DOI: 10.1148/rg.271065080
RadioGraphics 2007;27:145-159
© RSNA, 2007
Bowel Hot Spots at PET-CT1
Hima B. Prabhakar, MD,
Dushyant V. Sahani, MD,
Alan J. Fischman, MD, PhD,
Peter R. Mueller, MD and
Michael A. Blake, MRCPI, FRCR, FFR(RCSI)
1 From the Department of Radiology, Division of Abdominal Imaging and Interventional Radiology, Massachusetts General Hospital, 55 Fruit St, White 270, Boston, MA 02114. Presented as an education exhibit at the 2005 RSNA Annual Meeting. Received April 25, 2006; revision requested May 24 and received September 11; accepted September 25. All authors have no financial relationships to disclose.
Address correspondence to D.V.S. (e-mail: dsahani{at}partners.org).
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Abstract
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Positron emission tomography (PET) with 2-[fluorine-18]fluoro-2-deoxy-D-glucose (FDG) has been shown to be sensitive in the detection of many bowel malignancies, but its specificity is lower because of various physiologic and pathologic patterns of bowel FDG uptake. PET–computed tomography (CT) can be useful in localizing and characterizing foci of increased FDG uptake within the bowel. As the use of PET-CT in the staging and monitoring of oncologic disease continues to expand, familiarity with these patterns of bowel FDG uptake is essential and can help determine the need for and the relative urgency of further testing. Although a variety of imaging protocols are used for PET-CT, the use of negative oral contrast agent allows improved bowel distention while eliminating potential artifacts caused by high-density oral contrast agents. In addition, correlation with the CT portion of the combined PET-CT examination can sometimes help identify the cause of focal or segmental bowel uptake. The radiologist should be aware of potential pitfalls in the evaluation of FDG-avid foci within the abdomen, including bowel motility and low-attenuation lesions mimicking bowel. Nevertheless, even though the precise role of combined PET-CT for bowel assessment has yet to be determined, the application of sound basic principles of image interpretation will help ensure the accurate interpretation of bowel findings seen with this combined modality.
© RSNA, 2007
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:- Discuss the role and technical aspects of PET-CT in the evaluation of bowel disease, including imaging protocols and the use of contrast material.
- Identify physiologic and pathologic patterns of FDG uptake within bowel.
- Describe potential pitfalls in the evaluation of bowel with PET-CT.
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Introduction
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The clinical indications for positron emission tomography (PET)–computed tomography (CT) are similar to those for 2-[fluorine-18]fluoro-2-deoxy-D-glucose (FDG) PET alone, namely, the diagnosis, staging, and follow-up of a variety of malignancies. At PET, FDG uptake within bowel is often nonspecific; however, focal, multifocal, segmental, and (on occasion) diffuse increased bowel uptake may be indicative of underlying bowel disease. Abdominopelvic CT can be useful in detecting both nonfocal bowel disease and focal lesions; however, its sensitivity can be limited by lack of bowel distention. PET-CT combines the sensitivity of PET with the increased specificity of CT for the evaluation of the underlying cause of focal, segmental, or diffuse bowel uptake (1).
To evaluate bowel disease with PET-CT, it is important to understand patterns of physiologic bowel uptake. Moreover, correlation with CT findings is useful for evaluating focal, segmental, or diffuse bowel abnormalities. In this article, we briefly review the role of PET-CT in evaluating the causes of bowel FDG uptake. In addition, we discuss imaging protocols for PET-CT and describe various physiologic and pathologic patterns of FDG uptake within the esophagus, stomach, small bowel, and colon. Finally, we highlight pitfalls and challenges in the interpretation of bowel FDG uptake at PET-CT and review current recommendations for the evaluation of bowel disease.
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Evaluating the Causes of Bowel FDG Uptake with PET-CT
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The variable patterns of bowel uptake at FDG PET beg the question regarding the usefulness of combined PET-CT in the evaluation of bowel disease. Combined PET-CT is especially useful in localizing focal or unexpected FDG-avid bowel abnormalities. This capacity is of particular importance in evaluating abnormalities in the mid-abdomen, where lymphadenopathy and small bowel disease may be difficult to differentiate at PET alone (1).
The evaluation and monitoring of inflammatory bowel disease (IBD) has been another proposed use for FDG PET, particularly in the pediatric population (2–4). CT findings of IBD, in combination with FDG activity, may assist in monitoring disease activity and evaluating for complications of disease.
