Direct Conversion Detectors Research Articles

Testing of an organic metal halide perovskite for fast neutron detection

In this work, we synthesized and characterized the Methylhydrazinium Lead Trichloride MHyPbCl3(CH3NH2NH2PbCl3) perovskite as a fast neutron detector. The high hydrogen density of MHyPbCl3 enables efficient energy conversion from a fast neutron into a recoiled proton through the 1H(n,n)1H elastic scattering interaction, thereby, allowing for direct charge detection. Through IV characterization and X-Ray excitation, the crystal demonstrated a high resistivity at 4.43E11Ωcm and a good mobility-lifetime product (μτ) of 9.1E-3 cm2V, respectively, under C60/BCP/Cu and Au contact configuration to form an ohmic-ohmic detector. The crystal showed a good sensitivity to X-rays using an x-ray tube. The feasibility of the direct neutron conversion detector is demonstrated using the fast neutron beam at a Research Reactor. Waveforms from a charge-sensitive pre-amplifier showed distinct radiation-induced pulses from the MHyPbCl3 detector in response to the reactor neutron beam. Using a thermal neutron filter and gamma shielding in the beam, we showed that the pulses produced were more likely from neutron interactions, despite the Pb containing MHyPbCl3 is also sensitive to gamma-rays. With those fast neutron pulses, a post-pulse processing code was used to conduct pulse height analysis (PHA) and reconstruct an energy spectrum.

In vivo low-dose phase-contrast CT for quantification of functional and anatomical alterations in lungs of an experimental allergic airway disease mouse model.

Synchrotron-based propagation-based imaging (PBI) is ideally suited for lung imaging and has successfully been applied in a variety of in vivo small animal studies. Virtually all these experiments were tailored to achieve extremely high spatial resolution close to the alveolar level while delivering high x-ray doses that would not permit longitudinal studies. However, the main rationale for performing lung imaging studies in vivo in small animal models is the ability to follow disease progression or monitor treatment response in the same animal over time. Thus, an in vivo imaging strategy should ideally allow performing longitudinal studies. Here, we demonstrate our findings of using PBI-based planar and CT imaging with two different detectors-MÖNCH 0.3 direct conversion detector and a complementary metal-oxide-semiconductor (CMOS) detector (Photonics Science)-in an Ovalbumin induced experimental allergic airway disease mouse model in comparison with healthy controls. The mice were imaged free breathing under isoflurane anesthesia. At x-ray dose levels below those once used by commercial small animal CT devices at similar spatial resolutions, we were able to resolve structural changes at a pixel size down to 25 μm and demonstrate the reduction in elastic recoil in the asthmatic mice in cinematic planar x-ray imaging with a frame rate of up to 100 fps. Thus, we believe that our approach will permit longitudinal small animal lung disease studies, closely following the mice over longer time spans.

State-of-the-art challenges and emerging technologies in radiation detection for nuclear medicine imaging: A review

Single photon emission computed tomography (SPECT) and positron emission tomography (PET) are established medical imaging modalities that have been implemented for decades, but improvements in detector design and camera electronics are needed for advancement of both imaging technologies. Detectors are arguably the most important aspect of the systems. Similar to SPECT, PET typically relies on indirect conversion of gamma radiation via scintillators coupled with photosensors used to convert optical photons produced by the scintillator into an electrical signal. PET detectors are defined by their energy resolution, timing resolution, and spatial resolution, all of which affect and determine the image quality. Improvements in energy resolution have been shown by increasing the brightness of the scintillator utilizing materials like cerium bromide (CeBr3) or switching to a direct conversion detector, such as cadmium zinc telluride (CZT) or thallium bromide (TlBr). Timing resolution for PET is a focal point of the current research. Improving the timing resolution improves the signal-to-noise of the PET system and is integral to the implementation of time-of-flight PET. By utilizing novel configurations, such as side readouts on scintillators, timing resolution has been improved dramatically. Similarly, metascintillators, which use complex combinations for the scintillator material, have also shown improvements to the timing resolution. Additional research has focused on using Cherenkov light emission in scintillators to further improve the timing resolution. Other research is focused on using convolutional neural networks and other signal processing to enhance timing resolution. Lastly, aside from acollinearity and positron range, spatial resolution is impacted by the PET detector, therefore improving the intrinsic spatial resolution of the detector will allow for smaller features to be imaged. One method for improving the spatial resolution is to use unique configurations with layered scintillators. Additionally, monolithic scintillators have also been shown to have reduced spatial resolution. The future for both SPECT and PET image system advancement will depend on continued development of the detectors via many different pathways including materials, signal processing, physics, and novel configurations. In this review article, we will discuss challenges and emerging technologies for state-of-the-art radiation detectors utilized in PET and SPECT.

