A Completely MRI-Conditional External Defibrillator for 1.5T and 3T Scanners: Validation During Imaging and Resuscitation

Background: MRI is an established medical imaging tool; however, imaging of hemodynamically unstable patients remains restricted, since if cardiac arrest occurs during imaging, conventional external defibrillators are unsafe for use in that strong field environment. Patients with Cardiac Implantable Electronic Devices (CIEDs), whose sensors are deactivated during MRI [1], may require close defibrillator availability due to issues with timely VT detection [2]. The standard defibrillator can be pulled into the bore and interference from the electronics, cables, and surface electrodes can impact image quality. Thus, the responding Code Blue team removes arrested patients from the MRI room before defibrillation, which takes time and may impact recovery [3-5]. Our team [6] initially developed a 1.5 T MRI-conditional external defibrillator by attenuating the radiofrequency interference (RFI) with low-pass filters and resonant Baluns [7] that allowed imaging of subjects instrumented with defibrillator pads. We later added a tetanizing module [8] that reduced subject motion during defibrillation. We lately prototyped a 1.5 and 3T MRI-conditional defibrillator and hypothesized that this system could perform immediate defibrillation inside the bore following cardiac arrest.

Methods: We developed a completely MRI-compatible external defibrillator system, including RFI-shielded chassis, cables, and electrodes, that can operate inside the MRI bore without compromising image quality or patient safety (Figure 1). To eliminate ferromagnetic components, standard transformers were replaced with a non-magnetic voltage-multiplier, and electronics were re-engineered to function in strong magnetic fields, aiming to generate high-voltage (HV) without producing magnetic interference. A tetanizing module was integrated to mitigate motion artifacts caused by high-voltage shocks, employing a low-voltage, long-duration pre-pulse that reduces body displacement. The system was tested in 1.5T and 3T MRI environments in a swine model in which ventricular fibrillation (VF) was induced.

Results: The prototype defibrillator delivered cardiac-synchronized HV therapeutic shocks within the MRI bore without evidence of ferromagnetic attraction, operational failure, and a minimal reduction in MRI signal-to-noise ratio. The system delivered HV pulses equivalent to those of commercial defibrillators (Figure 2), while the tetanizing pre-pulse reduced abdominal-wall acceleration during shocks by a factor of 5, enabling uninterrupted imaging. Successful defibrillation was achieved in 5 swine undergoing cardiac arrests inside 1.5 and 3T bores (Figure 3).

Conclusion: This study presents the first integrated MRI-compatible external defibrillator system capable of safe operation inside the MRI bore. It enables emergency cardiac care during MRI scans without removing patients from the scanner, supporting imaging of patients currently excluded from MRI as well as for patient transfer from the ED/ ICU and MR-guided cardiac interventions.

Figure 1. Comparison of key components inside a conventional defibrillator versus our MRI-conditional defibrillator.
Figure 2. Biphasic waveforms from (A) a standard defibrillator (Medtronic Lifepak 20) compared with (B) those from the MRI-conditional defibrillator during delivery of a 200 J shock. Peak voltages (1500 V), and currents (16 A; A=Amps) were the same.
Figure 3. Experimental setup of the MRI-conditional defibrillator system inside a 3T Philips MRI suite (top), with induced ventricular fibrillation before defibrillation (bottom left) and the restored sinus rhythm after the shock (bottom right).