Michael E. Herrtage, MA BVSc DVR DVD DSAM MRCVS DipECVIM DipECVDI
It is possible to image the normal adrenal glands of most dogs and cats with a 7.5 MHz transducer. Large, obese and deep-chested dogs provide the most difficulty.
A lateral approach is used for each adrenal with the dog lying in lateral recumbency on the contralateral side. The right adrenal is more difficult to image than the left. It is consistently located craniomedial to the right kidney lying between the cranial pole of the kidney and CVC. The right adrenal is dorsal or dorsolateral to the CVC just cranial to the cranial mesenteric artery. The optimum image is obtained by scanning laterally between the last 2 intercostal spaces, therefore a real-time sector scanner is necessary to obtain a good image. The exact location varies with the position of the kidney. The left adrenal is more variable in location. It may be found anywhere from craniomedial to the left kidney to a more midline position near the aorta adjacent to the first or second lumbar vertebra. The left adrenal is ventrolateral to the aorta between the origin of the cranial mesenteric artery and the left renal artery.
The normal adrenal is somewhat flattened, bilobed and hypoechoic compared with the surrounding tissues. The cranial lobe is larger with a spatulate shape and the caudal lobe tapers distally. The mid-region of the adrenal is narrowed and frequently has a groove in which lies the phrenicoabdominal vein. The medulla of the normal adrenal is slightly hyperechoic compared to the cortex. The maximum and minimum dimensions (length x thickness) of normal canine adrenal glands have been reported as 10-50 mm x 2-12 mm (but are usually in the range 12-26 mm x 3-6 mm). There is poor correlation between these dimensions and body weight.
Assessment of the adrenal glands should include size, shape, echogenicity and corticomedullary architecture. Each kidney and the aorta and caudal vena cava should also be assessed from this approach.
The challenge for the ultrasonographer is to consistently distinguish between normal, hyperplastic and neoplastic glands. If both adrenal glands were of similar size and of normal shape, the cause of confirmed hyperadrenocorticism (HAC) is pituitary-dependent. However, if one adrenal appears abnormally enlarged and rounded with the contralateral adrenal of normal shape, the cause of HAC is adrenal-dependent.
Although the adrenal glands of dogs with pituitary-dependent hyperadrenocorticism have been characterised as being symmetrically enlarged with maintenance of the normal adrenal shape and contour, the diagnosis of adrenal hyperplasia is a somewhat subjective evaluation. The measurement of the thickness of the adrenal gland is considered more sensitive than the either the length or width of the gland. The hyperplastic adrenal is larger and much easier to image than a normal one, but has a normal, homogeneous hypoechoic pattern. A thickness of > 7.5 mm for the left adrenal is considered to provide the best sensitivity and specificity as a diagnostic test for pituitary-dependent hyperadrenocorticism, but this does not necessarily relate to function.
Adrenal masses are diagnosed by the location of the mass and clinical signs exhibited by the animal. Adrenocortical tumours may be function or non-functional and are difficult to differentiate ultrasonographically from tumours of the adrenal medulla (phaeochromocytoma). There is a propensity for malignant adrenal tumours to invade nearby vessels and surrounding tissues particularly the adjacent kidney, therefore a thorough ultrasonographic examination of adjacent vessels should be performed. Mineralisation is frequently associated with benign and malignant adrenocortical tumours in the dog and may aid in localising the adrenal. If an adrenal mass is identified, the liver should also be imaged for evidence of hepatic metastases.
In hypoadrenocorticism or Addison's disease the adrenal glands may appear atrophied.
The normal pancreas can be difficult to recognise on an ultrasonographic examination. The area of the pancreas is identified using surrounding anatomical structures. The area of the right lobe is defined caudal to the duodenal flexure, ventral to the right kidney and cranial to the colon. The body of the pancreas is caudal to the liver and ventral to the portal vein. The left lobe of the pancreas is inconsistently seen but may be found dorsomedial to the spleen in the left cranial abdomen. The area of the pancreas is mildly hyperechoic, presumably due to peripancreatic fat. Scanning planes used to image the pancreas include sagittal, ventral transverse, right lateral transverse and right ventral oblique. It may be advantageous to image in sternal or right lateral recumbency in order to avoid bowel gas. Gentle pressure on the abdomen with the transducer may help in displacing obscuring bowel gas.
Acute pancreatitis images ultrasonographically as a non-homogeneous, hypoechoic mass. The hypoechoic areas are related to oedema and/or necrosis. The borders of the inflamed pancreas are ill defined. Ascitic fluid and a thickened, hypoechoic duodenal wall may be identified. The inflamed pancreas may obscure normally visualised deeper structures, such as the portal vein. With chronic pancreatitis, the pancreas is slightly larger and more hyperechoic than normal. The increased echogenicity is due to the fibrous tissue formation that results from acute or recurrent pancreatitis.
Small pancreatic tumours (for example, insulinomas which may be causing clinical signs) are difficult to visualise. This may be due to improper scanning technique, interpretative inexperience or overlying stomach or bowel gas obscuring the mass. Therefore a negative scan does not rule out the presence of a tumour. In one series, insulinomas > 7 mm could be detected as spherical or lobular hypoechoic nodules. Large pancreatic tumours tend to have a complex echo pattern. It is often difficult to ascertain that the pancreas is the site of origin because of the large size of the mass. There is often diffuse metastatic disease present and examination of the hepatic parenchyma and regional lymph nodes is worthwhile. Pancreatitis and pancreatic neoplasia may cause bile duct obstruction that can be detected on liver scans.
The thyroid gland consists of two fusiform lobes each of which can be imaged in the long axis (sagittal plane) by positioning the transducer just caudal to the larynx, identifying the common carotid artery and then scanning medial to it or in the short axis (transverse plan) at the same level. Each lobe appears as a well-marginated, homogenous structure, which is less echogenic than the surrounding adventitia, but more echogenic than the cervical musculature. Each lobe normally measures 2-3 cm x 0.4-0.6 cm in medium sized dogs and 2 cm x 0.2-0.3 cm in cats. The right lobe may be more difficult to identify than the left.
Mass lesions may be recognised by glandular enlargement. The principal differential diagnoses for a thyroid mass in the dog or cat are benign and malignant neoplasia, which may be functional or non-functional. Functional adenomas, the most common cause of feline hyperthyroidism, vary between a well-defined hypoechoic nodular lesion to diffuse enlargement of the one or both lobes with a rather heterogeneous echo pattern. Thyroid carcinomas, the most common tumour in the dog, appear as large inhomogeneous masses with variable margination. Invasion of the carotid arteries, jugular veins and oesophagus may be detected.
There are normally four parathyroid glands in the dog and cat. The cranial pair of parathyroid glands is usually found cranial to the cranial pole of the thyroid gland. The caudal pair of parathyroid glands is usually embedded in the parenchyma of the caudal pole of the thyroid. The normal parathyroid glands are not always visualised ultrasonographically.
Most enlarged parathyroid glands in the dog are visible ultrasonographically as rounded hypoechoic or anechoic structures associated with the thyroid gland. Finding a solitary parathyroid gland in a hypercalcaemic dog supports a diagnosis of primary hyperparathyroidism whereas finding multiple enlarged parathyroid glands is compatible with secondary hyperparathyroidism. There are no reports of distinguishing a parathyroid adenoma from an adenocarcinoma.