FLUID

Simple fluid is easily identified on imaging studies. It is typically anechoic with sonography, low attenuation (less than 20 HU) on computed tomographic (CT) images, low signal intensity on T1-weighted MR images, very bright on T2-weighted magnetic resonance (MR) images, and does not enhance after administration of intravenous contrast media. Simple-appearing fluid is most often present in cysts, pseudocysts, seromas, bilomas, urinomas, lymphangiomas, and lymphoceles. The presence of simple-appearing fluid does not confirm benignity, however, because necrotic or cystic neoplasms may contain simple-appearing cystic areas depending on the modality. Furthermore, mucin (a substance produced by a variety of benign and malignant tumors) mimics simple fluid on imaging studies (Fig. 23-1). Therefore, it may be difficult or impossible to distinguish some mucinous neoplasms from nonneoplastic lesions that contain simple fluid. Fluid that appears simple with one modality will appear complex on another modality. For example, ultrasound (US) is often more useful than CT for evaluation and characterization of the internal complexity of cystic structures (Fig. 23-2).

Figure 23-1 Axial enhanced computed tomographic scan in a patient with pseudomyxoma peritonei.

Figure 23-2 A, Axial computed tomographic image in a patient with extraperitoneal fluid collection. Fluid appears slightly dense but otherwise uncomplicated. B, Ultrasound image of the same collection as (A) shows its complicated nature more clearly.

Box 23-1 lists some collections of simple-appearing fluid that may be encountered on imaging studies of the abdomen and pelvis. Table 23-1 lists some ways in which simple peritoneal fluid differs from other types of peritoneal fluid. A variety of processes may alter the attenuation value or signal intensity of peritoneal fluid on CT or MR images, including hemorrhage, leakage of contrast material from the urinary tract or bowel, or even intravenous extracellular contrast media administration (on delayed images).

Figure 23-3 Axial T2-weighted magnetic resonance image in a patient with lymphangioma.

Figure 23-4 Axial enhanced computed tomographic image in a patient with a large pancreatic pseudocyst.

Figure 23-5 A, Axial enhanced computed tomographic (CT) scan in a patient with urinoma after partial nephrectomy for a renal tumor. B, Delayed axial CT scan performed several hours after (A) shows contrast accumulation in urinoma.

Table 23-1 Differential Diagnosis of Peritoneal Fluid

Figure 23-6 A, Axial unenhanced computed tomographic (CT) image through the midabdomen in a patient with chronic pancreatitis, splenic artery pseudoaneurysm, and hemoperitoneum. B, Axial enhanced CT scan in the same patient as A shows hemoperitoneum more clearly. Note that hemoperitoneum measures more than 40 HU.

Figure 23-7 Axial computed tomographic scan in a trauma patient with hemoperitoneum caused by liver injury demonstrates sentinel clot sign.

Figure 23-8 Axial enhanced T1-weighted equilibrium-phase magnetic resonance image with fat suppression in a patient with fungal and bacterial peritonitis.

FAT

Macroscopic fat is usually easy to identify on imaging studies. It is typically echogenic on sonograms and may demonstrate mild acoustic shadowing or sound attenuation. However, this appearance is not specific for fat. For example, a hyperechoic renal mass often represents an angiomyolipoma, although some renal cell carcinomas may have a similar appearance (the latter entity is more likely to have a hypoechoic rim). In addition, fat does not always appear hyperechoic; for example, fat can appear hypoechoic in the anterior abdominal wall beneath the rectus muscles. This occasionally results in confusion with fluid or abscess. Lipomas can occasionally display echogenicity similar to that of muscle.

On CT, macroscopic fat appears of low attenuation (less than −220 HU). On T1-weighted MR images, fat appears significantly brighter than most other tissue types. The signal intensity of fat on T2-weighted images varies somewhat with the pulse sequence, although fat is typically bright on long-echo-train spin-echo sequences performed without fat suppression. With MRI, macroscopic fat can be identified by the presence of chemical shift artifact when it shares an interface with soft tissue or fluid. Fat will become dark on T1- or T2-weighted MR pulse sequences that utilize fatsuppression techniques such as short tau inversion recovery, spectral presaturation inversion recovery, fat saturation, or water excitation. Chylous collections within the abdomen may demonstrate fat–fluid levels on CT or MRI scans (Fig. 23-9). Box 23-2 lists some entities of the abdomen and pelvis that may contain macroscopic fat.

Figure 23-9 Axial unenhanced computed tomographic image in a patient who had previously undergone partial nephrectomy. Low-attenuation material floating in bladder was a persistent finding on multiple imaging examinations.

Figure 23-10 Axial enhanced computed tomographic image in a patient with left upper quadrant abscess after splenectomy. Drainage yielded oily yellow material (fat) and pus.

Figure 23-11 Axial enhanced computed tomographic image in a patient with angiomyolipoma of the left kidney.

Figure 23-12 Longitudinal sonogram of the kidney in a patient with renal angiomyolipoma.

Figure 23-13 Axial enhanced computed tomographic image of a patient with benign cystic teratoma of left ovary.

Figure 23-14 Axial enhanced computed tomographic image of a patient with epiploic appendagitis presenting with right lower quadrant pain.

Figure 23-15 Axial unenhanced computed tomographic image in a patient with cirrhosis and hepatocellular carcinoma containing areas of fatty metamorphosis.

Figure 23-16 Axial enhanced computed tomographic image in a patient with extraperitoneal pelvic liposarcoma.

Figure 23-17 Sagittal reformation of an enhanced computed tomographic scan in a patient with mesenteric panniculitis.

Figure 23-18 A, Axial, T1-weighted, opposed-phase magnetic resonance (MR) image in a patient with a myelolipoma of left adrenal gland. B, Axial, enhanced, T1-weighted, MR image in same patient performed with chemically selective fat saturation.

Figure 23-19 Axial computed tomographic image shows segmental omental infarction in a patient with recent abdominal surgery and midepigastric pain.

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