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DOI: 10.1148/rg.265055208
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RadioGraphics 2006;26:1335-1353
© RSNA, 2006

EDUCATION EXHIBIT

Pearls and Pitfalls in Interpretation of Abdominal and Pelvic PET-CT1

Michael A. Blake, FFR (RCSI), FRCR, Ajay Singh, MD, Bindu N. Setty, MD, James Slattery, FFR (RCSI), Mannudeep Kalra, MD, Michael M. Maher, MD, Dushyant V. Sahani, MD, Alan J. Fischman, MD and Peter R. Mueller, MD

1 From the Department of Radiology, Division of Abdominal Imaging and Intervention (M.A.B., A.S., B.N.S., J.S., M.K., M.M.M., D.V.S., P.R.M.) and Division of Nuclear Medicine (A.J.F.), Massachusetts General Hospital, White 270, 55 Fruit St, Boston MA 02114. Recipient of an Excellence in Design award for an education exhibit at the 2004 RSNA Annual Meeting. Received December 8, 2005; revision requested January 4, 2006 and received February 1; accepted February 6. All authors have no financial relationships to disclose. Address correspondence to B.N.S. (e-mail: bsetty{at}partners.org).


   Abstract
Top
 Abstract
LEARNING OBJECTIVES FOR TEST...
 Introduction
 Abdominopelvic PET-CT Protocol
 General Interpretation Issues
 Specific Interpretation Issues
 Conclusions
 References
 
The interpretation of images obtained in the abdomen and pelvis can be challenging, and the coregistration of positron emission tomographic (PET) and computed tomographic (CT) scans may be especially valuable in the evaluation of these anatomic areas. PET-CT represents a major technologic advance, consisting of generally complementary modalities whose combined strength tends to overcome their respective weaknesses. However, this combined functional-structural imaging approach raises a number of controversial questions and presents some unique interpretative challenges. Accurate PET-CT scan interpretation requires awareness of the various pitfalls associated with the imaging components, both individually and in combination. The results of recent PET-CT studies have been very encouraging, but larger prospective studies will be needed to establish optimal hybrid scanning protocols. Applying sound imaging principles, paying attention to detail, and staying abreast of advances in this exciting new modality are necessary for harnessing the full diagnostic power of abdominopelvic PET-CT.

© RSNA, 2006


   LEARNING OBJECTIVES FOR TEST 2
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
Introduction
 Abdominopelvic PET-CT Protocol
 General Interpretation Issues
 Specific Interpretation Issues
 Conclusions
 References
 
After reading this article and taking the test, the reader will be able to:


   Introduction
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
Abdominopelvic PET-CT Protocol
 General Interpretation Issues
 Specific Interpretation Issues
 Conclusions
 References
 
With the advent of positron emission tomography (PET)–computed tomography (CT), the metabolic information obtained with fluorine 18 (18F) fluorodeoxyglucose (FDG) PET can be combined with the morphologic information obtained with CT. FDG PET provides both qualitative and quantitative metabolic information that is valuable for diagnosis and management. Indeed, PET can allow the early detection of increased metabolic activity in diseased tissues, which can sometimes appear morphologically normal with other imaging modalities. PET can assist in the differentiation of benign from malignant tumors and in the follow-up of cancer patients who have undergone surgery, radiation therapy, or chemotherapy. Accurate anatomic localization of foci of increased metabolic activity can be difficult or impossible at stand-alone PET, particularly in the abdomen and pelvis, which are characterized by a lack of reliable identifiable anatomic structures and variable physiologic FDG uptake. On the other hand, CT provides valuable multiplanar information regarding the morphologic features and attenuation values of lesions and demonstrates the behavior of orally and intravenously administered contrast material. With combined PET-CT, the superimposition of the precise structural findings provided by CT allows more accurate and reproducible correlation of a hypermetabolic focus seen at PET with the correct anatomic or pathologic equivalent (1–3). Although the fusion of the two independent data sets results in both a more comprehensive examination and more accurate localization of abnormalities, it also introduces some unique potential pitfalls and interpretative difficulties. Again, this situation is especially true in the abdomen and pelvis, where physiologic FDG uptake can be misleading and CT has tissue characterization limitations, especially following surgery.

Many of the artifacts and pitfalls related to the interpretation of independent PET and CT scans are well documented, with more being reported as clinical experience grows (4–8). As mentioned earlier, however, combined PET-CT has its own unique pitfalls and artifacts (1,9,10). Physicians interpreting PET-CT scans should be familiar with the artifacts associated with the modalities, both individually and in combination—as well as with the principles of PET-CT—to ensure accurate scan interpretation and optimal patient care.

In this article, we review imaging protocols for PET-CT of the abdomen and pelvis and briefly discuss some general interpretation issues. We also discuss and illustrate a variety of specific interpretation issues, including attenuation correction and misregistration; hypermetabolic and hypometabolic lesions; and issues relating to specific anatomic locations (eg, liver and spleen; kidneys, ureters, and bladder; pancreas and adrenal glands; gastrointestinal tract; reproductive tract; bone).


   Abdominopelvic PET-CT Protocol
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Abdominopelvic PET-CT Protocol
General Interpretation Issues
 Specific Interpretation Issues
 Conclusions
 References
 
The introduction of CT-based attenuation correction and its integration with PET has led to the need for modified PET-CT scanning protocols. The two general approaches adopted for PET-CT scanning have essentially been to either (a) use CT simply to provide faster attenuation correction and coregistration; or (b) make full use of CT for attenuation correction, coregistration, and diagnosis (11). The former approach makes use of a very low radiation dose, which still provides satisfactory attenuation correction but adversely affects scan quality. The latter approach makes use of a standard radiation dose, thereby making diagnostic-quality CT possible. The specific indications and protocols for low-dose or standard-dose CT have yet to be defined or agreed upon. We combine the two approaches, first performing low-dose unenhanced CT primarily to provide attenuation information and then performing fully diagnostic standard-dose contrast material–enhanced CT immediately following the PET acquisition.

