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Abdominal
Imaging
ª Springer Science+Business Media, LLC 2012
Published online: 27 July 2012
Abdom Imaging (2013) 38:22–31
DOI: 10.1007/s00261-012-9933-z
Perspective on radiation risk in CT imaging
Joel G. Fletcher,1 James M. Kofler,1 John A. Coburn,2 David H. Bruining,3
Cynthia H. McCollough1
1
Department of Radiology, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA
Mayo Medical School, Rochester, MN, USA
3
Division of Gastroenterology, Department of Internal Medicine, Mayo Clinic, Rochester, MN, USA
2
Abstract
Awareness of and communication about issues related to
radiation dose are beneficial for patients, clinicians, and
radiology departments. Initiating and facilitating discussions of the net benefit of CT by enlisting comparisons to more familiar activities, or by conveying that the
anticipated radiation dose to an exam is similar to or
much less than annual background levels help resolve the
concerns of many patients and providers. While radiation risk estimates at the low doses associated with CT
contain considerable uncertainty, we choose to err on the
side of safety by assuming a small risk exists, even
though the risk at these dose levels may be zero. Thus,
radiologists should individualize CT scans according to
patient size and diagnostic task to ensure that maximum
benefit and minimum risk is achieved. However, because
the magnitude of net benefit is driven by the potential
benefit of a positive exam, radiation dose should not be
reduced if doing so may compromise making an accurate
diagnosis. The benefits and risks of CT are also highly
individualized, and require consideration of many factors
by patients, clinicians, and radiologists. Radiologists can
assist clinicians and patients with understanding many of
these factors, including test performance, potential patient benefit, and estimates of potential risk.
Key words: CT—Radiation dose—Radiation risk
Advances in multidetector CT and post-processing have
led to increasing utilization of CT cross-sectional imaging to approximately 88 million scans in 2010, an over
20-fold increase since 1980 [1]. This increase has resulted
in an ~600% increase in U.S. per capita exposure to
ionizing radiation from medical procedures from 1980 to
2006, with CT imaging contributing to nearly 50% of this
Correspondence to: Joel G. Fletcher; email: [email protected]
increase [2]. During this time, the increasing temporal
and spatial resolution of CT, as well as the ability to
individualize and decrease radiation dose, have led to an
increasing number of specialized CT exams for screening
and organ-specific imaging [3–8]. While familiar with the
hyperbole of sensationalistic media reports about the
risks of radiation from CT, we have noticed that the lack
of discussion between patients, radiologists, and referring
physicians has led to fear of CT imaging on the part of
many patients—leading them to forgo CT imaging all
together—and eliminating the potential benefit CT may
provide for their medical care. For example, we have
observed physicians unwilling to undergo CT themselves
even though alternative MR imaging is less accurate for
their suspected diagnosis; patients with hepatic malignancies worried about the increased risk of death from
repeat CT examinations; and symptomatic Crohn’s
patients who are hesitant to obtain a CT enterography
for the assessment of disease severity and complications.
While it may be scientifically accurate to say that a
typical abdominopelvic CT examination requires 400
times the dose of a chest X-ray [9], a chest X-ray cannot
diagnose Crohn’s disease or metastatic colon cancer,
providing information that would likely lead to beneficial therapy for the patient (Fig. 1). While CT radiation
dose has fallen over the past three decades [3], new
technical innovations such as automatic exposure control, automatic tube potential selection, iterative reconstruction, and other noise reduction techniques provide a
new opportunity for radiologists to re-engage patients
and clinicians in the discussion of the benefits and risks
of CT.