Numerous studies have shown FDG PET to play an important part in the evaluation and staging of colorectal cancer (5). In addition, recent studies have shown combined PET-CT to be superior to PET, CT, and the computer fusion of independently acquired PET and CT scans in the staging of colorectal cancer (6,7).
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Imaging Protocols
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One of the main advantages of dual PET-CT scanning in the evaluation of bowel is the ability to acquire CT scans during the same scanning session as PET scans. Bowel peristalsis and motility, particularly within small bowel, can make it difficult to correlate foci of FDG uptake with CT findings obtained at a different time.
At our institution, whole-body FDG PET scans are obtained on a REVEAL-XV1 scanner (CTI Molecular Imaging, Knoxville, Tenn) with a spatial resolution of 5.0 mm full width at half maximum and a section thickness of 3.5 mm. Patients fast for at least 6 hours before undergoing scanning, and blood glucose levels are measured prior to the administration of FDG. Bowel cleansing is not used as part of pretest preparation. Sixty minutes after the administration of a bolus of approximately 15 mCi of FDG, static and emission PET scans are obtained. A transmission CT scan is obtained in these patients at 60 mAs, primarily but not exclusively for attenuation correction, using a SOMATOM Sensation 16 scanner (Siemens Medical Systems, Schaumburg, Ill). Fully diagnostic contrast material–enhanced CT scans (standard radiation dose) are acquired during the same session with the REVEAL-XV1 and SOMATOM Sensation 16 scanners, with a section thickness of 5.0 mm and variable milliampere-seconds (average maximum, 180 mAs).
Transmission scans are obtained without intravenously administered contrast material because the presence of high-density contrast material on CT scans obtained for attenuation correction can lead to artifacts on corrected PET images. Although some European investigators have used contrast-enhanced scanning for attenuation correlation, doing so has not yet been accepted as standard practice (8). Nevertheless, intravenous contrast material is important in the acquisition of optimized CT scans in terms of vascular enhancement and parenchymal organ enhancement (particularly within the abdomen). Specifically, in the evaluation of bowel disease, intravenous contrast material can be helpful in evaluating bowel wall enhancement patterns as well as in distinguishing lymphadenopathy or masses from bowel (particularly small bowel). All patients without a known contraindication (eg, contrast material allergy, renal insufficiency) receive intravenous contrast material. The unenhanced transmission scan is used primarily for attenuation correction, as well as for evaluation of calcifications or disease that may not correlate as well on the enhanced CT scans (acquired after the PET portion of the study) secondary to bowel peristalsis or change in patient position.
In the evaluation of bowel disease at PET-CT, orally administered contrast material is helpful for bowel distention. However, because of the way CT data are used for attenuation correction in PET-CT, the use of high-density barium-based contrast agents can cause overestimation of PET activity. Briefly, CT attenuation correction algorithms create an attenuation map for the emitted 511-keV photons from FDG with use of a scaling factor for either bone or soft tissue. High-density barium-based oral contrast material will attenuate more CT x-rays, with energies of 70–140 keV, than 511-keV photons. This phenomenon can lead to an overestimation in PET activity of up to 20% in regions where barium-based oral contrast material is present. More pronounced overestimation may occur with both (a) delayed passage of oral contrast material, with higher concentrations of intraluminal barium; and (b) rapid transit of oral contrast material, with areas previously demonstrating high attenuation now being clear (8).
For our PET-CT examinations, all patients receive VoLumen (E-Z-EM, Lake Success, NY), a low-density barium sulfate suspension that acts as a negative oral contrast agent (Fig 1). Because negative oral contrast material causes bowel distention without increasing CT attenuation, it helps avoid the potential issues with attenuation artifact that can be seen with high-density barium-based oral contrast material (9–11). VoLumen (E-Z-EM) contains no mannitol and has a 0.1% wt/vol of barium sulfate.
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Figure 1a. Bowel distention. Axial and coronal fused PET-CT images obtained after the oral administration of 1800 mL of VoLumen (E-Z-EM) demonstrate distention of the stomach (a), small bowel (b), and colon (c) (arrow).
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Figure 1b. Bowel distention. Axial and coronal fused PET-CT images obtained after the oral administration of 1800 mL of VoLumen (E-Z-EM) demonstrate distention of the stomach (a), small bowel (b), and colon (c) (arrow).
