Tools of the Trade: CIRS MR Safe MRgRT Motion Management QA Phantom


The integration of MR imaging in radiation therapy facilitates real time motion management. The CIRS MRgRT Motion Management QA phantom is designed to address such needs. The phantom is MR Safe due to the use of piezoelectric motors and non-ferro-magnetic materials. The two piezoelectric motors move a cylindrical insert, which contain a tracking target, through a gel/liquid fillable body by rotating it independently from the motion in the inferior-superior direction. The moving insert contains an organic shaped target (tumor) filled with gel, which is surrounded by the same background gel used to fill the body. The body represents a heterogenous background due to simulated lungs, liver, kidney and spine. The simulated organs are anatomical in shape and have a life-like spatial relationship. They are filled with gels that provide contrast in CT and MR versus the background gel, which fills the void between the organs. Besides imaging, all organs, except for the lungs, offer ion chamber dosimetry cavities, which allow for completing an entire QA process; from imaging to planning to verification of dose delivered. The phantom is designed as a single unit with a piezo actuator fixed permanently to a base plate on which the MRI body “snaps.” This allows for quick setup, removal, filling and storage purposes. The phantom’s base plate has machined slots on the bottom, which allow for the use of indexing bars for precise and repeatable/ reproducible phantom-MRI (MRI-Linac) alignment. CIRS Motion Control software drives this phantom. In addition to multiple built-in motion profiles, which are more appropriate for commissioning and routine QA, the software allows for import of complex patient specific respiratory waveforms. These waveforms can be edited for amplitude, sample rate, cycle time, phase shift and baseline position. It also allows to setup independently controllable waveforms for linear and rotation motion of the insert. The inferior-superior motion of the insert/moving target can be gated based on amplitude to allow verification of beam latency.


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