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What is the difference between X-ray film, CT and MRI?

X-Ray is the most common test performed today. In 1895, Wilhelm Conrad Roentgen discovered the x-ray. His remarkable achievement radically changed the practice of medicine. For the first time physicians could see beyond the skin and underlying soft tissues to the skeleton without autopsy. Roentgen did not entirely understand these unusual rays. He used the letter "x" to describe the rays because in Algebra "x" refers to an unknown.

When the spine is x-rayed the beams pass through the skin and underlying soft tissues (e.g. muscle, ligaments, tendons). When the beams meet bone (vertebra) it stops creating a white shadow on the x-ray film. A bone abnormality is reflected on the finished film. Shades of gray mirror the density of the different tissues. X-rays are best for looking at bone. They are not helpful for looking at soft trauma.

X-ray films are widely used today and are often called radiographs. These tests are not performed at random. An x-ray would most likely be performed when spine or extremity pain (e.g. leg, arm) is severe or chronic and progressive. An x-ray may rule out particular problems involving bone and some soft tissue disorders. When an x-ray proves inconclusive additional tests may be ordered especially if something suspicious is detected.

CT Scan (Computerized Axial Tomography) or CAT Scan was developed in 1970. The CT Scan evolved from Tomograms; multiple x-rays taken at different levels to check the depth of an abnormality. The advent of computers in medicine has meant less radiation exposure and shorter study times. The CT Scan has become an important adjunct to x-rays. The CT Scan uses multiple x-ray beams projected at many angles in conjunction with computer resources to create three-dimensional cross-sectional images. Each image or picture reveals a different level of tissue that resembles slices.

MRI (Magnetic Resonance Imaging) is one of the most sensitive diagnostic tools. This medical miracle was first used on humans in 1971. MRIs differ from CT Scans in that there is no exposure to radiation. The MRI equipment is basically two powerful magnets; one external and one internal. Within the human body there are millions of negative and positive charged atoms. When these atoms are exposed to the electromagnetic waves produced by the MRI equipment, the atoms act like mini-magnets. By means of a computer, the data is collected, combined, and manipulated using complex mathematical equations. The final product reveals detailed anatomical images transferred onto film. MRI represents the gold standard in imaging. MRI is best for looking at soft tissues such as discs or nerves.

To appreciate the details rendered by an MRI consider the following contrast. Under x-ray, an intervertebral disc resembles a pocket of air. Using MRI the structure of the same disc is revealed in fine detail. Additionally, contrast dye introduced into the patient intravenously further defines and highlights particular aspects of the spine.

There are a few drawbacks to MRI. For example, take 100 normal people who appear to have nothing wrong with their spines and perform an MRI on each. The results may reveal that 20-25% of asymptomatic participants (without symptoms) have a herniated or bulging disc, or an arthritic condition. These patients are pain free and their lives go on without interruption at that particular time. The disadvantage is the results of an MRI may create a false positive. This means the MRI revealed a disorder for which there are no corresponding clinical symptoms. The point is this - the clinical symptoms must coincide with test results. It is not uncommon for a patient to come to the physician with a stack of MRIs indicating a herniated disc.

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