Mechanisms and prevention of thermal injury from gamma radiosurgery headframes during 3T MR imaging.

Magnetic resonance imaging (MRI) is regularly used for stereotactic imaging of Gamma Knife (GK) radiosurgery patients for GK treatment planning. MRI-induced thermal injuries have occurred and been reported for GK patients with attached metallic headframes. Depending on the specific MR imaging and headframe conditions, a skin injury from MRI-induced heating can potentially occur where the four headframe screws contact the skin surface of the patient's head. Higher MR field strength has a greater heating potential. Two primary heating mechanisms, electromagnetic induction and the antenna effect, are possible. In this study, MRI-induced heating from a 3T clinical MRI scanner was investigated for stereotactic headframes used in gamma radiosurgery and neurosurgery. Using melons as head phantoms, optical thermometers were used to characterize the temperature profile at various points of the melon headframe composite as a function of two 3T MR pulse sequence protocols. Different combinations of GK radiosurgery headframe post and screw designs were tested to determine best and worst combinations for MRI-induced heating. Temperature increases were measured for all pulse sequences tested, indicating that the potential exists for MRI-induced skin heating and burns at the headframe attachment site. This heating originates with electromagnetic induction caused by the RF fields inducing current in a loop formed by the headframe, mounting screws, and the region of the patient's head located between any of the two screws. This induced current is then resistively dissipated, with the regions of highest resistance, located at the headframe screw-patient head interface, experiencing the most heating. Significant heating can be prevented by replacing the metallic threads holding the screw with electrically insulated nuts, which is the heating prevention and patient safety recommendation of the GK manufacturer. Our results confirm that the manufacturer's recommendation to use insulating nuts reduces the induced currents in the headframe nearly to zero, effectively preventing heating and minimizing the likelihood of thermal injury.

Improved volumetric imaging in tomosynthesis using combined multiaxial sweeps.

This study explores the volumetric reconstruction fidelity attainable using tomosynthesis with a kV imaging system which has a unique ability to rotate isocentrically and with multiple degrees of mechanical freedom. More specifically, we seek to investigate volumetric reconstructions by combining multiple limited-angle rotational image acquisition sweeps. By comparing these reconstructed images with those of a CBCT reconstruction, we can gauge the volumetric fidelity of the reconstructions. In surgical situations, the described tomosynthesis-based system could provide high-quality volumetric imaging without requiring patient motion, even with rotational limitations present. Projections were acquired using the Digital Integrated Brachytherapy Unit, or IBU-D. A phantom was used which contained several spherical objects of varying contrast. Using image projections acquired during isocentric sweeps around the phantom, reconstructions were performed by filtered backprojection. For each image acquisition sweep configuration, a contrasting sphere is analyzed using two metrics and compared to a gold standard CBCT reconstruction. Since the intersection of a reconstructed sphere and an imaging plane is ideally a circle with an eccentricity of zero, the first metric presented compares the effective eccentricity of intersections of reconstructed volumes and imaging planes. As another metric of volumetric reconstruction fidelity, the volume of one of the contrasting spheres was determined using manual contouring. By comparing these manually delineated volumes with a CBCT reconstruction, we can gauge the volumetric fidelity of reconstructions. The configuration which yielded the highest overall volumetric reconstruction fidelity, as determined by effective eccentricities and volumetric contouring, consisted of two orthogonally-offset 60° L-arm sweeps and a single C-arm sweep which shared a pivot point with one the L-arm sweeps. When compared to a similar configuration that lacked the C-arm component, it is shown that the C-arm improves the delineation of volumes along the transverse axis. The results described herein suggest that volumetric reconstruction using multiple, unconstrained orthogonal sweeps can provide an improvement compared with traditional cone beam CT using standard axial rotations.