Download Imaging strategies to reduce the risk of radiation in CT studies

JOURNAL OF MAGNETIC RESONANCE IMAGING 25:900 –909 (2007)
Review Article
Imaging Strategies to Reduce the Risk of Radiation
in CT Studies, Including Selective Substitution
With MRI
Richard C. Semelka, MD,1* Diane M. Armao, MD,1 Jorge Elias Junior, MD, PhD,1,2 and
Walter Huda, PhD2
“When one admits that nothing is certain one must, I
think, also admit that some things are much more nearly
certain than others.” Bertrand Russell (1872–1970)
Computed tomography (CT) is one of the largest contributors
to man-made radiation doses in medical populations. CT currently accounts for over 60 million examinations in the United
States, and its use continues to grow rapidly. The principal
concern regarding radiation exposure is that the subject may
develop malignancies. For this systematic review we searched
journal publications in MEDLINE (1966 –2006) using the
terms “CT,” “ionizing radiation,” “cancer risks,” “MRI,” and
“patient safety.” We also searched major reports issued from
governmental U.S. and world health-related agencies. Many
studies have shown that organ doses associated with routine
diagnostic CT scans are similar to the low-dose range of radiation received by atomic-bomb survivors. The FDA estimates
that a CT examination with an effective dose of 10 mSv may be
associated with an increased chance of developing fatal cancer for approximately one patient in 2000, whereas the BEIR
VII lifetime risk model predicts that with the same low-dose
radiation, approximately one individual in 1000 will develop
cancer. There are uncertainties in the current radiation risk
estimates, especially at the lower dose levels encountered in
CT. To address what should be done to ensure patient safety,
in this review we discuss the “as low as reasonably achievable” (ALARA) principle, and the use of MRI as an alternative
to CT.
J. Magn. Reson. Imaging 2007;25:900 –909.
© 2007 Wiley-Liss, Inc.
Key Words: computed tomography; ionizing radiation;
cancer risks; MRI; patient safety
1
Department of Radiology, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina, USA.
2
Department of Radiology, School of Medicine of Ribeirao Preto, University of Sao Paulo, Ribeirao Preto, Brazil.
3
Department of Radiology, SUNY Upstate Medical University, Syracuse,
New York, USA.
Contract grant sponsor: National Council of Scientific and Technological Development of Brasil.
*Address reprint requests to: R.C.S., Department of Radiology, University of North Carolina at Chapel Hill, CB# 7510, 101 Manning Drive,
Chapel Hill, NC 27599-7510. E-mail: [email protected]
Received October 20, 2006; Accepted November 28, 2006.
DOI 10.1002/jmri.20895
Published online in Wiley InterScience (www.interscience.wiley.
com).
© 2007 Wiley-Liss, Inc.
AN IMPORTANT TRANSLATION from laboratory investigation to clinical practice occurred with the discovery
of x-rays by Wilhelm Roentgen in 1895, which gave
birth to the profession of diagnostic radiology. In the
1970s Godfrey Hounsfield developed the first commercial computed tomography (CT) scanner, which used
computer technology to help transform the practice of
radiology. The diagnostic value of CT when it is used
judiciously for specific indications is unquestioned.
For example, CT is unmatched in its ability to demonstrate findings in major trauma, renal stone disease, and interstitial lung disease (1,2). Recent advances in high-speed multidetector row CT technology
have enabled radiologists to obtain clearer images in
shorter times, and have markedly increased the use
of CT (3).
However, with increased use comes growing concern
regarding the possible risks associated with ionizing
radiation from diagnostic CT. In this article we review
publications regarding the hazards of x-rays, and discuss the lack of awareness by both medical practitioners and the general population concerning the potential harmful effects of x-rays. We also address current
uncertainties regarding the radiation risks of CT doses,
and propose ways in which the medical community can
modify its behavior in the light of the controversies
associated with the radiation risk of low doses. We outline how one can reduce individual and collective patient doses from CT without detracting from the valuable diagnostic information that this important imaging
modality affords health-care providers. One component
of this strategy is to substitute CT with MRI studies
when clinically appropriate.
