MRI-Grade Photoplethysmography Using Bundled Fiber Optics for Contactless Heart Rate Monitoring and Real-Time Gating

Abstract

Magnetic resonance imaging (MRI) relies on physiological triggering for cardiac gating, typically using the R-peak of an electrocardiogram (ECG). However, ECG-based triggering faces limitations in MRI environments due to challenges in electrode placement, magnetic field interference, and RF-induced heating. Alternatively, contact photoplethysmography (PPG) offers a feasible solution; however, it suffers from reduced accuracy and requires the fixation of a probe on the finger tip. To overcome these limitations, this paper proposes an MRI-compatible system for contactless forehead PPG using bundled fiber-optic guides. The proposed approach eliminates electrical interference and ensures safety. A low-power sensor node is proposed to investigate the trade-offs among signal fidelity, energy efficiency, and system latency in on-device PPG. By combining programmable optical sources, an analog front-end, and BLE connectivity, the platform enables reproducible MRI experimentation. It fully processes the PPG data in just 2.8 ms onboard, utilizing a low-power ARM Cortex-M33 core running at 128 MHz. A feasibility study involving 8 subjects was conducted to evaluate the proposed system and demonstrate the effectiveness of the MRI-compatible contactless PPG using green/red light. Several fiducial points of the PPG waveform - foot, onset, and peak - were evaluated for trigger generation. Despite a physiological delay of ∼100-150 ms relative to the R-peak of a reference ECG, the PPG-based R-peak point is reliably estimated with a jitter of 5.14 ms. The sensor node demonstrated the efficiency of the proposed solution operating with a 520 mAh battery for over 23 hours and integrates custom adapters for 2 m optical guides, ensuring safe electronic placement outside the MRI bore. These results confirm that our system can enable prospective gating of MRI measurements without the practical challenges of securing ECG leads or a fingertip PPG. The proposed system paves the way for a safe, contactless, low-power, self-contained sensor node with onboard processing and an interference-free gating method, with the potential to redefine physiological monitoring workflows in MRI environments, enabling precise synchronization without electromagnetic interference.