Studies have shown that oral and intravenous contrast material can be used for diagnostic CT to aid lesion localization and support characterization; however, modifications are necessary to avoid artifacts on the PET scans and to ensure appropriate attenuation correction (11–13). Some authors propose eliminating unenhanced CT, instead using contrast-enhanced CT for the PET attenuation correction. Doing so has the advantages of reducing radiation dose and shortening the time required for examination, although experience with this protocol has been limited. Artifacts may occur due to beam hardening from metallic orthopedic implants, affecting the CT-based attenuation correction of PET scans (see "Specific Interpretation Issues") (14–16). In addition, mismatch of the respective imaging data due to breathing movements and inconsistent patient positioning must be minimized to allow accurate PET-CT coregistration in abdominal studies (17,18). Normal "free" breathing or the normal expiratory phase for the acquisition of CT scans has been reported to provide better coregistration than the maximum inspiratory or maximum expiratory phases. However, breath-hold imaging is optimal because it provides the highest-quality CT scans.


   General Interpretation Issues
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Abdominopelvic PET-CT Protocol
 General Interpretation Issues
Specific Interpretation Issues
 Conclusions
 References
 
In general, FDG uptake, which is false positive with respect to tumor, can occur at PET-CT as a result of granulomatous disease or inflammation. Some benign tumors such as colonic adenomas and uterine fibroids may also demonstrate intense FDG uptake. False-negative PET findings can result if tumors are either too small or non–FDG avid. The latter group includes some neuroendocrine tumors, renal cell carcinoma (RCC), and certain types of lymphoma, although most lymphomas are FDG avid. Many previous studies have demonstrated that PET is usually more specific than CT for the staging of lymphoma in patients who have undergone treatment (19). However, there are subtypes of lymphoma that are poorly avid at PET, including marginal zone lymphoma (of which mucosa-associated lymphoid tissue lymphoma is a subtype) and peripheral T-cell lymphoma. Therefore, CT may play a particularly important role in the staging of these lymphoma subtypes at diagnosis and follow-up (20). Some mucinous and low-grade tumors are also known to be sometimes poorly FDG avid (5). High neighboring background activity can also obscure FDG uptake.

It is hoped that misinterpretation may be avoided by attributing FDG activity to the correct abdominal structure depicted at concurrent CT. A dedicated PET-CT workstation is mandatory for optimal viewing of the combined scans. Reviewing the CT data with appropriate window settings and examining the displays of both the attenuation-corrected and non-attenuation-corrected PET data serve to provide a comprehensive, integrated interpretation.


   Specific Interpretation Issues
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Abdominopelvic PET-CT Protocol
 General Interpretation Issues
 Specific Interpretation Issues
Conclusions
 References
 
Attenuation Correction
On PET-CT scanners, the emission data can be corrected for photon attenuation using the CT scan to generate an attenuation map. Doing so confers the following advantages: (a) there is less statistical noise from the CT, compared with germanium 68 (68Ge) transmission data on stand-alone PET scanners; (b) the scanning time for CT is much shorter than for radionuclide imaging, thus reducing overall scanning time by 15–20 minutes; and (c) the need for PET transmission hardware and the cost of replacing germanium source rods are eliminated (21).

There is a potential risk of overestimating the true FDG activity with CT-based attenuation correction. Nakamoto et al (21) demonstrated that the activity as measured with CT-based attenuation correction was overestimated by an average of 11% in bone and 2.1% in soft tissue compared with 68Ge-based attenuation correction. Attenuation correction with CT data may lead to artifacts caused by overcorrection of photopenic areas corresponding to high-attenuation structures at CT. This overcorrection inaccurately renders such areas hypermetabolic on the attenuation-corrected PET scans. Some studies have shown that the use of very high-density positive oral and intravenous contrast material may lead to attenuation-correction artifacts on the PET scan, particularly if there is movement of the contrast material during the time interval between PET and the CT used for attenuation correction (Fig 1) (3,22–25). Metallic prosthetic implants such as hip replacements, intrauterine contraceptive devices, or surgical clips can also lead to beam hardening at CT with consequent attenuation-correction artifact on the PET scan.


Figure 1
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  		Figure 1a.  Attenuation correction artifact in a patient with a bicornuate uterus. PET (a) and CT (b) scans show apparent increased activity (circled) in small bowel loops containing high-density barium, a finding that was not present on the non-attenuation-corrected scan. Genuine increased activity is seen in the endometrium (arrow).

 

Figure 1
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  		Figure 1b.  Attenuation correction artifact in a patient with a bicornuate uterus. PET (a) and CT (b) scans show apparent increased activity (circled) in small bowel loops containing high-density barium, a finding that was not present on the non-attenuation-corrected scan. Genuine increased activity is seen in the endometrium (arrow).

 
Physicians must be cognizant of this overcorrection phenomenon on the attenuation-correction PET scans during PET-CT interpretation. In clinical practice, Dizendorf et al (23) reported only a 4% overestimation of standardized uptake values with use of positive oral contrast material, a finding that suggests a negligible effect. Artifacts representing intense focal accumulations of positive oral contrast material can usually be resolved by viewing the CT and non-attenuation-corrected PET scans, which are not affected by the high-density material. The use of negative-attenuation oral contrast material also serves to resolve the problem (26). Algorithms are also being developed to account for this overestimation of activity by CT attenuation correction. Recently, Yau et al (27) demonstrated that coregistration with contrast-enhanced CT scans does not result in significant artifacts following CT attenuation correction.

Misregistration
The term misregistration refers to the superimposition of FDG activity on inappropriate anatomic structures seen at CT. Misregistration can be due to breathing, patient motion, bowel motility, or distention of the urinary bladder and can cause false-positive or -negative PET findings when radiotracer activity is superimposed on the wrong anatomic structure (Fig 2) (6,7).


