Digital flat-panel-based volume CT (VCT) represents a unique design capable of ultra-high spatial resolution, direct volumetric imaging, and dynamic CT scanning. This innovation, when fully developed, has the promise of opening a unique window on human anatomy and physiology. For example, the volumetric coverage offered by this technology enables us to observe the perfusion of an entire organ, such as the brain, liver, or kidney, tomographically (e.g., after a transplant or ischemic event). By virtue of its higher resolution, one can directly visualize the trabecular structure of bone. This paper describes the basic design architecture of VCT. Three key technical challenges, viz., scatter correction, dynamic range extension, and temporal resolution improvement, must be addressed for successful implementation of a VCT scanner. How these issues are solved in a VCT prototype and the modifications necessary to enable ultra-high resolution volumetric scanning are described. The fundamental principles of scatter correction and dose reduction are illustrated with the help of an actual prototype. The image quality metrics of this prototype are characterized and compared with a multi-detector CT (MDCT).

Ultra-high resolution flat-panel volume CT: fundamental principles, design architecture, and system characterization

The objective of this publication is to provide guidance on radiological protection in the new technology of cone beam computed tomography (CBCT). Publications 87 and 102 dealt with patient dose management in computed tomography (CT) and multi-detector CT. The new applications of CBCT and the associated radiological protection issues are substantially different from those of conventional CT. The perception that CBCT involves lower doses was only true in initial applications. CBCT is now used widely by specialists who have little or no training in radiological protection. This publication provides recommendations on radiation dose management directed at different stakeholders, and covers principles of radiological protection, training, and quality assurance aspects. Advice on appropriate use of CBCT needs to be made widely available. Advice on optimisation of protection when using CBCT equipment needs to be strengthened, particularly with respect to the use of newer features of the equipment. Manufacturers should standardise radiation dose displays on CBCT equipment to assist users in optimisation of protection and comparisons of performance. Additional challenges to radiological protection are introduced when CBCT-capable equipment is used for both fluoroscopy and tomography during the same procedure. Standardised methods need to be established for tracking and reporting of patient radiation doses from these procedures. The recommendations provided in this publication may evolve in the future as CBCT equipment and applications evolve. As with previous ICRP publications, the Commission hopes that imaging professionals, medical physicists, and manufacturers will use the guidelines and recommendations provided in this publication for implementation of the Commission's principle of optimisation of protection of patients and medical workers, with the objective of keeping exposures as low as reasonably achievable, taking into account economic and societal factors, and consistent with achieving the necessary medical outcomes.

https://journals.sagepub.com/doi/pdf/10.1177/0146645315575485

ICRP Publication 129: Radiological Protection in Cone Beam Computed Tomography (CBCT)

Micro Computed Tomography (micro-CT) was suggested in biomedical research to investigate tissues and small animals. Its use to characterize bone structures, vessels (e.g. tumor vascularization), tumors and soft tissues such as lung parenchyma has been shown. When co-registered, micro-CT can add structural information to other small animal imaging modalities. However, due to fundamental CT principles, high-resolution imaging with micro-CT demands for high x-ray doses and long scan times to generate a sufficiently high signal-to-noise ratio. Long scan times in turn make the use of extravascular contrast agents difficult. Recently introduced flat-panel based mini-CT systems offer a valuable trade- off between resolution (~200 µm), scan time (0.5 s), applied x-ray dose and scan field-of-view. This allows for angiography scans and follow-up examinations using iodinated contrast agents having a similar performance compared to patient scans. Furthermore, dynamic examinations such as perfusion studies as well as retrospective motion gating are currently implemented using flat-panel CT. This review summarizes applications of experimental CT in basic research and provides an overview of current hardware developments making CT a powerful tool to study tissue morphology and function in small laboratory animals such as rodents.

Small Animal Computed Tomography Imaging

Objectives:Implementation and evaluation of retrospective respiratory and cardiac gating of mice and rats using a flat-panel volume-CT prototype (fpVCT).Materials and Methods:Respiratory and cardiac gating was implemented by equipping a fpVCT with a small animal monitoring unit. ECG and breathing ex

Retrospective motion gating in small animal CT of mice and rats.

