Microsecond-Scale Characterization of Radiation-Induced Dynamics on a 0.35T MR-Linac
Abstract
Purpose
To characterize the microsecond-scale dynamics of the free induction decay (FID) of a 0.35T Magnetic Resonance (MR)-guided Linear Accelerator (MR-Linac) during irradiation.
Methods
We implemented a SPEEDI (Submillisecond Periodic Event Encoded Dynamic Imaging) sequence on a 0.35T MR-Linac for ultra-high temporal resolution. This version achieves 1-µs sampling of the same FID across multiple repetition times (TRs) to enhance the effective signal-to-noise-ratio (SNR). Measurements were acquired in multiple chemical phantom media (air, pure water, ex vivo tissue) under irradiation across several field sizes to isolate signal changes attributable to radiation-induced effects. FIDs were pre-processed by removing T2* decay and global signal drifts. Signals were then interpolated 10-fold and temporally aligned according to the expected desynchronization factor (5.038 µs per TR) of our 0.35T MR-Linac to enable µs-accurate comparison across TRs and enhance periodic features.
Results
Microsecond sampling of the FIDs captured the irradiation beam manifested as a transient peak superimposed on the expected T2* decay of the signal. Peak timing was consistent with the linac pulse frequency of 135 Hz. The transient reached maximum amplitude in approximately 5µs, also consistent with pulse duration, before decaying exponentially within 10µs. These peaks were observable in all phantom media and the distribution of peak-signal-to-noise-ratio (PSNR) varied significantly by field size and by type of phantom, indicating geometry- and medium-dependent contributions to the observed effect.
Conclusion
Microsecond-scale FID signal dynamics on a 0.35T MR-Linac demonstrate repeatable, synchronous transients with an observable duration of 15µs per radiation pulse. Field-size dependence suggests a hardware-coupling component (radiation or RF-related contamination in the imaging volume), while medium dependence suggests additional interactions related to the irradiated volume. Thus, the MRI signal dynamics observed at a microsecond scale suggest a dual nature from both radiation generation and radiation effects. Future work will further investigate the origin of these media-dependent measurements.