Oral contrast medium is routinely given to delineate bowel loops and to avoid mistaking bowel for an adrenal mass. Contrast agent is given 45 to 60 minutes before the CT examination and again 10 to 15 minutes before the start of scanning (1,2). (See Chapter 1 for more detailed information.) Water may be useful as an oral contrast agent when CT angiography is anticipated.

To improve delineation of abdominal and pelvic organs and identification of pathology, retroperitoneal CT studies are routinely performed with intravenous contrast medium, 2 mL/kg, not to exceed 125 mL. Flow rates vary with angiocatheter size; typical flow rates are: 1.5 to 2.5 mL per second for a 22-gauge catheter and 3 to 4 mL per second for a 20-gauge catheter.

Similar to other parts of the body, the choice of slice collimation and pitch will vary with detector technology and the size of the structure of interest. For a 16-row detector, 0.75- to 1.5-mm collimation with a pitch of 1 to 1.5 suffices. For a 64-row detector, 0.6- to 1.25-mm collimation and a pitch of 1 to 1.5 suffice. Because most retroperitoneal masses are large in children, a thicker collimation (>1 mm) usually suffices. Thinner collimation (<1 mm) is used to improve visualization of small lesions and for CT angiography. A 5-mm section thickness at 5-mm intervals is usually adequate for routine viewing of the volumetric data, with thinner sections reconstructed as needed. Thin reconstructions are used if multiplanar and 3D reconstructions are needed (1,2). (See Chapter 1 for more detailed discussion.)

Scanning should begin at the diaphragm and continue to the pubic symphysis. Noncontrast scans are not routinely performed. However, they may be useful for evaluating the presence of suspected calcifications or hemorrhage, if this information is important for diagnosis. Precontrast imaging can be limited to the region of interest and the milliamperage (mA) and kilovoltage (kV) reduced to the lowest possible that maintain image quality. Contrast-enhanced scanning is usually performed during the portal
venous phase of imaging, approximately 50 to 60 seconds after the initiation of contrast agent administration (1). This scan timing optimizes not only evaluation of the adrenal glands but also the liver for possible metastases. The start of scanning will be slightly earlier in smaller children. In the latter group of patients, in whom the contrast injection finishes before 50 seconds, the scan delay time needs to be shorter to ensure adequate enhancement of the solid organs. Scanning should start within 10 to 15 seconds of the end of the contrast administration (4).

Most diagnoses can be made from review of the axial image set. Multiplanar reformations in sagittal and coronal planes can be useful for demonstrating the craniocaudal extent of neoplastic, inflammatory, or traumatic lesions and are helpful for displaying the relationship of large adrenal masses to surrounding vasculature (5).

Congenital absence of the adrenal glands is a rare anomaly. Patients with renal agenesis or ectopy will almost always have an ipsilateral adrenal gland present (5,6). In these patients, the gland is elongated, with a linear shape, and lies either in its expected location or slightly more caudal (11). By comparison, the adrenal glands have a normal appearance in patients who have had a nephrectomy. Other organs, such as bowel, tail of the pancreas, and spleen, may move into the empty space in patients with renal agenesis or nephrectomy.

Developmental abnormalities of the adrenal gland are rare. These include adrenal fusion, ectopic or accessory tissue, and heterotopia (5,6). Adrenal fusion or horseshoe adrenal is usually discovered at autopsy and is associated with other anomalies including asplenia. It is rarely seen at imaging (12). Ectopic or accessory adrenal tissue occurs when fragments of adrenal tissue separate off during development. This ectopic tissue usually lies adjacent to the adrenal gland; however, it may lie elsewhere within the retroperitoneum or extend down to the gonads (8). Adrenal heterotopia occurs when accessory adrenal tissue is incorporated into adjacent viscera including kidney and liver (8).

Various normal structures may mimic an adrenal mass. Pseudotumors are more common on the left than on the right (13). They usually appear as a rounded structure in the region of the adrenal gland. Most pseudotumors are due to the presence of an accessory spleen, splenic lobulations, or tortuous splenic vessels associated with portal hypertension, producing a masslike appearance. Scanning after intravenous contrast administration should delineate these structures as pseudotumors. Splenic tissue will have the same attenuation and enhancement pattern as the normal spleen. Vascular pseudotumors can be recognized by their tubular shape and intense enhancement. Occasionally, unopacified bowel, a gastric diverticulum, or the tail of the pancreas may extend into the left suprarenal area, simulating an adrenal mass. The use of oral contrast medium allows these structures to be easily recognized. Sagittal and coronal reformatting, the use of gas-producing agents, and rotating the patient so that the segment of interest is in a nondependent (gas-filled) position are effective means of allowing these pseudotumors to be distinguished from true adrenal masses.

