Computed tomography (CT) studies account for the largest population radiation dose from medical diagnostic studies. While only a small fraction of radiation exposures lead to the formation of cancer, radiation dose is an important issue. Radiologists must be committed to dose reduction and should educate their patients and referring physicians about the radiation dose and alternative imaging choices.
is a Professor and the Director of MRI, Department of Radiology,
Emory University School of Medicine, Atlanta, GA.
is a Professor of Radiology, the Director of MRI, and the Vice
Chair of Clinical Research, Department of Radiology, University
of North Carolina at Chapel Hill, Chapel Hill, NC.
It is scientifically accepted that the ionizing radiation
generated by medical diagnostic X-ray machines induces neoplasms
through damage and alterations to cellular DNA, and that this risk
is related to the dose administered. Computed tomography (CT)
examinations account for the largest population radiation dose from
medical diagnostic studies, with more than 60 million CT
examinations performed per year in the United States that represent
an estimated 70% of all medical X-ray exposure.
Consensus on risk estimates are more challenging because of the low
doses administered, the stochastic nature of radiation effects, and
the time course between cause and effect.
Only a small fraction of X-ray photons will lead to DNA changes,
and only a fraction of those changes will lead to cancer formation.
The time between a DNA-altering event and progression to cancer
formation may require periods as long as 10 to 20 years in most
cases. Statistical analysis requires large numbers of people to be
studied to yield useful data. The atomic bomb survivors represent
the largest and most studied population. Although atomic bomb
radiation produces gamma rays, and medical exposure produces
X-rays, the carcinogenic potential is considered to be equivalent
for an equivalent dose.
Even so, the fact that approximately 42% of the U.S. population die
from cancer-related causes makes statistical analysis difficult.
Several national and international organizations have been given
the task of producing periodic reviews of the cumulative data on
human low-dose population exposure to determine health risks. The
seventh National Academy of Science report on Biological Effects of
Ionizing Radiation (BEIR VII)
is the most recent update from a highly respected organization. The
findings from BEIR VII are summarized here and reviewed in the
context of other studies, with a view to discuss how radiologists
may consider approaches to imaging that may moderate radiation
exposure to our patients. Given that the typical CT studies
resulting in the highest doses involve studies of the torso (chest,
abdomen, and pelvis), the potential for supplementing or replacing
CT examinations using magnetic resonance imaging (MRI) will be
discussed regarding the strengths and limitations of this
Health risks from exposure to low levels of ionizing
The National Academies' National Research Council has produced a
series of reports on BEIR, and the seventh report of this series
has been released.
This report represents an exhaustive evaluation of the literature
and updates prior health-risk estimates using the latest
epidemiological data. The report specifically examines low-dose,
low linear energy transfer (LET) ionizing radiation, as is produced
by CT or plain-film imaging systems.
In this study, low doses are defined as <100 mSv, while the
typical CT examination of the abdomen and pelvis will result in an
exposure of between 10 and 20 mSv.
In contrast, the exposure from a chest radiograph is approximately
and the estimated dose for a body scan with a combined positron
emission tomography (PET) and computed tomography (CT) scanner is
Although in the past it has been argued that the biological damage
and health risks from ionizing radiation may not have a linear
relationship with dose, the BEIR committee concluded that the
biological and biophysical data supports a linear no-threshold
(LNT) risk estimate model that indicates that even the smallest
dose of low-level ionizing radiation has the potential to cause an
increase in health risks to humans.
Biological effects of ionizing radiation may be mediated by
breaking biomolecular bonds of cellular DNA. In somatic cells
(non-germ cells), DNA damage may lead to initiation or promotion of
tumor formation by destroying important regulatory mechanisms
encoded in the DNA that are used for control of cell division and
Leukemias are the types of tumors that have been most studied
epidemiologically in relation to ionizing radiation exposure in
humans, and these studies have primarily concentrated on mortality
risks. The new epidemiological information and approaches used in
BEIR VII include estimation of risks for development of leukemias
and of solid tumors.
In addition, risk estimates for both mortality and incidence have
been determined. In germ cells, DNA damage could yield growth and
developmental defects passed on to offspring. The BEIR VII Report
examines data to attempt assessment of such risk.
Risk estimates at low doses
Estimates have been based on models other than the LNT model,
and some investigators believe that there is evidence to support
the conclusion that low doses of ionizing radiation may have a
cancer risk in excess of what is predicted by a linear model.