Risk estimates, uncertainty, and
recommendations
CT volume dose index (CTDIvol) is a measure of the
radiation delivered by a CT scanner for a given exam [10,
11]. It reflects the mean dose to the center of the scan
J. G. Fletcher et al.: Perspective on radiation risk in CT imaging
23
Fig. 1. 35-year-old female symptomatic Crohn’s patient with
fear of radiation-induced cancer from CT imaging. Despite
extreme difficulty with eating solid foods, patient was unwilling
to undergo CT enterography and was waiting weeks for MR
enterography examination under general anesthesia (due to
claustrophobia). After benefits and risks of enterography
were explained, patient underwent CT enterography exam the
next day, demonstrating multifocal, long-segment jejunal
(white arrows-A and C) and ileal inflammation (open arrows-B
and C), facilitating treatment with resolution of symptoms.
volume to a standardized acrylic cylinder, and it is
automatically reported by CT scanners. It tells users
what amount of radiation the scanner is producing, but
other factors need to be taken into consideration to
estimate the dose absorbed by the patient [12]. The
CTDIvol for each CT exam is automatically recorded in
the patient’s imaging record. While this information is
useful for radiologists and physicists monitoring and
comparing doses between acquisition protocols, patients
and their referring clinicians want to know the biologic
risk of CT examinations, not a measure of scanner
radiation output. Biologic risk is conveyed by effective
dose, which normalizes partial body radiation exposure
to a whole body exposure based on equivalent risk. To
calculate effective dose, organ doses must first be esti-
mated. Organ-specific doses are then multiplied by
weighting factors (which change depending on expert
consensus at the International Commission on Radiation
Protection) and then summed to calculate the effective
dose. This weighted sum is a broad measure of biologic
risk for radiation protection purposes, but ‘‘cannot be
measured and cannot be used for individual risk assessment’’ [1, 13, 14]. Effective dose is useful for exam
optimization, comparison between exams, and Institutional Review Boards, but does not convey the risk for
any particular patient. This is particularly true since the
weighting factors used to calculate effective dose are an
average value over both genders and all ages. In addition, individual patients have differing baseline risks
[14, 15].
24
J. G. Fletcher et al.: Perspective on radiation risk in CT imaging
The National Academy of Sciences report on the
Biological Effects of Ionizing Radiation VII (BEIR VII)
evaluated the biologic impact of low doses of radiation
based on a combination of data from atomic bomb
survivors, medical and environmental radiation studies,
and occupationally exposed workers [16]. They concluded that the available evidence is consistent with a
linear non-threshold (LNT) risk model for cancer risk,
although the LNT model was not statistically significantly better than nonlinear or threshold-based models
at doses in the range of diagnostic imaging exposures.
The atomic bomb survivor data demonstrate increases in
excess relative risk of cancer for acute, high dose-rate
exposures having effective doses above 150–200 mSv, but
there is considerable disagreement on the excess relative
risk of cancer in the dose range below this level [17–21].
The 15-country workers study conducted by the International Agency for Research on Cancer (in over 407,000
nuclear workers with an average cumulative dose of
19 mSv) reported a small but significant excess relative
risk estimates for all cancers, excluding leukemia [22].
However, this result has since been discredited due to
errors identified in the Canadian data, which accounted
for the majority of cancers. In June 2011, the Canadian
Nuclear Safety Commission issued a report noting no
significant increase in cancer risk in radiation workers
[23]. Muirhead examined 174,000 British radiation
workers and found that for leukemias (excluding CLL)
and solid neoplasms, confidence intervals did not exclude
the absence of risk until exposures were over 200 mSv
[24].
Cumulative doses above 200 mSv can be reached in
patients with a long history of medical imaging. Fractionation is a process whereby a larger cumulative dose
of ionizing radiation is given in smaller increments.
Howe [25] studied a cohort of over 64,000 Canadian
tuberculosis patients, of whom 39% were exposed to
highly fractionated multiple chest fluoroscopies leading
to a mean lung radiation dose of 1020 mSv, and observed no significant increases in cancer risk. He concluded there is ‘‘strong support…for a substantial
fractionation/dose-rate effect,’’ although the magnitude
of this effect may vary between different target organs
[26].
