Detective Quantum Efficiency Values Research Articles

Light emission efficiency and imaging properties of YAP:Ce granular phosphor screens

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.

TH-C-L100F-04: A Fast Method for Measurement of Modulation Transfer Function (MTF) and Detective Quantum Efficiency (DQE) in Presence of Phantom Scatter in Image Guided Radiotherapy

Purpose: To present an improved bar‐pattern method that allows for fast measurement of modulation transfer function(MTF) and detective quantum efficiency (DQE) in conventional no‐phantom scatter and phantom scatter conditions to characterize spatial resolution of both 2D and 3D imagingsystems for image guided radiation therapy(IGRT) with both kilovoltage and megavoltage x‐ray sources. Method and Materials: X‐ray imaging requires MTFmeasurements under phantom scatter free condition for DQE measurements. While slit/edge MTFmeasurements are laborious, the bar‐pattern method allows for a simplified approach for discrete MTFmeasurement although its implementation for megavoltage x‐ray imaging may lead to errors due to inadequate normalization. We introduce a new bar‐pattern method based on based on improved normalization techniques that provide extremely accurate MTFmeasurement. It is well suited for MTF and DQE measurements under phantom scatter conditions, which more closely mimic patient imaging and cannot be obtained using slit/edge methods. Comparisons of the bar‐pattern method are presented with slit/edge MTFs. We present a methodology to image bar‐patterns placed on a multi‐axial rotation jig that allows measurement of spatial resolution of not only 2D detectors(EPIDs) but also volumetric imagingsystems, i.e. CBCT and MVCT. Results: Comparisons of MTFmeasured with custom tungsten bar‐patterns show good statistical agreement with measurements with the slit and edge for two clinical EPIDsystems. Conclusions: The proposed bar‐pattern method provides fast accurate measurement of discrete MTF and DQE values, and is much simpler and faster to use than traditional slit and edge methods. This method also allows for MTF and DQE measurements under scatter conditions that more closely mimic patient imaging conditions. It can be used for EPID and CBCT/MVCT imaging quality assurance within minutes of imaging time and is therefore well suited for clinical measurements.

Performance of a high fill factor, indirect detection prototype flat‐panel imager for mammography

Empirical and theoretical investigations of the performance of a small-area, high-spatial-resolution, active matrix flat-panel imager, operated under mammographic conditions, is reported. The imager is based on an indirect detection array incorporating a continuous photodiode design, as opposed to the discrete photodiode design employed in conventional flat-panel imagers. Continuous photodiodes offer the prospect of higher fill factors, particularly for arrays with pixel pitches below approximately 100 microm. The array has a pixel-to-pixel pitch of 75 microm and a pixel format of 512 x 512, resulting in an active area of approximately 3.8 x 3.8 cm2. The array was coupled to two commercially available, structured CsI: Tl scintillators of approximately 150 microm thickness: one optimized for high light output (FOS-HL) and the other for high spatial resolution (FOS-HR), resulting in a pair of imager configurations. Measurements of sensitivity, modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE) were performed with a 26 kVp mammography beam at exposures ranging from approximately 0.5 to approximately 19 mR. MTF results from both CsI:Tl scintillators show that the array demonstrates good spatial resolution, indicating effective isolation between adjacent pixels. The effect of additive noise of the system on DQE was observed to be significantly higher for the FOS-HR scintillator compared to the FOS-HL scintillator due to lower sensitivity of the former. For the FOS-HL scintillator, DQE performance was generally high at high exposures, limited by the x-ray quantum efficiency, Swank factor and the MTF of the scintillators. For both scintillators, the DQE performance degrades at lower exposures due to the relatively large contribution of additive noise. Theoretical calculations based on a cascaded systems model were found to be in general agreement with the empirically determined NPS and DQE values. Finally, such calculations were used to predict potential DQE performance for hypothetical 50 microm pixel pitch imagers, employing similar continuous photodiode design and realistic inputs derived from the empirical measurements.

Investigation of the effect of the scintillator material on the overall X-ray detection system performance by application of analytical models

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 (Y 3Al 5O 12:Ce, Y 2O 3:Eu, ZnSCdS:Ag, Lu 3Al 5O 7, CdWO 4) are presented. Typical radiographic conditions were considered as input parameters. For simulation purposes, the intrinsic conversion efficiency ( η c), the total number of optical photons produced per incident X-ray ( m 0), 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 Y 2O 3: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 Lu 3Al 5O 7 and CdWO 4 prevailed.

