Ultrasound-to-Ultrasound deformable image registration for pseudo-CT simulation for Brachytherapy treatment planning: A preliminary study on tissue-mimicking phantom

Radiation therapy is often used to treat cancerous tumors, with most cases involving a combination of external and internal radiation treatments. To aid the clinician to plan for the treatment, image-guided radiotherapy is used, in which computed tomography (CT) images are acquired multiple times during treatment. This puts the patient at unnecessary risk of additional radiation exposure. Simulating pseudo-CT scans at some time points that would obviate acquiring CT scans has the potential to reduce the risks related to radiation exposure. In this study, we propose a methodology in which pseudo-CT images for brachytherapy planning can be simulated from the pre-radiotherapy CT and ultrasound and pre-brachytherapy ultrasound images. The primary step in this is the registration between pre-radiotherapy and pre-brachytherapy ultrasound images. In this study, the performance of the demons deformable registration algorithm and mid-space independent image registration algorithm on ultrasound images obtained from a tissue-mimicking phantom are evaluated. The resultant deformation field obtained from the registration step is used to simulate the pseudo-CT image, which is then compared with the ground-truth CT image acquired from the same phantom. The results show very little variability in the similarity metrics between the algorithms, indicating the feasibility of the proposed methodology.