Mechanical Wind-up Motion Phantom for MR-Linac Daily Quality Assurance.
Purpose: Visualizing organ motion in real-time is a primary advantage of combined MRI and linear accelerator treatment systems. However, the cine sequences used for motion visualization often trade image quality and spatial distortion for temporal resolution. Here we present a simple MR-safe phantom for evaluating daily consistency of such sequences.
Methods: We designed the components of the phantom in AutoDesk Fusion 360™. To avoid eddy currents, we utilized plastic for all moving components of the phantom. We made design choices for ease of 3D printing and simple laser cut production. We used glass bearings to reduce friction in the system, and leaded plastic (density of ~4 g/cc) for counterweights and energy storage.
Results: The phantom is 25cm in height and width, and 15cm in depth. A weighted tray is wound to the top of the phantom, and the stored energy is released via an escapement mechanism attached to an external oscillating arm. Attached to the oscillating arm is a 3cm diameter water-filled ball. The period of oscillation is adjusted via a slidable counterweight. The motion phantom produces an adjustable periodic oscillation in a frequency range of roughly 0.4 to 2Hz. This corresponds to a run time of between 15 to 3 minutes. This range allows enough time to setup the phantom and run a test of the cine sequences.
Conclusion: It is possible to construct a simple, lightweight, MR-safe mechanical phantom that oscillates with a long enough period to provide a useful motion test for cine MR sequences. The phantom can be built with a combination of 3D printed components, laser-cut plastic, and inexpensive plastic/glass bearings. STL files for all pieces will be uploaded to facilitate others wishing to instantiate the phantom.
Snyder M, Halford R, Yan D. Mechanical wind-up motion phantom for MR-linac daily quality assurance. Med Phys. 2021 Jun;48(6).