 5                        2008

delivered in chest CT examinations and support the use of the               TABLE 1. METHODS OF QUANTIFYING IONIZING RADIATION
ALARA principle (As Low As Reasonably Achievable) for
                                                                                                                                  International System
these exams.                                                                Method                  Conventional Units                of Units, or SI
    However, there are limitations to these new data. Because
workers were studied, there is no information on the effect in              Radiation exposure      Roentgens (R)            Coulombs per kilogram (C/kg)
                                                                            Absorbed dose           Rads (rad)               Grays (Gy)†
children; and since 90% of the workers were men who received
                                                                            Equivalent dose         Rems                     Sieverts (Sv)‡
over 98% of the cumulative dose, minimal information is avail-              Effective dose          Effective dose
able on the effect in women. The largest excess mortality from all                                     equivalent (Sv)*
contributing countries is found in the data from Canada, and
statistical significance is lost if this cohort is not included. Finally,      Abbreviations of the units of measure are in parentheses.
                                                                              * 1977 tissue-weighting factors.
the largest discrepancy between this study and the atomic bomb                †
                                                                                D multiplied by ICRP radiation weighting factor WR. The WR for x rays is 1.
cohort arises in the lung cancer mortality, suggesting that the               ‡
                                                                                1990 tissue-weighting factors.
confounding effects of smoking may have been inadequately                     Reprinted by permission from Reference 37.
allowed for.
    The influence of age at exposure and of sex has been studied             body risk estimate or be used to facilitate comparisons between
in the nuclear explosion cohort, showing that radiation risk is             examinations in different parts of the body. Equivalent dose is
substantially modified by these subject factors (16, 17). The                a modification of absorbed dose that incorporates weighting
increased radiation sensitivity of children is felt to arise from           factors to account for the different biologic effect of various
two biological facts: they have more time to express the cancer-            sources of radiation. For X-rays, the radiation weighting factor
inducing effect of radiation and they have more rapidly dividing            is 1 and the equivalent dose has the same numerical value as
cells than adults which are inherently more radiation sensitive.            absorbed dose.
    It has been found that women have approximately twice the                   Effective dose is a further refinement of radiation dose
risk compared with males for the same level of radiation ex-                measurement that estimates the whole body dose that would be
posure. Increased female risk is heightened in chest CT by the              required to produce the same stochastic risk as the partial body
presence of radiosensitive breast tissue in the radiated field.              dose that was actually delivered in a localized CT scan. It is useful
Radiation dose to breast tissue in chest CT examinations has                because it allows comparison of CT dose to that delivered in other
been calculated (18) and directly measured (19, 20), with reports           medical examinations. Effective dose is calculated by summing
showing wide variation in average values, ranging from 10 to                the absorbed doses to individual organs weighted for their ra-
70 mGy. The variation in values is related to CT parameter                  diation sensitivity (11). The measurement unit is the sievert (Sv)
settings, differences in size and configuration of breast tissue, and        or milli-sievert (mSv). Since effective dose requires determina-
methods to calculate or directly measure radiation dose. There is           tion of absorbed dose to each body organ multiplied by their
no debate that all CT-associated breast radiation dose values are           radiation sensitivity, the distribution of radiation dose in the body
substantially greater than the average glandular dose of 3 mGy for          must be determined. Chest CT has a markedly asymmetric dose
standard two-view screening mammography. It is important to                 distribution, with higher dose found peripherally and lower dose
note there is a strong age at exposure effect for breast tissue, with       centrally due to the shielding effects of body tissue. This makes it
lower risk for subjects above the age of 40 (21). These factors must        difficult to calculate the exact effective dose for each patient.
be taken into account in setting chest CT radiation dose param-             Instead, a simpler calculation is performed (Figure 1). Scanner
eters in CT chest examinations for women. Breast shields, thyroid
shields (22, 23), and X-ray tube current modulation techniques
have been employed to decrease radiation dose to these super-
ficial and radiosensitive tissues within the chest. These techniques
have been shown to decrease breast radiation exposure delivered
in chest CT scans. However, these dose-modifying techniques
must be used with consideration of their impact on image quality.

