[Magnetic resonance with fat-saturation sequences in studying the upper abdomen: the normal semeiological aspects]

The fat-saturation (FAT-SAT) MR technique decreases the signal intensity of fat in tissues, though yielding the T1, T2 and proton-density (PD) information available on spin-echo (SE) sequences. To investigate the potentials of FAT-SAT sequences in MRI of the upper abdomen, the authors carried out a prospective study including 129 subjects, namely 12 normal volunteers and 117 patients with different abdominal conditions. The patients were submitted to T1-weighted SE sequences (TR 500-600 ms, TE 15 ms), T2W SE (TR 1600-1730 ms, TE 80 ms) and PD SE (TR 1600-1730 ms, TE 20 ms). The images obtained with and without fat suppression were compared both qualitatively and quantitatively, with a special emphasis on the normal anatomy of the upper abdomen: we investigated the efficacy of subcutaneous and retroperitoneal fat suppression (116/129 cases, 90%), the reduction in respiratory and chemical shift artifacts (112/129 cases, 87%) and the better visualization of parenchyma (119/129 cases, 93%) and of other abdominal structures. Concerning the quantitative study, we calculated the signal-to-noise ratio (S/N) for liver, spleen, pancreas, adrenal glands, renal cortex and medulla, the improvement of contrast in these organs after retroperitoneal fat suppression (conspicuity) and the increase in contrast between organs (dynamic range). The statistical analysis showed significant differences between the sequences with and those without fat suppression. Correlations were found between observers' and quantitative evaluations, suggesting that the better yield of FAT-SAT sequences is probably due to three factors: 1) retroperitoneal and subcutaneous fat suppression; 2) increase in S/N ratio for pancreas and renal cortex on T1W images; 3) reduction in the dynamic range of signal intensity, which increases contrast between pancreas, adrenal glands and renal cortex relative to adjacent structures, especially on T2W or PD sequences. The results of this study suggest that FAT-SAT sequences are useful because fat suppression increases contrast and improves image quality, reducing respiratory and chemical shift artifacts.