A perspective on the device physics of lead halide perovskite semiconducting detector for gamma and x-ray sensing

Metal-halide perovskites are emerging materials for radiation sensing featuring high absorption cross section, high charge mobility, and long carrier lifetime enabling superior detection efficiencies even at room temperature. Perovskite detector technologies are more complex involving different detecting modes and device architectures. To elucidate the working principles, we provide a Perspective on the device structures and the operational principles of halide perovskite direct conversion detectors for radiation sensing. We will first discuss different detection modes and introduce the commonly used device architectures and their working principles. Based on our work and pioneer developments in the field, we will next compare the performances of the reported detectors. Additionally, we will summarize the single crystal detectors and polycrystalline thin film devices and discuss their advantages and disadvantages for different applications. Finally, we provide a future Perspective on the critical problems that must be addressed to commercialize this technology.

Next generation high resolution perovskite direct conversion detector: Monte Carlo design optimisation and virtual clinical trial

We implement virtual clinical integration of next-generation perovskite detectors into common x-ray imaging devices. This was achieved by performing Monte Carlo (MC) optimisation of the design and benchmarking of low cost, high spatial resolution, direct conversion perovskite crystal x-ray flat panel imagers for a next generation of breast-, MV-, and kV-cone beam CT detectors. Semiconductor methylammonium lead bromide perovskite crystals energy deposition efficiencies calculated in TOPAS were benchmarked against four common detector materials for twelve detector crystal thicknesses between 40 to 15 mm and ten beam energies ranging from 20 keV to 6 MeV. Based on these simulations, Koning's dedicated breast CT, and Varian's Truebeam kV- and MV-cone beam CT systems were designated as suitable applications for perovskite detectors. System specific Fastcat hybrid MC cone beam CT image simulation was subsequently used to optimise the perovskite detector design and conduct virtual clinical trials. Device-specific optimal perovskite crystal thicknesses were calculated to be 0.30, 0.86, and 1.99 mm for Koning breast CT and Truebeam kV- and MV-cone beam CT systems, respectively. Replacing the current detectors on these machines with low cost perovskite crystal detectors could be advantageous as it would simultaneously yield 12.1%, 9.5% and 86.1% improvements in detective quantum efficiency as well as increases in contrast to noise ratio in brain, lung, and bone tissues.

Direct Conversion X-ray Detector with Micron-Scale Pixel Pitch for Edge-Illumination and Propagation-Based X-ray Phase-Contrast Imaging.

In this work, we investigate the potential of employing a direct conversion integration mode X-ray detector with micron-scale pixels in two different X-ray phase-contrast imaging (XPCi) configurations, propagation-based (PB) and edge illumination (EI). Both PB-XPCi and EI-XPCi implementations are evaluated through a wave optics model—numerically simulated in MATLAB—and are compared based on their contrast, edge-enhancement, visibility, and dose efficiency characteristics. The EI-XPCi configuration, in general, demonstrates higher performance compared to PB-XPCi, considering a setup with the same X-ray source and detector. However, absorption masks quality (thickness of X-ray absorption material) and environmental vibration effect are two potential challenges for EI-XPCi employing a detector with micron-scale pixels. Simulation results confirm that the behavior of an EI-XPCi system employing a high-resolution detector is susceptible to its absorption masks thickness and misalignment. This work demonstrates the potential and feasibility of employing a high-resolution direct conversion detector for phase-contrast imaging applications where higher dose efficiency, higher contrast images, and a more compact imaging system are of interest.

