Adrenal imaging with technetium-99m-labelled low density lipoproteins

Evaluation of adrenal cortical function by external imaging is currently accomplished by injection of radiolabelled analogs of cholesterol. Although the adrenals do utilize exogenous cholesterol for steroid hormone synthesis, the cholesterol is delivered to the glands not as free cholesterol but through the uptake of low density lipoproteins (LDL), which are subsequently degraded within the adrenal cortical cells to provide cholesterol. Thus, we sought to assess the use of 99mTc-labelled LDL injected into rabbits to obtain external images of the adrenal glands. Adrenal images of all nine rabbits tested were obtained within 18 to 21 hours after injection of 99mTc-LDL. Seven of the rabbits were subjected to adrenal cortical suppression with dexamethasone and then all nine rabbits were imaged a second time. In the untreated animals, visualization of the adrenal glands was accompanied by normal serum cortisol concentrations and accumulation of radiolabel in the adrenals, whereas in the dexamethasone-treated animals, lack of visualization of the adrenal glands was correlated with low serum cortisols, and greatly decreased accumulation of the radionuclide in the adrenals. These findings demonstrate for the first time that LDL, when labelled with 99mTc, can be used to evaluate adrenal cortical function by external imaging.