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Certain imaging performance metrics are examined for a state-of-the-art 20 μm pixel pitch CMOS sensor (RadEye HR), coupled to a Gd2O2S:Tb scintillator screen. The signal transfer property (STP), the modulation transfer function (MTF), the normalized noise power spectrum (NNPS) and the detective quantum efficiency (DQE) were estimated according to the IEC 62220-1-1:2015 standard. The detector exhibits excellent linearity (coefficient of determination of the STP linear regression fit, R2 was 0.9978), while its DQE peaks at 33% and reaches 10% at a spatial frequency of 3 cycles/mm, for the measured with a Piranha RTI dosimeter (coefficient of variation CV = 0.03%) exposure value of 28.1 μGy DAK (detector Air Kerma). The resolution capabilities of the X-ray detector under investigation were compared to other commercial CMOS sensors, and were found in every case higher, except from the previous RadEye HR model (CMOS—Gd2O2S:Tb screen pair with 22.5 μm pixel pitch) version which had slightly better MTF. The present digital imager is designed for industrial inspection applications, nonetheless its applicability to medical imaging, as well as dual-energy is considered and certain approaches are discussed in this respect.

Quality Control (QC) tests in mammography are very important, since mammograms have been used as a population-based screening test for more than 30 years and QCs lead to better image quality and radiation safety for patients. European guidelines, EUREF and EFOMP protocols provide comprehensive QC guidelines for digital mammography (DM) and digital breast tomosynthesis (DBT) units, respectively. We developed a novel, fast, free and platform independent software (named MAMMO_QC) for QC performance tests in DM and tomosynthesis, based on the aforementioned guidelines. MAMMO_QC consists of a series of ImageJ plugins for DM and DBT. It does not require any programming knowledge and can be used to evaluate several performance parameters, such as pre-sampled modulation transfer function (pMTF), normalised noise power spectrum (NNPS), detective quantum efficiency (DQE), contrast-detail analysis based on the CDMAM 3.4 and 4.0 test tools, homogeneity and artefacts, automatic exposure control (AEC) performance. The user can use MAMMO_QC for acceptance, commissioning and routine QC performance analysis based on the European guidelines. We validated our results against well-established software products used in mammography and DBT (i.e., COQ, OBJ_IQ_reduced and Artinis CDMAM Analyzer). All the average relative differences were within 5.5%, and several years of usage and testing allows us to consider MAMMO_QC as an accurate and reliable tool for QC on DM and DBT systems. Our developed software for DM and DBT computes almost all the parameters stated in the European, EUREF and EFOMP guidelines. To the best of our knowledge, no such software has been developed so far.

The aim of this study is to explore the use of a quality (Q) factor in digital mammography as a figure-of-merit. Q factors take a reference value into account and can be compared to a theoretical value. They are also intended to summarise the performance of any unit based on a number. The mean glandular dose (MGD)-normalised Q factors were also introduced based on the relationship of the Q factors with MGD. Interestingly, the automatic exposure control exposures did not render the maximum normalised Q factor values as expected, which could indicate the need for further optimisation. It was also noted that the Q factors and the CDMAM-related quality parameters can be confidently predicted for a given MGD which in turn may be compared to the measured values. This might be another way to consider or perform optimisation in digital mammography.

High-resolution, low-noise x-ray detectors based on the complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) technology have been developed and proposed for digital breast tomosynthesis (DBT). In this study, we evaluated the three-dimensional (3D) imaging performance of a 50 ��m pixel pitch CMOS APS x-ray detector named DynAMITe (Dynamic Range Adjustable for Medical Imaging Technology). The two-dimensional (2D) angle-dependent modulation transfer function (MTF), normalized noise power spectrum (NNPS), and detective quantum efficiency (DQE) were experimentally characterized and modeled using the cascaded system analysis at oblique incident angles up to 30��. The cascaded system model was extended to the 3D spatial frequency space in combination with the filtered back-projection (FBP) reconstruction method to calculate the 3D and in-plane MTF, NNPS and DQE parameters. The results demonstrate that the beam obliquity blurs the 2D MTF and DQE in the high spatial frequency range. However, this effect can be eliminated after FBP image reconstruction. In addition, impacts of the image acquisition geometry and detector parameters were evaluated using the 3D cascaded system analysis for DBT. The result shows that a wider projection angle range (e.g. ��30��) improves the low spatial frequency (below 5 mm-1) performance of the CMOS APS detector. In addition, to maintain a high spatial resolution for DBT, a focal spot size of smaller than 0.3 mm should be used. Theoretical analysis suggests that a pixelated scintillator in combination with the 50 ��m pixel pitch CMOS APS detector could further improve the 3D image resolution. Finally, the 3D imaging performance of the CMOS APS and an indirect amorphous silicon (a-Si:H) thin-film transistor (TFT) passive pixel sensor (PPS) detector was simulated and compared.

In this study, the light output of a zinc selenide activated with tellurium (ZnSe: Te) single crystal was measured for X-ray radiography applications. A cubic crystal (10 &times, 10 &times, 10 mm) was irradiated using X-rays with tube voltages from 50 to 130 kV. The resulting energy absorption efficiency, detective quantum efficiency, and absolute luminescence efficiency were compared to published data for equally sized GSO: Ce (gadolinium orthosilicate) and BGO (bismuth germanium oxide) crystals. The emitted light was examined to estimate the spectral compatibility with widely used optical sensors. Energy absorption efficiency and detective quantum efficiency of ZnSe: Te and BGO were found to be similar, within the X-ray energies in question. Light output of all three crystals showed a tendency to increase with increasing X-ray tube voltage, but ZnSe: Te stood at least 2 EU higher than the others. ZnSe: Te can be coupled effectively with certain complementary metal&ndash, oxide&ndash, semiconductors (CMOS), photocathodes, and charge-coupled-devices (CCD), as the effective luminescence efficiency results assert. These properties render the material suitable for various imaging applications, dual-energy arrays included.

