Tissue correction strategy impacts GABA quantification: a study in healthy aging

Ashley D Harris^{1,2,3,4}, Eric Porges^{5}, Adam J Woods^{5,6}, Damon G Lamb^{5,7}, Ronald A Cohen^{5}, John B Williamson^{5,8}, Nicolaas AJ Puts^{3,4}, and Richard AE Edden^{3,4}

GABA-edited MRS data were collected in 94 participants (40 male) over an age range of 44 to 92, average age 73.1 ± 9.9 years. MEGA-PRESS data were collected on a 3T Philips Achieva scanner, using a 32-channel head coil and the following acquisition parameters: TR/TE = 2s/68ms, 14 ms-editing pulses at 1.9 ppm (‘On’) and 7.46 ppm (‘Off’), 320 averages, 2048 data points sampled at 2 kHz, VAPOR water suppression and 8 unsuppressed water averages for quantification. Data were acquired from an anterior voxel and a posterior voxel as shown in Figure 1. Data were processed using Gannet2.0 [3], with integrated voxel-to-image coregistration and segmentation using SPM [4]. Three different tissue correction strategies were compared: no tissue correction, CSF-correction (division by the voxel tissue fraction), and α-correction including the normalization to the group-average voxel-tissue fractions [2]. The α-correction (equation below) used an α = 0.5, thus assumes there is twice as much GABA in GM compared to WM. $$ \alpha-corrected GABA=\frac{I_{G}MM}{I_{w}\kappa}\frac{({\sum_i^{GM,WM,CSF}c_{w,i}}e^{-TE/T_{2w,i}}(1-e^{TR/T_{1w,i}})f_i)}{e^{-TE/T_{2G}}(1-e^{-TR/T_{1G}})}(\frac{\mu_{GM}+\alpha\mu_{WM}}{(f_{GM}+\alpha f_{WM})(\mu_{GM}+\alpha\mu_{WM})}) $$

where i designates
the tissue compartments GM, WM and CSF, I_{G} and I_{w} are the
signal integrals for GABA and water, respectively, c_{w,i} is the water
visibility, MM is the macromolecular fraction and κ is the editing efficiency
of GABA, f_{i} is the voxel fraction, and μGM and μWM is the
group average voxel fraction for GM and WM.

NIH grants: UL1TR000064, KL2 TR000065, R01 EB 016089, R21 NS077300, P41 EB015909, the Center for Cognitive Aging and Memory at the University of Florida, the McKnight Brain Research Foundation, and the Claude D. Pepper Center at the University of Florida.

Figure 1. Anterior and posterior voxel placement
(both midline 3x3x3 cm^{3}).

Figure 2. GABA levels plotted against age
for the uncorrected, the CSF-corrected and the α-corrected GABA levels in
the (A) posterior and (B) anterior voxels.

Figure 3. WM, GM and CSF voxel fractions
for the posterior and anterior voxels across the age range.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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