Effect Of Tissue Composition And Macromolecule In Brain GABA-MRS Measurement

y-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the human brain, and plays an essential role in normal brain function and brain energy metabolism. Due to low concentration of GABA in the brain, the results measured by magnetic resonance spectroscopy (MRS) are affected by many factors. This paper investigated the effect of tissue composition and macromolecule on GABA measurement, which included three aspects:1. Calculating the fraction of brain tissue compents in the volume of interest(VOI)Using a C++ program developed in-house, we masked the high-resolution segmented image to consider only tissue inside the spectroscopic voxed and calculated the volume of each tissue class relative to total voxel volume. With this procedure, we determind the gray matter, white matter and cerebrospinal fluid percentages in the spectroscopic voxles.2. The correlation between GABA levels and varied tissue composition in the anterior cingulate cortexThe tissue contribution in GAB A measurement remains to be resolved. We aims to explore the correlation between GABA content and tissue composition in the same brain area. GABA levels from three closely adjacent voxels with varied tissue composition in the anterior cingulate cortex were acquired by J-difference editing. The percentages of white matter and gray matter in the voxels were calculated using an in-house developed program, which refer to part one of this paper. When the voxel was shifted from the center to the right, the fraction of gray matter decreased gradually. GABA level depended significantly on fraction of gray matter(r=0.425, p=0.007). The regression analysis suggested GABA signal ratio of pure gray matter to pure white matter was 1.72. This study emphasizes the importance of tissue composition analysis and correction in the research of GABA.3. The effect of macromolecule in brain GABA-MRS measurementThough J-difference editing has been widely used in editing the 3.02 ppm GABA signal, there was macromolecule (MM) signal at the same chemical shift by MEGA-PRESS sequence. The macromolecule signal can be suppressed by using symmetrical editing, however, this approach is rarely applied at field strength of 3T due to insufficiently frequency selective pulse. Long duration of editing pulse can improve frequency selectivity. We measured the GABA+(GABA+MM) and macromolecule-suppressed GABA in the occipital lobe with symmetrical editing by increasing the duration of editing pluse from 14 ms to 20 ms, and echo time(TE) from 68 ms to 80 ms in this study. We found that the fraction of the total signal retained following MM suppression ([GABA(TE=80 ms)]/[GABA+(TE=80 ms)]) was 0.73. The symmetric macromolecule-suppressed editing can be applied to acquire MM-suppressed GABA, suggesting that it is perhaps more sensitive to interindividual differences in MRS-measured GABA.