量子检测效率在平板探测器评估中的研究进展

平板探测器(flat panel detector,FPD)是X射线摄影系统中至关重要的部分,其性能直接影响所采集图像的质量。量子检测效率(detective quantum efficiency,DQE)涉及探测器的噪声、分辨率、剂量、调制传递函数、噪声功率谱等多项参数,被公认是X射线成像性能最准确的评估指标,DQE越高,说明影像系统在低X射线入射剂量的情况下,获得高质量影像的能力越强。我们简单介绍了平板探测器的种类,对其DQE的检测技术的研究现状、进展及应用作了综述,归纳了在DQE检测过程中的有关影响因素,并进行了总结展望。     

Determination of the detective quantum efficiency of a digital x-ray detector: comparison of three evaluations using a common image data set

The detective quantum efficiency (DQE) of an x-ray digital imaging detector was determined independently by the three participants of this study, using the same data set consisting of edge and flat field images. The aim was to assess the possible variation in DQE originating from established, but slightly different, data processing methods used by different groups. For the case evaluated in this study differences in DQE of up to +/-15% compared to the mean were found. The differences could be traced back mainly to differences in the modulation transfer function (MTF) and noise power spectrum (NPS) determination. Of special importance is the inclusion of a possible low-frequency drop in MTF and the proper handling of signal offsets for the determination of the NPS. When accounting for these factors the deviation between the evaluations reduced to approximately +/-5%. It is expected that the recently published standard on DQE determination will further reduce variations in the data evaluation and thus in the results of DQE measurements.     

A CdZnTe slot-scanned detector for digital mammography

A new high-resolution detector has been developed for use in a slot-scanned digital mammography system. The detector is a hybrid device that consists of a CCD operating in time-delay integration mode that is bonded to a 150-microm-thick CdZnTe photoconductor array. The CCD was designed with a detector element pitch of 50 microm. Two devices were evaluated with differing crystalline quality. Incomplete charge collection was a source of reduction in DQE. This occurs in both devices due to characteristically low mobility-lifetime products for CdZnTe, with the greatest losses demonstrated by the multicrystalline sample. The mobility-lifetime products for the multicrystalline device were found to be 2.4 x 10(-4) and 4.0 x 10(-7) cm2/V for electrons and holes, respectively. The device constructed with higher quality single crystal CdZnTe demonstrated mobility-lifetime products of 1.0 x 10(-4) and 4.4 x 10(-6) cm2/V for electrons and holes. The MTF and DQE for the device were measured at several exposures and results were compared to predictions from a linear systems model of signal and noise propagation. The MTF at a spatial frequency of 10 mm(-1) exceeded 0.18 and 0.56 along the scan and slot directions, respectively. Scanning motion and CCD design limited the resolution along the scan direction. For an x-ray beam from a tungsten target tube with 40 microm molybdenum filtration operated at 26 kV, the single crystal device demonstrated a DQE(0) of 0.70 +/- 0.02 at 7.1 x 10(-6) C/kg (27 mR) exposure to the detector, despite its relatively poor charge collection efficiency.     

Screen optics effects on detective quantum efficiency in digital radiography: zero-frequency effects

Indirect flat panel imagers have been developed for digital radiography, fluoroscopy and mammography, and are now in clinical use. Screens made from columnar structured cesium iodide (CsI) scintillators doped with thallium have been used extensively in these detectors. The purpose of this article is to investigate the effect of screen optics, e.g., light escape efficiency versus depth, on gain fluctuation noise, expressed as the Swank factor. Our goal is to obtain results useful in optimizing screens for digital radiography systems. Experimental measurements from structured CsI samples were used to derive their screen optics properties, and the same methods can also be applied to powder screens. CsI screens, all of the same thickness but with different optical designs and manufacturing techniques, were obtained from Hamamatsu Photonics Corporation. The pulse height spectra (PHS) of the screens were measured at different x-ray energies. A theoretical model was developed for the light escape efficiency and a method for deriving light escape efficiency versus depth from experimental PHS measurements was implemented and applied to the CsI screens. The results showed that the light escape efficiency varies essentially linearly as a function of depth in the CsI samples, and that the magnitude of variation is relatively small, leading to a high Swank factor.     

