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How do MRIs detect medical problems?

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.

MRI; Huntington's
Image: THE WHOLE BRAIN ATLAS
MRI IMAGES reveal many parts of the body that were previously only seen during autopsy. This picture depicts brain changes associated with Huntington's Chorea.

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.

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