Magnetic Resonance Imaging (MRI)

MRI, or Magnetic Resonance Imaging, is a dynamic way for doctors to diagnose certain problems. MRI does not rely on X-rays. Instead, it uses strong magnets and radio waves to create a very sharp picture of the internal body structures. MRI uses sensitive instruments and specially designed computers to create images of the entire area being scanned

Unlike an x-ray beam, there is no radiation with the magnet so the MRI scans may be done for pregnant women. However, patients with a pacemaker implanted in their heart should not have an MRI scan because the magnetic field will cause the pacemaker to malfunction. Also, anyone who works around metal should first have an x-ray of their eye sockets to ensure that they do no have any metal filings in their eyes, which the magnetic field may cause to migrate and damage the eye during the MRI scan.

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MRI vs CT

A computed tomography (CT) scanner uses X-rays, a type of ionizing radiation, to acquire its images, making it a good tool for examining tissue composed of elements of a relatively higher atomic number than the tissue surrounding them, such as bone and calcifications (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire its images and is best suited for non-calcified tissue.

CT may be enhanced by use of contrast agents containing elements of a higher atomic number than the surrounding flesh (iodine, barium). Contrast agents for MRI are those which have paramagnetic properties. One example is gadolinium.

Both CT and MRI scanners can generate multiple two-dimensional cross-sections (slices) of tissue and three-dimensional reconstructions. Unlike CT, which uses only X-ray attenuation to generate image contrast, MRI has a long list of properties that may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to detect different features.

MRI can generate cross-sectional images in any plane (including oblique planes). CT was limited to acquiring images in the axial (or near axial) plane in the past. The scans used to be called Computed Axial Tomography scans (CAT scans). However, the development of multi-detector CT scanners with near-isotropic resolution, allows the CT scanner to produce data that can be retrospectively reconstructed in any plane with minimal loss of image quality.

For purposes of tumor detection and identification, MRI is generally superior. However, CT usually is more widely available, faster, much less expensive, and may be less likely to require the person to be sedated or anesthetized.

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MRI Contraindications

Contraindications for undergoing an MRI scan for spine-related pain in the back, neck or leg include:

Patients who have a heart pacemaker may not have an MRI scan
Patients who have a metallic foreign body (metal sliver) in their eye, or who have an aneurysm clip in their brain, cannot have an MRI scan since the magnetic field may dislodge the metal
Patients with severe claustrophobia may not be able to tolerate an MRI scan, although more open scanners are now available, and medical sedation is available to make the test easier to tolerate
Patients who have had metallic devices placed in their back (such as pedicle screws or anterior interbody cages) can have an MRI scan, but the resolution of the scan is often severely hampered by the metal device and the spine is not well imaged.

In regards to this last point, if stainless steel was used for the fusion, the artifact is much more of a problem than if titanium was used. Recently, titanium has been used for most fusions, and stainless steel has largely fallen out of favor because of the inability to get a postoperative MRI scan.