Determination of the two-dimensional detective quantum efficiency of a computed radiography system

Based on a recently described method for determining the two-dimensional presampling modulation transfer function (MTF), the aperture mask method, a method for determining the two-dimensional detective quantum efficiency (DQE) of a digital radiographic system was developed. The method was applied to a new computed radiography (CR) system and comparisons with one-dimensional determinations of the presampling MTF and the DQE were performed. The aperture mask method was shown to agree with the conventional tilted slit method for determining the presampling MTF along the axes. For the particular CR system studied, the mean of one-dimensional determinations of the DQE in orthogonal directions led to a representative measure of the average DQE behavior of the system up to the Nyquist frequency along the axes, but a deviation was observed above this frequency. In conclusion, the method developed for determining the two-dimensional DQE can be used to determine the imaging properties of a digital radiographic detector system over almost the entire frequency domain, the exception being the lowest frequencies (< or = 0.1 mm(-1)) at which the validity and the reliability of the method are low.     

Retrospective analysis of a detector fault for a full field digital mammography system

This paper describes objective and subjective image quality measurements acquired as part of a routine quality assurance (QA) programme for an amorphous selenium (a-Se) full field digital mammography (FFDM) system between August-04 and February-05. During this period, the FFDM detector developed a fault and was replaced. A retrospective analysis of objective image quality parameters (modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE)) is presented to try and gain a deeper understanding of the detector problem that occurred. These measurements are discussed in conjunction with routine contrast-detail (c-d) results acquired with the CDMAM (Artinis, The Netherlands) test object. There was significant reduction in MTF over this period of time indicating an increase in blurring occurring within the a-Se converter layer. This blurring was not isotropic, being greater in the data line direction (left to right across the detector) than in the gate line direction (chest wall to nipple). The initial value of the 50% MTF point was 6 mm(-1); for the faulty detector the 50% MTF points occurred at 3.4 mm(-1) and 1.0 mm(-1) in the gate line and data line directions, respectively. Prior to NNPS estimation, variance images were formed of the detector flat field images. Spatial distribution of variance was not uniform, suggesting that the physical blurring process was not constant across the detector. This change in variance with image position implied that the stationarity of the noise statistics within the image was limited and that care would be needed when performing objective measurements. The NNPS measurements confirmed the results found for the MTF, with a strong reduction in NNPS as a function of spatial frequency. This reduction was far more severe in the data line direction. A somewhat tentative DQE estimate was made; in the gate line direction there was little change in DQE up to 2.5 mm(-1) but at the Nyquist frequency the DQE had fallen to approximately 35% of the original value. There was severe attenuation of DQE in the data line direction, the DQE falling to less than 0.01 above approximately 3.0 mm(-1). C-d results showed an increase in threshold contrast of approximately 25% for details less than 0.2 mm in diameter, while no reduction in c-d performance was found at the largest detail diameters (1.0 mm and above). Despite the detector fault, the c-d curve was found to pass the European protocol acceptable c-d curve.     

Model of the spatial-frequency-dependent detective quantum efficiency of phosphor screens

We have developed a theoretical model to predict the modulation transfer function (MTF), the shape of the x-ray quantum noise power spectrum (NPS), and the spatial-frequency-dependent detective quantum efficiency (DQE) of an x-ray phosphor screen. The transfer of energy through the screen is modelled as a series of cascaded stochastic processes assuming that the screen consists of many thin phosphor layers. In this way, the model is able to account for the possibility of secondary-quantum noise and the difference in shape between MTF2 and the x-ray quantum NPS. Modelling a Kodak Min-R screen we were able to predict both the number of light quanta emitted per absorbed x-ray and MTF(f) to better than +/- 5%, and the scintillation efficiency to within 10% of experimentally measured values. The shape of the x-ray quantum NPS is predicted to within +/- 5% for spatial frequencies less than about 6 mm-1 and to within +/- 20% for higher frequencies.     