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Figure 1c. Bowel distention. Axial and coronal fused PET-CT images obtained after the oral administration of 1800 mL of VoLumen (E-Z-EM) demonstrate distention of the stomach (a), small bowel (b), and colon (c) (arrow).
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Recent studies have shown the superiority of negative oral contrast material containing mannitol and small bowel to both water and oral contrast material containing high-density barium for distention of the stomach. With increased bowel distention, disease affecting the bowel wall can be better assessed. Potential side effects of using mannitol-based negative oral contrast material include watery diarrhea. A recent study comparing a combined mannitol- and locust bean gum–based contrast agent with mannitol and water alone found that two of 20 patients had watery diarrhea with the combined agent compared with 83% of patients with mannitol alone. In addition, patients who received the mannitol- and locust bean gum–based contrast agent did not experience the brisk diuresis that can follow the ingestion of large amounts of water (8).
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Causes of Physiologic FDG Uptake within Bowel
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Within the body, FDG uptake is highest in the brain and heart. The causes of physiologic FDG uptake within bowel are not known; however, this uptake has been postulated to be secondary to metabolically active smooth muscle and mucosa, swallowed secretion, or microbial uptake (12).
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Physiologic versus Pathologic Patterns of Bowel Uptake
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FDG uptake within the normal gastrointestinal tract is highly variable. Patterns of physiologic bowel uptake have been described as ranging from diffuse low-level uptake (Fig 2) to heterogeneous and multifocal uptake (Fig 3) (12).
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Figure 2a. Diffuse physiologic bowel uptake in a 47-year-old human immunodeficiency virus–positive man with T-cell lymphoma. Coronal fused PET-CT image (a) and unfused PET scan (b) show diffuse low-level physiologic FDG uptake throughout the small and large bowel. No focal FDG uptake is identified. Note the slightly more prominent uptake within the cecum and right colon, which may be secondary to a relatively high concentration of lymphocytes in this region.
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Figure 2b. Diffuse physiologic bowel uptake in a 47-year-old human immunodeficiency virus–positive man with T-cell lymphoma. Coronal fused PET-CT image (a) and unfused PET scan (b) show diffuse low-level physiologic FDG uptake throughout the small and large bowel. No focal FDG uptake is identified. Note the slightly more prominent uptake within the cecum and right colon, which may be secondary to a relatively high concentration of lymphocytes in this region.
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Figure 3a. Heterogeneous physiologic bowel uptake in a 43-year-old woman with nasopharyngeal lymphoma. Coronal fused PET-CT image (a) and unfused PET scan (b) show mild FDG uptake at the fundus of the stomach and heterogeneous FDG uptake throughout the small and large bowel, without prominent focal-segmental uptake. The patient had no small or large bowel disease; thus, these findings again represent a normal variant of physiologic bowel uptake (cf Fig 2).
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Figure 3b. Heterogeneous physiologic bowel uptake in a 43-year-old woman with nasopharyngeal lymphoma. Coronal fused PET-CT image (a) and unfused PET scan (b) show mild FDG uptake at the fundus of the stomach and heterogeneous FDG uptake throughout the small and large bowel, without prominent focal-segmental uptake. The patient had no small or large bowel disease; thus, these findings again represent a normal variant of physiologic bowel uptake (cf Fig 2).
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In the absence of combined PET-CT, it is particularly important to recognize patterns of FDG uptake that could correspond to bowel so as to distinguish physiologic from pathologic uptake. For example, linear and J-shaped uptake in the upper abdomen can be seen with FDG uptake within the gastric wall. Physiologic focal uptake —sometimes intense—has been described within the right lower quadrant, corresponding to the cecum and right colon, and is thought to be related to the high concentration of glucose-metabolizing lymphatic cells in this region (12). On earlier studies, intense focal colonic uptake has been correlated with focal primary or metastatic neoplasm, whereas segmental uptake has been described with inflammatory processes (2,13).