RADIATION RISKS
Patients undergoing a CT examination typically receive
organ doses on the order of tens of mGy. As a result,
organ doses in CT are well below the threshold doses for
the induction of deterministic effects such as skin
burns and epilation, and the induction of eye cataracts
(4). In the middle of the 20th century, when atmospheric testing of atomic weapons resulted in radioactive fallout on a worldwide scale, the genetic effects of
radiation were deemed to be a major concern. Nowa-
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Reducing Risks of Radiation
days, however, any genetic risk is considered to be
minor compared to the risk of carcinogenesis. Individual genetic risks are regarded as negligible, and any
corresponding societal impact from diagnostic radiology is deemed to be very low. The principal concern at
patient doses on the order of tens of mGy is the induction of fatal and nonfatal cancers (5–7).
Radiation delivered by x-ray-based imaging modalities has been shown to result in deleterious health
effects. Publications in the 1940s and 1950s described
the increased incidence of leukemia in radiologists
(8,9). Fluoroscopy was a major form of imaging investigation at that time, which explains why radiologists had
a higher incidence of leukemia (since with this method
the radiologist and patient share the same radiation
exposure). One of the earliest reports that described the
development of leukemia in children as a result of x-ray
exposure of the maternal abdomen during pregnancy
was published in 1956 (10). This study involved a nationwide survey across England, and the preliminary
findings were that 85 patients with leukemia had been
exposed to radiation from maternal abdominal x-rays,
while 45 patients had not. The authors concluded that
“so large a total difference between the cases and controls can hardly be fortuitous” (10). They also explained
that “one reason for attempting a nation-wide survey
was the possibility that the peak of leukemia mortality
noted by Hewitt (11) might be explained if weak irradiation could initiate changes in a fetus or very young
child.” Their concluding statement was that “this apparently harmless examination may occasionally cause
leukemia or cancer in the unborn child.” A follow-up
review published in 1987 (12) considered that a causal
explanation is supported by evidence indicating an appropriate dose-response relationship, as well as by animal experiments, and concluded that radiation doses
on the order of 10 mGy received by the fetus in utero
produce a consequent increase in the risk of childhood
cancer (12). They reported that the excess absolute risk
coefficient is approximately 6% per gray of radiation
exposure (12). Historical data from the early days of
diagnostic x-ray imaging indicate that malignancies
arose from excessive exposure to the x-rays. A number
of articles in major medical journals described various
malignancies those arose in both patients and physicians (8,9). Other data showed an increased risk of
breast cancer in female patients who underwent serial
spine x-ray examinations for investigation of scoliosis
(13), and an increased incidence of childhood leukemia
associated with postnatal diagnostic x-rays (14). These
studies involved relatively high organ doses. Unfortunately the doses were not recorded, but they may be
much higher than the organ doses associated with individual CT examinations.
Organ doses associated with routine diagnostic and
elective CT scans are similar to the low-dose range of
radiation received by atomic-bomb survivors, who have
manifested significant increases in cancer incidence
and mortality (15–17). The individuals in the lowest
dose group of atomic-bomb survivors who showed a
significant rise in cancer incidence and mortality re-
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ceived organ equivalent doses1 in the range of 5–100
mSv (mean equivalent dose ⫽ 29 mSv) and 5–125 mSv
(mean equivalent dose ⫽ 34 mSv), respectively (15,16).
Typical equivalent doses in directly irradiated organs
are in the range of 20 –30 mSv for a single routine adult
CT examination (18,19). Although the form of radiation
delivered by atomic bombs (gamma radiation) is different from that used in radiology (x-radiation), there is
fundamentally no difference in the carcinogenic potential (4). The average number of CT scans conducted for
a given medical problem is two, yielding an average total
equivalent dose of 40 – 60 mSv (17,19,20). Of particular
concern are CT studies involving the liver, pulmonary
embolism, and renal calculi. This reflects the high dose
per procedure (liver), the territory of coverage (pulmonary embolism), and the likelihood of repeat studies
(renal calculi). CT pulmonary angiography delivers a
high radiation dose to the breasts of young women, and
thus has a tremendous potential to induce breast cancer (21). An important area of potential CT overuse is
the investigation of renal calculi. One recent study assessed repetitive studies in patients with suspected renal stones, and found that over a six-year period 4% of
patients underwent three or more CT studies for this
indication, with estimated effective doses ranging from
19.5 to 153.7 mSv (22). One factor hindering the widespread recognition of the harmful effects of CT is the
absence of direct data regarding cancer induction from
CT. One study reported on induction of DNA doublestrand breaks with CT in lymphocytes of subjects undergoing CT examination (23). DNA double-strand
breaks may be viewed as a step in the pathway to
carcinogenesis. Thus, there are data indicating an increased cancer risk at doses used in diagnostic CT, and
the potential for adverse consequences that may arise
from increasing CT utilization needs to be addressed.
