The retroperitoneal space is defined by several fascial planes. The inner and anterior border of the retroperitoneum is formed by the posterior peritoneum. The transversalis fascia forms the outer or posterior border. Anterior and posterior renal fasciae fuse behind the colon and form a single lateroconal fascia. These fascial boundaries form three extraperitoneal compartments (Fig. 26.1).
1. The anterior pararenal space extending from the posterior parietal peritoneum to the anterior renal fascia and confined laterally by the lateroconal fascia. It contains the ascending and descending colon, the duodenal loop, and the pancreas. Fluid collections in the anterior pararenal space, unless intrapancreatic, are usually confined to the site of origin but may sometimes extend into the small-bowel mesentery and the transverse mesocolon, respectively.
2. The two perirenal spaces encompass each one kidney, adrenal gland, and perirenal fat. Both spaces do not communicate across the midline, although the anterior renal fascia below the level of the renal hila occasionally appears non-interrupted across the midline. Normal thickness of the renal fascia ranges from 1 to 2 mm. The posterior renal fascia is usually better depicted than the anterior. An abnormally thick (> 2–3 mm) renal fascia may be caused by edema, hyperemia, fibrosis, lipolysis, inflammation, malignancy, or trauma.
3. The posterior pararenal space extending from the posterior renal fascia to the transversalis fascia. This relatively thin layer of fat continues uninterruptedly as the properitoneal fat of the abdominal wall, external to the lateroconal fascia. It is medially confined by the psoas muscle and contains no organs.
anterior pararenal space
anterior renal fascia
inferior vena cava
posterior pararenal space
posterior renal fascia
Normal retroperitoneal lymph nodes appear on CT scans as small soft-tissue densities in the vicinity of abdominal and pelvic blood vessels and may range from 3 to 10 mm in size. Pancreatic, celiac, and superior mesenteric lymph nodes are usually not visible unless they are enlarged. Whereas in the abdomen and pelvis, only lymph nodes > 1.5 cm in diameter are considered abnormal, retroperitoneal lymph nodes > 6 mm in diameter are suspicious. Also, a solitary pelvic or abdominal lymph node > 10 mm in diameter or a cluster of multiple small nodes are conspicuous. A ppropriate bowel loops opacification with oral contrast medium is important to allow differentiation from adenopathy. Intravenous contrast material helps to distinguish strongly enhancing vascular cross sections from lymph nodes, which usually enhance less strongly.
The abdominal aorta measures < 3 cm in diameter and tapers gradually in caliber before bifurcating into the common iliac arteries at the level of L3–L4. Abnormalities of the aorta and its branches such as aneurysms, atherosclerosis, thrombus formation, or dissection are best evaluated using contrast-enhanced scans acquired during the arterial phase. Optimal vessel opacification is achieved when applying bolus triggering technique (with the monitor scan placed on the aorta at the level of the celiac trunk) and contrast media flow rates of 4 to 5 mL/s. In addition, the iodine concentration of the contrast medium should be at least 350 mg I/mL. Thin collimation multidetector CT technology allows multiplanar reconstruction of equal quality due to an isotropic voxel size.
The inferior vena cava (IVC) is formed by the junction of the right and left common iliac veins just caudal to the aortic bifurcation and parallels the abdominal aorta on the right side. The cross-sectional diameter of the IVC usually amounts to 2 to 3 cm, and its shape may vary from slitlike to rounded. A collapsed IVC at multiple levels can be observed in patients with severe hypovolemia. A round and widened IVC may be indicative of heart failure. Forced inspiration and expiration usually provoke changes in the vascular diameter of the IVC. The differential diagnosis of various retroperitoneal abnormalities is given in Table 26.1.