Evaluation of imaging performance of a taper optics CCD `FReLoN' camera designed for medical imaging

The purpose of this work was to assess the imaging performance of an indirect conversion detector (taper optics CCD; FReLoN' camera) in terms of the modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE). Measurements were made with a synchrotron radiation laminar beam at various monochromatic energies in the 20-51.5 keV range for a gadolinium-based fluorescent screen varying in thickness; data acquisition and analysis were made by adapting to this beam geometry protocols used for conventional cone beams. The pre-sampled MTFs of the systems were measured using an edge method. The NNPS of the systems were determined for a range of exposure levels by two-dimensional Fourier analysis of uniformly exposed radiographs. The DQEs were assessed from the measured MTF, NNPS, exposure and incoming number of photons. The MTF, for a given screen, was found to be almost energy independent and, for a given energy, higher for the thinnest screen. At 33 keV and for the 40 (100) microm screen, at 10% the MTF is 9.2 (8.6) line-pairs mm(-1). The NNPS was found to be different in the two analyzed directions in relation to frequency. Highest DQE values were found for the combination 100 microm and 25 keV (0.5); it was still equal to 0.4 at 51.5 keV (above the gadolinium K-edge). The DQE is limited by the phosphor screen conversion yield and by the CCD efficiency. At the end of the manuscript the results of the FReLoN characterization and those from a selected number of detectors presented in the literature are compared.

Performance evaluation of a computed radiography imaging device using a typical “front side” and novel “dual side” readout storage phosphors

The Fourier-space modulation transfer function (MTF), normalized noise power spectrum (nNPS), and detective quantum efficiency (DQE) of a computed radiography (CR) x-ray imaging device were measured. Two different storage phosphor screens were used in conjunction with a single CR reader (Fuji, Clearview, CS). One of the storage phosphor plates (ST-BD) had a clear backing material which allowed "dual side read" of the latent image from both the "front" and "back" sides of the phosphor. The other phosphor plate had a light occluding backing material, limiting the readout to front side only (ST-VI). The standard RQA-5 beam quality was used. The MTF was measured using a 1 mm thick tungsten edge device. Compared to the ST-55BD phosphor, the ST-VI phosphor was found to have modestly higher MTF at all spatial frequencies. The nNPS(f) and DQE(f) were measured for nominal incident exposure levels ranging from 0.1 to 10 mR. The dual side read phosphor demonstrated superior DQE, especially at low spatial frequencies. At the frequency 0.5 cycles/mm, the DQE values for the 1 mR exposure were 0.36 and 0.21 for the ST-55BD and ST-VI phosphor plates, respectively. The differences between the spatial-frequency dependent DQE of the two plates can be attributed to the increased signal collection efficiency of the dual side read plates and differences in storage phosphor structure noise.

SU-FF-T-235: Detective Quantum Efficiency Analysis of An Electronic Portal Imaging Device

Purpose: To evaluate the portal image quality of an Elekta iViewGT system based on detective quantum efficiency (DQE) measurements.Method and Materials: An amorphous silicon based imager was irradiated with 6MV and 18MV beams of an Elekta Precise linear accelerator. The detector panel size is 1024 × 1024 with 0.4 mm pixel size. The source to panel distance is 150 cm. The presampling modulation transfer function(MTF) was computed from angulated slit images. Flat field images were acquired at 2 MUs and 10 MUs for the noise power spectra (NPS) measurements. For the calculation of the DQE, published dose‐to‐fluence conversion factors were used, namely 1.19×107 photons/mm2 per cGy for 6MV and 3.23×106 photons/mm2 per cGy for 18 MV. [Med. Phys. 25, 689–702 (1998)] Results: 1. The resolution (MTF) for the 6MV beam (f1/2=0.25 lp/mm) is comparable to a proto‐type flat‐panel array [Med. Phys. 31, 985–996 (2004)]. The MTF declines for the 18 MV beam (f1/2=0.16 lp/mm); 2. The NPS are nearly flat up to the Nyquist frequency for both energies. Fixed pattern noises are present at several frequencies and become less apparent for high MUs due to frame‐averaging effect; 3. The DQEs are approximately 1–2% at lower frequencies for both beams. The DQE values are similar for low and high MUs, and fall off rapidly due to decreased MTF at higher frequencies. Conclusion: The performance of a clinical portal imagingsystem has been characterized in terms of MTF, NPS and DQE. Our data demonstrate that this imager is X‐ray quantum‐limited. The DQE evaluation procedures are being further tested for accuracy, efficiency and robustness. The clinical impact of the MTF, DQE and fixed pattern noise will be investigated further.

