Small (< 1.5 cm) angiomyolipomas of the kidney: characterization by the combined use of in-phase and fat-attenuated MR techniques.
The purpose of this study was to evaluate the ability of in-phase spoiled gradient echo combined with chemically selective fat suppression or out-of-phase spoiled gradient echo MR images to characterize small renal masses as angiomyolipomas. Eleven patients with a total of 35 small (< 1.5 cm) angiomyolipomas underwent MR examination at 1.5T. Eight patients had solitary and three patients had multiple angiomyolipomas. One of the patients with multiple angiomyolipomas had tuberous sclerosis and the number of angiomyolipomas were quantified as 20. MR examinations included in-phase spoiled gradient echo (all patients), chemically selective fat suppressed spin echo (six patients), chemically selective fat suppressed spoiled gradient echo (three patients), selective water excitation spoiled gradient echo (one patient) and out-of-phase spoiled gradient echo (seven patients). Angiomyolipomas were minimally (4 lesions) or moderately (31 lesions) high in signal intensity relative to renal cortex on in-phase spoiled gradient echo images. On out-of-phase spoiled gradient echo images, demonstration of signal void fat-water phase cancellation was present in all eight lesions in the seven patients who were imaged with this sequence. Small lesion size rendered the entire angiomyolipoma signal void in seven of these eight lesions due to phase cancellation artifact. Signal void phase cancellation of lesion border or signal void of the entire angiomyolipoma resulted in high lesion conspicuity in all lesions. Mild loss of signal intensity was observed in 7 lesions and moderate loss of signal intensity noted in 25 lesions on chemically selective fat suppressed images. On chemically selective fat-suppressed images, seven lesions were difficult to identify due to limited signal differences between medium intensity cortex and low intensity angiomyolipomas (six lesions in two patients) and moderate breathing artifact (one lesion in one patient). Characterization of small renal masses as angiomyolipomas may be reliably performed using the combination of in-phase and fat-attenuating MR sequences. Consistent image quality and high conspicuity of fat-water phase cancellation interfaces renders out-of-phase imaging a reliable fat attenuating method to demonstrate the presence of fat in small angiomyolipomas. Breath-hold fat-suppressed spoiled gradient echo is also effective; however, this technique demonstrates less signal loss in fatty lesions and is subject to problems with inhomogeneity of fat suppression.