Magnetic resonance imaging plays a vital role in the preoperative diagnosis of and surgical planning for instability of the cervical vertebral column.1–4 However, implant-associated magnetic susceptibility artifacts detected after surgery raise concerns about the diagnostic use of MRI. Surgical implants made of metals with high magnetic susceptibilities relative to biological tissues introduce inhomogeneities in the static magnetic field, which causes artifacts attributable to signal loss from displaced resonance frequencies and signal dephasing.5,6 Susceptibility artifacts can obstruct or misrepresent portions of a displayed image, which can severely compromise an evaluator's diagnostic interpretation. The most commonly recognized susceptibility artifacts are represented by dark black regions indicative of a lack of signal, geometric distortions of anatomy, bright white regions of signal accumulation, and failure of fat saturation.7,8
Several imaging parameters, such as the type of pulse sequence, field strength, matrix size, field of view, slice thickness, bandwidth, and echo train length, can alter magnetic susceptibility artifacts.7,9–20 The ability to mitigate implant-related susceptibility artifacts by use of various imaging techniques, such as view-angle tilting, slice-encoding metal artifact correction, short echo-time projection reconstruction acquisitions, single-point imaging, prepolarized MRI, and postprocessing image correction, has been evaluated.21 In 1 veterinary study,22 a spin-warp TSE sequence improved depiction of the spinal cord margin by reducing the size of susceptibility artifacts related to stainless steel screws used to stabilize the atlantoaxial joint in small-breed dogs. Despite promising results for these proprietary sequences, many of these software packages are currently cost prohibitive and not readily available for use in veterinary medicine.
In addition to special sequences and adjustable parameters, susceptibility artifacts are also affected by the size, shape, orientation, and metallic composition of surgical implants.20,23–27 Although the titanium and stainless steel alloys commonly used in surgical implants are both within the paramagnetic range, the magnetic susceptibility of titanium is much more closely matched with that of biological tissues.28 Therefore, fewer local field inhomogeneities develop near titanium implants than near stainless steel implants. Studies9,22,29–34 have indicated that various titanium implants used in maxillofacial, orthopedic, and vertebral surgeries of humans induce less severe susceptibility artifacts than do stainless steel implants. Similar results have been obtained for the canine appendicular skeleton.32 However, evaluation of titanium-related MRI susceptibility artifacts in the cervical vertebrae of dogs is limited to the description of a single specimen that was part of the planning phase of 1 study.22 The authors of that study reported that the susceptibility artifacts related to titanium screws did not interfere with assessment of the spinal cord by use of conventional TSE sequences.22
The purpose of the study reported here was to characterize and compare magnetic susceptibility artifacts related to titanium and stainless steel monocortical screws in the cervical vertebrae of canine cadavers by use of routine TSE sequences. We hypothesized that titanium screws placed in a monocortical manner would cause mild susceptibility artifacts within the implanted vertebrae, but that evaluation of the spinal cord and intervertebral disks would not be impaired. Furthermore, we hypothesized that stainless steel screws would cause significantly greater susceptibility artifacts that would hinder interpretation of the neuroanatomic structures across 4 contiguous cervical vertebrae.
This manuscript represents a portion of a thesis submitted by Dr. Jones to the Department of Veterinary Sciences at The Ohio State University as partial fulfillment of the requirements for a Master of Science degree.
The authors thank Dr. Michael Knopp for facilitating MRI imaging.
Region of interest
Short tau inversion recovery
Turbo spin echo
Synthes Vet, West Chester, Pa.
Simplex P bone cement, Stryker Corp, Mahwah, NJ.
Philips Achieva, Cleveland, Ohio.
eFilm, Merge Healthcare Inc, Chicago, Ill.
Stata, version 11.0, StataCorp, College Station, Tex.
IBM SPSS Statistics, version 21, IBM Corp, Armonk, NY.
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Summary of MRI sequences and imaging parameters used for the assessment of the cervical vertebrae and spinal cord of canine cadavers implanted with stainless steel or titanium screws.
|Interslice gap (mm)||0||0||0||0||0||0||0.3|
|Matrix||180 × 180||280 × 264||280 × 198||180 × 180||280 × 280||200 × 200||280 × 265|
|TSE factor (ETL)||24||22||22||3||5||5||21|
|FOV (mm)||384 × 384||560 × 560||400 × 400||384 × 384||560 × 560||400 × 400||560 × 560|
|Acquisition duration (s)||205.18||240.16||182.52||365.78||131.71||131.71||202.79|
— = Not applicable. ETL = Echo train length. FOV = Field of view. NEX = Number of excitations. TE = Echo time. TI = Inversion time. TR = Repetition time.