One can assess the radiation risk to any single tissue
by considering the organ dose. A CT scan will normally
irradiate many organs and tissues, and it is the total
patient risk that is of interest. The effective dose takes
into account all of the irradiated organs and tissues, as
well as their radiosensitivities, and is the best single
parameter for quantifying how much radiation an individual will receive during any radiologic examination
(24). The effective dose, expressed in mSv, is the product of an organ’s equivalent dose and radiosensitivity,
and is obtained by summing over all exposed organs
and tissues (5). An effective dose provides the wholebody dose that produces the same risk as a partial dose
delivered by a localized radiologic procedure (25), and a
uniform whole-body dose of 1 mGy corresponds to an
effective dose of 1 mSv. Effective doses may also be
converted into a numerical risk of mortality from radiation-induced cancer (26), which is currently taken to
be on the order of 5%/Sv when averaged over a whole
population (5). The pediatric population represents an
especially vulnerable group of individuals who are at
increased risk for cancer (27). This fact is underscored
1
The equivalent dose (mSv) is the absorbed dose (mGy) multiplied by
the radiation weighting factor wr, which depends on the type of radiation. x-Rays, gamma rays, and beta particles have wr equal to 1, while
alpha particles and neutrons have wr values of 20.
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Semelka et al.
Table 1
Average Radiation Doses Associated With Common Imaging
Studies
Diagnostic examination
X-rays
Chest (PA film)
Limbs/joints
Head
Screening mammogram
Cervical spine
Abdomen
Pelvis/hip
Thoracic spine
Lumbar spine
Upper GI
Lower GI
CT
Head
Abdomena
Chesta
Pulmonary angiographya
PET-CT
Effective dose (mSv)
0.02
0.06
0.07
0.13
0.3
0.53
0.83
1.4
1.8
3.6
6.4
2.0
10.0a
10–40a
20–40a
25
a
CT protocols that overlap scanned regions or rescan the same
anatomic region of interest, (e.g., noncontrasted and contrast-enhanced scans), impart two to three times the radiation dose.
PA ⫽ postero-anterior, GI ⫽ gastrointestinal, PET ⫽ positron emission tomography.
by a recent article in the Journal of the American Medical Association regarding a long-term follow-up (55–58
years) of atomic-bomb survivors exposed to radiation
(28). The authors concluded that a significant linear
dose-response relationship existed in the prevalence of
malignant thyroid tumors, and that the relationship
was significantly higher in individuals exposed at
younger ages (⬍20 years). At the same technique factors, as used in adults, pediatric doses are substantially higher (29), and young children have a higher
radiosensitivity than adults (19,30,31). For example,
recent studies have shown that 600,000 abdominal and
head CT examinations annually in children under the
age of 15 years could result in 500 deaths from cancer
attributable to CT radiation (32). The most common
malignancies associated with radiation exposure include leukemia and breast, thyroid, lung, and stomach
cancers (33). It is important to note that the latency
period for solid cancers is generally long (10 –20 years
on average) and persists throughout life (34). Of all
malignancies, leukemia has the earliest latency period,
with increased risks being noted at two to five years
following radiation exposure (34).
HOW WELL ARE CT DOSES (AND RISKS)
UNDERSTOOD?
It is important to realize that CT doses are much higher
than conventional radiography doses (7,35–38), and
that the effective dose for a chest CT is approximately
100 –1000 times larger than that for a corresponding
chest x-ray examination. Table 1 shows the average
effective doses used in 16 common x-ray procedures in
19 western countries, and illustrates that patient doses
in CT are much higher than those in most common
radiographic examinations (7). Furthermore, published
frequency data mask the number of times a single patient receives multiple CT examinations (37) and the
common practice of multiphase CT scanning, both of
which increase the radiation dose (and potential risk) in
a cumulative manner (20,39). It has been estimated
that 30% of all individuals who undergo CT will be
examined at least three times (37).