Optical factors affecting the detective quantum efficiency of radiographic screens

Parameters related to the detective quantum efficiency (DQE) of several representative screens of different thicknesses, phosphor grain sizes, and optical properties were measured by the scintillation spectrum method, using monoenergetic x rays produced from x-ray fluorescence. The experimental results, including those for spectral shape and average light energies (EA) emitted, are compared with conventional theories of the operation of screens. It was hoped that this would vindicate the theory of the effect of optical properties and so permit the simple calculation of all parameters related to DQE from standard x-ray attenuation tables. Rather more substantial energy-dependent deviations of EA are found than was previously realized, which preliminary analysis suggests are due to both optical effects and photoelectron escape. We conclude that although DQE for a single energy can be calculated by simplified methods to within +/- 10%, the effective DQE when polyenergetic beams are used is much less accurately estimated and requires a fuller theoretical treatment.     

Effect of finite phosphor thickness on detective quantum efficiency

In this paper we describe theoretically the relationship between the finite thickness of a phosphor screen and its spatial-frequency-dependent detective quantum efficiency DQE(f-). The finite thickness of the screen causes a variation in both the total number of light quanta emitted from the screen in a burst from a given x-ray interaction and in the spatial distribution of the quanta within the light burst [i.e., shape or point spread function (PSF) of the light burst]. The variation in magnitude of the burst gives rise to a spatial-frequency-independent reduction in DQE, characterized by the scintillation efficiency As. The variation in PSF causes a roll off in DQE with increasing spatial frequency which we have characterized by the function Rc(f). Both As and Rc(f) can be determined from the moments of the distribution of the spatial Fourier spectrum of light bursts emitted from the phosphor and thus they are related: As is a scaling factor for Rc(f). Our theory predicts that it is necessary for all light bursts which appear at the output to have the same magnitude to maximize As and the same shape to maximize Rc(f). These requirements can lead to the result that the fluorescent screen with the highest modulation transfer function will not necessarily have the highest DQE(f) even at high spatial frequencies.     

A slot-scanned photodiode-array/CCD hybrid detector for digital mammography

We have developed a novel direct conversion detector for use in a slot-scanning digital mammography system. The slot-scan concept allows for dose efficient scatter rejection and the ability to use small detectors to produce a large-area image. The detector is a hybrid design with a 1.0 mm thick silicon PIN photodiode array (the x-ray absorber) indium-bump bonded to a CCD readout that is operated in time-delay integration (TDI) mode. Because the charge capacity requirement for good image quality exceeds the capabilities of standard CCDs, a novel CCD was developed. This CCD consists of 24 independent sections, each acting as a miniature CCD with eight rows for TDI. The signal from each section is combined off-chip to produce a full signal image. The MTF and DQE for the device was measured at several exposures and compared to a linear systems model of signal and noise propagation. Because of the scanning nature of TDI imaging, both the MTF(f) and DQE(f) are reduced along the direction of the scanning motion. For a 26 kVp spectrum, the DQE(0) was measured to be 0.75+/-0.02 for an exposure of 1.29 x 10(-5) C/kg (50 mR).     

Physical characterization of a prototype selenium-based full field digital mammography detector

The purpose of this study was to measure experimentally the physical performance of a prototype mammographic imager based on a direct detection, flat-panel array design employing an amorphous selenium converter with 70 microm pixels. The system was characterized for two different anode types, a molybdenum target with molybdenum filtration (Mo/Mo) and a tungsten target with rhodium filtration (W/Rh), at two different energies, 28 and 35 kVp, with approximately 2 mm added aluminum filtration. To measure the resolution, the presampled modulation transfer function (MTF) was measured using an edge method. The normalized noise power spectrum (NNPS) was measured by two-dimensional Fourier analysis of uniformly exposed mammograms. The detective quantum efficiencies (DQEs) were computed from the MTFs, the NNPSs, and theoretical ideal signal to noise ratios. The MTF was found to be close to its ideal limit and reached 0.2 at 11.8 mm(-1) and 0.1 at 14.1 mm(-1) for images acquired at an RQA-M2 technique (Mo/Mo anode, 28 kVp, 2 mm Al). Using a tungsten technique (MW2; W/Rh anode, 28 kVp, 2 mm Al), the MTF went to 0.2 at 11.2 mm(-1) and to 0.1 at 13.3 mm(-1). The DQE reached a maximum value of 54% at 1.35 mm(-1) for the RQA-M2 technique at 1.6 microC/kg and achieved a peak value of 64% at 1.75 mm(-1) for the tungsten technique (MW2) at 1.9 microC/kg. Nevertheless, the DQE showed strong exposure and frequency dependencies. The results indicated that the detector offered high MTFs and DQEs, but structured noise effects may require improved calibration before clinical implementation.     

