The commonest lesion of the adrenal gland is that of a benign adrenal adenoma, usually lipid-rich. This lipid is, however, microscopic fat. Microscopic fat can be equated to intra-voxel fat and is best demonstrated by paired T1 in and out of phase sequences, with loss of signal on the out of phase imaging. The demonstration of loss of signal uniformly in the right adrenal on these frequency encoded fat saturated images indicate the presence of bulk fat. Note that there is also suppression of the bulk fat in the peritoneum and subcutaneous tissues which is not demonstrated on typical T1 out of phase imaging. In this instance, therefore, the appearances are suggestive of a bulk (macroscopic) fat containing right adrenal lesion. This appearance is typically due to an adrenal myelolipoma.
Adrenal myelolipomas are benign lesions, typically detected incidentally as in this case. The to the ill-defined or ill-defined. Occasionally when large there may be associated with a haemorrhagic presentation although this is rare and I have may be encountered asymptomatic lesions measuring greater than 5 or 10 cm. Occasional calcifications can be demonstrated.
Adrenal myelolipomas are a composition of mature lipid laden adipocytes (lipoma) and some erythroid cells (myelo). They are themselves nonfunctioning although there are some occasional associations with other endocrine disorders such as Cushing's disease or Conn's syndrome. When diffusely fatty these lesions are easy to characterise and diagnose. There is a more complex consideration when these lesions contain significant soft tissue components as it is difficult differentiate the possibility of a collision tumour where a benign lesion such as a fat containing myelolipoma is incidentally adjacent to neoplastic lesion. Such lesions may require PET, follow-up or biopsy.
The differential for bulk fat containing adrenal lesions is limited. There are rare described adrenal angiomyolipomas (comparable to renal angiomyolipoma). Other bulk fat containing lesions within the adrenal glands are also unusual and limited to liposarcoma or rarer fat containing adrenocortical carcinomas.