MRI safety and imaging artifacts evaluated for a cannulated screw used for guided growth surgery

Objective: Percutaneously-placed cannulated screws are the implant of choice for treatment of skeletal deformity associated with growing children that have spastic cerebral palsy (CP). These patients often require MRI examinations throughout their childhood to evaluate associated comorbidities and frequently for research protocols. There are concerns related to the use of MRI when metallic implants are present. Therefore, this study characterized MRI safety and imaging artifacts for a cannulated screw commonly used for guided growth.

Methods: Standardized and well-accepted in vitro techniques were used to evaluate a cannulated screw (4.5 mm diameter x 50 mm length, 316 L stainless steel) for MRI issues. Static magnetic field interactions (i.e., translational attraction and torque) and artifacts were tested at 3-Tesla. Radiofrequency-related heating was assessed at 1.5-Tesla/64-MHz and 3-Tesla/128-MHz using relatively high levels of RF energy (whole-body averaged specific absorption rates of 2.7 W/kg and 2.9-W/kg, respectively). Artifacts were determined using T1-weighted, spin echo and gradient echo pulse sequences.

Results: The cannulated screw exhibited minor magnetic field interactions (14° deflection angle, no torque). The highest temperature changes at 1.5-Tesla/64-MHz and 3-Tesla/128-MHz MRI were 2.1 °C and 2.4 °C, respectively. The maximum artifact size on a gradient echo sequence extended 20 mm relative to the dimensions of the implant.

Conclusions: The in vitro tests performed on the cannulated screw indicated that there were no substantial concerns with respect to the use of 1.5- and 3-Tesla MRI. Therefore, a patient with this cannulated screw can safely undergo MRI by following specific conditions to ensure safety.