DOI: 10.1148/rg.251045074
RadioGraphics 2005;25:69-85
© RSNA, 2005
CT and MR Imaging of Extrahepatic Fatty Masses of the Abdomen and Pelvis: Techniques, Diagnosis, Differential Diagnosis, and Pitfalls1
Jose M. Pereira, MD,
Claude B. Sirlin, MD,
Pedro S. Pinto, MD and
Giovanna Casola, MD
1 From the Department of Radiology, Porto Medical School, Hospital S. João, Porto, Portugal (J.M.P., P.S.P.); and the Department of Radiology, University of California Medical Center, 200 W Arbor Dr, San Diego, CA 92103-8756 (C.B.S., G.C.). Presented as an education exhibit at the 2003 RSNA Scientific Assembly. Received April 14, 2004; revision requested May 18 and received June 29; accepted June 30. All authors have no financial relationships to disclose. Address correspondence to C.B.S. (e-mail: csirlin@ucsd.edu).
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Abstract
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The differential diagnosis of extrahepatic abdominopelvic masses is wide. Demonstration of fat within a lesion at noninvasive imaging is an important clue for narrowing the differential diagnosis. Macroscopic fat is readily identified with both computed tomography (CT) and magnetic resonance (MR) imaging. Demonstration of microscopic fat is more difficult and may require special techniques. Identification of fat with CT is based on x-ray resorption and therefore on the attenuation (typically less than –20 HU). Several MR imaging techniques have been developed for fat suppression. Two of the most widely available are spectroscopic fat saturation and chemical shift (in-phase/opposed-phase) imaging. Entities with predominantly macroscopic fat include myelolipoma, angiomyolipoma, teratoma, liposarcoma, lipoma, epiploic appendagitis, omental infarction, and mesenteric panniculitis. Lesions with predominantly microscopic fat include adrenal adenoma and some teratomas. Other fat-containing entities involve the mesentery and bowel wall; these include fibrofatty mesenteric proliferation and submucosal fat deposition.
© RSNA, 2005
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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 how to identify intralesion fat with CT and different MR imaging techniques.
- List the major differential diagnoses for the presence of fat in an extrahepatic abdominopelvic lesion.
- Describe the main clinical and radiologic features of fat-containing lesions in the abdomen and pelvis.
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Introduction
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Extrahepatic fat-containing masses of the abdomen and pelvis represent a broad spectrum of congenital, metabolic, inflammatory, traumatic, degenerative, and neoplastic processes. They can be divided into fat-containing neoplasms, fat-containing nonneoplastic masses, and other abdominopelvic fatty masses. Lesions with predominantly macroscopic fat include myelolipoma, angiomyolipoma, lipoma, liposarcoma, teratoma, epiploic appendagitis, fat infarction, and mesenteric panniculitis. Lesions with predominantly microscopic fat include adrenal adenoma and some teratomas. Other less common fatty masses include diaphragmatic and abdominal wall hernias, intussusception, fibrofatty mesenteric proliferation, and submucosal fat deposition.
Knowledge of clinical, anatomic, and imaging features is important in formulating an appropriate differential diagnosis and guiding patient care. The location of the fat-containing lesion is a critical parameter in formulating the correct diagnosis. Definitive noninvasive diagnosis of fatty lesions is usually possible with computed tomography (CT) or magnetic resonance (MR) imaging. Ultrasound (US) findings may be suggestive but are rarely diagnostic, and CT or MR imaging confirmation is usually necessary. Consequently, this review emphasizes CT and MR imaging. Correlative US findings are shown in select cases.
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How to Identify Intralesion Fat
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Computed Tomography
Identification of fat at computed x-ray tomography (CT) is based on x-ray resorption and therefore attenuation. Fat has lower attenuation than water. Hence, if the proportion of fat within a voxel is large, the corresponding image pixel will be dark, typically measuring less than –20 HU (Fig 1) (1). Identification of macroscopic fat is usually simple, although high spatial resolution may be required to depict minute quantities. Microscopic fat may be difficult to identify reliably, as mixing of higher-attenuation water and protein increases the mean CT number. For this reason, CT is not as sensitive for detecting microscopic fat as MR imaging (2).
MR Imaging
Fat has short T1 and relatively long T2 relaxation and thus appears hyperintense on T1-weighted and intermediately intense to hyperintense on T2-weighted fast spin-echo and gradient-echo images. However, these signal intensities are nonspecific, and signal intensity on T1- and T2-weighted images does not allow reliable identification of fat. To reliably identify fat, it is necessary to exploit the different resonance frequencies of water and fat protons.
Chemical shift artifacts are caused by spatial mismapping of fatty and water-based tissue along the frequency-encoding axis. The shift in pixels is equal to the chemical shift (in hertz) between fat and water protons (approximately 220 Hz at 1.5 T and 440 Hz at 3 T) divided by the bandwidth (in hertz per pixel). Thus, this artifact is more pronounced at high field strength or with low bandwidth pulse sequences. This kind of chemical shift artifact occurs when spin-echo and gradient-echo sequences are used (3).
Chemical shift also explains in-phase/opposed-phase imaging. Water and fat signal may be in phase (signal is additive) or opposed phase (signal is canceled), depending on the echo time (3). Typically, at 1.5 T, fat and water signal are maximally out of phase when an echo time of 2.2–2.3 msec is used (or 6.6–6.9 msec, 11.0–11.5 msec, etc) and maximally in phase when an echo time of 4.4–4.6 msec is used (or 8.8–9.2 msec, 13.2–13.8 msec, etc) (Fig 2).