Similarly, others have argued that low doses of radiation may have
no effect, or may even be beneficial by inducing the production of
DNA repair mechanisms within cells exposed to a low radiation dose.
The BEIR VII Report has taken into account a comprehensive
examination of the literature on this subject and has concluded
that there exists a "preponderance of scientific" experimental and
epidemiological data that predominantly supports the use of an LNT
model to predict cancer risk.
The main source of epidemiological data has been the study of
Japanese atomic bomb survivors from Hiroshima and Nagasaki. Other
data was derived from studies of medically exposed individuals and
nuclear workers. BEIR VII specifically examined the incidence and
mortality related to leukemia and to 11 different solid tumors,
including breast and thyroid carcinomas. Since the last BEIR Report
issued in 1990, an additional 15 years of follow-up of exposed
individuals has been evaluated and an updated dosimetry metric has
been implemented. New data included the evaluation of individuals
exposed to radiation for medical reasons. The individuals in the
lowest-dose group of atomic bomb survivors who showed a significant
rise in cancer incidence and mortality received total body exposure
with organ equivalent doses in the range of 5 to 100 mSv (mean
equivalent dose 29 mSv) and 5 to 125 mSv (mean equivalent dose 34
mSv), respectively. Typical equivalent doses in directly irradiated
organs are in the range of up to 20 to 30 mSv for a single,
routine, adult abdominal CT examination.
Therefore, risk estimates for radiation exposure from CT
examinations can be directly related to the findings among atomic
bomb survivors, without a need for extrapolation.
In summary, the BEIR VII Report indicates that in a population
with the demographic distribution of the U.S. population, a single
dose of 100 mSv is associated with an estimated lifetime
attributable risk (LAR) for developing a solid cancer or leukemia
of 1 in 100. The overall risk of developing a solid cancer or
leukemia from causes other than ionizing radiation in this same
population would be 42 in 100.
A single dose of 10 mSv is associated with a LAR of 1 in 1000 for
developing a cancer. Given that an estimated 60 million CT
examinations are performed each year,
using the assumptions of 1 whole-body 10 mSv dose per examination
per patient, this would predict an estimated LAR of 60,000 solid
cancers and leukemia, every year. In comparison, the estimated risk
of serious complications and death from receiving iodinated
intravenous CT contrast is approximately 1 in 400,000,
which is lower than the LAR from a single 10 mSv dose. We recognize
that these estimates make simplistic assumptions regarding
population exposure, but they are useful in order to appreciate a
discussion of the magnitude of current population medical exposure
levels associated with CT studies.
Non-neoplastic health risks
Ionizing radiation has been associated with the development of
other, non-neoplastic disorders, including cardiovascular disease.
However, these disorders will develop with doses >100 mSV. The
BEIR VII Report indicated that the data was insufficient to
quantify the risk for nonneoplastic disorders that are associated
with low-dose ionizing radiation <100 mSv. Similarly, the risk
of low-dose radiation for genetically transferred defects could not
be shown. However, it is further noted that the most likely outcome
of a fetus suffering from a radiation-induced DNA defect would be a
spontaneous abortion, but this has not been studied.
Other risk-estimate reports
Several other recent publications have attempted to estimate
risks in specific risk groups. Of particular note, the 2005 report
from the Department of Health and Human Services listed ionizing
radiation as a known human carcinogen in the Eleventh Report on
Major international scientific organizations that are responsible
for evaluating health risks of ionizing radiation and for
establishing policies include the International Commission on
Radiological Protection (ICRP)
and the United Nations Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR).
These organizations have come to the same conclusions as the BEIR
VII Report with regard to using an LNT model for dose-risk
estimates, thus concluding that there is no entirely safe dose of
ionizing radiation. The U.S. Food and Drug Administration estimates
that a CT examination with an effective dose of 10 mSv (eg, 1 CT of
the abdomen) may be associated with the increase in the possibility
of fatal cancer of approximately 1 chance in 2000.
Furthermore, published frequency data masks the number of times a
single patient receives multiple CT examinations
or receives multiphase CT scanning. Radiation risk may be related
to the cumulative dose of medical radiation, and there is limited
data to track the number of repeated CT scans or the number of
multiphase scans that individuals are receiving, although studies
have shown that up to half of the patients receiving a CT scan will
have multiple scans, and that the average number of scans for a
given indication is 2, yielding an average dose that may exceed 40
Other estimates indicate that 30% of all individuals having a CT
will have a total of at least 3 examinations.
