Fig. 10.1
Triangle-shaped normal left adrenal gland (arrow)
10.4 The Right Adrenal Gland
The patient is placed in left lateral decubitus and asked to breathe in deeply to improve the acoustic window offered by the liver. The right adrenal is better identifiable by sonography than the left, thanks to the presence of anatomical landmarks like the vena cava, diaphragm pillar, and liver, as well as the better scanning options. In fact, on the right, as well as intercostal scans, it is possible to perform anterior subcostal scans, whereas these are more difficult to achieve on the left.
The right adrenal gland is posterior to the left gland, located at the level of the 12th dorsal vertebra and retrocavally. Medially, the reference point is the diaphragm pillar and laterally the eighth liver segment. The lower part of the right adrenal gland is anteromedial to the right kidney, and the upper part extends upward toward the bare liver area (a liver portion that is devoid of the peritoneum covering, this portion lies between the two layers of coronary ligaments), separated from it by a thin layer of areolar fat. On intercostal scans, it appears as a hypoechogenic linear structure between the bare liver area and the diaphragm. On transverse scans it is visible at about the level of the liver hilum. On subcostal transverse scans, it appears as a hypoechogenic linear structure between the bare liver area and the diaphragm (it can sometimes be masked by the liver hilum structures).
10.5 The Left Adrenal Gland
The left adrenal is located more anteriorly than the right gland, extends anteriorly to the superior pole of the homolateral kidney, and lies vertically between the bodies of T12 and L1, being about 4 cm long. It can be examined with the patient in lateral right decubitus, making longitudinal scans along the medial or posterior axillary line, exploiting the acoustic window offered by the spleen. Visualization of the left adrenal gland may be unsuccessful due to colonic and/or gastric meteorism, but it is typically located between the lateral margin of the aorta and anteriorly to the superior pole of the left kidney. Medially, the point of reference is the diaphragm pillar, while laterally it is visible in contact with the inferior-medial face of the spleen; anteriorly it borders the tail of the pancreas and the splenic vein. The gland can be seen posteriorly to the splenic vein as a hypoechogenic linear structure, but it is frequently masked by the stomach shadow. In thin patients, compression can improve the acoustic window, by shifting the stomach. In left lateral decubitus, the left hepatic lobe can improve the acoustic window.
10.6 Ultrasound Characteristics of Adrenal Lesions
The indication for US study of adrenal gland disorders is to assess all those diseases that cause an increased volume of the organ: adenoma, carcinoma, trauma, hemorrhage, thrombosis, infections, TBC, metastases, histoplasmosis, and medullary tumors such as pheochromocytoma and pheochromoblastoma. Adrenal lesions are frequently diagnosed as “incidentalomas” that account for 13 % of all US examinations of the abdomen: 2/3 of incidentalomas are cortical adenomas and metastases [12].
10.6.1 Hyperplasia of the Glands and Adenomas
In adrenal hyperplasia, the gland is larger and presents clear-cut margins with an isoechogenic structure as compared to the renal parenchyma. Adenomas are generally rounded neoformations, mostly small and hypoechogenic (Fig. 10.2). A left adrenal adenoma can be confused with an accessory spleen. It has recently been reported that the use of contrast medium while performing the sonography can enable a differential diagnosis between adenomas and nonadenomatous lesions, with a comparable sensitivity to that of CT and MRI [13].
Fig. 10.2
Adrenal adenoma: hypoechogenic rounded mass (arrow)
10.6.2 Myelolipomas
Adrenal myelolipomas are rare, nonfunctioning tumors consisting of variable proportions of myeloid elements and fat, deriving from the cortical fascicular area. The age incidence of this tumor is between the 50s and 60s, with no difference between males and females. Because they are benign and nonsecreting, they are typically diagnosed incidentally. The usual size is between 2 and 10 cm (more rarely, 10–20 cm). For myelolipoma, US can have a specificity value: in general, it presents as a rounded mass with clear-cut margins and a hyperechogenic structure (Fig. 10.3), although this may not be true if the myeloid component exceeds the adipose portion. In this case it may present as hypoechogenic and difficult to differentiate from hyperplasia. If the tumor is large (>4 cm) and has a high-fat content, it is reasonable to expect artifacts due to variations in the propagation of the ultrasound wave through the fat that can provoke an apparent deformity of the diaphragm [14]. Differential diagnosis must be made with hemorrhagic cysts (generally well defined and circular, with a heterogeneous aspect if seen at onset, but hypoechogenic if long standing), pheochromocytoma (highly vascularized with hemorrhagic areas and necrosis), adjacent tumors (exophytic renal clear cell carcinoma, renal angiomyolipoma, exophytic liver hemangioma, or hepatocellular carcinoma), liposarcoma (primary retroperitoneal sarcoma involving the perirenal space, simulating an adipose tumor), and adrenal adenoma.
Fig. 10.3
Adrenal myelolipoma: hyperechogenic mass (arrow)
10.6.3 Pheochromocytomas
Pheochromocytomas typically derive from the neuroectodermic tissue of the adrenal medulla. They are associated with many neuroectodermic diseases (tuberous sclerosis, neurofibromatosis, von Hippel-Lindau disease, MEN IIa and IIb). These tumors affect patients aged between 40 and 60, and the right/left gland ratio is 2:1. Most pheochromocytomas are poorly defined at US and sometimes appear isoechogenic, sometimes heterogeneous, depending on the presence of necrosis or internal hemorrhagic spots (Fig. 10.4). Because of their rich vascularization, in some cases, they may look like cystic lesions with a hemorrhagic content and necrotic fragments accounting for the heterogeneous ultrasound appearance. Calcifications are rare. A differential diagnosis needs to be made between pheochromocytoma and adenoma or hyperplasia if it is small, while it may be confused with a carcinoma if it is larger.
