CBCT Artifacts and Implant Planning
A 62-year-old man is being planned for a mandibular implant. A CBCT scan is performed. The scan shows a significant streaking artifact around a metallic restoration in the adjacent tooth. The implant site is in the posterior mandible, and the inferior alveolar nerve canal is visible but partially obscured by the artifact. The patient has a history of smoking and hypertension. The clinician needs to assess bone density and the exact position of the canal.
1. What are the causes of the streaking artifact, and how can it be mitigated?
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Streaking artifacts are primarily caused by beam hardening and photon starvation due to the high attenuation of metal. In CBCT, the polychromatic X-ray beam and the lack of sophisticated artifact correction algorithms (compared to medical CT) exacerbate this. Mitigation strategies include: increasing kVp (if adjustable) to reduce beam hardening, using a metal artifact reduction (MAR) algorithm if available, adjusting the patient's position to avoid the metal being in the same plane as the region of interest, and using a smaller field of view to reduce scatter. However, MAR algorithms can introduce other artifacts. In this case, the artifact obscures the canal, so a second scan with a different angulation or a medical CT may be considered. Also, software tools to interpolate across the artifact may help.
2. How would you assess bone density and the position of the inferior alveolar nerve canal in the presence of this artifact?
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Bone density can be assessed using Hounsfield units (HU) if the CBCT is calibrated, but CBCT is not as reliable as medical CT for HU. Alternatively, visual assessment of trabecular pattern and cortical thickness can be used. For the canal, cross-sectional images and tracing the canal from adjacent slices where the artifact is less severe can help. Multiplanar reconstructions (axial, sagittal, coronal) and curved planar reformation along the canal can be used. If the canal is still not visible, a medical CT or MRI (which is not affected by metal artifacts) may be necessary. In addition, surgical planning software can help by allowing manual tracing. Given the importance of avoiding nerve injury, if uncertainty remains, a second imaging modality is warranted.