[Low flip-angle spin-echo imaging of the liver. Basic study and its application to hepatic space-occupying lesions]

Abstract

Dependence on T1 contrast can be reduced by changing the excitation flip angle. Low flip-angle spin-echo imaging can reduce imaging time because repetition time (TR) is reduced. The authors assessed the efficacy of low flip-angle spin-echo images in phantoms and in liver. MR phantoms made from polyvinyl alcohol gel to model the properties of normal liver, HCC, and hemangioma were scanned with various flip angles at TR 2400 and 1200 msec. Measured signal intensities fitted well with theoretical values. The T1 contrast of signal intensity decreased as the flip angle was reduced, accompanied by a decrease in signal-to-noise ratio (S/N). Thirty patients with hepatic space-occupying lesions (23 with HCC, three with metastases and four with hemangioma) were studied by conventional SE (CSE) at 2400/60/2 (TR/TE/NEX [number of excitations]) (10 min 46 sec imaging time) and low flip-angle SE (LFSE) at 1200/60/30 degrees/2 (TR/TE/FA/NEX) (5:20) and/or 1200/60/30 degrees/4 (10:18). The sensitivity of CSE in detecting lesions was 93% (44/47). It was 92% (35/38) for LFSE with two NEX and 94% (34/36) for LFSE with four NEX pulse sequences. The contrast-to-noise ratio (C/N) for images (HCC/liver, hemangioma/liver) obtained by LFSE with four NEX was significantly higher than that for those obtained by CSE (4.8 vs 3.5, p less than 0.01; 13.4 vs 9.7, p less than 0.01, respectively). Although the C/N (lesion/liver) for LFSE with two NEX sequences was lower than that of CSE for any type of lesion (3.0 vs 3.5 for HCC; 5.1 vs 6.3 for metastases; 8.3 vs 9.7 for hemangioma), the difference was not significant. Although reducing the flip angle from 90 degrees to 30 degrees with two NEX resulted in a decrease in S/N (10.7 to 8.9 for HCC; 15.3 to 11.9 for metastases; 20.0 to 18.1 for hemangioma; 7.4 to 6.3 for normal liver; 10.7 to 10.1 for spleen), the difference was not significant. For hepatic space-occupying lesions, low flip-angle spin-echo imaging is useful to obtain T2-weighted images in a shorter imaging time without sacrificing lesion detectability.