Anatomical and Functional Lung Imaging with Volumetric Computed Tomography in Non-Small Cell Lung Cancer

Non-small cell lung cancer (NSCLC) is one of the most diagnosed cancers in Canada, and the leading cause of cancer deaths. A significant challenge in treating NSCLC is balancing aggressive treatment with the potentially severe side effects. In radiation therapy, the management of respiratory motion and the risks of radiation-induced lung injury (RILI) pose significant challenges. 4-dimensional computed tomography (4D-CT) is an important part of motion management, but images often suffer from motion-induced artifacts. Volumetric CT scanners with wide axial field-of-view (aFOV) may reduce these artifacts and present an opportunity to advance CT-based functional lung imaging. Chapter 2 presents a phantom imaging study to investigate the suitability of a 256-slice volumetric CT (vCT) scanner for radiotherapy treatment planning. The density of the highest density materials was under-estimated by the scanner, which can be addressed with the use of an appropriate relative electron density (RED) curve. An average RED curve for all aFOV settings may be used. Chapter 3 presents a study of phantom and NSCLC patient 4D-CT images acquired on a clinical scanner and a vCT scanner. The v4D-CT images were re-sampled to simulate a conventional acquisition using a narrow aFOV clinical scanner. The phantom images demonstrated that target contouring variability decreased in v4D-CT imaging as compared to clinical 4D-CT. In the patient images, mean Hausdorff distance between organs at risk (OAR) contours was significantly correlated to respiratory phase, indicating that motion artifacts contribute to this variability. Chapter 4 presents a novel acquisition and analysis pipeline to image lung ventilation (V), perfusion (Q) and V/Q ratio in a single volumetric CT scan. In a porcine study, these images of V and Q were significantly correlated to standard Xe-enhanced ventilation and PET perfusion images in voxel-wise analysis. In a NSCLC patient study, the images were sensitive to changes in V and Q between baseline imaging and follow-up 6 weeks after radiotherapy. In this thesis, I demonstrate that volumetric CT scanners are suitable for use in radiation therapy simulation and treatment planning, and detail two scanning protocols which may reduce the challenges posed by respiratory motion and RILI risk in NSCLC

Investigation of CT dosimetry techniques for use in optimisation of Automatic Tube Current Modulation (ATCM) performance

The PhD project aimed to develop methodologies for optimisation of automatic tube current modulation (ATCM) performance for the four most important computed tomography (CT) scanner manufacturers (Toshiba, GE, Philips and Siemens scanners). Since the human body to which ATCM response is elliptical in shape, the project started with an investigation of differences between dosimetry in cylindrical and elliptical phantoms. The ATCM systems modulate the tube current in both the x-y plane and z-axis, therefore phantoms made from multiple sections of different size ellipses were designed for quality control of the ATCM performances and evaluated by comparing with a conical phantom developed by imaging performance and assessment of CT (ImPACT) evaluation centre, UK. In order to link the project into patient dose optimisation, CT scanners in which patient doses were high were identified and the link with patient size evaluated. Since the large variations in patient dose may be influenced by scan parameters, the phantom developed was used to carry out measurements on CT scanners and investigate some factors and attribute reasons for the high doses. Finally changes in CT scanner protocols were recommended. The results from the elliptical phantom showed that the doses in the centre and anterior were larger than in the cylinder, while doses in the lateral periphery were similar. Differences in ratios of doses between the two phantoms for different CT scanners are linked to the beam profiles produced by the individual bow tie filters. Phantoms made from multi elliptical sections demonstrated similar trends for the Philips and Siemens ATCM systems. However, the abrupt changes in attenuation provoked the ATCMs to increase tube current aggressively with the GE and Toshiba systems. A phantom like a wedding cake with broader sections and smaller differences in attenuation circumvented these effects. The volume weighted CT dose index (CTDIvol) increased significantly with patient size for Toshiba and GE scanners whilst the changes for Siemens and Philips scanners were less marked. However, the use of Philips the D-DOM ATCM option led to a significant increase in patient dose. The reconstruction filter and image thickness are major factors influencing patient dose for the Toshiba CT scanner