Measurement of the detective quantum efficiency (DQE) of digital X-ray detectors according to the novel standard IEC 62220-1

A mobile measurement facility which complies with IEC 62220-1 has been set up to determine the detective quantum efficiency (DQE) of digital X-ray detector systems. Exemplary measurements were performed for two similar CR detector systems, a CsI-based indirect detector and an Se-based direct detector. The standardised radiation quality RQA 5 was applied for measurement and for three of these systems RQA 9 was also applied. A pronounced dependence of DQE on radiation quality was observed for the direct detector, where the DQEs for RQA 5 and RQA 9 differ by a factor of approximately 2. The uncertainty (95% confidence interval) associated with the measured DQE values is within 0.01 and 0.04 depending on, for example, the spatial frequency. Thus, it has been demonstrated that the DQE can be measured accurately and reliably with the accuracy required by the international standard IEC 62220-1. It is now possible to objectively measure and compare DQE values of digital X-ray detector systems.

Physical characteristics of a low‐dose gas microstrip detector for orthopedic x‐ray imaging

A new scanning slit gas detector dedicated to orthopedic x-ray imaging is presented and evaluated in terms of its fundamental imaging characteristics. The system is based on the micromesh gaseous structure detector and achieves primary signal amplification through electronic avalanche in the gas. This feature, together with high quantum detection efficiency and fan-beam geometry, allows for imaging at low radiation levels. The system is composed of 1764 channels spanning a width of 44.8 cm and is capable of imaging an entire patient at speeds of up to 15 cm/s. The resolution was found to be anisotropic and significantly affected by the beam quality in the horizontal direction, but otherwise sufficient for orthopedic studies. As a consequence of line-by-line acquisition, the images contain some ripple components due to mechanical vibrations combined with variations in the x-ray tube output power. The reported detective quantum efficiency (DQE) values are relatively low (0.14 to 0.20 at 0.5 mm(-1)) as a consequence of a suboptimal collimation geometry. The DQE values were found to be unaffected by the exposure down to 7 microGy, suggesting that the system is quantum limited even for low radiation levels. A system composed of two orthogonal detectors is already in use and can produce dual-view full body scans at low doses. This device could contribute to reduce the risk of radiation induced cancer in sensitive clientele undergoing intensive x-ray procedures, like young scoliotic women.

Segmented phosphors: MEMS-based high quantum efficiency detectors for megavoltage x-ray imaging

Current electronic portal imaging devices (EPIDs) based on active matrix flat panel imager (AMFPI) technology use a metal plate+phosphor screen combination for x-ray conversion. As a result, these devices face a severe trade-off between x-ray quantum efficiency (QE) and spatial resolution, thus, significantly limiting their imaging performance. In this work, we present a novel detector design for indirect detection-based AMFPI EPIDs that aims to circumvent this trade-off. The detectors were developed using micro-electro-mechanical system (MEMS)-based fabrication techniques and consist of a grid of up to approximately 2 mm tall, optically isolated cells of a photoresist material, SU-8. The cells are dimensionally matched to the pixels of the AMFPI array, and packed with a scintillating phosphor. In this paper, various design considerations for such detectors are examined. An empirical evaluation of three small-area (approximately 7 x 7 cm2) prototype detectors is performed in order to study the effects of two design parameters--cell height and phosphor packing density, both of which are important determinants of the imaging performance. Measurements of the x-ray sensitivity, modulation transfer function (MTF) and noise power spectrum (NPS) were performed under radiotherapy conditions (6 MV), and the detective quantum efficiency (DQE) was determined for each prototype SU-8 detector. In addition, theoretical calculations using Monte Carlo simulations were performed to determine the QE of each detector, as well as the inherent spatial resolution due to the spread of absorbed energy. The results of the present studies were compared with corresponding measurements published in an earlier study using a Lanex Fast-B phosphor screen coupled to an indirect detection array of the same design. The SU-8 detectors exhibit up to 3 times higher QE, while achieving spatial resolution comparable or superior to Lanex Fast-B. However, the DQE performance of these early prototypes is significantly lower than expected due to high levels of optical Swank noise. Consequently, the SU-8 detectors presently exhibit DQE values comparable to Lanex Fast-B at zero spatial frequency and significantly lower than Fast-B at higher frequencies. Finally, strategies for reducing Swank noise are discussed and theoretical calculations, based on the cascaded systems model, are presented in order to estimate the performance improvement that can be achieved through such noise reduction.

Cassette-based digital mammography.