Multiple organizations and reports have given recommendations for describing biologic risks to patients
from medical radiation (Table 1). The American Association of Physicists in Medicine has said that predictions
of hypothetical cancers and cancer deaths are highly
speculative, and the Health Physics Society has recommended against a quantitative estimation of health risks
below effective doses of 50 mSv [27, 28]. While the BEIR
VII report notes that statistical limitations make it difficult to evaluate cancer risks in humans less than
100 mSv, it also states that it is ‘‘unlikely that a threshold
exists for the induction of cancers…[and] that the
Table 1. Consensus statements regarding estimating the describing
biologic risks to patients from medical radiation
Organization/source
Recommendation/conclusion
Health Physics Society
Recommend against quantitative
estimation of health risks below
an individual dose of 50 mSv in
1 year or a lifetime dose of
100 mSv (above that received
from natural sources) [28]
At doses of 100 mSv or less, statistical limitations make it difficult to evaluate cancer risk in
humans [16]
Empirical risk relationships only
validated above 150 mSv
Risks of medical imaging at effective doses below 50 mSv for
single procedures or 100 mSv for
multiple procedures over short
time periods are too low to be
detectable and may be non-existent. Predictions of hypothetical
cancer incidence and deaths in
patient populations exposed to
such low doses are highly speculative and should be discouraged [27]
2006 BEIR VII Report
French Academies of Science
American Association
of Physicists in Medicine
occurrence of radiation-induced cancers at low doses will
be small’’ [29].
We have found that it is extremely useful in discussing
the biologic risk of CT examinations to refer to relative
risks for which the patients may be more familiar
(Fig. 2). For example, the overall probability of dying
from cancer is 23%. Using an estimated dose of 10 mSv
for an abdominopelvic CT scan and the linear nothreshold model, a conservative estimate of the increased
probability of dying from cancer after an abdominal
pelvic CT would be 0.05%. In comparison, the increase in
risk from the radon in U.S. homes is estimated to be
0.3% (six times higher), and the estimated increase in risk
from arsenic in drinking water in the average U.S. home
is 0.1% (twice as high) [30]. Such comparisons are particularly helpful if used to engage a patient in thinking
about potential benefit and potential risk. For example,
the risk of dying from drowning is 0.09%, and the risk of
dying from bicycling is 0.02%, and patients often think
about swimming or biking as safe and derive benefit
from participating in these activities. Extending this
benefit-risk analogy, if a CT might potentially increase
their survival by leading to potentially life-saving therapy
by even a mere 1%, the CT would be 20 times more likely
to be beneficial than harmful.
Net benefit
CT exams must be justified so that there is a net benefit
to the patient, i.e., the exam is medically indicated and
benefits exceed the risks. Physicians often weigh the
benefits vs. risks for many types of drugs. In instances
where there is a small risk, such as CT imaging, the
J. G. Fletcher et al.: Perspective on radiation risk in CT imaging
25
Fig. 2. Visual representation of probability of death from
various causes, compared to dying from a radiation-induced
malignancy from abdominal or head CT, using risk assump-
tions as delineated in BEIR VII and linear no-threshold
hypothesis. Sources are www.cdc.gov/nchs/fastats/deaths.
htm and Gerber et al. [30].
quantification of the net benefit is driven largely by the
perceived benefit. When the potential benefit for CT
scanning is lower, such as in a screening study in
asymptomatic patients, it is desirable to use lower radiation doses and no intravenous contrast in order to
minimize risk (e.g., screening CT colonography (CTC) or
low dose lung cancer screening). Berrington de González
et al. [31] examined the benefit to risk ratio in CTC,
assuming the radiation dose levels used in the National
Colonography Study and colonographic screening every
5 years from ages 50–80, along with three microsimulation models for colorectal cancer development, and
compared potential lives saved using screening CTC to
potential deaths from fatal cancers due to medical radiation. They estimated a large benefit to risk ratio for
screening CTC, which varied from 24:1 to 35:1.
The potential net benefit from any particular CT
exam rests largely on patient risk for a particular disease,
the potential impact of medical or surgical therapy on
that disease, and the ability of CT to detect the relevant
abnormality. All three of these factors must be taken into
account when thinking about potential benefit. For
example, patients who have a condition that CT can
accurately detect and stage, and/or patients who are
symptomatic or in whom the likelihood of disease is
high, are likely to derive a high degree of benefit from CT
imaging. In screening cohorts, patients are asymptomatic, and the potential benefit may be less due to lower
disease prevalence (e.g., about 15% of screening CTC
patients will have a colorectal polyp ‡6 mm). Benefits of
CT compared to alternative imaging strategies must also
be considered, in addition to the risk of not obtaining a
CT exam. If multi-organ imaging is necessary (for
example, in the trauma patient), CT imaging offers a
clear advantage to alternative MR and ultrasound
imaging because of its ability to image multiple organs
and large portions of the body in a rapid period of time,
a factor which has been cited by referring clinicians as
advantageous to CT [32]. The availability of CT, ultrasound, or MR imaging and local radiologic expertise is
also important, especially if conditions need to be ruled
in or out quickly [32]. Comorbidities or idiosyncratic
patient factors (e.g., renal failure, claustrophobia,
metallic implants, inability to hold still, and compliance
with screening recommendations) are all important patient-specific factors that clinicians might consider. Table 2 lists factors that clinicians and radiologists should
consider when thinking about the potential benefit and
risks for a CT examination.