Multispectral photon-counting for medical imaging and beam characterization — A project review

Central focus of the MPMIB project – funded via the Academy of Finland’s RADDESS 2018–2021 programme – has been research towards a next-generation radiation detection system operating in a photon-counting (PC) multispectral mode: The extraction of energy spectrum per detector pixel data will lead to better efficacy in medical imaging with ionizing radiation. Therefore, it can be an important asset for diagnostic imaging and radiotherapy, enabling better diagnostic outcome with lower radiation dose as well as more versatile characterization of the radiation beam, leading for example to more accurate patient dosimetry.We present our approach of fabricating direct-conversion detectors based on cadmium telluride (CdTe) semiconductor material hybridized with PC mode capable application-specific integrated circuits (ASICs), and will give a review on our achievements, challenges and lessons learned. The CdTe crystals were processed at Micronova, Finland’s national research infrastructure for micro- and nanotechnology, employing techniques such as surface passivation via atomic layer deposition, and flip chip bonding of processed sensors to ASIC. Although CdTe has excellent photon radiation absorption properties, it is a brittle material that can include large concentrations of defects. We will therefore also emphasize our quality assessment of CdTe crystals and processed detectors, and present experimental data obtained with prototype detectors in X-ray and Co-60 beams at a standards laboratory.

Recent Progress in Halide Perovskite Radiation Detectors for Gamma-Ray Spectroscopy

Detecting hard X-rays and γ-rays with high energy resolution is critical for medical and industrial applications, high-energy fundamental physics, and homeland security. Two types of radiation detectors, indirect-conversion (scintillators) and direct-conversion (solid-state) detectors, are the most widely used technologies. Semiconductor-based detectors that can directly convert γ-rays into an electrical signal and operate at room temperature are especially important as portable and cost-efficient detectors with high sensitivity and energy resolution. Recently, lead halide perovskites have attracted enormous interest for γ-ray detection, and significant progress has been made toward practical detectors using perovskites as active materials. In this Review, we start with the fundamentals of γ-ray detection and review the recent developments in halide perovskite γ-ray detectors. The key factors affecting the detector performance are summarized. We also give an outlook on the field, with emphasis on the challenges to be overcome.

Development of a high-spatial-resolution X-ray detection system with silicon separator

In this study, a high-spatial-resolution scintillator-type X-ray detection system with a silicon separator was developed. The image characteristics are evaluated, and the approaches required for the practical application of the proposed system were discussed. X-ray inspection is an indispensable benefit in various fields; however, the spatial resolution of a scintillation-type X-ray detector is not as high as that of a direct-conversion X-ray imaging detector due to the scattering of the scintillator emissions. Therefore, the purpose of this study was to suppress such diffusion and improve the resolution to achieve a high-spatial-resolution scintillator-type X-ray imaging detector. Silicon wafers were processed to create small square holes in pixel patterns, and these holes were filled with the scintillator. The spatial resolution of the sample was then measured to confirm its improvement. The high-spatial-resolution of the scintillator-type detector was achieved using silicon separators. Moreover, appropriate noise characteristics and detection efficiency must be ensured to achieve good detection capability. Based on X-ray irradiation experiments and image quality evaluations, indicators were obtained for improving the image detection performance of the silicon separator in terms of noise and detection efficiency. Thus, the scintillator-type imaging detector with the high-spatial-resolution could be of practical use.

Silicon photomultiplier-based scintillation detectors for photon-counting CT: A feasibility study.

