The BOLD signal is typically expressed in arbitrary units (A.U.s) or as a percent change from baseline. Although limited successes at BOLD calibration have been achieved in some research laboratories (see Advanced Discussion), for the most part BOLD signals cannot be easily converted into absolute units, such as blood flow per minute or number of depolarization spikes per second.
Hence for most BOLD/fMRI studies the signal from an "activated" brain region must be differentiated from "non-activated" areas using statistical techniques based on changes measured between "on task" and "off task" states.
Advanced Discussion (show/hide)»
Notwithstanding these limitations, some limited success at calibrated BOLD imaging has been achieved. Existing methods include observing changes in BOLD signal using respiratory challenges inducing hypercapnia (↑CO2) and hyperoxia (↑O2), combined with an arterial spin labeling (ASL) technique to measure blood flow. Cerebral blood volume may be measured using exogenous contrast (e.g. gadolinium) or by endogenous contrast methods such as vascular space occupancy (VASO). Together with sophisticated mathematical modeling oxygen extraction fraction (OEF) may then be estimated.
Blockley NP, Griffeth VEM, Simon AB, Buxton RB. A review of calibrated blood oxygenation level-dependent (BOLD) methods for the measurement of task-induced changes in brain oxygen metabolism. NMR Biomed 2013; 26:987-1003.
Hoge RD. Calibrated fMRI. Neuroimage 2012; 62:930-937. (review)
Lu H, Golay X, Pekar JJ, van Zijl PCM. Functional magnetic resonance imaging based on changes in vascular space occupancy. Magn. Reson. Med. 2003; 50: 263–274.
How do you design a BOLD/fMRI study?