There are many methods currently in use for quantifying ionizing
radiations (Table 1) (24). The fact that several methods exist
attests to the complexity of this issue. The simplest parameter,
radiation exposure, is determined by measuring ionization in air
caused by the X-ray beam. The measurement unit is coulombs per
kilogram (abbreviation, C/kg). It has limited clinical value, as it
does not take into account the area irradiated, the penetrating
power of the radiation, or the radiation sensitivity of the irra-
diated organs. From radiation exposure we can calculate the skin
entrance dose, which is important when examining deterministic
effects such as skin erythema. Although deterministic effects are
not encountered in routine CT, they are of potential concern in
CT fluoroscopy. A more refined measurement is absorbed dose,
determined by measuring the energy absorbed per unit mass
within an object. The measurement unit is the gray (abbreviation,
Gy). Unlike radiation exposure, the gray is dependent on the
composition of the object or subject placed in the radiation beam.          Figure 1. Diagram shows algorithm for the estimation of radiation
However, absorbed dose does not account for the differing                   exposure risk from computed tomography (CT) using the metric of
radiation sensitivity of organs, and it cannot provide a whole-             effective dose.
Mayo: Radiation Dose in CT                                                                                                                     937

manufacturers use dose data derived from measurements made in                 These serve as a guide to those planning or evaluating research
head and body phantoms to determine a weighted CT dose index                  (e.g., ethics committees). Research radiation exposure is divided
(CTDI) for each CT scanner model at all available selections of               into four categories (I, IIA, IIB, and III) corresponding to
tube voltage (kVp), tube current (mA), and rotation time. The                 effective dose limits of less than 0.1 mSv, 0.1 to 1mSv, 1 to
selected pitch value is then incorporated to produce a CT dose                10 mSv, and over 10 mSv. The increasing dose limits in the four
index called the CTDIVOL. Once the scan length is determined                  categories are related to increasing potential benefit from the
from the topogram, the appropriate CTDIVOL is combined with                   research as indicated by the category guidance notes: I, expected
the actual length scanned in the patient to calculate the dose                to only increase knowledge; IIA, increase in knowledge leading to
length product (DLP). Since the administered radiation dose is                a health benefit; IIB, increase in knowledge aimed directly at the
linearly related to the length scanned in the patient, technologists          diagnosis, cure, or prevention of disease; III, increase in knowl-
should ensure that the scanned volume is confined to the region of             edge to have substantial benefit and usually directly related to the
interest to avoid excessive radiation dose.                                   saving of life or the prevention or mitigation of serious disease. In
    The DLP is a measure of the radiation dose delivered to that              addition, these research guidelines account for the substantial age
patient during the scan. An estimated effective dose for the                  variation in radiation sensitivity allowing an increase by a factor
specified CT scan can be calculated by multiplying the DLP                     of 5 to 10 for subjects over 50 years old and decreasing the limits
value by the normalized effective dose coefficients (Table 2) for              by a factor of 2 to 3 for children. It is noted that research that
the scanned body part. This normalized effective dose coeffi-                  involves serial radiologic investigations must calculate the cumu-
cient accounts for the radiation sensitivity of the body region               lative radiation dose over the course of the study to determine the
scanned based on exposed organ radiosensitivities. The DLP                    exposure category.