The demand for adequate image quality with low radiation doses for patients has greatly increased. This is especially true in the case of position verification in radiotherapy which requires a high number of images per patient. This study presents a physical characterisation of a new clinical detector named “Lassena (CsI)” based on a thick layer of structured thallium activated caesium iodide and complementary metal-oxide semiconductor technology with active pixel sensor architecture for general X-ray imaging and cone-beam computed tomography (CBCT) applications. We made a critical appraisal of its performance for the first time and determined its signal transfer property (STP) and its detective quantum efficiency (DQE) by acquiring the pre-sampling modulation transfer function (pMTF) and normalised noise power spectrum (NNPS) in addition to the dark current calculation. The investigation was conducted with the application of three X-ray beam qualities: (50 kV (RQA3), 70 kV (RQA5) and 90 kV (RQA7)) in compliance with the International Electrotechnical Commission (IEC 62220-1(2003)) standard. The STP was found to be linear with the coefficient of determination (R2) more than 0.9995 in all cases. The spatial resolution and NNPS results led to acceptable DQE values at all energies; in particular the DQE values at 0.5 line pairs per mm (DQE(0.5)) which were 0.46 for RQA3, 0.52-0.56 for RQA5 and 0.55-0.59 for RQA7. Lastly, the dark current was 2.51 pA/cm2 for a 50 μ m pixel pitch. For CBCT applications, Lassena (CsI) showed very promising results.

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 (

The demand for adequate image quality with low radiation doses for patients has greatly increased. This is especially true in the case of position verification in radiotherapy which requires a high number of images per patient. This study presents a physical characterisation of a new clinical detector based on a thick layer of structured Thallium activated Cesium Iodide. We made a critical appraisal of its performance for the first time and determined its detective quantum efficiency (DQE) by acquiring the pre-sampling modulation transfer function and normalised noise power spectrum (NNPS). The investigation was conducted with the application of three x-ray beam qualities in compliance with the International Electrotechnical Commission (IEC 62220-1:2003) standard. The spatial resolution and NNPS results led to relatively high DQE values at all energies: DQEs (0.5) were 0.46 for 54kV, 0.52-0.56 for 74kV and 0.55-0.59 for 92kV.

Purpose: Large area x-ray imagers based on complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) technology have been proposed for various medical imaging applications including digital breast tomosynthesis (DBT). The low electronic noise (50–300 e{sup −}) of CMOS APS x-ray imagers provides a possible route to shrink the pixel pitch to smaller than 75 μm for microcalcification detection and possible reduction of the DBT mean glandular dose (MGD). Methods: In this study, imaging performance of a large area (29 × 23 cm{sup 2}) CMOS APS x-ray imager [Dexela 2923 MAM (PerkinElmer, London)] with a pixel pitch of 75 μm was characterized and modeled. The authors developed a cascaded system model for CMOS APS x-ray imagers using both a broadband x-ray radiation and monochromatic synchrotron radiation. The experimental data including modulation transfer function, noise power spectrum, and detective quantum efficiency (DQE) were theoretically described using the proposed cascaded system model with satisfactory consistency to experimental results. Both high full well and low full well (LFW) modes of the Dexela 2923 MAM CMOS APS x-ray imager were characterized and modeled. The cascaded system analysis results were further used to extract the contrast-to-noise ratio (CNR) for microcalcifications with sizes of 165–400 μm at variousmore » MGDs. The impact of electronic noise on CNR was also evaluated. Results: The LFW mode shows better DQE at low air kerma (K{sub a} < 10 μGy) and should be used for DBT. At current DBT applications, air kerma (K{sub a} ∼ 10 μGy, broadband radiation of 28 kVp), DQE of more than 0.7 and ∼0.3 was achieved using the LFW mode at spatial frequency of 0.5 line pairs per millimeter (lp/mm) and Nyquist frequency ∼6.7 lp/mm, respectively. It is shown that microcalcifications of 165–400 μm in size can be resolved using a MGD range of 0.3–1 mGy, respectively. In comparison to a General Electric GEN2 prototype DBT system (at MGD of 2.5 mGy), an increased CNR (by ∼10) for microcalcifications was observed using the Dexela 2923 MAM CMOS APS x-ray imager at a lower MGD (2.0 mGy). Conclusions: The Dexela 2923 MAM CMOS APS x-ray imager is capable to achieve a high imaging performance at spatial frequencies up to 6.7 lp/mm. Microcalcifications of 165 μm are distinguishable based on reported data and their modeling results due to the small pixel pitch of 75 μm. At the same time, potential dose reduction is expected using the studied CMOS APS x-ray imager.« less

Digital breast tomosynthesis (DBT) is an advanced imaging application for breast cancer detection, which makes use of a number of 2D X-ray projection images over a limited angular range to reconstruct quasi 3D reconstruction images. As such, first an introduction to digital X-ray tomosynthesis is given, after which existing limited angle tomosynthesis methods are presented. In the next section DBT geometry and its development to date is discussed. Details are given on geometries currently available from different manufacturers, and recent advances. Part of this section presents the available relevant reconstruction methods. Next, we look into the DBT detectors and their performance evaluation. Current DBT detectors are based on amorphous silicon (a-Si) and amorphous selenium (a-Se) thin film transistor (TFT) technology. However, complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) digital X-ray detectors have the potential to replace a-Si:H TFT detectors in DBT in the near future, due to a smaller pixel pitch, low electronic noise, faster frame rate and negligible image lag. The performance of current DBT detectors is evaluated mainly by: automatic exposure control performance, response function, noise analysis, detector element failure, and system projection modulation transfer function (MTF). Following this we discuss image quality measurements, because they are essential for the evaluation and optimization of DBT systems. They should represent relevant clinical tasks, such as the detection of microcalcifications and masses in mammographic backgrounds. Currently the CDMAM phantom is used for contrast-detail analysis (i.e., the required threshold contrast to detect discs of various diameters) of the reconstructed images. The TOR MAM phantom can also be used to score the visualization of discs, filaments and specks for various contrast levels. The parameter Z-resolution is used to assess the inter-plane spread of reconstruction artifacts associated with 1 mm diameter aluminum spheres (contained in a specific three-dimensional phantom). Furthermore, the system MTF in the x–y plane is used to take into account all sources of blurring in the DBT system: detector MTF, additional sources of unsharpness and the reconstruction algorithm. The final part of the chapter describes image simulation methods for DBT optimization. Briefly, DBT is currently under consideration for its use in breast cancer screening in Europe, in combination with 2D mammography or alone. Several parameters (such as image acquisition parameters, detector response, system geometry, radiation dose, and image processing and reconstruction algorithms) are studied for their effect on image quality and the investigation of the optimum use of DBT. Traditionally, large clinical trials are required to evaluate these parameters over a large number of women. Such trials are time consuming, expensive and require irradiating asymptomatic women. Alternatively, several research groups use virtual clinical trials (based on image simulation methods) to optimize DBT parameters in fast, radiation-free, and cost-effective ways. This part of the chapter reviews several simulation methods in DBT and the applications in evaluating its effectiveness.

