C-arm x-ray systems equipped with flat panel detectors (FPD) lack spectral and ultra-high-resolution (UHR) capabilities desired by physicians for image guided interventions (IGIs), for example to discriminate between and/or quantify different materials such as iodine and calcium, or in the visualization of very fine structures or devices used in interventional procedures. Photon counting detectors (PCDs) can introduce these capabilities to the interventional suite: In this work, we propose a new dagger-shaped PCD design tailored for IGIs to upgrade the imaging capabilities in the C-arm interventional system while preserving the functionality of the existing FPD and reducing the system cost compared to completely replacing the FPD with a large-area PCD. The design consists of two modules integrated together: One is a long-strip shape for narrow-beam spectral and UHR CT with full axial coverage, and one is rectangle-shaped for volume- and region-of-interest 2D and 3D spectral and UHR imaging. As a proof of concept, prototypes of each module were used to perform phantom and in vivo animal experiments. Results show the potential of the proposed design in discriminating between and quantifying iodine and calcium by leveraging the spectral information provided by PCDs. UHR 2D and 3D PCD images show the improved capabilities of the dagger PCD in delineating small blood vessels with improved contrast-to-noise ratios, as well as resolving fine structures such as stents commonly used in IGIs.
In photon counting detectors (PCDs), the detected energy of an x-ray photon often deviates from its true energy due to a variety of reasons such as Compton scattering. For a given input x-ray energy or spectrum, the statistical distribution of the detected energy is referred to as the energy response function. Knowledge of the energy response function of a given PCD can greatly facilitate the evaluation of the spectral imaging performance and the optimization of PCD imaging systems. In this work, a physics-based analytical model of the energy response function of CdTe-based PCDs was developed. The model covers the whole diagnostic x-ray energy range and incorporates all relevant physical processes into consideration (e.g., K-fluorescence photon reabsorption or escape, Compton scattering). The model is applicable to PCDs operated under the anti-coincidence (i.e., anti-charge sharing) mode by modifying the parameters related to the severity of charge sharing. Those parameters can be experimentally calibrated to adapt the model to specific PCD systems. To validate the proposed model, experiments were performed using a CdTe-based PCD system. The results demonstrated that the model can accurately predict the PCD energy response functions of a variety of input x-ray spectra, under the anti-coincidence or the single pixel detection mode.
In recent years, much effort has been committed to lowering radiation dose in CT. However, when the radiation dose is lowered, not only is image noise elevated, but the CT number also becomes more inaccurate. Note that the CT number bias issue is intrinsically rooted in the statistical nature of photons and the standard image formation process that has been used for the past 50 years in medical CT practices: after CT data are acquired, a log-transform is applied to generate the sinogram projection data, then an image reconstruction algorithm is applied to reconstruct the CT images. However, there is a fundamental flaw in this image formation process: the log-transform itself is a statistically biased estimator since the statistical mean of the log-transformed data is different from the log-transform of the statistical mean of the data. In medical CT applications, we are forced to take the log transform of a single sample of the measured CT data and then images are reconstructed from the log-transformed data. Consequently, CT images will then have inaccurate CT numbers. In this work, we investigated the imaging physics foundation of the CT number inaccuracy issue in low dose CT and developed a simple, yet extremely effective correction method to address this long-standing issue in CT imaging. This correction scheme was experimentally validated in the context of photon counting detector CT (PCD-CT). Our experimental results demonstrated that the correction scheme addresses the CT number bias problem and improves material quantification accuracy of spectral PCD-CT images.
In this work, a unified framework was developed to jointly address scatter artifacts, detector nonuniformity-induced concentric artifacts, and beam hardening artifacts in C-arm photon counting detector (PCD) cone beam CT. By leveraging the energy-resolving capability of PCDs, a better estimation of the scattered photon signal was obtained via a photoelectric-Compton scattering decomposition. Next, detector nonuniformity and beam hardening artifacts were jointly corrected via a second-round projection domain pixel-wise material decomposition. Both phantom and in vivo animal results demonstrated that the proposed correction method generated high-quality and quantitative PCD cone beam CT images for image-guided interventions.
