Localised 1H-MR spectroscopy for metabolic characterisation of diffuse and focal brain lesions in patients infected with HIV

OBJECTIVES—To evaluate the role of proton MRspectroscopy (¹H-MRS) in detecting metabolic changes indiffuse or focal lesions in the brain of patients infected with HIV.
METHODS—Sixty HIV seropositive patients (25 with HIV related encephalopathies, 20 with toxoplasmosis, eight withprogressive multifocal leukoencephalopathies (PMLs), and sevenwith lymphomas) and 22HIV seronegative neurologicalcontrols were examined with a combined MRI and¹H-MRStechnique using a Siemens 1.5 Tesla Magnetom. Spectra (Spin Echosequence, TE 135 ms) were acquired by single voxel, localised on focallesions in toxoplasmosis, PML, lymphomas, and HIV encephalopathies andon the centrum semiovale of neurological controls. Choline (Cho),creatine (Cr), N-acetyl aspartate (NAA), lactate, and lipids wereevaluated in each spectrum and NAA/Cr, NAA/Cho, and Cho/Cr ratios were calculated.
RESULTS—A significant decrease in NAA/Cr andNAA/Cho ratios were found in all HIV diagnostic groups in comparisonwith neurological controls (p<0.003), suggesting neuronal or axonaldamage independent of brain lesion aetiology. However, the NAA/Cr ratiowas significantly lower in PML and lymphomas than in HIVencephalopathies (p<0.02) and toxoplasmosis (p<0.05). HIVencephalopathies, lymphomas, and toxoplasmosis showed a significantincrease in the Cho/Cr ratio in comparison withneurological controls (p<0.03) without between groupdifferences. The presence of a lipid signal was more frequent inlymphomas (71%) than in other HIV groups (Fisher's test, p=0.00003). The presence of mobile lipid resonance together with a high Cho/Cr ratio in lymphomas may be related to an increased membrane synthesis and turnover in tumour cells. A lactate signal (marker of inflammatory reaction), was found in all but one patient with PML lesions (75%), but had a lower incidence in the other HIV diagnostic groups (Fisher's test, p=0.00024).
CONCLUSION—¹H-MRS shows a highsensitivity in detecting brain involvement in HIV related diseases, buta poor specificity in differential diagnosis of HIV brain lesions.Nevertheless, the homogeneous metabolic pattern that characterises PMLsuggests the usefulness of ¹H-MRS as an adjunct to MRI indifferentiating CNS white matter lesions, such as HIV encephalopathies,from PML.