Lesions that may simulate an adrenal mass on the right side include an exophytic hepatic mass and interposition of fluid-filled colon into the hepatorenal space. An exophytic renal mass on either side may also mimic an adrenal mass. Other extra-adrenal suprarenal masses include extralobar sequestration (Fig. 10.2) and gastric duplication cysts (14).

Adrenal masses may be caused by neoplasms, hemorrhage, abscesses, and cysts. Adrenal neoplasms may arise from either the adrenal medulla or the cortex. Neoplasms arising in the adrenal medulla include neuroblastoma, ganglioneuroblastoma, ganglioneuroma, and pheochromocytoma. Adrenal cortical neoplasms include adrenocortical carcinoma and adenoma.

Adrenal hemorrhage is more common in neonates than in older children. In neonates, hemorrhage is usually secondary to birth trauma or perinatal anoxia, but it has been seen in the setting of overwhelming septicemia and anticoagulation therapy (5,6). Affected newborns present with a palpable abdominal mass, jaundice, and anemia. Adrenal insufficiency does not develop, because the major insult is to the regressing fetal cortex rather than to the adult cortex. Sonography is the examination of choice in neonates suspected of having adrenal hemorrhage. When the diagnosis is equivocal, CT can be helpful to document the presence of blood, based on characteristic attenuation values.

The primary diagnostic problem in the neonate is congenital neuroblastoma presenting as a complex or cystic mass without evidence of metastatic disease. In general, masses owing to adrenal hemorrhage decrease in size rather quickly and disappear in several weeks, whereas the size of a neuroblastoma is unlikely to decrease. Serial imaging with sonography is an acceptable method of separating the two lesions if clinical and laboratory data are not diagnostic.

Adrenal hemorrhage in the older infant and child is usually due to abdominal trauma, but it also can occur as a complication of anticoagulation therapy or overwhelming infection, especially infection caused by Neisseria meningitides. Posttraumatic hematomas are usually unilateral, more often on the right than the left side, and commonly associated with hepatic and splenic lacerations (59,60,61). Most are asymptomatic and detected on CT studies performed for evaluation of other abdominal injury.

Orthotopic liver transplant is another cause of adrenal hemorrhage. In this procedure, a segment of the recipient's inferior vena cava is excised, requiring ligation and division of the right adrenal vein, which can result in infarction and sometimes hemorrhage in the right adrenal gland (62).

Adrenal cysts and abscesses are uncommon adrenal lesions in childhood (5,6). Adrenal cysts may be true cysts, lined by epithelium, or pseudocysts, which lack an endothelial lining. Pseudocysts result from prior hemorrhage or infection, often parasitic in origin (63). At CT, adrenal cysts are well circumscribed, round, and of near-water attenuation (Fig. 10.14). Higher attenuation, septations, fluid–fluid levels, and wall calcification may be seen in the presence of hemorrhage or infection (63). Multilocular adrenal cysts have been reported with hemihypertrophy and Beckwith–Wiedemann syndrome (64).

Wolman disease is a cause of bilateral adrenal enlargement. This familial disorder of lipid metabolism results in the accumulation of cholesterol esters and triglycerides in most tissues of the body. Affected patients have enlarged adrenal glands with bilateral calcifications (Fig. 10.15) (65). Differential considerations for adrenal calcifications include adrenal teratoma (66), neuroblastoma, ganglioneuroma, cyst, carcinoma, and pheochromocytoma.

Initial noncontrast scans are generally not needed in pancreatic CT examinations but may be obtained using low
radiation exposure techniques to identify calcifications or suspected hemorrhage. Scanning for evaluation of trauma and pancreatitis is usually performed during the portal venous phase of imaging (50- to 60-second delay, with shorter delays in smaller patients) (5). In the evaluation of a known or suspected pancreatic mass, thin section scanning in the pancreatic parenchyma phase (30- to 35-second delay) followed by scanning in the portal venous phase can be useful in detecting and localizing hypovascular masses, whereas scanning during the arterial phase (15- to 20-second delay) may help to identify hyperenhancing lesions, such as neuroendocrine tumors, and to evaluate critical arterial structures (70,71). Portal venous phase imaging usually begins at the level of the diaphragm and extends to the pubic symphysis. Arterial and parenchymal phase imaging is limited to the pancreas and peripan-creatic area.

