Functional Magnetic Resonance Imaging or fMRI is the medical imaging technique used to measure the haemodynamic response of the brain in relation to the neural activities. fMRI is the advanced form of MRI Scanning which uses the magnetic resonance property of the nucleus of the atoms. It accesses the functions of the brain using the magnetic resonance property of haemoglobin.
The blood flow and oxygen transport in the brain are closely related. When the nerve cells in the brain are actively metabolizing, they consume more glucose and becomes less energetic and shift to anaerobic Glycolysis. The haemodynamic response to this brain activity causes more blood flow to the sites of increased activity. This changes the oxyhaemoglobin level of blood and local blood volume.
Magnetic resonance is the property of atoms to vibrate when subjected to a strong magnetic field. The MRI uses this property to align the magnetization of atoms in the brain cells using a strong magnetic field. The externally applied magnetic field causes the nuclei of atoms to produce a rotating magnetic field which can be recorded. The strong magnetic field causes the rotation of nuclei at different speeds in different areas. The nuclei of active and inactive areas of the brain show difference in magnetic resonance, so it is easy to detect defective areas of the brain.
Haemodynamic response of Brain
The brain is composed of delicate nerve cells which utilize only glucose for energy production. The nerve cells do not store any glucose or oxygen and continuous supply of these materials is necessary for normal brain activity. The haemodynamic response of the brain is meant for rapid supply of oxygen and glucose to the sites of higher activities. As a result of the haemodynamic response, the blood releases more oxygen and glucose to Neurons and Astrocytes of the brain. This creates a higher Oxyhaemoglobin – De-oxyhaemoglbin ratio in the local areas of the brain.
Magnetism in tissues
Living tissue exhibits some magnetic property which is the character of protons and electrons. MRI exploits this magnetic property to identify dead tissues in organs. The MRI collects the signals from the water protons. In live tissue, water is abundant compared to dead tissue. This will help to identify the defective tissues. Magnetic relaxation in tissues can be increased using Contrast agents which can be introduced into the body intravenously to identify the areas of hypervascularity. The contrast agents contain magnetic centers that create a magnetic field very much higher than that of water protons.
Haemoglobin is Dimagnetic and Paramagnetic
The haemoglobin molecule exhibits a magnetic resonance property and is Dimagnetic in the oxyhaemoglobin state and Paramagnetic in the de-oxyhaemoglobin state. Dimagnetism is the property to create a magnetic field in opposition to an externally created magnetic field thus causing a repulsive effect. The external magnetic field alters the orbital velocity of electrons spinning around the nucleus of the atom. This causes a change in the magnetic dipole movement. Paramagnetic property means the ability of magnetism in the presence of an external magnetic field. The paramagnetic materials have a magnetic permeability greater or equal to unity and hence attracted to magnetic fields.
Haemoglobin and BOLD
Haemoglobin is the major endogenous oxygen-binding molecule, responsible for binding oxygen in the lung and transporting it to the tissues by means of the circulation. Haemoglobin is the Red colored pigment in the blood which contains Iron which is always kept in the Ferrous state. That is why the haemoglobin is red in colour both in the oxidized and de oxidized states. When the haemoglobin combines with oxygen, it becomes oxyhaemoglobin and when it releases oxygen to the cells, it becomes De-oxyhaemoglobin. The parameter used in fMRI is BOLD (Blood-Oxygen-Level Dependence) which is the relative concentration of two states of haemoglobin like Oxyhaemoglobin and Deoxyhaemoglobin.
The Magnetic Resonance of the brain depends on the level of oxygen, so by collecting the data of magnetic resonance, it is easy to access the utilization of oxygen and glucose by the brain cells. That is, active cells use glucose and dead cells do not use glucose.
fMRI is a Non Invasive ( Not causing injury) technique used to record brain signals without the risk of radiation as seen in CT or PET scanning. It gives a spatial resolution of 2-3 mm and can record signals from all parts of the brain and spinal cord. fMRI working is based on the volumetric acquisition of images with good spatial and temporal resolutions. Images are taken every 1-4 seconds and the Voxels (Voxel is the volume element, representing a value on a regular grid in three dimensional space. It is similar to Pixel in digital image) in the image represents tissues of 2-4 mm cube size.