Abstract
Task and resting-state functional MRI (fMRI) is primarily based on the same blood-oxygenation level-dependent (BOLD) phenomenon that MRI-based cerebrovascular reactivity (CVR) mapping has most commonly relied upon. This technique is finding an ever-increasing role in neuroscience and clinical research as well as treatment planning. The estimation of CVR has unique applications in and associations with fMRI. In particular, CVR estimation is part of a family of techniques called calibrated BOLD fMRI, the purpose of which is to allow the mapping of cerebral oxidative metabolism (CMRO2) using a combination of BOLD and cerebral-blood flow (CBF) measurements. Moreover, CVR has recently been shown to be a major source of vascular bias in computing resting-state functional connectivity, in much the same way that it is used to neutralize the vascular contribution in calibrated fMRI. Furthermore, due to the obvious challenges in estimating CVR using gas challenges, a rapidly growing field of study is the estimation of CVR without any form of challenge, including the use of resting-state fMRI for that purpose. This review addresses all of these aspects in which CVR interacts with fMRI and the role of CVR in calibrated fMRI, provides an overview of the physiological biases and assumptions underlying hypercapnia-based CVR and calibrated fMRI, and provides a view into the future of non-invasive CVR measurement.
Highlights
Reviewed by: Joana Pinto, University of Oxford, United Kingdom Peiying Liu, Johns Hopkins Medicine, United States
This review addresses all of these aspects in which cerebrovascular reactivity (CVR) interacts with functional MRI (fMRI) and the role of CVR in calibrated fMRI, provides an overview of the physiological biases and assumptions underlying hypercapnia-based CVR and calibrated fMRI, and provides a view into the future of non-invasive CVR measurement
The step of the calibrated fMRI framework per the Davis model is to estimate the oxidative metabolism component of the bloodoxygenation level-dependent (BOLD) signal measured in response to a task, by combining the M parameter already measured, the BOLD signal measured in response to the task, the cerebral-blood flow (CBF) signal measured in response to the same task, and the same alpha and beta parameters mentioned in the calibration procedure
Summary
Functional MRI (fMRI) is predominantly performed using the blood-oxygenation level-dependent (BOLD) signal. In Komori et al (2007) for example, this increase was shown to be of 21.6% for arteriolar diameter and 34.5% flow velocity for a 50% change in CO2 partial pressure in rabbit arterioles This sensitivity can be captured using MRI, within our own data, a 12.0% change in inhaled CO2 concentration resulting in a 24.9% change in gray matter CBF measured using arterial spin labeling (ASL) and a 1.5% change in the gray matter BOLD signal (Gauthier and Hoge, 2013). The various methods for producing CO2 variations are summarized in a number of recent reviews (Fierstra et al, 2013; Chen, 2018; Pinto et al, 2020)
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