Phase-contrast magnetic resonance angiography for the determination of cerebrovascular reserve

Cerebrovascular reserve (CVR) is the potential for cerebral arteriolar dilatation to occur, in response to decreased cerebral perfusion pressure, in order to maintain constant cerebral blood flow. Diminution or absence of CVR is considered a risk factor for stroke. Current methods for determining CVR include single-photon emission computed tomography, positron emission tomography and transcranial Doppler (TCD) ultrasonography. However, significant advantages could derive from the utilization of magnetic resonance angiography (MRA) based on the concurrent acquisition of hemodynamic information (CVR and collateral flow) with phase-contrast (PC) techniques and vascular morphology with three-dimensional, time-of-flight methods. With a 1.5-T scanner and acetazolamide (AZM), an arteriolar dilator, CVR was determined in 7 normal subjects. Mean flow velocity in the middle cerebral arteries was determined by PC MRA before and after AZM administration. For comparative purposes, mean flow velocities in the same middle cerebral arteries were determined by TCD before and after AZM administration. The mean flow velocities were as follows (mean +/- standard deviation, n = 7): 40 +/- 8 (PC MRA) versus 61 +/- 10 cm/sec (TCD) before AZM treatment and 58 +/- 11 (PC MRA) versus 85 +/- 15 cm/sec (TCD) after AZM administration. The increase in mean flow velocity (before vs after AZM), that is, the CVR, was 45 +/- 11% as shown by PC MRA and 39 +/- 14% as shown by TCD. Although significant differences were present between the mean flow velocities measured before and those after AZM administration, as determined by PC MRA and TCD, the CVR was not significantly different (45 vs 39%, respectively). These preliminary results suggest that PC MRA may be a method for determining CVR.