MRI and Radiology Schools Directory

MRI machines make use of the fact that body tissue contains lots of water, and hence protons (1H nuclei), which get aligned in a large magnetic field. Each water molecule has two hydrogen nuclei or protons. When a person is inside the powerful magnetic field of the scanner, the average magnetic moment of many protons becomes aligned with the direction of the field. A radio frequency current is briefly turned on, producing a varying electromagnetic field. This electromagnetic field has just the right frequency, known as the resonance frequency, to be absorbed and flip the spin of the protons in the magnetic field. After the electromagnetic field is turned off, the spins of the protons return to thermodynamic equilibrium and the bulk magnetization becomes re-aligned with the static magnetic field. During this relaxation, a radio frequency signal (electromagnetic radiation in the RF range) is generated, which can be measured with receiver coils.

Information about the origin of the signal in 3D space can be learned by applying additional magnetic fields during the scan. These additional magnetic fields can be used to only generate detectable signal from specific locations in the body (spatial excitation) and/or to make magnetization at different spatial locations precess at different frequencies, which enables k-space encoding of spatial information. The 3D images obtained in MRI can be rotated along arbitrary orientations and manipulated by the doctor to be better able to detect tiny changes of structures within the body. These fields, generated by passing electric currents through gradient coils, make the magnetic field strength vary depending on the position within the magnet. Because this makes the frequency of the released radio signal also dependent on its origin in a predictable manner, the distribution of protons in the body can be mathematically recovered from the signal, typically by the use of the inverse Fourier transform.

Protons in different tissues return to their equilibrium state at different relaxation rates. Different tissue variables, including spin density, T1 and T2 relaxation times, and flow and spectral shifts can be used to construct images. By changing the settings 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.

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.
Magnetic field

MRI scans require a magnetic field with two properties, uniform field density and strength. The magnetic field cannot vary more than 1/10,000 of 1% and field strength ranges (depending on the scanner) from 0.2 to 3 teslas in strength in currently clinically used scanners, with research scanners investigating higher field strengths such as seven teslas. The lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners, for claustrophobic patients. Higher field strengths can be achieved only with superconducting magnets. An MRI with a 3.0 tesla strength magnet may be referred to as a "3-T MRI" or "3-tesla 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 [9] (see also the subsection on acoustic noise).
Contrast agents and implants

MRI 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. There are many electronically activated devices that have approval from the US FDA to permit MRI procedures in patients under highly specific MRI conditions . 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.

Prepolarized MRI

In 2001, a research team at Stanford invented a new technique which came to be called "Prepolarized MRI" or PMRI. The team demonstrated that the magnets do not have to be both uniform and strong, rather two magnets can be used together, where one is strong and the other one is uniform.

The first magnet in a PMRI scanner is strong, but not uniform. This magnet creates a very strong magnetic field which varies in uniformity by as much as 40%. This is the "prepolarize" component. A second much weaker (only requiring the electric power necessary to run two hairdryers) but far more precise magnet then creates a homogeneous magnetic field. These two magnets can be ordinary copper wound magnets, which greatly lowers the cost of an MRI scanner. Because the magnetic field is "tuned" by the second magnet, a PMRI scan can be obtained immediately adjacent to a metal prosthetic, unlike an MRI scan.


When are MRI scans used?

An MRI ( magnetic resonance imaging) scan is a radiology technique that uses magnetism, radio waves, and a computer to produce images of body structures. The MRI scanner is a tube surrounded by a giant circular magnet. The patient is placed on a moveable bed that is inserted into the magnet. The magnet creates a strong magnetic field that aligns the protons of hydrogen atoms, which are then exposed to a beam of radio waves. This spins the various protons of the body, and they produce a faint signal that is detected by the receiver portion of the MRI scanner. The receiver information is processed by a computer, and an image is produced.

The image and resolution produced by MRI is quite detailed and can detect tiny changes of structures within the body. For some procedures, contrast agents, such as gadolinium, are used to increase the accuracy of the images.

An MRI scan can be used as an extremely accurate method of disease detection throughout the body. In the head, trauma to the brain can be seen as bleeding or swelling. Other abnormalities often found include brain aneurysms, stroke, tumors of the brain, as well as tumors or inflammation of the spine.

Neurosurgeons use an MRI scan not only in defining brain anatomy but in evaluating the integrity of the spinal cord after trauma. It is also used when considering problems associated with the vertebrae or intervertebral discs of the spine. An MRI scan can evaluate the structure of the heart and aorta, where it can detect aneurysms or tears.

It provides valuable information on glands and organs within the abdomen, and accurate information about the structure of the joints, soft tissues, and bones of the body. Often, surgery can be deferred or more accurately directed after knowing the results of an MRI scan.