Existing clinical C-arm interventional x-ray systems equipped with flat panel detectors (FPDs) can generate fluoroscopic, angiographic, and cone-beam CT (CBCT) images with sufficient volumetric coverage for interventional imaging tasks. However, FPD-CBCT does not provide sufficient low-contrast detectability, resolution, or spectral imaging capability desired for certain interventional procedures. To overcome these limitations, a C-arm photon counting detector (PCD) CT prototype was developed by installing an interchangeable strip PCD on the C-arm gantry. The narrow z width of the PCD reduces detector cost and reduces scatter when paired with a narrow beam collimation. However, it does not provide sufficient volumetric coverage compared to the standard FPD. The purpose of this work was to develop a step-and-shoot data acquisition method to enlarge the effective z-coverage of the C-arm strip PCD-CT system. A total of 10 back-and-forth short-scan C-arm gantry rotations were used with image object translation. By using an Arduino board to process the x-ray-on pulse signals in real-time, a motorized patient table prototype was synchronized with the C-arm system such that it translates the object by the PCD width during the rest time in between gantry rotations. To evaluate whether this multisweep step-and-shoot acquisition mode can generate high-quality and volumetric PCD-CT images, experiments were performed using an anthropomorphic head phantom, and a stent. The multi-sweep step-and-shoot C-arm protocol resulted in volumetric PCD-CT images with lower image noise and improved low-contrast visualization over the FPD-CBCT in the head phantom, as well as improved visibility of small iodinated blood vessels using maximum intensity projections. Under an ultra-high-resolution PCD mode, the fine structures of the stent were visualized more clearly by the PCD-CT than the highest-available resolution provided by the FPD-CBCT. The multi-sweep step-and-shoot acquisition can therefore extend the z-coverage of the C-arm PCD-CT prototype by a factor of 10 to enable high-quality and volumetric C-arm PCD-CT images acquired with a narrow beam-narrow detector setup for image-guided interventions.
The purpose of this work is develop a novel multi-contrast chest x-ray radiography (MC-CXR) imaging system to enable the simultaneous generation of three mutually complementary x-ray contrast mechanisms to enhance the diagnostic performance of CXR for respiratory diseases. The developed grating-based MC-CXR system employs a scanning beam image acquisition scheme in which the patient table is translated at a speed of up to 9 cm/s. The system is capable of accomplishing MC-CXR imaging of an anthropomorphic chest phantom in under 4 seconds, with an air kerma and effective dose that are well below that of a conventional CXR exam.
This work reports the development of a C-arm photon counting detectors (PCD)-CT system for evaluating the potential clinical utility of PCD-CT for intraoperative imaging. A dual-threshold CdTe-based strip PCD was mounted on a Siemens Artis Zee C-arm interventional system. A new geometric calibration method was developed to correct for the geometric distortion of the C-arm system during rotation. Experimental results show that, under clinically relevant conditions, the C-arm PCD-CT system can reliably and reproducibly generate high quality MDCT-like images without any noticeable geometric distortion or banding artifact.
Endovascular procedures performed in the angio suite have gained considerable popularity for treatment of ischemic stroke as well as aneurysms. However, new intracranial hemorrhage (ICH) may develop during these procedures, and it is highly desirable to arm the angio suite with real-time and reliable ICH monitoring tools. Currently, angio suites are equipped with scintillator-based flat panel detector (FPD) imaging systems for both planar and cone beam CT (CBCT) imaging applications. However, the reliability of CBCT for ICH imaging is insufficient due to its poor low-contrast detectability compared with MDCT and lack of spectral imaging capability for differentiating between ICH, calcifications, and iodine staining from periprocedural contrast-enhanced imaging sequences. To preserve the benefits of the FPD for 2D imaging and certain high-contrast 3D imaging tasks while adding a high quality, quantitative, and affordable CT imaging capability to the angio room for intraoperative ICH monitoring, a hybrid detector system was developed that includes the existing FPD on the C-arm gantry and a strip photon-counting detector (PCD) that can be translated into the field-of-view for high quality PCD-CT imaging at a given brain section-of-interest. The hybrid system maintains the openness and ease of use of the C-arm system without the need to remodel the angio room and without installing a slidinggantry MDCT (aka Angio CT) with orders of magnitude higher costs. Additionally, the cost of the strip PCD is much less than the cost of a large-area PCD. To demonstrate the feasibility and potential benefits of the hybrid PCD-FPD system, a series of physical phantom studies, and human cadaver studies were performed at a gantry rotation speed (7 s) and radiation dose level that closely match those of clinical CBCT acquisitions. The experimental images of C-arm PCD-CT demonstrated MDCT-equivalent low-contrast detectability of PCD-CT and significantly reduced artifacts compared with FPD-based CBCT.
