Can Hounsfield units be measured on a non‑contrast MRI for a 1.7 cm adrenal incidentaloma?

Can Hounsfield Units Be Measured on Non-Contrast MRI?

No, Hounsfield units (HU) cannot be measured on MRI—HU are a CT-specific measurement scale that quantifies X-ray attenuation, and MRI uses entirely different physics (magnetic resonance signal intensity) that does not produce HU values. 1

Why HU Are CT-Specific

• Hounsfield units are defined exclusively for CT imaging and represent the linear attenuation coefficient of tissue relative to water, measured on a standardized scale where water = 0 HU and air = -1000 HU. 1

• CT uses X-ray attenuation to generate images, while MRI uses radiofrequency pulses and magnetic field gradients to measure hydrogen proton behavior—these are fundamentally incompatible measurement systems. 1

• The ACR Appropriateness Criteria explicitly state that non-contrast CT is required to measure HU for characterizing adrenal incidentalomas, with a threshold of ≤10 HU indicating a benign lipid-rich adenoma. 1, 2

The Correct Imaging Approach for Your 1.7 cm Adrenal Incidentaloma

Initial Characterization

• Obtain a dedicated non-contrast CT of the abdomen to measure the lesion's attenuation in HU—this is the ACR's highest-rated first-line test (appropriateness rating 8/9) for adrenal incidentalomas. 1, 2

• If the mass measures ≤10 HU and appears homogeneous, it is definitively a benign lipid-rich adenoma requiring no further imaging. 1, 2, 3

• If the mass measures >10 HU, proceed to second-line imaging with either delayed contrast-enhanced CT (washout protocol at 10-15 minutes) or chemical shift MRI to further characterize the lesion. 1, 2

MRI's Role in Adrenal Imaging

• Chemical shift MRI (with in-phase and opposed-phase sequences) is an alternative second-line test that detects signal intensity loss in lipid-rich adenomas, but it does not measure HU. 1, 2

• MRI has an appropriateness rating of 8/9 for adrenal incidentalomas when non-contrast CT is indeterminate (>10 HU), particularly useful in younger patients or when radiation exposure is a concern. 1

• Dynamic contrast-enhanced MRI has lower utility (appropriateness rating 2/9) compared to chemical shift MRI for routine adrenal characterization. 1

Critical Concurrent Step

• All patients with an adrenal incidentaloma require hormonal evaluation regardless of imaging characteristics, as 5-12% of radiologically benign incidentalomas have subclinical hormone production requiring treatment. 2, 4, 3

• Screen for pheochromocytoma with plasma free metanephrines or 24-hour urinary fractionated metanephrines before any intervention, as unrecognized pheochromocytomas can cause life-threatening hypertensive crises. 2, 5, 3

• Perform a 1 mg overnight dexamethasone suppression test (cortisol >50 nmol/L or >1.8 µg/dL indicates mild autonomous cortisol secretion). 2, 3

Size-Specific Management for Your 1.7 cm Lesion

• Lesions <3 cm are almost universally benign in patients without cancer history, making extensive workup less urgent but still necessary. 2, 3

• If the lesion measures ≤10 HU on non-contrast CT and hormonal workup is normal, no follow-up imaging is required according to ACR and European Society of Endocrinology guidelines. 2, 3

• If the lesion measures >10 HU but shows benign features on washout CT (>50% washout at 10-15 minutes) or chemical shift MRI (signal loss on opposed-phase), repeat imaging at 6-12 months is reasonable. 2, 3, 6

Common Pitfall to Avoid

• Never assume MRI signal characteristics can substitute for HU measurements—while both modalities can characterize adrenal adenomas, they use entirely different diagnostic criteria and are not interchangeable. 1, 7