Figure 2
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  		Figure 2.  Misregistration artifact. Fused PET-CT scan shows apparent increased radiotracer uptake (arrow) in segment VI of the right hepatic lobe, a finding that is secondary to misregistration of the physiologic renal activity over the right lobe.

 
Normal "free" breathing (normal expiratory phase) has been found to be more suitable than maximum inspiratory or expiratory phases for the acquisition of CT scans for coregistration. However, the use of breath-hold imaging has distinct advantages in terms of CT scan quality. Minimizing or at least reducing the time delay between PET and CT is also beneficial, as is preventing patient motion between scans. The interpreting physician must be able to recognize misregistration and to correct it with software coregistration. Awareness of potential misregistration and its causes is important for ensuring accurate PET-CT interpretation.

Nonneoplastic Hypermetabolic Lesions
As mentioned earlier, hypermetabolic lesions do not always indicate malignancy. In general, false-positive FDG uptake can occur at PET-CT due to granulomatous disease, abscess, surgical changes, foreign body reaction, or inflammation (eg, in diverticulitis, gastritis, or arteriosclerosis) (Fig 3).


Figure 3
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  		Figure 3a.  Nonneoplastic hypermetabolic activity. (a) Coronal PET scan shows FDG uptake by an aortic graft (arrowheads) secondary to either reendothelialization of the graft or a sterile inflammatory response. (b) Coronal CT scan shows the graft (arrowheads) extending from the lower abdominal aorta to the common iliac arteries.

 

Figure 3
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  		Figure 3b.  Nonneoplastic hypermetabolic activity. (a) Coronal PET scan shows FDG uptake by an aortic graft (arrowheads) secondary to either reendothelialization of the graft or a sterile inflammatory response. (b) Coronal CT scan shows the graft (arrowheads) extending from the lower abdominal aorta to the common iliac arteries.

 
 Some benign tumors may also demonstrate intense FDG uptake. On the other hand, high neighboring activity can mask malignant FDG uptake.

The CT component of the PET-CT study can readily reveal clinically pertinent nonneoplastic conditions such as abscesses, aortic aneurysms, and bowel obstruction. In addition, the use of oral and intravenous contrast material can augment CT performance far beyond mere anatomic correlation and attenuation correction for PET (12). Contrast-enhanced CT can often allow detection of the responsible nonneoplastic hypermetabolic conditions (eg, abscess, inflammation), thereby preventing false-positive interpretation. CT will also often provide complementary diagnostic information when there is high background FDG activity surrounding a malignancy. Allowing a sufficient amount of time to pass between radiation therapy or surgical treatment and PET-CT is important in preventing false-positive interpretation caused by persistent posttreatment FDG activity. It is best to wait at least 6 weeks if tumor recurrence is suspected in the surgical or irradiated bed. Interpreting physicians should be aware of any pertinent clinical symptoms such as fever and pain, which may point to underlying inflammatory disease.

Liver and Spleen
Liver activity is usually mildly intense with a uniform mottled appearance. There is usually less activity in the spleen than in the liver at FDG PET, and splenules can cause some confusion at PET alone. Caution must also be exercised in the interpretation of liver dome lesions, since these lesions can be erroneously projected into the lung base due to respiratory artifact (26). In addition, up to 36%–50% of hepatomas are reported to lack FDG avidity (28,29). Well-differentiated hepatocellular carcinomas are non–FDG avid, unlike moderately well-differentiated to poorly differentiated hepatocellular carcinomas. Necrotic and mucinous metastatic adenocarcinomas are also known to be poorly FDG avid (Fig 4) (28,30–33).


Figure 4
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  		Figure 4a.  False-negative findings in a patient with known pancreatic cancer. (a) Fused PET-CT scan shows multiple focal non-FDG-avid hepatic lesions (arrowheads). (b) Contrast-enhanced CT scan shows multiple focal lesions (arrowheads) representing metastases in the liver.

 

Figure 4
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  		Figure 4b.  False-negative findings in a patient with known pancreatic cancer. (a) Fused PET-CT scan shows multiple focal non-FDG-avid hepatic lesions (arrowheads). (b) Contrast-enhanced CT scan shows multiple focal lesions (arrowheads) representing metastases in the liver.

 
The coregistered CT scans can be of particular help in identifying and characterizing both hepatic and splenic FDG-avid lesions. Contrast-enhanced CT can provide the necessary localization information for surgical planning, including relationships to vessels and liver segments (Fig 5), and is also helpful in identifying intrasplenic lesions and splenules.


Figure 5
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  		Figure 5a.  Recurrence of metastasis in a patient with known hepatic metastasis from rectal carcinoma. The patient had undergone partial hepatectomy. (a) Fused PET-CT scan shows increased FDG uptake (arrow) adjacent to the partial hepatectomy bed. (b) CT scan shows a focal hypoattenuating lesion (arrow) corresponding to the site of increased FDG activity.

 

Figure 5
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  		Figure 5b.  Recurrence of metastasis in a patient with known hepatic metastasis from rectal carcinoma. The patient had undergone partial hepatectomy. (a) Fused PET-CT scan shows increased FDG uptake (arrow) adjacent to the partial hepatectomy bed. (b) CT scan shows a focal hypoattenuating lesion (arrow) corresponding to the site of increased FDG activity.

 
Kidneys and Urinary Tract
18F-FDG is an analogue of glucose; unlike glucose, however, it is not reabsorbed by the renal tubules. Thus, any part of the urinary tract can show increased activity from excreted FDG. In particular, dilated or redundant ureters and bladder diverticula can cause confusion in PET interpretation (34).