value is displayed on the scanner console once the topogram has
been obtained and the scan prescribed. In chest CT, multiplying               RADIATION DOSE REDUCTION
the DLP by 0.017 allows the radiologist or technologist to
calculate the estimated effective dose of the examination before              Reduction in CT radiation exposure results in increased image
scan acquisition. The DLP value can be archived in the picture                noise and decreased image quality. Studies assessing the sub-
archiving and communication system (PACS) by storing the                      jective evaluation of chest CT scans have demonstrated that
protocol page. Newer DICOM standards for CT enable the                        radiologists consistently gave higher image-quality scores to
storage of dose data in the DICOM header of each examination.                 images obtained with a higher radiation dose (30, 31). Image
    It is noted that effective dose, while easy to calculate and              noise can be measured by placing a region of interest (. 100 pixels)
convenient, is also an imperfect dose descriptor. Since tissue-               in an area of uniform density (e.g., the thoracic aorta). The
weighting factors are averaged over sex and age, effective dose               standard deviation of the pixel values represents image noise and
risk assessment is appropriate to a 30-year-old hermaphrodite.                is a measure of the uncertainty of quantitative CT measures. It is
An alternate approach has been described (25) that uses mea-                  noted that the choice of reconstruction algorithm affects image
sured or calculated organ radiation doses, applies them to age-               noise, with higher noise associated with high-spatial-frequency
and sex-specific organ risk estimates (from the BEIR 7 report)                 reconstruction algorithms (e.g., bone or lung algorithms) com-
and calculates an effective risk from the examination. Effective              pared with low-spatial-frequency algorithms (e.g., standard, soft-
risk would attempt to estimate the risk of developing cancer from             tissue algorithm). Since high-spatial-frequency reconstruction
the partial body irradiation of the examination. It would not                 algorithms are most commonly used to assess bones or lung
consider hereditary effects that are currently embodied in the                parenchyma, tissues with high radiographic contrast, increased
effective dose calculation. Effective risk could be adjusted for age          noise is usually not a diagnostic problem. However, increased
and sex. Although this is a new approach requiring further                    noise may interfere with quantitative measures of disease such
evaluation, it has the potential to improve communication of                  as computer-calculated emphysema scores. New adaptive re-
the risk from CT radiation exposure to patients and physicians.               construction algorithms are being developed that can decrease
    Radiation dose surveys have noted wide variations in DLP                  image noise, providing improved image quality at lower radiation
settings for identical examinations between institutions (2, 3, 26,           dose. These advanced algorithms should facilitate radiation dose
27). To decrease this variation and protect the public from in-               reduction.
advertent overexposure, the European communities have pub-                        Radiation dose can be adjusted at the time of image acquisi-
lished suggested reference dose values (28) for chest CT exami-               tion by changing the X-ray tube current or voltage and the scan
nations, with a DLP value of 650 mGy cm. This reference dose                  time. In practice, the tube current is most frequently adjusted to
value was obtained by surveying a large number of institutions in             change the radiation dose and image noise. In most CT scanners,
Europe and adopting the 75th percentile of responses as the                   the tube current is adjustable in steps from 20 mA to approxi-
reference dose values. This value serves as a guide to acceptable             mately 400 mA. Decreasing the tube voltage also decreases the
practice.                                                                     radiation dose, but also affects subject contrast and can impact CT
    The European Commission has also provided guidelines for                  number measurements. Finally, radiation dose is linearly related
radiation exposure in medical and biomedical research (29).                   to the scan time. However, in most cases scan time is minimized to
                                                                              reduce motion artifact. It is noted that the radiation exposure
                                                                              delivered at a given tube voltage and current setting will vary
TABLE 2. DOSE LENGTH PRODUCT TO EFFECTIVE DOSE                                greatly between CT scanners of different models and manufac-
CONVERSION COEFFICIENTS                                                       turers because of differences in scanner geometry (X-ray tube-to-
                                                                              patient separation) and X-ray tube filtration.