Gout is the most prevalent inflammatory arthritis found in men. A prompt diagnosis and early treatment of gout are crucial in preventing eventual functional impairment and reduction in comorbidities. In this work, the quantitative material information provided by a multi-contrast x-ray (MCXR) imaging acquisition is leveraged to develop a rapid, non-invasive, and low dose diagnostic method for gout detection and gout-pseudogout differentiation. This work establishes a theoretical foundation to demonstrate how a single-kV MCXR acquisition is capable of differentiating gout from pseudogout via a projection domain two-material decomposition. Experimental results from a benchtop MCXR system are presented. The imaging performance of the proposed MCXR technique is compared to dual-energy radiography to further validate the method.
Due to the subtle variations of energy response functions across photoconductive panels, photon counting detector CT are subject to severe banding artifacts. This work presents a physics based method to correct for these artifacts. It employs calibration objects with known thicknesses and composition to estimate the panel-specific response functions, which are used to concert the raw photon counting projection data of an arbitrary image object into acrylic- and aluminum-equivalent pathlengths. Experimental results show the method not only remove the banding artifacts but also address the beam hardening artifacts.
Virtual non-contrast (VNC) CT images derived from multi-energy CT data have demonstrated many valuable clinical applications. However, VNC CT has not yet established itself as a technology that can reliably replace the true non-contrast CT. One commonly observed phenomenon is an erroneous removal or reduction of calcium signal in VNC images. The purpose of this work is to develop a photon counting CT-based method to decouple the iodine signal from the calcium signal to achieve VNC CT imaging. Thanks to the energy resolving capability of the photon counting detector (PCD), a photon counting CT enables all single kV CT data to carry additional spectral information needed for VNC CT reconstruction. However, the energy discriminating capability of a real PCD system is far from ideal, which can severely degrade the fidelity of the encoded spectral information and the efficiency of the material decomposition. In this work, a physics-based model of the PCD energy response function was developed and experimentally validated. By leveraging this model, a method was developed to correct the distorted spectral information in the measured PCD energy bin data, allowing the true post image object spectrum to be estimated to accomplish accurate three-material decomposition and VNC CT reconstruction. Both numerical simulation and experimental results demonstrated that the proposed spectral distortion correction method can effectively improve the CT number accuracy of both iodine-containing vessels and calcium-containing bony structures in VNC CT images.
Iodine K-edge CT imaging utilizes the sudden increase in the attenuation coefficient of iodine when the x-ray energy exceeds the K-shell binding energy of iodine. Early works on K-edge CT used multiple K-edge filters to generate different quasi-monoenergetic spectra with mean energies that straddled the iodine K-edge, and then multiple projections acquired with these spectra were processed to enhance the sensitivity of imaging iodine. Recent developments in energy-resolving photon counting detector (PCD) technology offer the potential for single-shot K-edge CT imaging. However, the performance of PCD-based iodine K-edge CT is often limited by the relatively low energy of the iodine K-edge (33.2 keV) compared with the mean energy of a polychromatic spectrum used in CT. This work explored the potential of introducing an iodine beam filter to PCD-based iodine K-edge CT to improve its imaging performance. To optimize the beam filtration condition, a realistic energy response function of an experimental PCD system was used when calculating the Cramér-Rao Lower Bounds (CRLBs) of three-material (iodine, bone, and water) decomposition estimators for each filtration condition. Experimental studies with a benchtop PCD CT system were performed to confirm the CRLB results. Both theoretical and experimental results demonstrated that by using an optimized iodine filter, quantitative accuracy of material basis images was improved. Compared with a commercial dual-energy-CT system, the optimized experimental K-edge CT system effectively reduced residual iodine signal in the bone basis image and reduced residual bone signal in the water-basis image.
