Imaging Studies And Other Testing - When To See A Doctor: Back Pain

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Imaging studies and other testing

Although it's often tempting to enlist technology in the effort to identify the cause of vexing symptoms, most experts now question the value of early imaging tests for garden-variety back pain. In fact, the current consensus is that diagnostic imaging tends to be overused at the onset of back pain symptoms — the very time when it is least useful.

Here's why: In most cases, x-rays and other imaging tests, such as CT and MRI scans, are not useful in determining the cause of low back pain. CT and MRI are expensive tests, a factor that becomes something to consider if you have a copayment or deductible requirement on your insurance plan. And probably most convincing of all, roughly 90% of people with low back pain recover on their own without such tests, often in a matter of weeks.

These tests are rarely normal, yet the abnormalities they reveal are seldom related to the person's symptoms. Indeed, the abnormalities may not cause any pain at all. For example, nearly two-thirds of MRI examinations of adults with no back pain reveal disk bulges or protrusions in the lumbar spine. Similarly, in people over age 50, an astounding 90% of x-rays and other imaging scans will show abnormalities resulting from aging, but such age-related changes are often visible regardless of whether the individuals have any back pain. In other words, even if you have wear from aging, that may not be the source of your pain.

However, there are times when imaging tests may be appropriate. If an initial medical evaluation leads your physician to suspect a serious condition such as a tumor, infection, fracture, or compression of the cauda equina, or if the pain becomes chronic (continues with little or no improvement for three months or more), imaging studies and other tests can help identify the source of your back pain.

The bottom line? Unless a physician suspects a severe or systemic condition that needs immediate attention, it's wise to wait four weeks before considering the more sophisticated imaging tests. Chances are, you'll recover on your own. But if you and your physician agree an imaging test may be helpful, here's what to expect.

X-rays

X-rays primarily show your bones and other calcium-containing tissues (see Figure 8). Although they provide some idea of the condition of the vertebrae, x-rays of the lumbar spine offer little information about the disks, ligaments, muscles, and other soft tissues. X-rays expose you to some radiation, but the doses are low and, in the absence of other, significant radiation exposure, shouldn't pose a danger.

Figure 8: X-ray of the spine An x-ray primarily shows bones and other tissues that contain calcium. X-ray courtesy of Daniel I. Rosenthal, M.D., Radiology Dept., Massachusetts General Hospital

Nonetheless, despite their limited diagnostic value in some situations, x-rays are indispensable for identifying fractures, as well as bone changes caused by tumors, infection, and certain forms of arthritis.

Computed tomography (CT) scans

CT has done wonders for the diagnosis of back problems, but as mentioned above, it can't always find the source of back pain. For this test, you must lie still for about 20 minutes on a table that slides into a tunnel-like scanner. An x-ray device moves along your body taking multiple pictures, each from a slightly different angle. Instead of sending a single x-ray beam through your body, this device uses many narrow beams. The beams are collected by a detector that rotates around you and then sent to a specialized computer, which instantaneously analyzes and synthesizes multiple images of your back. The result is a set of remarkably detailed composite views of nearly any anatomic plane. One downside: CT scans expose you to more radiation than conventional x-rays.

CT scans provide distinct outlines of the various structures of the back (see Figure 9). While they can show arthritis or spinal stenosis, for example, they may not indicate clearly whether a herniated disk is causing problems, such as sciatica.

Figure 9: CT scan Computed tomography (CT) provides images of the body in different planes and shows distinct outlines of the various structures. CT scan courtesy of Daniel I. Rosenthal, M.D., Radiology Dept., Massachusetts General Hospital

Magnetic resonance imaging (MRI)

MRI technology uses electromagnetic waves to create images of your tissues, thus avoiding the radiation hazard of x-rays and CT scans. MRI scans also delineate soft tissues, including intervertebral disks, spinal nerves, and tumors (see Figure 10).

Figure 10: Magnetic resonance imaging Like a CT scan, magnetic resonance imaging provides images of your body in different planes. It also shows soft tissues in some detail. MRI courtesy of Daniel I. Rosenthal, M.D., Radiology Dept., Massachusetts General Hospital

The MRI procedure usually takes half an hour or longer, during which you lie motionless in a large tube located in the center of a room-sized machine. Detailed images are obtained from the minute electromagnetic waves emitted by body tissues that are subjected to an intense magnetic field. Although MRI is so noisy that the patient must wear ear protectors, the procedure is not invasive in any way and is believed to be completely harmless. However, some people feel claustrophobic in the tube.

Although MRI scans and, to a lesser extent, CT scans are expensive tests, when used in conjunction with a detailed medical history and the results of a physical examination, they can eliminate some of the guesswork involved in making a diagnosis. But even when an imaging technique uncovers an abnormality, it isn't necessarily the cause of your back pain.

Myelography

This form of diagnostic imaging can reveal the positions of the lumbar nerve roots. It can also indicate distortions in the shape of the fluid-filled sheath that surrounds the spinal cord and the cauda equina. In this test, a contrast medium (a fluid opaque to x-rays) is injected into the sheath, and its flow is observed with a fluoroscope (a special x-ray machine). Myelography thus allows a clinician to locate any abnormalities — such as herniated disks, stenosis, or spinal tumors — that cause distortion of the sheath.

However, myelography is expensive. It must be performed in a hospital or clinic. It is also invasive, in that it requires injecting fluid into your spinal cord. And some people find it to be quite uncomfortable. Possible complications include headache (which can occasionally be severe), allergic reaction to the contrast fluid, and even infection resulting from the injection of the fluid, although this is rare. Since the advent of the imaging techniques described below, myelography is performed only when a diagnosis is particularly difficult or when it's necessary to pinpoint the location of a problem in preparation for surgery. In such cases, myelography is often performed in combination with CT imaging.

Other techniques

Upon occasion, a physician will recommend other tests. As with the tests already described, these are useful once your physician has ruled out some probable causes of your pain and is therefore using the tests to further narrow the possibilities.

Bone scan. For this test, the doctor injects a virtually harmless, short-lived radioactive substance into your bloodstream. Your bones absorb this substance at rates that vary according to the activity of the bone cells. An electronic device records this absorption on photographic film, thereby locating areas of rapid bone formation. A tumor, an infection, or a healing fracture will appear as a "hot spot" on the image. Once the location of an abnormality is known, other techniques can be used to make the diagnosis. A bone scan, which is nearly as expensive as CT, takes about four hours, starting from the time of the injection of the radioactive material. The exposure to radioactivity associated with this procedure is a fraction of that required for a regular x-ray of the lower spine. The only discomfort is in having the injection and lying face-down for up to an hour while the scan is completed.

Electromyography (EMG) and nerve conduction testing. For electromyography, fine needles are inserted into your muscles to detect and record electrical patterns generated either in response to electrical stimulation of nerves or through voluntary muscle activity. Pressure and irritation affecting the spinal nerves and certain diseases can change the nature and speed of the signals. Electromyography can help provide a more detailed analysis of nerve function, enabling your doctor to monitor the condition of your nerves. It is often painful. EMG is done along with nerve conduction testing (which determines how fast individual nerves relay signals), another approach to monitoring the function of spinal nerve roots.

Last updated:	January 23, 2007