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Pathologic Uptake at PET-CT
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Esophagus
Although PET-CT is not a screening test for benign disease of the esophagus, esophageal uptake is sometimes identified in patients who undergo scanning for other indications. Faint linear physiologic uptake has been described within the distal esophagus, anterior to the spine, and is thought to represent both swallowed secretions and smooth muscle activity. Mild to moderate linear FDG uptake in the esophagus has been associated with reflux esophagitis (Fig 4). In patients with a history of gastroesophageal reflux, the pattern is most commonly seen in the distal esophagus (14). In patients with a history of radiation therapy, the pattern of involvement may reflect radiation esophagitis within the irradiation zone (15). In addition, more intense linear uptake has been described with Barrett esophagus, usually within the distal third of the esophagus (14,16). Correlation with fused PET-CT images may be helpful for excluding focal abnormality; in general, however, the collapsed esophagus can make evaluation more challenging. Because Barrett esophagitis is a premalignant condition, it is reasonable to recommend endoscopic correlation in patients with moderate to intense linear esophageal uptake.
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Figure 4a. Esophagitis in a 63-year-old woman with nasopharyngeal carcinoma who presented with complaints of xerostomia and dysphagia. The patient was undergoing chemotherapy and radiation therapy. (a) Coronal staging CT scan shows mild esophageal thickening involving the middle to distal esophagus (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate linear low-level FDG uptake (arrow), a finding that corresponds to the esophageal thickening seen at CT.
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Figure 4b. Esophagitis in a 63-year-old woman with nasopharyngeal carcinoma who presented with complaints of xerostomia and dysphagia. The patient was undergoing chemotherapy and radiation therapy. (a) Coronal staging CT scan shows mild esophageal thickening involving the middle to distal esophagus (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate linear low-level FDG uptake (arrow), a finding that corresponds to the esophageal thickening seen at CT.
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Figure 4c. Esophagitis in a 63-year-old woman with nasopharyngeal carcinoma who presented with complaints of xerostomia and dysphagia. The patient was undergoing chemotherapy and radiation therapy. (a) Coronal staging CT scan shows mild esophageal thickening involving the middle to distal esophagus (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate linear low-level FDG uptake (arrow), a finding that corresponds to the esophageal thickening seen at CT.
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Although benign entities such as glycogenic acanthosis have been described as demonstrating focal esophageal uptake (16), when esophageal FDG uptake is distinctly focal, and particularly when it is associated with a mass on fused PET-CT images, esophageal cancer is a serious concern. Prior studies have described increased FDG uptake in both primary squamous cell carcinoma and adenocarcinoma of the esophagus. Whereas esophageal adenocarcinoma tends to occur distally near the gastroesophageal junction (Fig 5), squamous cell carcinoma tends to occur more frequently within the proximal esophagus (17). Thus, when focal esophageal FDG uptake is identified, it is important to recommend endoscopic correlation to exclude malignant and pre-malignant causes, since the early detection of such causes could potentially have significant treatment implications.
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Figure 5a. Esophageal adenocarcinoma in a 59-year-old man who presented with progressive dysphagia and weight loss. Upper endoscopy demonstrated a distal esophageal mass, and biopsy revealed poorly differentiated esophageal adenocarcinoma. (a) Coronal CT scan demonstrates a partially obstructive distal esophageal mass with proximal esophageal dilatation (arrow). (b, c) Coronal PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake (arrow) corresponding to the CT findings.
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Figure 5b. Esophageal adenocarcinoma in a 59-year-old man who presented with progressive dysphagia and weight loss. Upper endoscopy demonstrated a distal esophageal mass, and biopsy revealed poorly differentiated esophageal adenocarcinoma. (a) Coronal CT scan demonstrates a partially obstructive distal esophageal mass with proximal esophageal dilatation (arrow). (b, c) Coronal PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake (arrow) corresponding to the CT findings.
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Figure 5c. Esophageal adenocarcinoma in a 59-year-old man who presented with progressive dysphagia and weight loss. Upper endoscopy demonstrated a distal esophageal mass, and biopsy revealed poorly differentiated esophageal adenocarcinoma. (a) Coronal CT scan demonstrates a partially obstructive distal esophageal mass with proximal esophageal dilatation (arrow). (b, c) Coronal PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake (arrow) corresponding to the CT findings.