Study                                                   E/DLP (mSv/mGy cm)
                                                                                  In the past, the tube current of CT scanners was uniform at all
Head                                                           0.0023         angles around the patient and for the full longitudinal (cranial
Chest                                                          0.017          caudal) extent of the scan. However, the chest is an elliptical
Abdomen                                                        0.015          object that has higher attenuation from left to right than from
Abdomen-pelvis                                                 0.017
Pelvis                                                         0.019
                                                                              anterior to posterior. Attenuation also varies as the chest is
                                                                              scanned cranial to caudal because of the shoulders. CT image
 Definition of abbreviations: DLP 5 dose length product; E 5 effective dose.   quality is disproportionately degraded by views with few photons
938                                                                    PROCEEDINGS OF THE AMERICAN THORACIC SOCIETY VOL 5                         2008

(photon starvation) compared with the image quality improve-            Since children, young adults, women, and pregnancy have been
ment associated with views with high photon counts. To address          shown to increase radiation sensitivity, the most strident dose
this issue, manufacturers have introduced programs that adjust          reduction efforts should be focused on these groups. Finally, it is
the tube current depending on the attenuation of the object in          noted that the complexity of CT requires a close collaboration
both the transverse (x, y) and longitudinal (z) directions to           between radiologists and medical physicists to successfully re-
minimize either photon-starved or photon-rich projections, max-         duce radiation dose while maintaining diagnostic accuracy.
imizing image quality while minimizing radiation dose. This tube
                                                                        Conflict of Interest Statement: J.R.M. does not have a financial relationship with
current modulation technique has been shown to produce a sub-           a commercial entity that has an interest in the subject of this manuscript.
stantial reduction in radiation dose (32–34) with minimal degra-
dation of image quality. Routine use of dose modulation systems
is recommended, as they compensate for asymmetry in the size
and density of the body section being scanned, resulting in
a signal-to-noise ratio that is adequate for diagnosis but is not       References
excessive (35). Advanced tube current modulation schemes with            1. IMV. 2006 CT Market Summary Report. Des Plains, IL: IMV Medical
novel reconstruction algorithms are being developed to reduce                  Information Division; 2006.
radiation dose to superficial radiation sensitive-tissues such as the     2. Shrimpton PC, Edyvean S. CT scanner dosimetry. Br J Radiol 1998;
breast and thyroid. Further experience with these new radiation          3. Aldrich JE, Bilawich AM, Mayo JR. Radiation doses to patients
dose modulation systems is required before they can be widely                  receiving computed tomography examinations in British Columbia.
employed.                                                                      Can Assoc Radiol J 2006;57:79–85.
    However, dose modulation systems can produce variations in           4. Sprawls P Jr. AAPM tutorial. CT image detail and noise. Radiographics
image noise that may modify the numeric evaluation of emphy-                   1992;12:1041–1046.
sema in serial follow-up examinations. In addition, dose modu-           5. Rothenberg LN, Pentlow KS. Radiation dose in CT. Radiographics
lation systems may interact with the patient position and X-ray
                                                                         6. Lee C, Haims A, Monico E, Brink J, Forman H. Diagnostic CT scans:
beam filters, producing increased radiation dose in patients who                assessment of patient physician and radiologist awareness of radiation
are incorrectly centered in the CT gantry. Algorithms to auto-                 dose and possible risks. Radiology 2004;231:393–398.
matically center patients are being developed (36). In longitudi-        7. Di Marco AF, Briones B. Is chest CT performed too often? Chest 1993;
nal research studies careful attention to scan parameters and                  103:985–986.
patient positioning is an important component of both radiation                                                           ¨
                                                                         8. Naidich DP, Pizzarello D, Garay SM, Muller NL. Is thoracic CT
dose reduction and inter-scan reproducibility. Finally, repeated               performed often enough? Chest 1994;106:331–332.
                                                                         9. Di Marco AF, Renston JP. In search of the appropriate use of chest
scanning of the same region increases the radiation dose in
                                                                               computed tomography. Chest 1994;106:332–333.
a linear fashion. Therefore, the timing of follow-up examinations       10. Ullrich R, Jernigan M, Satterfield L, Bowles N. Radiation carcinogen-
involves a trade off between additional information and the                    esis: time-dose relationships. Radiat Res 1987;111:179–184.
radiation dose detriment of the associated dose buildup effect.         11. ICRP-60. Recommendations of the International Commission on Ra-
                                                                               diological Protection. Oxford: Pergamon Press; 1991.
