Optimization and evaluation of scanning-beam digital X-ray tomosynthesis for real-time 3D image guidance for lung tumor biopsies [electronic resource]
Keywords: Bioengineering.

Biopsy methods used in lung cancer typically have either significant complication rates or high rates of false negatives. The safest biopsy method (transbronchial biopsy) has high false negative rates. This thesis takes the initial steps towards adding real-time 3-dimensional imaging to this process via tomosynthesis. Currently transbroncial biopsies are guided by an Electromagnetic Navigation Bronchoscopy (ENB) system. This guidance assumes little physiological change between the CT scan and the procedure and also assumes that there is no movement of the catheter while removing the ENB probe from the working channel and inserting the needle or cytobrush for the biopsy. Thus the pulmonologist is 'blind' during the actual biopsy. The scanning-beam digital x-ray (SBDX) system (an inverse geometry fluoroscopic system) can add tomosynthesis imaging to the biopsy process. The SBDX system has a 2D array of small beams incident from multiple angles onto a small detector. The multiple angles allows 3-dimensional information to be reconstructed in the images via tomosynthesis. To maximize the tomographic angle of the SBDX system, the tomosynthetic angle as a function of tumor-to-detector distance (TDD) was calculated. Monte Carlo Software (PCXMC) was used to calculate organ doses and effective dose for source-to-detector distances (SDDs) for multiple system geometries. These calculations were performed for both the SBDX system and for standard fluoroscopy. The effect of system geometry on patient dose, both in absolute terms and in terms of image quality were investigated. The tomographic angle had more significant changes with SDD in the region near the detector, at a source to tumor distance that is 69.7% of the SDD assuming constant source and detector size. Changing the patient location in order to increase tomographic angle has a significant effect on organ dose distribution due to geometrical considerations. When tumor signal to noise ratio (SNR) is held constant (ie. x-ray fluence is scaled appropriately), SBDX gives 2-10 times less dose than fluoroscopy for the same conditions within the typical range of patient locations. These models suggested an ideal SDD of 100 cm because of practical considerations of varying patient size, and adequate room between the patient and the bracing securing the detector. To show that the SBDX could improve targeting of tumor lesions, realistic phantoms were needed. Pig lungs and hearts were preserved, the vasculature was filled, and lesions were created in them. The lungs were capable of breathing motion when placed in a custom vaccum chamber. A chest phantom with a rib cage was cut to fit in the vacuum chamber. This provided a very lifelike phantom model for the lungs including the capacity to mimic breathing motion. The prepared lungs were scanned by CT and the scans were used for ENB planning. The pulmonologist placed fiducials which represented the tissue sample site using the ENB system. During this process, the lungs were deformed to simulate the real clinical condition of breathing. After all the fiducials were placed, the lungs were scanned on the SBDX system. The shortest distance from the distal tip of the fiducial to the center of the lesion was measured using the SBDX images. The lungs were again scanned by CT and the fiducial-to-lesion distances were measured. The SBDX system fiducial-to-lesion distances had significant (p< 0.0001) agreement with the CT fiducial-to-lesion distances, indicating that SBDX images can accurately provide image guidance for ENB procedures. The ENB tip-to-target distances were not accurate. When auto-registered lungs were compared with manually registered lungs, it was found that the auto-registered lungs, which had less CT-to-body divergence, had more agreement with the CT fiducial-to-lesion measurements. This indicates that CT-to-body divergence both from rigid registration of primary bronchial passages and non-rigid breathing motion is responsible for the ENB inaccuracy in the distal regions of the lungs. Additional steps to bring this method to the clinic include testing with both guidance systems concurrently, continued development of the reconstruction algorithm, and testing the system in an in vivo model.