Dissertations / Theses on the topic 'Four-dimensional images'

Purpose – We propose a multidisciplinary approach based on an extensive data base which provides digitalized photographic material from the end of the 19th century up to recent times. Thus a large amount of photographic evidence will be exploited, structured and enriched by additional sources to serve as a foundation for an application relying on 3D visualizations. The application addresses scholars as well as the general public and will provide different kinds of information and tools for research and knowledge transfer. Design/methodology/approach – The method applied will be diachronic: the virtual model may show one point in urban history depicting a certain state of past Dresden and also its development through the various eras. In addition the method works in a dualistic mode: on the one hand the physical development of the urban area will be explored and presented in detail, on the other hand the analysis of the pictures will give profound insights in the specific perception of the urban space. Originality/value – This methodology aims to make large repositories more accessible and proactive in information-seeking. Using a 3D application as an access for media repositories, research tools and functionalities which can improve the scientific handling of the data will be considered. How should the data and information be processed to meet the researcher’s needs? Which information can be retrieved from the visual media? What needs to be considered to ensure scientific standards and motivation while working with the image repositories? Users of the virtual archives can benefit extensively form effective searching functions and tools which work not only content- and theme-based but also location-based. Practical implications – The outcomes of the research will be presented in a 4D browser and available in an Augmented Reality presentation. The design will comply with the requirements of the field of application, whether aiming at a scientific, educative or touristic purpose. The paper itself considers three different approaches to the topic highlighting the multidisciplinary strategy and opportunities of the project. The first one considers research questions from art history. The second one reflects on concepts from information science, photogrammetry and computer vision for visualizations and the third one introduces an interaction concept for an AR application for the Zwinger in Dresden.

Electrical Impedance Tomography (ElT) has been used as a visualisation and measurement tool in many fields such as medical imaging, geophysical prospecting and industrial process applications. To date, single sensing ring strategies and two-dimensional (2D) electric field reconstruction algorithms are mostly used in ElT applications. The quality of measurement will be affected by the three-dimensional (3D) effects that cause imaging errors in both the near-sensor-region and the spatial coordination of a conventional 2D sensor. The typical errors include the object off-plane sensing and offpath trajectory effects - objects lying a short axial distance from the image plane are reconstructed closer to the central axis than their true position. There is a distinct possibility that it may also give a rise to erroneous velocity components normal to the axial direction. The aim of this thesis is to reduce the 3D effects by designing and implementing a full 3D pipeline sensing strategy which takes into account of the 3D nature of the ElT sensing field. The main approaches of the thesis are: (1) a new sensing system, the Zigzag sensor, which represents a new electrode configuration has been designed; (2) a fast forward solver, using Finite Element Modelling, has been implemented with the aim of achieving real-time processing of tomographic measurements; (3) the Sensitivity Conjugate Gradient (SCG) Algorithm has been adapted to 3D ElT for the first time. Moreover, the thesis contributes towards the application of the developed 3D ElT system for dynamic flow visualisation and velocimetry with 3D auto-correlation method which provided a balance between the requested imaging precision and computation speed. The thesis details both theoretical and experimental approaches as well as evidences that the zigzag sensor with the 3D SCG offers some advantage over conventional methods to reduce the 3D effects on ElT pipeline imaging.

Respiratory motion remains a significant challenge for radiation therapy in targeting the tumour. The use of planning margins to avoid geometrical miss of the target volume during respiration results in excessive lung tissue irradiation that limits the prescribed dose to be safely delivered and escalated for better therapeutic gain. The purpose of this study was to develop effective dose planning techniques for treatment to be performed under natural patient breathing. The techniques accounted for the dosimetric influences of tumour movement and aimed to provide an optimized treatment volume by minimizing the internal target volume (ITV) without compromising the target coverage. In the study, the accumulated 4D dose distribution over the tumour volume was calculated using deformable image registration (DIR). A DICOM-RT based tool-box was specially developed for automated 4D dose calculation and evaluations. A new concept of defining the internal target volume from 4D dose coverage, namely inverse ITV (iITV) was introduced via the dose volume enclosed by the minimum accumulated dose in the tumour during the respiratory cycle. The dosimetric advantages of using this iITV with reference to the conventional ITV were confirmed in nine clinical cases by an average dose volume reduction of 16.4% (ranging from 2.3% to 29.9%). 4D radiotherapy involves complex dose distribution which was found to be affected by a number of factors including tumour size, magnitude of tumour displacement, tumour motion characteristics and the reference phases selected for dose planning. Our findings indicate that optimal dose planning was generally, but not always, achieved with the planning CT performed at the temporal mean tumour position and the degree of target coverage maximization strongly depends on the nature of tumour movement. Moreover, the conventionally geometric defined treatment margin could over estimate the treatment volume for a required target coverage. In conclusion, 4D dose calculation based on DIR offers realistic dose estimation, as both geometric and temporal factors are considered, and also provides optimal dose plans by minimizing the treatment volume. However, 4D radiation planning involves a number of factors resulting from the properties of tumours (eg. tumour size, amplitude and characteristics of tumour motion, etc) and from the procedure of treatment planning (eg. reference phase for dose planning, penumbra of dose beam, employed treatment volume etc) that interactively affect the resultant dosimetry. Since these factors vary patient-by-patient, there is no single formula or universal solution that can be used to obtain optimal dose planning. The 4D dose toolbox developed in this study could however provide a user friendly platform for 4D dose calculation and analysis, and allow the optimal treatment modalities and planning techniques to be determined for individuals.