The pediatric population is more vulnerable to the risks of CT
radiation, and an estimate of the lifetime cancer mortality risk
attributable to the radiation exposure from a single abdominal CT
examination in a 1-year-old child is approximately 1 in 550.
Additional concern derives from the potential for use of suboptimal
CT technique and excessive exposures in smaller pediatric patients:
at the same technique factors, pediatric doses are substantially
It has been estimated that 600,000 abdominal and head CT
examinations were performed annually in children under the age of
15 years with an estimated LAR of 500 deaths from cancer, and that
the number of CT studies is growing rapidly.
Implications for standards of patient care
Informed patient consent
Currently, most institutions require a signed consent form from
patients undergoing a CT scan. Typically, the risks described to
the patient are related to the intravenous iodinated contrast
utilized for a contrast-enhanced CT examination. However, the need
to discuss the radiation risk of CT examinations is not mandatory
and is rarely a component of the standard consent form. In
addition, informed consent requires that the radiologist, or a
representative, describe any reasonable alternatives to having the
CT examination; this is also not commonly done. A recent study in
health policy and practice by Lee et al
considered adult patients with mild-to-moderate abdominal/pelvic or
flank pain who underwent CT examination during a 2-week period in
the emergency department of a U.S. academic medical center and who
were subsequently surveyed after acquisition of the CT scan. Only
7% of patients stated that they were informed about the risks and
benefits of a CT scan. More importantly, only 3% of patients
reported that they were informed about the increased lifetime
cancer risk associated with CT.
Evidence suggests that referring emergency department physicians
are largely unaware that there are potentially harmful effects from
CT radiation exposure,
with only 9% aware of increased cancer risk. In fact, data show
that radiologists performing the CT examination consider the
radiation exposure of limited concern, with only 47% of
radiologists familiar with the increased risk of cancer associated
with the CT examination.
Even many radiologists are unaware of the dose of radiation
delivered to the patient during the CT examination.
Another recent study from a large hospital in the United Kingdom
used a simple questionnaire to assess the knowledge of primary care
and specialist physicians on radiation doses and risks.
Results of this study showed an urgent need to improve physicians'
understanding of radiation exposure. Only 27% of physicians and
only 57% of radiologists and radiology-related subspecialists
achieved a test score >45%.
In addition, the CT imaging protocol used is entirely at the
discretion of the radiologist or CT technologist. If meticulous
care is not employed, it is technically feasible to scan a patient
using multipass techniques for the purpose of achieving multiple
phases of images before and after the administration of iodinated
contrast and to use tube current settings that could radiate a
patient with tube current levels that are set up to maximum levels-
far in excess of the minimum required to achieve a diagnostic
examination. Details of imaging strategies must also include the
consideration of the use of slice profiles, since the settings of
slice thickness and table pitch (the speed with which the patient
passes through the scanner) also have direct effects on radiation
exposure dose. For the purpose of dose minimization, these
parameters-and the consideration of the pathology and organ systems
being evaluated-must be considered.
In addition, imaging requirements and risks will differ according
to the size and age of the patients. The use of faster CT scanners
has promoted scanning beyond the indicated field of view. For
example, CT technologists may regularly perform an abdominal
examination to include most of the pelvis, even though the pelvic
images were not requested and the additional pelvic slices will
increase the total radiation dose.
Currently, there are no industry, federal governmental, or medical
regulatory bodies that have control over these CT scan parameters
or practice patterns, and a wide range of methodologies are applied
at different centers.
In most cases, ultrasound (US) and MRI represent safer
alternatives to CT for abdominal and pelvic imaging. Additionally,
MRI is useful and may be the preferred alternative for assessing
certain diseases of the thorax and for brain and spine imaging.
Limitations of MRI include its high cost, limited availability,
contraindications, and the complexity associated with operating the
MRI has been considered an expensive diagnostic test. However,
the actual cost of MRI equipment, service, and site expenses has
steadily diminished during the past 20 years by factors of
approximately 65%, 55%, and 80%, respectively, while the fixed
costs associated with a state-of-the-art CT have increased. The
average MRI technical fee in 2004 fell by 70% compared with the
average fee in 1985, and the trend in decreasing costs of MRI is
expected to continue for the next 20 years.
