Hounsfield Units (HU) are a measure of the radiodensity of different body tissues as determined by Computed Tomography (CT). This measure of the attenuation of an x-ray beam was named after Sir Godfrey Hounsfield.
Air has an HU of -1000 while distilled water has an HU of 0. Bone is +1000 and blood is 70. Different tissues have different HUs.
Many, but not all, CT scans that evaluate the adrenal glands report HU of identified lesions or masses. Adrenal adenomas usually are <10 HU. Lesions with an HU >10 have a higher likelihood of being some other lesion and even a possible malignancy. For example, pheochromocytomas usually are 10-30 HU. Metastases to the adrenal glands usually have HU ranging from 20-50.
Contrast enhanced CT scans are complementary in the evaluation of adrenal masses. Pheochromocytomas and malignancies are usually hypervascular relative to lipid-rich adenomas. Adrenal myelolipomas have a characteristic appearance. Biochemical testing is also useful and can confirm pheochromocytoma or one of the subtypes of adrenocortical tumors.
Kirsch and others evaluated the utility of the 10HU cutoff or threshold for diagnosis of adrenal adenomas (Am J Surg. 2020 Apr 21;220(4):920-924. doi: 10.1016/J.amjsurg.2020.04.021 2). Their study was a retrospective review using surgical and pathology findings to characterize adrenal lesions. They found that using the criterion of <10HU had a sensitivity of 47.6% and a specificity of 93.3%. Using a cutoff of <16 they found the sensitivity increased to 65.4% and the specificity remained the same. When they added an estimation of the contrast washout during the CT using a value of >60% for lesions with an HU >16 the sensitivity was 93.4% and the specificity remained unchanged. The positive predictive value was 96.6 and the negative predictive value was 85.7. These criteria employ not only the radiodensity of lesions but also the contrast washout of the lesion during the CT with contrast. This is why CT scans to evaluate the adrenal glands should be ordered as “Adrenal protocol with and without contrast.” Simply ordering a CT of the abdomen does not allow the radiologist to study the adrenals glands in the way they should be evaluated.
Interestingly, most incidentally detected adrenal nodules are never investigated to further define their nature. They should be. If you’ve had a nodule detected by CT obtained for something else ask your physician to refer you to an endocrinologist for further evaluations.
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