The physics of functional magnetic resonance imaging (fMRI)

Richard B Buxton
2013 Reports on progress in physics (Print)  
Functional magnetic resonance imaging (fMRI) is a methodology for detecting dynamic patterns of activity in the working human brain. Although the initial discoveries that led to fMRI are only about 20 years old, this new field has revolutionized the study of brain function. The ability to detect changes in brain activity has a biophysical basis in the magnetic properties of deoxyhemoglobin, and a physiological basis in the way blood flow increases more than oxygen metabolism when local neural
more » ... tivity increases. These effects translate to a subtle increase in the local magnetic resonance signal, the blood oxygenation level dependent (BOLD) effect, when neural activity increases. With current techniques, this pattern of activation can be measured with resolution approaching 1 mm 3 spatially and 1 s temporally. This review focuses on the physical basis of the BOLD effect, the imaging methods used to measure it, the possible origins of the physiological effects that produce a mismatch of blood flow and oxygen metabolism during neural activation, and the mathematical models that have been developed to understand the measured signals. An overarching theme is the growing field of quantitative fMRI, in which other MRI methods are combined with BOLD methods and analyzed within a theoretical modeling framework to derive quantitative estimates of oxygen metabolism and other physiological variables. That goal is the current challenge for fMRI: to move fMRI from a mapping tool to a quantitative probe of brain physiology. The fMRI experiment The NMR signal NMR is a highly developed field with many sophisticated methods for manipulating the magnetization associated with nuclear spins to yield informative signals. The basic method for generating the signal used in fMRI, though, is perhaps the simplest imaginable NMR signal. The central physical principles underlying NMR are the following: 1. Equilibrium magnetization. When placed in a magnetic field B 0 , the magnetic moments of nuclei with nonzero spin tend to weakly align with B 0 , creating a net macroscopic magnetization M 0 . Functional MRI manipulates the magnetization due to hydrogen nuclei (protons), and the hydrogen nuclei in the brain are overwhelmingly in water molecules (a proton concentration of about 78M, compared with mM concentrations of most other metabolites). Precession. If the magnetization M 0 is tipped away from alignment with B 0 , it will precess around the B 0 axis with angular frequency ω 0 = γB 0 , where γ (gyromagnetic ratio) is a constant for any given nucleus. For protons γ = 2.675 × 10 8 rad T −1 , and a typical magnetic field for fMRI is 3 Tesla (T), so the precession frequency ν 0 (=ω 0 /2π) is approximately 128 MHz. After tipping the magnetization away from B 0 , the net magnetization vector can be described as two components: the remaining longitudinal magnetization along the B 0 axis, and the rotating transverse magnetization perpendicular to B 0 . The rotating component generates an oscillating magnetic field that induces a current in a nearby coil, creating the basic measured NMR signal. Relaxation. Over time, the transverse magnetization decays exponentially to zero with a time constant T 2 , and the longitudinal magnetization recovers exponentially toward its equilibrium value M 0 with a time constant T 1 . In gray matter in the human brain at a field strength of 3 T, T 1 ~1.0 s and T 2 ~ 0.1 s.
doi:10.1088/0034-4885/76/9/096601 pmid:24006360 pmcid:PMC4376284 fatcat:3r24a6k54jcpbkrcpzo62rwl3q