Calibration of a C-arm X-ray system for its use in tomography.

Intraoperative radiotherapy (IORT) is a procedure that allows the administration of a high dose of ionizing radiation directly on the tumor bed just after the surgery. The main advantage with respect to external radiotherapy (RT) is that it allows exposing the tumor bed at maximum avoiding radiation on surrounding healthy tissue.

The first tool for IORT planning was developed by GMV in collaboration with the imaging group at Gregorio Marañón Hospital inside a line of research devoted to IORT. The tool was based on a previous CT image as it is done for external radiotherapy planning. The problem in the case of IORT lies in the changes suffered by the patient’s anatomy during surgery and after resecting the tumor, making the dose calculations imprecise, as they were done based on CT images previous to the intervention. Within this line of research, obtaining volumetric images of the patient during the intervention using a C-arm as a tomograph has been proposed to evaluate and/or improve the results on IORT planning.

A C-arm is a portable system with a wide variety of movements that allows obtaining projective images (radiographies) from multiple perspectives of the operating table during an intervention, avoiding the need of moving the patient. For the development of this project a C-arm by SIEMENS (model: Siremobil Compact L) is available.

In order to use the system, designed originally to obtain planar images, as a tomograph, it is necessary to make an evaluation of the possible non-idealities (distortion in the detector, repeatability and geometrical misalignments) and its effects on the reconstructed images. Furthermore, it is necessary to develop a calibration algorithm that allows obtaining the correction value to compensate them in reconstruction. The objective of this thesis is to give answers to these two needs.

In the first place, the evaluation of the projection images has been done by making a distortion and repeatability study based on acquisitions of different phantoms. Secondly, the effects of geometrical misalignments on both projections and reconstructions have been evaluated. To this end, a software tool has been developed to emulate the acquisition done by the C-arm, producing a set of projections of synthetic phantoms based on the parameters that describe the real system. This tool allows modeling the geometrical misalignments applying inclinations, rotations and translations on the detector and changes on the value of source-to-detector distance.

Finally a software calibration tool has been developed, based on the method proposed by Cho et al. in 2005, an algorithm specifically designed for its use with a C-arm. It allows the estimation of a set of parameters that describe the system geometry. With these parameters, the reconstruction algorithm can compensate the non-idealities to obtain a good quality reconstruction.