[Metal abrasion as an imaging artefact in nuclear spin tomography]

Eighteen patients with vertebral body fractures that had been stabilized by an internal spinal skeletal fixation system were prospectively examined by magnetic resonance (MR) imaging between February 1989 and November 1990 at the Department od Diagnostic Radiology of the University Hospital Freiburg 3-7 days after removal of the metallic implants. In most cases imaging artifacts in the paraspinal extensor muscles were evident on MR studies. These were found especially in the region of the previous site of the metal clip jaw bearing. In a few cases artifacts were also present within the vertebral body and/or vertebral arch, but only if the vertebral body had been surgically reconstructed by transpedicular spongiosa implantation. None or only minor artifacts by abrasion of metal were detected if the (modified Schanz's) screws appeared to be tight at surgical removal of the implants. Therefore, marked metal artifacts on MR imaging retrospectively indicate a chronic straining of the implants, which has been shown to be a risk factor for implant loosening. Additional in vitro studies with powdered metallic alloy showed that as little as 1 mg of metal could be detected as artifacts in routine spin-echo sequences. When the metallic pieces were large enough to be seen on conventional radiographs or computed tomograms, they caused severe, distorting artifacts on MR imaging. It is concluded that MR imaging is the method of choice for detection of small amounts of metal.