What is the normal appearance of the adrenal glands on CT and MRI?
The adrenal glands are anterosuperior to the upper poles of the kidneys and are located within the perinephric space of the retroperitoneum. The right adrenal gland is located anterior and lateral to the right diaphragmatic crus, posterior to the inferior vena cava, and medial to the right hepatic lobe. The left adrenal gland is located anterior and lateral to the left diaphragmatic crus, posterior to the splenic vessels at the level of the pancreas or stomach, and medial to the spleen.
On CT, the adrenal glands have soft tissue attenuation, and on MRI, they have intermediate T1-weighted and slightly high T2-weighted signal intensity relative to skeletal muscle. Each adrenal gland is comprised of a body, a medial limb, and a lateral limb and often has an inverted letter “Y” or “V” configuration. Each adrenal gland limb is usually ≤ 6 mm in thickness. However, the left adrenal gland is generally thicker and more nodular than the right adrenal gland, and the adrenal glands often increase in thickness/volume and become more nodular with increasing age.
The adrenal glands typically receive arterial blood supply from three separate arteries, although variations can occur: the superior adrenal arteries arise from the inferior phrenic arteries; the middle adrenal arteries arise from the lateral aspect of the abdominal aorta; and the inferior adrenal arteries arise from the superior aspect of the renal arteries. The right adrenal vein drains into the inferior vena cava, whereas the left adrenal vein joins with the left inferior phrenic vein before draining into the left renal vein, although variants are common.
Unenhanced CT images (but not MR images) are useful to detect adrenal calcifications. Unenhanced images on both CT and MRI are useful to (1) detect presence of macroscopic fat within an adrenal nodule to establish a specific diagnosis of adrenal myelolipoma, (2) detect presence of microscopic lipid content within an adrenal nodule to establish a specific diagnosis of adrenal adenoma, and (3) serve as a reference of comparison for contrast-enhanced images for adrenal nodule characterization.