On the risk side of the equation, the appropriateness
of CT imaging must be compared to potential alternatives, especially for younger or pregnant patients, where
the potential risk of radiation-induced malignancy is
higher [1, 33, 34]. Alternative ultrasound or MR imaging
may be more justified if these imaging techniques can be
performed and interpreted with similar accuracy. Intravenous contrast may lead to nephrotoxicity in at-risk
individuals, is unnecessary to accomplish certain diagnostic tasks and may be reduced for others [35]. Lower
tube potential and noise reduction techniques are newer
26
J. G. Fletcher et al.: Perspective on radiation risk in CT imaging
Table 2. Factors that clinicians and radiologists should consider when
thinking about the potential benefit and risks for a CT examination
Factor impacting CT benefit to Comment
risk ratio
Patient risk
Existing disease
Symptomatic
Asymptomatic
Disease/abnormality under
consideration
Single organ
Potential for multiple organs
(trauma, potential metastases)
Clinical urgency
Consequences of delay
in diagnosis
Appropriateness of CT
Other patient factors
Claustrophobia
When patients are asymptomatic
or disease prevalence is low;
lower doses are needed
Clinical need for multi-organ
coverage and clinical urgency
favor CT
Compare diagnostic performance
to alternatives
Examples include: willingness to
undergo endoscopyGenerally
favor CT due to non-invasive
nature and low requirement for
compliance
Patient compliance toward CT
or alternative strategies
Patient comorbidities
Need for sedation or anesthesia
Local radiology practice
Scanner technology and
infrastructure for organ-specific
exams (CT enterography, CT
coronary angiography,
dual-energy CT)
Availability of required
post-processing tools
Local expertise and comfort
Availability of CT vs. alternative imaging strategies, especially
in emergent cases
methods to permit reductions in intravenous contrast
and radiation dose without compromising diagnostic
benefit in many situations (see ‘‘Optimization, individualization, and implementation’’) [35–40].
Communication with referring
clinicians and patients
Clearly, communication between patients, clinicians, and
radiologists is critical to ensure a CT exam is medically
justified and appropriate for each patient’s individual
context, not to mention that s/he should understand the
procedure and potential benefits of the exam. Patients
are aware of idiosyncratic factors, such as their symptoms and family history and their willingness to undergo
an exam (oral contrast, bowel preparation, IV stick,
claustrophobia, fear of radiation) that may affect which
types of exams are likely to provide the most benefit.
Referring clinicians are likely best positioned to estimate
the net benefit from CT examination as they can weigh
the likelihood of potential disease(s) for a particular
patient, its potential severity, the ability of CT to detect
and characterize or stage the disease, the impact of
alternative therapies, potential impacts of false negative
and false positive exams, and the risk of not obtaining a
CT exam. Radiologists can answer questions about test
performance and radiation risk, and certainly need to
understand the concerns of the referring clinician when
optimizing the CT acquisition methods and parameters,
and the potential need for post-processing and/or subspecialized interpretation. Unfortunately, a recent survey
of a multispecialty practice found that only 30% of clinicians discuss the risks and benefits of CT imaging with
their patients prior to obtaining a scan, potentially in
part because they do not understand the risk of radiation
themselves [32]. Discussion of CT benefit and risk can be
facilitated proactively by radiologists by providing resources to referring clinicians and answering questions as
soon as they arise. Radiologists can provide referring
clinicians with pertinent and individually relevant resources that focus on common types of CT examinations,
so that referring clinicians are able to have relevant
knowledge at hand when considering, ordering, and
discussing CT exams with their patients. Some radiology
practices discuss exam appropriateness with different
subspecialties and provide pamphlets and materials to
referring clinicians to facilitate this discussion [41]. An
example of bullet points and individually pertinent facts
facilitating clinician–patient discussion of CT benefit and
risk is provided for CT enterography and colonography
(Figs. 3, 4). Other resources may take the form of decision support systems, hyperlinks to ACR appropriateness criteria [42], or institutionally agreed upon
guidelines for common imaging questions (for example,
imaging of the pregnant patient).