Blunt abdominal trauma remains the leading cause of pancreatitis in children and is most often caused by motor vehicle or bicycle accidents or nonaccidental injury (98). Other causes include systemic diseases (vasculitis and Crohn disease), drug toxicity (valproic acid, L-asparaginase, and corticosteroids), obstructive biliary tract diseases (gallstones, sludge, and biliary ascariasis), surgery, post–endoscopic retrograde cholangiopancreatography (ERCP), viral infection (mumps, coxsackievirus B), and developmental or hereditary disorders. Developmental anomalies include pancreas divisum and duodenal duplication, whereas hereditary diseases include cystic fibrosis and hereditary pancreatitis (98). In pancreas divisum, it has been suggested that the duct of Santorini and the accessory papilla are too small to transmit the increased volume of pancreatic secretions that must flow through them. The result is a relative stenosis at the accessory ampulla and pooling of secretions, leading to pancreatitis. Duodenal duplications may be located next to the duodenum adjacent to the ampulla of Vater or they may arise in the pancreas itself, most often in the pancreatic head and tail (99). Pancreatitis is thought to develop when there is compression of the pancreatic duct by the duplication cyst. The cause of acute pancreatitis is unknown in as many as 30% of children (98).

Chronic pancreatitis indicates persistent pancreatic inflammation associated with permanent morphologic changes in the gland and/or functional abnormalities (71). Affected children usually present with recurrent abdominal pain. Most chronic pancreatitis in children is associated with hereditary pancreatitis and pancreaticobiliary ductal anomalies (e.g., pancreas divisum and anomalous insertion of the common bile duct) (98). Hereditary pancreatitis is familial and often an autosomal dominant disorder, characterized by recurrent episodes of acute pancreatitis beginning in childhood and continuing over many years. Several genetic factors are now recognized as potential causes, including mutations within the CFTR gene and the cationic trypsinogen gene (98). Complications include exocrine and endocrine pancreatic insufficiency, pseudocyst formation, and adenocarcinoma of the pancreas. Less common causes of chronic pancreatitis are malnutrition, hyperparathyroidism, and cystic fibrosis. About 10% to 20% of children with acute pancreatitis develop recurrent or chronic pancreatitis (98).

The characteristic CT manifestations of chronic pancreatitis are parenchymal and ductal calcifications, an atrophic gland with or without fatty replacement, and pancreatic or biliary ductal dilatation. Chronic pancreatitis can be associated with pseudocyst formation and splenic and portal venous obstruction (110).

Other rarer causes of chronic pancreatitis are idiopathic fibrosing pancreatitis and autoimmune pancreatitis (111,112,113). Idiopathic fibrosing pancreatitis is associated with a fibrotic process in the pancreatic head, leading to obstruction of the common bile duct (98,112). CT findings of fibrosing pancreatitis include focal pancreatic head enlargement and biliary ductal dilatation (111) (Fig. 10.33).

Autoimmune pancreatitis is associated with elevation of serum IgG levels, autoantibodies to pancreatic antigens, irregular narrowing of the main pancreatic duct, and biliary strictures. There is an association with other autoimmune disorders, such as primary sclerosing cholangitis and systemic lupus erythematosus (113). CT findings of auto-immune pancreatitis include focal or diffuse pancreatic enlargement, delayed enhancement, a low-attenuation rim surrounding the pancreas, and minimal or no peripancreatic inflammation (113). Diagnosis of this disease is important because it is reversible with steroid therapy.

Pancreatic tumors are uncommon in children, accounting for 0.2% of all pediatric tumors (114,115,116,117). They can be divided histologically into epithelial and nonepithelial types, and epithelial tumors may be further classified as exocrine or endocrine (94). Exocrine tumors include pancreatoblastoma, ductal and acinar cell carcinoma, and solid pseudopapillary tumors. Endocrine tumors include nonfunctioning and functioning islet cell tumors (insulinoma, gastrinoma, glucagonoma, VIPoma, and somatostatinoma) and neuroendocrine adenomatosis (congenital hyperinsulinism, see above discussion). Nonepithelial pancreatic tumors arise from the connective, lymphatic, vascular, and neuronal tissues of the pancreas (114,117). Rare cystic neoplasms are microcystic adenoma and mucinous cystic tumor.

Cystic lesions of the pancreas may be congenital or acquired. True congenital cysts are epithelial lined and
thought to be due to the abnormal sequestration of primitive pancreatic ducts. They are generally asymptomatic and found incidentally, although abdominal distension, vomiting, jaundice, or pancreatitis have been reported. They are most often found prenatally or in children younger than 2 years or age, but they may be seen at any age (148,155,156). At CT, congenital cysts appear as thin-walled, unilocular or multilocular, water attenuation masses (94,149,155) (Fig. 10.43). They do not enhance following contrast administration.