Stereotactic body radiotherapy (SBRT) is a promising treatment strategy for early–stage lung cancers. Conventional three–dimensional (3D) SBRT based on a static patient geometry is an insufficient model of reality, posing constraints on accurate Monte Carlo (MC) dose calculation and intensity–modulated radiotherapy (IMRT) optimization. Four–dimensional (4D) radiotherapy explicitly considers temporal anatomical changes by characterizing the organ motion and building a 4D patient model, generating a treatment plan that optimizes the doses to moving tissues, i.e., 4D dose (as opposed to the static 3D dose to tissue), and delivering this plan by synchronizing the radiation with the moving tumor. This thesis focuses on 4D robotic tracking lung SBRT. By recalculating the conventional 3D plan on the 4D patient model using MC simulation, it was found that 4D moving dose distributions could detect increase of normal tissue doses and complication probabilities (NTCP), and decrease of tumor dose and control probability. For one patient, the risk of myelopathy was estimated at 8% and 18% from the 3D equivalent path–length corrected (EPL) and the 4D MC doses, respectively. Such increased NTCP suggests that better estimations of different dosimetric quantities using 4D MC dose calculation are crucial to improve the existing dose–response models. Dosimetric error in 4D robotic tracking SBRT was found to be caused predominately by tissue heterogeneities, as assessed by the comparisons of the 4D moving tissue doses calculated using the conventional EPL and MC algorithms. At 3% tolerance level, our results indicated clinically significant dose prediction errors only in tumor but not in other major normal tissues. Furthermore, 4D tracking radiotherapy was found to have greater ability to limit the normal tissue volume receiving high to medium doses than the other advanced SBRT strategy combining volumetric–arc radiotherapy with 4D cone–beam CT verification. Invariant target motion was found to be an unrealistic assumption of 4D radiotherapy from the analysis of probability motion function (pmf) of motion data. Systematic and random variations of motion amplitude, frequency, and baseline were found to reduce the reproducibility of pmfs, on average, to just 30% for the principal motion of 3400 seconds. Experimental evaluations showed that systematic motion change reduced the gamma passing rate of radiochromic film measurements at 3mm distance–to–agreement and 3% dose difference criteria from 91% for 4D dose calculated with MCand EPL algorithms to 47% and 53% in the static object, respectively,. For moving target object, gamma passing rates of the 4D MC doses hardly changed with reproducible and non–reproducible motion (95% vs. 93%), and barely differed between conventional 3D and 4D MC doses (95% vs. 95% with reproducible, and 96% vs. 93% with non–reproducible motions). Distortions due to image artifacts and registration errors were consistently observed in the 4D dose distributions but not the 3D dose distributions. In conclusion, 4D Monte Carlo planning shall be considered for robotic target tracking only if robustness against uncertainties of patient geometry, and accuracy of 4DCT imaging and deformation registration are significantly improved.
published_or_final_version
Clinical Oncology
Doctoral
Doctor of Philosophy

Four dimensional imaging has become part of the standard of care for diagnosing and treating non-small cell lung cancer. In radiotherapy applications 4D fan-beam computed tomography (4D-CT) and 4D cone-beam computed tomography (4D-CBCT) are two advanced imaging modalities that afford clinical practitioners knowledge of the underlying kinematics and structural dynamics of diseased tissues and provide insight into the effects of regular organ motion and the nature of tissue deformation over time. While these imaging techniques can facilitate the use of more targeted radiotherapies, issues surrounding image quality and accuracy currently limit the utility of these images clinically. The purpose of this project is to develop methods that retrospectively compensate for anatomical motion in 4D-CBCT and correct motion artifacts present in 4D-CT to improve the image quality of reconstructed volume and assist in localizing respiration-influenced, diseased tissue and mobile structures of interest. In the first half of the project, a series of motion compensation (MoCo) workflow methods incorporating groupwise deformable image registration and projection-warped reconstruction were developed for use with 4D-CBCT imaging. In the latter half of the project, novel motion artifact observation and artifact- weighted groupwise registration-based image correction algorithms were designed and tested. Both deliverable components of this project were evaluated for their ability to enhance image quality when applied to clinical patient datasets and demonstrated qualitative and quantitative improvements over current state-of-the-art.

