| Popis: |
The number of magnetic resonance imaging (MRI) systems and the number of patient scans continues to annually increase along with the increasing prevalence of patients with implanted medical devices. Although MRI has an impressive track record of safety, there is a unique set of safety concerns for the millions of patients with implanted medical devices. This coupled with the fact that 50-75% of patients with cardiovascular devices will be indicated for an MRI shows the importance of MRI safety for the population of patients with implants. Current techniques used for evaluating MR safety of medical devices may be overly conservative since they do not account for physiological conditions. Furthermore, these techniques lack established thermal dose acceptance criterion for vascular tissue. The ultimate goal of this research is to improve patient safety and access to MRI by developing physiologically relevant radio frequency (RF) induced heating safety limits for vascular tissue and facilitating more appropriate magnetic resonance (MR) safety evaluations of medical devices. The extent of vascular tissue change as a function of thermal treatment dose was used to establish a thermal dose acceptance criterion for RF induced heating of implanted vascular devices during MRI. Thermal dose treatments were delivered in a swine model via RF ablation and the thermal doses across all experiments were normalized using the well-established CEM43 equation. The carotid arteries were harvested after at least 4 days post-ablation and histopathology was used to score the observed tissue changes. For vascular tissue in a swine model, CEM43 thermal doses of less than or equal to 150 minutes and histologic vascular injury scores of less than or equal to 11 were interpreted as clinically unimportant with no adverse events expected with longer healing times.A computer simulation of RF induced heating of a peripheral vascular stent during MRI was developed and validated with flow phantom experiments. The results of this study demonstrate that flow significantly reduces the temperature rise of a stent and the surrounding medium during RF induced heating, and the computer simulation accurately predicts the thermal effects of flow. Further validation of the computer simulation included in vivo pig experiments. RF induced heating of stents implanted in the carotid arteries of six pig was experimentally measured and accurately simulated in a virtual pig anatomy. The results of this study demonstrate that the bulk mass of the anatomy, positioning of the stent, and blood flow significantly affects the RF induced temperature rise of a stent during MRI. |
