Program Information

A Three-Dimensional Head-And-Neck Phantom for Validation of Kilovoltage- and Megavoltage-Based Deformable Image Registration

N Kirby*, K Singhrao , J Pouliot , UC San Francisco, San Francisco, CA

Presentations

MO-C-17A-5 Monday 10:15AM - 12:15PM Room: 17A

Purpose: To develop a three-dimensional (3D) deformable head-and-neck (H&N) phantom with realistic tissue contrast for both kilovoltage and megavoltage computed tomography and use it to objectively evaluate deformable image registration (DIR) algorithms.

Methods: The phantom represents H&N patient anatomy. It is constructed from thermoplastic, which becomes pliable in boiling water, and hardened epoxy resin. Using a system of additives, the Hounsfield unit (HU) values of these materials were tuned to mimic anatomy for both kilovoltage (kV) and megavoltage (MV) imaging. The phantom opened along a sagittal midsection to reveal non-radiopaque markers, which were used to characterize the phantom deformation. The deformed and undeformed phantom was scanned with kV and MV computed tomography. Additionally, a calibration curve was created to change the HUs of the MV scans to be similar to kV HUs, (MC). The extracted ground-truth deformation was then compared to the results of two commercially available DIR algorithms, from Velocity Medical Solutions and MIM Software.

Results: The phantom produced a 3D deformation, representing neck flexion, with a magnitude of up to 8 mm and was able represent tissue HUs for both kV and MV imaging modalities. The two tested deformation algorithms yielded vastly different results. For kV-kV registration, MIM made the lowest mean error, and Velocity made the lowest maximum error. For MV-MV, kV-MV, and kV-MC Velocity produced both the lowest mean and lowest maximum errors.

Conclusion: The application of DIR across different imaging modalities is particularly difficult, due to differences in tissue HUs and the presence of imaging artifacts. For this reason, DIR algorithms must be validated specifically for this purpose. The developed H&N phantom is an effective tool for this purpose.

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