Analysis of the kinestatic charge detection system as a high detective quantum efficiency electronic portal imaging device

Megavoltage x-ray imaging suffers from reduced image quality due to low differential x-ray attenuation and large Compton scatter compared with kilovoltage imaging. Notwithstanding this, electronic portal imaging devices (EPIDs) are now widely used in portal verification in radiotherapy as they offer significant advantages over film, including immediate digital imaging and superior contrast range. However video-camera-based EPIDs (VEPIDs) are limited by problems of low light collection efficiency and significant light scatter, leading to reduced contrast and spatial resolution. Indirect and direct detection-based flat-panel EPIDs have been developed to overcome these limitations. While flat-panel image quality has been reported to exceed that achieved with portal film, these systems have detective quantum efficiency (DQE) limited by the thin detection medium and are sensitive to radiation damage to peripheral read-out electronics. An alternative technology for high-quality portal imaging is presented here: kinesatic charge detection (KCD). The KCD is a scanning tri-electrode ion-chamber containing high-pressure noble gas (xenon at 100 atm) used in conjunction with a strip-collimated photon beam. The chamber is scanned across the patient, and an external electric field is used to regulate the cation drift velocity. By matching the scanning velocity with that of the cation (i.e., ion) drift velocity, the cations remain static in the object frame of reference, allowing temporal integration of the signal. The KCD offers several advantages as a portal imaging system. It has a thick detector geometry with an active detection depth of 6.1 cm, compared to the sub-millimeter thickness of the phosphor layer in conventional phosphor screens, leading to an order of magnitude advantage in quantum efficiency (>0.3). The unique principle of and the use of the scanning strip-collimated x-ray beam provide further integration of charges in time, reduced scatter, and a significantly reduced imaging dose, enhancing the imaging signal-to-noise ratio (SNR) and leading to high DQE. While thick detectors usually suffer from reduced spatial resolution, the KCD provides good spatial resolution due to high gas pressure that limits the spread of scattered electrons, and a strip-collimated beam that significantly reduces the inclusion of scatter in the imaging signal. A 10 cm wide small-field-of-view (SFOV) prototype of the KCD is presented with a complete analysis of its imaging performance. Measurements of modulation transfer function (MTF), noise power spectrum (NPS), and DQE were in good agreement with Monte Carlo simulations. Imaging signal loss from recombination within the KCD chamber was measured at different gas pressures, ion drift velocities, and strip-collimation widths. Image quality for the prototype KCD was also observed with anthropomorphic phantom imaging in comparison with various commercial and research portal imaging systems, including VEPID, flat-panel imager, and conventional and high contrast film systems. KCD-based imaging provided very good contrast and good spatial resolution at very low imaging dose (0.1 cGy per image). For the prototype KCD, measurements yielded DQE(0)=0.19 and DQE(1 cy/mm)=0.004.     