PurposeThe implementation of photon‐counting detectors is widely expected to be the next breakthrough in X‐ray computed tomography (CT) instrumentation. A small number of prototype scanners equipped with direct‐conversion detectors based on room‐temperature semiconductors, such as CdTe and CdZnTe (CZT), are currently installed at medical centers. Here, we investigate the feasibility of using silicon photomultiplier (SiPM)‐based scintillation detectors in photon‐counting computed tomography (PCCT) scanners, as a potential alternative to CdTe and CZT detectors.MethodsWe introduce a model that allows us to compute the expected energy resolution as well as the expected pulse shape and associated rate capability of SiPM‐based PCCT detectors. The model takes into account SiPM saturation and optical crosstalk, because these phenomena may substantially affect the performance of SiPM‐based PCCT detectors with sub‐mm pixels. We present model validation experiments using a single‐pixel detector consisting of a 0.9 × 0.9 × 1.0 mm3 LuAP:Ce scintillation crystal coupled to a 1 × 1 mm2 SiPM. We subsequently use the validated model to compute the expected performance of the fast scintillators LYSO:Ce, LuAP:Ce, and LaBr3:Ce, coupled to currently available SiPMs, as well as to a more advanced SiPM prototype with improved dynamic range, for sub‐mm pixel sizes.ResultsThe model was found to be in good agreement with the validation experiments, both with respect to energy resolution and pulse shape. It shows how saturation progressively degrades the energy resolution of detectors equipped with currently available SiPMs as the pixel size decreases. Moreover, the expected pulse duration is relatively long (~200 ns) with these SiPMs. However, when LuAP:Ce and LaBr3:Ce are coupled to the more advanced SiPM prototype, the pulse duration improves to less than 60 ns, which is in the same order of magnitude as pulses from CdTe and CZT detectors. It follows that sufficient rate capability can be achieved with pixel sizes of 400 μm or smaller. Moreover, LaBr3:Ce detectors can provide an energy resolution of 11.5%‐13.5% at 60 keV, comparable to CdTe and CZT detectors.ConclusionsThis work provides first evidence that it may be feasible to develop SiPM‐based scintillation detectors for PCCT that can compete with CdTe and CZT detectors in terms of energy resolution and rate capability.

High-energy micrometre-scale pixel direct conversion X-ray detector.

The objective of this work was to fabricate and characterize a new X-ray imaging detector with micrometre-scale pixel dimensions (7.8 µm) and high detection efficiency for hard X-ray energies above 20 keV. A key technology component consists of a monolithic hybrid detector built by direct deposition of an amorphous selenium film on a custom designed CMOS readout integrated circuit. Characterization was carried out at the synchrotron beamline 1-BM-B atthe Advanced Photon Source of Argonne National Laboratory. The direct conversion detector demonstrated micrometre-scale spatial resolution with a 63 keV modulation transfer function of 10% at Nyquist frequency. In addition, spatial resolving power down to 8 µm was determined by imaging a transmission bar target at 21 keV. X-ray signal linearity, responsivity and lag were also characterized in the same energy range. Finally, phase contrast edge enhancement was observed in a phase object placed in the beam path. This amorphous selenium/CMOS detector technology can address gaps in commercially available X-ray detectors which limit their usefulness for existing synchrotron applications at energies greater than 50 keV; for example, phase contrast tomography and high-resolution imaging of nanoscale lattice distortions in bulk crystalline materials using Bragg coherent diffraction imaging. The technology will also facilitate the creation of novel synchrotron imaging applications for X-ray energies at or above 20 keV.

An X-ray detector based on a Pt-MgZnO-Pt structure

Direct-conversion X-ray detectors made of wide-bandgap semiconductors have many advantages and important applications. In this study, a direct-conversion X-ray detector based on Pt-MgZnO-Pt structure was fabricated by radio frequency magnetron sputtering. The X-ray detection characteristics of the device were studied. The optimized device with a structure of Pt-MgZnO-Pt exhibited a responsivity of 72.6 nC Gyair –1 cm–2, response time of ∼0.1 s (rise)/∼0.4 s (fall), and a high SNR of 206 at a bias of 30 V under a dose rate of 100 mGyair/s. The experimental results demonstrate the good application prospects of the wide-bandgap MgZnO-based X-ray detectors.

Abstract No. 454 Photon counting C-arm computed tomography for thoracoabdominal interventional imaging: an in vivo porcine feasibility study

