Empirical electro-optical and x-ray performance evaluation of CMOS active pixels sensor for low dose, high resolution x-ray medical imaging

Monolithic complementary metal oxide semiconductor (CMOS) active pixel sensors with high performance have gained attention in the last few years in many scientific and space applications. In order to evaluate the increasing capabilities of this technology, in particular where low dose high resolution x-ray medical imaging is required, critical electro-optical and physical x-ray performance evaluation was determined. The electro-optical performance includes read noise, full well capacity, interacting quantum efficiency, and pixels cross talk. The x-ray performance, including x-ray sensitivity, modulation transfer function, noise power spectrum, and detection quantum efficiency, has been evaluated in the mammographic energy range. The sensor is a 525 x 525 standard three transistor CMOS active pixel sensor array with more than 75% fill factor and 25 x 25 microm pixel pitch. Reading at 10 f/s, it is found that the sensor has 114 electrons total additive noise, 10(5) electrons full well capacity with shot noise limited operation, and 34% interacting quantum efficiency at 530 nm. Two different structured CsI:Tl phosphors with thickness 95 and 115 microm, respectively, have been optically coupled via a fiber optic plate to the array resulting in two different system configurations. The sensitivity of the two different system configurations was 43 and 47 electrons per x-ray incident on the sensor. The MTF at 10% of the two different system configurations was 9.5 and 9 cycles/mm with detective quantum efficiency of 0.45 and 0.48, respectively, close to zero frequency at approximately 0.44 microC/kg (1.72 mR) detector entrance exposure. The detector was quantum limited at low spatial frequencies and its performance was comparable with high resolution a: Si and charge coupled device based x-ray imagers. The detector also demonstrates almost an order of magnitude lower noise than active matrix flat panel imagers. The results suggest that CMOS active pixel sensors when coupled to structured CsI:Tl can be used for conventional and advanced digital mammography due to their low noise, high resolution performance.     

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.     

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.     

A comparison of x-ray detectors for mouse CT imaging

There is significant interest in using computed tomography (CT) for in vivo imaging applications in mouse models of disease. Most commercially available mouse x-ray CT scanners utilize a charge-coupled device (CCD) detector coupled via fibre optic taper to a phosphor screen. However, there has been little research to determine if this is the optimum detector for the specific task of in vivo mouse imaging. To investigate this issue, we have evaluated four detectors, including an amorphous selenium (a-Se) detector, an amorphous silicon (a-Si) detector with a gadolinium oxysulphide (GOS) screen, a CCD with a 3:1 fibre taper and a GOS screen, and a CCD with a 2:1 fibre taper and both GOS and thallium-doped caesium iodide (CsI:Tl) screens. The detectors were evaluated by measuring the modulation transfer function (MTF), noise power spectrum (NPS), detective quantum efficiency (DQE), stability over multiple exposures, and noise in reconstructed CT images. The a-Se detector had the best MTF and the highest DQE (0.6 at 0 lp mm(-1)) but had the worst stability (45% reduction after 2000 exposure frames). The a-Si detector and the CCD with the 3:1 fibre, both of which used the GOS screen, had very similar performance with a DQE of approximately 0.30 at 0 lp mm(-1). For the CCD with the 2:1 fibre, the CsI:Tl screen resulted in a nearly two-fold improvement in DQE over the GOS screen (0.4 versus 0.24 at 0 lp mm(-1)). The CCDs both had the best stability, with less than a 1% change in pixel values over multiple exposures. The pixel values of the a-Si detector increased 5% over multiple exposures due to the effects of image lag. Despite the higher DQE of the a-Se detector, the reconstructed CT images acquired with the a-Si detector had lower noise levels, likely due to the blurring effects from the phosphor screen.     

System performance of a prototype flat-panel imager operated under mammographic conditions

The results of an empirical and theoretical investigation of the performance of a high-resolution, active matrix flat-panel imager performed under mammographic conditions are reported. The imager is based upon a prototype, indirect detection active matrix array incorporating a discrete photodiode in each pixel and a pixel-to-pixel pitch of 97 microm. The investigation involved three imager configurations corresponding to the use of three different x-ray converters with the array. The converters were a conventional Gd2O2S-based mammographic phosphor screen (Min-R) and two structured CsI:Tl scintillators: one optimized for high spatial resolution (FOS-HR) and the other for high light output (FOS-HL). Detective quantum efficiency for mammographic exposures ranging from approximately 2 to approximately 40 mR at 26 kVp were determined for each imager configuration through measurements of x-ray sensitivity, modulation transfer function (MTF), and noise power spectrum (NPS). All configurations were found to provide significant presampling MTF at frequencies beyond the Nyquist frequency of the array, approximately 5.2 mm(-1) , consistent with the high spatial resolution of the converters. In addition, the effect of additive electronic noise on the NPS was found to be significantly larger for the configuration with lower system gain (FOS-HR) than for the configurations with higher gain (Min-R, FOS-HL). The maximum DQE values obtained with the CsI:Tl scintillators were considerably greater than those obtained with the Min-R screen due to the significantly lower Swank noise of the scintillators. Moreover, DQE performance was found to degrade with decreasing exposure, although this exposure-dependence was considerably reduced for the higher gain configurations. Theoretical calculations based on the cascaded systems model were found to be in generally good agreement with these empirically determined NPS and DQE values. In this study, we provide an example of how cascaded systems modeling can be used to identify factors limiting system performance and to examine trade-offs between factors toward the goal of maximizing performance.     