Recently CMOS Active Pixels Sensors (APSs) have become a valuable alternative to amorphous Silicon and Selenium Flat Panel Imagers (FPIs) in bio-medical imaging applications. CMOS APSs can now be scaled up to the standard 20 cm diameter wafer size by means of a reticle stitching block process. However despite wafer scale CMOS APS being monolithic, sources of non-uniformity of response and regional variations can persist representing a significant challenge for wafer scale sensor response. Non-uniformity of stitched sensors can arise from a number of factors related to the manufacturing process, including variation of amplification, variation between readout components, wafer defects and process variations across the wafer due to manufacturing processes. This paper reports on an investigation into the spatial non-uniformity and regional variations of a wafer scale stitched CMOS APS. For the first time a per-pixel analysis of the electro-optical performance of a wafer CMOS APS is presented, to address inhomogeneity issues arising from the stitching techniques used to manufacture wafer scale sensors. A complete model of the signal generation in the pixel array has been provided and proved capable of accounting for noise and gain variations across the pixel array. This novel analysis leads to readout noise and conversion gain being evaluated at pixel level, stitching block level and in regions of interest, resulting in a coefficient of variation ≤ 1.9%. The uniformity of the image quality performance has been further investigated in a typical X-ray application, i.e. mammography, showing a uniformity in terms of CNR among the highest when compared with mammography detectors commonly used in clinical practise. Finally, in order to compare the detection capability of this novel APS with the currently used technology (i.e. FPIs), theoretical evaluation of the Detection Quantum Efficiency (DQE) at zero-frequency has been performed, resulting in a higher DQE for this detector compared to FPIs. Optical characterization, X-ray contrast measurements and theoretical DQE evaluation suggest that a trade off can be found between the need of a large imaging area and the requirement of a uniform imaging performance, making the DynAMITe large area CMOS APS suitable for a range of bio-medical applications.

Complementary metal-oxide-semiconductors (CMOS) active pixel sensors (APS) have been introduced recently in many scientific applications. This work reports on the performance (in terms of signal and noise transfer) of an X-ray detector that uses a novel CMOS APS which was developed for medical X-ray imaging applications. For a full evaluation of the detector's performance, electro-optical and X-ray characterizations were carried out. The former included measuring read noise, full well capacity and dynamic range. The latter, which included measuring X-ray sensitivity, presampling modulation transfer function (pMTF), noise power spectrum (NPS) and the resulting detective quantum efficiency (DQE), was assessed under three beam qualities (28 kV, 50 kV (RQA3) and 70 kV (RQA5) using W/Al) all in accordance with the IEC standard. The detector features an in-pixel option for switching the full well capacity between two distinct modes, high full well (HFW) and low full well (LFW). Two structured CsI:Tl scintillators of different thickness (a thin one for high resolution and a thicker one for high light efficiency) were optically coupled to the sensor array to optimize the performance of the system for different medical applications. The electro-optical performance evaluation of the sensor results in relatively high read noise (∼360 e -), high full well capacity (∼1.5×10 6 e -) and wide dynamic range (∼73 dB) under HFW mode operation. When the LFW mode is used, the read noise is lower (∼165) at the expense of a reduced full well capacity (∼0.5×10 6 e -) and dynamic range (∼69 dB). The maximum DQE values at low frequencies (i.e. 0.5 lp/mm) are high for both HFW (0.69 for 28 kV, 0.71 for 50 kV and 0.75 for 70 kV) and LFW (0.69 for 28 kV and 0.7 for 50 kV) modes. The X-ray performance of the studied detector compares well to that of other mammography and general radiography systems, obtained under similar experimental conditions. This demonstrates the suitability of the detector for both mammography and general radiography, with the use of appropriate scintillators. The high DQE values obtained under low mammographic exposures (up to 0.65 for 22.3 μGy) matches the demand for high detectability in imaging of the dense breast. © 2012 Elsevier B.V.

PURPOSE: The x-ray performance evaluation of digital x-ray detectors is based on the calculation of the modulation transfer function (MTF), the noise power spectrum (NPS), and the resultant detective quantum efficiency (DQE). The flat images used for the extraction of the NPS should not contain any fixed pattern noise (FPN) to avoid contamination from nonstochastic processes. The "gold standard" method used for the reduction of the FPN (i.e., the different gain between pixels) in linear x-ray detectors is based on normalization with an average reference flat-field. However, the noise in the corrected image depends on the number of flat frames used for the average flat image. The aim of this study is to modify the standard gain correction algorithm to make it independent on the used reference flat frames. METHODS: Many publications suggest the use of 10-16 reference flat frames, while other studies use higher numbers (e.g., 48 frames) to reduce the propagated noise from the average flat image. This study quantifies experimentally the effect of the number of used reference flat frames on the NPS and DQE values and appropriately modifies the gain correction algorithm to compensate for this effect. RESULTS: It is shown that using the suggested gain correction algorithm a minimum number of reference flat frames (i.e., down to one frame) can be used to eliminate the FPN from the raw flat image. This saves computer memory and time during the x-ray performance evaluation. CONCLUSIONS: The authors show that the method presented in the study (a) leads to the maximum DQE value that one would have by using the conventional method and very large number of frames and (b) has been compared to an independent gain correction method based on the subtraction of flat-field images, leading to identical DQE values. They believe this provides robust validation of the proposed method.

