Benchtop experiments have proven the utility of a stationary computed tomography (CT) scanner for head imaging. The purpose of this study was for system control development and integration in a clinical setting for clinical use and evaluation. Software and interfaces for technologist operation of the complete system during patient scanning were also developed. A clinical imaging bed was integrated with the x-ray control system with off-the-shelf microcontrollers to drive the development of the system for clinical evaluation. The clinical imaging system is composed of three carbon nanotube (CNT) x-ray source arrays and nine strip detectors. 135 projections are acquired per slice at 120kVp, 10mA, and 2.95ms exposure per projection. Anthropomorphic phantoms have been imaged in preparation for the first patients. Reconstruction was performed using adaptive steepest descent projection onto convex set (ASD-POCs) method. Scanner performance parameters were measured. Images are evaluated by neuroradiologists. A working stationary head CT (sHCT) system has been developed for patient imaging evaluation. Image quality is sufficient for starting the observational clinical trial as shown in images of the ACR accreditation phantom and KYOTO head phantoms. This preliminary study has shown that the sHCT system is ready for patient imaging studies. Clinical utility will be assessed in a patient study with patients with prior head trauma.
X-ray Computed Tomography (CT) is an indispensable imaging modality in the medical field, notably in the diagnosis of traumatic brain injury and brain hemorrhage. Existing clinical CT systems, all of which use rotating x-ray sources, are too complex for practical deployment in resource-poor environments. A stationary array of sources and detectors with sufficient angular coverage and focal spot density could potentially simplify the image acquisition hardware and generate adequate projection data for CT reconstruction. In this study, we present a stationary head CT (s-HCT) prototype which combines projection data from three separate but parallel imaging planes into a complete volumetric iterative CT reconstruction. The fully operational scanner features three carbon nanotube (CNT) x-ray source arrays with 45 distributed focal spots each, and an Electronic Control System (ECS) for high speed control of the x-ray exposure from each focal spot. Projection data is acquired by translating the object along the z-axis at constant speed to expose all three imaging planes for the full dataset. As the object is moving at constant speed, 135 views are collected every 0.44 s (cycle time). The 3D iterative reconstruction designed specifically for the s-HCT configuration has been used to produce phantom images for initial assessment of the spatial resolution. A linear collimator was designed and constructed for the reduction of cross-plane scatter. Finally, the prototype has been able to acquire these images in scan times comparable to those of commercial scanners (<1min), indicating the CNT x-ray and s-HCT technologies are developed enough for clinical trials.
Purpose: The purpose of this study was to evaluate a novel stationary head CT system (sHCT) enabled by the carbon nanotube (CNT) field emission x-ray source array for volumetric head imaging. A data processing and image reconstruction package was developed and demonstrated for the system. Methods: The experimental sHCT system consisted of three CNT x-ray source arrays placed in parallel imaging planes separately. During imaging, the sources stayed stationary while only the bed advancing through a tunnel formed by the parallel planes. In each imaging cycle, 135 projections were acquired over 232 degrees of view. A CT ACR 464 phantom and a Kyoto head phantom were imaged to evaluate the system performance. Image uniformity, signal to noise ratio, spatial resolution, CT number and detectability were investigated for the proposed system. All images were processed and reconstructed with an iterative reconstruction-based package. Total-variation (TV) regularization methods such as the adaptive steepest descent projection onto convex set (ASDPOCS) algorithm were implemented to reduce noises and artifacts caused by the reduced projection views. Results: Volumetric data with good uniformity, high spatial resolution, and detectability for both high- and low-resolution features were demonstrated for the proposed sHCT scanner. Two scan protocols, step-and-shoot and continuous mode were compared and proved to provide similar image quality while the latter increased the total scan speed. The SIRTASDPOCS algorithm effectively suppressed the sparse-view and limited-view artifact and enhance the contrast noise ratio. Conclusion: We demonstrated the feasibility of sHCT for volumetric head imaging using multiple CNT x-ray source arrays. The device provides 3D images with high fidelity. The prototype sHCT system is being installed at the UNC hospital for a patient imaging study.