KEYWORDS: Signal to noise ratio, Imaging systems, Error analysis, Modulation transfer functions, Signal detection, Photons, Numerical simulations, X-ray imaging
In 1963, Shaw applied Fourier analysis to the zero-frequency DQE and developed the frequency-dependent DQE or DQE(k) and made it clear that DQE(k) is applicable to every frequency level within the system bandwidth, including the zero frequency. Over time, especially after entering the modern era of digital x-ray imaging, the experimental measurement methods of DQE(k) (particularly the measurements of the NPS which is an important element in DQE(k)) have evolved, and some measurement methods may generate nonphysical NPS and DQE results at k=0. As a result, an experimental DQE(k) curve is often cut off at certain low frequency above zero. This work presents a new experimental method to deal with two challenges: severe NPS(k=0) underestimation due to polynomial-based background detrending; severe NPS(k=0) overestimation due to the presence of faint but non-negligible system drift. Based on a theoretical analysis of the impact of drift to the measured autocovariance function, the error introduced by drift can be isolated, and corresponding correction can be applied to NPS(k=0). Both numerical simulation with known ground truth and experimental studies demonstrated that the proposed method enables accurate DQE(k=0) measurement.
KEYWORDS: Signal to noise ratio, Computed tomography, Arteries, Sensors, Photon counting, Angiography, Signal detection, X-ray computed tomography, Spatial resolution, Head
Cerebral CT angiography (CTA) is widely used for the diagnosis of various cerebrovascular diseases, including strokes, vasculitis, aneurysms, and etc.2–4 For the diagnosis of ischemic strokes, the availability of high quality CTA images not only helps in identifying the presence/location of large vessel occlusion but also facilitates the assessment of collateral blood supply. As another example, accurate rendering of the superficial temporal arteries is valuable in identifying vessel inflammations induced by giant cell arteritis.5 While CTA is an established clinical gold standard for imaging large cerebral arteries and veins,1 an important challenge that currently remains for MDCT-based CTA is its limited performance in imaging small perforating arteries with a diameter below 0.5 mm.4 As a consequence, the relativley invasive artery biopsy procedure remains the current clinical gold standard for the diagnosis of giant cell arteritis.6 The use of indirect conversion energy integrating detectors puts intrinsic limit on the spatial resolution of MDCT, both in-plane and along the z direction. Severe partial volume averaging effect (PVE) and the preferential weighting of high energy photons7 are among major reasons for the relatively poor performance of MDCT-based CTA for imaging iodinated small vessels. Photon counting detector-based CT (PCD-CT) offers potential technological solutions to these challenges MDCT systems face for CTA. When compared to MDCT, the direct conversion design of PCD reduces limitations on both in-plane and through-plane spatial resolution, and the inherent equal weighting of high and low energy photons of PCD-CT systems offers an improvement in the CNR of iodinated vessels. The purpose of this work was to theoretically and experimentally study the potential impacts of the PCD-CT technology to an important component of CTA image package: the maximum intensity projection (MIP) image. MIP is a simple 3D image visualization method to display CTA data sets. Based on source images alone, it can be very challenging to evaluate occlusion conditions since most vessels extend to different z positions. In comparison, a MIP image that extracted information from a much longer z range can provide clearer evidence for an occlusion; in addition, it can effectively enhance the visibility of small collateral vessels. This work first derived the statistical properties of the MIP image, then analyzed how each of the benefits of PCD (improved z resolution; reduced noise autocovariance along z) propagates from the source CT images to the final MIP image. Finally, experiments were performed using a benchtop PCD-CT system and an anthropomorphic CTA phantom to showcase the significantly improved visibility of small perforating arteries.
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