Multi-energy CT can be used to decompose CT images into different material bases to allow for the quantification of materials. Three-material decomposition is desirable for clinical applications such as K-edge imaging. However, three-material decomposition requires measurements at 3 or more energies or additional assumptions such as volume conservation. This work concerns the following questions: 1) For photon counting detector (PCD) CT with only two energy bins, can the joint use of the PCD and kV-switching improve the performance of three-material decomposition for K-edge imaging? 2) If the PCD energy threshold must be fixed for both kV levels, how can its optimal value be pre-determined for a given K-edge imaging task? 3) While in conventional kV-switching multi-energy CT the separation of the two kV levels needs to be as wide as achievable, is this still the case when kV-switching is jointly used with a dual-bin PCD? These questions were answered by performing Cram´er-Rao lower bound (CRLB) analysis for dual-bin PCD-CT with kV-switching to optimize the PCD threshold and kV levels for the decomposition of water, iodine, and gadolinium bases using a previously modeled CdTe PCD energy response function. Experimental dual-bin PCD-CT images were acquired with kV-switching, and without kV-switching but with the volume conservation constraint, using the optimal acquisition parameters obtained from the CRLB analysis. Dual-bin PCD-CT with kV-switching allowed separation of all three basis materials with improved quantitative accuracy over dual-bin PCD-CT without kV-switching, which also suffered from poorer material separation and residual or missing signals from other materials.
KEYWORDS: X-rays, Photon counting, X-ray detectors, Spatial resolution, Capacitors, X-ray computed tomography, Design and modelling, Analog to digital converters, Polymethylmethacrylate, Analog electronics
In photon counting detectors (PCDs), electric pulses induced by two or more x-ray photons can pileup when their temporal separation is less than the detector deadtime. Pulse pileups degrade the quantitative accuracy of PCD projection and CT images. In contrast, conventional energy integrating detectors (EIDs) that integrate x-ray induced electric charge are linear with x-ray flux without suffering from pileup losses even at high flux levels. This work presents a novel PCD readout circuit design that leverages the fact that the total induced charge is linearly related to the true input flux to correct for pileup-induced count losses in PCDs. The method only requires an inexpensive upgrade to the existing PCD circuits to record the accumulated charges. After collecting PCD counts and the integrated charges, a lookup table was used to estimate the true photon events. Proof-of-concept experiments were performed with a CdTe-based PCD, and an EID was used to emulate the proposed charge summing component of the readout circuit. With the proposed correction, PCD counts became linear with input flux, even under severe pulse pileup conditions where the PCD is paralyzed and the uncorrected counts start to decrease with increasing flux. Without the correction, post-log sinograms for the test objects were severely overestimated; with the proposed correction, the sinograms accurately represent the true radiological path lengths of the objects. Lastly, no impact on spatial resolution was observed after the proposed correction in images of a spatial resolution test pattern.
Flat panel detectors (FPDs) with two separate scintillator layers enable dual-energy (DE) 2D and 3D x-ray imaging with perfect temporal registration. A thick (1 mm) Cu layer is permanently sandwiched between the two scintillators to widen the spectral separation. Because the majority of clinical applications of FPDs do not benefit from DE imaging, the existence of the Cu layer during those applications degrades the overall dose efficiency. In this work, a novel dual-layer FPD design with a removable liquid filter design was presented to improve the dose efficiency of dual-layer FPDs when they are used for non-DE imaging applications.
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 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.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.