PURPOSE: This paper proposes the further development of a resolution modification routine which is used to simulate the presampling modulation transfer function (pMTF) of digital x-ray detectors. METHODS: It suggests a method to reconstruct anisotropic two dimensional (2D) pMTF matrices from the experimentally measured horizontal and vertical 1D pMTFs. In this study, the horizontal dimension of the detector is 17.3 cm, while the vertical one is 24 cm. This matrix is multiplied with the 2D Fourier transform of the super-sampled ideal input image to simulate blurring. Then, the restored image is sampled to form the pixels of the digital image. The authors suggest convolution with the comb function instead of the rectangular function to avoid the correction with the sinc function required by the latter. It is demonstrated that this correction is avoided when the comb function is used. Moreover, this study suggests a way to effectively sample the images in the case when the ratio between the "analog" pitch of the super-sampled input image and the pixel pitch of the digital x-ray detector is a semi-integer. RESULTS: The validation of the simulation algorithm demonstrated that when the comb function was used the average absolute difference between the pMTF measured from the output images and the input ones was less than 1%, while this was of 13% when the rectangular function was used. When a sinc correction was applied in the latter case the difference decreased again to less than 1%. CONCLUSIONS: The developed modification routine provides the means to simulate the spatial resolution of digital x-ray detectors under a wider range of conditions.

Periodic noise is a common problem in many digital sensors, limiting their performance. The general aim of the research of which this work forms part is to use CMOS sensors in dual energy X-ray mammography. This imaging methodology requires the subtraction of two subsequent images, and it was observed that the resulting image is severely affected by this type of noise. Therefore, methods to reduce periodic noise are required as a preliminary step in the project. This study proposes a software solution to reduce periodic noise, in the form of three cross-shaped notch-reject frequency filters, namely brick wall, Gaussian and interpolation. The suggested filters have been applied during the optical characterisation of a CMOS active pixel sensor, named "Large Area Sensor" (LAS). The optical characterisation methods used were the "photon transfer" and "nonlinear compensation" methods. The former was used to calculate an input parameter for the latter method, which is considered sufficiently precise for a CMOS imager, because it takes into account its inherent nonlinearity. The above filters gave similar results using both the aforementioned methods. Using the latter method, it was found that the periodic noise limited the calculation of the performance parameters of the LAS. More specifically, it leads to an underestimation of the sensor's signal and noise gains by 7% and 13%, respectively. However, its biggest effect is the overestimation of the read noise by 29%. Finally, it underestimated the sensor's full well capacity and dynamic range by 9% and 4 dB, respectively. © 2010 Elsevier B.V.

Stitched, large area, complementary metal-oxide-semiconductor (CMOS), active pixel sensors (APS) show promises for X-ray imaging applications. In this paper we present an investigation of the effects of stitching on uniformity of sensor response for an experimental APS. The sensor, known as LAS (large area sensor), was made by reticular stitching onto a single silicon wafer of a 5×5 array of regions consisting of 270×270 pixels with 40 μm pixel pitch, to yield 1350×1350 pixels and an imaging area of 54×54 mm. Data acquired from two different sensors of the same type were filtered to remove spiking pixels and electromagnetic interference (EMI). The non-linear compensation (NLC) technique for CMOS sensor analysis was used to determine the variation in gain, read noise, full well capacity and dynamic range between stitched regions. Variations across stitched regions were analysed using profiles, analysis of pixel variations at stitch boundaries and using a measurement of non-uniformity within a stitched region. The results showed that non-uniformity variations were present, which increased with signal (1.5–3.5% at dark signal, rising to 3–8%). However, these were found to be smaller than variations caused by differences in readout electronics, particularly at low signal levels. The results suggest these variations should be correctable using standard calibration methods.

The Large Area Sensor (LAS) is a 1350 times1350 array of active pixels on a 40 Im pitch fabricated in a 0.35 im CMOS process. Stitching technology is employed to achieve an area of 5.4 cm times5.4 cm. The sensor includes 'regions of reset', whereby three different integration times can be set on the array to achieve a large imaging range for static scenes. Characterization of the noise performance included temporal and fixed pattern sources. LAS was found to have a read noise of 62 e-, a full well capacity of 61 times103 e- and a conversion gain of 5 e- per digital number (DN). The fixed pattern noise (FPN) was evaluated at half saturation; within a single stitched section of the array, column-to-column FPN was found to be 0.6%, while the pixel-to-pixel FPN was 3%. Both FPN sources were found to be gain related and could be corrected via flat fielding. Based on the results of characterization, LAS was coupled to a structured CsI:Tl scintillator and included in an X-ray diffraction system developed for the analysis of breast biopsy samples. Data acquired with plastic test objects agrees with that acquired by a previous prototype sensor. It is demonstrated that an imaging output range of 140 dB can be achieved using integration times of 0.1 ms to record the transmitted X-ray beam and 2.3 s to record the lower intensity scattered radiation.

LYSO:Ce and LuYAP:Ce are single crystal non-hygroscopic scintillators of high density, high light yield and short decay time, which have been successfully used in small animal PET imagers. In the present study, the luminescence emission properties of (Lu0.9, Y0.1)2SiO5:Ce (LYSO:Ce) and (Lu0.7, Y0.3)AIO3:Ce (LuYAP:Ce) crystals were investigated for use in X-ray medical imaging. Both crystals had dimensions of 2 times 2 times 8 mm3, with all surfaces polished. Evaluation was performed by determining the X-ray luminescence efficiency (XLE) (emitted light energy flux over incident X-ray energy flux) and the detector optical gain (DOG) (emitted light photons per incident x-ray photon) in a wide range of X-ray energies employed in mammography (22-49 kVp) and in general X-ray imaging (50-140 kVp). Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The emission spectrum under X-ray excitation was measured using an optical grating monochromator to determine the spectral compatibility to various optical photon detectors incorporated in medical imaging detectors. Optical characteristics such as transmission and absorption spectra were investigated in addition to the scintillation properties. The light emission performance of the two scintillation materials studied was found adequately high for X-ray imaging.

