Test-retest reliability of key metabolite measurements in the anterior cingulate cortex using single voxel 1H Press at 3T

1 H-MRS were determined with LCModel software. This software used a library of reference spectra in a basis set recorded at 3T and calibrated using the tissue water signal as an internal standard. The LCModel fitting algorithm uses the multiple peaks contributing to an individual metabolite spectrum to estimate the tissue content of each metabolite. The residual signal corresponds to, and is fitted by, additional broad peaks representing unknown metabolites and other factors such as macromolecular components with short T1 relaxation times. Given the voxel used was identical for all three MRS acquisitions, we did not correct for tissue content. MRS parameters used for this study provided robust signals with a signal-to-noise ratio of 15.80 and a full-width-half-maximum of 7.6Hz. All metabolites showed good reliability of fit as judged from the average Cramer-Rao lower bounds (CRLB). The mean CRLB values (estimates of the standard deviation (SD) of the fit) for Cr, NAA, TMA, mI and Glx were 3.5, 3.9, 5.4, 6.1 and 8.6 respectively (all CRLB values were <20). The variability of key metabolites in the ACC was assessed using the coefficient of variation (CV). The appeal of the CV as a measure is that the standard deviations (SDs) of such metabolites generally increase or decrease proportionally with changes in the mean, so that division by the mean removes it as a factor in the variability. The CV is therefore a standardization of the SD that allows comparison of variability estimates regardless of the overall magnitude of metabolite concentration. Results Figure 1D illustrates that while all the metabolites were variable between subjects (range from 7-15%; blue bars), the test-retest variability was relatively smaller (range 2-8%; yellow bars). CV measures indicate that Cr (2.2%) NAA (4.0%), TMA (4.7%) and mI (5.5%) were especially stable across the three within subject acquisitions (examples of Cr and NAA are displayed on Figure 1C). However, the CV was higher for Glx (8.4%). These findings are also indicated in the ratio measures (displayed on the blue bars) of the two sources of variance (inter-subject : test-retest) for all metabolites. Discussion Our findings suggest that anterior cingulate measures of Cr, Cho, NAA, and mI can be reliably indexed in a flexible manner at 3T using short TEs. mI may be moderately reactive to (and interact with) the mental demands of the scanning protocol prior to the acquisition, while Glx showed a relatively large test-retest variability, suggesting patient-control differences in this metabolite must be interpreted cautiously when measured in uncontrolled situations. A. C.