2023 - Setup-Free Cardiorespiratory Physiology Synchronization for MRgRT with Fiber Bragg Gratings

Purpose/Objective(s): MRI-guided radiotherapy (MRgRT) in the central thorax requires monitoring cardiorespiratory motion to optimize treatment margins or to freeze out motion during imaging. For the latter, the MRI acquisition is synchronized with physiology monitors, such as ECG, SpO­₂ or respiratory belts. Typically, these surrogate signals suffer from MRI-induced distortions and/or time delays. In addition, setup time adds to workflow complexity and duration. We demonstrate how a monitor based on optical fiber Bragg gratings (FBG) alleviates these issues whilst yielding both cardiac and respiratory signatures for MR-sequence synchronization.

Materials/Methods: FBGs are mechanical strain sensors operating according to fiber optic sensing principles. Strain-induced changes in grating spacings are measured as shifts in the peak detected wavelength (in picometer, pm) of a fiber-coupled laser reflected on the FBG. Silica glass fibers have low electrical conductivity and are non-magnetic, so no interference from the MRI magnetic and RF fields needs to be considered. In this work we used an FBG glued onto a rigid plate, serving as a broadband mechanical resonator. In addition to slowly varying strain due to motion, vibrations from sound could also be measured in this way. The FBG was mounted underneath a 1.5 T MR-linac anterior coil, which was resting on a volunteer's chest. The FBG peak positions were recorded with 8 kHz sampling rate during a clinical 2D TSE scan. As check for RT compatibility we irradiated the FBG on its own in the MR-linac with a 100 MU 10x10 cm² field.

Results: Respiratory motion was detected at the low-frequency end of the signal spectrum (<20 Hz) with a peak-to-peak (pp) signal magnitude of 180 pm. Cardiac signatures in the form of a ballistocardiogram (<20 Hz, 30 pm pp) and phonocardiogram (near 100 Hz, 3 pm pp) were also observed. Acoustic noise generated by the MR sequence was recorded as well (mostly around 1 kHz, 7 pm pp). The sequence parameters of the TSE sequence (e.g. TR, TE, and echo-train length) were readily recognized from these sounds. As such, this method resulted in both cardiorespiratory and MR-sequence signatures on a single time basis, yielding an excellent feedback signal for cardiorespiratory-synchronized MRgRT. No signal distortions due to irradiation were observed.

Conclusion: We presented an FBG physiology monitor for measuring cardiorespiratory signatures in an MR-linac environment. The co-detection of sounds from the MR sequence serves as an asset for low-latency synchronization. Integrating FBG sensors in the MRI coil array will provide unparalleled physiology-based MR-sequence triggering without adding to patient setup time.