Bunched phase encoding (BPE): A new fast data acquisition method in MRI

Abstract

A new fast data acquisition method, "Bunched Phase Encoding" (BPE), is presented. In conventional rectilinear data acquisition, only a readout gradient (and no phase encoding gradient) is applied when k-space data are acquired. Reduction of the number of phase encoding lines by increasing the phase encoding step size often leads to aliasing artifacts. Papoulis's generalized sampling theory asserts that in some cases aliasing artifact-free signals can be reconstructed even if the Nyquist criterion is violated in some regions of the Fourier domain. In this study, Papoulis's theoretical construct is exploited to reduce the number of acquired phase encoding lines. To achieve this, k-space data are sampled along a "zigzag" trajectory during each readout; samples are acquired at a sampling frequency higher than that of the normal rectilinear acquisition. The total number of TR cycles and, hence, the total scan time can be reduced. Data acquisition speed is often critical in practical MR imaging (MRI) applications. Conventional rectilinear MRI data acquisition time is equal to TR * N pe , where N pe is equal to the number of phase encoding (PE) lines in the rectilinear data acquisition. Reduction of the number of PE lines often leads to undesirable artifacts (e.g., aliasing) in the reconstructed image. To date, various kinds of reconstruction techniques have been developed to reduce MRI scan time. These methods include partial Fourier imaging (1-4) and parallel imaging (5-8). In partial Fourier imaging, data are acquired in only a limited part of the target k-space. The missing k-space data are "estimated" using special reconstruction techniques. However, image quality often depends on the original k-space coverage and a priori estimated information, e.g., an image phase map, used in the reconstruction process.In parallel imaging, parts of k-space data are collected using multiple receiver channels. Image reconstruction can be performed based on knowledge of the receiver coils' sensitivities. Although parallel imaging is a promising fast data acquisition method, it requires more than one receiver channel. Furthermore, an appropriate coil arrangement and accurate sensitivity maps are essential for image reconstruction with sufficient reduction of aliasing artifacts.A generalized sampling theory was proposed by Papoulis in 1977 (9). This theory asserts that in certain cases (e.g., under the condition that f(t) is zero outside a finite interval, i.e., time-limited) the original function f(t) can be reconstructed without aliasing artifacts even if the Nyquist criterion is violated in portions of the Fourier domain (9). For example, if m bunched samples are acquired at 1/m-th the Nyquist rate in the Fourier domain, it is possible to reconstruct the original time-limited function f(t) without aliasing artifacts.In this study we take advantage of this theoretical construct to reduce the number of TR cycles in rectilinear MRI data acquisitions. In the newly proposed method, the PE step size is set larger than that of the conve...

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