High-Speed Spiral-Scan Echo Planar NMR Imaging-I

Abstract

An improved echo planar high-speed imaging technique using spiral scan is presented and experimental advantages are discussed. This proposed spiral-scan echo planar imaging (SEPI) technique employs two linearly increasing sinusoidal gradient fields, which results in a spiral trajectory in the spatial frequency domain (k-domain) that covers the entire frequency domain uniformly. The advantages of the method are: 1) circularly symmetric T2 weighting, resulting in a circularly symmetric point spread function in the image domain; 2) elimination of discontinuities in gradient waveforms which in turn will reduce initial transient as well as steady-state distortions; and 3) effective rapid spiral-scan from dc to high frequency in a continuous fashion, which ensures multiple pulsing with interlacing for further resolution improvement without T2 decay image degradation. Some preliminary experimental results will be presented and further possible improvements suggested.

Spiral Scan Peripheral Nerve Stimulation

“…Defining S x ϭ ѨB x /Ѩt, etc., setting S z ϭ 0, and substituting Eqs. [18] through [20] into Eq. [17] gives…”

BOLD signal compartmentalization based on the apparent diffusion coefficient

“…The isotropic diffusion weighting gradients were embedded into a gradient-recalled spiral imaging sequence [11,12]. The use of spiral acquisition allowed us to achieve large b factors without using excessively long echo time in that spiral imaging starts data acquisition from the center of k-space.…”

Modified block uniform resampling (BURS) algorithm using truncated singular value decomposition: Fast accurate gridding with noise and artifact reduction

“…The URS algorithm is based on the sampling theorem which asserts that the original analog signal can be recovered if the Nyquist criterion is satisfied. Equivalently, the column vector x of the data at N Cartesian grid points and the column vector b of the measured data at M sampled points can be related in matrix form as Ax ϭ b [1] where A is an M ϫ N matrix with elements a ji given by a ji ϭ sinc͑ ͉ j Ϫ k i ͉/dk͒, i ϭ 1, . .…”

“…[3] with respect to x, the SVD of A needs to be performed to calculate A # (the Moore-Penrose pseudoinverse matrix of A ). Suppose that A is an m ϫ n matrix of rank r. There exist orthogonal m ϫ m and n ϫ n matrices U and V such that A ϭ U⌺V T and ⌺ ‫؍‬ U T A V where the m ϫ n matrix ⌺ is a zero matrix except the first r diagonal elements 1 . In the BURS algorithm, one particular element of A # b which corresponds to the central grid point in a grid window is employed as the gridded value and all other elements are ignored.…”

“…Nonuniform data sampling under time-varying gradients, such as spiral (1,2) and rosette scans (3), is often performed for fast data acquisition in magnetic resonance imaging (MRI). Most of the image reconstruction methods currently used in nonuniformly-sampled k-space generally consist of two steps: gridding, and then 2D-inverse Fourier transform (2D-IFT) reconstruction.…”