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Figure 2a. Mature cystic teratoma of the ovary in a 27-year-old woman. (a) Axial in-phase gradient-echo MR image (repetition time msec/echo time msec = 120/4.4) shows a heterogeneous predominantly hyperintense mass in the right ovary (arrow). The hyperintense component is consistent with fat or hemorrhage. (b) Axial out-of-phase gradient-echo MR image (120/2.2) shows partial signal loss in the central nodule. More important, there is dramatic signal loss at the interface between the lesion and the water-based ovarian tissue stretched around it (arrowheads). This finding indicates that the lesion contains fat. (c) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal loss (arrow) relative to the signal intensity on the in-phase image (a), an appearance indicative of a predominantly macroscopic fatty component.
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Figure 2b. Mature cystic teratoma of the ovary in a 27-year-old woman. (a) Axial in-phase gradient-echo MR image (repetition time msec/echo time msec = 120/4.4) shows a heterogeneous predominantly hyperintense mass in the right ovary (arrow). The hyperintense component is consistent with fat or hemorrhage. (b) Axial out-of-phase gradient-echo MR image (120/2.2) shows partial signal loss in the central nodule. More important, there is dramatic signal loss at the interface between the lesion and the water-based ovarian tissue stretched around it (arrowheads). This finding indicates that the lesion contains fat. (c) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal loss (arrow) relative to the signal intensity on the in-phase image (a), an appearance indicative of a predominantly macroscopic fatty component.
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Figure 2c. Mature cystic teratoma of the ovary in a 27-year-old woman. (a) Axial in-phase gradient-echo MR image (repetition time msec/echo time msec = 120/4.4) shows a heterogeneous predominantly hyperintense mass in the right ovary (arrow). The hyperintense component is consistent with fat or hemorrhage. (b) Axial out-of-phase gradient-echo MR image (120/2.2) shows partial signal loss in the central nodule. More important, there is dramatic signal loss at the interface between the lesion and the water-based ovarian tissue stretched around it (arrowheads). This finding indicates that the lesion contains fat. (c) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal loss (arrow) relative to the signal intensity on the in-phase image (a), an appearance indicative of a predominantly macroscopic fatty component.
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Frequency-selective fat suppression techniques reduce the signal from lipid-containing voxels by applying a preparatory pulse with an appropriate frequency to saturate fat protons (4). This technique effectively reduces macroscopic fat signal, such as in lipomas and most teratomas. Frequency-selective fat suppression requires a homogeneous external magnetic field. In regions of magnetic heterogeneity, such as at air–soft tissue interfaces, frequency-selective fat suppression frequently fails.
Chemical shift (in-phase/opposed-phase) imaging is more sensitive to microscopic quantities of fat than frequency-selective fat suppression (or attenuation-based CT methods). The reason is that opposed-phase imaging causes signal from fat and water to cancel out. Frequency-selective fat suppression simply eliminates signal from fat, so a larger amount of fat is required for its effect to be noticeable (5).
An advantage of frequency-selective fat suppression is that it is available for both spin-echo and gradient-echo pulse sequences. In-phase/opposed-phase chemical shift imaging generally is possible only with gradient-echo imaging because the 180o refocusing pulse(s) of spin-echo and fast spin-echo sequences bring fat and water protons back into phase and eliminate the phase differences.
Signal intensity from fat can also be suppressed with short inversion time inversion-recovery (STIR) techniques (6). However, as signal suppression in this technique depends on T1 properties of tissues, other tissue components with a T1 similar to that of fat (subacute hemorrhagic products) also lose signal. Therefore, STIR should not be used to characterize fat. STIR should also not be used after administration of a paramagnetic contrast agent such as a gadolinium chelate, because the T1 of tissues accumulating the agent may become similar to the T1 of fat, resulting in signal reduction of the target tissue and loss of contrast enhancement.
Other methods of identifying fat are water-selective spatial spectral techniques, Dixon techniques, and MR spectroscopy. These are not widely used in abdominopelvic applications now and are not discussed herein.
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Fat-containing Neoplasms
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Myelolipoma
Myelolipoma is an uncommon benign tumor composed of mature adipose cells and hematopoietic tissue. The prevalence in autopsy series is between 0.08% and 0.2% (7). Typically, myelolipoma arises in the adrenal gland. Extra-adrenal myelolipoma is rare and is found most commonly in the presacral and other retroperitoneal areas (8). Usually asymptomatic and discovered incidentally at cross-sectional imaging, myelolipoma occasionally causes discomfort due to compression or hemorrhage.
The CT features are characteristic. Lesions usually have a negative Hounsfield unit value owing to macroscopic fat (Fig 1). Because of intermixed hematopoietic tissue, the attenuation is usually higher than that of retroperitoneal fat (Figs 3, 4). High-attenuation regions may be seen due to hemorrhage or calcifications.
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Figure 3. Adrenal myelolipoma. Axial CT image shows a well-defined mass with predominantly soft tissue attenuation in the left adrenal gland. Note the nodule of fat attenuation (arrow), which is characteristic of myelolipoma.
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Figure 4. Adrenal myelolipoma. Axial CT image enhanced with intravenous contrast material shows a heterogeneous left adrenal mass (arrows) with fatty and enhancing soft tissue components. The presence of fat permits reliable diagnosis of a benign myelolipoma despite the soft tissue elements.
