Metal Artifact Reduction Sequence MRI (MARS) – Cervical Spine
Cervical Spinal Fusion: The Advantages of Combining Metallic Artifact Reduction Sequences (MARS Protocol) with Low Filed Strength MRI on an Esaote G-Scan Brio Tiltable MRI™.
Metallic implants and fixation plates are an inherent part of spinal fusion surgery and metallic implants have the potential of creating “metallic artifact” during magnetic resonance imaging (MRI) because implants that have any forromagnetic properties will induce a local magnetic field when placed within a magnetic field. This induced field or artifact has the appearance of a black area surrounded by a bright ring on MRI.
The size of the artifact is related to three primary things: 1. the size, composition and orientation of the implant; 2. the strength of the MRI (i.e. the stronger the MRI (high field strength) the greater the artifact; and 3. use of any metallic artifact reduction sequences (MARS protocol).
Implant Composition: Many contemporary implants are constructed of Titanium and the induced artifact is inherently small. However, there may be impurities in the production method that introduce contaminants with ferromagnetic properties. For this reason, even identical implants from different production batches may have strikingly different artifact profiles.
MRI Field Strength: One of the greatest advantages of lower field strength MRI scanners for spinal MRI is that they produce much less artifact than high field strength MRI. In the example below, an individual was scanned in a 1.5 Tesla GE MRI using MARS protocol to reduce the artifact from the cervical fixation plate Figure 1). Despite use of MARS protocol, the black metallic artifact extended into the spinal canal and obscured visualization of the spinal cord. In order optimize evaluation of the spinal cord and fixation plate, the same individual was re-examined using a the Salubrio Esaote G-scan Brio Tiltable MRI (TM symbol) using MARS protocol within a few days of the initial MRI. There is minimal artifact. The artifact on the spinal cord is absent and the marrow signal adjacent to the fixation screws can be evaluated. The signal-to-noise is comparable to the high field strength MRI.
MARS Protocol: Metallic artifact reduction sequences (MARS protocol) were created to mitigate the inherent problems problems with high field strength MRI scanners. High field filed strength MRI scanners (1.5T and 3.0T) have the advantages of faster scanning and higher signal-to-noise ratios. These are very important for many types of imaging but come at a cost- larger metallic artifacts. Even micro-metallic artifact from a needle injection or shaving of bone from a laminotomy can produce limiting artifact using high field strength MRI. In most cases the inherent low metallic artifact induced by low filed strength MR makes the use of MARS protocols unnecessary although MARS protocols can further mitigate metallic artifact in problem cases.
I believe that low field MR should be the first MRI of choice when there are metallic implants in the spine that would produce undesirable metallic artifact using high field strength MRI.
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