Gurmeen Kaur, MBBS
@kaurgurmeen
A major challenge faced by vascular neurologists and neuro-radiologists is accurately identifying the subset of patients with chronic cerebrovascular steno-occlusive disease, predicting those that have a propensity to develop hemodynamic failure and, as a result, have an increased risk of stroke.
Over the past few years, with development of imaging technology, multiple CT and MRI-based techniques have been developed to assess the degree of hemodynamic failure. Cerebral blood flow measurement using 15O PET scan has been recognized as the gold standard. There is a baseline measurement followed by a second scan with acetazolamide (Diamox) challenge.
The Diamox challenge helps in identifying a ‘perfusion reserve’ with stages of hemodynamic failure as described:
Stage 0: normal CBF, normal perfusion reserve
Stage 1: normal CBF, decreased perfusion reserve
Stage 2: decreased CBF, decreased perfusion reserve
While this 15O PET scan is the gold standard, there are significant issues with regular use of it because it needs a radioactive tracer and is available in only a few specialized centers. Because of these limitations, the authors explore alternate forms of measuring and quantifying hemodynamic failure. Blood oxygen level dependent (BOLD) fMRI emerges as a good option because of its ease of acquisition and the fact that it can be potentially added to every clinical MRI, which is routinely performed in patients with strokes. In the BOLD technique, changes in signal per mm of Hg change in CO2 are used to determine cerebrovascular reserve (CVR) capacity in a given vascular bed.
To assess whether BOLD compares well with the gold standard, the authors compared BOLD and 15O PET scan in 19 scans of 16 patients with chronic steno-occlusive disease and 10 normal healthy controls, which were performed within 6 weeks of each other.
In the BOLD group, quantitative CVR was significantly lower in the patients compared to controls, and there was no difference in left and right hemispheres in the control group. Significant differences between the affected and unaffected hemisphere were found for both BOLD and PET.
Given the good agreement in the 2 techniques, it was concluded that BOLD CVR can be used as a surrogate imaging marker for 15O PET scan for determining hemodynamic failure of the affected hemisphere and MCA territory and for hemodynamic failure stage II, increased stroke risk for individual chronic cerebrovascular pathologies, such as carotid artery occlusion.
The authors predict that in the future, BOLD CVR, or even simply ASL (arterial spin labeling) could replace the 15O PET scan because of the ease of acquisition and low cost. By grouping BOLD CVR studies of healthy subjects, a healthy reference atlas can be created that may assist in better determination of specific cutoff values for hemodynamic failure. If this is successful, conventional stroke protocol MRIs could include BOLD or ASL and help us in identifying the patient cohort at the risk of re-occlusion and clinical worsening and hence expedite recognition and treatment.
