3.6.2\ Differential Diagnosis

Ulcerative colitis is a mucosal disease that primarily affects the rectum. It is typically left-sided or diffuse, and only rarely involves the right colon exclusively (Philpotts et al. 1994). The mean wall thickness in Crohn’s disease is usually greater than in ulcerative colitis (Fishman et al. 1987). The halo sign, a low-attenuation ring in the bowel wall caused by deposition of submucosal fat, is seen more commonly in ulcerative colitis than in Crohn’s disease. Proliferation of mesenteric fat is almost exclusively seen in Crohn’s disease, whereas proliferation of perirectal fat is nonspecific and can result from Crohn’s disease, ulcerative colitis, pseudomembranous colitis, or radiation colitis (Philpotts et al. 1994). Abscesses are almost exclusively found in Crohn’s disease and not in ulcerative colitis (Gore et al. 1996).

3.6.3\ Value of Imaging

Cross-sectional imaging is able to demonstrate transmural extent, skip lesions beyond severe luminal stenoses, and intraperitoneal extraintestinal complications. MRI is preferred because of its lack of ionizing radiation and high diagnostic accuracy (sensitivity and specificity of up to 84% and 100%, respectively) (Laghi et al. 2003). MRI is also better at detecting complications such as fistulae that can be missed on CT. However, CT and MR imaging are both inferior compared to enteroclysis in the depiction of early disease manifestations (Furukawa et al. 2004).

3.7\ Rectus Sheath Hematoma

Rectus sheath hematoma is an uncommon and often misdiagnosed condition resulting from either rupture of the epigastric vessels or the rectus muscle itself. The hematoma may be caused by coagulation disorders, trauma, or anticoagulation therapy (Fishman et al. 1987). Clinically, most patients present with acute abdominal pain, a perior infraumbilical mass, and anemic syndrome. Some patients also have a history of severe coughing episodes due to bronchial infection.

3.7.1\ Imaging Findings

The shape of rectus sheath hematomas depends on the relationship to the arcuate line, which is 3.5–5 cm below the umbilical level (Fukuda et al. 1996). Above this level, they usually appear as spindle-shaped due to encasement by firm aponeurotic sheaths (Fig. 23). Below the arcuate line, hematomas tend to appear spherical and may communicate with extraperitoneal pelvic and perivascular pelvic spaces (Fukuda et al. 1996). On CT, hematomas present as homogeneous hyperdense lesions with thin circumferential halos of low density. Clot resorption leads to diminution of density and fluid–fluid levels because a hematocrit effect may be found within hematomas (Berna et al. 1996; Wolverson et al. 1983). Additional findings of rectus sheath hematoma include increased density of the adjacent subcutaneous fat and enlargement of the anterolateral muscles (Fukuda et al. 1996). On MRI, rectus sheath hematomas demonstrate heterogeneous signal intensities with areas of high signal intensity on T1-weighted and T2-weighted images. Fluid–fluid levels and a concentric ring sign can also be noted (Blum et al. 1995).

Fig. 23  Rectus sheath hematoma on CT. A spindleshaped lesion is seen in the left rectus muscle (arrow). It shows homogenous high density and is surrounded at its anterior periphery by a minimal hypodense rim. Only minimal thickening of the adjacent lateral abdominal muscles can be noted