Physiology and Physics of the fMRI Signal

The functional magnetic resonance imaging (fMRI) signal results from a complex interplay of basic physiological processes, namely cerebral metabolic rate of oxygen (CMRO2), cerebral blood flow (CBF), and cerebral blood volume (CBV). The ensuing change in the concentration of deoxygenated hemoglobin (dHb) affects the susceptibility of the tissue and gives rise to the so-called blood oxygenation level-dependent (BOLD) signal. The BOLD effect, together with the CBV change, which varies the amount of water protons in the extra- and intravascular space, is picked up as the fMRI signal by gradient- and spin-echo MRI sequences. In this chapter, we describe in detail the spatiotemporal properties of CMRO2, CBV, and CBF and how to model them mathematically. A special focus is on the resulting hemodynamic transients, such as the initial dip and the poststimulus undershoot. In the second half of the chapter, we review the fMRI physical mechanisms using the endogenous dHb-based contrast. In particular, we describe the relaxation rate as a function of susceptibility, diameter of blood vessels, MRI sequences, and magnetic field strength. With these parameters at hand, the fMRI signal and its intra- and extravascular contributions can be calculated from dHb and CBV changes. In summary, underlying the fMRI signal are numerous physiological and physical processes. The spatiotemporal information content of the fMRI signal, thus, depends on the dynamic properties of brain vascular and metabolic processes and the MRI parameters used (e.g., field strength, echo time, MRI sequence) to detect them.