It is important to realize that the imaging-based radiation risk for the individual patient is small compared
to the overall risk for developing cancer from all causes
(approximately 42 per 100 individuals) (33). However,
from a public health risk perspective, this small individual cancer risk must be multiplied by a large and
ever-increasing population of individuals undergoing
CT examinations. A small excess mortality risk of
0.05% for cancer attributable to ionizing radiation exposure for an individual translates into 50 radiationinduced deaths per 100,000 people exposed (40). Such
a risk could be extrapolated from an effective dose of 10
mSv, which is incurred from a single CT examination
(40).
The FDA estimates that a CT examination with an
effective dose of 10 mSv (e.g., one CT of the abdomen)
may be associated with an increased chance of developing fatal cancer for approximately one patient in
2000 (41). An estimate of the lifetime cancer mortality
risk attributable to the radiation exposure from a single
abdominal CT examination in a one-year-old child is
approximately one in 550 (32). A recent report by de
Gonzalez and Darby (42) estimated that, per year, diagnostic x-ray use in the United States causes 0.9% of the
cumulative risk of cancer to age 75 in men and women,
which is equivalent to 5695 cases. These statistics indicate the potential for a future public health problem,
given that over 60 million CT scans are currently performed annually in the United States (20,43) and the
use of this modality is continuing to grow rapidly (44).
Despite the current evidence regarding the risk of
low-dose radiation offered by radiation biologists, there
is a lack of information transfer between researchers
and clinicians, including radiologists. In a recent study
of health policies and practices in radiology, Lee et al (3)
considered adult patients who were seen in the emergency department of a U.S. academic medical center for
mild to moderate abdominopelvic or flank pain and
underwent a CT investigation. Only 7% of the patients
stated that they were informed about the risks and
benefits of a CT scan. More importantly, only 3% of the
patients reported that they were informed about the
increase lifetime cancer risk associated with CT. Referring emergency department physicians were also
largely unaware that there were any potential harmful
effects from the radiation exposure (only 9% were aware
of the increased cancer risk). Of even greater concern
are data showing that the radiologists who performed
the CT examinations considered the radiation exposure
to be of limited importance, and were unaware of the
amount of radiation that was delivered to the patients
by CT. Another recent study from a large hospital in the
UK used a simple questionnaire with a multiple-choice
format to assess the knowledge of primary care and
specialist physicians concerning radiation doses and
Reducing Risks of Radiation
risks (45). The results of the study revealed an urgent
need to improve physicians’ understanding of radiation
exposure. Only 27% of doctors attained the generous
cutoff of a 45% passing mark, and only 57% of radiologists and radiology-related subspecialists passed the
test.
This lack of awareness of potential radiation risks
from CT is well illustrated by a recent article on lung
cancer screening in a premier medical journal (46). The
authors described the various benefits and drawbacks
of lung-cancer screening, but made no mention of the
potential risk of inducing cancer by radiation. The
breach in communication has created a deficit in public
awareness regarding the untoward effects of radiation
doses associated with CT examinations. The obligation
of physicians to provide information on diseases and
the risk/benefit ratio of diagnostic procedures and
treatment options has come under scrutiny in the modern medical environment. The radiology community
should be committed to dose reduction, which will require a team approach involving major radiology and
radiology-related groups, including radiological societies, manufacturers, standards organizations, regulators, and public agencies (43). It is also the responsibility of the radiology community to increase consumer
education by describing radiation risks and alternative,
safe imaging modalities. Educational efforts should be
directed toward referring physicians, regulators, public
health officials, and patients. Key components for ensuring patient safety include limiting the improper use
of CT, and minimizing exposure to levels that are essential for achieving satisfactory image quality.
RADIATION RISK UNCERTAINTIES
No major scientific body that investigates radiation
risks, including the International Commission on Radiological Protection (ICRP) (5), the National Academy of
Sciences Committee on the Biological Effects of Ionizing
Radiation (BEIR) (6), and the United Nations Scientific
Committee on the Effects of Atomic Radiation (UNSCEAR) (7), recommends the use of a threshold dose for
the induction of cancer. The Department of Health and
Human Services first listed ionizing radiation as a
known human carcinogen in 2005 (47). The most recent report on mortality from the Life Span Study (LSS)
cohort of atom-bomb survivors followed by the Radiation Effects Research Foundation (RERF) showed that
excess solid cancer risks appear to be linear in dose
even for equivalent doses in the 0 –150 mSv range, with
no evidence for a threshold (16). In the United States,
the National Council on Radiation Protection and Measurements recently concluded that “there is no conclusive evidence on which to reject the assumption of a
linear-nonthreshold dose-response relationship for
many of the risks attributable to low-level ionizing radiation. . .” (48). The results from the largest study of
nuclear workers ever conducted showed that radiation
risk estimates were “higher, but statistically compatible
with, the risk estimates used for current radiation protection standards” (49).