Full breast digital mammography with an amorphous silicon-based flat panel detector: physical characteristics of a clinical prototype

The physical characteristics of a clinical prototype amorphous silicon-based flat panel imager for full-breast digital mammography have been investigated. The imager employs a thin thallium doped CsI scintillator on an amorphous silicon matrix of detector elements with a pixel pitch of 100 microm. Objective criteria such as modulation transfer function (MTF), noise power spectrum, detective quantum efficiency (DQE), and noise equivalent quanta were employed for this evaluation. The presampling MTF was found to be 0.73, 0.42, and 0.28 at 2, 4, and 5 cycles/mm, respectively. The measured DQE of the current prototype utilizing a 28 kVp, Mo-Mo spectrum beam hardened with 4.5 cm Lucite is approximately 55% at close to zero spatial frequency at an exposure of 32.8 mR, and decreases to approximately 40% at a low exposure of 1.3 mR. Detector element nonuniformity and electronic gain variations were not significant after appropriate calibration and software corrections. The response of the imager was linear and did not exhibit signal saturation under tested exposure conditions.     

Dual-energy digital mammography for calcification imaging: scatter and nonuniformity corrections

Mammographic images of small calcifications, which are often the earliest signs of breast cancer, can be obscured by overlapping fibroglandular tissue. We have developed and implemented a dual-energy digital mammography (DEDM) technique for calcification imaging under full-field imaging conditions using a commercially available aSi:H/CsI:Tl flat-panel based digital mammography system. The low- and high-energy images were combined using a nonlinear mapping function to cancel the tissue structures and generate the dual-energy (DE) calcification images. The total entrance-skin exposure and mean-glandular dose from the low- and high-energy images were constrained so that they were similar to screening-examination levels. To evaluate the DE calcification image, we designed a phantom using calcium carbonate crystals to simulate calcifications of various sizes (212-425 microm) overlaid with breast-tissue-equivalent material 5 cm thick with a continuously varying glandular-tissue ratio from 0% to 100%. We report on the effects of scatter radiation and nonuniformity in x-ray intensity and detector response on the DE calcification images. The nonuniformity was corrected by normalizing the low- and high-energy images with full-field reference images. Correction of scatter in the low- and high-energy images significantly reduced the background signal in the DE calcification image. Under the current implementation of DEDM, utilizing the mammography system and dose level tested, calcifications in the 300-355 microm size range were clearly visible in DE calcification images. Calcification threshold sizes decreased to the 250-280 microm size range when the visibility criteria were lowered to barely visible. Calcifications smaller than approximately 250 microm were usually not visible in most cases. The visibility of calcifications with our DEDM imaging technique was limited by quantum noise, not system noise.     

Physical characteristics of GE Senographe Essential and DS digital mammography detectors

The purpose of this study was to investigate physical characteristics of two full field digital mammography (FFDM) systems (GE Senographe Essential and DS). Both are indirect conversion (x ray to light) alpha-Si flat panels coupled with a CsI(Tl) scintillator. The examined systems have the same pixel size (100 microm) but a different field of view: a conventional size 23 x 19.2 cm2 and a large field 24 X 30.7 cm2, specifically designed to image large breasts. In the GE Senographe Essential model relevant improvements in flat panel design were implemented and new deposition tools for metal, alpha-Si, and CsI(Tl) were introduced by GE. These changes in detector design are expected to be beneficial for advanced applications such as breast tomosynthesis. The presampling modulation transfer function (MTF), normalized noise power spectrum (NNPS), and detective quantum efficiency (DQE) were measured for a wide range of exposure (25-240 microGy) with a RQA-M2 technique (28 kVp with a Mo/Mo target/filter combination and 2 mm of additional aluminum filtration). At 1, 2, and at 4 lp/mm MTF is equal to 0.9, 0.76, and 0.46 for the conventional field detector and to 0.85, 0.59, and 0.24 for the large field detector. The latter detector exhibits an improved NNPS due to a lower electronic noise and a better DQE that reaches 60%. In addition a contrast-detail analysis was performed with CDMAM 3.4 phantom and CDCOM software: GE Senographe DS showed statistically significant poorer detection ability in comparison with the GE Senographe Essential. These results could have been expected, at least qualitatively, considering the relative DQE of the two systems.     

