Jay Pillai
We decided to write this review to highlight a relatively under-recognized phenomenon that substantially impacts interpretation of clinical fMRI examinations that are performed for presurgical mapping. This rather poorly understood phenomenon is known as “neurovascular uncoupling” (NVU). The NVU problem can be adequately addressed with breath-hold cerebrovascular reactivity mapping (BH CVR) in cases of focal resectable brain lesions such as brain tumors. We describe this methodology in this review and discuss its strengths and limitations, particularly in comparison with exogenous carbon dioxide administration methods that provide more quantitative estimates of CVR via more controlled hypercapnia challenges. We further propose that BH CVR mapping be considered an emerging standard of care for clinical fMRI because it is capable of detecting NVU that may substantially impair our ability to elicit BOLD responses in eloquent cortex. The potential adverse consequences of failure of detection of NVU may include improper utilization of complementary intraoperative mapping and inadvertent resection of eloquent cortex; the latter may lead to unexpected and sometimes permanent postsurgical neurologic deficits. At the Johns Hopkins Hospital, we have used BH CVR mapping routinely for the last 5 years and have seen many cases where it has made a real difference in patient care by preventing incorrect interpretation of presurgical fMRI activation maps. Although we deal primarily with brain tumor cases, the BH CVR approach may also be used in cases of malformations of cortical development/epilepsy, arteriovenous malformations, and other vascular malformations.
We have been expanding our research efforts with respect to BH CVR mapping by extending our work to ultra-high-field imaging at 7T and also exploring other ways of detecting NVU, as well as attenuating its effects on interpretation of fMRI activation maps. I am currently editing an issue of