Comments on MR heating tests of critical implants

MAGNETIC RESONANCE IMAGING (MRI)-related heating for certain implants may be excessive, causing substantial patient injury. The determination of implant heating is particularly challenging because of the numerous variables that must be considered in order to properly identify both MRIand implant-related conditions that can impact the findings (1,2). Therefore, I applaud the recent commentary by Kainz (3), a member of the Food and Drug Administration (FDA), for presenting several of the confounding factors that complicate tests used to characterize implant heating. Kainz’s (3) guest editorial refers to possible issues associated with the in vitro assessment of MRI-related heating, with an emphasis on “MR critical implants.” MR critical implants were defined as: “active implantable medical devices (AIMDs); semiactive implants, i.e., implants powered from outside the body; and elongated metallic structures that are in the range of the critical length (defined later in the guest editorial) (3). Notably, there was no mention of devices that have ”resonant conducting loops,“ which may likewise generate excessive heating in association with MRI (4). Examples include cervical fixation devices and external fixation systems. As a member of the MRI community involved in the evaluation of implants since 1988 (5,6), I welcome the opportunity to respond to the content of this guest editorial insofar as our group and other investigators have extensive experience testing many of the so-called MR critical implants (7–23). In addition, we have been involved in the development of the methodology utilized to evaluate these devices (24). Importantly, findings from many studies have resulted in labeling approved by the U.S. Food and Drug Administration (FDA) to permit MRI procedures to be performed in patients with active, semiactive, and elongated devices, including cochlear implants, bone fusion stimulators, Foley catheters with temperature sensors, implantable infusion pumps, a deep brain stimulation system, spinal cord stimulators, a vagus nerve stimulator, a microstimulator (Bion), relatively long, orthopedic implants, and many others (7–23). As a result of this work, patients with “MR critical implants” throughout the world undergo MRI procedures safely, as MR healthcare workers follow FDA-approved guidelines (or guidelines approved by other agencies), which usually state specific instructions for a given device with respect to MRI parameters and other instructions. These instructions often indicate a particular whole-body averaged specific absorption rate (WB-SAR) that should not be exceeded during the MRI examination. For the aforementioned devices, the evaluation of MRI safety included an assessment of heating caused by MRI procedures. Interestingly, as the guest editorial indicates, “Most MR heating tests refer to [American Society for Testing and Materials International] ASTM Standard 2182-02a which is for passive implants only” (3). One of the reasons for this is that, currently, there is no standard for testing MR critical implants and, therefore, the ASTM standard used for passive implants has been modified to test active, semiactive, and elongated implants. In spite of this issue, the fact remains that the FDA approved MRI-related labeling for implants that utilized findings from heating tests suggested to be problematic. Notably, to date, there is no evidence that patient injuries have occurred in relation to the use of labeling approved by the FDA for these devices relative to the heating information. By comparison, there have been serious patient injuries related to not following specific instructions intended to prevent excessive implant heating (25–27).