It is beneficial to minimize urinary stasis within the renal collecting system, ureters, and bladder, particularly when there is pelvic disease. Good hydration and regular voiding can help minimize urinary stasis. Some authors advocate the use of intravenous diuretics or bladder catheterization (35). Care must be taken when interpreting PET scans in the context of RCC, since a study by Kang et al (36) demonstrated that PET has a sensitivity of only 60% for RCC (Fig 6). When no intravenous contrast material is given, FDG excretion provides some renal physiologic information and can help distinguish parapelvic cysts from hydronephrosis. On the other hand, CT is helpful in distinguishing lymphadenopathy in the periaortic and periureteral regions from horseshoe kidney and ureteral activity, respectively (Fig 7).


Figure 6
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  		Figure 6a.  False-negative finding in a patient with RCC. (a) Fused PET-CT scan shows a solid mass (arrow) in the right renal cortex. No corresponding area of increased activity was seen at PET, whose sensitivity for RCC is approximately 60%. (b) Contrast-enhanced CT scan shows a 2.5-cm enhancing mass (arrow) arising from the lower pole of the right kidney, a finding that represents an RCC.

 

Figure 6
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  		Figure 6b.  False-negative finding in a patient with RCC. (a) Fused PET-CT scan shows a solid mass (arrow) in the right renal cortex. No corresponding area of increased activity was seen at PET, whose sensitivity for RCC is approximately 60%. (b) Contrast-enhanced CT scan shows a 2.5-cm enhancing mass (arrow) arising from the lower pole of the right kidney, a finding that represents an RCC.

 

Figure 7
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  		Figure 7a.  Retroperitoneal lymphadenopathy mimicking ureteral activity in a patient with lymphoma. (a) PET scan shows focal increased FDG activity (arrow) in the right retroperitoneum. Without PET-CT superimposition, the hypermetabolic activity may be mistaken for physiologic activity in the right ureter. (b) Contrast-enhanced CT scan shows enlarged right retroperitoneal lymph nodes (arrow) adjacent to the right midureter.

 

Figure 7
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  		Figure 7b.  Retroperitoneal lymphadenopathy mimicking ureteral activity in a patient with lymphoma. (a) PET scan shows focal increased FDG activity (arrow) in the right retroperitoneum. Without PET-CT superimposition, the hypermetabolic activity may be mistaken for physiologic activity in the right ureter. (b) Contrast-enhanced CT scan shows enlarged right retroperitoneal lymph nodes (arrow) adjacent to the right midureter.

 
Pancreas and Adrenal Glands
The fact that early-stage pancreatic cancers can be falsely negative at FDG PET suggests a major limitation of using FDG PET alone to detect this neoplasm (37). False-positive FDG PET results for pancreatic cancer can also occur in chronic active pancreatitis and autoimmune pancreatitis (38). Similarly, focal FDG uptake by inflamed parenchyma and irradiated tissues may be indistinguishable from pancreatic malignancy (39). Focal FDG uptake due to portal vein thrombosis, hemorrhagic pseudocysts, peripancreatic lymph nodes, and retroperitoneal fibrosis has also been reported (37–39).

False-positive findings may occur at adrenal FDG PET, particularly with respect to pheochromocytoma and adrenal hyperplasia, although approximately 5% of adenomas and, rarely, myelolipomas may also yield false-positive findings (40–43). Increased suprarenal FDG uptake due to brown fat, which may mimic an adrenal lesion, is another pitfall of adrenal PET (44).

PET-CT improves the independent performance of its component modalities in diagnosing pancreatic cancer by allowing better recognition of false-positive PET findings and greater confidence in making the correct diagnosis at CT (Fig 8).


Figure 8
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  		Figure 8a.  Pancreatic carcinoma. (a) Fused PET-CT scan shows a hypermetabolic focus (arrow) in the head of the pancreas, a finding that represents a pancreatic adenocarcinoma. (b) Unenhanced CT scan shows a small, poorly visualized hypoattenuating lesion (arrow) in the pancreatic head. (c) Follow-up contrast-enhanced CT scan obtained 6 months later clearly depicts the adenocarcinoma as a heterogeneous pancreatic mass (circled).

 

Figure 8
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  		Figure 8b.  Pancreatic carcinoma. (a) Fused PET-CT scan shows a hypermetabolic focus (arrow) in the head of the pancreas, a finding that represents a pancreatic adenocarcinoma. (b) Unenhanced CT scan shows a small, poorly visualized hypoattenuating lesion (arrow) in the pancreatic head. (c) Follow-up contrast-enhanced CT scan obtained 6 months later clearly depicts the adenocarcinoma as a heterogeneous pancreatic mass (circled).

 

Figure 8
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  		Figure 8c.  Pancreatic carcinoma. (a) Fused PET-CT scan shows a hypermetabolic focus (arrow) in the head of the pancreas, a finding that represents a pancreatic adenocarcinoma. (b) Unenhanced CT scan shows a small, poorly visualized hypoattenuating lesion (arrow) in the pancreatic head. (c) Follow-up contrast-enhanced CT scan obtained 6 months later clearly depicts the adenocarcinoma as a heterogeneous pancreatic mass (circled).

 
Adrenal PET-CT allows determination of the attenuation characteristics and the metabolic activity of adrenal lesions with a single examination and, in the majority of cases, should provide information that is diagnostic. The characterization of adrenal lesions with FDG PET depends on increased glucose metabolism in malignancy. Several studies have reported on the capacity of FDG PET to help differentiate benign from malignant adrenal lesions. Lesions with activity that is less or much greater than that of the liver can confidently be diagnosed as benign or malignant, respectively (Fig 9). Lesions with slightly increased activity relative to the liver are classified as indeterminate. Authors have reported up to 100% sensitivity and specificity for FDG PET in distinguishing benign from malignant lesions (40–42).