Availability of MRI lags behind that of CT, thus limiting
patient access to MRI. However, increased demand for MRI studies
would place a burden on centers to increase the numbers of
scanners. In addition, the technology continues to evolve, bringing
forth developments that are designed to shorten the scan times,
thus increasing throughput on a given MR system. Individual scans
may not be approved because of reticent medical insurance
providers. Documentation and continuing education of the insurance
providers is required. State and federal governments have a vested
interest in supporting actions that promote health maintenance, and
they may have to establish insurance support guidelines.
The complexity of MR scanner operation may be largely resolved
through engineering hardware and software solutions. For example,
many steps performed by a technologist for an abdominal MRI study
(which is considered one of the more complex techniques) are
repetitive and lend themselves to automation. Solutions may result
from automated patient and organ detection methods that assist the
operater in patient positioning, field-of-view, surface coils, and
optimized sequence parameter selections. This optimization
procedure could be automatically individualized for each patient
and propogated throughout the protocol to minimize the need for
operator inputs. Combined with currently evolving coil hardware and
software solutions, multiple-station examinations of multiple body
parts, including whole-body imaging, has become feasible in
relatively short examination times. Simplified user interfaces have
lagged behind current software capabilities, but they represent an
area of more dedicated development effort from vendors;
increasingly simplified MR console software is beginning to appear
on the market with user interfaces that borrow from more familiar
personal computer operating systems.
Contraindications in MRI are mostly related to cardiac pacemaker
devices, ferrous vascular aneurysm clips in the brain, and ferrous
foreign bodies in the eye. The commonly used gadolinium chelates
are a family of extremely safe contrast agents that can be used in
patients who normally could not tolerate iodinated CT contrast
because of allergy or renal insufficiency.
For most clinical problems for which CT is used to evaluate
disease in the body, MRI would provide an acceptable or favorable
alternative. In many cases, MRI would offer equal or superior
imaging results with regard to diagnostic sensitivity and
specificity. This has been shown most clearly in most disorders of
intra-abdominal and pelvic origin. Evidence to demonstrate the
potential for evaluation of intra-thoracic disorders, including
pulmonary disease, is developing. The soft tissues of the chest,
abdomen, and the pelvis represent the most radiosensitive tissues
and are the focus of the following review of systems.
Although CT imaging remains the standard imaging test for
evaluation of interstitial lung diseases, the ability to evaluate
pulmonary nodules on contrast-enhanced 3-dimensional gradient-echo
MRI has been shown to have sensitivity equal to that of CT on
nodules as small as 6 mm.
In addition, MRI can provide similar or better imaging of the chest
wall and mediastinal soft tissues with regard to the vasculature,
lymph nodes, and masses. Cardiac MRI can also provide dynamic
information about cardiac morphology and function and for the
evaluation of myocardial scar tissue that results from myocardial
Breast MRI has been shown to be more sensitive than standard
mammography, although its specificity is relatively lower.
With regard to routine chest imaging, current MR technology can
be used in cancer work-up, including the staging or restaging of
metastatic disease, and can be used routinely in patients with an
intolerance to iodinated contrast. The authors frequently combine
the chest examination with an abdominal study when assessing
Evaluation of pulmonary emboli has become a common request for
CT imaging of the chest, particularly in acute or emergency care
settings. These patients are frequently young adults and are
predictably more sensitive to radiation effects. The use of MR
angiography for the evaluation of pulmonary emboli is a developing
area, and its feasibility has been reported.
It is expected that MRI will become at least an alternative
modality for selected patients, such as young patients or those who
have renal insufficiency and are therefore at a relatively higher
risk for iodinated contrast-induced renal failure.
The examination of the liver and pancreas using MRI has the
support of a considerable body of literature to argue in favor of
the use of MRI as a first-line imaging study for the evaluation of
tumors and for the evaluation of diffuse diseases, such as liver
fatty accumulation, hepatitis, cirrhosis, hemochromatosis,
pancreatitis, and pancreatic or bile duct disorders.
CT imaging of the liver may show good sensitivity, but MRI provides
superior sensitivity and specificity.
