Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Medical Biophysics

Supervisor

Chronik, Blaine A.

Abstract

Magnetic resonance imaging (MRI) has cemented itself as the gold standard for imaging of soft tissues and is only increasing in popularity. Given the rising number of MRI scanners and medical device being implanted into patients, it is becoming increasingly likely that patients undergoing MRI will have an implanted medical device (IMD). The presence of an elongated metallic IMD inside a scanner could result in dangerous interactions with the radiofrequency fields during MRI, thus some of these IMDs preclude the patients from being scanned.

Orthopedic devices typically fall into this category due to their high potential for RF induced heating, and typically perform poorly in the current standard test method for RF heating. That said, there exists a subset of orthopedic IMDs that still ‘fail’ the current safety testing by heating slightly above the current acceptance criterion. It is hypothesized that such IMDs are not truly a hazard to the patient but are likely failing due to the conservative nature of the current RF heating test (ASTM F2182-19a).

In this thesis, novel test platforms are presented for more realistic evaluation of RF heating in orthopedic IMDs, which were used to experimentally challenge the behavior of their simulated counterparts. These test platforms were designed to address the simplifications in the current ASTM test standard that led to exaggerated heating compared to what is expected in patients, namely geometry/material mimicking and perfusion cooling. Heating of a sample implant was simulated (Sim4Life) in these novel test platforms, along with experimental verification of two phantoms to determine agreement with simulation.

Simulations (and experimental work) indicated that IMD heating in these realistic phantoms could be anywhere from 20-50% lower than the current ASTM phantom, which is a reasonable estimate of the magnitude of the safety margin involved. It appears perfusion cooling is most effective at reducing IMD heating (compared to geometry/tissue mimicking differences), though improved experimental verification is required before these simulations can influence regulatory change. Introducing empirical evidence of perfusion cooling to regulatory conversations around implant safety would improve access to MRI for the millions living with such marginally unacceptable orthopedics.

Summary for Lay Audience

Magnetic resonance imaging (MRI) is an excellent method for imaging soft tissues in the human body and is essentially harmless to patients being scanned, provided they don’t have any implants. For patients with particular implants (e.g., plates and screws), MRI can cause dangerous heating due to interactions between the scanner and such long metallic objects. These implants are tested to determine how much they could heat inside a patient, but unfortunately patients can sometimes be banned from undergoing MRI if their implant fails this test by a large margin.

We hypothesized that the current test method (ASTM F2182) exaggerates implant heating due to its’ simplistic nature (i.e., it does not represent the human body), which leads to higher implant heating than would be expected in the patient. While this ‘better safe than sorry’ approach is good for patient safety, some patients were being unfairly banned from MRI. Although some implants have truly dangerous potential for heating, this thesis is focused on implants that failed this heating test by a small margin. These devices are hypothesized not to be true hazards to the patient, but rather simply victims of this ‘better safe than sorry’ testing.

This thesis presents novel test platforms that more closely mimic the human body compared to the current test method, which is simply a box filled with gel. Some of these platforms were designed to challenge the shape and material of the current ‘box of gel’, while another platform designed to evaluate blood flow cooling of implant heating. These test platforms were simulated to compare predicted heating to the current ASTM test, and two were chosen for experimental verification; allowing us to challenge the simulated predictions to determine how much we can trust simulations.

Simulations (and some experimental results) indicate that heating of some implants could be anywhere from 20-50% lower than the current ASTM test method, though more experimental work is required to improve agreement with simulation. Regardless, these results lay the groundwork for regulatory changes that should allow improved access to MRI for patients with such implants.

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