Introduction 

FMRI (Functional magnetic resonance imaging) 

Types 

Neural activities lead to metabolic responses, such as increased blood flow and oxygen supply, which are  detected and used in fMRI by the following methods:

• Contrast fMRI: involves an injection of contrast agents such as iron oxide, coated with sugar or starch. The agents reach the brain and generate a signal that is proportional to the cerebral blood volume (CBV). This method provides relatively strong signals; however, due to the introduction of agents into the bloodstream, it is considered a semi-invasive method.

• Blood-Oxygen-Level Dependent (BOLD) fMRI: this widely used technique detects differences in magnetic susceptibility between oxygenated hemoglobin (diamagnetic) and deoxygenated hemoglobin (paramagnetic). It is considered an indirect measure of neuronal activity because it measures changes in blood oxygenation rather than direct neuronal firing. Evidence strongly correlates BOLD signals with neuronal activity (Logothetis et al., 2001). The method still relies on the magnetic properties of hydrogen atoms in water molecules, which are influenced by the local magnetic environment altered by oxygenation levels.

• Perfusion fMRI:  this technique employs ‘arterial spin labeling’ (ASL) to magnetically label hydrogen nuclei in arterial blood and  image their distribution in the brain. This method is sensitive to cerebral blood flow (CBF), which is considered a good correlate of neuronal activity and does not require any contrast agents. Compared to the BOLD responses, the signal in perfusion fMRI is more stable and the noise is whiter. However, it has a weaker signal and higher image acquisition time, so it’s less used.