Development of a breathing lung phantom for proton CT imaging

<i>Objective.</i>To report on the design of a deformable lung phantom capable of imitating breathing motion with realistic tissue surrogate properties for proton imaging applications.<i>Approach</i>. The phantom was manufactured via 3D printing and silicone moulding, with a customised structural design for motor-controlled breathing motion. The overall size of the phantom was rescaled to fit in the experimental proton computed tomography (pCT) scanner prototype, featuring a 284 mm maximum size for the imaging field-of-view. Several flexible resins were evaluated in perspective of flexibility by varying ultraviolet exposure times, as increased exposure results in resin hardening at each layer. We optimised the structure to achieve ideal lung compression properties, while preserving its integrity to hold the weight of a solid tumour. Phantom material properties were characterised by segmentation of each component in x-ray CT and pCT images, to determine the CT number expressed in Hounsfield units and the relative stopping power (RSP) with respect to water.<i>Main results</i>. We achieved non-homogenous compression in the lung using a grid structure with gradient thickness. The rigid ribcage was 3D printed using granite based material. The tumour motion implemented in the phantom design, as measured using template-matching in fluoroscopic x-ray imaging, revealed hysteretic motion with 10 mm peak-to-peak in the superior-inferior direction.<i>Significance.</i>The developed deformable lung phantom imitated lung motion characteristics, featuring CT number and RSP values in the range comparable to human tissues. The developed breathing phantom is put forward for experimental motion studies in pCT imaging.