                                                                        12. Tubiana M, Aurengo A, Averbeck D, Bonnin A, Le Guen B, Masse R,
CONCLUSIONS                                                                    Monier R, Valleron A, de Vathaire F. Dose effect relationships
                                                                               and estimation of the carcinogenic effect of low doses of ionizing
The introduction of helical and multi–detector row CT scanners                 radiation. Paris: Academie des Sciences and Academie Nationale de
has resulted in an increase in the number of indications for and               Medicine; 2005.
the diagnostic accuracy of chest CT examinations. However,              13. Strzelczyk J, Damilakis J, Marx M, Macura K. Facts and controversies
despite increasing education and awareness, the current level                  about radiation exposure, Part 2: Low level exposures and cancer risk.
of radiation exposure from CT remains high. The 15-country                     J Am Coll Radiol 2007;4:32–39.
study has added further information to support the linear, no-          14. Berrington A, Darby S, Weiss H, Doll R. 100 years of observation on
                                                                               British radiologists: mortality from cancer and other causes 1897–
threshold approach to the detrimental effects of radiation dose in
                                                                               1997. Br J Radiol 2001;74:507–519.
the range below 20 mSv, similar to that received in chest CT scans      15. Cardis E, Vrijheid M, Blettner M, Gilbert E, Hakama M, Hill C, Howe
(3–6 mSv). Current radiation dose surveys continue to indicate                 G, Kaldor J, Muirhead C, Schubauer-Berigan M, et al. The 15 country
that there is large variation in the technical factors employed by             collaborative study of cancer risk among radiation workers in the
radiologists (3), and there is a resultant large variation in the              nuclear industry: estimates of radiation related cancer risks. Radiat
radiation dose to patients between institutions. Reference dose                Res 2007;167:396–416.
values for chest CT have been developed and published. Radi-            16. Pierce D, Shimizu Y, Preston D, Vaeth M, Mabuchi K. Studies of the
                                                                               mortality of atomic bomb survivors: report 12, part 1. Cancer: 1950–
ologists need to monitor the radiation dose delivered in exami-                1990. Radiat Res 1996;146:1–27.
nations within their institutions, adopt and adhere to national         17. Brenner DJ, Elliston CD, Hall EJ, Berdon WE. Estimated risks of
radiation dose guidelines, and investigate further dose reduction              radiation-induced fatal cancer from pediatric CT. AJR Am J Roent-
strategies within their own practices. Further research into the               genol 2001;176:289–296.
complex relationship between radiation exposure, image noise,           18. Parker MS, Hui FK, Camacho MA, Chung JK, Broga DW, Sethi NN.
and diagnostic accuracy should be encouraged to scientifically                  Female breast radiation exposure during CT pulmonary angiography.
                                                                               AJR Am J Roentgenol 2005;185:1228–1233.
establish the minimum radiation doses that provide adequate
                                                                        19. Hurwitz L, Yoshizumi T, Reiman R, Paulson E, Frush D, Nguyen G,
diagnostic information for standard clinical questions, disease                Toncheva G, Goodman P. Radiation dose to the female breast from
quantitation, and disease follow up. Once these minimum levels                 16 MDCT protocols. AJR Am J Roentgenol 2006;186:1718–1722.
of image quality are determined and validated, automatic expo-          20. Milne E. Female breast radiation exposure. AJR Am J Roentgenol 2006;
sure controls for CT scanners should be programmed to ensure                   186:E24. (letter).
that all patients undergo CT with techniques that conform to the        21. Brenner D. Radiation risks potentially associated with low dose CT
ALARA (As Low As Reasonably Achievable) principle. Finally,                    screening of adult smokers for lung cancer. Radiology 2004;231:440–
new approaches to image reconstruction should be addressed to           22. Fricke B, Donnelly L, Frush D, Yoshizumi T, Varchena V, Poe S,
maximize the relationship between dose and image quality.                      Lucaya J. In plane bismuth breast shields for pediatric CT: effects on
    As dispensers of this known carcinogen, radiologists must take             radiation dose and image quality using experimental and clinical data.
the lead in promoting all of these measures for patient protection.            AJR Am J Roentgenol 2003;180:407–411.
