Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize detailed internal structures. MRI makes use of the property of Nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body.
An MRI machine uses a powerful magnetic field to align the magnetization of some atoms in the body, and radio frequency fields to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner—and this information is recorded to construct an image of the scanned area of the body. Strong magnetic field gradients cause nuclei at different locations to rotate at different speeds. 3-D spatial information can be obtained by providing gradients in each direction.
MRI provides good contrast between the different soft tissues of the body, which make it especially useful in imaging the brain, muscles, the heart, and cancers compared with other medical imaging techniques such ascomputed tomography (CT) or X-rays. Unlike CT scans or traditional X-rays, MRI uses no ionizing radiation.
How it Works
The body is largely composed of water molecules. Each water molecule has two hydrogen nuclei or protons. When a person goes inside the powerfulmagnetic field of the scanner, the magnetic moments of some of these protons change and align with the direction of the field. A radio frequency transmitter is briefly turned on, producing a further varying electromagnetic field. The photons of this field have just the right energy, known as the resonance frequency, to be absorbed and flip the spin of the aligned protons in the body. The frequency at which the protons resonate depends on the strength of the applied magnetic field. After the field is turned off, those protons which absorbed energy revert back to the original lower-energy spin-down state. They release the difference in energy as a photon, and the released photons are detected by the scanner as an electromagnetic signal, similar to radio waves.
As a result of conservation of energy, the resonation frequency also dictates the frequency of the released photons. The photons released when the field is removed have an energy — and therefore a frequency — which depends on the energy absorbed while the field was active. It is this relationship between field-strength and frequency that allows the use of nuclear magnetic resonance for imaging. An image can be constructed because the protons in different tissues return to their equilibrium state at different rates, which is a difference that can be detected. Five different tissue variables — spin density, T1 and T2 relaxation times and flow and spectral shifts can be used to construct images. By changing the parameters on the scanner, this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI.
The 3D position from which photons were released is learned by applying additional fields during the scan. This is done by passing electric currents through specially-wound solenoids, known as gradient coils. These fields make the magnetic field strength vary depend on the position within the patient, which in turn makes the frequency of released photons dependent on their original position in a predictable manner, and the original locations can be mathematically recovered from the resulting signal by the use of inverse Fourier transform.
Contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms, MRI images of joints. Unlike CT, MRI uses no ionizing radiation and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implants and cardiac pacemakers. In the case of cochlear implants, the US FDA has approved some implants for MRI compatibility. In the case of cardiac pacemakers, the results can sometimes be lethal,so patients with such implants are generally not eligible for MRI.
Since the gradient coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to damp this noise, it can approach 130 decibels (dB) with strong fields.
MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain,muscle, connective tissue and most tumors.