When one considers types of CT examinations that
were unavailable or not routine two decades ago, it becomes obvious that perceived patient benefit largely
drives CT justification: for example, dual-energy for uric
acid stone detection and treatment; CT enterography for
detection of Crohn’s disease and its complications; CT
urography for known or suspected transitional cell carcinoma; CTC to detect colorectal polyps and cancers;
multiphase CT enterography to detect small bowel cancers and masses in occult GI bleeding are all relatively
new diagnostic tasks for CT imaging that are linked with
potential efficacious therapies [4, 6, 7, 43, 44].
Conversations between clinicians and radiologists can
lead to some important but uncomfortable conversations. For example, it may be necessary to explain that
larger cumulative doses are justified in very sick patients
(for example, severe acute pancreatitis—where the risk of
organ failure and sepsis is high) or when the risk of
misdiagnosis is high (for example, detection of pancreatic
cancer or HCC—where potentially increased radiation
risk is offset by the higher risk of death). In addition,
dose reduction hardware and software can be very
J. G. Fletcher et al.: Perspective on radiation risk in CT imaging
Fig. 3.
27
Individually relevant facts facilitating clinician–patient discussion of CT benefit and risk is provided for CT colonography.
expensive. Most departments will be unable to deploy
these technologies throughout their entire practice, so it
will be necessary to strategically place these technologies
where they can provide the most benefit, and/or triage
particularly important groups of patients (such as children) to this technology. It will likely require some time
for radiology practices to sort out these types of tradeoffs
for themselves, much less discuss them with referring
clinicians. In addition, while patients with many CT
exams may not wish to receive additional CT imaging, a
necessary correlative to the linear no-threshold hypothesis
is that any cumulative threshold is completely arbitrary
and without clinical meaning—the risk for the 10th CT
exam should be the same as the first. Justification should
28
J. G. Fletcher et al.: Perspective on radiation risk in CT imaging
Fig. 4. Individually relevant facts facilitating clinician–patient discussion of CT benefit and risk for CT enterography in patients
with known or suspected Crohn’s disease.
be based on clinical history and anticipated benefit of
considered exam, regardless of prior exposures [45].
Optimization, individualization,
and implementation
Exam optimization is the responsibility of the entire
imaging team, composed of radiologists, technologists,
and physicists. It includes thoughtfully selecting acqui-
sition methods and interpretive tools for a CT exam so
that disease detection is maximized while minimizing
dose and non-radiation risk [46]. However, because the
magnitude of net benefit is driven by the potential benefit
of a positive exam, radiation dose should not be reduced
if doing so may compromise accomplishment of the
diagnostic task. For example, increasing CT slice thickness and decreasing dose is not beneficial to the patient if
it means that a pancreatic cancer could be missed
J. G. Fletcher et al.: Perspective on radiation risk in CT imaging
29
Fig. 5. Routine portal-phase contrast-enhanced CT with
6-mm slice thickness at level of pancreatic head and body
(A, B). Exam interpreted as negative by non-abdominal
radiologist. Due to continuing clinical suspicion and re-evaluation by GI radiologist, exam was repeated 2 days later with
dual-phase technique, increased tube current, and 2.5-mm
slice thickness, demonstrating improved visualization of
low attenuation mass in pancreatic head (C) and dilated
pancreatic duct (D) and leading to diagnosis of pancreatic
cancer.
