Purpose: The increased use of MR in radiation oncology for general patient simulation has led to many studies of target visualization during the breathing cycle. Quality assurance phantoms for these 4D techniques require MR-specific considerations, most notably the absence of ferromagnetic materials. We present a design for such a phantom.
Methods: The phantom concept is based on the elimination of motors and electronic equipment, thus allowing construction with no ferromagnetic materials that would present a safety hazard in a high magnetic field. The design also eliminates other conductive metals in which eddy currents could be generated during motion. The design is largely constructed of 3D printed parts made from polylactic acid (PLA), with nylon fasteners, leaded plastic, rubber drive belts, and a water fillable target. Bearings are nylon and glass, and functional components rotate about polycarbonate axles.
Results: The phantom design relies upon driven damped harmonic motion of a weighted dual pendulum. At the base of the pendulum sits the water-filled target along with a leaded plastic weight, which is contra another smaller leaded weight that adjusts periodicity. The energy to drive the oscillator is stored in a stout elastic band and transferred through the mechanism via a 3 stage belt and pulley system. The stages allow for longer cycles once the phantom has been set in motion. The period of the phantom can be reliably adjusted from 0.5 seconds up to roughly 2 seconds. The overall time of oscillation before the stored energy has been exhausted is roughly 1 minute at present.
Conclusion: A driven damped harmonic phantom is a viable design for MR safe motion quality assurance. Refinements in both construction and design should lead to a phantom with time factors appropriate for typical motion scans in an MR simulator.