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.
Purpose: The invention of carbon nanotube (CNT) x-ray source arrays has enabled the development of novel imaging systems, including stationary tomosynthesis and stationary computed tomography (CT) with fast data acquisition, mechanically robust structures, and reduced image blur from source–detector motion. In this work, we report the results of simulation studies of potential system configurations for a stationary head CT (s-HCT) using linear CNT x-ray sources and detector arrays.
Approach: We explored s-HCT configurations that utilize one, two, and three linear CNT source arrays. Simulations were implemented using three digital phantoms with both CPU and GPU computing. Sinogram coverage was used for qualitative evaluation of the CT projection collection efficiency for each configuration. A modified low-contrast Shepp–Logan (SL) phantom was implemented for image quality assessment using quantitative metrics. Different iterative reconstruction (IR) methods were compared with both qualitative and quantitative assessments.
Results: Sinogram coverage of s-HCT configurations was sensitive to the number of CNT source arrays and geometry. The simulations suggest that a s-HCT configuration with three planes gives near complete sinogram coverage. Such a configuration enables accurate reconstruction of the low-contrast SL phantom and considerably diminished artifacts caused by the system geometry.
Conclusions: An optimized s-HCT system configuration with three linear CNT x-ray source arrays is feasible. IR algorithms can diminish artifacts caused by sparse and asymmetrical scans. The proposed s-HCT system configuration is currently under construction.
Purpose: Carbon nanotube (CNT) based field-emission x-ray source arrays allow the development of robust stationary computed tomography (CT) imaging systems with no gantry movement. There are many technical considerations that constrain the optimal system design. The aim of this work is to assess the image quality of a proposed Stationary Head CT (sHCT) system through simulation. Methods: In our previous work, we defined a system design consisting of three parallel imaging planes. Each plane consists of a CNT x-ray source array with a large number of linearly distributed focal spots and three strip detector modules. Each imaging plane is rotated 120° with respect to the adjacent plane to provide maximum projection view coverage of the region of interest (ROI). An iterative reconstruction algorithm based on the ASTRA toolbox was developed for the specific sHCT system. The ACR 464 phantom and a set of clinical head CT data were used to assess the system design and image quality. Imaging performance was evaluated both quantitatively and qualitatively. Results: The simulation results suggest that the proposed sHCT design is feasible and high-fidelity CT images can be obtained. The reconstructed image of the ACR 464 phantom reproduces accurate CT numbers. The reconstructed CT images for the human head confirm the capability of this prototype for identifying low contrast pathologies. Conclusion: A three-plane sHCT system is evaluated in this work. The iterative reconstruction algorithm produces high image quality in terms of uniformity, signal-to-noise ratio, signal-to-contrast ratio and structural information. Further work on the optimization of the current sHCT system will focus on speed up of volumetric image data collection in system hardware and further improvement of the reconstruction image quality through regularization and incorporating of machine leaning techniques.
X-ray Computed Tomography (CT) is an indispensable imaging modality in the diagnosis of traumatic brain injury and brain hemorrhage. While the technology and the associated system components have been refined over the last several decades, all modern CT systems still rely on the principle of rotating sources and detectors. The rotating gantry adds a high degree of complexity to the overall system design, and could be eliminated in favor of a configuration of stationary x-ray sources and detectors. Such a change could potentially enable CT systems to be better suited for austere environments. Furthermore, the image acquisition speed would no longer be limited by the maximum rotating speed of the gantry. Unfortunately, due to the size and bulk of existing commercial x-ray sources, such a configuration is impossible to build with a sufficient number of focal spots. Recently, carbon nanotube (CNT) x-ray source arrays have been used in various stationary imaging configurations to generate diagnostic quality tomosynthesis images in the fields of mammography, dentistry, and orthopedics. In this study, we present a potential stationary head CT (s-HCT) design which combines projection data from 3 separate but parallel imaging planes for a complete CT fan-beam reconstruction. The proposed scanner consists of 3 CNT x-ray source arrays with a large number distributed focal spots each, and an Electronic Control System (ECS) for high speed control of the x-ray exposure from each focal spot. The projection data was collected by an array of multi-row detectors. For this unique imaging configuration, a customized geometry calibration procedure was developed. A linear collimator was designed and constructed for the reduction of cone-angle scatter. Finally, volumetric CT slice data was acquired through z-axis translation of the imaging object.