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Stomach
Gastric wall inflammation is a common entity that has a number of predisposing factors, including medications (eg, aspirin or other nonsteroidal anti-inflammatory drugs) and infectious causes such as Helicobacter pylori. CT findings are often nonspecific, and an inflammatory cause of gastric wall thickening can be difficult to distinguish from a possible underlying infiltrative neoplasm such as lymphoma or gastric adenocarcinoma (18). FDG PET uptake in the stomach has been described as typically mild with a J-shaped configuration (Fig 3) (12). Diffusely increased gastric uptake has been described in a patient with non-specific gastritis as well as in a patient with infectious gastritis (19,20). Although neither CT nor PET demonstrates high specificity for the detection of gastritis, the intensity and configuration of FDG uptake (J shaped or diffuse), in combination with gastric wall thickening, particularly in a distended stomach, may indicate underlying inflammatory or infectious gastritis (Fig 6). In these instances, correlation with endoscopic biopsy findings or with the patient’s symptoms may be useful, particularly in excluding the possibility of an underlying infiltrative gastric neoplasm.
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Figure 6a. Gastritis in a 57-year-old man with Hodgkin lymphoma of the left groin. (a) CT scan shows gastric wall thickening (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake involving the gastric fundus and body (arrow), a finding that corresponds to the gastric wall thickening seen at CT. (d) CT scan obtained following treatment with ranitidine shows a moderate decrease in gastric wall thickening (arrow). (e, f ) Axial fused PET-CT image (e) and PET scan (f ) demonstrate markedly decreased FDG uptake involving the stomach (arrow) compared with the pretreatment images (cf a–c).
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Figure 6b. Gastritis in a 57-year-old man with Hodgkin lymphoma of the left groin. (a) CT scan shows gastric wall thickening (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake involving the gastric fundus and body (arrow), a finding that corresponds to the gastric wall thickening seen at CT. (d) CT scan obtained following treatment with ranitidine shows a moderate decrease in gastric wall thickening (arrow). (e, f ) Axial fused PET-CT image (e) and PET scan (f ) demonstrate markedly decreased FDG uptake involving the stomach (arrow) compared with the pretreatment images (cf a–c).
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Figure 6c. Gastritis in a 57-year-old man with Hodgkin lymphoma of the left groin. (a) CT scan shows gastric wall thickening (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake involving the gastric fundus and body (arrow), a finding that corresponds to the gastric wall thickening seen at CT. (d) CT scan obtained following treatment with ranitidine shows a moderate decrease in gastric wall thickening (arrow). (e, f ) Axial fused PET-CT image (e) and PET scan (f ) demonstrate markedly decreased FDG uptake involving the stomach (arrow) compared with the pretreatment images (cf a–c).
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Figure 6d. Gastritis in a 57-year-old man with Hodgkin lymphoma of the left groin. (a) CT scan shows gastric wall thickening (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake involving the gastric fundus and body (arrow), a finding that corresponds to the gastric wall thickening seen at CT. (d) CT scan obtained following treatment with ranitidine shows a moderate decrease in gastric wall thickening (arrow). (e, f ) Axial fused PET-CT image (e) and PET scan (f ) demonstrate markedly decreased FDG uptake involving the stomach (arrow) compared with the pretreatment images (cf a–c).
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Figure 6e. Gastritis in a 57-year-old man with Hodgkin lymphoma of the left groin. (a) CT scan shows gastric wall thickening (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake involving the gastric fundus and body (arrow), a finding that corresponds to the gastric wall thickening seen at CT. (d) CT scan obtained following treatment with ranitidine shows a moderate decrease in gastric wall thickening (arrow). (e, f ) Axial fused PET-CT image (e) and PET scan (f ) demonstrate markedly decreased FDG uptake involving the stomach (arrow) compared with the pretreatment images (cf a–c).
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Figure 6f. Gastritis in a 57-year-old man with Hodgkin lymphoma of the left groin. (a) CT scan shows gastric wall thickening (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate intense FDG uptake involving the gastric fundus and body (arrow), a finding that corresponds to the gastric wall thickening seen at CT. (d) CT scan obtained following treatment with ranitidine shows a moderate decrease in gastric wall thickening (arrow). (e, f ) Axial fused PET-CT image (e) and PET scan (f ) demonstrate markedly decreased FDG uptake involving the stomach (arrow) compared with the pretreatment images (cf a–c).