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At MR imaging, the fatty component is usually hyperintense on T1-weighted images and heterogeneously hyperintense on T2-weighted images due to nonuniform admixture of fat and marrow components. This appearance is nonspecific and may be mimicked by adrenal metastases or primary adrenal cancers. Frequency-selective fat suppression allows the diagnosis of myelolipoma to be confirmed by demonstrating signal loss (Fig 5). Fat-containing malignancies of the adrenal gland are exceedingly rare (9); thus, adrenal malignancies would not be expected to lose signal on fat-suppressed images. Chemical shift in-phase/opposed-phase imaging may also be diagnostic by demonstrating india ink or etching artifacts between fat- and water-based components. India ink or etching artifacts are produced at boundaries between fat- and water-based tissues. The reason is that the voxels along those boundaries contain both fat and water protons and hence lose signal on opposed-phase relative to in-phase images.
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Figure 5a. Adrenal myelolipoma. (a) Axial in-phase MR image shows a heterogeneous predominantly hyperintense lesion (arrows), which is a nonspecific appearance. (b) Axial out-of-phase MR image shows dramatic signal loss at the interface between water- and lipid-based components (arrows), thus permitting noninvasive diagnosis of the lesion. (c) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows diminished signal intensity in the macroscopic fatty component (arrowhead), a finding that confirms the diagnosis.
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Figure 5b. Adrenal myelolipoma. (a) Axial in-phase MR image shows a heterogeneous predominantly hyperintense lesion (arrows), which is a nonspecific appearance. (b) Axial out-of-phase MR image shows dramatic signal loss at the interface between water- and lipid-based components (arrows), thus permitting noninvasive diagnosis of the lesion. (c) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows diminished signal intensity in the macroscopic fatty component (arrowhead), a finding that confirms the diagnosis.
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Figure 5c. Adrenal myelolipoma. (a) Axial in-phase MR image shows a heterogeneous predominantly hyperintense lesion (arrows), which is a nonspecific appearance. (b) Axial out-of-phase MR image shows dramatic signal loss at the interface between water- and lipid-based components (arrows), thus permitting noninvasive diagnosis of the lesion. (c) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows diminished signal intensity in the macroscopic fatty component (arrowhead), a finding that confirms the diagnosis.
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Adenoma
Adrenal adenoma is a common tumor, with a prevalence of 3% in autopsy series (10). As the majority of lesions are small and nonfunctional, most adenomas are incidental findings ("adrenal incidentalomas"). Hyperfunctional adenomas also occur and are responsible for important endocrine disorders such as Cushing syndrome and Conn syndrome.
At pathologic analysis, both hyperfunctioning and nonfunctional adenomas contain a variable amount of intracytoplasmic fat. Lipid-rich adenomas (approximately 80% of adenomas) are easily identified at both CT and MR imaging. At CT, adrenal adenomas appear as small (<3 cm), well-defined homogeneous masses that are typically hypoattenuating relative to the liver (Fig 6). Lee et al (9) demonstrated that an attenuation value of less than 0 HU at unenhanced CT is diagnostic of an adenoma with 100% confidence; however, this threshold has only 47% sensitivity. In a series by Korobkin et al (11), a cutoff of 18 HU permitted a diagnosis of adenoma with 100% specificity and 85% sensitivity, compared to the specificity:sensitivity ratio of 68%:100% with a more conservative cutoff of 10 HU. A rational approach advocated by some authorities is to choose the CT number threshold on the basis of the patient’s risk for metastatic disease. For example, a threshold of 10 HU could be applied to older patients or to those with known primary malignancies. A threshold of 18 HU could be applied to younger patients without underlying cancer.
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Figure 6a. Adrenal adenoma. Axial unenhanced (a) and contrast material-enhanced (b) CT images show a low-attenuation mass in the left adrenal gland (arrow). The mass has an attenuation value of 3 HU on the unenhanced image (a) and 9 HU on the contrast-enhanced image (b). This result suggests the presence of microscopic fat, a finding compatible with adenoma.
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Figure 6b. Adrenal adenoma. Axial unenhanced (a) and contrast material-enhanced (b) CT images show a low-attenuation mass in the left adrenal gland (arrow). The mass has an attenuation value of 3 HU on the unenhanced image (a) and 9 HU on the contrast-enhanced image (b). This result suggests the presence of microscopic fat, a finding compatible with adenoma.
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Lipid-poor adenomas are more difficult to diagnose because the CT numbers increase and approach those of soft tissue. Contrast-enhanced imaging with 10-minute-delayed CT scans may be helpful in these cases. By using a threshold of 30 HU, the sensitivity and specificity for delayed contrast-enhanced CT in the characterization of benign disease are 80% and 100%, respectively. A relative percentage washout of more than 50% in the delayed study represents a sensitivity and specificity of 98% and 100%, respectively, for the detection of adenoma (12).
Chemical shift MR imaging is a reasonable second imaging test for further characterization when CT results are indeterminate (13). Because of the high sensitivity of chemical shift MR imaging to minute amounts of intravoxel fat, MR imaging demonstrates signal intensity loss on opposed-phase images in the majority of adenomas, with a sensitivity of 89% for lesions with an attenuation of 10–30 HU and 100% for lesions with an attenuation of 10–20 HU with a maintained specificity of 100% (Fig 7) (14). In a recent study (15), dynamic postgadolinium contrast-enhanced MR imaging was suggested to provide useful complementary information to the appearance of adrenal masses on in-phase and out-of-phase images. Twenty-five of 35 adrenal adenomas showed homogeneous capillary blush on immediate postgadolinium images, and 33 of 35 adrenal adenomas demonstrated rapid washout on 45-second postgadolinium images. Fat-suppressed images and STIR techniques have less sensitivity for detecting minute amounts of lipid and are of little if any benefit in lesion characterization.