LSO:Ce, LYSO:Ce and GSO:Ce single-crystal scintillator light emission characteristics were studied in the low γ-ray energy range (99mTc source) used in nuclear medical imaging. The absolute luminescence efficiency and the optical emission spectrum of the three scintillators were measured, under γ-ray excitation using an integration sphere coupled to a photomultiplier and an optical spectrometer, respectively. Spectral compatibility of all scintillators to optical sensors was also estimated. The absolute luminescence efficiency of all crystals was found adequately high (8.7 μW m-2/μGy s-1 for GSO:Ce, 15.3 μW m-2/μGy s-1 for LYSO:Ce and 20.0 μW m-2/μGy s-1 for LSO:Ce). Their emission spectra were found compatible (57-94%) to currently employed optical photon detectors. © 2007 Elsevier B.V. All rights reserved.

Phosphor materials are used in medical imaging combined with radiographic film or other photodetectors. Cerium (Ce3+) -doped scintillators are of particular interest for medical imaging, because of their very fast response. YAP:Ce scintillator-based image detectors have already been evaluated in single-crystal form and under conditions of positron emission tomography and synchrotron or γ-ray irradiation. Furthermore, YAP:Ce phosphor has been evaluated in conjunction with radiographic films. The present work reports experimental and theoretical data concerning the light output absolute luminescence efficiency (AE) of the YAP:Ce screens under irradiation conditions employed in medical X-ray projection imaging (i.e., in diagnostic radiology). projection imaging (i.e., in diagnostic radiology). YAP:Ce phosphor screens with surface densities ranging between 53 and 110 mg/cm2 were prepared by sedimentation on fused silica substates in our laboratory. The resulted surface density of the screens was determined by dividing the phosphor mass deposited on the screen surface with the area of the surface. Additionally this work addresses the imaging performance of YAP:Ce by estimation of the detective quantum efficiency (DQE), i.e., the square of the signal to noise ratio transfer. Absolute efficiency was found to decrease with X-ray tube voltage for for YAP:Ce phosphor. The highest experimental efficiency was obtained for the 53.7 mg/cm2 and 88.0 mg/cm2 YAP:Ce screens. The highest DQE value was found for the 88.0 mg/cm2 screen irradiated at 60 kVp.

The purpose of the present work was to model a modern X-ray detection system and to investigate the effect of the scintillator material on the detector's output signal. The scintillators were used in the form of screens. The parameters investigated were the Modulation Transfer Function (MTF), the Detective Quantum Efficiency (DQE) and the Energy Absorption Efficiency (EAE). The results for some well-known scintillators (Y3Al5O12:Ce, Y2O3:Eu, ZnSCdS:Ag, Lu3Al5O7, CdWO4) are presented. Typical radiographic conditions were considered as input parameters. For simulation purposes, the intrinsic conversion efficiency (ηc), the total number of optical photons produced per incident X-ray (m0), the attenuation coefficients and other optical parameters of the scintillator materials, were taken as input data. The complete simulation procedure was performed in a specially designed Graphical User Interface (GUI). The results showed that the Y2O3:Eu scintillator presented similar behavior to that of ZnSCdS:Ag, exhibiting higher DQE at zero spatial frequencies. For higher frequencies, however, the DQE values of Lu3Al5O7 and CdWO4 prevailed. © 2006 Elsevier B.V. All rights reserved.

Wafer-scale CMOS active pixel sensors (APSs) have been developed recently for x-ray imaging applications. The small pixel pitch and low noise are very promising properties for medical imaging applications such as digital breast tomosynthesis (DBT). In this work, we evaluated experimentally and through modeling the imaging properties of a 50 μm pixel pitch CMOS APS x-ray detector named DynAMITe (Dynamic Range Adjustable for Medical Imaging Technology). A modified cascaded system model was developed for CMOS APS x-ray detectors by taking into account the device nonlinear signal and noise properties. The imaging properties such as modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) were extracted from both measurements and the nonlinear cascaded system analysis. The results show that the DynAMITe x-ray detector achieves a high spatial resolution of 10 mm(-1) and a DQE of around 0.5 at spatial frequencies

The aim of the present study was to evaluate the imaging performance of YAP:Ce powder scintillating screens under exposure conditions employed in diagnostic radiology (50-140 kV). Various screens were prepared in our laboratory from YAP: Ce powder (Phosphor Technology, Ltd.), with coating thickness ranging from 53 to 110 mg/cm2. The imaging performance of the screens was assessed by experimental determination of the modulation transfer function (MTF) and the noise transfer function (NTF). MTF was determined by the edge spread function (ESF) method while NTF was estimated by noise power spectrum (NPS) measurements after uniform screen irradiation. In addition, parameters related to overall image quality, such as the signal-to-noise ratio transfer (MTF/NTF), were estimated. MTF curves were affected by the beam hardening effects caused by the patient simulating 20 mm thick aluminum phantom. Under these conditions MTF values were found to increase with the mean X-ray photon energy. A similar effect was observed for NTF curves. Results were compared with data obtained on CsI:Tl scintillator. Taking into consideration the very fast response of YAP:Ce, these data may be of interest in designing X-ray imaging detectors. © 2006 Elsevier B.V. All rights reserved.

This work investigates the effect of the energy-weighting technique on the signal to noise ratio (SNR) response of X-ray imaging detectors. So far in the literature all scintillation-detector characteristics (detection efficiency, conversion efficiency, light-attenuation effects, etc) that degrade image quality have been ignored. A theoretical evaluation of the scintillator's SNR output was carried out. An algorithm was produced to describe the variation of the weighting factor, and SNR, with respect to the anode material (Mo or W), in a particular energy range (25-40 keV), typical for mammography, using two different phantoms. Results show that under non-ideal conditions the ratio of the weighted SNR to the original SNR appears to be increasing from values that are close to unity, and under specific conditions, can reach values up to 30. For the further investigation of this method, a more complex, simulated computed tomography breast imaging system was modeled and studied for various parameters such as breast software phantoms, scintillation materials and reconstruction filters. © 2006 Elsevier B.V. All rights reserved.