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In contrast, when focal uptake is identified within the stomach, PET-CT images should be carefully evaluated for the presence of an underlying gastric mass. A number of studies have shown FDG PET to be sensitive for the detection of primary as well as metastatic and recurrent gastric adenocarcinoma (21–23). FDG uptake within gastric adenocarcinoma has been found to be variable, with some poorly differentiated tumors showing less intense uptake than other histologic subtypes (21). Gastrointestinal stromal tumors (GISTs) of the stomach, on the other hand, are more consistently FDG avid (Fig 7) (24). In addition, with treatment, GISTs can show decreased FDG avidity prior to any detectable change at CT, so that combined PET-CT is a useful adjunct in monitoring treatment in these patients (24,25).
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Figure 7a. GIST in a 67-year-old man. The mass was incidentally detected at upper endoscopy performed to evaluate for guaiac-positive stool. (a) CT scan shows a proximal soft-tissue gastric mass (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate increased FDG uptake (arrow), a finding that corresponds to the mass seen at CT. Biopsy of the mass revealed a GIST.
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Figure 7b. GIST in a 67-year-old man. The mass was incidentally detected at upper endoscopy performed to evaluate for guaiac-positive stool. (a) CT scan shows a proximal soft-tissue gastric mass (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate increased FDG uptake (arrow), a finding that corresponds to the mass seen at CT. Biopsy of the mass revealed a GIST.
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Figure 7c. GIST in a 67-year-old man. The mass was incidentally detected at upper endoscopy performed to evaluate for guaiac-positive stool. (a) CT scan shows a proximal soft-tissue gastric mass (arrow). (b, c) Axial fused PET-CT image (b) and PET scan (c) demonstrate increased FDG uptake (arrow), a finding that corresponds to the mass seen at CT. Biopsy of the mass revealed a GIST.
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Small Bowel
The small bowel typically demonstrates heterogeneous low-level FDG uptake; however, this uptake can appear more focally intense in the pelvis with overlying bowel loops (26). Multiple patterns of bowel wall thickening and enhancement have been described at contrast-enhanced CT, findings that can aid in the interpretation of underlying bowel disease. One recognized pitfall in the interpretation of small and large bowel disease is underdistention, which can simulate the appearance of bowel wall thickening (27).
Infectious processes within the colon have been shown to demonstrate diffuse or segmental uptake; to our knowledge, however, no cases of infectious enteritis have been described in the literature. Presumably, small bowel uptake would be similar to colonic uptake and demonstrate either a segmental or a diffuse pattern, depending on the extent of bowel involvement (28). In combination with typical patterns of bowel wall enhancement seen at contrast-enhanced CT (27), diffuse FDG uptake within the small bowel could potentially be ascribed to an infectious or inflammatory cause with greater certainty (Fig 8).
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Figure 8a. Infectious enteritis in a 50-year-old human immunodeficiency virus–positive man with diffuse adenopathy of unknown cause. (a) Coronal CT scan obtained after the administration of negative oral contrast material for small bowel distention clearly demonstrates mild ileal wall thickening (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate diffuse low-level uptake within loops of ileum (arrow). Stool cultures were positive for Salmonella and Campylobacter species.
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Figure 8b. Infectious enteritis in a 50-year-old human immunodeficiency virus–positive man with diffuse adenopathy of unknown cause. (a) Coronal CT scan obtained after the administration of negative oral contrast material for small bowel distention clearly demonstrates mild ileal wall thickening (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate diffuse low-level uptake within loops of ileum (arrow). Stool cultures were positive for Salmonella and Campylobacter species.
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Figure 8c. Infectious enteritis in a 50-year-old human immunodeficiency virus–positive man with diffuse adenopathy of unknown cause. (a) Coronal CT scan obtained after the administration of negative oral contrast material for small bowel distention clearly demonstrates mild ileal wall thickening (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate diffuse low-level uptake within loops of ileum (arrow). Stool cultures were positive for Salmonella and Campylobacter species.
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FDG PET has been proposed as a useful adjunct in the monitoring of IBD, particularly within the pediatric population. In patients with known Crohn disease, for example, FDG PET has been shown to be both sensitive and specific in the detection of active sites of disease within both the small bowel and the colon (Fig 9) (2,29). Because of its high sensitivity, FDG PET has been proposed as a noninvasive way of monitoring pediatric IBD, both Crohn disease and ulcerative colitis (30). CT findings in IBD are well known, and the usefulness of CT in assessing complications such as abdominal abscess has been shown. In combining the demonstrated sensitivity of FDG PET with the increased resolution of CT, PET-CT may be helpful in the evaluation of IBD.
