Metabolite proton T₂ mapping in the healthy rhesus macaque brain at 3 T

The structure and metabolism of the rhesus macaque brain, an advanced model for neurologic diseases and their treatment response, is often studied noninvasively with MRI and ¹H-MR spectroscopy. Due to the shorter transverse relaxation time (T₂) at the higher magnetic fields these studies favor, the echo times used in ¹H-MR spectroscopy subject the metabolites to unknown T₂ weighting, decreasing the accuracy of quantification which is key for inter- and intra-animal comparisons. To establish the "baseline" (healthy animal) T₂ values, we mapped them for the three main metabolites' T₂s at 3 T in four healthy rhesus macaques and tested the hypotheses that their mean values are similar (i) among animals; and (ii) to analogs regions in the human brain. This was done with three-dimensional multivoxel ¹H-MR spectroscopy at (0.6 x 0.6 x 0.5 cm)³ = 180 μL spatial resolution over a 4.2 x 3.0 x 2.0 = 25 cm³ (∼30%) of the macaque brain in a two-point protocol that optimizes T₂ precision per unit time. The estimated T ₂s in several gray and white matter regions are all within 10% of those reported in the human brain (mean ± standard error of the mean): N-acetylaspartate = 316 ± 7, creatine = 177 ± 3, and choline = 264 ± 9 ms, with no statistically significant gray versus white matter differences.