DynAMITe (Dynamic range Adjustable for Medical Imaging Technology) is a recently developed wafer scale (12.8 cm × 13.1 cm) X-ray detector consisting of a Complementary Metal-Oxide-Semiconductor (CMOS) Active Pixel Sensor (APS) coupled to a 150 µm Thallium-activated Cesium Iodide (CsI:Tl) screen. It consists of two cameras: a Sub-Pixel (SP) camera with 50 µm pitch and a Pixel (P) camera with 100 µm pixel pitch. Each camera can support multiple Regions of Interest (ROIs). The combination of different settings (reset voltages, frame rates and destructive or non-destructive readout) for the two cameras allows DynAMITe to achieve a high dynamic range in comparison to other CMOS devices. Mean-Variance analysis was used in this paper to characterise the detector using different reset voltages. A commercial X-ray source was used instead of optical light to perform the Mean-Variance analysis. Overall gain instead of conversion gain can be measured using this method. However, by using a cascaded linear system model, the conversion gain of the detector using different reset voltage can also be estimated. The experimental results have shown that by combining the two cameras at different reset voltages, the dynamic range can be increased to 93.7 ± 0.2 dB from 66.7 ± 0.4 dB (SP) and 66.8±0.1 (P).

The SYRMEP (SYnchrotron Radiation for MEdical Physics) beamline at Elettra is performing the first mammography study on human patients using free-space propagation phase contrast imaging. The stricter spatial resolution requirements of this method currently force the use of conventional films or specialized computed radiography (CR) systems. This also prevents the implementation of three-dimensional (3D) approaches. This paper explores the use of an X-ray detector based on complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) technology as a possible alternative, for acquisitions both in planar and tomosynthesis geometry. Results indicate higher quality of the images acquired with the synchrotron set-up in both geometries. This improvement can be partly ascribed to the use of parallel, collimated and monochromatic synchrotron radiation (resulting in scatter rejection, no penumbra-induced blurring and optimized X-ray energy), and partly to phase contrast effects. Even though the pixel size of the used detector is still too large – and thus suboptimal – for free-space propagation phase contrast imaging, a degree of phase-induced edge enhancement can clearly be observed in the images.

MI-3 is a consortium of 11 universities and research laboratories whose mission is to develop complementary metal-oxide semiconductor (CMOS) active pixel sensors (APS) and to apply these sensors to a range of imaging challenges. A range of sensors has been developed: On-Pixel Intelligent CMOS (OPIC)—designed for in-pixel intelligence; FPN—designed to develop novel techniques for reducing fixed pattern noise; HDR—designed to develop novel techniques for increasing dynamic range; Vanilla/PEAPS—with digital and analogue modes and regions of interest, which has also been back-thinned; Large Area Sensor (LAS)—a novel, stitched LAS; and eLeNA—which develops a range of low noise pixels. Applications being developed include autoradiography, a gamma camera system, radiotherapy verification, tissue diffraction imaging, X-ray phase-contrast imaging, DNA sequencing and electron microscopy.

Medical x-ray imaging is used to provide sufficient information to detect specific aspects of human body structure or function. Hence, the quality of a medical x-ray image (either two- or three-dimensional depending on the imaging modality) needs to be sufficient enough to provide the required information for a given diagnostic task. Image quality in all x-ray-based modalities is mainly affected by spatial resolution, contrast, and noise. The current chapter describes these three primary physical parameters of image quality and their relationship, as well as derived parameters obtained from them. In particular, it explains fundamental objective and subjective image quality metrics such as modulation transfer function, noise power spectrum, contrast, contrast transfer function, signal-to-noise ratio, contrast-to-noise ratio, and low-contrast detectability.

Digital detectors based on complementary metal-oxide-semiconductors (CMOS) active pixel sensor (APS) technology have been introduced recently in many scientific applications. This work is focused on the X-ray performance evaluation of a novel CMOS APS detector in low energy medical imaging applications using monochromatic synchrotron radiation (i.e., 17-35 keV), which also allows studying how the performance varies with energy. The CMOS sensor was coupled to a Thallium-activated structured cesium iodide (CsI:Tl) scintillator and the detector's X-ray performance evaluation was carried out in terms of sensitivity, presampling modulation transfer function (pMTF), normalized noise power spectrum (NNPS) and the resulting detective quantum efficiency (DQE). A Monte Carlo simulation was used to validate the experimentally measured low frequency DQE. Finally, the effect of iodine's secondary generated K-fluorescence X-rays on pMTF and DQE results was evaluated. Good agreement (within 5%) was observed between the Monte Carlo and experimentally measured low frequency DQE results. A CMOS APS detector was characterized for the first time over a wide range of low energies covering the mammographic spectra. The detector's performance is limited mainly by the detectability of the scintillator. Finally, we show that the current data could be used to calculate the detector's pMTF, NNPS and DQE for any mammographic spectral shape within the investigated energies.

X-ray diffraction studies are used to identify specific materials. Several laboratory-based x-ray diffraction studies were made for breast cancer diagnosis. Ideally a large area, low noise, linear and wide dynamic range digital x-ray detector is required to perform x-ray diffraction measurements. Recently, digital detectors based on Complementary Metal-Oxide- Semiconductor (CMOS) Active Pixel Sensor (APS) technology have been used in x-ray diffraction studies. Two APS detectors, namely Vanilla and Large Area Sensor (LAS), were developed by the Multidimensional Integrated Intelligent Imaging (MI-3) consortium to cover a range of scientific applications including x-ray diffraction. The MI-3 Plus consortium developed a novel large area APS, named as Dynamically Adjustable Medical Imaging Technology (DynAMITe), to combine the key characteristics of Vanilla and LAS with a number of extra features. The active area (12.8 × 13.1 cm 2) of DynaMITe offers the ability of angle dispersive x-ray diffraction (ADXRD). The current study demonstrates the feasibility of using DynaMITe for breast cancer diagnosis by identifying six breast-equivalent plastics. Further work will be done to optimize the system in order to perform ADXRD for identification of suspicious areas of breast tissue following a conventional mammogram taken with the same sensor. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

