Magnetic Resonance Spectroscopy
MRS is a relatively new technique that, by enabling measurement of ATP, lactate levels, and pH at discrete locations within the brain [Dunbabin DW, Sandercock PAG, 1991], can distinguish areas that have no viable neurons from areas that may be salvageable (i.e., the ischemic penumbra) [Feiber SR, et al. J Neurol. 1990;237:156].
DWI involves measuring the diffusion of water molecules over short distances. The apparent diffusion coefficient of water is reduced in areas of ischemia within minutes after onset. DWI studies obtained 2 to hours after stroke onset have demonstrated the ischemic region, while standard MRI studies did not. Subsequent MRI and CT scans confirm the nature of the abnormal areas on the DWI study. Thus, DWI can detect areas of ischemic brain injury more quickly than standard MRI or CT. This technique may substantially improve the initial diagnostic accuracy and potentially provide a method for evaluating the effects of the therapeutic interventions on the ischemic region [Minematsu K, et al. Neurology. 1993;43:397-403. Lo EH, et al. J Cereb Blood Flow Metab. 1994;14:597-603].
Perfusion imaging is usually performed by injecting a contrast agent and then obtaining a rapid series of MRIs using an ultrafast technique. The images track the passage of the contrast agent through the brain. This technique can be used to assess cerebral blood flow and blood volume in various brain regions. Ultrafast MRI techniques are currently used in research setting [Minematsu K, et al. 1993].