Angiomyolipoma (AML) without visible fat: Ultrasound, CT and MR imaging features with pathological correlation

Objectives To compare imaging findings with histopathology in AML without visible fat (AML(wvf)). Methods With IRB approval, we identified 18 AML(wvf) that underwent CT between 2002-2014. A radiologist measured NECT-attenuation, corticomedullary (CM) and nephrographic (NG) enhancement, echogenicity relative to renal cortex (RC) (N = 5), T2W (T2(AML)/T2(RC)) signal-intensity (SI), and chemical-shift SI ([SIIN-PHASE SIOPPOSED-PHASE]/SIIN-PHASE) indices (N = 6). A pathologist re-evaluated 15/18 AML(wvf) for 1)< or > 25 % adipocytes/high-power-field (HPF), 2) "many or few" blood vessels. Comparisons were performed using chi-square and independent t-tests. Results 73.3 %(11/15) of AML(wvf) had < 25 % adipocytes/HPF and 86.7 %(13/15) had "many" blood vessels. NECT-attenuation was 41.8(+/- 6.9) HU. 61.1 %(11/18) of AML(wvf) were hyper-attenuating and 38.9 %(7/18) iso-attenuating; attenuation was associated with %-adipocytes/HPF, (p = 0.01). CM/NG enhancement were 63.3(+/- 20.8)/51.7(+/- 15.5) HU. 72.2 %(13/18) of AML(wvf) had wash-out enhancement, with no association with amount of blood vessels at pathology, (p = 0.68). No difference in echogenicity was noted by histology (p > 0.05). All AML(wvf) were T2-hypointense (SI ratio = 0.61 [+/- 0.1]). 2/6 AML(wvf) showed SI drop on chemical-shift MRI; both were iso-attenuating and were associated with > 25 % adipocytes/HPF (p = 0.04). Conclusions AML(wvf) are typically T2-hypointense and hyper-attenuating with wash-out enhancement due to abundant smooth muscle and vessels respectively. Iso-attenuating AML(wvf) with microscopic fat on MRI contain more adipocytes/HPF.