Figure 9
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  		Figure 9a.  Adrenal adenoma. (a) Fused PET-CT scan shows bilateral lesions (arrowheads) in the adrenal glands with no significant FDG activity, findings that are consistent with adrenal adenomas. (b) Unenhanced CT scan shows the bilateral adrenal masses (arrowheads) with a uniform attenuation of 4 HU, findings that are also consistent with adrenal adenomas.

 

Figure 9
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  		Figure 9b.  Adrenal adenoma. (a) Fused PET-CT scan shows bilateral lesions (arrowheads) in the adrenal glands with no significant FDG activity, findings that are consistent with adrenal adenomas. (b) Unenhanced CT scan shows the bilateral adrenal masses (arrowheads) with a uniform attenuation of 4 HU, findings that are also consistent with adrenal adenomas.

 
CT can be useful in situations in which nonneoplastic lesions manifest with increased FDG uptake by helping identify the cause of the increased uptake. Indeed, the majority of adrenal lesions can be characterized with CT criteria alone if unenhanced CT is performed. Approximately 70% of adrenal adenomas contain sufficient intracytoplasmic fat to lower the CT attenuation to 10 HU or less (45). Adrenal lesions can be further characterized on the basis of washout characteristics on delayed contrast-enhanced CT scans (46–48). Thus, both components of PET-CT provide useful information for adrenal lesion characterization. The integrated PET-CT information may be particularly useful in the rare case of collision tumors, in which an adenoma and metastasis coexist. The characteristic brown fat pattern is most commonly seen 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 makes use of increased glucose and is sympathetically innervated. The distinctive pattern is more common in thin patients and during the winter months (44). Brown fat uptake is usually recognizable on the basis of its characteristic pattern, coregistration to areas of fat, and lack of a corresponding adrenal mass. Awareness of this entity together with careful CT correlation usually results in the correct diagnosis (44).

Hypometabolic Lesions
Although PET has high sensitivity for the detection of a number of cancerous lesions (eg, lung, colorectal, esophageal, breast, thyroid, and head and neck cancers; sarcoma; melanoma; lymphoma), there are a number of neoplasms that are not hypermetabolic and thus not FDG avid. These cancers include certain renal cell cancers and lymphomas, neuroendocrine tumors, bronchoalveolar carcinomas, colonic mucinous adenocarcinomas, prostate carcinomas, and carcinoid tumors (5–7,20).

The information provided by the diagnostic CT component of PET-CT is clearly extremely valuable in situations in which tumors are not FDG avid. Contrast-enhanced CT facilitates tumor detection and characterization (Fig 10). The use of alternate contrast agents is possible for more specific PET evaluation and is being investigated further. The use of PET for follow-up is generally not advisable when pre-chemotherapy PET fails to show FDG uptake, although it can reveal transformation to higher-grade tumors in some patients with lymphoma. PET is also generally less helpful when one of the known hypometabolic histologic types is being investigated.


Figure 10
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  		Figure 10a.  Pancreatic metastases from RCC. (a) Fused PET-CT scan shows a focal pancreatic nodule (arrow) with no FDG activity. (b) On a contrast-enhanced CT scan, the nodule (arrow) is enhancing, a finding that indicates a metastasis in the pancreatic body.

 

Figure 10
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  		Figure 10b.  Pancreatic metastases from RCC. (a) Fused PET-CT scan shows a focal pancreatic nodule (arrow) with no FDG activity. (b) On a contrast-enhanced CT scan, the nodule (arrow) is enhancing, a finding that indicates a metastasis in the pancreatic body.

 
Gastrointestinal Tract
Radiotracer uptake within the gastrointestinal tract is highly variable, which can create difficulty in diagnosing a neoplastic process in the vicinity of bowel loops (Fig 11) (49,50). The esophagus generally does not demonstrate markedly increased uptake in the absence of inflammation or malignancy. Homogeneous increased uptake in the stomach wall and gastroesophageal junction is relatively common (Fig 12). 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 in the rectoanal region.


Figure 11
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  		Figure 11a.  Response of a gastrointestinal stromal tumor to chemotherapy. (a) Fused PET-CT scan shows a mesenteric mass (arrows) with no FDG activity, a finding that indicates a lack of viable tumor cells. (b) FDG PET scan shows heterogeneously increased activity (arrows) within the partially necrotic tumor. (c) CT scan obtained 6 months after b shows a marked interval decrease in the size of the mass (arrows).

 

Figure 11
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  		Figure 11b.  Response of a gastrointestinal stromal tumor to chemotherapy. (a) Fused PET-CT scan shows a mesenteric mass (arrows) with no FDG activity, a finding that indicates a lack of viable tumor cells. (b) FDG PET scan shows heterogeneously increased activity (arrows) within the partially necrotic tumor. (c) CT scan obtained 6 months after b shows a marked interval decrease in the size of the mass (arrows).

 

Figure 11
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  		Figure 11c.  Response of a gastrointestinal stromal tumor to chemotherapy. (a) Fused PET-CT scan shows a mesenteric mass (arrows) with no FDG activity, a finding that indicates a lack of viable tumor cells. (b) FDG PET scan shows heterogeneously increased activity (arrows) within the partially necrotic tumor. (c) CT scan obtained 6 months after b shows a marked interval decrease in the size of the mass (arrows).

 

Figure 12
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  		Figure 12a.  Increased FDG uptake representing gastritis in a patient with a history of partial gastrectomy. (a) Fused PET-CT scan shows increased radiotracer uptake (arrow) in the gastric wall. (b) CT scan shows inflammation (arrow) of the portion of the stomach that remained after partial gastrectomy.

 

Figure 12
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  		Figure 12b.  Increased FDG uptake representing gastritis in a patient with a history of partial gastrectomy. (a) Fused PET-CT scan shows increased radiotracer uptake (arrow) in the gastric wall. (b) CT scan shows inflammation (arrow) of the portion of the stomach that remained after partial gastrectomy.