Purpose: Today’s state-of-the-art CT systems rely on a rotating gantry to acquire projections spanning up to 360 degrees around the head and/or body. By replacing the rotating source and detector with a stationary array of x-ray sources and line detectors, a head CT scanner could be potentially constructed with a small footprint and fast scanning speed. The purpose of this project is to design and construct a stationary head CT (s-HCT) scanner capable of diagnosis of stroke and head trauma patients in limited resource areas such as forward operating bases. Here we present preliminary imaging results which demonstrate the feasibility of such a system using carbon nanotube (CNT) x-ray source arrays.
Methods: The feasibility study was performed using a benchtop setup consisting of an x-ray source array with 45 distributed focal spots, each operating at 120kVp, and an Electronic Control System (ECS) for high speed control of the x-ray output from individual focal spots. The projection data was collected by an array of detectors configured specifically for head imaging. The basic performance of the CNT x-ray source array was characterized. By rotating the object in discrete angular steps, a potential s-HCT configuration was emulated. The collected projection images were reconstructed using an iterative reconstruction algorithm developed specifically for this configuration. Evaluation of the image quality was completed by comparing this image of the ACR CT phantom obtained with the s-HCT to that obtained by a clinical CT scanner.
Results: The CNT x-ray source array was found to have a consistent focal spot size of 1.3×1.1 mm2 for all beams (IEC 1.0). At 120 kVp the HVL was measured to be 5.8 mm Al. Axial images have been acquired with slice thickness 2.5 mm to evaluate the imaging performance of the s-HCT system. Contrast-noise-ratio was measured for the acrylic (120 HU) and water (0 HU) materials in the ACR CT 464 phantom Module 01. A value of 5.2 is reported for the benchtop setup with an entrance dose of 2.9 mGy, compared to the clinical measurement of 30.5 found at 74.5 mGy. These images demonstrate that the s-HCT system based on CNT x-ray source arrays is feasible.
Conclusion: Customized CNT x-ray sources were developed specifically for head CT imaging. The feasibility of using this source array to construct a s-HCT scanner has been demonstrated by emulating a potential CT configuration. It is shown that diagnostic quality CT images can be obtained using the proposed system geometry. These preliminary images provide confidence that a s-HCT system can be constructed for clinical evaluation.
Purpose: The invention of carbon nanotube (CNT) x-ray source array has allowed development of many novel imaging systems including stationary tomosynthesis devices for breast, chest and dental imaging. This technology enables stationary computed tomography with potentially a fast data acquisition rate and a mechanically robust structure by eliminating the rotating gantry. It reduces the image blur caused by the mechanical motion. The purpose of this work is to explore possible system configurations of stationary head CT (s-HCT) using fixed-position linear CNT x-ray source arrays and detector arrays. Methods: Sinogram coverage is used for qualitative evaluation on the CT projection data collection efficiency for a given configuration. Accordingly, the configuration is optimized based on the coverage in sinogram space. To evaluate the system feasibility on imaging low-contrast brain tissues, a modified low-contrast Shepp-Logan phantom is implemented for quality assessment using quantitative metrics. Different Iterative Reconstruction methods are compared for both qualitative and quantitative assessment as well. Results: The sinogram coverage of s-HCT configurations changes significantly with different number of CNT source arrays used, as well as the layout of the geometry. Preliminary results suggest that a s-HCT configuration with three planes gives a nearly completed sinogram coverage which provides enough information to reconstruct image with good quality. Different reconstruction techniques are used for such configuration with a low-contrast head phantom. High-quality images are reconstructed for the proposed configuration. Conclusion: An optimized s-HCT system configuration can be built with few linear CNT x-ray source arrays. Given such configuration, Iterative Reconstruction algorithms in conjunction with Total-Variation Regularization provides highquality images even for low-contrast objects.
