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Thesis Format

Integrated Article

Degree

Master of Science

Program

Physics

Supervisor

Chronik, Blaine A.

Abstract

Magnetic resonance imaging (MRI) is a medical imaging modality that has seen continuous growth in the decades since its introduction. In conjunction with this increase in the use of MRI, there has also been a growth in the number of patients having implanted medical devices, such as pacemakers. These devices can have undesirable interactions with the MR system. The safety of these interactions must be guaranteed while ensuring that safety limits are not so conservative that they would preclude too many patients from benefiting from MRI. One factor that could make limits less restrictive is spatial averaging of the fields in MRI.

The objective of this investigation was to determine the effect that spatial averaging would have on the predicted time-varying magnetic field (dB/dt) values within realistic MRI gradient systems. ISO/TS 10974:2018(E) contains simulated data describing the peak dB/dt values that active implantable medical devices (AIMDs) could be exposed to when within varying volumes within the bore of the MRI scanner to account for the device location dependence of the dB/dt. For devices with realistic spatial extent (e.g. a 5 cm diameter component), the dB/dt relevant for testing would need to include information about the spatial average of the dB/dt over the device.

This investigation involved the development and validation of a numerical software system to simulate the fields produced by arbitrary gradient coil designs at any point within the gradient and for any device geometry placed within that environment. The software was shown to accurately predict physical measurements, and it was shown that the mean and peak dB/dt values differ between 2 and 17%. The difference between the peak and mean values increases monotonically with the distance from the central axis of the gradient coils.

Summary for Lay Audience

Magnetic resonance imaging (MRI) is a medical imaging modality that is becoming more and more common. For healthy patients, it is known to be a very safe imagining modality, as it does not make use of any of the damaging radiation present in other imaging techniques like x-rays. However, when patients have an implanted medical device, like a pacemaker, as part of their treatment, this can lead to undesirable interactions with the MRI scanner. Patients with health complications benefiting from these medical devices are often those who would most benefit from MRI, so there is a need to guarantee that patient safety is assured while still allowing as many people as possible to be scanned.

One of the most important parts of every MRI system is the gradient coil. This coil produces a time varying magnetic field, called a dB/dt. In order to take full advantage of the power of MRI, it is desirable for this dB/dt value to be as large as safely possible. However, the larger the dB/dt, the greater the interaction with a medical device.

Current safety standards only refer to the peak dB/dt values that devices could be exposed to within the bore of the MRI scanner. But for devices realistic devices, the dB/dt relevant for testing would need to include the spatial average of the dB/dt over the device.

This investigation examined the effect that this spatial averaging would have on dB/dt exposure to ensure that safety limits are not too conservative, so that the most people possible can safety benefit from the diagnostic capabilities of MRI.

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