The development of positron emission tomography (PET)/ computed tomography (CT) provided the fusion of functional and anatomic information. But it also has specific pitfalls and artifacts. Accurate PET/CT interpretation and optimized patient care require that the radiologist have a working knowledge of these pitfalls and the principles of PET/CT. This article addresses some of the problems with abdominal PET/CT
performance as well as protocol and interpretation issues.
is the Director of Abdominal PET/CT,
is a Clinical Fellow,
is the Medical Director of CT services and, at the time this
article was written,
was a Research Assistant, Department of Radiology, Division of
Abdominal Imaging and Intervention, Massachusetts General
Hospital, Boston, MA.
is now the Director of CT Research, Emory University, Atlanta,
Imaging continues to play a major role in the management of
oncologic patients. Most imaging modalities yield purely anatomic
and morphologic tumor detail without addressing tumor metabolism.
The advent of positron emission tomography (PET) with
18Ffluorodeoxyglucose (FDG) has provided tumor-related qualitative
and quantitative metabolic information critical to patient
diagnosis and management. PET enables the detection of increased
metabolic activity in tissue that can appear morphologically normal
on other imaging modalities. It can also assist in the
differentiation of benign from malignant lesions and in the imaging
follow-up of cancer patients following surgery, radiation, or
PET, however, is limited by relatively poor spatial resolution,
whereby accurate anatomic localization of foci of increased
metabolic activity can be difficult or impossible. This is
particularly the case in the abdomen, where there may be a lack of
identifiable anatomic structures.
One of the most exciting technological advances in recent years
is the clinical application of combined PET and computed tomography
(CT) scanning. First described by Beyer et al,
PET/CT provides high-resolution cross-sectional information from CT
coregistered with metabolic information from PET. The imaging data
are acquired using a combined scanner during a single examination
and are subsequently fused. The CT images are used for more precise
and rapid attenuation correction of the PET data. The combination
of CT detail with PET data leads to more assured anatomic
localization of areas of increased metabolic activity (Figure
In addition, both modalities have further individual strengths
that are highly complementary. PET can be used with different
radiotracers to provide other useful biologic information, while CT
provides valuable multiplanar information regarding the
morphological features and attenuation values of lesions as well as
oral and intravenous (IV) contrast behavior. In addition to
reducing the PET imaging time per patient from 45 to 60 minutes
with a conventional dedicated PET scanner to 15 to 30 minutes, a
PET/CT scanner also reduces the number of equivocal PET and CT
PET/CT offers many other possible advantages for improved patient
care. These include improved diagnosis and staging of abdominal
cancers, more accurate identification and localization of
disseminated malignancy and recurrent disease, and planning and
monitoring of surgical and adjuvant therapy.
The artifacts and pitfalls related to interpretation of
independent PET and CT images are well documented, and more are
being reported as clinical experience grows.
The combined modality PET/CT provides its own specific pitfalls and
It is essential that a working knowledge of these pitfalls as well
as the principles of PET/CT be acquired in order to ensure accurate
image interpretation and to optimize patient care. This review will
highlight some of the problems regarding abdominal PET/CT
performance and address some specific protocol and interpretation
Abdominal PET/CT protocol
The introduction of CT-based attenuation correction and its
integration with PET necessitates different PET/CT scanning
protocols. The 2 general approaches adopted for PET/CT scanning are
essentially to limit the CT to simply perform faster attenuation
correction, or to make full use of the CT for both attenuation
correction and diagnosis and coregistration.
The former approach employs the lowest possible CT radiation dose
to achieve attenuation correction. The latter approach allows
diagnostic quality CT to be achieved with a standard radiation
dose. The specific indications and protocols for low or standard
radiation dose CT have yet to crystallize. We combine the 2
approaches by employing a low-dose CT scan without IV contrast to
provide attenuation information and a fully diagnostic
contrast-enhanced scan acquired at standard radiation dose
immediately following the PET acquisition. Other contentious issues
include the reimbursement for the study, the validation of
indications for use, and who should interpret the exam.
Although recent studies have shown that oral and IV contrast
media can be administered for the diagnostic CT to aid in lesion
localization and support characterization, modifications are
necessary to avoid image artifacts in the PET images and to ensure
appropriate attenuation correction (AC).
Beam-hardening artifacts can occur from metallic orthopedic and
dental implants, which affect CT-based attenuation correction of
In addition, mismatch of internal organs due to breathing movements
and inconsistent patient positioning must be minimized to allow
precise PET/CT coregistration in abdominal studies.
Normal "free" breathing or normal expiratory phase for acquisition
of CT images has been found to be more suitable than maximum
inspiratory or maximum expiratory phases for coregistration.
Breath-hold imaging, however, clearly confers significant
advantages in CT image quality. Our dual CT acquisition protocol
described above allows us to acquire, at a minimum, the
contrast-enhanced scan during a breath-hold.