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Figure 7a. Adrenal adenoma. Axial in-phase (a) and out-of-phase (b) chemical shift MR images show an adrenal nodule (arrow). There is homogeneous signal loss on the out-of-phase image (b) relative to the signal intensity on the in-phase image (a), a finding that confirms the presence of water- and lipid-based components.
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Figure 7b. Adrenal adenoma. Axial in-phase (a) and out-of-phase (b) chemical shift MR images show an adrenal nodule (arrow). There is homogeneous signal loss on the out-of-phase image (b) relative to the signal intensity on the in-phase image (a), a finding that confirms the presence of water- and lipid-based components.
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However, neither CT densitometry nor chemical shift MR imaging is infallible in differentiating atypical adenoma, especially if large, from metastasis. Atypical adenomas may contain calcification, hemorrhage, or necrosis (16). In addition, adenomas may lack detectable amounts of intracytoplasmic lipid even at chemical shift MR imaging (17). In these cases, comparison to results of previous studies, short-term imaging follow-up, or biopsy may be necessary.
Angiomyolipoma
Angiomyolipoma, also called renal hamartoma, is a benign tumor composed of fat, smooth muscle, and thick-walled blood vessels. Individual cells have a mature, benign appearance, but the tissue architecture is abnormal. The fat component typically predominates. Although intratumoral blood vessels are thick-walled, hemorrhage is a frequent complication (18). Angiomyolipoma is the cause for bleeding in 16%–20% of patients with spontaneous perinephric hemorrhage (19). Necrosis and calcification are rare. The vast majority arise from the renal cortex. Typically, renal angiomyolipoma is discovered incidentally at cross-sectional imaging; the prevalence is 0.3%–3%. Extrarenal angiomyolipoma is rare and usually occurs in the liver (20).
There are two distinct epidemiologic forms. A sporadic form accounts for about 90% of cases and characteristically occurs in middle-aged women; the remaining cases are associated with tuberous sclerosis (21). The two forms are histologically indistinguishable but vary in their imaging features. In patients with tuberous sclerosis, angiomyolipomas tend to be multiple and bilateral, may be massive, and often are associated with multiple cysts; extrarenal manifestations of tuberous sclerosis are often seen, including lymphangioleiomyomatosis. Patients with the sporadic form often have small solitary tumors.
CT permits confident noninvasive diagnosis of renal angiomyolipoma. At CT, these lesions typically appear as well-defined, cortically based, predominantly fat-attenuation masses (Fig 8). However, intratumoral fat cannot be visualized at CT in approximately 4.5% of all angiomyolipomas; this can be explained by the predominance of blood vessels, muscle, or immature fat or the scattering of a small amount of fat within other components (22). The size at discovery is usually less than 5 cm. Heterogeneous soft tissue attenuation (due to hemorrhage or fibrosis or representing muscular and vascular components) may be evident (23). Contrast enhancement is variable and depends on the amount of soft tissue and vascularity (Fig 9).
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Figure 8a. Renal angiomyolipoma. (a) Longitudinal US image shows a hyperechoic lesion in the inferior pole of the left kidney (cursors). The differential diagnosis includes renal cell carcinoma and angiomyolipoma. (b) Axial CT image shows fat attenuation within the lesion (arrow), a finding indicative of an angiomyolipoma.
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Figure 8b. Renal angiomyolipoma. (a) Longitudinal US image shows a hyperechoic lesion in the inferior pole of the left kidney (cursors). The differential diagnosis includes renal cell carcinoma and angiomyolipoma. (b) Axial CT image shows fat attenuation within the lesion (arrow), a finding indicative of an angiomyolipoma.
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Figure 9a. Renal angiomyolipoma with a predominant soft tissue component and a tiny fatty component. Axial unenhanced (a), corticomedullary phase (b), nephrographic phase (c), and excretory phase (d) CT images show a heterogeneous mass in the lateral left kidney. The mass is predominantly of soft tissue attenuation and superficially resembles a renal cell carcinoma; however, the presence of focal areas of fat attenuation (CT number, –20 HU or lower) (arrows) permits confident diagnosis of an angiomyolipoma.
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Figure 9b. Renal angiomyolipoma with a predominant soft tissue component and a tiny fatty component. Axial unenhanced (a), corticomedullary phase (b), nephrographic phase (c), and excretory phase (d) CT images show a heterogeneous mass in the lateral left kidney. The mass is predominantly of soft tissue attenuation and superficially resembles a renal cell carcinoma; however, the presence of focal areas of fat attenuation (CT number, –20 HU or lower) (arrows) permits confident diagnosis of an angiomyolipoma.
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Figure 9c. Renal angiomyolipoma with a predominant soft tissue component and a tiny fatty component. Axial unenhanced (a), corticomedullary phase (b), nephrographic phase (c), and excretory phase (d) CT images show a heterogeneous mass in the lateral left kidney. The mass is predominantly of soft tissue attenuation and superficially resembles a renal cell carcinoma; however, the presence of focal areas of fat attenuation (CT number, –20 HU or lower) (arrows) permits confident diagnosis of an angiomyolipoma.
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Figure 9d. Renal angiomyolipoma with a predominant soft tissue component and a tiny fatty component. Axial unenhanced (a), corticomedullary phase (b), nephrographic phase (c), and excretory phase (d) CT images show a heterogeneous mass in the lateral left kidney. The mass is predominantly of soft tissue attenuation and superficially resembles a renal cell carcinoma; however, the presence of focal areas of fat attenuation (CT number, –20 HU or lower) (arrows) permits confident diagnosis of an angiomyolipoma.
