Effects of physiologic human brain motion on proton spectroscopy: quantitative analysis and correction with cardiac gating

Abstract

Proton MR spectroscopy is a powerful noninvasive method that enables measurement of certain brain metabolites in healthy subjects and patients with diseases. A major difficulty with clinical and research applications of in vivo proton MR spectroscopy is the variability of metabolite concentrations, especially in regions with substantial physiologic motion. In our preliminary evaluation, we tested the hypothesis that physiologic brain motion leads to lower mean metabolite concentrations and higher SDs for the measured metabolite concentrations.

Fig 1.

Schematic representation shows the standard STEAM sequence used. The gradients (x, y, z) are applied along one of the three orthogonal axes for voxel selection, and the gradient waveforms (A1, A2) and shaded gradient waveform during the mixing time (TM) are applied along all three orthogonal axes. The distance between the vertical dashed lines under A1 and A2 represent δ; the distance of the horizontal dashed line represents Δ.

Fig 2.

Axial T2-weighted image shows the placement of the 2 × 2 × 2-cm voxel in the left basal ganglia region, where the water-suppressed MR spectroscopic data were acquired.

Fig 3.

Plot shows the global phase difference in the water signal (global phase at each time point minus the mean global phase) in the five subjects. The large change in the global phase difference during the systolic phase (75–300 milliseconds) of the cardiac cycle indicates a large amount of brain motion during that period. A relatively small global phase difference is observed after 300 milliseconds; however, the error bars are large ( 300–600 milliseconds). These indicate that some brain motion occurred in some subjects. The global phase difference and the error bar is small at the 675-millisecond delay; this finding indicates minimal brain motion in all of the subjects.

Fig 4.

Typical STEAM spectrum acquired with (A) and without (B) cardiac gating . The area under each of the metabolite peaks is larger with the data obtained from the gated spectrum compared with that of the nongated spectrum.