Angiomyolipomas are hamartomas of the kidney, which means they are benign tumors containing fat tissue, smooth muscle cells, and atypical blood vessels. Angiomyolipomas reveal a variable appearance on MR images depending on the ratio of the abovementioned three different tissue types contained in them.
Angiomyolipomas occur more often in patients suffering from tuberous sclerosis and autosomal dominantly inherited phacomatoses. Angiomyolipomas in these patients can be multiple in up to 20% of cases or even bilateral. In all other patients’ angiomyolipomas occur as solitary tumors most of the time. Women are more often affected than men are. Even though angiomyolipomas are benign tumors they have a tendency to grow and rare cases of tumor growth into the renal vein and inferior vena cava has been described. With increasing size, angiomyolipomas have a tendency to rupture, with subsequent hemorrhage.
MR images demonstrate angiomyolipomas often as round lesions that exhibit high signal intensity on T1-weighted images and a distinct signal drop on fat-saturated T1-weighted images or opposed-phased T1-weighted images. In opposed-phased images, the signal cancellation occurs particularly at the boundary between the angiomyolipomas and the adjacent renal parenchyma in case of macroscopic fatty tissue and within the lesion itself in case of mainly intracellular, microscopic fatty tissue.
Rare cases in which the muscular or the vascular elements of the angiomyolipomas prevail may demonstrate moderate SI in all sequences with no distinct signal drop. Angiomyolipomas show little (<15%) enhancement. Intratumoral hemorrhage manifests with high SI on T1-weighted images.