Over the past several years, digital mammography systems have been installed clinically across North America in small but growing numbers. A photostimulable phosphor-based full-field digital mammography image was evaluated in this investigation. Commonly known as computed radiography (CR), its use closely mimics the screen-film mammography paradigm. System performance using modulation transfer function (MTF) and detective quantum efficiency (DQE) metrics show MTF(2.5 mm(-1)) = 0.5, DQE(2.5 mm(-1)) = 0.3, and MTF(5.0 mm(-1)) = 0.2, DQE(5.0 mm(-1)) = 0.05, for a 26 kVp beam, 0.03 mm molybdenum tube filtration, 4.5 cm tissue attenuation, and 15 mR incident exposure to the detector. Slightly higher DQE values were measured at 32 kVp with 0.025 mm rhodium tube filtration. CR mammography advantages include the ability to use existing mammography machines, where multiple rooms can be converted to "digital" operation, which allows overall cost savings compared to integrated digital mammography systems. Chief disadvantages include the labor-intensive handling of the cassettes prior to and after the imaging exam, lack of a direct interface to the x-ray system for recording technique parameters, and relatively slow processing time. Clinical experience in an IRB-approved research trial has suggested that digital mammography with photostimulable storage phosphors and a dedicated CR reader is a viable alternative to conventional screen-film mammography.

Slot scintillation detector modeling for digital radiography

A scanning slot X-ray digital detector operating at relatively high photon energies, mainly 70 kVp for dental radiography, has been designed and modeled. In this energy range, further improvements are required in order to attain optimal image quality for the lowest achievable mean absorbed dose to the patient. This paper concerns the investigation of the physics of imaging using a structured screen composed of a scintillating fiber optic (LKH-6/CCD system) and its performance by Monte Carlo (MC) simulation. The A s factor, X-ray cross-talk, quantum noise effect on resultant values of Detective Quantum Efficiency (DQE) and Modulation Transfer Function (MTF) are discussed. The results of this paper should serve as a useful guide in the engineering design of future structured screen/CCD based systems.

Effects of x-ray spectra on the DQE of a computed radiography system

The effect of incident x-ray beam quality on the measured detective quantum efficiency (DQE) of a computed radiography system was investigated. The incident x-ray beams used had peak tube potentials of 70, 95, and 120 kVp, were filtered with various thicknesses of a "patient equivalent phantom" (PEP), aluminum, and copper, and provided a consistent exposure to the storage phosphor. For each peak tube potential and filter combination, the one-dimensional modulation transfer function and noise power spectrum were measured and the square of the incident signal-to-noise ratio was estimated. The spatial frequency dependent DQE was calculated from these data. The DQE was integrated to provide an overall estimate of the efficiency and frequency response of the computed radiography system for the various x-ray beams. There was found to be a wide range of integral DQE (IDQE) values for the peak tube potential and filter combinations used. For example, the IDQE ranged from 3.0 to 0.9 mm(-2) using the peak tube potential and filter combinations 70 kVp with 5.1 cm PEP and 120 kVp with 30.3 cm PEP, respectively. Finally, peak tube potential and filter combinations 70 kVp with 10.2 cm PEP and 120 kVp with 20.2 cm PEP were chosen as standard x-ray beams that will be used at our facility to measure the DQE of digital radiographic imaging systems for evaluation and acceptance testing.

The feasibility of boron containing phosphors in thermal neutron image plates, in particular the systems M 2B 5O 9X : Eu 2+ (M=Ca, Sr, Ba; X=Cl, Br). Part I: simulation of the energy deposition process

A thermal neutron image plate (IP) consisting of a mixture of BaFBr : Eu 2+ and Gd 2O 3 (IP-Gd) is compared with an alternative plate consisting of M 2 B 5 O 9 X : Eu 2+ ( M= Ca, Sr, Ba; X= Cl,Br) (IP-B). In particular, the Detective Quantum Efficiency (DQE) of both plates is considered. Monte Carlo simulations of the energy deposition process after the absorption of a neutron show that the variance of the energy deposition in the IP-Gd affects the DQE to a large extent. Because of this, the average number of photoelectrons detected per incident neutron, 〈 N phe〉, has to be only 1.3 for the haloborates to obtain an IP-B with a higher DQE value than the maximum possible with a IP-Gd.

Estimation of the information content of medical images produced by scintillators interacting with diagnostic X-ray beams

The information content of medical images produced by X-ray beam–scintillator interaction was assessed by a method involving detective quantum efficiency (DQE) and information capacity determination. The method was based on emitted light fluence and modulation transfer function (MTF) measurements and it was employed to evaluate the performance of (Gd 0.5La 0.5) 2O 2S:Tb scintillator, which was used in the form of laboratory prepared scintillator screens with coating weights ranging between 20 and 139 mg/cm 2. Screens were excited to luminescence using 80 and 120 kVp X-ray beams. The 83 mg/cm 2 scintillator layer exhibited the highest DQE at zero spatial frequency. Thick scintillator layers displayed higher DQE values in the low frequency range while at high frequencies the DQE of thin scintillators were higher. Additionally, thin scintillator layers showed slow DQE variation with increasing frequency. The information capacity of scintillator layers decreased with increasing coating weight indicating that the information content of images produced by thin scintillating layers is higher and more evenly distributed with respect to spatial frequencies.