Mayo: Radiation Dose in CT                                                                                                                             939

23. Hopper KD, King S, Lobell M, TenHave T, Weaver J. The breast: in-          30. Haaga JR, Miraldi F, MacIntyre W, LiPuma JP, Bryan PJ, Wiesen E.
      plane X-ray protection during diagnostic thoracic CT-shielding with             The effect of mAs variation upon computed tomography image
      bismuth radioprotective garments. Radiology 1997;205:853–858.                   quality as evaluated by in vivo and in vitro studies. Radiology 1981;
24. Huda W. Radiation dosimetry in diagnostic radiology. AJR Am J                     138:449–454.
      Roentgenol 1997;169:1487–1488.                                                                                                         ¨
                                                                               31. Mayo JR, Hartman TE, Lee KS, Primack SL, Vedal S, Muller NL. CT of
25. Brenner DJ. Effective dose: a flawed concept that could and should be              the chest: minimal tube current required for good image quality with
                                                                                      the least radiation dose. AJR Am J Roentgenol 1995;164:603–607.
      replaced. Br J Radiol 2008;81:521–523.
                                                                               32. Kalender WA, Wolf H, Suess C, Gies M, Greess H, Bautz WA. Dose
26. Panzer W, Scheurer C, Zankl M. Dose to patients in computed
                                                                                      reduction in CT by on-line tube current control: principles and
      tomography examinations: results and consequences from a field
                                                                                      validation on phantoms and cadavers. Eur Radiol 1999;9:323–328.
      study in the Federal Republic of Germany. In: Moores BM, Wall
                                                                               33. Kalender WA, Wolf H, Suess C. Dose reduction in CT by anatomically
      BF, Eriskat H, Schibilla H, editors. Optimization of image quality and
                                                                                      adapted tube current modulation: II. Phantom measurements. Med
      patient exposure in diagnostic radiology. London: British Institute of
                                                                                      Phys 1999;26:2248–2253.
      Radiology Report 20; 1989.
                                                                               34. Greess H, Wolf H, Baum U, Lell M, Pirkl M, Kalender W, Bautz W.
27. Nishizawa K, Maruyama T, Takayama M, Okada M, Hachiya J, Furuya
                                                                                      Dose reduction in computed tomography by attenuation-based on-
      Y. Determinations of organ doses and effective dose equivalents from            line modulation of tube current: evaluation of six anatomical regions.
      computed tomographic examination. Br J Radiol 1991;64:20–28.                    Eur Radiol 2000;10:391–394.
28. European Commission. European guidelines on quality criteria for           35. Tack D, De Maertelaer V, Gevenois PA. Dose reduction in multi-
      computed tomography. EUR 16262EN. Luxembourg: Office for                         detector CT using attenuation-based online tube current modulation.
      Official Publication of the European Communities; 2000.                          AJR Am J Roentgenol 2003;181:331–334.
29. Commission of the European Communities DX, Radiation Protection            36. Li J, Udayasankar UK, Toth TL, Seamans J, Small WC, Kalra MK.
      Division. Guidance on medical exposures in medical and biomedical               Automatic patient centering for MDCT: effect on radiation dose.
      research. Luxembourg: Radiation Protection publications of the                  AJR Am J Roentgenol 2007;188:547–552.
      Commission of the European Communities DX, Radiation Protection                                         ¨
                                                                               37. Mayo JR, Aldrich J, Muller NL. Radiation exposure at chest CT:
      Division. RP99; 1998.                                                           a statement of the Fleischner Society. Radiology 2003;228:15.