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MR imaging allows correct characterization of most angiomyolipomas because of intratumoral fat deposition. The intratumoral fat can be confidently identified by using fat suppression techniques or by demonstrating an india ink or etching artifact at the interface of the water-based kidney and the fat-containing tumor (Fig 10). However, if a lesion is small relative to the section thickness and the voxel size, signal loss may artifactually appear to be complete or nearly complete rather than peripheral (Fig 10b). The reason is that volume averaging may occur between the hyperintense center of a small lesion and its hypointense out-of-plane edge. Intuitively, the degree of central signal loss can be predicted by the size of the lesion, the section thickness, and the voxel size. MR imaging offers no definite diagnostic advantage over CT (24,25) but should be considered in younger patients because of the absence of ionizing radiation.
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Figure 10a. Angiomyolipoma. (a) Axial in-phase gradient-echo MR image (echo time = 4.4 msec) shows a hyperintense focus in the right kidney (arrow). The differential diagnosis includes a fatty lesion and a hemorrhagic cyst. (b) Axial out-of-phase gradient-echo MR image (echo time = 2.2 msec) shows signal loss at the interface of the fatty lesion and the water-based renal parenchyma (arrow). This finding indicates that the lesion is an angiomyolipoma and not a hemorrhagic cyst.
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Figure 10b. Angiomyolipoma. (a) Axial in-phase gradient-echo MR image (echo time = 4.4 msec) shows a hyperintense focus in the right kidney (arrow). The differential diagnosis includes a fatty lesion and a hemorrhagic cyst. (b) Axial out-of-phase gradient-echo MR image (echo time = 2.2 msec) shows signal loss at the interface of the fatty lesion and the water-based renal parenchyma (arrow). This finding indicates that the lesion is an angiomyolipoma and not a hemorrhagic cyst.
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The most important differential diagnosis for large exophytic angiomyolipomas is retroperitoneal (perirenal) liposarcoma. Multiplanar acquisition (MR imaging) or reformation (CT) may be necessary to differentiate the renal-based angiomyolipoma from a retroperitoneal liposarcoma. A careful evaluation for a sharp defect in the renal parenchyma and the presence of enlarged vessels and associated angiomyolipomas should enable accurate differentiation of these lesions in almost all cases. Liposarcoma does not cause a defect in the renal parenchyma, and the interface of the lesion with the kidney is smooth (26). Angiomyolipomas commonly contain enlarged vessels that can be seen at contrast-enhanced CT. In comparison, well-differentiated liposarcomas are relatively avascular. There is a potential for CT and MR angiography in the evaluation of difficult cases by demonstrating the renal artery origin of the feeding arteries supplying most angiomyolipomas (26).
Bleeding occurs frequently in angiomyolipomas and may mask the fat component (Fig 11). In such cases, imaging features may suggest renal cell carcinoma (RCC) (27,28). Angiomyolipomas with minimal fat can mimic RCC, leading to unnecessary surgery. Biphasic helical CT may be useful in the differentiation of these two entities (22). Homogeneity of tumor enhancement and the prolonged enhancement pattern of angiomyolipomas with minimal fat, in contrast to the heterogeneous enhancement and early washout pattern seen in most cases of RCC, are the most valuable CT findings. Rarely, RCC may contain small amounts of fat, usually because the tumor has engulfed a portion of the renal sinus or perirenal fat. Although clear cell type RCC could conceivably have a similar appearance to angiomyolipoma, signal loss at opposed-phase imaging is extremely rare in RCC and differentiation is usually possible on the basis of other findings. RCC rather than angiomyolipoma should be suspected when one or more of the following findings are present: intratumoral calcification, invasion of perirenal or sinus fat, small foci of fat within a large necrotic tumor, and enlarged nonfatty lymph nodes or venous invasion (29).
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Figure 11a. Massive right renal angiomyolipoma with hemorrhage in a patient with tuberous sclerosis several years after left nephrectomy. (a) Axial unenhanced CT image shows a huge heterogeneous mass of predominantly fat attenuation replacing the right kidney (arrows). The kidney itself is virtually unrecognizable. (b) Axial CT image obtained inferior to a shows a nodular area of high attenuation (arrow), which is suggestive of hemorrhage. Note the surrounding hemorrhage on both images (arrowhead in a, arrowheads in b).
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Figure 11b. Massive right renal angiomyolipoma with hemorrhage in a patient with tuberous sclerosis several years after left nephrectomy. (a) Axial unenhanced CT image shows a huge heterogeneous mass of predominantly fat attenuation replacing the right kidney (arrows). The kidney itself is virtually unrecognizable. (b) Axial CT image obtained inferior to a shows a nodular area of high attenuation (arrow), which is suggestive of hemorrhage. Note the surrounding hemorrhage on both images (arrowhead in a, arrowheads in b).
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Renal lipoma is an unusual benign tumor. CT and MR imaging findings do not differ from those of angiomyolipoma with a predominant fatty component. The diagnosis can be established only by means of histologic findings, which are rarely available because these cases do not require surgery.
Liposarcomas arising from the kidney are extremely rare. They generally are larger at presentation, tend to be invasive, and are treated surgically if possible (30).
Cortical defects, either congenital or from scarring, may simulate small angiomyolipomas, especially at US. Moreover, small renal cell carcinomas and angiomyolipomas can have an identical sonographic appearance. However, some sonographic features are typical of angiomyolipomas, including acoustic shadowing and a stronger echogenicity than the renal sinus (31,32), and when present may not warrant further imaging. When an echogenic mass is less typical, further imaging should be performed.