La tomographie d'emission monophotonique fournit une sequence d'images 3d representatives de la distribution du traceur administre au patient, pour differents instants du cycle cardiaque. Elle met en evidence les zones mal irriguees du myocarde. Afin d'eviter une trop longue immobilisation, les temps d'acquisition sont limites, ce qui conduit a des mesures tres bruitees. La reconstruction etant un probleme inverse mal-pose, les images tomographiques sont alors tres degradees. Nous proposons dans cette these deux methodes permettant d'ameliorer la qualite statistique de ces images en regularisant temporellement le processus de reconstruction. La simple moyenne temporelle conduisant a un flou cinetique, le mouvement du myocarde est d'abord estime a partir du suivi de trois surfaces caracteristiques du myocarde, puis integre dans l'algorithme de reconstruction. La premiere methode developpee consiste a reconstruire une phase particuliere connaissant l'ensemble des mesures acquises et la loi d'evolution. La reconstruction s'inscrit dans le cadre theorique du filtrage de kalman. La solution formelle necessitant l'inversion d'une matrice de tres grande dimension, nous proposons une solution sous-optimale mais rapide. L'algorithme recursif repose sur des operations de type filtrage-retropropjection. Nous presentons ensuite une deuxieme approche dans laquelle l'ensemble des phases est reconstruite simultanement. La reconstruction de la sequence est effectuee par minimisation d'une fonction quadratique etablie dans le cadre d'une regularisation spatio-temporelle. Le calcul iteratif des images est effectue a partir de l'algorithme du gradient conjugue. Les resultats experimentaux montrent la validite de notre approche et mettent en evidence l'apport d'une regularisation temporelle

Les nouvelles technologies mises en œuvre actuellement suscitent des demandes croissantes auprès de l’industrie électronique dont la capacité des circuits électroniques et de leurs microprocesseurs croît de façon explosive en suivant la loi de Moore. Le nombre croissant de transistors par unité de surface entraîne des échauffements considérables qui sont nuisibles au bon fonctionnement des systèmes et posent des problèmes d’évacuation de la chaleur générée, de façon très localisée, dans les composants électroniques. Afin de maîtriser les flux de chaleur créés, il est indispensable d’utiliser des matériaux nouveaux capables de conduire très rapidement et efficacement, c’est à dire de façon unidirectionnelle, la chaleur vers un puits thermique. Les travaux présentés dans cette thèse s’inscrivent dans cette problématique et proposent l’étude de matériaux, isolants électriques, afin d’éviter des courts circuits dans la fabrication de composants électroniques, mais aussi présentant une conductivité thermique fortement anisotrope afin d’évacuer la chaleur dans une seule direction. Pour cela des matériaux très conducteurs, à l’état monocristallin, sont nécessaires. Pour réaliser des mesures de conductivité thermique dans les meilleures conditions, de tels échantillons, d’excellente qualité et parfaitement homogènes ont été synthétisés. Pour obtenir une telle qualité d’échantillons, la méthode de la zone solvante (TSZM : Travelling Solvent Zone Method) a été utilisée. Cette méthode de croissance cristalline, n’utilisant pas de creuset, permet l’obtention de monocristaux exempts d’impuretés, de plusieurs centimètres de longueur. Les matériaux étudiés dans ce travail sont les cuprates de basse dimensionnalité Sr2CuO3, SrCuO2 et La5Ca9Cu24O41 présentant dans leur structure un arrangement d’ions cuivre Cu2+, de spin ½, sous forme de chaînes linéaires ou d’échelles, présentant un caractère 1D marqué. Leur conductivité thermique, dans la direction 1D, est décrite par la somme de deux contributions, l’une, phononique et, l’autre, d’origine magnétique, liée aux spins des ions cuivre. Pour obtenir une meilleure compréhension des différents mécanismes d’interaction en compétition, l’influence de la pureté de ces composés ainsi que celle du dopage sur le site des ions Cu2+ sur la conduction thermique d’origine magnétique, a été étudiée. La pureté des échantillons joue un grand rôle, à basse température, sur la conductivité thermique magnétique du fait d’une diminution des interactions spinons-défauts. Par ailleurs, une étude structurale par diffraction des rayons X et de neutrons sur chacun des composés a été réalisée et a mis en évidence la présence de distorsions dans la structure du composé La5Ca9Cu24O41
Today’s new technologies bring increasing demands to the electronics industry whose capacity of electronic circuits and related microprocessors increases very rapidly, following Moore’s law. The increasing number of transistors per unit area brings about significant heating which may be harmful to the good functioning of the systems and creates problems in the evacuation of the very localized heat generated in the electronic components. In order to control the heat flow which is produced, it is essential to use new materials able to conduct rapidly and efficiently, i. e. unidirectionally, the heat toward a heat sink. The present thesis work deals with the above described issues and presents the study of materials which have to be insulating in order to avoid short circuits in the electronic components and also exhibit a strong anisotropy of the thermal conductivity in order to evacuate the heat exclusively in one direction. Single crystals are therefore required. In order to realize thermal conductivity measurements in the best conditions, perfect homogeneous single crystals of excellent quality were synthesized by the Travelling Solvent Zone Method. This no-crucible crystal growth method allows the synthesis of impurity-free single crystals several cm long. The investigated materials are the low dimensional cuprates Sr2CuO3, SrCuO2 and La5Ca9Cu24O41 exhibiting in their structures an alignment of Cu2+ ions of spin ½ as linear chains or ladders, showing thus a distinct 1D character. Their thermal conductivity in the 1D direction is described as the sum of two contributions, one phononic and the other of magnetic origin. In order to obtain a better understanding of the different competitive interaction mechanisms, the influence on thermal conductivity, of the purity of the compounds and also of doping on the copper site has been investigated. Furthermore, structural refinement was done (X-ray and neutron diffraction) and has permitted to highlight distortions in the La5Ca9Cu24O41 samples