We have developed a clinically ready first generation stationary breast tomosynthesis system (s-DBT). In the s-DBT system, focal spot blur associated with x-ray source motion is completely eliminated, allowing for rapid acquisition of projection images over a larger angular span without changing the acquisition time.
In the phantom studies the 1st generation s‐DBT system has demonstrated 30% higher spatial resolution than the corresponding continuous motion DBT systems. The system is currently being evaluated for its diagnostic performance in 100 patient clinical evaluation against FFDM. Initial results indicate that the s‐DBT system can produce increased lesion conspicuity and comparable MC visibility. However due to x‐ray flux limitations, certain large size patients have to be excluded. Recent studies have shown that increasing the angular span beyond 30° can be beneficial for enhanced depth resolution. We report the preliminary characterization of the 2nd generation s-DBT system with a new CNT x-ray source array, increased tube flux and a larger angular span. Increasing x‐ray tube flux allows for a larger patient population and dual energy imaging. Results indicate that the system delivers more than twice the flux, allowing for imaging of all size patients with acquisition time of 2‐4 seconds. A 7° increase in angular span over 1st generation decreased the ASF by 37%. Additionally, the 2nd generation s‐DBT system utilizing a specific AFVR reconstruction method resulted in a 92% increase in the in plane resolution over CM DBT system, and a 37% increase in spatial resolution over the 1st generation s-‐DBT system.
XinRay Systems Inc has a rectangular x-ray computed tomography (CT) imaging setup using multibeam x-ray tubes.
These multibeam x-ray tubes are based on cold cathodes using carbon nanotube (CNT) field emitters. Due to their
unique design, a CNT x-ray tube can contain a dense array of independently controlled electron emitters which generate a linear array of x-ray focal spots. XinRay uses a set of linear CNT x-ray tubes to design and construct a stationary CT setup which achieves sufficient CT coverage from a fixed set of views. The CT system has no moving gantry, enabling it to be enclosed in a compact rectangular tunnel. The fixed locations of the x-ray focal spots were optimized through simulations. The rectangular shape creates significant variation in path length from the focal spots to the detector for different x-ray views. The shape also results in unequal x-ray coverage in the imaged space. We discuss the impact of this variation on the reconstruction. XinRay uses an iterative reconstruction algorithm to account for this unique geometry, which is implemented on a graphics processing unit (GPU). The fixed focal spots prohibit the use of an antiscatter grid. Quantitative measure of the scatter and its impact on the reconstruction will be discussed. These results represent the first known implementation of a completely stationary CT setup using CNT x-ray emitter arrays.
A prototype stationary digital breast tomosynthesis (s-DBT) system has been developed by retrofitting a Hologic Selenia
Dimension rotating gantry tomosynthesis scanner with a spatially distributed carbon nanotube (CNT) x-ray source array.
The goal is to improve the system spatial resolution by removing the x-ray tube motion induced focal spot blurring. The
CNT x-ray source array comprises 31 individually addressable x-ray beams covering 30° angular span. Each x-ray beam
has a minimum focal spot size of 0.64×0.61mm
(full-width-at-half-maximum), a stationary W anode operating up to
50kVp, and 1mm thick Al filter. The flux from each beam is regulated and varied using dedicated control electronics.
