Spectral phase‐corrected GRAPPA reconstruction of three‐dimensional echo‐planar spectroscopic imaging (3D‐EPSI)

Abstract

MR spectroscopic (MRS) images from a large volume of brain can be obtained using a 3D echo-planar spectroscopic imaging (3D-EPSI) sequence. However, routine applications of 3D-EPSI are still limited by a long scan time. In this communication, a new approach termed "spectral phase-corrected generalized autocalibrating partially parallel acquisitions" (SPC-GRAPPA) is introduced for the reconstruction of 3D-EPSI data to accelerate data acquisition while preserving the accuracy of quantitation of brain metabolites. In SPC-GRAPPA, voxel-by-voxel spectral phase alignment between metabolite 3D-EPSI from individual coil elements is performed in the frequency domain, utilizing the whole spectrum from interleaved water reference 3D-EPSI for robust estimation of the zero-order phase correction. The performance of SPC-GRAPPA was compared with that of fully encoded 3D-EPSI and conventional GRAPPA. Analysis of whole-brain 3D-EPSI data reconstructed by SPC-GRAPPA demonstrates that SPC-GRAPPA with an acceleration factor of 1. Key words: phase correction; partial parallel imaging; echoplanar spectroscopic imaging; brain metabolites; short echo time Volumetric echo-planar spectroscopic imaging (3D-EPSI) is currently the most efficient strategy to obtain 3D maps of brain metabolite distributions (1). Compared to conventional spectroscopic imaging, sampling efficiency is gained with EPSI by simultaneously encoding the spectral dimension together with one spatial dimension, thus reducing the time-consuming spatial encoding in spectroscopic imaging to two dimensions. Although the acquisition speed of 3D-EPSI is one order of magnitude faster than that of conventional spectroscopic imaging methods, applications of 3D-EPSI are still limited by relatively long scan times, especially when high spatial resolution is desired. With the advent of multichannel phased-array coils, faster acquisition has become possible with parallel imaging that requires fewer numbers of phase-encoding (PE) steps for image reconstruction, albeit at the expense of a reduced signal-to-noise ratio (SNR) (2). Parallel imaging was initially applied to 2D spectroscopic imaging employing the sensitivity encoding (SENSE) method (3,4). However, phase cancellations reduced metabolite signal and introduced artifacts (2). To avoid phase cancellation, coherent phase alignment between the EPSI data from the individual coils in a phased array is required. According to the Fourier shift theorem, the phase alignment can be done in either the time domain or the frequency domain. In the time domain, phase alignment can be done by "time-shifting" the free induction decay (FID) signals from different coil elements until the FID signals start with the same phase, which is usually measured from the first sampling point of the FID signal. In the frequency domain, phase alignment can be achieved by maximizing the integral of the real component of each spectrum (5,6) from the different coil elements. Such spectral phase correction (SPC) for resonance peaks, which takes the entire 1 H sp...

Strategies for rapid in vivo 1H and hyperpolarized 13C MR spectroscopic imaging

“…A number of studies have investigated the use of SENSE and GRAPPA for 1 H MRSI were presented by Dydak et al (28,29), Tsai et al (30), Lin et al (31), Ozturk-Isik et al (32,33), Bannerjee et al (34) and Zhu et al (35). While the results demonstrated were promising, there were clearly trade-offs in terms of artifacts and reconstruction times that should be considered in using such approaches.…”

“…While this technology would clearly benefit from the use of parallel imaging with receiver coil arrays (35), there have been a relatively small number of such studies to date. This is partly due to the limited number of applications to larger animals or humans and partly to the limited availability of multi-nuclear, multi-channel rf coils.…”

Quantitative SENSE-MRSI of the human brain

“…Therefore, there have been efforts to reduce MRSI scan time using various different approaches (many of which were first developed for fast MRI experiments). Some of the fast MRI techniques that have been adapted for use in MRSI include echo train acquisition [3], echo planar techniques [4], reduced k -space acquisition [5,6] and parallel imaging methods [7,8]. …”

A radial self‐calibrated (RASCAL) generalized autocalibrating partially parallel acquisition (GRAPPA) method using weight interpolation

“…One of the most successful methods to shorten scan time has been parallel imaging, which utilizes data from multiple receiver coils with varying spatial sensitivities to reduce aliasing artifact when sampling below the Nyquist limit (1–4). Parallel imaging has had a profound impact on all MRI techniques, including cardiac functional imaging (5,6), angiography (7,8), diffusion weighted liver imaging (9), and spectroscopy (10). …”