 
Misinterpretation may be avoided by attributing FDG activity to the correct abdominal structure. Focal large bowel activity that exceeds hepatic activity is unusual and should suggest the presence of disease (Figs 13, 14). A thorough regional review of the coregistered CT scans is warranted to look for focal masses or adjunct signs of inflammation, bearing in mind that peristalsis, patient motion, and breathing may lead to misregistration artifact. Preferably, this CT should be diagnostic, performed with oral and intravenous contrast material. Even in the absence of corresponding CT findings, intense focal colonic activity at PET warrants further investigation.


Figure 13
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  		Figure 13a.  Villonodular colonic polyp. (a) Fused PET-CT scan shows a hypermetabolic focus (arrow) in the ascending colon, a finding that represents a tubulovillous colonic polyp. (b) Contrast-enhanced CT scan shows a soft-tissue nodule (arrow) in the ascending colon, a finding that corresponds to the colonic polyp.

 

Figure 13
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  		Figure 13b.  Villonodular colonic polyp. (a) Fused PET-CT scan shows a hypermetabolic focus (arrow) in the ascending colon, a finding that represents a tubulovillous colonic polyp. (b) Contrast-enhanced CT scan shows a soft-tissue nodule (arrow) in the ascending colon, a finding that corresponds to the colonic polyp.

 

Figure 14
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  		Figure 14a.  Acute sigmoid diverticulitis. (a) Fused PET-CT scan shows segmental hypermetabolism (circled) secondary to inflammation of the sigmoid colon. (b) Contrast-enhanced CT scan shows wall thickening of the sigmoid colon (circled) along with diverticulitis, a benign but significant cause of abnormal FDG uptake.

 

Figure 14
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  		Figure 14b.  Acute sigmoid diverticulitis. (a) Fused PET-CT scan shows segmental hypermetabolism (circled) secondary to inflammation of the sigmoid colon. (b) Contrast-enhanced CT scan shows wall thickening of the sigmoid colon (circled) along with diverticulitis, a benign but significant cause of abnormal FDG uptake.

 
 Animal studies have shown that the majority of physiologic intestinal activity is due to intestinal mucosa or bowel contents. Bowel uptake usually has a recognizable linear appearance (Fig 15). Physiologic FDG uptake in pelvic organs, which are displaced posteriorly after abdominoperineal resection, is the main cause for false-positive interpretations in patients being evaluated for rectal cancer recurrence. This false-positive rate is reduced with PET-CT scan coregistration. Indeed, PET-CT has been found to be accurate in the detection of pelvic recurrence after surgical removal of rectal cancer and is reported to allow differentiation of a benign lesion from a neoplastic abnormality with a sensitivity of 100% and a specificity of 96% (50).


Figure 15
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  		Figure 15a.  Physiologic bowel activity. Coronal (a) and axial (b) fused PET-CT scans show physiologic increased activity (circled) in the right colon and ileum, respectively.

 

Figure 15
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  		Figure 15b.  Physiologic bowel activity. Coronal (a) and axial (b) fused PET-CT scans show physiologic increased activity (circled) in the right colon and ileum, respectively.

 
Reproductive Tract
FDG PET of the reproductive tract can depict lymph node and distant metastases, sometimes before these changes can be visualized or recognized as malignant at CT. To date, FDG PET has been used for the evaluation of patients with prostate, ovarian, cervical, and testicular cancer. However, CT is helpful for all of these indications and provides useful complementary information in combined PET-CT (1,9,10).

Uterus. — In premenopausal women, the endometrial FDG uptake changes cyclically, increasing during the ovulatory and menstrual phases (Fig 16). Postmenopausal endometrial uptake is abnormal.


Figure 16
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  		Figure 16a.  Endometrial FDG uptake during menses. (a) Sagittal fused PET-CT scan shows physiologic increased activity (arrow) in the endometrium. The activity is located below the uterus due to misregistration produced by filling of the urinary bladder between pelvic PET and contrast-enhanced CT. (b) Fused PET-CT scan obtained with use of unenhanced CT that was performed just prior to pelvic PET shows accurate superimposition of the endometrial FDG activity (arrow) over the center of the uterus.

 

Figure 16
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  		Figure 16b.  Endometrial FDG uptake during menses. (a) Sagittal fused PET-CT scan shows physiologic increased activity (arrow) in the endometrium. The activity is located below the uterus due to misregistration produced by filling of the urinary bladder between pelvic PET and contrast-enhanced CT. (b) Fused PET-CT scan obtained with use of unenhanced CT that was performed just prior to pelvic PET shows accurate superimposition of the endometrial FDG activity (arrow) over the center of the uterus.

 
Knowledge of the patient’s pertinent menstrual history is extremely helpful for informed interpretation, with expected increased activity during the ovulatory and menstrual phases. A focus of increased uptake in the endometrium adjacent to a cervical tumor does not necessarily reflect endometrial tumor invasion (51). Uterine fibroids are also known to occasionally show increased FDG activity (Figs 17, 18). Saksena et al (52) reported 18% of fibroids in their series to be hypermetabolic.


Figure 17
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  		Figure 17a.  Uterine fibroid. (a) Fused PET-CT scan shows a uterine fibroid (arrow) with no FDG activity, the most common FGD uptake pattern in fibroids. Most uterine fibroids are hypo- or isometabolic relative to the myometrium. (b) Contrast-enhanced CT scan shows a hypoattenuating lesion (arrow) that represents the uterine fibroid.

 

Figure 17
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  		Figure 17b.  Uterine fibroid. (a) Fused PET-CT scan shows a uterine fibroid (arrow) with no FDG activity, the most common FGD uptake pattern in fibroids. Most uterine fibroids are hypo- or isometabolic relative to the myometrium. (b) Contrast-enhanced CT scan shows a hypoattenuating lesion (arrow) that represents the uterine fibroid.