Prior studies on conventional CT demonstrate improved contrast resolution and radiation dose efficiency using direct conversion and energy-resolving photon counting detectors (PCD). The objective of this study was to determine the feasibility of using a prototype C-arm based PCD-CT system for thoracoabdominal interventional imaging and assess the potential benefits in comparison to flat panel detector-based cone-beam CT (FPD-CBCT). A 51 cm × 0.6 cm CdTe-based PCD strip with 100 μm pixel size was mounted to the front of the FPD of a Siemens Artis Zee interventional system. The PCD has a max frame rate of 150 fps and two adjustable energy thresholds for high-speed dual energy imaging. Eight-second rotational FPD-CBCT and PCD-CT acquisitions of the upper abdomen and lower thorax were performed in three female domestic swine (49.3 ± 2.5 kg). Acquisitions in one of the animals were performed after a stent graft was placed during a TIPS procedure. To assess radiation dose reduction potential, PCD-CT scans were performed at 25% (7.5 mGy) of the standard CTDIvol level (30 mGy) used by the conventional FPD-CBCT protocol. Images were assessed for artifacts, noise, contrast and spatial resolution. PCD-CT images had lower noise due to rejection of electronic noise by the PCD. This resulted in higher spatial and contrast resolution for PCD-CT, even at a lower radiation dose (25% of the FPD-CBCT). Blooming and streak artifacts from iodine accumulated in the kidney and bone were lower on the PCD images. Increased soft tissue contrast on the PCD images improved visualization of visceral detail, especially of bowel. Blooming and shading artifacts near the TIPS stent graft were suppressed in PCD-CT images. In this preliminary study, a prototype C-arm based PCD-CT system demonstrated potential benefits in image quality and radiation dose efficiency when compared to FDP-CBCT. Further investigation of C-arm PCD-CT is warranted in order to assess the potential for applying this technology in improving thoracoabdominal interventions.

Radiologia de tecnologia digital aplicada à pintura: avaliação da qualidade de imagens obtidas em sistemas clínicos de radiologia

Introduction: The area’s studies have a great relevance in artwork. Laboratories committed to this purpose are scarce and employ professionals without specific knowledge in handling of ionizing radiation. It is not known to what extent, in Portugal, clinical radiological centers could provide added value to technology of digital radiography applied to painting, in order to expand their business areas and the performance of radiographers, thus enhancing synergies between these two areas. Goals: It was intended to analyse a possible applicability, to painting, of the Digital Radiology systems (CR and DR), used in the clinic settings, through the evaluation of the variable “radiographic image quality”, since the systems Clinical radiological tests, with indirect or direct conversion detectors of different energies and photodetectors of the electrical signal, show significant improvements in imaging patterns. Material and Methods: 21 radiographs were obtained from exposures made over three paintings in three different radiological systems with an emphasis on exposure parameters (potential difference (kV) and current intensity (mAs)). The sample was directed to works that presented different techniques, materials and compositions and selected by a museologist. The quality of the radiographs, operationalized by the analysis’s dimensions used in the evaluation of artworks: preservation of the support; distinction of details; contrast; contour definition; creative and technical process and components / materials. Four observers (2 specialists in medical image and 2 in art), under conditions of independence, evaluated the radiographs acquired according to a 4-point Likert scale. The analysis dimensions were organized into 11 criteria and the conditions for structured observation of the radiographs were standardized for display on a 15.6 ”GeChic 1503H monitor. Results: The most graded images were obtained with the Computerized Radiography (CR) system (mode: 3) given that this technology, in the opinion of the observers, provided a better view of the analysis criteria previously defined. The results were presented using descriptive statistics and discrete categorical variables, grouped into modal classes. Conclusions: Clinical digital radiography, especially Computed Radiography (CR), applied to painting, showed considerable dimensions of image quality and its use can contribute to the conservation, restoration, validation, analytical study, dating and authentication of artworks.

Engineering of a Blocking Layer Structure for Low-Lag Operation of the a-PbO-Based X-Ray Detector

Direct conversion flat panel detectors are of great significance to the field of medical X-ray imaging since they offer imaging performance and diagnostic capabilities not achievable with other methods. Currently, mammographic direct conversion detectors employ a layer of amorphous selenium (a-Se) photoconductor. Although its properties ideally fit the requirements of mammography, where “soft” X-rays are used, a-Se cannot be used in high-energy X-ray procedures. To extend the diagnostic capabilities of the direct conversion detectors, amorphous lead oxide (a-PbO) is proposed as an alternative photoconductor. It is a high effective atomic number material and thus has a higher X-ray stopping power over the wide X-ray energy range. a-PbO is, therefore, a suitable candidate for applications in radiography, fluoroscopy, and digital tomosynthesis. Here, we report on the development of a blocking structure with a polyimide (PI) layer needed to maintain low dark current at high electric fields. We demonstrate that a 1- $\mu \text{m}$ -thick PI blocking layer allows the operation of the detector at strong electric fields (≥10 V/ $\mu \text{m}$ ) while suppressing the dark current to an innocuous level (< 1 pA/mm2). It also improves temporal performance by reducing signal lag. No ghosting effect was observed at exposure rates up to 1 R/s; however, at high radiation levels, the detector’s sensitivity degraded. This degradation is not permanent as the detector restores its original sensitivity after several hours of rest in the dark without bias applied.