An experimental method for the determination of spatial-frequency-dependent detective quantum efficiency (DQE) of scintillators used in X-ray imaging detectors

The spatial-frequency-dependent detective quantum efficiency (DQE) of imaging scintillators was studied independently of the optical detector (film, photocathode, or photodiode) employed in medical imaging devices. A method was developed to experimentally determine the scintillator DQE in terms of its luminescence efficiency (LE), quantum detection efficiency, modulation transfer function, and light emission spectrum. Gd 2O 2S : Tb, La 2O 2S : Tb, Y 2O 2S : Tb and ZnSCdS : Ag scintillating screens were prepared in laboratory and were excited to luminescence by a medical X-ray tube. Maximum DQE values varied between 0.13 and 0.33 depending on the scintillator material, the screen coating weight, and the tube voltage; Gd 2O 2S : Tb was superior to La 2O 2S : Tb followed by ZnSCdS : Ag and Y 2O 2S : Tb. This ranking was maintained at frequencies up to 100 cycles/cm. Considering the same material, DQE of thin screens was found superior to DQE of thicker screens at medium-to-high frequencies. The proposed method allows for the comparison of imaging characteristics of scintillating materials without the inclusion of optical detector imaging properties.

Absorption and noise in cesium iodide x-ray image intensifiers.

The measured and theoretically predicted values of detective quantum efficiency (DQE) for a CsI x-ray image intensifier are compared for nine monoenergetic beams of x rays. The agreement between measurement and theory of better than +/- 5% indicates that we have a sound understanding of the physical parameters controlling the DQE. It is shown that the fraction of K-fluorescent x rays escaping from the input phosphor is independent of incident energy. The number of electrons released within the x-ray image intensifier (XRII) by an incident x ray has been measured. The mechanism for energy broadening within the XRII is shown to be predominantly the limited number of electrons and not light absorption.

Some Detector Characteristics of the Photographic Process

The influence of the quantum nature of radiation on the sensitivity-resolution-noise characteristics of photographic processes is discussed. It is described how practical quantum efficiency and resolution properties may be compared with those of an ideal imaging process in terms of detective quantum efficiency (DQE). Some of the principles which are involved in the photographic application to detection problems are reviewed. A model which includes most of the established photographic quantum inefficiencies is used to interpret the measured values of DQE, and to indicate the various ways in which the photographic process is open to future improvements.

The resolution of photographic emulsions for electrons in the energy range 7-60 keV

The quality of images recorded photographically with electrons with energies from 7-60 keV is examined by measurements on the granularity of large areas exposed to a uniform optical density and the density profiles found in the images of sharp opaque edges. The work has been mainly carried out with Ilford Special Lantern Contrasty (SLC), Ilford Nuclear Research G5 and Kodak C-type DC3 emulsions. The granularity of SLC and G5 at 60 and 7 keV respectively has been examined over a range of densitometer scanning aperture diameters from 65 to 63 μm and these measurements are related to the corresponding correlation curves. Grain correlation is found for SLC at 60 kV extending over image regions about 15 μm in diameter; no correlation was found for G5 at 7 kV, or at 60 kV for which case a few measurements were also made. The changes in image granularity with electron energy were examined for SLC, G5 and two other nuclear emulsions using a scanning aperture 45 μm in diameter. Similar measurements on DC3 were limited to the energy range 7-20 keV. The detective quantum efficiencies were evaluated from the granularity measurements for all these emulsions. As examples of the results, over the energy range from 7-60 keV the values of detective quantum efficiency for G5 and SLC fell by factors of about 2 and 30 respectively. A comparison between observed and calculated values of the granularity shows reasonable agreement allowing for the assumptions made. The edge density profiles for SLC, G5 and DC3 were measured at 60 and 7 kV and, after the profiles are fitted to a function of exponential form, the corresponding modulation transfer functions (MTF) are considered. Inter-relating the measurements on the image granularity and the MTF allows the determination of the minimum detectable density difference corresponding to a Fourier image component of a given wavelength, and results are given for SLC at 60 kV and G5 at 7 kV. A comparison of these results shows that electron images of good quality may be recorded on G5 at low voltages and at readily realized image magnifications.