Nesta tese, buscou-se avaliar se a tomografia computadorizada de duplaenergia pós-contraste (TCDE) é capaz de detectar diferenças regionais da perfusão pulmonar em um modelo animal suíno incluindo variações de decúbito, lesão alveolar e oclusão da artéria pulmonar com balão, comparando estes resultados com os obtidos pela perfusão de primeira passagem com a tomografia computadorizada dinâmica (TCD). Dez suínos landrace foram divididos em Grupos A (N = 5, controle) e B (N = 5). Animais do Grupo B foram submetidos ao protocolo de lesão alveolar induzida por ventilação mecânica (LPIV). O volume sanguíneo perfundido e o fluxo sanguíneo pulmonar foram, respectivamente, estimados pela TCDE (%VSPTCDE) e pela TCD (FSPTCD), em diversas condições experimentais: posição supina versus prona, presença versus ausência de LPIV, presença ou ausência de oclusão da artéria pulmonar. A correlação entre %VSPTCDE e FSPTCD foi moderada (R = 0,60) com ampla variabilidade (intervalo 0,35-0,91) entre animais. %VSPTCDE e FSPTCD demonstraram padrões similares de heterogeneidade da perfusão pulmonar nas diferentes condições experimentais. Entretanto, reduções do %VSPTCDE causadas pela oclusão com balão foram em média -29,32 %, enquanto reduções do FSPTCD foram em média -86,78 % (p < 0,001). Estimativas quantitativas do VSPTCDE tiveram um erro médio de +4.3 ml/100g em comparação com o FSPTCD, com limites de concordância de 95 % entre -16,6 ml/100g e 25,1 ml/100g. A TCDE póscontraste é capaz de prover estimativas semiquantitativas que refletem a heterogeneidade regional da perfusão pulmonar causada por mudanças de decúbito, lesão alveolar e oclusão da artéria pulmonar com balão, apresentando moderada correlação com a perfusão de primeira passagem pela TCD
We aimed to evaluate whether contrast-enhanced dual-energy CT (DECT) detects regional pulmonary perfusion changes in a swine model of acute lung injury, with variations in decubitus and transient occlusion of the pulmonary artery, comparing these results with those obtained with dynamic CT perfusion (DynCT). Ten landrace swine were assigned to Groups A (N = 5, control) and B (N = 5). Group B was subjected to ventilator-induced lung injury (VILI). Perfused blood volume and pulmonary blood flow were quantified by DECT (PBVDECT) and DynCT (PBFDynCT), respectively, under different settings: supine versus prone, and with/without balloon occlusion of a pulmonary artery (PA) branch. Correlation of regional PBVDECT versus PBFDynCT was moderate (R = 0.60) with high variability (range 0.35-0.91) among the animals. Regional pulmonary perfusion changes assessed by %PBVDECT agreed with PBFDynCT in response to decubitus changes, lung injury and balloon occlusion in the multivariate analysis. However, reductions in %PBVDECT caused by balloon occlusion were in average -29.32 %, whereas reductions in PBFDynCT were in average -86.78 % (p < 0.001). Quantitative estimates of PBVDECT had a mean bias of +4.3 ml/100g in comparison with PBVDynCT, with 95 % confidence intervals between -16.6 ml/100g and 25.1 ml/100g. Semiquantitative contrastenhanced DECT reflects regional changes in perfusion caused decubitus changes, acute lung injury, and balloon occlusion of the PA, with moderate correlation in comparison with DynCT