In PET/CT scanners, the PET emission data can be corrected for
photon attenuation using the CT image to generate a 511-keV
attenuation map. This is achieved by algorithmically converting the
Hounsfield units (HU) in CT to attenuation coefficients appropriate
for energies of 511 keV. All manufacturers of PET/CT scanners
incorporate CT-based AC algorithms in their systems; in fact, for
some PET/CT scanners, it is the only option offered. This technique
has several advantages. First, there is less statistical noise from
the CT data compared with transmission data acquired by a
radionuclide source, such as 68Ge in standard PET scanning. Second,
scan time for the acquisition of CT data is much shorter than
radionuclide sources, thus reducing overall scan time by 15 to 20
minutes. Finally, the need for PET transmission hardware is
However, there is a potential risk of overestimating the true
tracer activity with CT-based AC. Nakamoto et al
has shown that the measured activity with CT-based AC was
overestimated by an average of 11% in bone and 2.1% in soft tissue
when compared with 68Ge-based AC. Thus, it is important to take
this into account when interpreting quantitative or
semi-quantitative (standardized uptake value [SUV]) changes between
a study performed with CT AC and a study performed with 68Ge AC.
Attenuation correction using CT data may lead to some artifacts
that are caused by overcorrection of photopenic areas corresponding
to high-attenuation structures on CT. This incorrectly renders such
areas hypermetabolic on the AC-PET images. Some studies have shown
that the use of high-density positive oral and IV contrast media
may lead to AC artifacts in the PET image.
Antoch et al
reported that artifacts from IV contrast are limited to the
transient bolus passage of undiluted contrast through the great
vessels in the thorax. Artifacts from oral positive contrast agents
can also arise where there are focal concentrations of high-density
contrast, such as bar-ium.
Low-density oral contrast usually does not result in major
artifacts. In clinical practice, Dizendorf et al
reported only a 4% overestimation of SUVs in relation to positive
oral contrast media, suggesting a negligible effect. Where focal
accumulations of positive oral contrast media do give rise to
artifacts, these can be resolved by viewing the non-AC corrected
images. In an effort to avoid contrast-induced artifacts, negative
oral contrast media are being evaluated.
Metallic prosthetic implants (such as hip replacements,
intrauterine contraceptive devices, or surgical clips) can lead to
beam hardening on CT, with consequent AC artifact on the PET image.
It is important to view the uncorrected PET images to avoid
false-positive interpretation, particularly when there is a
question regarding periprosthetic inflammation or infection
Algorithms are being developed to allow for this overestimation
of activity by CT AC and may minimize these effects in the future.
Availability of diagnostic CT is extremely valuable in situations
in which tumor is not 18F-FDG-avid, since contrast agents
facilitate tumor detection and characterization. Diagnostic CT can
also reveal clinically pertinent non-neoplastic conditions, such as
aortic aneurysms and bowel obstruction. Therefore, the use of oral
and IV contrast media can augment CT performance beyond mere
anatomic correlation and attenuation correction for PET.
In general, false-positive FDG uptake can occur in PET/CT in
relation to granulomatous disease or inflammation. Some benign
tumors, such as colonic adenomas and fibroids, may also demonstrate
intense FDG uptake. False negatives can arise in tumors that are
either small or non-FDG-avid. These include some neuroendocrine
tumors, renal cell carcinoma, and certain types of lymphoma. Many
previous studies have demonstrated that PET is more specific than
CT for staging of posttreatment lymphoma patients.
However, there are subtypes of lymphoma that are poorly avid on
PET, including, most notably, marginal zone lymphoma (of which
mucosa-associated lymphoid tissue [MALT] lymphoma is a subtype) and
peripheral T-cell lymphoma. Therefore, CT may play a more important
role in the staging of these patients at diagnosis and follow-up.
Some mucinous and low-grade tumors are also known to be poorly
High neighboring background activity can also lead to obscuration
of FDG uptake.
It is hoped that misinterpretation may be avoided by the ability
to accurately attribute FDG activity to the correct abdominal
structure. Radiotracer uptake within the gastrointestinal tract is
highly variable (Figure 3). The esophagus generally does not
demonstrate increased uptake in the absence of inflammation or
malignancy. Homogeneous uptake in the stomach wall is relatively
common, however. Small-bowel uptake is variable but usually of low
grade. Uptake within the colon may be quite avid, particularly in
the cecum/ascending colon and within the rectosigmoid region. This
usually has a recognizable linear appearance. Focal large-bowel
activity greater than hepatic activity is unusual and should alert
the radiologist to the possible presence of pathology. A thorough
regional review of the coregistered CT images is warranted to look
for focal masses or adjunct signs of inflammation; one must bear in
mind that peristalsis, patient motion, and breathing may lead to
misregistration artifact. Preferably, this should be a diagnostic
CT with oral and IV contrast media. In the absence of corresponding
CT findings, focal intense colonic activity on PET still warrants
further investigation. The exact cause of the variable FDG uptake
in the intestinal tract is as yet unclear. Animal studies have
shown that the majority is due to intestinal mucosa or bowel
Although 18F-FDG is an analogue of glucose, unlike glucose, it
is not reabsorbed by the renal tubules. Thus, any part of the
urinary tract can show increased activity. In particular, dilated
or redundant ureters as well as bladder diverticula can be
It is beneficial to minimize urinary stasis within the renal
collecting system, ureters, and bladder, particularly when there is
pelvic pathology. Good hydration and regular voiding can achieve
this. Some authors advocate the use of IV diuretics
or bladder catheterization.
