History
A 46-year-old male patient, presenting with a comminuted tibia fracture, underwent pseudarthrosis revision with plate osteosynthesis and locking screws. Six weeks later, a follow-up CT examination was scheduled to evaluate the status of osseous healing. A scan in ultra-high resolution (UHR) mode with spectral shaping (tin filtration) was performed on a photon-counting CT (PCCT), NAEOTOM Alpha®.
Diagnosis
In the standard polychromatic UHR images, metal artifacts caused by the implants were present and partially obscured the visualization of the fracture clefts. Virtual monoenergetic images (VMI), reconstructed at 120 keV, 150 keV and 190 keV, showed substantial suppression of metal artifacts and improved visibility of the fracture clefts. No sign of osseous consolidation was evident. Spotty osteopenia was present in the tibia and the foot. Following the CT scan, the patient was instructed to walk using forearm crutches and applying only 30% of his own body weight onto his feet. The patient was scheduled to return 6 weeks later for the next follow-up examination.
Fig. 1: Sagittal MPR images show a fracture cleft (arrows) that is partially obscured by metal artifacts on the polychromatic image (Fig. 1a), and is however clearly visible on VMI images reconstructed at 120 keV (Fig, 1b), 150 keV (Fig. 1c) and 190 keV (Fig. 1d). No sign of osseous consolidation is evident.
Fig. 2: Cinematic volume rendered images (a–d) show a 3D overview of the plate osteosynthesis and locking screws in relation to the fractured tibia and fibula.
Comments
CT assessment following surgery is important to identify whether a fracture consolidates during the healing process. This information is pivotal for surgeons to decide about patient mobility, or whether revision surgery is necessary in case of complications such as delayed union or non-union. Metal artifacts caused by implants, due to photon starvation and beam hardening, can significantly compromise image interpretability. It has been shown that a combined approach, using spectral shaping (tin filtration) and virtual monoenergetic image (VMI) reconstructions, is effective in reducing metal artifacts at improved dose efficiency compared to standard scans [1]. While spectral shaping excludes low-energy photons that contribute little to high contrast structures (i.e., bone, metal), VMIs further shift the mean photon energy to higher levels, thereby reducing beam hardening artifacts. VMIs can be easily reconstructed and implemented in routine imaging as spectral information is inherently available in PCCT. Furthermore, PCCT can acquire UHR images without using additional combs or grids to reduce the detector aperture, resulting in an increased spatial resolution at full dose efficiency.
In this particular case, metal artifacts are better suppressed in VMIs reconstructed at higher keV levels (150 and 190 keV). However, a tradeoff with reduced image contrast has to be recognized. Clear visualization of the fracture clefts is observed in VMIs reconstructed at 120 keV, while image contrast is still satisfactory. The improved visibility can increase diagnostic confidence for providing better patient care.
Examination Protocol