A tenet of modern radiotherapy (RT) is to identify the treatment target accurately, following which the high-dose treatment volume may be expanded into the surrounding tissues in order to create the clinical and planning target volumes. Respiratory motion can induce errors in target volume delineation and dose delivery in radiation therapy for thoracic and abdominal cancers. Historically, radiotherapy treatment planning in the thoracic and abdominal regions has used 2D or 3D images acquired under uncoached free-breathing conditions, irrespective of whether the target tumor is moving or not. Once the gross target volume has been delineated, standard margins are commonly added in order to account for motion. However, the generic margins do not usually take the target motion trajectory into consideration. That may lead to under- or over-estimate motion with subsequent risk of missing the target during treatment or irradiating excessive normal tissue. That introduces systematic errors into treatment planning and delivery. In clinical practice, four-dimensional (4D) imaging has been popular in For RT motion management. It provides temporal information about tumor and organ at risk motion, and it permits patient-specific treatment planning. The most common contemporary imaging technique for identifying tumor motion is 4D computed tomography (4D-CT). However, CT has poor soft tissue contrast and it induce ionizing radiation hazard. In the last decade, 4D magnetic resonance imaging (4D-MRI) has become an emerging tool to image respiratory motion, especially in the abdomen, because of the superior soft-tissue contrast. Recently, several 4D-MRI techniques have been proposed, including prospective and retrospective approaches. Nevertheless, 4D-MRI techniques are faced with several challenges: 1) suboptimal and inconsistent tumor contrast with large inter-patient variation; 2) relatively low temporal-spatial resolution; 3) it lacks a reliable respiratory surrogate. In this research work, novel 4D-MRI techniques applying MRI weightings that was not used in existing 4D-MRI techniques, including T2/T1-weighted, T2-weighted and Diffusion-weighted MRI were investigated. A result-driven phase retrospective sorting method was proposed, and it was applied to image space as well as k-space of MR imaging. Novel image-based respiratory surrogates were developed, improved and evaluated.

Dissertation

Imaging respiratory induced tumor motion in the radiation therapy treatment room could eliminate the necessity for large motion encompassing margins that result in excessive irradiation of healthy tissues. Currently available image guidance technologies are ill-suited for this task. Two-dimensional fluoroscopic images are acquired with sufficient speed to image respiratory motion. However, volume information is not present, and soft tissue structures are often not visible because a large volume is projected onto a single plane. Currently available volumetric imaging modalities are not acquired with sufficient speed to capture full motion trajectory information. Four-dimensional cone-beam computed tomography (4D CBCT) using a gantry mounted 2D flat panel imaging device has been proposed but has been limited by high doses, long scan times and severe under-sampling artifacts. The focus of the work completed in this thesis was to find ways to improve 4D imaging using a gantry mounted 2D kV imaging system. Specifically, the goals were to investigate methods for minimizing imaging dose and scan time while achieving consistent, controllable, high quality 4D images.

First, we introduced four-dimensional digital tomosynthesis (4D DTS) and characterized its potential for 3D motion analysis using a motion phantom. The motion phantom was programmed to exhibit motion profiles with various known amplitudes in all three dimensions and scanned using a 2D kV imaging system mounted on a linear accelerator. Two arcs of projection data centered about the anterior-posterior and lateral axes were used to reconstruct phase resolved DTS coronal and sagittal images. Respiratory signals were obtained by analyzing projection data, and these signals were used to derive phases for each of the projection images. Projection images were sorted according to phase, and DTS phase images were reconstructed for each phase bin. 4D DTS target location accuracies for peak inhalation and peak exhalation in all three dimensions were limited only by the 0.5 mm pixel resolution for all DTS scan angles. The average localization errors for all phases of an assymetric motion profile with a 2 cm peak-to-peak amplitude were 0.68, 0.67 and 1.85 mm for 60 o 4D DTS, 360 o CBCT and 4DCT, respectively. Motion artifacts for 4D DTS were found to be substantially less than those seen in 4DCT, which is the current clinical standard in 4D imaging.