Analogous Lubberts effect in photon counting detectors.

For a light burst generated by an x ray-scintillator interaction, the quasi-isotropic spreading of optical quanta determines that both the light collection efficiency and the spread function of the light burst depend on the depth (z) of interaction: the closer the initial position of light burst to the light sensor, the less light loss and spatial resolution degradation a detector system encounters. For scintillator-based indirect-conversion detectors, the variation of the x-ray conversion gain and the MTF along the z direction contributes to a significant fraction of DQE loss and is known as the Lubberts effect. Understanding the physics behinds the effect and developing solutions to reduce the effect have led to tremendous improvements in the performance of indirect-conversion detectors. In contrast, for direct-conversion photon counting detectors (PCDs), since no optical quanta generation or transport is involved, there is no classical Lubberts effect. However, the transport properties of the generated charge carriers in semiconductors do depend on their travel distance. As a result, the performance of direct conversion detectors may depend on z. This dependence can be considered as a counterpart of the classical Lubberts effect. The purpose of this work is to analyze the contributions of a variety of physical mechanisms that may contribute to a Lubberts-like effect in direct-conversion PCDs. The results show that the Lubberts-like effect is primarily caused by the depth-dependent charge trapping and recombination likelihood. Based upon the physics analysis, PCD electrode design can be optimized to minimize the Lubberts-like effect and the associate DQE loss.

Investigation of detector uniformity issues for Siemens Inspiration systems

PurposeDetector uniformity is an important parameter in digital mammography to guarantee a level of image quality adequate for early detection of breast cancer.Many problems with digital systems have been determined through the uniformity measurement, primarily as a result of incorrect flat-field calibration and artifacts caused by image receptor defects.The European guidelines suggest a method for the image uniformity assessment based on measurement of Signal-to-Noise ratio (SNR) and Pixel Value (PV) across a uniform image.Nineteen mammography systems from the same manufacturer installed in our organization incorporate an a-Se direct conversion detector. Since their installation, instability and inconsistency of image uniformity has attracted medical physicist attention.A number of different tests have been carried out in order to understand and establish reasons for this instability. MethodsThree different tests have been performed to evaluate the impact of the heel effect, detector temperature and ghosting on the uniformity images. All the tests are based on the acquisition of uniform images as suggested by the European Guidelines for Quality Assurance in Breast Cancer Screening and Diagnosis. ResultsResults show that an increase in detector temperature produces an increase of SNR and decrease of uniformity. A further decrease of uniformity ranging between 20% and 30% is due to the ghosting while a decrease of about 10% is due the heel effect. ConclusionsX-ray tube, system geometry and detector have an impact on the system uniformity and an understanding of the contribution of each is necessary in order to obtain comparable image quality among all the systems.

Simulation Study of a Monochromatic X-ray Beam with K-edge Filter for Low Dose Mammography

X-ray mammography can be used to acquire images useful for early breast cancer detection in medical applications. There have been a number of approaches to diagnostic low-dose mammography. The purpose of this study was to design a concept for use of monochromatic X-ray beams by using K-edge filters for low dose mammography. The SRS-78 X-ray simulation code and Monte Carlo simulation GATE code were used to model the X-ray tube, target material, filter material, breast phantom, and detector. To achieve a monochromatic X-ray beam, molybdenum (Mo) and rhodium (Rh) were used as filter materials in various thicknesses. The direct conversion detector (FDXD 1417, Drtech, Seongnam, Korea) composed of thin-film transistor (TFT)-amorphous selenium (a-Se) was modelled through Monte Carlo simulation. According to the CVR and EER results, the filter thickness for optimal mammographic imaging is set to 6 and 3 HVL for Mo and Rh, respectively. SNR values with the Rh filter improved by 1.01, 6.28, 5.60, 5.60, 5.60, 5.60 % over the range from 0.1 to 0.6 μGy compared to analogous SNR values without filter. The present work demonstrates that monochromatic X-ray beams can be generated for low dose mammography. According to the results, Rh filter could be useful for enhancing calcification while absorbed dose is reduced.