(Fig. 5). Other CT acquisition parameters, such as the
phase of enhancement, tube potential selection, and oral
and intravenous contrast can dramatically impact the
ability to identify disease for any given diagnostic task
[39, 47]. Imaging parameters must also be tailored to
individual patient size using tube current modulation,
tube potential selection or technique charts, and consideration of the patient’s age [48]. The easiest and most
important way to reduce radiation dose in large and
small CT practices is to eliminate CT exams that are not
clinically indicated, as well as superfluous acquisitions
(such as routine unenhanced or delayed scans), that do
not contribute to making an accurate medical diagnosis.
Several challenges exist to optimization. There is lack
of information on the specific image quality requirements
(e.g., spatial and contrast resolution) needed to achieve
acceptable performance for specific diagnostic tasks, so
radiologists are often left to utilizing the dose levels used
in large retrospective or prospective studies. Generally,
radiation dose can be greatly reduced when the image
contrast differences needed for diagnosis are very high
(renal stone identification, CTC, CT angiography), but
radiation doses will need to be higher when subtle soft
tissue attenuation differences need to be detected (e.g.,
liver mass identification/characterization). In addition, a
frequently overlooked aspect to optimization is the
expertise of the interpreting radiologist (Fig. 5). This
consideration is essential in terms of obtaining potential
benefit from CT exams. A GI radiologist may be attuned
to the imaging findings of autoimmune pancreatitis that
can be seen on even a four slice scanner, but not attuned
to the imaging findings of medullary sponge kidney of
CT urography. Similarly, software infrastructure must be
established throughout departments so that appropriate
image reconstruction and post-processing facilitate the
accomplishment of diagnostic tasks, such as dual-energy
post-processing or interactive 3D imaging for CTC.
In the current age of CT dose reduction with noise
30
J. G. Fletcher et al.: Perspective on radiation risk in CT imaging
reduction techniques, departments will need to carefully
evaluate and compare image-based vs. scanner-based
noise reduction approach. Scanner-based alternatives
may have less impact on workflow, but may be more
expensive to implement in a large department.
In this age of dose awareness, departmental issues to
be addressed by radiology departments include the triage
of appropriate patients to low dose techniques, and
implementation of low dose protocols throughout the
CT practice. Observer performance studies demonstrate
that radiologists are very good at evaluating noisy low
dose images without an impact on disease identification
or staging for a variety of diseases, so expensive technology is not always needed [49–53]. Radiologists across
a department need to agree upon standard protocols for
common clinical situations. Thus, the transition to a
lower dose CT practice involves efficiency, reproducibility, and communication. It also involves weighing
potential alternatives, such as an investment in decision
support systems (to eliminate unnecessary exams) vs.
investing in noise reduction hardware and software.
Conclusion
Dose awareness and communication by radiologists and
radiology departments can be beneficial for patients,
clinicians, and radiology departments. Initiating and
facilitating discussions of the net benefit of CT by
enlisting comparisons to more familiar activities (swimming or bicycling), or by conveying that the anticipated
radiation dose to an exam is similar to or much less than
annual background levels helps resolve the concerns of
many patients and providers. For example, our pediatric
oncologists had eliminated surveillance chest CT and
performed surveillance chest X-ray instead, until we
made a low dose chest CT exam available to them.
Demonstrating that these CT exams could be performed
using radiation dose levels similar to 3 weeks of natural
background exposure in Rochester, Minnesota, overcame their reservations. Similarly, implementation of low
dose CT enterography across our practice has helped our
gastroenterologists to understand that we are considering
patient risk and benefit in conjunction with them when
scanning their Crohn’s disease patients.
While radiation risk estimates regarding CT contain
considerable uncertainty, we choose to err on the side of
safety by assuming a small risk exists in order to protect
our patients. We also understand that the benefits and
risks of CT are highly individualized and require consideration of many factors by patients, clinicians, and
radiologists; however, many of these factors can be
understood and communicated before CT exams are
ordered, so that appropriate discussions can occur with
the patient. Radiologists should assist clinicians and
patients with understanding test performance, potential
benefit, and estimates of potential risk. Radiologists
should individualize CT scans according to patient size
and diagnostic task, to ensure that maximum benefit and
minimum risk can be achieved, and should work across
departments so that a standard and thoughtful approach
can be taken for every patient.
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