The strength in MRI is the ability to provide more information
regarding the exact pathology within the liver to differentiate
between benign and malignant lesions. In addition, compared with
CT, MRI is better able to evaluate inflammatory changes that are
associated with hepatitis and cholangiohepatitis with greater
sensitivity. MRI can combine tissue imaging sequences with MR
cholangiopancreatography techniques to more comprehensively
evaluate disorders related to the intra- and extrahepatic bile
ducts and gallbladder.
Imaging of the pancreas is another relative strength of MRI,
with its greater sensitivity and specificity for the evaluation of
mild or severe changes of pancreatitis
and its demonstrated capacity for the detection of small, and
potentially more treatable, pancreatic cancers.
In addition, it has been argued that sensitivity and specificity
for small tumors is superior on MRI. Figure 1 shows characteristic
examples of tumors of the liver or pancreas that are shown with
greater conspicuity and specificity on MRI than on
contrast-enhanced CT. The adrenal glands
and kidneys may be similarly evaluated for tumors on CT and MRI,
although CT can be used to detect even 1 mm of renal calculi, and
MRI is generally insensitive to the presence of nonobstructing
calculi. However, MRI has been shown to have the potential to
replace CT and intravenous pyelography (IVP) for the evaluation of
renal uptake and excretion, renal function, and the collecting
Given the potential for inducing acute tubular necrosis with the
use of iodinated contrast in patients with known or suspected renal
dysfunction, this is an additional attractive attribute of MRI.
Adrenal glands and retroperitoneal structures, including lymph
nodes and the major vascular structures, may be similarly analyzed
by CT or MRI. It should also be noted that a routine MRI
examination should include dynamically acquired contrast-enhanced
T1-weighted gradient-echo technique, with images acquired during
the arterial, venous, and delayed phases of contrast distribution.
The additional information acquired by this approach may be
critical in many cases of tumor or in cases of inflammatory or
fibrotic processes of the liver and pancreas. Routinely using
multiphase contrast-enhanced imaging on CT studies would represent
a significant increase in radiation dose. As MRI offers greater
specificity for most cases in which multiphase imaging is
indicated, it would be reasonable to replace the majority of
multiphase CT examinations with MRI and reserve CT for only those
patients in whom MRI fails or is contraindicated.
Soft tissues of the female pelvis generally produce only a small
range of image densities on CT, leading to low contrast between
structures. Generally, US and MRI provide significantly superior
visualization of the soft tissues, including the uterus, with clear
distinction of the endometrium and myometrium, cervix, and vagina,
as well as parametrial and adnexal structures including the
ovaries. Compared with US, MRI can generate superior contrast
between the different soft tissue structures and may offer
additional benefits by providing a full field of view to include
the entire pelvis, which can be more challenging on sonographic
Examination of the prostate on MRI provides excellent soft tissue
detail for the demonstration of the zonal anatomy and for the
evaluation of the prostatic capsule and provides important
structural delineation for prostatic carcinoma local staging.
The results of CT and MRI may be similar in the detection of pelvic
lymph nodes and vessels.
Experience using MRI for the evaluation of the bowel has been
increasing, and results show that MRI may be an excellent imaging
study for the evaluation of inflammatory bowel disease,
including Crohn's disease. It has been shown that CT findings of
Crohn's disease do not correlate well with disease activity.
Feasibility for using MRI for the evaluation of Crohn's disease and
for the determination of disease activity has been reported.
In particular, the use of dynamically gadolinium-enhanced
gradient-echo imaging, in combination with fat-suppressed
single-shot echo-train T2- weighted imaging, may yield information
that correlates with the degree of active inflammation in and
around the involved bowel segments. Feasibility for the use of MRI
for the investigation of other acute gastrointestinal disease also
has been shown, such as for acute appendicitis.
It may be that MRI could be used as the primary imaging modality
for the evaluation of acute appendicitis in young adults and in
pediatric age patients.
Excellent depiction of a full range of disease processes and
safety (due to lack of radiation) make MRI ideal for evaluating
Ultrasound is a similarly safe imaging modality but has lesser
reliability to visualize disease processes and cannot provide the
overall topographical display.