The most important factors that affect the image quality are contrast, spatial resolution and noise. These factors and their relationship are quantitatively described by the Contrast-to-Noise Ratio (CNR), Signal-to-Noise Ratio (SNR), Modulation Transfer Function (MTF), Noise Power Spectrum (NPS) and Detective Quantum Efficiency (DQE) parameters. The combination of SNR, MTF and NPS determines the DQE, which represents the ability to visualize object details of a certain size and contrast at a given dose. In this study the performance of a novel large area Complementary Metal-Oxide-Semiconductor (CMOS) Active Pixel Sensor (APS) X-ray detector, called DynAMITe (Dynamic range Adjustable for Medical Imaging Technology), was investigated and compared to other three digital mammography systems (namely a) Large Area Sensor (LAS), b) Hamamatsu C9732DK, and c) Anrad SMAM), in terms of physical characteristics and evaluation of the image quality. DynAMITe detector consists of two geometrically superimposed grids: a) 2560 × 2624 pixels at 50 μm pitch, named Sub-Pixels (SP camera) and b) 1280 × 1312 pixels at 100 μm pitch, named Pixels (P camera). The X-ray performance evaluation of DynAMITe SP detector demonstrated high DQE results (0.58 to 0.64 at 0.5 lp/mm). Image simulation based on the X-ray performance of the detectors was used to predict and compare the mammographic image quality using ideal software phantoms: a) one representing two three dimensional (3-D) breasts of various thickness and glandularity to estimate the CNR between simulated microcalcifications and the background, and b) the CDMAM 3.4 test tool for a contrast-detail analysis of small thickness and low contrast objects. The results show that DynAMITe SP detector results in high CNR and contrast-detail performance. © 2012 IEEE.

The x-ray performance evaluation of digital x-ray detectors is based on the calculation of the modulation transfer function (MTF), the noise power spectrum (NPS), and the resultant detective quantum efficiency (DQE). The flat images used for the extraction of the NPS should not contain any fixed pattern noise (FPN) to avoid contamination from nonstochastic processes. The "gold standard" method used for the reduction of the FPN (i.e., the different gain between pixels) in linear x-ray detectors is based on normalization with an average reference flat-field. However, the noise in the corrected image depends on the number of flat frames used for the average flat image. The aim of this study is to modify the standard gain correction algorithm to make it independent on the used reference flat frames.Many publications suggest the use of 10-16 reference flat frames, while other studies use higher numbers (e.g., 48 frames) to reduce the propagated noise from the average flat image. This study quantifies experimentally the effect of the number of used reference flat frames on the NPS and DQE values and appropriately modifies the gain correction algorithm to compensate for this effect.It is shown that using the suggested gain correction algorithm a minimum number of reference flat frames (i.e., down to one frame) can be used to eliminate the FPN from the raw flat image. This saves computer memory and time during the x-ray performance evaluation.The authors show that the method presented in the study (a) leads to the maximum DQE value that one would have by using the conventional method and very large number of frames and (b) has been compared to an independent gain correction method based on the subtraction of flat-field images, leading to identical DQE values. They believe this provides robust validation of the proposed method.

This paper proposes the further development of a resolution modification routine which is used to simulate the presampling modulation transfer function (pMTF) of digital x-ray detectors.It suggests a method to reconstruct anisotropic two dimensional (2D) pMTF matrices from the experimentally measured horizontal and vertical 1D pMTFs. In this study, the horizontal dimension of the detector is 17.3 cm, while the vertical one is 24 cm. This matrix is multiplied with the 2D Fourier transform of the super-sampled ideal input image to simulate blurring. Then, the restored image is sampled to form the pixels of the digital image. The authors suggest convolution with the comb function instead of the rectangular function to avoid the correction with the sinc function required by the latter. It is demonstrated that this correction is avoided when the comb function is used. Moreover, this study suggests a way to effectively sample the images in the case when the ratio between the "analog" pitch of the super-sampled input image and the pixel pitch of the digital x-ray detector is a semi-integer.The validation of the simulation algorithm demonstrated that when the comb function was used the average absolute difference between the pMTF measured from the output images and the input ones was less than 1%, while this was of 13% when the rectangular function was used. When a sinc correction was applied in the latter case the difference decreased again to less than 1%.The developed modification routine provides the means to simulate the spatial resolution of digital x-ray detectors under a wider range of conditions.

Breast tomosynthesis is a promising technology for breast imaging Although existing tomosynthesis systems using detector technology developed for FFDM and uniform acquisition parameters have demonstrated the potential to improve the effectiveness of breast screening, the full potential of tomosynthesis is yet to be realised The effectiveness of tomosynthesis depends on multiple factors, including acquisition geometry, number of projections, reconstruction software and X-ray detector performance In this study, the authors investigated the use of a specially designed 29 cm x 23 cm CMOS flat panel X-ray detector with a novel Active Pixel Sensor with high spatial resolution, high speed read-out, low noise, negligible image lag and a unique ability to reconfigure imaging parameters such as resolution and gain during an acquisition Advanced tomosynthesis acquisition methods were used with the new detector including non-uniform spacing of projection views This combination of optimised X-ray detector and optimised acquisition methods provides enhanced imaging performance.

The aim of this study was to estimate the influence of K-characteristic radiation on the performance of x-ray scintillating screens containing two heavy elements by Monte Carlo methods. K-characteristic radiation is produced within materials of at least one heavy (high atomic number) element. This radiation may result either in spatial resolution degradation or in emission efficiency decrease. The scintillators studied were the following: LYSO (Lu1.8Y0.2SiO5 and LuYSiO5), CsI and YTaO4. All the aforementioned scintillators have two heavy elements, thus the K-characteristic radiation of the high-Z element can produce additional K-characteristic photons on the low-Z element, resulting in further degradation. Scintillator performance was described in terms of the: (a) Probability of generation and reabsorption of a K-characteristic photon (PKR) and (b) Spatial distribution of K-characteristic radiation within the scintillator material. A custom validated Monte Carlo model was used, in order to simulate the transport of K-characteristic radiation within the above scintillator materials. Results showed that, depending on screen thickness (20-100 mg/cm2) and incident photon energy (20-80 keV) the scintillator's emission efficiency may be significantly reduced. © 2009 IOP Publishing Ltd and SISSA.