The maximum tube current is determined by the heat load of the stationary anode and depends on the energy, pulse
width and the focal spot size used. Stable operation at 28kVp, 27mA tube current, 250msec pulse width and 38mA tube
current, 183msec pulse width per exposure was achieved with extended lifetime. The standard ACR phantom was
imaged and analyzed to evaluate the image quality. The actual scanning speed depends on the number of views and the
readout time of the x-ray detector. With the present detector, 6 second scanning time at either 15 views or 31 views can
be achieved at 100mAs total imaging dose with a detector readout time of 240msec.
KEYWORDS: Modulation transfer functions, Digital breast tomosynthesis, Imaging systems, X-ray sources, X-rays, Image quality, Signal to noise ratio, Mammography, Sensors, Carbon nanotubes
The stationary Digital Breast Tomosynthesis System (s-DBT) has the advantage over the conventional DBT systems as
there is no motion blurring in the projection images associated with the x-ray source motion. We have developed a
prototype s-DBT system by retrofitting a Hologic Selenia Dimensions rotating gantry tomosynthesis system with a
distributed carbon nanotube (CNT) x-ray source array. The linear array consists of 31 x-ray generating focal spots
distributed over a 30 degree angle. Each x-ray beam can be electronically activated allowing the flexibility and easy
implementation of novel tomosynthesis scanning with different scanning parameters and configurations. Here we report
the initial results of investigation on the imaging quality of the s-DBT system and its dependence on the acquisition
parameters including the number of projections views, the total angular span of the projection views, the dose
distribution between different projections, and the total dose. A mammography phantom is used to visually assess image
quality. The modulation transfer function (MTF) of a line wire phantom is used to evaluate the system spatial resolution.
For s-DBT the in-plan system resolution, as measured by the MTF, does not change for different configurations. This is
in contrast to rotating gantry DBT systems, where the MTF degrades for increased angular span due to increased focal
spot blurring associated with the x-ray source motion. The overall image quality factor, a composite measure of the
signal difference to noise ratio (SdNR) for mass detection and the z-axis artifact spread function for microcalcification
detection, is best for the configuration with a large angular span, an intermediate number of projection views, and an
even dose distribution. These results suggest possible directions for further improvement of s-DBT systems for high
quality breast cancer imaging.
Tomosynthesis requires projection images from different viewing angles. Using a distributed x-ray source this can be
achieved without mechanical motion of the source with the potential for faster image acquisition speed. A distributed xray
tube has been designed and manufactured specifically for breast tomosynthesis. The x-ray tube consists of 31 field
emission x-ray sources with an angular range of 30°. The total dose is up to 100mAs with an energy range between 27
and 45 kVp. We discuss the source geometry and results from the characterization of the first prototype. The x-ray tube
uses field emission cathodes based on carbon nanotubes (CNT) as electron source. Prior to the manufacturing of the
sealed x-ray tube extensive testing on the field emission cathodes has been performed to verify the requirements for
commercial tomosynthesis systems in terms of emission current, focal spot size and tube lifetime.
Tomosynthesis imaging requires projection images from different viewing angles. Conventional systems use a moving xray
source to acquire the individual projections. Using a stationary distributed x-ray source with a number of sources that
equals the number of required projections, this can be achieved without any mechanical motion. Advantages are a
potentially faster image acquisition speed, higher spatial and temporal resolution and simple system design. We present
distributed x-ray sources based on carbon nanotube (CNT) field emission cathodes. The field emission cathodes deliver
the electrons required for x-ray production. CNT emitters feature a stable emission at high current density, a cold
emission, excellent temporal control of the emitted electrons and good configurability. We discuss the use of stationary
sources for two applications: (i) a linear tube for stationary digital breast tomosynthesis (sDBT), and (ii) a square tube for
on-board tomosynthesis image-guided radiation therapy (IGRT). Results from high energy distributed sources up to
160kVp are also presented.
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