The BEIR VII report recently released by the National
Academy of Sciences provides the most up-to-date and
903
comprehensive risk estimates for cancer from exposure
to low-level ionizing radiation (33). The BEIR VII executive summary states that “a comprehensive review of
available biological and biophysical data supports a
linear-no-threshold (LNT) risk model- that the risk of
cancer proceeds in a linear fashion at lower doses without a threshold and that the smallest dose has the
potential to cause a small increase in risk to humans”
(33). A “low dose” is defined by the BEIR VII committee
as ranging from almost zero to ⬃100 mSv (0.1 Sv). On
average, assuming an age and sex distribution similar
to that of the entire U.S. population, the BEIR VII lifetime risk model predicts that approximately one individual in 1000 will develop cancer from an exposure to
10 mSv (0.01 Sv) of low-dose radiation (33). The BEIR
VII report concludes by stating that future medical radiation studies are warranted, and emphasizes that “of
concern for radiological protection is the increasing use
of computed tomography (CT) scans and diagnostic X
rays” (www.nap.edu). It is therefore clear that the carcinogenic potential of radiation at doses encountered in
CT needs to be taken seriously by the medical imaging
community.
Nonetheless, it is important to recognize that there
are considerable uncertainties in the current radiation
risk estimates as provided by organizations such as the
ICRP, UNSCEAR, and BEIR, especially at the lower dose
levels encountered in CT (50). The available evidence is
not conclusive, primarily because of the absence of definitive epidemiological data at low dose levels. Some
investigators have reviewed the available epidemiological and radiobiological evidence and concluded that the
current ICRP risk estimates are too high (51–53). It has
also been suggested that low-dose radiation may be
beneficial, and that our concerns about the radiation
risks of CT doses is unwarranted (54,55). At the same
time, it is also necessary to acknowledge that some
researchers have suggested that radiation risks may be
even higher than those currently adopted for practical
use by radiation protection bodies (56 –58). Radiation
risk estimates for low radiation doses are a very controversial topic, and there is currently no scientific consensus regarding the existence of any threshold dose
below which the risk of cancer induction would be zero.
Given the uncertainty as to whether there is a threshold dose for radiation-induced cancer, any decision that
is made may eventually turn out to be erroneous. If we
assume that there are radiation risks when there are
none, we will be expending efforts and resources to
minimize nonexistent risks; however, if there truly are
radiation risks and we choose to ignore them, we will be
subjecting our patients to long-term detrimental consequences. Making decisions as to how to act in the presence of scientific uncertainty is essentially a political
process, and therefore will require the use of value
judgments (59).
WHAT SHOULD BE DONE?
In the absence of definitive evidence on the effects of low
dose radiation, and a corresponding consensus in the
scientific community, the prudent course of action is to
act on the assumption that low-dose radiation may well
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cause cancer. If we assume that CT doses may cause
cancer, it would be reasonable for the radiology community to seek ways to eliminate unnecessary radiation
exposure while ensuring appropriate utilization of CT. A
CT examination may be considered indicated when the
benefits of the CT would outweigh any assumed radiation risks. Adoption of this course of action is in no way
expected to impede the undoubted benefits of CT technology for all indicated examinations. Below we describe three essential components of an effective strategy to promote patient safety and minimize any
detrimental effects of CT radiation exposures.
Consider Alternatives to CT
Until recent times, MRI has been considered as an alternative modality to CT investigation for cases in which
1) there are contraindications to CT (e.g., allergy to
contrast agents or poor renal function) or 2) the CT
findings are inconclusive. However, cumulative experience has demonstrated that MRI surpasses CT in several respects:
1. MRI avoids the significant health risk associated
with a radiation dose from a CT procedure. The
ionizing radiation dose makes CT especially unsuitable for studies that require serial assessments in which the doses will become additive.
2. MRI provides inherently superior soft-tissue contrast even before the administration of intravenous contrast.
3. Greater patient safety is achieved with the gadolinium (Gd)-based contrast agents used in MRI
compared to the iodine-based contrast agents
used in CT (60)
4. More different types of tissue can be interrogated
during a single MRI examination, and thus data
reflecting a greater range of chemical and physical
properties of both normal and abnormal tissue
can be obtained (61).