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Figure 9a. Terminal ileitis in a 62-year-old man with a solitary pulmonary nodule and a prior history of Crohn disease. (a) Coronal CT scan shows wall thickening in the terminal ileum (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate segmental increased FDG uptake (arrow), a finding that corresponds to the CT finding and is consistent with the patient’s history of Crohn disease.
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Figure 9b. Terminal ileitis in a 62-year-old man with a solitary pulmonary nodule and a prior history of Crohn disease. (a) Coronal CT scan shows wall thickening in the terminal ileum (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate segmental increased FDG uptake (arrow), a finding that corresponds to the CT finding and is consistent with the patient’s history of Crohn disease.
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Figure 9c. Terminal ileitis in a 62-year-old man with a solitary pulmonary nodule and a prior history of Crohn disease. (a) Coronal CT scan shows wall thickening in the terminal ileum (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate segmental increased FDG uptake (arrow), a finding that corresponds to the CT finding and is consistent with the patient’s history of Crohn disease.
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Both primary and metastatic malignancies can affect the small bowel, and both can demonstrate increased FDG uptake. GISTs typically show FDG avidity and can be found in the small bowel as focal lesions. In addition, lymphomatous involvement of the small bowel has been described as showing marked FDG avidity. Metastatic involvement of the small bowel has been described most frequently in patients with melanoma, which typically shows high FDG avidity (31,32). Conventional imaging (including CT) is typically insensitive for the detection of bowel metastases; however, with the increased ability to localize bowel lesions provided by fused images, PET-CT has been shown to help detect unsuspected bowel metastases (33,34).
Colon
FDG activity within the colon is typically heterogeneous and can vary in distribution from mild focal to diffuse uptake. Often, there is higher uptake within the cecum and right colon, presumably due to the higher concentration of lymphocytes in this region (12).
Diffusely increased FDG uptake has been described with inflammatory enterocolitis, including collagenous colitis. In patients with collagenous colitis, conventional imaging (including CT) is often nondiagnostic (Fig 10) (35).
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Figure 10a. Collagenous colitis in a 69-year-old woman with bladder, ovarian, and lung cancer. (a) Coronal CT scan demonstrates no evidence of colonic wall thickening (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate increased FDG uptake involving the ascending colon (arrow). Colonoscopy and subsequent biopsy revealed collagenous colitis, a rare condition that is diagnosed in patients with chronic watery diarrhea in whom conventional radiography or endoscopy demonstrates a healthy colon but colonic biopsies show unique inflammatory changes. The characteristic features of collagenous colitis are infiltration of lymphocytes into the colonic epithelium and distinctive thickening of the subepithelial collagen table.
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Figure 10b. Collagenous colitis in a 69-year-old woman with bladder, ovarian, and lung cancer. (a) Coronal CT scan demonstrates no evidence of colonic wall thickening (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate increased FDG uptake involving the ascending colon (arrow). Colonoscopy and subsequent biopsy revealed collagenous colitis, a rare condition that is diagnosed in patients with chronic watery diarrhea in whom conventional radiography or endoscopy demonstrates a healthy colon but colonic biopsies show unique inflammatory changes. The characteristic features of collagenous colitis are infiltration of lymphocytes into the colonic epithelium and distinctive thickening of the subepithelial collagen table.
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Figure 10c. Collagenous colitis in a 69-year-old woman with bladder, ovarian, and lung cancer. (a) Coronal CT scan demonstrates no evidence of colonic wall thickening (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate increased FDG uptake involving the ascending colon (arrow). Colonoscopy and subsequent biopsy revealed collagenous colitis, a rare condition that is diagnosed in patients with chronic watery diarrhea in whom conventional radiography or endoscopy demonstrates a healthy colon but colonic biopsies show unique inflammatory changes. The characteristic features of collagenous colitis are infiltration of lymphocytes into the colonic epithelium and distinctive thickening of the subepithelial collagen table.
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Focally increased FDG uptake within the bowel has been described for both malignant and benign processes. Nonneoplastic causes that have been described include appendicitis, diverticulitis (Fig 11), and focal abdominal or pelvic abscesses (Fig 12) (28,33,36). PET-CT findings in these cases may be diagnostic, since the CT manifestations of these entities are well described in the literature.
