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Program Information

An External Respiratory Surrogate for MRI-Based Radiation Therapy Simulation

C Williams

J de Arcos , R Cormack , E Schmidt , C Williams*, Brigham and Women's Hospital, Boston, MA


TU-D-FS4-2 (Tuesday, August 1, 2017) 11:00 AM - 12:15 PM Room: Four Seasons 4

Purpose: Abdominal marker blocks with infrared reflectors (IR) are commonly used as respiratory surrogates (RS) for gating in external beam radiotherapy treatments. However, in MR-based simulation, RF coils can interfere with the IR’s line-of-sight requirement to the block, making them impractical. The accuracy of gated delivery is dependent on the correlation between motions of the RS and the desired radiation target, so it is undesirable to use disparate surrogates (such as bellows, spirometry or navigators) for simulation if the treatment will use an IR marker block. This project is aimed at building and validating an MR-based external RS that is analogous to the external IR abdominal surrogates used on clinical linear accelerators.

Methods: A commercial respiratory gating block was modified by embedding five flexible printed circuits, each containing a micro-coil tuned to 123.2 MHz, above Vitamin-E capsules. The coils were connected to a 3T MRI scanner via patient isolation circuitry and a dedicated 8-channel receiver. An MR-tracking sequence (TR/TE/α=3.38 ms/2.14 ms/5°, 1.04x1.04x1.04mm³ resolution, bandwidth 175kHz, PFD=3, 3 averages) providing 14 frames-per-sec 3D localization, was interleaved with 1-3 (Sagittal, Coronal, Axial) orthogonal-plane balanced-SSFP MRI images. Images were acquired of phantoms and volunteers to validate the device function.

Results: A respiratory signal was successfully acquired using the coil-embedded surrogate block. The superior-inferior location of the diaphragm was correlated with the anterior-posterior motion of the surrogate with coefficient of determination r² = 0.95 and a best fit time lag of 0.033 seconds. 2.6 Hz Interleaved Imaging and tracking rates were achieved.

Conclusion: A standard IR external marker block can be embedded with micro-coils and actively tracked during MRI acquisition, enabling it to be used as a respiratory surrogate during MR-based radiotherapy simulation.

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