The magnetic resonance imaging (MRI) scanner was first built in New York in 1977. It has since been vital to the advancement of neuroscience and understanding disorders in the brain. The scanner itself is an enormous magnet that produces a magnetic field which is over 10,000 times stronger than that of the Earth. This field is used in conjunction with radio waves to produce detailed 3-dimensional images of the human body, with a resolution significantly superior to that of a CT scan or X-ray.
Composition of the scanner
The MRI scanner itself is composed of 3 layers. The outermost layer is the primary magnet, which produces a strong, homogeneous (uniform) magnetic field that is never turned off. The middle layer is composed of 3 gradient coils, which alter the strength of the primary magnetic field to localise the field in the x, y and z directions, so that one can distinguish where in the body that signals have originated. Radiofrequency coils, more commonly known as rf coils, constitute the innermost layers; these act as the ‘antennae’ of the MRI scanner, sending and receiving the radio waves to and from the tissue.
Water molecules (H2O) make up 65% of the human body. Inside each water molecule, are 2 Hydrogen (H) atoms, each containing 1 proton. All protons possess a quantum-mechanical property known as spin, which can be thought of as its intrinsic angular momentum. From this, arises an intrinsic magnetic moment that causes each proton to produce a small magnetic field, like a miniature bar magnet.
When the body enters the MRI scanner, the powerful magnetic field causes these protons to align either parallel or antiparallel to the field, the majority of which occupy the lower-energy parallel state. However, due to their intrinsic spin, the particles experience a torque-induced circular motion known as precession. This motion can be compared to the ‘wobbling’ that is observed as you try to bring a spinning top to a halt. The rate of this precession depends on the magnetic field strength which is applied across the body, otherwise known as the Lamor frequency after Joseph Lamor.
The gradient coils are simply loops of wire on a cylindrical shell. When a current is passed through these coils, a secondary magnetic field is produced, which alters the primary magnetic field strength so that the precession frequency of the protons varies with position. There are 3 independent gradient coils, each responsible for varying the field in the x, y and z axes, the purpose of this spatial encoding is to identify exactly where in the body that a received signal has originated.
Radio frequency (rf) coils generate a magnetic field perpendicular to the primary field. They are turned on for short periods of time, to produce rf pulses. It is important that these pulses have the same frequency (resonance) as that of the protons so that energy can be exchanged. Protons in the body absorb energy from the rf pulse, enabling them to spin out of equilibrium and align either perpendicular or antiparallel to the static magnetic field; this is known as transverse and longitudinal magnetisation, respectively. The rf pulse also causes the protons to spin in phase.
T1 and T2 relaxation
When the radiofrequency field is turned off, the protons realign with the magnetic field, releasing electromagnetic (EM) energy as they revert back to equilibrium position; protons in different tissues in the body release different amounts of energy. Spins that had been excited into the higher-energy antiparallel state revert back to the lower-energy parallel alignment in time T1, as they undergo longitudinal relaxation.
T2, the transverse magnetisation decay time, refers to the time taken for protons to lose all phase coherence once the rf pulse has been switched off, known as transverse relaxation. Unlike T1, T2 is unrelated to field strength and is tissue specific.
Signal to Image
The rf coil detects the energy released by the protons as they realign themselves with the primary field, converting this energy into an electrical signal. The computer then processes the information from this signal and it is Fourier transformed into a high-resolution image. The patient may be intravenously supplied with a contrast agent containing Gadolinium, which works by forcing the protons to realign with the magnetic field more quickly, reducing the relaxation time to produce a brighter image.
Painless, non-invasive, free of ionising radiation and capable of producing extremely detailed 3D images, the MRI technique is an extremely effective tool for medical diagnosis.