Care must be taken when interpreting PET images in the context of
renal cell carcinoma, as a recent study has reported only 60%
Liver activity is usually mildly intense with a uniformly
mottled appearance. Caution must be exercised when interpreting
liver dome lesions, as these can be erroneously projected in the
lung base due to respiratory artifact.
In addition, hepatomas are known to be poorly FDG-avid.
Splenic activity is usually less than liver on FDG-PET, and CT can
be of particular help with splenules that can cause some confusion
on PET imaging alone. Some studies have shown that very early stage
pancreatic cancers can give rise to false-negative results with
FDG-PET, thus suggesting a major limitation of FDG-PET imaging
alone in the detection of pancreatic cancer.
False-positive FDG-PET results for pancreatic cancer can occur in
chronic active pancreatitis and autoimmune pancreatitis.
Similarly, focal uptake of the tracer compound by inflamed
parenchyma and irradiated tissues may be indistinguishable from
There have also been reports of ocal FDG uptake caused by portal
vein thrombosis, hemorrhagic pseudocysts, peripancreatic lymph
nodes, and retroperitoneal fibrosis.
CT can be helpful in these situations by identifying the cause of
the increased FDG uptake.
Adrenal adenomas are relatively common in the general population
(2% to 9%) and occur in up to 5% of abdominal CT scans.
The majority of adrenal lesions can be characterized by CT
criteria. Approximately 70% of adrenal adenomas contain sufficient
intracytoplasmic fat to lower the CT attenuation to ≤10 HU.
Adrenal lesions can be further characterized by washout
characteristics on delayed CT.
Characterization of adrenal lesions by FDG-PET depends on increased
glucose metabolism in malignancy. Several series have investigated
the ability of FDG-PET to distinguish between benign and malignant
adrenal lesions. In a series of 50 adrenal lesions, Mijin et al
used background liver activity as a baseline. Lesions with activity
lower or much greater than liver activity could be confidently
diagnosed as benign or malignant, respectively. Lesions with
slightly increased activity relative to liver were classed as
indeterminate. Other authors have reported up to 100% sensitivity
and specificity for FDG-PET in distinguishing benign from malignant
False-positive scans do occur in FDG-PET adrenal imaging,
particularly in relation to pheochromocytoma, adrenal hyperplasia,
angiomyelolipoma and in approximately 5% of adenomas.
PET/CT combines both the attenuation characteristics and the
metabolic activity of adrenal lesions in one examination and should
provide information that is diagnostic for most of the cases
The integrated PET/CT information may be particularly useful in
the rare event of collision tumors when an adenoma and metastasis
coexist. Brown fat with a perinephric distribution is another
potential pitfall of PET/CT in adrenal imaging because of increased
FDG up-take (Figure 4). The characteristic brown-fat pattern is
more commonly recognized in the neck and supraclavicular regions
but can also be seen in the paraspinal and periadrenal regions.
Brown fat is considered a vestigial organ of thermogenesis that
utilizes increased glucose and is sympathetically innervated and
driven. The pattern is more common in thin patients and during the
winter months. Awareness of this entity together with careful CT
correlation usually results in correct diagnosis.
PET/CT is extremely useful in characterizing lesions in the
pelvis. There are some pitfalls to be wary of, however. Endometrial
uptake of tracer is seen in biphasic peaks at ovulation and
menstruation in premenopausal subjects (Figure 5).
Postmenopausal endometrial uptake is abnormal. Ovarian uptake
may be physiological in premenopausal subjects but is associated
with malignancy in postmenopausal women.
PET and CT can be complementary in displaying bony abnormalities,
with PET excelling in marrow abnormalities and CT performing better
with cortically based lesions. The diffuse increase in bone marrow
activity seen following colony stimulating factors is now well
recognized. Paget's disease and fibrous dysplasia may show
increased uptake but these entities can usually be recognized on
CT. A dedicated PET/CT workstation is mandatory for optimal viewing
of the abdominal and pelvic images. Reviewing the CT data using
appropriate window settings and examining the displays of both the
attenuation-corrected and noncorrected PET data serve to provide a
comprehensive, integrated interpretation.
Although the PET/CT scanner clearly represents a major
technologic advance, the alliance of functional imaging with
structural imaging raises a number of controversial issues.
Protocol and interpretation issues include examination
reimbursement, validation of indications for use, who should
interpret the examination, specific indications and protocols for
low radiation dose CT, and suitability and timing of oral and IV
contrast. Recent PET/CT publications have been highly encouraging,
but larger prospective studies will be necessary to establish the
optimal hybrid scanning protocols. We have outlined some specific
performance and interpretation principles, including some that are
still in development. Applying sound principles and staying abreast
of advances in this exciting new modality are necessary
requirements to harness the full diagnostic power of abdominal