Dimitrios Mitsouras, Presenter: Nothing to Disclose

Robert Vincent Mulkern PhD, Abstract Co-Author: Nothing to Disclose

Alan Edelman PhD, Abstract Co-Author: Nothing to Disclose

Gary Peter Zientara PhD, Abstract Co-Author: Nothing to Disclose

Frank John Rybicki MD, Abstract Co-Author: Nothing to Disclose

Combination of 2D selective RF excitation for reduced field-of-view (rFOV) imaging with echo trains produced via ultra-short (.5ms), non-selective, hard-pulse refocusing RF pulses (UFSE) appears intrinsically incompatible. However, it is highly desirable since it can enable high resolution 3D MRI with reduced FSE dephasing artifact. Based on the fact that present-day MR imager technology can offer highly effective 2D selection, we implement two rFOV UFSE methods for sub-millimeter voxel imaging of the brain and spine.

The first method employs a 2D-selective RF excitation on a short (~5.5ms) reversed spiral trajectory, minimizing excitation time and enabling 3-4ms echo spacings. Echoes are formed by the non-selective UFSE train, placing focus on excitation optimization to minimize outer volume signal. The second method employs dual orthogonally 1D-selective slab excitations, thus refocusing magnetization only within the slab intersection. Following the first echo formed by the slab-selective RF pair, subsequent echoes are formed by the UFSE train with minimum 3-4ms echo spacing, placing focus on avoiding spurious echoes from magnetization outside the slab intersection. This is accomplished by use of dephasing gradients, unbalanced for outer volume magnetization throughout the echo train. Brain and spine imaging is demonstated with both methods.

A 8x8cm FOV along both phase encoded dimensions was obtained, with 98% outer volume suppression suited for 50 SNR alias-free imaging. This achieves a 1.8 to 2.5-fold resolution improvement in the spine and brain compared to 2D multi-slice protocol scans, without SNR reduction, and resulted in 1mm³ and .77mm³ voxels, respectively. Short 2D selection and ultra-short echo spacings afforded by hard-pulse refocusers minimized distortions; for example artifact from susceptibility that is especially problematic at air-tissue interfaces. Also reduced was blurring due to dephasing, thus increasing effective spatial resolution.

3D reduced FOV UFSE methods maintain or increase image quality for 3D MR neuroimaging applications while increasing imaging efficiency.

Mitsouras, D, Mulkern, R, Edelman, A, Zientara, G, Rybicki, F, Neuroimaging Applications of Two Reduced Field-of-View 3D Fast Spin Echo Magnetic Resonance Imaging Methods with Ultrashort, Nonselective, Hard-pulse Refocusing Radiofrequency Pulses.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4417174.html