We then developed a comprehensive framework for relating patient respiratory parameters with acquisition and reconstruction parameters for slow gantry rotation 4D DTS and 4D CBCT imaging. This framework was validated and optimized with phantom and lung patient studies. The framework facilitates calculation of optimal frame rates and gantry rotation speeds based on patient specific respiratory parameters and required temporal resolution (task dependent). We also conducted lung patient studies to investigate required scan angles for 4D DTS and achievable dose and scan times for 4D DTS and 4D CBCT using the optimized framework. This explicit and comprehensive framework of relationships allowed us to demonstrate that under-sampling artifacts can be controlled, and 4D CBCT images can be acquired using lower doses than previously reported. We reconstructed 4D CBCT images of three patients with accumulated doses of 4.8 to 5.7 cGy. These doses are three times less than the doses used for the only previously reported 4D CBCT investigation that did not report images characterized by severe under-sampling artifacts.

We found that scan times for 200 o 4D CBCT imaging using acquisition sequences optimized for reduction of imaging dose and under-sampling artifacts were necessarily between 4 and 7 minutes (depending on patient respiration). The results from lung patient studies concluded that scan times could be reduced using 4D DTS. Patient 4D DTS studies demonstrated that tumor visibility for the lung patients we studied could be achieved using 30 o scan angles for coronal views. Scan times for those cases were between 41 and 50 seconds. Additional dose reductions were also demonstrated. Image doses were between 1.56 and 2.13 cGy. These doses are well below doses for standard CBCT scans. The techniques developed and reported in this thesis demonstrate how respiratory motion can be imaged in the radiotherapy treatment room using clinically feasible imaging doses and scan times.

Dissertation

To date, image localization of mobile tumors prior to radiation delivery has primarily been confined to 2D and 3D technologies, such as fluoroscopy and 3D cone-beam CT (3D-CBCT). Due to the limited information from these images, larger volumes of healthy tissue are often irradiated in order to ensure the radiation field encompasses the entirety of the target motion. Since the overarching goal of radiation therapy is to deliver maximum dose to cancerous cells and simultaneously minimize the radiation delivered to healthy surrounding tissues, it would be ideal to use 4D imaging to obtain time-resolved volume images of the tumor motion during respiration.

4D-CBCT imaging has been previously investigated, but has not yet seen large clinical translation due to the obstacles of long acquisition time and large image radiation dose. Furthermore, 4D-CBCT currently requires the use of external surrogates to correlate the patient's respiration with the image acquisition process. This correlation has been under question by a multitude of studies demonstrating the uncertainties that exist between the surrogate and the actual motion of the internal anatomy. Errors in the correlation process may result in image artifacts, which could potentially lead to reconstructions with inaccurate target volumes, thereby defeating the purpose of even using 4D-CBCT.

It is therefore the aim of this dissertation to initially highlight an additional limitation of using 3D-CBCT for imaging respiratory motion and thereby reiterate the need for 4D-CBCT imaging in the treatment room, develop a simple and efficient technique to achieve markerless, self-sorted 4D-CBCT and finally to comprehensively evaluate its robustness across a variety of potential clinical scenarios with a digital human phantom.

People often spend a longer period of time exhaling as compared with inhaling, and some do so in an extremely disproportionate manner. To demonstrate the disadvantage of using 3D-CBCT in such instances, a dynamic thorax phantom was imaged with a large variety of simulated and patient-derived respiratory traces of ratios of time spent in the inspiration phase versus time spent in the expiration phase (I/E ratio). Canny edge detection and contrast measures were employed to compare the internal target volumes (ITVs) generated per profile. The results revealed that an I/E ratio of less than one can lead to potential underestimation of the ITV with the severity increasing as the inspiration becomes more disproportionate to the expiration. This occurs because of the loss of contrast in the inspiration phase, due to the fewer number of projections acquired there. The measured contrast reduction was as high as 94% for small targets (0.5 cm) moving large amplitudes (2.0 cm) and still as much as 22.3% for large targets (3.0 cm) moving small amplitudes (0.5 cm). This is alarming because the degraded visibility of the target in the inspiration phase may inaccurately impact the alignment of the planning ITV with that of the FB-CBCT and thereby affect the accuracy of the localization and consequent radiation delivery. These potential errors can be avoided with the use of 4D-CBCT instead, to form the composite volume and serve as the verification ITV for alignment.

