Because of the inherent limitations, conventional cone beam computed tomography (CBCT) systems are inaccurate in quantifying bone mineral density (BMD). A multisource CBCT (ms-CBCT) was recently developed. The aim of this study was to evaluate the accuracy of the ms-CBCT system in estimating the BMD of mandibular and maxillary bones and to compare it with the conventional CBCT, utilizing a multi-detector CT (MDCT) as a reference standard. For this purpose, an anthropomorphic adult skull and tissue equivalent head phantom, along with a homemade calibration phantom containing inserts of calcium hydroxyapatite of varied densities, were imaged. The phantoms were imaged using the ms-CBCT, the ms-CBCT operating in the conventional CBCT mode using one source, two clinical dental CBCT scanners, and an MDCT, under similar imaging doses. The images were reconstructed, registered and segmented. The scanner-specific calibration curves were utilized to establish a relation to convert the measured CT Hounsfield Unit (HU) and Greyscale Value (GV) at multiple region-of-interests to the BMD. Statistical analysis showed that a significant enhancement in the accuracy of the HU and BMD values derived from the ms-CBCT as compared to the conventional clinical dental CBCT. These findings suggest that the ms-CBCT system may serve as a reliable and lower-dose alternative for precise BMD quantification.
Dual-energy CBCT acquires projection images using two different x-ray spectra, enabling material quantification and synthesis of virtual monoenergetic images to reduce metal artifacts. Current methods for dual-energy CBCT imaging include using either a fast kVp switching x-ray source or a dual-layer detector. In this work, we demonstrate a novel approach to dual-energy CBCT by spectral filtration of an x-ray source that contains two focal spots. The method generates two distinct x-ray spectra and independently programmable dose and dose rate for the two energies, without a significant cost increase over a conventional CBCT. Two spectral filters were attached to the x-ray exit windows of a dual-focus carbon nanotube x-ray source operating at a constant tube voltage. As the source and detector rotate around the object, alternating high and low energy projections were acquired by alternatively activating electron emission from the two cathodes. A one-step material decomposition based on separable quadratic surrogate functions was implemented. A water equivalent plastic phantom with calcium and iodine contrast inserts was imaged to evaluate the accuracy of material decomposition. Results demonstrate a functional low-cost dual-energy CBCT system by spectral filtration, with the accuracy for material quantification comparable to those from the dual-layer detector and fast kVp switching based DE-CBCT.
The purpose of this study is to develop and evaluate a functional multisource cone beam computed tomography (ms- CBCT) scanner to mitigate some of the main limitations of the current CBCT. The benchtop ms-CBCT utilizes a carbon nanotube (CNT) field emission source array to generate multiple narrowly collimated and rapidly scanning x-ray beams, each illuminating a section of the object and collectively covering the region of interest. A contrast phantom and a Defrise phantom were imaged by the ms-CBCT, the ms-CBCT operating in the conventional CBCT configuration, and a clinical CBCT. The results show the ms-CBCT reduces the spatial nonuniformity and root-mean-square error (RMSE) of the CT HU values by respectively 75% and 60%, essentially eliminates the cone beam artifacts, increases the effective axial coverage, and improves the CNR by 30%~50% compared to the conventional CBCT at a comparable imaging dose. The results show that the ms-CBCT can potentially provides the performance of an MDCT while maintaining the essential attributes of a CBCT including volumetric imaging, low dose, affordability, and compact design.
This study evaluated a multi-source cone beam computed tomography (ms-CBCT) design. The design replaces the single X-ray tube in the conventional CBCT with a linear array of collimated X-ray sources aligned along the axial direction. A single beam carbon nanotube (CNT) X-ray source with an adjustable external collimator was mechanically translated to different positions along the axial direction to simulate the configurations with different number of sources and corresponding beam cone angle. A flat panel detector was offset horizontally to extend the field of view. Several phantoms were scanned under different configurations for comparison. The scatter-primary-ratio was reduced essentially linearly with the increasing number of X-ray sources (𝑁𝑠𝑜𝑢𝑟𝑐𝑒) from 47% in the N1 (𝑁𝑠𝑜𝑢𝑟𝑐𝑒 = 1, cone angle 𝜃 = 10.3°) to below 20% in the N8 (𝑁𝑠𝑜𝑢𝑟𝑐𝑒 = 8, cone angle 𝜃 = 2.3°) configuration. The CT cupping artifact was reduced from 15% in N1 to 10% in N8. The nonuniformity of the CT numbers, as measured by the standard deviation of the Hounsfield Unit (HU) values from multiple regions of interest (ROIs) in the axial plane of a contrast phantom was reduced from 38.0 in the N1 to 19.8 in the N8 configuration. An anthropomorphic head phantom imaged with the N8 ms-CBCT configuration also has a more uniform line profile than the image with the N1 configuration in the central homogenous region.
Preliminary results have shown replacing the conventional x-ray tube with a stack of narrowly collimated beams along the axial direction reduces scatter and improves the spatial uniformity and the CT number accuracy in CBCT. The purpose of this study was to evaluate a carbon nanotube (CNT) x-ray source array designed for a prototype multisource CBCT (ms-CBCT) for maxillofacial imaging. The CNT x-ray source array comprises 8 evenly distributed focal spots (“sources”), and 8 corresponding CNT cathodes and gate electrodes, with an inter-focal spot spacing of 12mm. A multisource collimator was designed to confine the radiation from each focal spot to a narrow cone beam covering a section of the object in the axial direction. The focal spot sizes (FSS) were measured using a pinhole camera setup following the IEC standard procedure. The radiation fields of the collimated beams were measured using a flat panel detector. The x-ray source cathode and tube currents were recorded, and radiation dose was measured with a dose meter. Preliminary 3D volumetric imaging of an anthropomorphic head phantom was conducted using the CNT source array. The targeted 15mA tube current was achieved from each focal spot at 110kVp. The measured FSS IEC nominal value was 0.7. The average x-ray cone angle of each collimated beam was 2.3°. The prototype CNT x-ray source array achieved the specifications for the oral and maxillofacial ms-CBCT scanner. Work is in progress to evaluate the imaging system.
Cone-beam computed tomography (CBCT) provides volumetric scans while keeping a relatively low dose and cost. This characteristic makes CBCT favorable in the field of dentistry, but the frequent presence of metallic objects in patients causes metal artifact that severely degrades the resulting images. Dual-energy CT provides more information about the materials which can be used to synthesize virtual monoenergetic images (VMI) to reduce metal artifacts. In this work, we investigated and optimized a dual-energy cone-beam CT (DE-CBCT) system using a carbon-nanotube CNT x-ray source with dual focal spots and spectral filtrations. We used two x-ray spectra generated by applying spectral filters at a constant x-ray tube voltage. We imaged an anthropomorphic head phantom with metal beads. The projection images were reconstructed separately using an iterative CT reconstruction algorithm and bilateral filtering is applied to the reconstructed images for denoising. The VMIs were synthesized from the denoised reconstructed images using an image domain decomposition method and displayed noise comparable to the images obtained from a single energy. The resulting VMIs displayed fewer metal artifacts compared to the single energy images of the same object from a clinical CBCT scanner. To optimize the spectral separation, we implemented a filter selection algorithm and improved the mean energy separation from 13keV to 17.5keV while maintaining the same x-ray output. We also demonstrated the ability of this system to independently control the imaging dose of the two energy scans by varying the x-ray exposure current and time using the dual focal spot CNT source.
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