Quantitative analysis of the effects of physiologic brain motion on point-resolved spectroscopy

Abstract

Background and purpose: Although single-voxel proton MR spectroscopy is a noninvasive method that enables measurement of brain metabolite concentrations, it has been shown that physiologic brain motion causes inaccuracies in measurement of metabolite concentrations and increases the overall SD of the measurements when the stimulated echo acquisition mode (STEAM) is used. We tested the hypothesis that the point-resolved spectroscopy (PRESS) technique is less sensitive to physiologic brain motion than the STEAM technique.

Methods: In 10 healthy subjects, spectra were obtained from a voxel located in the left basal ganglia by using the PRESS sequence with cardiac gating and without water suppression to assess global phase change as a function of physiologic brain motion. This was accomplished by acquiring data at various time delays from the R wave throughout the cardiac cycle. Subsequently, spectra were obtained in 10 healthy subjects by using PRESS both without and with cardiac gating, and with water suppression, to determine whether brain motion resulted in a statistically significant difference in mean and SD of measured metabolite concentration.

Results: At various time delays from the R wave throughout the cardiac cycle, no significant global phase difference was noted in water signal intensity. In addition, when PRESS data were obtained both without and with cardiac gating (by using an optimal delay obtained from previously published data by using STEAM), no significant difference was seen in measured metabolite concentrations and SDs.

Conclusion: The PRESS technique is relatively insensitive to physiologic brain motion.

Fig 1.

Schematic representation of the PRESS sequence used to acquire spectral data. The unshaded gradients pulsed are applied in the x, y, and z axes for voxel selection. The shaded gradient pulses are dephase-rephase gradients, which remove the spurious signal intensity generated by RF inhomogeneities. The 90° and 180° RF pulses are indicated. The time window for data sampling is also indicated.

Fig 2.

T2-weighted axial image demonstrating voxel placement in the region of the left basal ganglia.

Fig 3.

Global phase difference for PRESS (solid line) as a function of gating delay from the R wave. Note that the global phase change of the water signal intensity is minimal throughout the cardiac cycle. The STEAM (dashed line) data are from previously published work.

Fig 4.

PRESS spectra obtained from one subject, illustrating the spectral quality of gated (A) and nongated (B) acquisitions. This figure demonstrates that there is no difference in spectral quality between the 2 methods.