Digital X-ray detectors based on Complementary Metal-Oxide- Semiconductor (CMOS) Active Pixel Sensor (APS) technology have been introduced in the early 2000s in medical imaging applications. In a previous study the X-ray performance (i.e. presampling Modulation Transfer Function (pMTF), Normalized Noise Power Spectrum (NNPS), Signal-to-Noise Ratio (SNR) and Detective Quantum Efficiency (DQE)) of the Dexela 2923MAM CMOS APS X-ray detector was evaluated within the mammographic energy range using monochromatic synchrotron radiation (i.e. 17-35 keV). In this study image simulation was used to predict how the mammographic beam quality affects image quality. In particular, the experimentally measured monochromatic pMTF, NNPS and SNR parameters were combined with various mammographic spectral shapes (i.e. Molybdenum/Molybdenum (Mo/Mo), Rhodium/Rhodium (Rh/Rh), Tungsten/Aluminium (W/Al) and Tungsten/Rhodium (W/Rh) anode/filtration combinations at 28 kV). The image quality was measured in terms of Contrast-to-Noise Ratio (CNR) using a synthetic breast phantom (4 cm thick with 50% glandularity). The results can be used to optimize the imaging conditions in order to minimize patient's Mean Glandular Dose (MGD).

There is a suggestion of a new approach to mammography whereby following a conventional mammogram, the radiologist could interrogate suspicious regions using X-ray diffraction whilst the patient is still present and to establish the true extent of disease. A starting point for this work is to quantify the minimum detectable amount of breast cancer within a realistic thickness phantom. Perspex has a similar diffraction pattern to healthy breast tissue whilst water is similar to breast tumour, hence these two materials are used as tissue equivalent test objects for X-ray diffraction measurements. The preliminary results show linear agreement between the ratio of Perspex to water and the ratio of the diffraction peak intensities at 0.7 nm-1 and 1.5 nm-1. The minimum detectable limit for a component of the two 'tissue' mix was found to be 4.1%. This suggests that X-ray diffraction can be used to quantify tissue like mixtures down to the 4.1% / 95.9% mix level and hence has a strong potential for delineating the extent of infiltration disease.

Medical imaging is a long established part of patient management in the treatment of disease. However, in most cases it only provides anatomical detail and does not provide any form of tissue characterisation. This is particularly true for X-ray imaging. Recent studies on tissue diffraction have shown that true molecular signatures can be derived for different tissue types. Breast cancer samples and liver tissue have been studied. It has been shown that diffraction profiles can be traced away from the primary tumour in excised breast tissue samples and that potentially 3mm fat nodules in liver tissue can be identified in patients at acceptable doses.

The current study uses a novel large area (12.8 cm × 13.1 cm) complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) X-ray detector, named Dynamic range Adjustable for Medical Imaging Technology (DynAMITe), for breast cancer diagnosis. The detector consists of two geometrically superimposed grids: a) 2560 × 2624 fine-pitch grid of pixels (50 μm pitch), named Sub-Pixels (SP camera), for low intrinsic noise and high spatial resolution and b) 1280 × 1312 large-pitch grid of pixels (100 μm pitch), named Pixels (P camera), for high dynamic range. X-ray performance characterization measurements show that the detective quantum efficiency (DQE) of the SP camera is in the range 0.7–0.75 at low spatial frequencies using a tungsten (W) anode X-ray source at 28 kV. Hence, the detector is suitable for mammography. Furthermore, we used the SP camera to combine mammograms with angle dispersive X-ray diffraction (ADXRD) measurements in order to apply the X-ray biopsy concept in one examination. The results show that ADXRD technique indicates the presence of cancer in suspicious areas on the mammogram. Hence, it could be used to determine the region affected by cancer and assist in planning surgery. This study is the proof of concept that mammography and ADXRD can be combined in one examination.

In many biomedical imaging applications Flat Panel Imagers (FPIs) are currently the most common option. However, FPIs possess several key drawbacks such as large pixels, high noise, low frame rates, and excessive image artefacts. Recently Active Pixel Sensors (APS) have gained popularity overcoming such issues and are now scalable up to wafer size by appropriate reticule stitching. Detectors for biomedical imaging applications require high spatial resolution, low noise and high dynamic range. These figures of merit are related to pixel size and as the pixel size is fixed at the time of the design, spatial resolution, noise and dynamic range cannot be further optimized. The authors report on a new rad-hard monolithic APS, named DynAMITe (Dynamic range Adjustable for Medical Imaging Technology), developed by the UK MI-3 Plus consortium. This large area detector (12.8 cm × 12.8 cm) is based on the use of two different diode geometries within the same pixel array with different size pixels (50 μm and 100 μm). Hence the resulting device can possess two inherently different resolutions each with different noise and saturation performance. The small and the large pixel cameras can be reset at different voltages, resulting in different depletion widths. The larger depletion width for the small pixels allows the initial generated photo-charge to be promptly collected, which ensures an intrinsically lower noise and higher spatial resolution. After these pixels reach near saturation, the larger pixels start collecting so offering a higher dynamic range whereas the higher noise floor is not important as at higher signal levels performance is governed by the Poisson noise of the incident radiation beam. The overall architecture and detailed characterization of DynAMITe will be presented in this paper.

We present an x-ray phase contrast imaging method based on coded apertures sensitive to phase effects in two directions simultaneously. To date, this is the only non-free-space propagation approach with this capability. Whereas the use of free-space propagation methods is limited to synchrotron radiation or microfocal x-ray sources, which impose severe limits in terms of practical applicability, coded-aperture based methods have been shown to provide synchrotronlike phase contrast enhancements with conventional x-ray sources. A two-directional sensitive method working with conventional sources could create a breakthrough in medical imaging, where two-directional sensitivity is often a mandatory requirement.