 

Figure 18
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  		Figure 18a.  Uterine fibroid. (a) Fused PET-CT scan shows a hypermetabolic focus (arrowhead) in the uterine body. This finding corresponds to a hypoattenuating fibroid that was seen at CT. Approximately one-fifth of fibroids demonstrate this pattern at FDG PET. (b) Contrast-enhanced CT scan shows a focal hypoattenuating fibroid (arrowhead) in the left wall of the uterine body.

 

Figure 18
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  		Figure 18b.  Uterine fibroid. (a) Fused PET-CT scan shows a hypermetabolic focus (arrowhead) in the uterine body. This finding corresponds to a hypoattenuating fibroid that was seen at CT. Approximately one-fifth of fibroids demonstrate this pattern at FDG PET. (b) Contrast-enhanced CT scan shows a focal hypoattenuating fibroid (arrowhead) in the left wall of the uterine body.

 
Thus far, FDG PET has been shown to be accurate and sensitive (>95%) in detecting recurrence of endometrial cancer and evaluating therapeutic response (Figs 19, 20). FDG also accumulates reliably in cervical cancer (53,54). CT allows localization of the abnormal focus of FDG to the pertinent gynecologic structure at PET-CT.


Figure 19
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  		Figure 19a.  Endometrial carcinoma. (a) Fused PET-CT scan shows a centrally located uterine mass (arrowheads) with increased FDG activity. (b) Contrast-enhanced CT scan shows an enlarged uterus with a centrally located heterogeneous mass (arrowheads), a finding that represents endometrial carcinoma.

 

Figure 19
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  		Figure 19b.  Endometrial carcinoma. (a) Fused PET-CT scan shows a centrally located uterine mass (arrowheads) with increased FDG activity. (b) Contrast-enhanced CT scan shows an enlarged uterus with a centrally located heterogeneous mass (arrowheads), a finding that represents endometrial carcinoma.

 

Figure 20
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  		Figure 20a.  Metastasis to the right ventricular wall from endometrial carcinoma. (a) Fused PET-CT scan shows a hypermetabolic focus (arrow) in the anterior wall of the right ventricle. No identifiable lesion was seen at CT. (b) Follow-up contrast-enhanced CT scan obtained 2 months later shows a hypoattenuating mass causing contour abnormality (arrowhead) in the right ventricular wall. The endometrial metastasis had been visualized at previous PET even before it produced a visible CT abnormality.

 

Figure 20
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  		Figure 20b.  Metastasis to the right ventricular wall from endometrial carcinoma. (a) Fused PET-CT scan shows a hypermetabolic focus (arrow) in the anterior wall of the right ventricle. No identifiable lesion was seen at CT. (b) Follow-up contrast-enhanced CT scan obtained 2 months later shows a hypoattenuating mass causing contour abnormality (arrowhead) in the right ventricular wall. The endometrial metastasis had been visualized at previous PET even before it produced a visible CT abnormality.

 
Ovary.— Although increased ovarian FDG uptake in postmenopausal women is associated with malignancy (Fig 21b, 21c), it may be functional in premenopausal women because a corpus luteum cyst can transiently increase ovarian uptake (Fig 21a). Postsurgical changes in the pelvis can make CT interpretation difficult.


Figure 21
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  		Figure 21a.  (a) Physiologic FDG uptake in the corpus luteum in the postovulation phase of the menstrual cycle. Fused PET-CT scan shows focal increased FDG activity (arrow) in the right ovary corresponding to a corpus luteal cyst. CT can readily depict corpus luteal cysts, especially if the dates of the patient’s last menstrual period are known. (b, c) Pathologic FDG uptake due to ovarian cancer in a different patient. (b) PET scan shows abnormal FDG uptake (arrow), a finding that represents the solid nodular component of a left ovarian carcinoma. (c) CT scan shows a complex left ovarian mass with a solid nodular component (arrow), findings that are consistent with left ovarian carcinoma.

 

Figure 21
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  		Figure 21b.  (a) Physiologic FDG uptake in the corpus luteum in the postovulation phase of the menstrual cycle. Fused PET-CT scan shows focal increased FDG activity (arrow) in the right ovary corresponding to a corpus luteal cyst. CT can readily depict corpus luteal cysts, especially if the dates of the patient’s last menstrual period are known. (b, c) Pathologic FDG uptake due to ovarian cancer in a different patient. (b) PET scan shows abnormal FDG uptake (arrow), a finding that represents the solid nodular component of a left ovarian carcinoma. (c) CT scan shows a complex left ovarian mass with a solid nodular component (arrow), findings that are consistent with left ovarian carcinoma.

 

Figure 21
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  		Figure 21c.  (a) Physiologic FDG uptake in the corpus luteum in the postovulation phase of the menstrual cycle. Fused PET-CT scan shows focal increased FDG activity (arrow) in the right ovary corresponding to a corpus luteal cyst. CT can readily depict corpus luteal cysts, especially if the dates of the patient’s last menstrual period are known. (b, c) Pathologic FDG uptake due to ovarian cancer in a different patient. (b) PET scan shows abnormal FDG uptake (arrow), a finding that represents the solid nodular component of a left ovarian carcinoma. (c) CT scan shows a complex left ovarian mass with a solid nodular component (arrow), findings that are consistent with left ovarian carcinoma.

 
Again, menstrual history is extremely helpful for interpretation, with expected increased uptake in corpus luteal cysts, which also characteristically appear as small, rim-enhancing cysts at contrast-enhanced CT. FDG PET is known to be helpful for differentiation between residual and recurrent disease and for diagnosis when CT is inconclusive due to anatomic distortion (55,56). In one study, the diagnostic accuracy of FDG PET, serum tumor marker CA-125 level, and CT–MR imaging in detecting recurrent ovarian cancer was shown to be 91.7%, 83.3%, and 66.7%, respectively (56). The results are less promising for microscopic metastases and peritoneal disease. FDG PET has a higher specificity than the CA-125 level or conventional CT–MR imaging for detecting recurrent ovarian cancers. Combining the PET and CT information in PET-CT should further improve accuracy (1,9,10).

