Absolute Quantification Using Turbo Spectroscopic Imaging in Multiple Sclerosis Patients

One of the drawbacks of scanning patients using multiple-voxel spectroscopic imaging is the long acquisition time. This is especially true when one is interested in obtaining absolute metabolite concentrations which requires acquisition of unsuppressed water spectra in addition to the suppressed spectra. In our experiment, turbo spectroscopic imaging (TSI) method with acquisition of three echoes per excitation was applied to reduce scanning time without lowering the spatial resolution. In 15 relapsing-remitting multiple sclerosis patients (mean age 37.07 years, mean disease duration 7.67 years), an MRSI scan at the level of centrum semiovale was obtained. The scan time was approximately 7 min including the unsuppressed spectra. Tissue water was used as an internal concentration reference to obtain absolute metabolite concentrations of N-acetyl-aspartate (NAA), creatine (Cr), and choline (Cho). The peak areas were corrected for differences in transversal and longitudinal relaxation times and a water concentration of 55.5 M was assumed. A three-dimensional high-resolution T ₁ scan was acquired and used to segment tissue in gray matter (GM), white matter (WM), and cerebrospinal fluid using FSL’S FAST segmentation method (a software library of the automated segmentation tool by the Center of Functional MRI of the Brain, Oxford, UK). Finally, a regression analysis was employed to address the metabolite concentrations and ratios in GM and WM, respectively. Our study shows that the metabolite concentrations (NAA, Cho, Cr) and metabolite ratios (NAA/Cr and Cho/Cr) in GM and WM obtained using the methods discussed earlier are comparable to the results found in other studies of similar patient groups. It also shows that TSI method can be used to obtain the absolute metabolite ratios in a reasonable scan time.