Effect of various noise sources on the detective quantum efficiency of phosphor screens

We have examined the effect of screen-structure, optical-detector, and secondary-quantum noise sources on detective quantum efficiency, DQE(f). This was done by using experimental measurements of screen-structure and optical-detector noise in combination with a theoretical model which predicts x-ray quantum and secondary-quantum noise for different optical and physical properties of a phosphor screen. The reduction in DQE(f) from noise sources other than x-ray quantum noise depends on the noise power spectra (NPS) of these other sources relative to the x-ray quantum NPS. Even though x-ray quantum noise may be the dominant noise source at low spatial frequencies, it decreases relatively rapidly with increasing frequency so that other noise sources, which may be small at low frequencies, dominate. Our model predicts that DQE(f) can be increased, at spatial frequencies less than 4 mm-1, by changing the optical properties of the screen even though modulation transfer function MTF(f) may decrease. Furthermore, if screen and optical-detector noise decrease with increasing frequency and secondary-quantum noise sufficiently small, then DQE(f) will also be improved at frequencies greater than 4 mm-1.     

Self-normalizing method to measure the detective quantum efficiency of a wide range of x-ray detectors.

The detective quantum efficiency (DQE) is widely accepted as the most relevant parameter to characterize the image quality of medical x-ray systems. In this article we describe a solid method to measure the DQE. The strength of the method lies in the fact that it is self-normalizing so measurements at very low spatial frequencies are not needed. Furthermore, it works on any system with a response function which is linear in the small-signal approximation. We decompose the DQE into several easily accessible quantities and discuss in detail how they can be measured. At the end we lead the interested reader through an example. Noise equivalent quanta and normalized contrast values are tabulated for standard radiation qualities.     

Imaging performance of an amorphous selenium digital mammography detector in a breast tomosynthesis system

In breast tomosynthesis a rapid sequence of N images is acquired when the x-ray tube sweeps through different angular views with respect to the breast. Since the total dose to the breast is kept the same as that in regular mammography, the exposure used for each image of tomosynthesis is 1/N. The low dose and high frame rate pose a tremendous challenge to the imaging performance of digital mammography detectors. The purpose of the present work is to investigate the detector performance in different operational modes designed for tomosynthesis acquisition, e.g., binning or full resolution readout, the range of view angles, and the number of views N. A prototype breast tomosynthesis system with a nominal angular range of +/-25 degrees was used in our investigation. The system was equipped with an amorphous selenium (a-Se) full field digital mammography detector with pixel size of 85 microm. The detector can be read out in full resolution or 2 x 1 binning (binning in the tube travel direction). The focal spot blur due to continuous tube travel was measured for different acquisition geometries, and it was found that pixel binning, instead of focal spot blur, dominates the detector modulation transfer function (MTF). The noise power spectrum (NPS) and detective quantum efficiency (DQE) of the detector were measured with the exposure range of 0.4-6 mR, which is relevant to the low dose used in tomosynthesis. It was found that DQE at 0.4 mR is only 20% less than that at highest exposure for both detector readout modes. The detector temporal performance was categorized as lag and ghosting, both of which were measured as a function of x-ray exposure. The first frame lags were 8% and 4%, respectively, for binning and full resolution mode. Ghosting is negligible and independent of the frame rate. The results showed that the detector performance is x-ray quantum noise limited at the low exposures used in each view of tomosynthesis, and the temporal performance at high frame rate (up to 2 frames per second) is adequate for tomosynthesis.     

Direct conversion detectors: the effect of incomplete charge collection on detective quantum efficiency

Direct conversion detectors offer the potential for very high resolution and high quantum efficiency for x-ray imaging, however, variations in signal can arise due to incomplete charge collection. A charge transport model was developed to describe the signal and noise resulting from incomplete charge collection. This signal to noise ratio (SNR) reduction was incorporated into the cascaded systems model for a simple x-ray detector. A new excess noise factor, A(c) (termed the collection noise factor) is introduced to describe the reduction in detective quantum efficiency (DQE). The DQE is proportional to the product of the quantum efficiency and the collection noise factor. If the trapping cross sections for electrons and holes are very different, and if the detector is biased improperly, the collection noise factor can drop to as low as 50%. In addition, the signal loss due to incomplete charge collection will reduce the DQE in the presence of added noise. Because of this, the DQE generally does not continue to improve with greater detector thickness. The collection noise factor and DQE are predicted for CdZnTe, PbI2, and Se. The optimization of detector thickness should be based not only on quantum efficiency but also on the charge collection statistics, which are influenced by bias field and polarity.