MAKING MRI SAFE FOR IMPLANTED DEVICES
An increasing number of patients have implanted devices, such as cardiac pacemakers and deep brain stimulators. These patients are often either excluded from MRI, or are scanned using low power "MR Conditional" protocals with compromised diagnostic quality. In addition, there are a wide range of interventional procedures that could be performed in an MRI, but are precluded by the possible interactions between the MRI scanner and the interventional devices. Attempts to address both of these areas have focused on making the devices safe for the MRI environment. This is difficult and expensive, and doesn't address the millions of patients who already have implanted devices.
In this presentation, we will describe an alternative approach, that focuses on making the MRI system safe for devices. Many devices are already made from non-magnetic materials (nitinol, titanium, etc). For these, the major safety concern is device heating from the high power RF transmit from the MRI system. We have developed an array of methods to detect, quantify, and mitigate these interactions. Before the patient is placed in the magnetic, potentially dangerous interactions are detected by looking for characteristic resonancances. Then, before the scan, a low power study visualizes the interacting devices, and measures the currents produced to quantify the risk. This can also be done by measuring impedence changes in the RF coil elements. Finally, a multiple channel transmit system is used to produce a transmit field pattern that minimizes the device interactions.
This approach has a number of advantages. It is specific to each patient, device, and MRI scanner. Interactions can be monitored and corrected continuously, which is important for interventional procedures where the device geometry is constantly changing. Finally, the modifications to the scanner are relatively straight forward, particularly as two-channel parallel transmit is becoming more common.