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

Objective To compare imaging findings with histopathology in AML without visible fat (AML wvf ). Material and 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 ([SI IN-PHASE − SI OPPOSED-PHASE ]/SI IN-PHASE ) indices ( N = 6). A pathologist re-evaluated 15/18 AML wvf for 1) 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 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. Key Points • Five percent of AML do not demonstrate detectable fat on imaging • These AML are hyperattenuating and T2-hypointense due to abundant smooth muscle • These AML show washout enhancement without association to vessel count at histopathology • Iso-attenuating AML with microscopic fat on MRI show >25 % adipocytes/HPF • The term “AML without visible fat” is proposed to reduce ambiguity