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E1333. Multimodal Imaging of Carbon Fiber-Based Implants for Orthopaedic Spinal Fixation
Authors
  1. Francis Delaney; Mater Misericordiae University Hospital
  2. Eoin Kavanagh; Mater Misericordiae University Hospital
Background
Metallic implants have traditionally been used in surgical fixation procedures for unstable or symptomatic spinal tumours. These implants, however, can cause significant artifact on post-operative CT and MRI limiting evaluation for surgical complications or fractures and for recurrent malignancy on surveillance imaging. In addition, metallic implants can hinder post-operative radiotherapy as CT metal artifact reduces the precision of radiotherapy planning, and absorption of x-ray photons by metallic implants affects the accuracy of dose delivery. New orthopedic implants made from composite carbon fiber-based materials have emerged in recent years for multiple orthopedic procedures and are now available for spinal fixation procedures. As the use of carbon fiber (CF) implants grows, an awareness of their appearance across all imaging modalities is important for radiologists.

Educational Goals / Teaching Points
• To review the difficulties caused by metal artifact in post-operative diagnostic imaging and radiotherapy planning/delivery after spinal fixation using metallic implants • To discuss the evolving use of CF implants for a variety of indications in orthopedic surgery, focusing on their emerging role in spinal fixation • To describe the important multimodality imaging features of CF implants, including important considerations for imaging protocols, and illustrate key findings using case examples from our institution.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
In CT, metal implants typically result in streak artifact due to beam hardening and photon starvation. CF implants do not significantly absorb x-ray photons, considerably improving the visualisation of surrounding tissues. This can be demonstrated both with basic visual evaluation or quantitatively by region of interest (ROI) measurements. Metal artifact in magnetic resonance imaging (MRI), known as susceptibility artifact, occurs due to the introduction of local magnetic field inhomogeneities. CF implants allow for better quality MR images while using standard spinal MR sequences. Where CF-based implants still contain some titanium, metal artifacts are produced to a lesser degree. These are worse at higher magnet strengths, and 1.5T MRI may be preferred over 3T in these patients. The radiolucent nature of CF implants is a disadvantage intra-operatively where fluoroscopic visualisation of screws is impaired due to reduced tissue contrast. A titanium tip may be added to the screw to improve intra-operative identification of positioning. Follow-up evaluation of implant position and integrity using plain radiography is another important consideration given the reduced tissue contrast. There may be a need for increased use of CT in certain cases where a hardware complication is suspected or plain radiographs are unclear.

Conclusion
Carbon fiber-based implants for spinal fixation are increasingly encountered in daily radiology practice and offer significant advantages in post-operative imaging. There are, however, important considerations across all imaging modalities of which radiologists must be aware.