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MRI and x-ray each have a role in assessing back pain.
The medical technologies of x-ray and MRI are essential imaging tools in assessing back pain. Each offers important clues to the underlying causes of back pain, and each plays a somewhat different role in the process. These technologies differ in how they produce images, the types of images they produce and in how they are used in back pain assessment. Each also has limitations or drawbacks in its usefulness as a diagnostic aid.
X-ray images are made by projecting a beam of x-rays from a tube, through a body part and onto a film or a digital sensor. The various densities of tissue absorb, or stop, the x-rays to varying degrees, such that those making it through to the film or sensor create images, that reflect those contrasting tissue densities. Bones, being quite dense, clearly stand out on x-ray images. X-rays provide fast and relatively inexpensive diagnostic images.
MRI technology does not use x-rays. To produce an MRI image, a patient is placed into a strong magnetic field. The magnetic field is tuned to interact with hydrogen atoms in the patient's body. The reorienting of hydrogen atoms caused by manipulation of the magnetic field is detected and recorded by sensors in the machine. Because different tissues in the body contain varying concentrations of hydrogen, the collected information can be reformatted by a computer to produce images of those tissues. MRI technology is expensive, and requires more time to create an image for the clinician to interpret.
X-rays produce detailed images of the bones of the spine. The so-called soft tissues, including discs, ligaments, nerves and muscles don't show up well on x-ray images. X-ray views are also limited to various front-to-back and side-to-side angles. MRI images may be manipulated to produce views from a bottom-to-top perspective, giving clinicians information not available in the traditional views. MRI images also demonstrate, in detail, the soft tissue structures within and around the spine.
Some common sources of back pain, such as muscle spasms, strains and minor inflammation, do not require medical imaging for a diagnosis. Guidelines from the American Chiropractic College of Roentgenology advise against routinely ordering x-rays for patients with back pain. In cases of trauma, unexplained weight loss, pain not relieved by rest, neurological deficits, a history of cancer or in patients with back pain over age 50, x-rays are appropriate.
MRI is a superior technology for imaging a number of conditions, including cancer, infection and disc herniations. MRI does not, according to an October 2002 review article in "Annals of Internal Medicine," offer a better diagnosis or lead to better treatment outcomes for other types of back pain. The authors of the study recommend MRI only when more serious conditions are suspected or when back surgery is being contemplated.
The major drawback of x-ray imaging for back pain is that it exposes the patient to ionizing radiation. Ionizing radiation can damage cells and is a potential cause of cancer. For this reason, x-rays should only be performed when medically necessary. Because MRI units expose patients to powerful magnetic fields, metallic implants or metal fragments lodged in the body may move while a patient is undergoing a scan. The technician will usually screen for metallic objects before scanning. The middle of an MRI scan is a poor time to discover you have a metal fragment in your eye. Pacemakers, prosthetic heart valves and certain metallic vascular clips are particular problems that make MRI a poor imaging choice for some patients.