Comparison of fat saturation techniques for single-shot fast spin echo sequences for 7-T body imaging

For T2-weighted abdominal images, homogenous fat suppression (FS) is crucial for diagnosis, but inherent B0/B1 inhomogeneities at 7 T lead to inhomogeneous FS and to tissue signal loss for most techniques. Here, 4 different FS techniques for single-shot fast spin echo were compared, whereby the recently proposed time-interleaved acquisition of modes (TIAMO) was used for the imaging portion of the sequence to reduce B1 artifacts.

Fat suppression techniques included a novel method using TIAMO (TIAMO FS: multiple fat-selective 90-degree radiofrequency pulses applied with alternating transmit radiofrequency modes), slice-selective gradient reversal (SSGR), slice-selective smaller bandwidth refocusing pulses (SSB), and the combination of SSGR and SSB with TIAMO FS. Ten volunteers were examined in 6 different ways in the following order: without any FS, with TIAMO FS, with SSGR, SSGR with preceding TIAMO FS, SSB, and SSB with preceding TIAMO FS. For evaluation of the techniques, regions of interests were placed identically for all 6 protocols per volunteer in subcutaneous fat, intra-abdominal fat, organs, and muscle. Overall image quality, artifacts, quality of subcutaneous/intra-abdominal/retroperitoneal FS, and homogeneity of FS were rated over the entire field of view by 2 experienced radiologists using a 5-point scale.

Comparing the different FS techniques, only SSGR and SSGR combined with TIAMO FS led to a nearly homogeneous FS over the entire field of view and all slices. All other techniques showed severe FS inhomogeneities. Results of a radiologic evaluation confirmed the observations made by the quantitative analysis.

Of the compared techniques, the most favorable was SSGR because, here, a homogeneous FS with moderate tissue signal loss of approximately 20% was achieved with no additional preparation pulses being necessary. Using this FS technique combined with TIAMO image acquisition, delineation between fat and bright liquids in single-shot fast spin echo images, which is essential for diagnosis, is possible at 7 T.