Shreyas Shreenivas Vasanawala MD, PhD, Abstract Co-Author: Nothing to Disclose

Brian A Hargreaves PhD, Abstract Co-Author: Nothing to Disclose

Dwight G Nishimura PhD, Abstract Co-Author: Nothing to Disclose

Francois Chan MD, PhD, Presenter: Nothing to Disclose

Two avenues of recent progress in rapid MRI are balanced steady-state free precession imaging (SSFP, True-FISP, FIESTA, balanced FFE) and parallel imaging (SMASH, SENSE). Short scan times of parallel imaging demand the high SNR efficiency of SSFP. As fat is bright in SSFP images, several methods of fat/water separation in SSFP have been proposed. The fastest is PS-SSFP, previously described for single coil imaging. We hypothesize that since image phase is retained in SENSE reconstruction through complex coil sensitivity maps and complex reduced FOV images, fat/water separation will still be feasible with the PS-SSFP method. The novel combination of PS-SSFP and SENSE will permit ultrarapid fat-suppressed 3D imaging.

With short TR and center frequency set between that of fat and water, both fat and water resonances are in high signal regions of the SSFP spectral response. If TE = TR/2, the MR signal phase is relatively constant aside from a sharp transition that is exploited by PS-SSFP to distinguish fat and water. As with SENSE, aliased high-resolution SSFP images from an array of coils are obtained along with a low-resolution reference image. A nonaliased full-FOV high-resolution complex-valued image is reconstructed, which is then partitioned into water and fat images based on the phase transition of the MR signal.

Corn oil and saline phantoms were imaged with SSFP: 35 x 17.5 cm FOV, 256 x 256 matrix, 32 slices, 5mm slice thickness, torso phased array (four coils). Also, coronal breathheld abdominal 3D SSFP images of volunteers were obtained in 19 seconds: 14 x 28 cm FOV, 256 x 256 matrix, 32 slices, 3 mm slice thickness, flip angle 60 degrees. These were then unfolded to full 28 x 28 cm FOV with a PS-SSFP SENSE reconstruction (acceleration factor = 2).Fat/water separation in phantoms and in vivo was excellent.

Despite SENSE reconstruction, phase-based separation of fat and water with the PS-SSFP method is still feasible. Excellent fat suppression is demonstrated in both phantoms and in vivo. Potential applications include cholangiography, where a SENSE acceleration factor of two distinguishes impractical and feasible suspended respiration times.

Vasanawala, S, Hargreaves, B, Nishimura, D, Chan, F, Incorporating Fat Suppression into Balanced Steady-State Free Precession Parallel Imaging.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4407784.html