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Figure 11a. Perforated diverticulitis in a 64-year-old man who had recently undergone left hemicolectomy for stage IIIB colon cancer. (a) Coronal CT scan shows diverticulitis with perforation, small extraluminal collections, and pericolonic inflammatory changes (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate FDG uptake in the sigmoid colon (arrow), a finding that corresponds to the CT findings.
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Figure 11b. Perforated diverticulitis in a 64-year-old man who had recently undergone left hemicolectomy for stage IIIB colon cancer. (a) Coronal CT scan shows diverticulitis with perforation, small extraluminal collections, and pericolonic inflammatory changes (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate FDG uptake in the sigmoid colon (arrow), a finding that corresponds to the CT findings.
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Figure 11c. Perforated diverticulitis in a 64-year-old man who had recently undergone left hemicolectomy for stage IIIB colon cancer. (a) Coronal CT scan shows diverticulitis with perforation, small extraluminal collections, and pericolonic inflammatory changes (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate FDG uptake in the sigmoid colon (arrow), a finding that corresponds to the CT findings.
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Figure 12a. Perianal fistula in a 49-year-old man with longstanding Crohn disease who presented with rectal cancer. (a) Coronal CT scan shows a perianal fistula with a small abscess (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate focal FDG uptake in the region of the anus (arrow), a finding that corresponds to the fistula seen at CT.
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Figure 12b. Perianal fistula in a 49-year-old man with longstanding Crohn disease who presented with rectal cancer. (a) Coronal CT scan shows a perianal fistula with a small abscess (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate focal FDG uptake in the region of the anus (arrow), a finding that corresponds to the fistula seen at CT.
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Figure 12c. Perianal fistula in a 49-year-old man with longstanding Crohn disease who presented with rectal cancer. (a) Coronal CT scan shows a perianal fistula with a small abscess (arrow). (b, c) Coronal fused PET-CT image (b) and PET scan (c) demonstrate focal FDG uptake in the region of the anus (arrow), a finding that corresponds to the fistula seen at CT.
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Colorectal cancer is the second leading cause of cancer-related death in the Western world. FDG PET has been shown to be highly sensitive in the detection of primary colonic neoplasm (Fig 13); however, its specificity is lower because of physiologic uptake as well as inflammatory causes.
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Figure 13a. Poorly differentiated rectal adenocarcinoma in a 71-year-old man. (a) Sagittal CT scan demonstrates an enhancing soft-tissue mass along the posterior rectum (arrow). (b, c) Sagittal fused PET-CT image (b) and PET scan (c) demonstrate marked FDG uptake in the rectum (arrow), a finding that corresponds to the mass seen at CT.
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Figure 13b. Poorly differentiated rectal adenocarcinoma in a 71-year-old man. (a) Sagittal CT scan demonstrates an enhancing soft-tissue mass along the posterior rectum (arrow). (b, c) Sagittal fused PET-CT image (b) and PET scan (c) demonstrate marked FDG uptake in the rectum (arrow), a finding that corresponds to the mass seen at CT.
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Figure 13c. Poorly differentiated rectal adenocarcinoma in a 71-year-old man. (a) Sagittal CT scan demonstrates an enhancing soft-tissue mass along the posterior rectum (arrow). (b, c) Sagittal fused PET-CT image (b) and PET scan (c) demonstrate marked FDG uptake in the rectum (arrow), a finding that corresponds to the mass seen at CT.
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The primary use of FDG PET in colon cancer staging relates to the detection of regional lymph nodes and distant metastatic disease, for which this modality has high sensitivity (5,37,38). Although some studies have shown PET to be superior to CT in the detection of hepatic metastases, others have shown it to have limited sensitivity in the detection of subcentimeter hepatic lesions. Consequently, CT remains the principal means of detecting hepatic metastases in patients with colorectal cancer (32).
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Interpretation Pitfalls and Challenges
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One of the most important potential pitfalls in the interpretation of FDG-avid foci within and adjacent to bowel is bowel motility. Whereas the retroperitoneal portions of the gastrointestinal tract remain fixed, the intraperitoneal portions are mobile. This fact may be more of an issue with separately acquired PET and CT scans because, depending on bowel distention and patient positioning, focal findings within the bowel may change location between scans. In some cases, depending on bowel motility, small lesions such as serosal implants may change location between unenhanced and enhanced scans if, as in our protocol, these scans are separated by the duration of the PET acquisition.
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Current Recommendations
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Abstract
LEARNING OBJECTIVES