In order to delineate accurate target volumes from 4D-CBCT phase images, it is crucial that the projections be properly associated with the patient's respiration. Thus, in order to improve previously developed 4D-CBCT techniques, the basics of Fourier Transform (FT) theory were utilized to extract the respiratory signal directly from the acquired projection data. Markerless, self-sorted 4D-CBCT reconstruction was achieved by developing methods based on the phase and magnitude information of the Fourier Transform. Their performance was subsequently compared to the gold standard of visual identification of peak-inspiration projections. Slow-gantry acquired projections of two sets of physical phantom data with sinusoidal respiratory cycles of 3 and 6 seconds as well as three patients were used as initial evaluation of the feasibility of the Fourier technique. Quantitative criteria consisted of average difference in respiratory phase (ADRP) and percentage of projections assigned within 10% respiratory phase of the gold standard (PP10). For all five projection datasets, the results supported feasibility of both FT-Phase and FT-Magnitude methods with ADRP values less than 5.3% and PP10 values of 87.3% and above.

Because the technique proved to be promising in the initial feasibility study, a more comprehensive evaluation was necessary in order to assess the robustness of the technique across a larger set of possibilities that may be encountered in the clinic. A 4D digital XCAT phantom was used to generate an array of respiratory and anatomical variables that affect the performance of the technique. The respiratory variables studied included: inspiration to expiration ratio, respiratory cycle length, diaphragmatic motion amplitude, AP chest wall expansion amplitude, breathing irregularities such as baseline shift and inconsistent peak-inspiration amplitude, as well as six breathing profiles derived from cine-MRI images of three healthy volunteers and three lung cancer patients. The anatomical variables studied included: male and female patient size (physical dimension and adipose content), body-mass-index (BMI) category, tumor location, and percentage of the lung in the field-of-view (FOV) of the projection data. CBCT projections of each XCAT phantom were then generated. Additional external imaging factors such as image noise and detector wobble were added to select cases with different percentages of lung in the projection FOV to investigate any effects on the robustness. FT-Phase and FT-Magnitude were each applied and quantitatively compared to the gold standard. Both methods proved to be robust across the studied scenarios with ADRP<10% and PP10>90%, when incorporating minor modifications to region-of-interest (ROI) selection and/or low-frequency location to certain cases of diaphragm amplitude and lung percentage in the FOV of the projection (for which a method may have previously struggled). Nevertheless, in the instance where one method initially faltered, the other method prevailed and successfully identified peak-inspiration projections. This is promising because it suggests that the two methods provide complementary information to each other. To ensure appropriate clinical adaptation of markerless, self-sorted 4D-CBCT, perhaps an optimal integration of the two methods can be developed.

Dissertation

A bench-top dual cone-beam computed tomography (CBCT) system was developed consisting of two orthogonally placed 40x30 cm2 flat-panel detectors and two conventional X-ray tubes with two individual high-voltage generators sharing the same rotational axis. The X-ray source to detector distance is 150 cm and X-ray source to rotational axis distance is 100 cm for both subsystems. The objects are scanned through 200° of rotation. The dual CBCT (DCBCT) system utilized 110° of projection data from one detector and 90° from the other while the two individual single CBCTs utilized 200° data from each detector. The system performance was characterized in terms of uniformity, contrast, spatial resolution, noise power spectrum and CT number linearity. The uniformity, within the axial slice and along the longitudinal direction, and noise power spectrum were assessed by scanning a water bucket; the contrast and CT number linearity were measured using the Catphan phantom; and the spatial resolution was evaluated using a tungsten wire phantom. A skull phantom and a ham were also scanned to provide qualitative evaluation of high- and low-contrast resolution. Each measurement was compared between dual and single CBCT systems.

Compared with single CBCT, the DCBCT presented: 1) a decrease in uniformity by 1.9% in axial view and 1.1% in the longitudinal view, as averaged for four energies (80, 100, 125 and 150 kVp); 2) comparable or slightly better contrast to noise ratio (CNR) for low-contrast objects and comparable contrast for high-contrast objects; 3) comparable spatial resolution; 4) comparable CT number linearity with R2 ≥ 0.99 for all four tested energies; 5) lower noise power spectrum in magnitude. DCBCT images of the skull phantom and the ham demonstrated both high-contrast resolution and good soft-tissue contrast.

One of the major challenges for clinical implementation of four-dimensional (4D) CBCT is the long scan time. To investigate the 4D imaging capabilities of the DCBCT system, motion phantom studies were conducted to validate the efficiency by comparing 4D images generated from 4D-DCBCT and 4D-CBCT. First, a simple sinusoidal profile was used to confirm the scan time reduction. Next, both irregular sinusoidal and patient-derived profiles were used to investigate the advantage of temporally correlated orthogonal projections due to a reduced scan time. Normalized mutual information (NMI) between 4D-DCBCT and 4D-CBCT was used for quantitative evaluation.