Until recently, MRI systems have been used to evaluate disease
processes that are limited to a single or a few body regions or
organ systems, while CT systems are capable of scanning the entire
chest, abdomen, and pelvis during a single breath-hold. Newer
engineering developments are being introduced to facilitate more
rapid imaging of larger body regions and to efficiently combine the
imaging of multiple stations, such as the chest, abdomen, and the
pelvis, or to include the entire head, neck, and body. For example,
recent studies have shown that imaging the entire body as a method
for tumor evaluation and staging,
or for whole-body MR angiography
Recommendations for the practicing radiologist: When and
how to use CT
Clearly, CT is an excellent modality for investigating many
disease processes. Indications for which CT should be considered a
primary diagnostic method include the evaluation of pa-tients with
severe trauma, the evaluation of the placement of tubes and
catheters in extremely ill patients, the detection of renal and
ureteric calculi, and the detection of interstitial lung disease.
Radiologists who are responsible for adjusting CT protocols should
be aware of the methods by which radiation dose may be minimized
with regard to using the lowest possible dose to achieve a
This includes using the minimum kilovoltage and tube current
according to the age or size of the patient and ensuring that the
thickest slices, least number of phases, and minimal coverage of
body parts is performed as is necessary to evaluate the clinical
areas of concern.
Based on our current understanding of the radiation risks
associated with certain CT studies, it is incumbent on the
radiology community to discuss with patients the concern in the
scientific community that X-rays may result in an increased risk of
cancer development. Patients should be given an explanation of the
expected benefit in relation to the risk for radiation-induced
cancer. It would be reasonable to provide the patient with the LAR
estimates for subsequent cancer quoted from the BEIR VII Report: 1
in 1000 for a single CT and cumulatively higher for a multipass CT
or for repeated examinations. The risks are lower for older
patients and may become <1:10,000 if the patient is older than
50 years of age, but are greater in younger patients and can be
described as approaching twice the risk for adult patients, which
is estimated to be 1 in 550 for a 1-year-old patient.
The current practice for imaging patients with known or
suspected intracranial or spinal neurological disorders has been
increasingly oriented toward using MRI as a primary imaging
technique. A large potential impact on practice patterns would be
expected from evaluating body imaging indications. Examples of
common indications for which alternatives to body CT should be
routinely recommended include patients with known or suspected
hepatobiliary, pancreatic, adrenal, and renal disease with the
exception of renal calculi. For example, in patients who have had
an unenhanced or single-phase enhanced CT scan for an initial
assessment of disease and who were found to have a nonspecific
hepatic or pancreatic lesion, a multiphase gadolinium-enhanced MRI
should be preferred in place of a multiphase CT study. Patients
with right upper quadrant pain may initially be screened by
abdominal US, and if nondiagnostic, a gadolinium-enhanced abdominal
MRI should be recommended. Similarly, female patients with
gynecologic or obstetric concerns should first be imaged by US,
followed by MRI if the sonographic examination is non-diagnostic.
It is essential that the radiology community be committed to
dose reduction. The community is responsibile for educating
patients about the radiation risks and the use of alternative, safe
imaging modalities--in particular, with emphasis on US and MRI--for
the foreseeable future. Educational efforts should be directed to
radiologists, referring physicians, regulators, public health
officials, and patients. A key component for optimizing patient
safety should be an aggressive assessment process to limit the
improper use of CT.
The BEIR VII Report represents an updated consensus evaluation
on low-dose ionizing radiation risks, in the range of exposures
associated with routine clinical body CT examinations. These
estimates show that the estimated risk is sufficiently significant
that clinicians, as patient advocates, should include radiation
risk as a component of obtaining informed consent from the patient
prior to a CT study. As a routine component to the process of
informed consent, clinicians should also discuss and be able to
offer an alternative, such as an MRI study, when it can be argued
that the MRI examination may provide similar or superior diagnostic
results with lower short- and long-term health risks.
When CT examinations are performed, the current responsibility
falls on the radiologist who is interpreting the study to ensure
that the optimal CT protocol is implemented and that this includes
utilizing a technique that will minimize radiation exposure while
maintaining the diagnostic sensitivity and specificity of the
examination. This includes restricting the dose rate and the field
of view imaged. Specific strategies could be considered for
particular indications. For example, in many cases, liver
examinations using multiple-pass contrast-enhanced studies may be
restricted to a single pass, and MRI can be used if the CT study is
nondiagnostic. If a patient and clinician choose an MR examination,
challenges in our current system include having adequate
availability, having adequate expertise in the performance and
interpretation of these studies, and obtaining approval for
coverage of the MRI costs from the medical insurer. These are
surmountable problems, however, and the solution will largely
depend upon a greater concerted effort of clinicians to act as