The impact of the heel effect and lag on the uniformity of a-Se detector for mammography application

Multiple studies have shown early detection of breast cancer through routine mammography screening can reduce mortality by up to 25% [ [1] World Health Organization; IARC handbooks of cancer prevention: handbook 7: breast cancer screening. IARC Press; 2001. Google Scholar ]. However, this reduction of mortality is possible only if the key goal of mammography related to the image quality is achieved. Detector uniformity is an important image quality parameter to measure as part of a routine mammography QA programme. Many problems with digital systems have been determined through this measurement, primarily as a result of incorrect flat-field calibration and artifacts caused by image receptor defects. The European guidelines [ [2] EUREF; European guidelines for quality assurance in breast cancer screening and diagnosis. 4th ed. Luxembourg; 2006. Google Scholar ] suggest a method for the image uniformity assessment based on measurement of Signal-to-Noise ratio (SNR) and Pixel Value (PV) across a uniform image. Nineteen mammography systems from the same manufacturer installed in our organisation incorporate an a-Se direct conversion detector. Since their installation, instability and inconsistency of image uniformity has attracted medical physicist attention. A number of different tests have been carried out in order to understand and establish reasons for this instability. In this work, we will present the impact of the heel effect and image lag on these uniformity tests. A test protocol involving a screening simulation has been adopted in order to quantify the variation of uniformity across the detector due the lag effect. Images have also been analysed before and after cropping the image to reduce the impact of heel effect. Results show increased non-uniformity ranging between 20% and 30% due to the lag effect and approximately 10% due to the Heel effect.

Cascaded systems analysis of a-Se/a-Si and a-InGaZnO TFT passive and active pixel sensors for tomosynthesis

Medical imaging systems like full field digital mammography (FFDM) and digital breast tomosynthesis (DBT) commonly use amorphous selenium (a-Se) based passive pixel sensor (PPS) direct conversion x-ray detectors. On one hand, direct conversion detectors inherently offer better resolution characteristics in terms of a higher modulation transfer function (MTF), in comparison to the indirect CsI:Tl PPS x-ray imager. On the other hand, especially at lower doses, this superior performance of the direct imager is seldom retained in its detective quantum efficiency (DQE) curves.It is well known that a-Se PPS x-ray imagers suffer from high additive electronic noise originating from the from the amorphous silicon (a-Si) thin film transistor (TFT) array that is being used in the current back-plane technology. This degrades the noise power spectrum (NPS) and subsequently the overall DQE. To address this deficiency, we propose to replace the PPS back-plane by active pixel sensor (APS) back-plane technology, which has the potential to reduce the back-plane electronic noise by amplifying the input signal, especially at low doses. The proposed APS is based on amorphous In–Ga–Zn–O (a-IGZO) TFT technology, which can offer high mobility (5–20 cm2 V−1 s−1), low leakage current (<10−13 A) and low flicker noise (Hooge’s parameter αH ~ 1.5 10−3), leading to better imager noise performance.To test our hypothesis, we used linear cascaded systems analysis to model the imaging performance (MTF, NPS and DQE) of the PPS and APS a-Se direct imagers. This model was first validated using experimentally measured data obtained for a 85 µm pixel pitch a-Se/a-Si TFT PPS imager. Using this model, we analyzed the noise performance of the direct a-Se and indirect CsI:Tl x-ray a-IGZO APS imagers at different dose and electronic noise levels. Obtained results clearly showed that lowering back-plane electronic noise can significantly improve the performance of the a-Se/a-IGZO TFT APS imager. Our simulated results showed that a higher DQE at lower radiation doses (maximum DQE of 0.6 can be achieved at an exposure level of 1 µGy) can be achieved with the a-Se detector, thereby making this combination a promising candidate for low dose applications like DBT.