Prostate Gland.— The evaluation of primary prostate cancer with FDG PET is difficult due to hypometabolism of the tumor.

A positive correlation has been shown between the serum prostate-specific antigen level and both C-acetate uptake and FDG uptake (11,57). C-acetate seems more useful than FDG in the detection of local recurrences and regional lymph node metastases, but its role is still under investigation. Fluorocholine PET-CT is considered a promising imaging modality for detecting local recurrence and lymph node metastases in patients with recurrent prostate cancer (58). The search for nodal disease and distant metastases with CT can again be facilitated with the addition of PET (PET-CT).

Bone
Some bone metastases are difficult to identify with CT alone, and it may be difficult to distinguish increased sclerosis due to progressive metastatic disease from treatment response. Paget disease and fibrous dysplasia may show increased uptake during their active phases (Fig 22).


Figure 22
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  		Figure 22a.  Paget disease. (a) PET scan shows increased FDG activity (arrow) in the left innominate bone, a finding that corresponds to the active phase of Paget disease. Inactive chronic Paget disease tends not to show increased FDG activity. (b) CT scan shows coarsening of the trabecular pattern in the left innominate bone (arrow).

 

Figure 22
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  		Figure 22b.  Paget disease. (a) PET scan shows increased FDG activity (arrow) in the left innominate bone, a finding that corresponds to the active phase of Paget disease. Inactive chronic Paget disease tends not to show increased FDG activity. (b) CT scan shows coarsening of the trabecular pattern in the left innominate bone (arrow).

 
Bone metastases involving the marrow are often more conspicuous at PET, whereas Paget disease and fibrous dysplasia can usually be recognized at CT. FDG PET–CT has high specificity for the detection of malignant involvement of the spine. It allows precise localization of lesions and helps identify accompanying soft-tissue involvement. Comparison of FDG PET findings with corresponding CT findings allows detection of degenerative spondylitis, which is the most common cause of increased FDG uptake in the spine at PET (Fig 23). PET and CT can be complementary in displaying bone abnormalities, with PET excelling in the depiction of marrow abnormalities and CT better depicting cortically based lesions.


Figure 23
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  		Figure 23.  Degenerative spondylitis. Fused PET-CT scan shows increased FDG activity in the vertebral endplates (arrowheads). Corresponding reactive sclerosis from degenerative disk disease of the spine was seen at CT.

 
 The lack of an appreciable abnormality at CT should not dissuade the physician interpreting a PET-CT study that demonstrates significant focal abnormal uptake in bone at PET from suspecting a malignant marrow process. MR imaging can usually help confirm the PET diagnosis if clinically necessary. The diffuse increase in bone marrow activity seen following chemotherapy and the administration of colony-stimulating factors is now well recognized (Fig 24). Indeed, the finding of a metabolic flare at FDG PET performed after institution of tamoxifen treatment appears to help predict responsiveness to antiestrogen therapy in patients with metastatic breast cancer.


Figure 24
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  		Figure 24a.  Diffuse increase in bone marrow activity following chemotherapy and the administration of colony-stimulating factors. (a) Fused PET-CT scan obtained prior to chemotherapy shows no increased bone activity. A focus of FDG activity (arrowhead) is seen in the lower thoracic region, a finding that represents focal degenerative disease. (b) Fused PET-CT scan obtained 5 weeks later following chemotherapy shows intense FDG activity in both the axial and proximal appendicular skeleton. (c) Coronal CT scan shows the normal axial and appendicular skeleton.

 

Figure 24
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  		Figure 24b.  Diffuse increase in bone marrow activity following chemotherapy and the administration of colony-stimulating factors. (a) Fused PET-CT scan obtained prior to chemotherapy shows no increased bone activity. A focus of FDG activity (arrowhead) is seen in the lower thoracic region, a finding that represents focal degenerative disease. (b) Fused PET-CT scan obtained 5 weeks later following chemotherapy shows intense FDG activity in both the axial and proximal appendicular skeleton. (c) Coronal CT scan shows the normal axial and appendicular skeleton.

 

Figure 24
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  		Figure 24c.  Diffuse increase in bone marrow activity following chemotherapy and the administration of colony-stimulating factors. (a) Fused PET-CT scan obtained prior to chemotherapy shows no increased bone activity. A focus of FDG activity (arrowhead) is seen in the lower thoracic region, a finding that represents focal degenerative disease. (b) Fused PET-CT scan obtained 5 weeks later following chemotherapy shows intense FDG activity in both the axial and proximal appendicular skeleton. (c) Coronal CT scan shows the normal axial and appendicular skeleton.

 

   Conclusions
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Abdominopelvic PET-CT Protocol
 General Interpretation Issues
 Specific Interpretation Issues
 Conclusions
References
 
Although PET-CT clearly represents a major technologic advance, the alliance of functional imaging with structural imaging raises a number of controversial issues and introduces some new interpretative challenges. Accurate PET-CT interpretation requires awareness of the pitfalls associated with the imaging components, both individually and in combination.

FDG PET and CT are complementary modalities whose combined strength tends to overcome their respective weaknesses. The results of recent PET-CT studies have been very encouraging, but larger prospective studies are needed to establish optimal hybrid scanning protocols. Applying sound imaging principles and staying abreast of advances in this exciting new modality are necessary for harnessing the full diagnostic power of abdominal PET-CT.


   Footnotes
 

Abbreviations: FDG = fluorodeoxyglucose, RCC = renal cell carcinoma


   References
Top
 Abstract
 LEARNING OBJECTIVES FOR TEST...
 Introduction
 Abdominopelvic PET-CT Protocol
 General Interpretation Issues
 Specific Interpretation Issues
 Conclusions
 References
 

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