Author: Zhaohui Han, Piotr Zygmanski 👨🔬
Affiliation: Brigham and Women’s Hospital, Dana Farber Cancer Institute, Harvard Medical School, Department of Radiation Oncology, Brigham and Women’s Hospital, Dana-Farber Cancer Institute, Harvard Medical School 🌍
Purpose: Real-time motion tracking/gating are oftentimes limited to a single sagittal plane on many magnetic resonance linear accelerators (MR-Linac). Out-of-plane motion is therefore difficult to detect and quantify. In this study, we aim to demonstrate the feasibility of using a very simple sensor with only two complementary elements for out-of-plane motion tracking in MR-guided radiotherapy (MRgRT).
Methods: A prototype two-element sensor was fabricated using the Superflab bolus material. Each element was triangularly shaped and was diagonally cut from a rectangular piece of 10cm×7cm×1cm. Cine tracking on a single sagittal plane was used to simulate the real-time MR-guidance and the sensor was used to detect the out-of-plane motion. The sensor was first calibrated to establish the relationship between the element signals and the lateral position of the sagittal plane. During calibration, the sensor remained centered on the isocenter while the sagittal tracking plane was driven to a series of lateral positions ranging from -2.1cm to 2.1cm with a step size of 3mm. To validate, six lateral couch shifts were applied to simulate the out-of-plane motion up to 1.5cm. At each couch position, the sensor was cine-imaged on a sagittal plane through the isocenter. MR signals from the sensor elements, together with the established calibration, were used to calculate the lateral shifts, which were then compared to the known couch positions to determine the detection accuracy.
Results: The sensor gave strong signals on our 0.35T MR-Linac. On a sagittal tracking cine, MR images of the two elements showed varying lengths, the difference of which demonstrated a linear relation with the lateral position of the sagittal plane. Sub-millimeter detection accuracy was achieved for six validation tests (Mean_error=0.5mm, Max_error= 0.7mm).
Conclusion: The sensor was able to detect out-of-plane motion with sub-millimeter accuracy and can be useful for MRgRT where only single-plane tracking was available.