Acquired cystic lesions include pseudocysts, cysts of parasitic origin, and retention cysts related to cystic fibrosis (see above). Multiple cysts may be seen in systemic diseases and syndromes, including von Hippel–Lindau disease, Beckwith–Wiedemann syndrome, autosomal dominant polycystic disease, and Meckel–Gruber syndrome (149).

Von Hippel–Lindau syndrome (VHL) is a dominantly inherited familial cancer syndrome caused by a mutation of a tumor suppressor gene, the VHL gene. VHL is associated with various neoplasms, most commonly retinal, cerebellar, and spinal hemangioblastoma; renal cell carcinoma; pheochromocytoma; and pancreatic tumors (microcystic adenomas, adenocarcinoma, and islet cell tumors). Cysts in the pancreas, kidney, liver, epididymis, and adrenal glands are also common (157,158). Most pancreatic cysts are clinically silent and discovered during routine screening examinations; rarely epigastric pain, diabetes mellitus, or steatorrhea have been reported in patients with extensive cystic disease. On CT, they have near-water-attenuation contents and thin or imperceptible walls (Fig. 10.44).

As in other parts of the body, the demonstration of small retroperitoneal lymph nodes or masses requires meticulous attention to technique. Both oral and intravenous contrast agents are usually needed to distinguish between normal structures (e.g., bowel and vessels) and lymph nodes. Scanning should be initiated during the portal venous phase of contrast enhancement.

Normal retroperitoneal lymph nodes are not routinely seen on CT examinations of infants and young children, owing to the small size of these nodes and the inherent paucity of retroperitoneal fat. Any node, regardless of size, in prepubertal patients should be regarded as abnormal. On the other hand, one or two small nodes may be seen in adolescent patients, appearing as round or oval soft tissue structures not exceeding 10 mm in size (1,159). Multiple nodes or nodes in the retrocrural area and pelvis should be viewed with suspicion, especially if there is a history of malignancy. Normal lymph nodes usually show little enhancement after administration of intravenous contrast agent. The attenuation of normal lymph nodes on unenhanced and contrast-enhanced CT scans is equal to that of muscle. Retroperitoneal lymph nodes usually have a perivascular distribution, surrounding the aorta and inferior vena cava.

The CT appearance of lymphadenopathy ranges from one or more enlarged individual lymph nodes to a large, soft tissue mass in which discrete nodes are no longer recognizable (160) (Fig. 10.45). A large mantle of confluent adenopathy may obscure the contours of adjacent structures and displace the great vessels and bowel anteriorly as well as the kidneys laterally. Both benign and malignant lymphadenopathy may show enhancement following administration of intravenous contrast material. The pattern of enhancement may be homogeneous, heterogeneous, central, or peripheral.

False-positive diagnoses of lymphadenopathy result either from misinterpretation of nodal enlargement owing to benign disease as malignancy or from mistaking unopacified collapsed bowel loops or normal vascular structures as nodal enlargement, a problem that is easily addressed by meticulous attention to technique. False-negative interpretations are almost always due to the inability to recognize replaced architecture in normal-sized nodes (161,162).

In some cases, the attenuation value of the enlarged nodes is helpful in suggesting a diagnosis (160). High-attenuation nodes may be seen when there is excess iron deposition, usually secondary to multiple transfusions for chronic anemia. Calcified nodes can be seen in old healed granulomatous infections, neuroblastoma (Fig. 10.46), and untreated or treated lymphoma. Low-attenuation (+10 to +30 HU) lymph nodes can be associated with testicular neoplasms, particularly teratocarcinoma (Fig. 10.47), lymphoma, Mycobacterium infection (usually M. tuberculosis) (163,164), and histoplasmosis. The relatively low attenuation may be the result of liquefaction, necrosis, or the deposition of fat or fatty acids in the lymph nodes.

Lymphoma, Wilms tumor, and neuroblastoma are the common causes of malignant retroperitoneal lymphadenopathy in children, but other neoplasms, such as pelvic rhabdomyosarcoma and ovarian and testicular malignancies, also can involve these nodes. In the latter two conditions, tumor usually first spreads to the retroperitoneal nodes before involving the pelvic nodes.

Retroperitoneal hemorrhage in children usually follows blunt abdominal trauma. Less frequently, it is a complication of percutaneous renal biopsy, surgery, anticoagulant therapy, a bleeding diathesis, malpositioned indwelling catheter, or retroperitoneal malignancy. The location and attenuation characteristics of the blood vary with the source and duration of the hemorrhage. Hemorrhage resulting from renal trauma or biopsy typically surrounds the kidney before extending into the retroperitoneum. Hemorrhage associated with anticoagulant therapy or a bleeding diathesis may initially diffusely infiltrate the retroperitoneum.

