Purpose: Fluoroscopically-guided orthopedic surgeries at our hospital have been aborted occasionally due to image distortion (e.g. S-distortion) when an image intensifier (II) system is used without any apparent source of magnetic field present. The theory that strong magnetic fields such as those near an MRI scanner produce S-distortion cannot explain this phenomenon. This study scrutinizes the root cause of the image distortion in order to fill the void between our theoretical knowledge and our observations clinically.
Methods: Operational rooms (ORs) and their accessory equipment (e.g., surgical table) were thoroughly screened using an AC/DC gaussmeter. Magnetic flux density and its distribution, if any, were identified and recorded. An upper extremity anthropomorphic phantom and rod phantoms, made of various magnetic susceptible materials (Ti, Steel, Al, and Cu), were scanned to investigate potential effects of the implanted material on S-distortion. A GE and a Siemens fluoroscopy II systems were used for the evaluation, while a GE flat panel detector (FPD) system was used for comparison. Two metrics, the angle of rotation (θ) and the deviation/length ratio, were defined and used to quantify the extent of S-distortion. Mu-metal shielding was applied to test the possible mitigation of S-distortion.
Results: The DC-mode magnetic fields up to 2500 µT were identified in random locations of ORs and from surgical tables. Our experiments demonstrated that a small magnetic flux density at the sub-gauss (<100 µT) level could generate a large distortion artifact, deemed as unacceptable (θ>4°). The image distortion was vendor/model dependent but independent of the material being imaged. Mitigation possibilities of this distortion include relocation of the fluoroscopic system to a low magnetic field area and the usage of external mu-metal as a shield.
Conclusion: Magnetic fields randomly present in ORs need to be considered when a fluoroscopically-guided orthopedic surgery is performed with an II system.
Magnetic Fields, Image Artifacts, Image Intensifier
IM/TH- Image-guided Surgery: Fluorescence/Optical-guided imaging