The use of MRI and ultrasound eliminates exposure
to the ionizing radiation encountered in CT. The existence of safe and robust imaging modalities such as
MRI should obviate altogether the risk/benefit ratio
invoked in many clinical scenarios involving CT use. CT
should be reserved for use as a problem-solving modality only when it is clearly the superior approach. CT is
indicated for the evaluation of primary lung disease
(e.g., interstitial lung disease), the majority of chest and
abdominal traumas, evaluation of tubes and catheters
in postoperative or intensive-care patients, and identification of renal calculi (62– 67). It is equally important
to acknowledge the limitations of MRI in terms of both
diagnostic accuracy and safety. Patients with cardiac
pacemakers and ferrous intracranial vascular clips are
not candidates for MRI (68), and indeed MRI has been
reported to cause death in such patients (69). The
known hazards associated with flying ferromagnetic objects, internal prostheses, and wire or metal objects in
contact with the skin of patients can be avoided by the
routine implementation of mandatory screening questionnaires filled out by patients before each MRI examination. A major current concern is that patients with
Semelka et al.
chronic renal failure may develop a serious medical
condition—nephrogenic systemic fibrosis (NSF)—if they
receive a high dose of Gd chelate (70,71). To date, however, it appears that Omniscan™ is the contrast agent
most likely to result in this condition. A prudent course
of action would be to use the more stable Gd chelates—
Magnevist® and MultiHance®—in all patients with renal failure.
Following the introduction of rapid high-quality scanning techniques and the development of new tissuespecific contrast agents, the application of MRI for
many organ systems has achieved unprecedented
growth. The majority of benign, malignant, and inflammatory diseases of the spleen, adrenals, kidneys, pancreas, and organs of the male and female pelvis are well
depicted by MRI, and in the hands of experienced practitioners are better elucidated than they would be on CT
(72–76). The superiority of MRI over CT for detecting
and characterizing focal liver lesions and liver disease is
well established (77– 80). In patients with malignancy,
MRI demonstrates an advantage over CT in detecting
extrahepatic abdominal disease in many sites (81). Further, although MRI is acknowledged as being superior
to CT for detecting disease in many individual organ
systems, a recent study showed that whole-body MRI
for evaluating metastatic disease was equivalent to, and
in some instances more accurate than “gold standard”
reference techniques such as CT (82). It is beyond the
scope or the intention of this review to describe the role
played by MRI in organ systems that are often studied
by CT investigation. The interested reader is directed to
a recent textbook on this subject (83) for further details.
Two key issues have served as impediments to a widespread preference for MRI over CT in the medical community: 1) health-care costs and 2) the ordering practices of referring physicians. Until recently, MRI has
been considered an expensive diagnostic test. However,
the actual costs of MRI equipment, service, and site
expenses have plummeted over the past 20 years by
factors of approximately 65%, 55%, and 80%, respectively. The average MRI technical fee in 2004 had fallen
by 70% compared to the average fee in 1985. Further,
the trend of decreasing costs for MRI is expected to
continue over the next 20 years (84). The charges for
MRI studies are generally about 20% higher than those
for CT studies (hospital charges).
Changes in practice, such as eliminating inappropriate referrals for CT, will exert the most dramatic impact
on avoiding the hazards of radiation but will be the
most challenging to implement. A logical first step to a
transition in imaging ordering patterns would be to
utilize MRI as the primary imaging tool to investigate
diseases of the liver. The rationale for this is fourfold: 1)
the intrinsic safety of the modality, contrast agents, and
intravenous injection process; 2) the established
greater accuracy of MRI over CT for liver investigations
(78) (Fig. 1); 3) the ease of performing MRI; and 4) the
ease of interpreting MRI studies, even without extensive
MR training. A number of practices by various radiology
agencies would have to be modified to aid in this conversion, including recommendations by various international colleges of radiology, and increasing the con-
Reducing Risks of Radiation
Figure 1. High-quality postcontrast (16 row multidetector) CT
(a) and postcontrast MR (b) T1-weighted images. More small
hypervascular tumors are visualized by MRI (arrows, b) than
by CT. This reflects the much greater sensitivity of MRI for
detecting contrast enhancement compared to CT.
tent of abdominal MRI examination material on
radiology certification examinations.
