Remember true FISP? A high SNR, near 1-second imaging method for T2-like contrast in interventional MRI at .2 T

Clinical requirements for interventional MRI (I-MRI) monitoring of needle placement or thermal ablation demand rapid (near-real-time) image acquisition rates, high spatial resolution, and T2 weighting. Experimental analysis performed earlier suggests that many sequences used for either rapid scanning or T2 weighting at high fields fail to meet both the speed (conventional spin echo [SE], turbo SE) or contrast (ie, fast low-angle shot [FLASH], fast imaging with steady state precession [FISP]) requirements when used at .2 T. In this work, we revisited a number of pulse sequences advocated primarily for higher field applications requiring T2 weighting and found that refocused steady state coherent pulse sequences, aka, true FISP sequences, performed superiorly in achieving both speed and T2 contrast requirements for I-MRI at .2 T. This work focuses on our experience with this new/old technique in the I-MRI setting and describes how one can take advantage of the low field strength and modest inhomogeneity of .2 T (and similar) systems to design pulse sequences that balance TE, TR (and hence T2 dephasing), and resonant offset frequency effects to provide images with the desired contrast and minimal artifactual field inhomogeneity "banding." At high flip angles (approximately 90 degrees ), reasonably short TEs (approximately 5 msec) and short TRs (approximately 10 msec), we have used this method in our last 25 I-MRI procedures (biopsies and/or radiofrequency [RF] thermal ablations) and found these sequences to be extremely useful in both needle localization phases of I-MRI biopsy procedures, RF thermal ablation electrode guidance, and posttherapy imaging assessment. Design methods and clinical I-MRI cases are presented that highlight these points.