Teratoma
Ovarian teratomas are the most common germ cell neoplasms. By far, most ovarian teratomas are mature cystic teratomas (MCTs), also known as dermoid cysts.
MCTs are composed of well-differentiated derivations from at least two germ cell layers (ectoderm, mesoderm, or endoderm). Ectodermal tissue (skin derivates and neural tissue) is invariably present. Mesodermal tissue (fat, bone, cartilage, muscle) is present in over 90% of cases, and endodermal tissue (gastrointestinal and bronchial epithelium, thyroid tissue) is seen in the majority (33). MCTs are the most common benign ovarian tumors in women less than 45 years old. The tumors are bilateral in 10% of cases (33). Although most mature teratomas are asymptomatic, abdominal pain or other nonspecific symptoms occur in a minority of patients. Important complications of MCT are torsion, rupture, and malignant degeneration (34,35). Because of the slow growth of these tumors (average of 1.8 mm per year), some authors advocate nonsurgical management of smaller (<6 cm) lesions (36).
MCTs are unilocular in 88% of cases and filled with sebaceous material. A raised protuberance known as the Rokitansky nodule (dermoid plug) may project into the cystic cavity. Bone or teeth, if present, tend to be located within this nodule.
At CT, fat attenuation within a cystic adnexal mass, with or without calcification in the wall, is diagnostic (Fig 12). Fat is reported is 93% of cases, and teeth or other calcifications are reported in 56% (37). Typically, the calcifications of MCTs are smooth and well defined.
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Figure 12a. Mature cystic teratoma of the left ovary. (a) Sagittal transabdominal US image shows a large, heterogeneous, predominantly hyperechoic mass (arrows). (b) Axial contrast-enhanced CT image shows the retrouterine mass (arrows), which is predominantly of fat attenuation. (c, d) Axial in-phase (c) and out-of-phase (d) MR images show signal loss at the interface between water- and lipid-based components on the out-of-phase image (arrows in d) relative to the signal intensity on the in-phase image (c), thus confirming the presence of a fat component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal suppression (arrowheads).
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Figure 12b. Mature cystic teratoma of the left ovary. (a) Sagittal transabdominal US image shows a large, heterogeneous, predominantly hyperechoic mass (arrows). (b) Axial contrast-enhanced CT image shows the retrouterine mass (arrows), which is predominantly of fat attenuation. (c, d) Axial in-phase (c) and out-of-phase (d) MR images show signal loss at the interface between water- and lipid-based components on the out-of-phase image (arrows in d) relative to the signal intensity on the in-phase image (c), thus confirming the presence of a fat component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal suppression (arrowheads).
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Figure 12c. Mature cystic teratoma of the left ovary. (a) Sagittal transabdominal US image shows a large, heterogeneous, predominantly hyperechoic mass (arrows). (b) Axial contrast-enhanced CT image shows the retrouterine mass (arrows), which is predominantly of fat attenuation. (c, d) Axial in-phase (c) and out-of-phase (d) MR images show signal loss at the interface between water- and lipid-based components on the out-of-phase image (arrows in d) relative to the signal intensity on the in-phase image (c), thus confirming the presence of a fat component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal suppression (arrowheads).
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Figure 12d. Mature cystic teratoma of the left ovary. (a) Sagittal transabdominal US image shows a large, heterogeneous, predominantly hyperechoic mass (arrows). (b) Axial contrast-enhanced CT image shows the retrouterine mass (arrows), which is predominantly of fat attenuation. (c, d) Axial in-phase (c) and out-of-phase (d) MR images show signal loss at the interface between water- and lipid-based components on the out-of-phase image (arrows in d) relative to the signal intensity on the in-phase image (c), thus confirming the presence of a fat component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal suppression (arrowheads).
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Figure 12e. Mature cystic teratoma of the left ovary. (a) Sagittal transabdominal US image shows a large, heterogeneous, predominantly hyperechoic mass (arrows). (b) Axial contrast-enhanced CT image shows the retrouterine mass (arrows), which is predominantly of fat attenuation. (c, d) Axial in-phase (c) and out-of-phase (d) MR images show signal loss at the interface between water- and lipid-based components on the out-of-phase image (arrows in d) relative to the signal intensity on the in-phase image (c), thus confirming the presence of a fat component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal suppression (arrowheads).
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At MR imaging, the sebaceous component has high signal intensity on T1-weighted images, similar to that of retroperitoneal fat. The signal intensity on T2-weighted images is variable, usually approximating that of fat. Hemorrhagic lesions including endometriomas may have similar signal intensities on T1-weighted images but are usually darker than MCTs on T2-weighted images. Frequency-selective fat saturation or chemical shift artifact enables accurate differentiation of teratomas from hemorrhagic lesions (Figs 2, 12) (38,39). A minority of MCTs do not demonstrate a sebum-filled cyst cavity. However, they usually demonstrate evidence of fat in the wall or in the Rokitansky nodule (40).
Immature teratomas differ from MCTs in that they contain immature or embryonic tissues at histologic analysis, demonstrate clinically malignant behavior, are much less common (<1% of teratomas), and affect a younger group (during the first 2 decades of life). Mature cystic elements similar to those seen in MCTs are invariably present. However, solid components usually dominate over small foci of fat, and calcifications tend to be coarse or ill-defined. Recognition of these differential imaging and demographic features permits the correct diagnosis to be suggested.
