A standard implant for determination of RF-induced local SAR in MRI heating tests
RF-induced heating of tissue surrounding implanted medical devices can occur during MRI scans due to device interactions with the B1 magnetic field. Current methods for determining local SAR in non-clinical RF-induced heating tests such as calorimetry are highly prone to temperature probe noise. A more sensitive measurement technique was developed to measure local SAR using a 10 cm long 1/8-inch diameter Titanium rod.^ Temperature measurements were conducted on eight metal rods. Of the eight rods, there were four different metals tested at lengths of 10 cm and 20 cm. It was decided based on preliminary measurement results to pursue Titanium as the metal of choice for the standard implant due to its ability to provide high signal-to-noise, low thermal conductivity, and low image distortion. The 10 cm long rod was selected based on its relatively high temperature rises at the common clinical MRI static magnetic field strengths of 1.5 Tesla and 3 Tesla.^ Further heating tests were conducted on the Titanium rod at various locations in a standard ASTM rectangular phantom. The results were compared to a simulated transfer function plot. Electric field measurements were also conducted on the Titanium rod for comparison with simulation. Measurements provided good matching with simulated local SAR values. A local SAR of 1.3°C/W/kg was calculated for the 10 cm Titanium rod at 64 MHz and a local SAR of 1.45°C/W/kg was calculated for 128 MHz. Overall results were in good agreement with the local SAR values determined from calorimetry. ^
John A. Nyenhuis, Purdue University.
Engineering, Biomedical|Engineering, Electronics and Electrical