FUNCTIONAL MAGNETIC-RESONANCE-IMAGING - THE BASICS OF BLOOD-OXYGEN-LEVEL DEPENDENT (BOLD) IMAGING

There are 2 principal techniques of functional MRI (fMRI): the blood-o xygen-level dependent (BOLD) technique, which is the favoured method b ecause no intravenous contrast medium is required, and the dynamic or exogenous technique. The BOLD technique takes advantage of the fact th at the change from diamagnetic oxyhemoglobin to paramagnetic deoxyhemo globin that takes place with brain activation results in decreased sig nal intensity on MRI. Commercially available scanners can be used to c onduct single-slice BOLD fMRI experiments, but echo-planar hardware is needed for multislice whole-brain experiments. Sequence choices in BO LD fMRI include spin-echo and gradient-echo sequences, to which rapid acquisition with relaxation enhancement and echo-planar techniques may be applied. Optimal imaging parameters (echo time, slice thickness, h eld of view and nip angle) are important in maximizing signal-to-noise ratios. Various statistical techniques and software programs have bee n developed to interpret the large amounts of data gathered from BOLD fMRI experiments, which presents one of the biggest challenges in perf orming this technique with clinical MR units. Controversy exists regar ding the effects of draining veins on cortical mapping, of inflow of b lood into the imaging slice or volume, and of motion artifact. BOLD fM RI has demonstrated good correlation with positron emission tomography , magnetoencephalography and electrocorticographic recordings of motor responses. It has been used to study cortical activity of visual, mot or, auditory and speech tasks as well as brain centres for smell, moto r imagery, complex motion and memory. As such, it holds promise for th e study of brain function, but must be subjected to larger studies com paring it with the gold standard of electrocorticographic mapping.