High‐resolution 31P echo‐planar spectroscopic imaging in vivo at 7T

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In vivo phosphorus (31P) MR spectroscopy (MRS) enables noninvasive studies of energy metabolism, intracellular pH, and phospholipid turnover in living tissue. Mapping 31P‐containing metabolites by means of 31P MR spectroscopic imaging (MRSI) yielded information concerning, for example, bioenergetics of human muscle 1, heart diseases 2, pathologies of the liver 4, pathologic phospholipid metabolism in breast cancer 5, and the signature of small metabolites in prostate tumors 7. A notorious problem of 31P MRSI is the poor spatial resolution owing to the low concentration of 31P‐containing compounds in tissue. In addition, 31P MRSI usually is performed with conventional chemical shift imaging (CSI) 8, which requires time‐consuming phase‐encoding for 2D or 3D spatial sampling.
The application of acquisition‐weighted 31P CSI techniques in combination with surface coils at B0 = 7T enabled voxel sizes in the range of 5 and 10 mL 10. The corresponding measurement times for 2D or 3D in vivo 31P CSI were in the order of 5 to 20 min.
To improve these acquisition parameters, MRSI techniques with efficient spatial sampling were proposed. A particularly interesting candidate is echo‐planar spectroscopic imaging (EPSI), which samples data in the presence of a rapidly oscillating readout gradient 15. The simultaneous encoding of spectral and spatial information strongly reduces measurement times in comparison to conventional CSI. Proton (1H) EPSI of the human brain was first demonstrated by Posse et al. 17.
A limitation of EPSI is the high demand on the performance of the gradient system. Instabilities and timing errors in applied gradient trains, as well as drifts and inhomogeneities in B0, manifest as ghost artifacts in spectra 18; uncompensated gradient eddy currents cause line shape distortions 19. Scan time is spared when employing EPSI instead of CSI, but that is at the expense of spectral quality and signal‐to‐noise ratio (SNR) per unit time and unit volume 20. The loss in SNR per unit time and unit volume can be kept small by means of ramp‐sampling techniques with appropriate gradient wave forms 21. To reduce problems related to gradient performance, multishot EPSI approaches 23 turned out to be useful.
Compared to 1H EPSI, application of EPSI to 31P is even more challenging because stronger gradients must be switched with shorter oscillation periods. The purpose of this study was to show feasibility of 31P EPSI in human calf muscle and brain at B0 = 7T and demonstrate volumetric mapping of phosphorus‐containing metabolites in vivo with high spatial resolution. To obtain sufficient spectral width with reduced demand on the gradient system, the multishot approach of Matsui et al. 24 was employed.

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