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...

Metabolite T 2 is necessary for accurate quantification of the absolute concentration of metabolites using long-echo-time (TE) acquisition schemes. However, lengthy data acquisition times pose a major challenge to mapping metabolite T 2 . In this study we used proton echo-planar spectroscopic imaging (PEPSI) at 3T to obtain fast T 2 maps of three major cerebral metabolites: N-acetyl-aspartate (NAA), creatine (Cre), and choline (Cho). We showed that PEPSI spectra matched T 2 values obtained using single-voxel spectroscopy (SVS). Data acquisition for 2D metabolite maps with a voxel volume of 0.95 ml (32 ؋ 32 image matrix) can be completed in 25 min using five TEs and eight averages. A sufficient spectral signal-to-noise ratio (SNR) for T 2 estimation was validated by high Pearson's correlation coefficients between logarithmic MR signals and TEs (R 2 ‫؍‬ 0.98, 0.97, and 0.95 for NAA, Cre, and Cho, respectively). In agreement with previous studies, we found that the T 2 values of NAA, but not Cre and Cho, were significantly different between gray matter (GM) and white matter (WM; P < 0.001). The difference between the T 2 estimates of the PEPSI and SVS scans was less than 9%. Consistent spatial distributions of T 2 were found in six healthy subjects, and disagreement among subjects was less than 10%. In summary, the PEPSI technique is a robust method to obtain fast mapping of metabolite Key words: proton echo-planar spectroscopic imaging; singlevoxel spectroscopy; T 2 relaxation time; cerebral metabolites; gray/white matter difference Estimation of the relaxation times of metabolites is necessary for accurate quantification of metabolite concentrations using long-echo-time (TE) acquisition schemes (1-3). Given the T2 relaxation time, metabolite signals acquired at different TEs can be extrapolated to obtain the signal at TE ϭ 0 and thus estimate the concentration of the metabolite (4). Differences in T2 decay are negligible only for short-TE methods (with TE below 10 ms). In several pathological conditions, such as edema and ischemic stroke (5-8), interactions between cerebral metabolites and macromolecules and proteins are known to modify the biochemical environments of the metabolites, which in turn alters the motion-sensitive T2 relaxation time. Accurate estimation of metabolite T2 is therefore especially important in clinical applications of magnetic resonance spectroscopy (MRS) to ascertain whether changes in metabolite MR signals are derived from fluctuations in the metabolite concentration or from changes in the metabolite relaxation time (2,9). In addition to providing information about metabolite concentration, T2 values may give complementary information about metabolite behavior, as has been suggested by studies of brain tumor (9), ischemic stroke (5-7,10), virus infection (11), drug abuse (12), and other neurological disorders (13-15).The T2 relaxation times of metabolites can be measured by collecting multiple spectra over a range of TEs and then fitting the MR signals as a function of TE (1-3,5,10). Se...

No abstract

No abstract

No abstract