Use CT Only When Indicated
Due to the popularity of volumetric imaging acquisition
techniques and the rapid speed at which CT images can
be obtained, the utilization of CT scans has greatly
increased in recent years (85). CT is responsible for the
largest component of medical radiation, which is by far
the largest source of man-made exposures of the population (20). Although CT in a typical U.S. hospital
represents only ⬃10% of all x-ray imaging, the radiation
from CT accounts for up to 67% of all medical radiation
(20,37). CT use in the U.S. has been growing rapidly
since the mid 1990s, and shows no sign of abating (44).
Coexistent with the increased volume of CT exams is
the growing proportion of unnecessary CT evaluations.
This principle is exemplified by data gathered from a
major medical center by Donnelly (85) in 2005, which
indicated a 65% interval increase in pediatric abdominal CT exam referrals from the emergency department
during the first quarter of the 2002 academic year compared to the same period in 2001. In this article, multiple factors were listed as contributing to the problem
of unnecessary referrals for CT studies, including 1)
overcautious ordering patterns of referring physicians
with concerns related to potential malpractice litigation, 2) monetary incentives to perform more CT studies
related to fee-for-service financial arrangements, and 3)
905
pressure from the American public to use high-end
technology. An example of a CT application that does
not improve diagnostic accuracy but is being used increasingly is investigation of children with suspected
appendicitis (86).
The current and ever-growing number of pediatric
and adult diagnostic CTs performed in western countries annually is compounded by the increasing number of independent radiology clinics that use full-body
CT screening in healthy adults. Despite the lack of
supporting data for a life-prolonging benefit, CT scanning continues to be popular for screening and early
detection of a variety of diseases, including coronary
artery disease, lung cancer, and colon cancer (18,87).
Unregulated direct-to-consumer marketing and self-referral are galvanizing this trend in CT whole-body
screening (88). Self-referred diagnostic screening recently expanded to functional brain imaging using single photon emission CT (88). This form of functional CT
imaging is advertised with a focus on school-aged children who are performing at marginal educational levels
and adults who are concerned about memory loss (89).
In advertisements for CT screening, references to the
health risk of irradiation associated with CT scans are
notably absent (62). In the private sector, the use of CT
scanners to perform elective total-body scans and cardiac CTs is a lucrative business. Supervision of dosimetry
at such scanning centers does not routinely occur (34).
Communication between health-care providers and
radiologists is critical for determining whether a CT
scan is appropriate. CT should only be performed when
the benefit to the patient outweighs any assumed radiation risk (90). Supplied with sufficient clinical information, the radiologist may be able to suggest an acceptable alternative that does not use ionizing
radiation, such as MRI or ultrasound. Dialogue between the referring physician and the radiologist is essential, especially given a generally uninformed medical
community with inadequate knowledge about radiation
exposure (45). With the advent of widespread hospital
and clinic-based electronic medical record (EMR) systems, an effective strategy for modifying behavior and
ordering practices may be to employ a decision support
system. When a referring physician orders an imaging
study, a computer-based decision support system
would list indications for various imaging modalities
and, when relevant, provide radiation dose and concomitant health risk information for CT. Additionally,
there should be an automated system in place to track
imaging-related radiation exposures in each patient’s
EMR. This system would serve two purposes: 1) it
would provide decision support for physicians ordering
radiation-based imaging modalities, especially for patients with previous exposures, and 2) it would create
an important database to be used in future epidemiological studies aimed at clarifying potential health risks
associated with low-dose ionizing radiation.
Keep Patient Doses as Low as Reasonably
Achievable (ALARA)
CT is a digital technology that suffers no obvious image
quality-related penalty for high doses of radiation.
906
Higher CT doses enhance image quality by reducing
quantum mottle, whereas in screen-film radiography
higher doses result in overexposed (dark) examinations
with little image contrast (20). Increasing CT radiation
doses above a certain level will not contribute to improving diagnostic image quality, and will only result in
depositing more radiation into the patient’s body
(25,91). Maximizing image quality in this way while
ignoring the patient’s exposure to radiation is inappropriate. For example, techniques that employ modern
multidetector CT technology, multiphase contrast-enhanced CT of the liver or kidneys, or CT urography, are
generally performed with the intention of acquiring sufficient data for maximal image quality and diagnostic
information, and often not enough attention is paid to
limiting radiation exposure.
An insufficient radiation dose in CT will result in
increased image mottle with concomitant degradation
of the radiologist’s ability to detect low-contrast lesions.