Liposarcoma
Liposarcoma is a malignant tumor of mesenchymal origin that may arise in any fat-containing region of the body. Liposarcoma is one of the most common primary neoplasms in the retroperitoneum. It rarely arises in the mesentery or peritoneum (41). Histologically, liposarcomas are classified, in increasing order of malignancy, as well-differentiated, myxoid, pleomorphic, and round cell subtypes. Liposarcomas may contain multiple histologic subtypes within the same lesion. CT and MR imaging appearances vary according to these histologic subtypes (42).
Well-differentiated liposarcomas resemble lipomas, with attenuation (CT) and signal intensity (MR imaging) equal to those of fat. Fibrous septa may be thicker, more irregular, or more nodular than those seen in lipoma. These septa have attenuation (CT) and signal intensity (MR imaging) similar to those of muscle and may enhance dramatically on fat-suppressed T1-weighted MR images after administration of a gadolinium chelate (Figs 13, 14) (42–44). These tumors often recur if only marginally excised but do not metastasize.
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Figure 13. Mesenteric liposarcoma. Axial contrast-enhanced CT image shows a well-defined, slightly heterogeneous mass with fatty components in the mesentery. The mass causes posterior displacement of some abdominal structures (arrowhead) and anterior displacement of the small intestine (arrows). The thin, fibrous internal septa of soft tissue attenuation suggest the diagnosis of liposarcoma. An identical mass arising from the adrenal gland or kidney would be considered benign.
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Figure 14a. Retroperitoneal liposarcoma. (a) Transabdominal US image shows a large, well-defined, hyperechoic mass (cursors) in the abdominal cavity. The mass demonstrates the attenuation of sound, which is indicative of fat. (b) Axial contrast-enhanced CT image shows heterogeneous fat attenuation and coarse, thickened septa in the mass (arrows), findings suggestive of a well-differentiated liposarcoma. (c, d) Axial in-phase (c) and out-of-phase (d) gradient-echo MR images show signal loss in the interface between lipid and water on the out-of-phase image (arrowheads in d) relative to the signal intensity on the in-phase image (c), a finding that confirms the presence of a fatty component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal loss of the macroscopic fatty components (arrows), a finding that confirms the predominantly fatty content of the mass.
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Figure 14b. Retroperitoneal liposarcoma. (a) Transabdominal US image shows a large, well-defined, hyperechoic mass (cursors) in the abdominal cavity. The mass demonstrates the attenuation of sound, which is indicative of fat. (b) Axial contrast-enhanced CT image shows heterogeneous fat attenuation and coarse, thickened septa in the mass (arrows), findings suggestive of a well-differentiated liposarcoma. (c, d) Axial in-phase (c) and out-of-phase (d) gradient-echo MR images show signal loss in the interface between lipid and water on the out-of-phase image (arrowheads in d) relative to the signal intensity on the in-phase image (c), a finding that confirms the presence of a fatty component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal loss of the macroscopic fatty components (arrows), a finding that confirms the predominantly fatty content of the mass.
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Figure 14c. Retroperitoneal liposarcoma. (a) Transabdominal US image shows a large, well-defined, hyperechoic mass (cursors) in the abdominal cavity. The mass demonstrates the attenuation of sound, which is indicative of fat. (b) Axial contrast-enhanced CT image shows heterogeneous fat attenuation and coarse, thickened septa in the mass (arrows), findings suggestive of a well-differentiated liposarcoma. (c, d) Axial in-phase (c) and out-of-phase (d) gradient-echo MR images show signal loss in the interface between lipid and water on the out-of-phase image (arrowheads in d) relative to the signal intensity on the in-phase image (c), a finding that confirms the presence of a fatty component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal loss of the macroscopic fatty components (arrows), a finding that confirms the predominantly fatty content of the mass.
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Figure 14d. Retroperitoneal liposarcoma. (a) Transabdominal US image shows a large, well-defined, hyperechoic mass (cursors) in the abdominal cavity. The mass demonstrates the attenuation of sound, which is indicative of fat. (b) Axial contrast-enhanced CT image shows heterogeneous fat attenuation and coarse, thickened septa in the mass (arrows), findings suggestive of a well-differentiated liposarcoma. (c, d) Axial in-phase (c) and out-of-phase (d) gradient-echo MR images show signal loss in the interface between lipid and water on the out-of-phase image (arrowheads in d) relative to the signal intensity on the in-phase image (c), a finding that confirms the presence of a fatty component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal loss of the macroscopic fatty components (arrows), a finding that confirms the predominantly fatty content of the mass.
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Figure 14e. Retroperitoneal liposarcoma. (a) Transabdominal US image shows a large, well-defined, hyperechoic mass (cursors) in the abdominal cavity. The mass demonstrates the attenuation of sound, which is indicative of fat. (b) Axial contrast-enhanced CT image shows heterogeneous fat attenuation and coarse, thickened septa in the mass (arrows), findings suggestive of a well-differentiated liposarcoma. (c, d) Axial in-phase (c) and out-of-phase (d) gradient-echo MR images show signal loss in the interface between lipid and water on the out-of-phase image (arrowheads in d) relative to the signal intensity on the in-phase image (c), a finding that confirms the presence of a fatty component. (e) Axial gadolinium-enhanced fat saturation T1-weighted MR image shows marked signal loss of the macroscopic fatty components (arrows), a finding that confirms the predominantly fatty content of the mass.
