Robert Gerstman D.O., a member of the American
              Osteopathic Association, provides the following
              explanation. Dr. Gerstman, who specializes in
              psychiatry, is board certified by the American
              Osteopathic Board of Neurology and Psychiatry and the
              American Board of Psychiatry and Neurology.

              Before Magnetic Resonance Imaging
              (MRI) entered the clinical arena in
              1982, the only way to get any sort
              of 3-D representation of the human
              body was by using Computerized
              Axial Tomography, otherwise known
              as CAT or CT scans. Although CT
              works well in certain contexts, it
              has limitations. It exposes patients
              to radiation and only shows the
              body on its axial (top to bottom)
              plane.

              In contrast, MRI does not rely on the absorption of x-rays. It
              is based instead on Nuclear Magnetic Resonance (NMR).
              When MRI was first introduced in research, it was actually
              called NMR. That name, though, scared many people who
              incorrectly assumed that the technique would expose them
              to nuclear radiation. In fact, the 'N' of NMR represents
              atomic nuclei and how they spin, not nuclear radiation.

              The basic physics involved is as follows: When atoms are
              placed in a magnetic field, the odd-numbered atoms (those
              having an unequal number of protons and neutrons, such as
              hydrogen) align within this field. In other words, their axes
              of rotation all point the same way. Hydrogen is the most
              abundant odd-numbered atom in the body, but all
              odd-numbered atoms are subject to this alignment process.
              When these atoms are then exposed to a brief interruption of
              the magnetic field (commonly referred to as a pulse), they
              shift away from the magnetic field. After the pulse is lifted,
              the atoms realign, emitting a radiofrequency signal.
              Scanners in an MRI machine collect all the signals from the
              individual nuclei and, with the help of computer analysis, use
              that information to create a series of dimensional images.

              Unlike CT, MRI can show pictures along many planes--the
              axial plane, the saggital plane (side to side) and the coronal
              plane (front to back)--enabling physicians to see images that
              were previously impossible to visualize except during
              autopsy. Of clinical significance, using different pulse signals
              results in different image types. The three most commonly
              used types are termed T1, T2 and proton density.

              T1 is a short, fast pulse that makes fat tissue appear bright
              and cerebral spinal fluid (CSF) dark. T1 images look like CT
              images and are more focused than the other MRI image
              types. T1 allows for the overall visualization of structures in
              the body--a view that can be enhanced by using a contrast
              medium. In the same way that iodine can be used in CT
              scans to stain blood vessels, gadolinium diethyylenetrinine
              pentaacetic acid (gadolinium DTPA) renders blood vessels in
              a T1 MRI image white. (Gadolinium does not routinely cross
              the blood-brain barrier unless the barrier has broken down
              due to, say, tumors or infections.)

              T2 pulses last four times as long as the T1 variety, which
              makes hydrogen nuclei, surrounded by water, a more
              suitable contrast. In T2 images, CSF appears white and areas
              that have an abnormally high water content (those affected
              by tumor, infection or stroke) look bright as well. In proton
              density images, CSF and the brain look the same, making it
              easier to see tissue changes next to ventricle structures.

              In addition to their clinical versatility, MRI scanners seem to
              cause no harm to biological tissue at exposures of 0.3-2.0
              teslas of electromagnetic energy. And the technique has
              numerous applications; new ones are being discovered all
              the time. MRI can show atrophy changes of the brain
              common in Alzheimer's Dementia. It can detect tumors at
              earlier stages of development than many other forms of
              medical imaging. And it better reveals parts of the body that
              are not easily shown on the axial plane, including the
              cerebellum, where telltale changes take place in Parkinson's
              disease, Huntington's Chorea and Multiple Sclerosis.

              Unfortunately, MRI can not be used for every patient. Those
              with pacemakers and ferromagnetic metal implants are not
              eligible. Also people with even mild claustrophobia can
              experience great discomfort during an MRI scan, which
              requires them to lie still inside a narrow tube. Increasingly
              many open MRI machines, which don't have tubes, are
              available. But the images they produce, though still superior
              to CT scans, are not as sharp.

              That said, CT scans are still an integral part of every hospital
              system, particularly in emergency settings because they
              don't take nearly as long as MRI scans to produce. Also, they
              depict bone more clearly than MRI. So in cases of the
              unconscious patient with possible head trauma, CT is the way
              to go.