Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI), diagnostic medical imaging technique utilizing the principles of nuclear magnetic resonance. Although magnetic resonance images have been produced for over two decades, the basic research in this field was conducted in the 1930s and 1940s and included fundamental investigations by physicists into the interaction of atomic nuclei with magnetic fields. By 1950 the basic physics that underlies magnetic resonance imaging was essentially well understood. However, three other circumstances had to be realized: the availability of a powerful and fast computer, the engineering of a human body-sized uniform stable magnet with associated radiofrequency electronics, and the idea that diagnostically useful interior human images were obtainable. P. C. Lauterbur, Raymond Damadian, and Peter Mansfield demonstrated the feasibility of this idea using the physical principles of nuclear magnetic resonance (Lauterbur and Mansfield were jointly awarded the 2003 Nobel Prize for Physiology or Medicine for their work on MRI). The first images using magnetic resonance were published in the early 1970s, and the medical applications have accelerated in laboratories and medical centres around the world during the decade 1983 to 1993.

The casual observer may be overwhelmed by the multitude of medical imaging techniques and applications available using magnetic resonance imaging. MRI is viewed by many as the most versatile, powerful, and sensitive diagnostic imaging modality available. Its medical importance can be summarized briefly as having the ability to non-invasively generate thin section, functional images of any part of the body at any angle and direction in a relatively short period of time. In addition, later techniques have allowed visualization of the heart with exquisite anatomical detail at any angle and direction using the technique of electrocardiographic gating. Other advances in MRI allow the visualization of arteries and veins using the technique called magnetic resonance angiography. Furthermore, magnetic resonance spectroscopic imaging allows maps of biochemical compounds corresponding to any anatomical slice of the human body. This produces exceedingly powerful basic biomedical and anatomical information with tremendous potential for fundamental new knowledge and early diagnosis of multiple diseases.

The principle of MRI is applicable in the human body because we are all filled with small biological magnets, the most abundant and responsive of which is the nucleus of the hydrogen atom, the proton. The principles of MRI take advantage of the random distribution of protons which possess fundamental magnetic properties. This process involves three basic steps. First, MRI generates a steady-state condition within the body by placing the body in a strong (30,000 times stronger than the Earth's magnetic field) steady magnetic field. Secondly, it changes the steady-state orientation of protons by stimulating the body with radiofrequency energy. Thirdly, it terminates the radiofrequency stimulation and “listens” to the body transmitting information about itself at the special “resonant” frequency using an appropriately designed antenna coil. The transmitted signal is detected and serves as the basis of the construction of internal images of the body using computer principles similar to those which were already developed for X-ray CAT (computerized axial tomography) or CT scanners.

Wherever available in current medical practice, MRI is the diagnostic modality of choice for essentially every disease of the brain and central nervous system. MRI scanners provide equivalent anatomical resolution and superior contrast resolution to X-ray CT scanners. They produce similar functional information as PET (position emission tomography) scanners but with superior anatomical detail. MRI scanners also provide complementary imaging to the X-ray images because of the ability of MRI to distinguish multiple soft tissue intensities in both normal and pathologic states. MRI is risk free, except for a few contraindications including patients with cardiac pacemakers, patients who might have iron filings next to their eye (for example, a sheet metal worker), inner ear transplants, and some aneurysm clips in the brain. While an MRI scan is relatively expensive, it is the most dramatic example of new and more accurate diagnostic information available at less risk and sometimes at less cost because of the ability to increasingly provide diagnostic evaluation for outpatients and avoid more expensive hospitalization.