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Myxoid liposarcoma is the most common subtype, accounting for 50% of all liposarcomas. At CT, they often have an inhomogeneous appearance, with CT attenuation values less than that of muscle (Figs 15, 16). Occasionally, fat and soft tissue elements are distributed homogeneously within the lesion, producing fluid attenuation at CT. Consequently, the lesion may appear cystic on nonenhanced CT images and cause diagnostic confusion.
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Figure 15. Myxoid liposarcoma. Axial contrast-enhanced CT image shows a minimally heterogeneous mass with a mean CT number of 10 HU (arrows). Prospectively, the mass was erroneously diagnosed as a benign cyst on the basis of the fluid attenuation value. The correct diagnosis was made at surgery when the surgeon unexpectedly encountered an incompletely resectable solid mass. In retrospect, although the mean attenuation value within the lesion was 10 HU, the range of attenuation values on a pixel-by-pixel basis was –100 HU to 60 HU. Recognition of the broad range of attenuation values and the somewhat irregular septa within the mass may have permitted correct diagnosis with CT.
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Figure 16a. Myxoid liposarcoma. Axial contrast-enhanced CT images show a lobulated inhomogeneous mass localized to the retroperitoneal space, with posterior displacement of the psoas muscle and anterior displacement of the retroperitoneal vessels. Portions of the mass have fluid attenuation owing to admixture of fatty and soft tissue components, similar to the lesion in Figure 15. Recognition of enhancing septa and mural components, which are more conspicuous on the delayed image (arrows in b), permitted the correct diagnosis.
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Figure 16b. Myxoid liposarcoma. Axial contrast-enhanced CT images show a lobulated inhomogeneous mass localized to the retroperitoneal space, with posterior displacement of the psoas muscle and anterior displacement of the retroperitoneal vessels. Portions of the mass have fluid attenuation owing to admixture of fatty and soft tissue components, similar to the lesion in Figure 15. Recognition of enhancing septa and mural components, which are more conspicuous on the delayed image (arrows in b), permitted the correct diagnosis.
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At MR imaging, myxoid liposarcomas exhibit signal intensity similar to that of water: low signal intensity on nonenhanced T1-weighted images and high signal intensity on T2-weighted images. Thus, these malignant lesions may superficially resemble benign cysts at unenhanced MR imaging, but careful inspection usually reveals lacy, linear, or amorphous regions of high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images. These represent intratumoral fat and permit the correct diagnosis (43). Recognition of chemical shift artifacts between water- and fat-based components also permits the correct diagnosis.
Although these lesions may superficially resemble cysts on unenhanced CT and MR images, they are readily differentiated after intravenous administration of contrast material. Slowly progressive, reticular enhancement is characteristic and reveals the solid nature of these tumors (43).
Pleomorphic and round cell liposarcomas are heterogeneous, nonfatty tumors. Therefore, it is usually impossible to differentiate them from other sarcomas.
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Fat-containing Nonneoplastic Masses
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Lipoma
Lipoma is a benign mesenchymal tumor that resembles normal fat. Fat within a lipoma cannot be distinguished histologically from normal fat; however, there are ultrastructural and biochemical differences. Most commonly, lipomas arise in soft tissues and skeletal muscle, but they may arise in any region of the body that contains fat, including the gastrointestinal tract. The colon is the most commonly affected bowel segment, accounting for 65%–75% of gastrointestinal lipomas, followed by the small bowel with about 20%–25% (45). Approximately 90%–95% of gastrointestinal lipomas arise in the submucosa. Most are asymptomatic and discovered incidentally. Complications of gastrointestinal lipomas include intussusception and intestinal bleeding.
Regardless of their location, lipomas have characteristic imaging features that permit reliable noninvasive diagnosis. Lipomas typically demonstrate homogeneous fatty attenuation at CT (Figs 17, 18) and homogeneous signal intensity identical to that of fat with all MR imaging pulse sequences (43). Thin fibrous septa of low signal intensity on T1- and T2-weighted images may traverse the lesion. Some lipomas have prominent fibrous septa and nodularity and may mimic well-differentiated liposarcomas at imaging.
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Figure 17. Lipoma of the pancreas. Axial contrast-enhanced CT image shows a large, homogeneous, fat attenuation mass within the pancreatic head (arrows), with lateral displacement of the common bile duct (arrowhead).
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Figure 18. Lipoma of the terminal ileum. Axial contrast-enhanced CT image shows a small, homogeneous, fat attenuation mass within the wall of the terminal ileum (arrow), an appearance that allows diagnosis of a lipoma.
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Epiploic Appendagitis
Appendices epiploicae are pedunculated adipose structures protruding from the external surface of the colon into the peritoneal cavity. Appendices epiploicae typically measure 1–2 cm thick and 2–5 cm long and extend from the cecum to the rectosigmoid junction. Appendices epiploicae are prone to torsion leading to ischemic or hemorrhagic infarction, due to their limited blood supply, pedunculated shape, and excessive mobility. Infarction results in a focal inflammatory process called epiploic appendagitis (46,47). The condition usually manifests as localized abdominal pain in one of the lower quadrants and clinically mimics acute appendicitis or diverticulitis.
CT is usually diagnostic, avoiding unnecessary surgery. Characteristic findings include (a) a paracolonic oval fatty mass representing the infarcted or inflamed appendix epiploica, (b) a well-circumscribed hyperattenuating rim that surrounds the mass and represents the inflamed visceral peritoneal lining, and sometimes (c) a high-attenuation central dot representing engorged or thrombosed central vessels or central areas of hemorrhage (Fig 19) (48,49). Mild local reactive thickening of the adjacent colonic wall is often seen (48).
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Abstract
LEARNING OBJECTIVES