For the simple sinusoidal profile, the average NMI for ten phases between two single 4D-CBCTs was 0.336, indicating the maximum NMI that can be achieved for this study. The average NMIs between 4D-DCBCT and each single 4D-CBCT were 0.331 and 0.320. For both irregular sinusoidal and patient-derived profiles, 4D-DCBCT generated phase images with less motion blurring when compared with single 4D-CBCT.

For dual kV energy imaging, we acquired 80kVp projections and 150 kVp projections, with an additional 0.8 mm tin filtration. The virtual monochromatic (VM) technique was implemented, by first decomposing these projections into acrylic and aluminum basis material projections to synthesize VM projections, which were then used to reconstruct VM CBCTs. The effect of the VM CBCT on metal artifact reduction was evaluated with an in-house titanium-BB phantom. The optimal VM energy to maximize CNR for iodine contrast and minimize beam hardening in VM CBCT was determined using a water phantom containing two iodine concentrations. The linearly-mixed (LM) technique was implemented by linearly combining the low- (80kVp) and high-energy (150kVp) CBCTs. The dose partitioning between low- and high-energy CBCTs was varied (20%, 40%, 60% and 80% for low-energy) while keeping total dose approximately equal to single-energy CBCTs, measured using an ion chamber. Noise levels and CNRs for four tissue types were investigated for dual-energy LM CBCTs in comparison with single-energy CBCTs at 80, 100, 125 and 150kVp.

The VM technique showed a substantial reduction of metal artifacts at 100 keV with a 40% reduction in the background standard deviation compared with a 125 kVp single-energy scan of equal dose. The VM energy to maximize CNR for both iodine concentrations and minimize beam hardening in the metal-free object was 50 keV and 60 keV, respectively. The difference in average noise levels measured in the phantom background was 1.2% for dual-energy LM CBCTs and equivalent-dose single-energy CBCTs. CNR values in the LM CBCTs of any dose partitioning were better than those of 150 kVp single-energy CBCTs. The average CNRs for four tissue types with 80% dose fraction at low-energy showed 9.0% and 4.1% improvement relative to 100 kVp and 125 kVp single-energy CBCTs, respectively. CNRs for low contrast objects improved as dose partitioning was more heavily weighted towards low-energy (80kVp) for LM CBCTs.

For application of the dual-energy technique in the kilovoltage (kV) and megavoltage (MV) range, we acquired both MV projections (from gantry angle of 0° to 100°) and kV projections (90° to 200°) with the current orthogonal kV/MV imaging hardware equipped in modern linear accelerators, as gantry rotated a total of 110°. A selected range of overlap projections between 90° to 100° were then decomposed into two material projections using experimentally determined parameters from orthogonally stacked aluminum and acrylic step-wedges. Given attenuation coefficients of aluminum and acrylic at a predetermined energy, one set of VM projections could be synthesized from two corresponding sets of decomposed projections. Two linear functions were generated using projection information at overlap angles to convert kV and MV projections at non-overlap angles to approximate VM projections for CBCT reconstruction. The CNRs were calculated for different inserts in VM CBCTs of a CatPhan phantom with various selected energies and compared with those in kV and MV CBCTs. The effect of overlap projection number on CNR was evaluated. Additionally, the effect of beam orientation was studied by scanning the CatPhan sandwiched with two 5 cm solid-water phantoms on both lateral sides and an electronic density phantom with two metal bolt inserts.

Proper selection of VM energy (30keV and 40keV for low-density polyethylene (LDPE), polymethylpentene (PMP), 2MeV for Delrin) provided comparable or even better CNR results as compared with kV or MV CBCT. An increased number of overlap between kV and MV projections demonstrated only marginal improvements of CNR for different inserts (with the exception of LDPE) and therefore one projection overlap was found to be sufficient for the CatPhan study. It was also evident that the optimal CBCT image quality was achieved when MV beams penetrated through the heavy attenuation direction of the object.

In conclusion, the performance of a bench-top DCBCT imaging system has been characterized and is comparable to that of a single CBCT. The 4D-DCBCT provides an efficient 4D imaging technique for motion management. The scan time is reduced by approximately a factor of two. The temporally correlated orthogonal projections improved the image blur across 4D phase images. Dual-energy CBCT imaging techniques were implemented to synthesize VM CBCT and LM CBCTs. VM CBCT was effective at achieving metal artifact reduction. Depending on the dose-partitioning scheme, LM CBCT demonstrated the potential to improve CNR for low contrast objects compared with single-energy CBCT acquired with equivalent dose. A novel technique was developed to generate VM CBCTs from kV/MV projections. This technique has the potential to improve CNR at selected VM energies and to suppress artifacts at appropriate beam orientations.

Dissertation