Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) is a medical technique which utilizes a magnetic field and the natural An example of a magnetic resonance imaging system (MRI). Photograph by Mason Morfit. FPG International Corp. Reproduced by permission.

resonance of atoms to provide an image of human tissue. While the foundation for its development first took place in the late 1930s, it was not until the late 1960s that it was used by doctors to view the inner workings of the human body.

The development of MRI began in the early 1900s with discoveries made in nuclear magnetic resonance (NMR). During this time, scientists were just starting to develop theories about the structure of atoms and the nature of visible and ultraviolet light. NMR was discovered to be related to the magnetic properties of an atom's nucleus. It is a phenomenon in which atomic nuclei absorb and emit radio waves when placed in a large magnetic field. These properties were first demonstrated in 1924 by the Austrian physicist Wolfgang Pauli (1900-1958).

In 1938, the first instrument to utilize an atom's nuclear magnetic resonance for analysis was developed. This device was able to provide data related to the magnetic properties of certain substances. However, this crude instrument had two major drawbacks including its ability to only analyze gaseous materials and its inability to provide direct measurements. These limitations were overcome in 1945 when two groups led by two scientists Felix Bloch and Edward M. Purcell independently developed improved NMR devices. These new devices were more useful than the first NMR, providing researchers with the ability to collect data on many different types of systems. After some technological improvements scientists were able to use this technology to investigate biological tissues in the mid-1960s.

The application of NMR to medicine soon followed. Scientists discovered that different types of tissue gave different magnetic signals. One of the first applications of NMR was using it to distinguish between normal and cancerous tissue. This was done by Raymond Damadian (1936-) in 1971. Later experiments showed that many different body tissues could be distinguished by NMR scans. In 1973, NMR data was integrated with computer calculations of tomography and the first magnetic resonance image (MRI) was produced. To test this new method, a device was designed to study a living mouse. While it took more than an hour to scan, an image of the mouse's internal organs was obtained. Human imaging followed a few years later. The first commercial MRI scanners were sold around 1981. Since then, various technological improvements have been made which helped reduce the scanning time required and improve the resolution of the images. Most notable improvements have been made in the three-dimensional application of MRI.

Modern MRI devices are used frequently by doctors to produce images of the interior tissues of their patients. First the patient is put on a flat bed device which is surrounded by several coils which can produce a strong constant magnetic field. A known radiofrequency (RF) signal is then applied to the system causing certain atoms within the patient to resonate. When the RF signal is stopped, the atoms continue to resonate for a short time. Eventually, when the resonating atoms return to their natural state they emit their own RF signal which is received by detectors on the MRI. The signals are then processed through a computer and converted into a visual image of the patient.

The signals that are emitted from the body are produced by protons within the body. In a typical MRI scan these signals come from hydrogen atoms in the body. The first magnetic resonance images were constructed solely on the concentration of protons within a given tissue. However, these images were fuzzy and did not have good resolution. When the relaxation time, which is the time it takes for the protons to emit their signals, was included in the calculations of the scan, MRI became much more useful for constructing an internal image of the body. In all body tissues, there are two types of relaxation times, T1 and T2, that can be detected. The different types of tissues exhibit different T1 and T2 values. For example the brain tissue has a different T1 and T2 value than blood. By using the three variables, proton density, T1 and T2 values, a clear image can be constructed.

MRI is now used for a variety of applications. It is used by far the most for creating images of the human brain. It is particularly useful for this purpose because the soft tissue emits a distinct signal making it easy to distinguish between healthy tissue and lesions. In addition to structural information, MRI also allows scientists to study brain function. This type of imaging is based on the fact that during brain activity the rate of blood flow changes. When the scans are taken with sufficient speed the blood can actually be seen moving through the organ. This has important consequences for studying the various parts of the brain. Another application for MRI is in sports medicine where it is used for muscular skeletal imaging. Using MRI, injuries to the ligaments and cartilage in the joints of the knees, wrists, and shoulder can be readily seen. This technique has eliminated the need for traditional invasive surgeries. A developing use for MRI is in tracking chemical components in the body. In these scans a person is injected with a compound containing molecules such as carbon 13 or phosphorus 31. These atoms produce distinguishable signals so they can be easily tracked in the body.