On enhanced CT, acute and subacute hemorrhage usually have an attenuation value equal to or less than that of muscle (185). Over time, the attenuation value decreases, usually in a centripetal fashion. A chronic hematoma appears as a well-circumscribed, low-attenuation mass (+20 to +40 HU) with a higher attenuation rim (186). The rim may later calcify. Hemorrhage may be localized or diffusely infiltrate the retroperitoneum (Fig. 10.56). Contrast-enhanced CT may show active arterial extravasation, appearing as a focal high-attenuation area surrounded by a large hematoma (185). On noncontrast CT, acute hemorrhage may appear denser than muscle.

Retroperitoneal fibrosis is uncommon in children (187,188,189). It is usually idiopathic, although it may be associated with some medications (methysergide) and
systemic diseases, including systemic lupus erythematosus, juvenile idiopathic arthritis, ankylosing spondylitis, malignancy, infection, and sickle cell disease (190). Retroperitoneal hemorrhage and urine leakage may also cause fibrosis (188). The fibrotic process typically begins at or below the aortic bifurcation and then extends cephalad along the anterior surface of the spine, encasing the blood vessels and ureters, or caudad into the pelvis, compressing the rectosigmoid colon and bladder (187). At CT, retroperitoneal fibrosis appears as a well-marginated soft tissue mass, often encasing the aorta and inferior vena cava. Loss of the normal fat planes surrounding these structures is common. There may be hydronephrosis or ureteral narrowing (191). Retroperitoneal fibrosis causes slight or no anterior displacement of the aorta; marked displacement is atypical and should suggest the possibility of a primary retroperitoneal tumor (187). Variable contrast enhancement is seen. Early-stage disease may show moderate to marked enhancement, whereas late-stage disease may be hypoattenuating.

The paired psoas muscles descend in a paravertebral location from the transverse processes of the 12th thoracic vertebra into the iliac fossa, where they merge with the iliacus to become the iliopsoas muscle. At CT, the psoas major muscles appear as paired soft tissue–attenuation, paraspinal structures. The cranial ends of the muscles have a triangular shape, whereas the caudal ends are
ovoid or round. The size of the muscles increases as they descend into the pelvis (192).

The psoas minor muscles are located immediately anterior to the psoas major muscles. At CT, they may be seen as small, rounded, soft tissue structures anterior to the psoas major muscles. These slender muscles should not be mistaken for adenopathy.

The psoas muscles are closely apposed to the posterior aspect of the retroperitoneum and its contents, including lymph nodes, kidneys, pancreas, duodenum, and the ascending and descending colon. Disease processes that involve the retroperitoneum and vertebral bodies can also involve the psoas muscles.

Psoas abscess is usually the result of direct extension of contiguous infection, such as appendicitis, inflammatory bowel disease, renal infection, or vertebral osteomyelitis. Less commonly, there is no known source and the abscess is presumed to be of hematogenous origin (192,193). The classic presentation is that of fever, back pain, and a limp. CT findings are an enlarged psoas muscle containing a focal low-attenuation fluid collection (194) (Fig 10.57). Enhancement of the abscess wall or surrounding soft tissues may be seen after administration of intravenous contrast material. The demonstration of gas bubbles within the psoas muscle is diagnostic of infection, but this finding is uncommon. Necrotic tumors and hematomas can mimic the CT appearance of psoas abscess. When imaging findings are nonspecific, CT can be used to guide percutaneous needle aspiration or biopsy for diagnosis (192,194).

The common neoplastic diseases involving the psoas muscles are lymphoma, Wilms tumor, and Ewing sarcoma. Like infection, tumor within the psoas muscle is more often the result of direct extension rather than intrinsic disease. At CT, psoas tumors usually appear as heterogeneous soft tissue masses with well-circumscribed or poorly defined margins. The involved muscle is enlarged (Fig. 10.58).

Hemorrhage into the psoas muscle usually is the result of trauma (Fig. 10.56A) or a coagulopathy, such as hemophilia, or anticoagulant therapy. As mentioned earlier, the attenuation of the hemorrhage depends on the age of the blood products (see discussion of retroperitoneal hemorrhage). The CT appearance of subacute or chronic hemorrhage can mimic that of neoplasm and abscess. In cases in which the findings are nonspecific, CT can be used to
guide percutaneous aspiration for culture and histologic examination.

The psoas muscle may atrophy as a result of neuromuscular disorders. The characteristic CT features are small size and low attenuation due to fatty replacement.