SIV infection and CD8 lymphocyte depletion results in rapid and robust neuronal injury. Upper panel: In vivo ¹H magnetic resonance spectra from frontal cortex of a rhesus macaque. Before SIV inoculation (left) and 10 weeks after SIV infection and CD8 depletion. Indicated on the spectrum are the metabolites utilized for the study, which included NAA, Cho, Cr and MI. The hatched line demonstrates a decline in the intensity of the NAA resonance that is observed 10 weeks after infection when the spectra are normalized to the Cr resonance (dotted line). Lower panel: Changes in NAA/Cr as a function of time after infection in SIV-infected, CD8 lymphocyte–depleted animals. Data are from 4 SIV-infected, CD8 lymphocyte–depleted animals. Three of 4 animals were scanned twice before infection, so there are 7 data points at time 0. All 4 animals were scanned at 2 weeks after infection. Three animals were scanned at 21 days and 3 were scanned at 40 days after infection. Thereafter, the animals were scanned at lower frequency. The measurements were derived from in vivo ¹H MR spectra acquired from 1.5 × 1.5 × 1.5 cm³ voxels localized to the frontal lobe, centrum semiovale and basal ganglia. A strong linear relationship between the time of infection and the NAA/Cr ratio was present for frontal cortex within the frontal lobe (r = –0.70; P = 0.0001) (left), basal ganglia (r = –0.57; P = 0.006) (center), and centrum semiovale white matter (r = –0.68; P = 0.0003) (right).