Quality of 186 child brain spectra using motion and B0 shim navigated single voxel spectroscopy

Abstract

Purpose: To evaluate B0 shim and motion navigated single voxel spectroscopy in children. Assess the repeatability of metabolite concentrations in three regions: medial frontal grey matter, peritrigonal white matter, and basal ganglia. Determine the extent of intra- and interacquisition movement in this population.

Methods: Linewidth and signal to noise ratio were calculated to assess spectral quality of 186 spectra at 3 Tesla. Repeatability was assessed on 31 repeat scans. Navigator images were used to assess localization errors, while navigator motion and shim logs were used to demonstrate the efficacy of correction needed during the scans.

Results: Average linewidths ± standard deviations of N-acetyl aspartate are 3.8 ± 0.6 Hz, 4.4 ± 0.5 Hz, and 4.7 ± 0.8 Hz in each region, respectively. Scan-to-scan measurement variance in metabolite concentrations closely resembled the expected variance. A total of 73% and 32% of children moved before and during the acquisition, causing a voxel shift of more than 10% of the voxel volume, 1.5 mm. The predominant movement directions were sliding out of the coil and nodding (up-down rotation). First-order B0 corrections were significant (>10 μT/m) in 18 % of acquisitions.

Conclusion: Prospective motion and B0 correction provides high quality repeatable spectra. The study found that most children moved between acquisitions and a substantial number moved during acquisitions.

Keywords: B0 correction; motion correction; navigator; single voxel spectroscopy.

Figure 1

Three different volumes of interest: medial frontal grey matter, right peritrigonal white matter, and right basal ganglia.

Figure 2

Spectral quality plot showing the association of poorer SNR with increased linewidth. mfgm: medial frontal grey matter, ptwm: peritrigonal white matter, bg: basal ganglia. Note the bg outlier at 9.8 Hz corresponds to a voxel reported in Figure 7 that overlapped the ventricle.

Figure 3

The best, median, and worst spectra acquired in each VOI. The spectra shown were phase corrected by LCModel after offline data pre-processing. For the worst spectrum in the bg, the subject moved before the scan causing the voxel to partly overlap the ventricle.

Figure 4

Box and whisker plots showing the range of Cramér-Rao minimum variance bounds (% standard deviation) for metabolite quantifications as calculated by LCModel for each VOI.

Figure 5

Bland Altman plots of differences in concentration at two time points as a function of the average concentration for the 31 repeated acquisitions.

Figure 6

Box and whisker diagrams showing the displacement of SVS voxels from their intended locations for each VOI. a. Inter acquisition shifts were assessed between the start of the scan session and the relevant acquisition; “water” denotes the displacement between the relevant water reference and the start of the relevant acquisition. b–d. Maximum, mean, and standard deviation of displacement during each acquisition.

Figure 7

Incorrect voxel location due to motion between structural and SVS acquisitions. a. Coronal and transverse slices from a subject for whom movement caused the voxel to move into the ventricle. This small shift had a significant impact on the spectral quality and corresponds to the outlier in Figure 2 with an SNR of 6. b. Coronal and sagittal slices for an acquisition in peritrigonal white matter where the voxel shifted 10.4 mm. The voxel remained predominantly in white matter.

Figure 8

Maximum first-order B0 gradient applied by the navigator at any time during an acquisition for the three VOIs. Gradients are Euclidean norms computed relative to the initial shim.