There is a lot of difference between MRI scanners. First, there is a large difference in image quality between MRI scanners.
- The strength of the magnet is very important, and high-field scanners (1.5 Tesla magnets) are generally regarded as the best magnet strength.
- Low-field scanners (0.25 to 0.5 Tesla magnets) may generate enough power in some cases, but where a lot of detail is needed (e.g. the foramen where the nerve exits the spine) they may not be adequate. Another disadvantage of a low-field scanner is that it often takes longer to do the scan.
For more information see Do I Need an MRI Scan?
From the standpoint of making a surgical decision, one important technical point is how the images are angled. If the images do not cut parallel to the disc space at each spinal level in question, the scan will be of limited usefulness in generating a surgical decision. This is a technical issue and all scanners are capable of doing this, but some technicians do not know how important this is to the surgeon. It takes a little more work on the technician’s part but it is worth doing.
The older MRI scanners are basically a tunnel, and for a complete scan a patient must lie still for about 40 to 60 minutes. However, due to varying needs of patients, a few different types of MRI scanners are now available. Some patients who may have difficulty with a traditional MRI scan include:
- Claustrophobia. A small percentage of patients are claustrophobic and cannot tolerate the confined space within the magnet. The vast majority of these individuals will be sufficiently relaxed with sedatives taken orally. All of the highest resolution MRI scanners are completed tunnels and tend to accentuate the claustrophobic reaction.
- Size. The MRI scanner is a confined space, and some patients may be too large to fit in the MRI scanner tunnel. Some MRI scanners also have weight limits.
- Pain. If a patient is suffering from severe back pain, neck pain or other symptoms, lying still for a prolonged period may not be well tolerated.
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Because the traditional tunnel scanners may not work well for some people, Open MRI scanners have been developed. While some patients may find the more open MRI scanners easier to tolerate, the machines use a lower-field magnet and do not generate as good an image. With a lower quality image (the images are not as clear or detailed), it is often difficult to make a definitive anatomic diagnosis.
Recently, short bore magnets have been developed that combine the accuracy of a tunnel scanner and the comfort of an open MRI scan. Although they are not completely open, they are much less constrictive because of the short bore magnet (shorter tunnels), but can produce a high- field.
Most modern, high-field magnets are now designed with these shorter tunnels (Short Bore architecture) and, theoretically, maintain the highest image quality while causing less claustrophobia and being more easily tolerated by patients who are in a lot of pain. The high-field units (1.5 Tesla or greater) tend to be faster, which means less time in the tube, less patient discomfort.
MRI imaging has revolutionized the diagnosis and treatment of spine-related pain syndromes. MRI technology is constantly improving, although at great financial costs to the end consumers (doctors and their patients). The newest and usually most expensive equipment tends to be available in the largest hospitals and imaging centers that specialize in spine care and other complicated medical disorders.
In many communities there is no choice in the scanner to be used, but where there is a choice it is advisable to get the best quality scan the first time - especially if a patient is considering surgery. MRI scans have taken a considerable amount of guesswork out of spine surgery as they can give very accurate anatomic detail that can be used to plan a surgical approach.