Shreyas Vasanawala MD, PhD, PRESENTER: Nothing to Disclose

Abstract: HTML Purpose: This work demonstrates a novel steady-state MRI technique for faster 3D fat-suppressed joint imaging with T2-like contrast. Improved gradient hardware has decreased field inhomogeneity sensitivity and increased T2 image contrast of fully gradient refocused imaging (SSFP or TrueFISP). Despite optimal SNR efficiency, SSFP is hindered by bright fat. Several SSFP modifications incorporate fat suppression by novel RF phase cycles or interleaved fat suppression pulses, but with increased scan time. A novel method, termed phase-sensitive SSFP (PS-SSFP), distinguishes fat and water by the inherent SSFP signal phase. Methods and Materials: PS-SSFP employs an SSFP sequence with TE half the TR. The spectral response of the signal with respect to resonance frequency is periodic, with periodicity decreasing with decreasing TR, resulting in less field inhomogeneity sensitivity. With TR less than 7 ms, and a center frequency set between that of fat and water, both fat and water resonance frequencies are in high signal regions of the spectral response. If TE = TR/2, the MR signal phase has a sharp transition, so fat and water signals are out of phase and can be separated. To determine feasibility of fat/water separation and the contrast between various tissues, six normal knees were scanned with PS-SSFP at 1.5 T: TR 4 to 7 ms, flip angle 25 to 45 degrees, 16 cm FOV, and 1 to 2 mm slices. Six symptomatic knees were imaged with PS-SSFP and fat-suppressed FSE. Two radiologists reviewed the images and graded the pathology. Results: PS-SSFP images entire knees with 0.625 x 0.625 x 1.0 mm3 resolution in less than three minutes, with excellent fat-water separation. Because of the short echo time, tissues such as menisci and ligaments that have no signal on standard musculoskeletal imaging sequences were found to have signal with PS-SSFP. Relative signals of muscle, cruciate ligaments, cartilage, and synovial fluid to menisci are 1.8, 2.0, 2.6, and 8.4 respectively. Contrast between cartilage and synovial fluid was judged similar to T2-weighted FSE. Overall, PS-SSFP images were graded as sharper, likely from the thin slices of the 3D technique and perhaps because there is no blurring from sampling at various echo times as with FSE. Bone marrow edema and cartilage defects graded similarly on FSE and PS-SSFP images. Conclusion: PS-SSFP is a novel fast, high-resolution SNR efficient fat-water separated method of 3D musculoskeletal imaging. Fat suppression and detection of joint pathologies is demonstrated. The method is faster than previous steady-state methods.      

Vasanawala MD, PhD, S, Phase-Sensitive Steady-State Free Precession Musculoskeletal Imaging.  Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL. http://archive.rsna.org/2003/3102543.html