Accelerated proton echo planar spectroscopic imaging (PEPSI) using GRAPPA with a 32‐channel phased‐array coil

Abstract

Parallel imaging has been demonstrated to reduce the encoding time of MR spectroscopic imaging (MRSI). Here we investigate up to 5-fold acceleration of 2D proton echo planar spectroscopic imaging (PEPSI) at 3T using generalized autocalibrating partial parallel acquisition (GRAPPA) with a 32-channel coil array, 1.5 cm 3 voxel size, TR/TE of 15/2000 ms, and 2.1 Hz spectral resolution. Compared to an 8-channel array, the smaller RF coil elements in this 32-channel array provided a 3.1-fold and 2.8-fold increase in signal-to-noise ratio (SNR) in the peripheral region and the central region, respectively, and more spatial modulated information. Comparison of sensitivityencoding (SENSE) and GRAPPA reconstruction using an 8-channel array showed that both methods yielded similar quantitative metabolite measures (P > 0.1). Concentration values of N-acetyl-aspartate (NAA), total creatine (tCr), choline (Cho), myo-inositol (mI), and the sum of glutamate and glutamine (Glx) for both methods were consistent with previous studies. Using the 32-channel array coil the mean Cramer-Rao lower bounds (CRLB) were less than 8% for NAA, tCr, and Cho and less than 15% for mI and Glx at 2-fold acceleration. At 4-fold acceleration the mean CRLB for NAA, tCr, and Cho was less than 11%. In conclusion, the use of a 32-channel coil array and GRAPPA reconstruction can significantly reduce the measurement time for mapping brain metabolites. Key words: proton echo planar spectroscopic imaging; PEPSI; MR spectroscopic imaging; parallel MRI; 32-channel phase array; GRAPPA MR spectroscopic imaging (MRSI) plays important roles in both clinical diagnosis and biomedical research. One of the main challenges of the conventional MRSI techniques is the lengthy data acquisition time, a result of the many phase-encoding steps required for complete spatial encoding. Several methods have been proposed to reduce scanning time using reduced or weighted k-space acquisition (1). Other methods acquire multiple (typically two to four) individually phase-encoded spin echoes within a single RF excitation to reduce encoding time (2). However, because the acquisition of multiple echoes requires shortened echo spacing, such a method is characterized by a limited spectral resolution. Alternatively, it is possible to acquire all the spatial information in a single shot using fast imaging readout modules, and to encode the spectral information by incrementing the spectral evolution time in separate RF excitations (3-6). In this way, spatial resolution is independent of scanning time, thus high spatial resolution can be achieved. However, the disadvantage of such approaches is the time-consuming spectral encoding process required to achieve high spectral resolution and bandwidth. Proton echo planar spectroscopic imaging (PEPSI) (7-9) uses an oscillating readout gradient to simultaneously acquire spatial and spectral information in a single RF excitation. PEPSI yields spectral resolution that approximates that of conventional MRSI and enables a reduction in the encoding tim...

High‐resolution 1H‐MRSI of the brain using SPICE: Data acquisition and image reconstruction

“…However, integration of parallel imaging, which has been shown useful in various MRSI studies (10), into the current SPICE acquisition and reconstruction framework can provide additional flexibility in trading off speed, SNR and resolution (10,(13)(14)(15), making SPICE more adaptive to various practical applications. More specifically, if sufficient SNR is available (e.g., for 3D or lower resolution acquisitions), the parallel imaging capability can be used for further acceleration.…”

“…Significant efforts have been made in the last several decades to achieve fast, high-resolution MR spectroscopic imaging (MRSI), through the development of fast sequences (1)(2)(3)(4)(5)(6)(7)(8)(9)(10) and advanced image reconstruction methods (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). SPICE (SPectroscopic Imaging by exploiting spatiospectral CorrElation) is a relatively new approach that we recently proposed to achieve high-resolution MRSI with good signal-to-noise ratio (SNR) and speed (26).…”

Potential Impact of a 32-Channel Receiving Head Coil Technology on the Results of a Functional MRI Paradigm

“…as the data acquisition and processing capabilities of modern magnetic resonance imaging scanners advance, there is a trend toward the use of multielement coils with a higher number of coil elements for magnetic resonance imaging (Mri) in general [1][2][3][4][5][6][7][8][9][10][11] and functional magnetic resonance imaging (fMri) in particular [12]. Two possible benefits for the use of higher-element coils have been discussed in the scientific community.…”

“…On the one hand, they should enable higher acceleration factors for parallel imaging techniques [1,3,4,8,9] which, in turn, lead to shorter examination times, reduction of acoustic scanner noise as well as reduced susceptibility artifacts for fMri due to shorter echo train durations during echo-planar imaging (ePi) [13,14]. On the other hand, one expects a higher signal-expressed as signal-to-noise ratio (Snr) [2,3,6,10,12]-which would be reflected in the ability of improved object discrimination holding different signal intensities. a major limitation of the fMri methodology is the extremely low physiological Snr of the blood oxygen leveldependent (BOlD) signal which is caused by physiological signal variations.…”

“…Here we aimed to (1) evaluate the potential of a 32-channel receiving (32ch) head coil in comparison to a standard eight-channel (8ch) coil for fMri using a standard motor paradigm, and to (2) compare Snrs for both coils during phantom measurements.…”