Accordingly, in digital imaging modalities there is a
tendency to increase the amount of radiation used to
avoid this problem (dose creep) (92,93). Increasing
doses in this manner does not necessarily improve diagnostic performance, but will always increase the patient dose. It is essential that the amount of radiation
used to perform a CT examination is appropriately sufficient to obtain the required diagnostic performance,
and that radiation exposures are kept ALARA (94). As
an example, multiphase examinations comprise studies in which repeat examinations of an anatomic area
are performed relative to the administration of intravenous contrast material. Precontrast studies are the
most familiar, but early- (arterial), mid- (venous), and
late-phase CT scans may also be performed (20). Every
additional phase increases the radiation dose and risk
by the multiple of the total number of phases (20). It has
been reported that multiphase scanning is used for
approximately 30% of pediatric patients undergoing
body CT, often in three phases (39). Multiple scan protocols should be held to a minimum (90).
It is important to note that the ALARA principle is
practical and can be achieved without sacrificing any of
the benefits of diagnostic information that CT currently
offers to medical practitioners and their patients. We
illustrate this principle by considering in the following
paragraphs how CT scanning that takes into account
the patient size, as well as the specific diagnostic task,
can reduce doses with no corresponding penalty in
terms of acquired diagnostic information.
Patient Size
Until very recently, children and adults were routinely
scanned with identical protocols that did not differentiate between the large differences in patient sizes and
the corresponding differences in transmitted x-ray intensities (25). In part, this was likely a result of the fact
that digital imaging systems can operate at any level of
radiation exposure, and there is no corresponding image quality “penalty” associated with higher doses. Recent studies in pediatric CT have shown that it is unacceptable to use a tube current setting that is
appropriate for an adult on a child (85,95,96). Reduc-
Semelka et al.
tions in tube current or gantry speed have recently been
introduced that decrease the radiation dose in pediatric
CT (97–99). It is a welcome sign that most major manufacturers have recently added dose-reduction features
specifically geared toward pediatric patients, and initiated user education programs for CT dose control (43).
Regulatory agencies and accreditation bodies now require operators of CT scanners to employ patient sizespecific technique charts (43,100). The recent emphasis of the ALARA principle in radiological practice and
the corresponding dose reductions do not appear to
have adversely affected CT diagnostic performance
(101).
Imaging Task
Another key component of dose reduction is the development of CT imaging protocols that explicitly take into
account the diagnostic task at hand (102). It is not
prudent to set technique factors without considering
the purpose of the scan, as it is often possible to significantly reduce patient doses without adversely affecting
diagnostic performance. CT scanning performed after
the administration of iodinated contrast agents may
produce much higher contrast-to-noise ratios (CNRs)
for enhancing lesions when performed at lower x-ray
tube voltages (80 kV), which could offer major dose
savings (103,104). CT pelvimetry is another task-dependent CT examination in which reductions of radiation dose are both essential and achievable (105,106).
CT screening protocols for lung cancer frequently reduce the x-ray tube output by an order of magnitude in
comparison with routine chest CT examinations, and
there is no evidence of any detrimental impact of these
dose reductions in terms of diagnostic performance
(107,108). Many researchers are investigating ways to
dramatically reduce radiation exposure while maintaining diagnostic information (109), and expanding
such studies could help to significantly reduce population CT doses. Dual-source CT was recently released. In
addition to providing faster imaging acquisition times
and higher temporal resolution, according to its manufacturer it uses 50% less radiation exposure than traditional CT scans, which may be especially useful for
cardiac imaging (110).
CONCLUSIONS
The fact that CT has exerted a tremendous impact on
diagnostic radiology over the past few decades is undeniable. However, there is growing concern about the
potential for significant health risks, on both individual
and public health levels, attributable to CT radiation.
Despite the diversity of opinion regarding the exact nature of the health risk, we hope there is a consensus of
commitment to hold the standard of patient safety at a
premium. Therefore, in this article we have attempted
to give a balanced view in defining the problem and
offering a practical approach to dose reduction. The
information contained in this article should not be construed as an indictment of current medical practices,
but rather as an incentive to increase awareness, pre-
Reducing Risks of Radiation
vent unnecessary health risks, and ultimately improve
patient care.
ACKNOWLEDGMENT
Jorge Elias, Jr., is supported by the National Council of
Scientific and Technological Development of Brasil.
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