For imaging during minimally-invasive treatment in the so-called catheter laboratory conventional X-ray projection imaging is classically used. Particularly in cardio-vascular angiography and neuroradiology, so-called Carm systems are used, enabling a flexible positioning of X-ray tube and detector. In the last years, these systems experienced the most important technical innovation with the introduction of 3D imaging functionality by means of cone-beam computed tomography (CBCT). With this technique a large number of projections is acquired during a rotation of the C-arm around the patient. Afterwards, these projections are reconstructed to volumetric images using algorithms similar to those used in classical computed tomography. The objective of current research is to improve the 3D image quality in order to extend the imaging capability to high quality low contrast imaging with C-arm X-ray systems. In this context, this thesis addresses the problem of scattered radiation. Because in CBCT with large area X-ray detectors the irradiated patient volume is substantially larger than in classical computed tomography, also the amount of scattered radiation reaching the detector is significantly larger and can even be superior to the amount of primary radiation. Therefore, scattered radiation is a major source of image degradation and nonlinearity in flat-detector based CBCT and is the most severe cause of inhomogeneity artifacts in reconstructed images. The primary objectives of this thesis are the detailed quantitative analysis of scattered radiation, the assessment of existing scatter compensation methods as well as the development of new effective methods for the reduction of scatter induced artifacts. After an introduction to the physical and algorithmic principles of CBCT in the first part of the thesis, at first a detailed quantitative analysis of the characteristics of scattered radiation in projections of CBCT is undertaken. This analysis is based on the advancements of a Monte-Carlo CBCT simulator allowing to study realistic and clinically relevant patient geometries obtained from real data sets of conventional computed tomography. With this method practically noise free reference data sets for typical measure-

Scattered radiation in cone beam computed tomography : analysis, quantification and compensation

We developed and evaluated a prototype flat-panel detector based Volume CT (VCT) scanner. We focused on improving the image quality using different detector settings and reducing x-ray scatter intensities. For the presented results we used a Varian 4030CB flat-panel detector mounted in a multislice CT-gantry (Siemens Medical Systems). The scatter intensities may severely impair image quality in flat-panel detector CT systems. To reduce the impact of scatter we tested bowtie shaped filters, anti-scatter grids and post-processing correction algorithms. We evaluated the improvement of image quality by each method and also by a combination of the several methods. To achieve an extended dynamic range in the projection data, we implemented a novel dynamic gain-switching mode. The read out charge amplifier feedback capacitance is changing dynamically in this mode, depending on the signal level. For this scan mode dedicated corrections in the offset and gain calibration are required. We compared image quality in terms of low contrast for both, the dynamic mode and the standard fixed gain mode. VCT scanners require different types of dose parameters. We measured the dose in a 16 cm CTDI phantom and free air in the scanners iso-center and defined a new metric for a VCT dose index (VCTDI). The dose for a high quality VCT scan of this prototype scanner varied between 15 and 40 mGy.

Evaluation of image quality and dose on a flat-panel CT-scanner

We developed and evaluated a prototype flat-panel detector based Volume CT (fpVCT) scanner. The fpVCT scanner consists of a Varian 4030CB a-Si flat-panel detector mounted in a multi slice CT-gantry (Siemens Medical Solutions). It provides a 25 cm field of view with 18 cm z-coverage at the isocenter. In addition to the standard tomographic scanning, fpVCT allows two new scan modes: (1) fluoroscopic imaging from any arbitrary rotation angle, and (2) continuous, time-resolved tomographic scanning of a dynamically changing viewing volume. Fluoroscopic imaging is feasible by modifying the standard CT gantry so that the imaging chain can be oriented along any user-selected rotation angle. Scanning with a stationary gantry, after it has been oriented, is equivalent to a conventional fluoroscopic examination. This scan mode enables combined use of high-resolution tomography and real-time fluoroscopy with a clinically usable field of view in the z direction. The second scan mode allows continuous observation of a timeevolving process such as perfusion. The gantry can be continuously rotated for up to 80 sec, with the rotation time ranging from 3 to 20 sec, to gather projection images of a dynamic process. The projection data, that provides a temporal log of the viewing volume, is then converted into multiple image stacks that capture the temporal evolution of a dynamic process. Studies using phantoms, ex vivo specimens, and live animals have confirmed that these new scanning modes are clinically usable and offer a unique view of the anatomy and physiology that heretofore has not been feasible using static CT scanning. At the current level of image quality and temporal resolution, several clinical applications such a dynamic angiography, tumor enhancement pattern and vascularity studies, organ perfusion, and interventional applications are in reach.

Combination of CT scanning and fluoroscopy imaging on a flat-panel CT scanner

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Evaluation of image quality and dose on a flat-panel CT-scanner

Combination of CT scanning and fluoroscopy imaging on a flat-panel CT scanner