Evaluation and clinical application of a new method of quantifying mean cerebral transit time
Recent work using Positron Emission Tomography has indicated that the best indicator of cerebral vascular reserve (CVR) is the ratio of cerebral blood flow to cerebral blood volume, which is the reciprocal of mean cerebral transit time (MCTT). However, previous attempts to quantify MCTT have ben unsuccessful. A new isotopic method of quantifying MCTT, which has overcome previous problems, is described and has been subjected to validation and application in two studies: (i) in patients with acute stroke, (ii) in patients undergoing carotid endarterectomy. In the validation study, MCTT was compared with blood flow velocity in the middle cerebral artery, using Transcranial Doppler (TCD) sonography. Both methods were reproducible and there was a linear relationship between MCTT and inter-hemispheric MCTT asymmetry has been defined. The transit time and TCD methods were employed in 32 patients with acute, first-time cerebral infarction. Patterns of underlying vascular pathology correlated with a clinical and CT scan/autopsy classification of cerebral infarction and there was good correlation between the transit time and TCD findings. The new technique, when applied to 55 patients undergoing carotid endarterectomy, showed that 31% had pre-operative evidence of impaired CVR in the symptomatic hemisphere, 75% returning to normal after surgery. Significant predictors for intra-operative sroke were; (i) age over 65, (ii) residual neurological deficit, (iii) complex plaque morphology, (iv) the combination of impaired CVR and CT infarction in the symptomatic hemisphere. No patient with recurrent symptoms after carotid endarterectomy has developed impaired CVR or recurrent disease in the operated internal carotid artery (ICA) during follow-up. One patient has developed impaired CVR in the non-operated hemisphere in association with disease progression in the non-operated ICA. The transit time method shows considerable potential as an inexpensive, quick and simple alternative to the previously available methods of evaluating CVR.