Mesentery, Omentum, Peritoneum: CT, Ultrasound and MRI

Abstract

Ultrasound is an accurate, safe, readily available, portable, and relatively inexpensive imaging modality. Traditionally assessment of the abdomen and pelvis with ultrasound has concentrated on the solid viscera, gallbladder, and bile ducts. Many believe that interference from bowel gas and poor visibility in general limit evaluation of the peritoneum and peritoneal cavity and that evaluation of these areas is very time consuming. As a result little time is spent on teaching techniques to optimally image the peritoneum and peritoneal cavity and in general there is a poor understanding of the appearance of peritoneal disease with ultrasound. Ultrasound, however, has the potential to be sensitive and specific when imaging the peritoneum (Hanbidge et al. 2003). To be successful two criteria must be met: (1) the operator performing the scan must be aware of the potential involvement of the peritoneum and peritoneal cavity with a disease process and (2) once aware, must carry out a detailed evaluation of these areas. The initial survey should be performed with a general purpose 3.5–5 MHz transducer. The field of view should be adjusted to include the full depth of the peritoneal cavity with regular change of the focal zone to optimize visualization of different levels within the field of view. The power and gain settings should also be regularly adjusted using low settings to better characterize hypoechoic nodules and masses and high settings to better characterize free or loculated fluid as simple or particulate. Variable degrees of compression are used to help displace bowel gas and change in patient position may also facilitate better visualization of specific areas of interest. Ultrasound has the advantage of direct patient interaction and specific sites of pain and/or tenderness may be evaluated in more detail. Once the initial survey is complete focused assessments of specific areas of interest are performed using a higher frequency curvilinear or linear transducer. Once again the focal zone, power, and gain settings are adjusted to facilitate optimal detection and characterization of suspected abnormalities. Finally a transvaginal ultrasound should be performed in all female patients as the Pouch of Douglas is commonly involved in both benign and malignant peritoneal processes (Damani and Wilson 1999; Serafini et al. 2001). The high-frequency transvaginal transducer affords detailed assessment not only of the uterus and ovaries but also the parietal peritoneum in the Pouch of Douglas and along the pelvic side walls (Fig. 99.1) and also the visceral peritoneum on the serosal surfaces of pelvic bowel loops. The transvaginal transducer is also very sensitive at detecting and characterizing small volumes of free fluid. In general the presence of free fluid facilitates the detection and characterization of peritoneal thickening, masses, and nodules with ultrasound but much information can be obtained with ultrasound in the absence of free fluid once a meticulous and thorough examination is performed. Ultrasound is also a fast and efficient means of image guidance for diagnostic or therapeutic paracentesis or for biopsy of omental cakes or peritoneal masses. A. Hanbidge (*) Department of Medical Imaging, University of Toronto, Toronto, ON, Canada