X-ray imaging performance of structured cesium iodide scintillators

Columnar structured cesium iodide (CsI) scintillators doped with Thallium (Tl) have been used extensively for indirect x-ray imaging detectors. The purpose of this paper is to develop a methodology for systematic investigation of the inherent imaging performance of CsI as a function of thickness and design type. The results will facilitate the optimization of CsI layer design for different x-ray imaging applications, and allow validation of physical models developed for the light channeling process in columnar CsI layers. CsI samples of different types and thicknesses were obtained from the same manufacturer. They were optimized either for light output (HL) or image resolution (HR), and the thickness ranged between 150 and 600 microns. During experimental measurements, the CsI samples were placed in direct contact with a high resolution CMOS optical sensor with a pixel pitch of 48 microns. The modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of the detector with different CsI configurations were measured experimentally. The aperture function of the CMOS sensor was determined separately in order to estimate the MTF of CsI alone. We also measured the pulse height distribution of the light output from both the HL and HR CsI at different x-ray energies, from which the x-ray quantum efficiency, Swank factor and x-ray conversion gain were determined. Our results showed that the MTF at 5 cycles/mm for the HR type was 50% higher than for the HL. However, the HR layer produces approximately 36% less light output. The Swank factor below K-edge was 0.91 and 0.93 for the HR and HL types, respectively, thus their DQE(0) were essentially identical. The presampling MTF decreased as a function of thickness L. The universal MTF, i.e., MTF plotted as a function of the product of spatial frequency f and CsI thickness L, increased as a function of L. This indicates that the light channeling process in CsI improved the MTF of thicker layers more significantly than for the thinner ones.     

Theoretical analysis of high-resolution digital mammography

The performance of a high-resolution charge coupled device-based full-field digital mammography imager was analysed using a mathematical framework based on an adaptation of cascaded linear systems theory described by other investigators. This work has been conducted in order to understand the impact of various design parameters on the physical performance characteristics of the imager. Specifically, the effect of pixel size, scintillator thickness and packing density, x-ray spectra, air kerma, dark current, charge integration time, and pixel fill-factor on the frequency dependent detective quantum efficiency was studied using a charge-coupled device as a reference platform. The imaging system was modelled as a series of physical processes with gain and spatial spreading. For each stage, the signal and noise power spectra were computed and propagated through the imaging chain as inputs to subsequent stages. Good agreement between experimental and theoretical predictions was obtained for various x-ray spectral conditions that were investigated. The modulation transfer function, MTF(f) and detective quantum efficiency DQE(f) characteristics obtained in this study are encouraging and comparable to other digital mammography systems. The results of this study strongly suggest the feasibility of large area scintillator-based digital mammography imagers with pixel sizes below 100 microm.     

Imaging characteristics of different mammographic screens.

A study of mammography systems with green-emitting screens was conducted to determine how the image quality parameters (apart from dose requirement), such as modulation transfer function (MTF) and Wiener spectrum (WS), depend on the dye content of the compound and coating weight of the screen. In addition, the contribution to total noise of the individual components, i.e., film, screen, and quantum noise, was studied. The quantities derived from MTF and WS, namely detective quantum efficiency (DQE) and noise equivalent quanta (NEQ), were also investigated in regard to their dose dependency. It can be demonstrated that the MTF of the screens becomes more favorable when the dye content is increased, while noise is not significantly affected. This suggests the use of a mammography screen capable of greater detail recognition, requiring at least double the dose of today's conventional systems with approximately 80 microGy system dose. On the other hand, the manufacture of a screen with about 60% of the dose of the conventional system is possible with very little loss in image quality. For the systems in common use today (80 microGy), quantum noise represents a considerable share of the total noise at low spatial frequencies, whereas in high spatial frequencies, the graininess of the film dominates quantum noise and screen structure.     

Image quality assessment in digital mammography: part I. Technical characterization of the systems

In many European countries, image quality for digital x-ray systems used in screening mammography is currently specified using a threshold-detail detectability method. This is a two-part study that proposes an alternative method based on calculated detectability for a model observer: the first part of the work presents a characterization of the systems. Eleven digital mammography systems were included in the study; four computed radiography (CR) systems, and a group of seven digital radiography (DR) detectors, composed of three amorphous selenium-based detectors, three caesium iodide scintillator systems and a silicon wafer-based photon counting system. The technical parameters assessed included the system response curve, detector uniformity error, pre-sampling modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE). Approximate quantum noise limited exposure range was examined using a separation of noise sources based upon standard deviation. Noise separation showed that electronic noise was the dominant noise at low detector air kerma for three systems; the remaining systems showed quantum noise limited behaviour between 12.5 and 380 μGy. Greater variation in detector MTF was found for the DR group compared to the CR systems; MTF at 5 mm(-1) varied from 0.08 to 0.23 for the CR detectors against a range of 0.16-0.64 for the DR units. The needle CR detector had a higher MTF, lower NNPS and higher DQE at 5 mm(-1) than the powder CR phosphors. DQE at 5 mm(-1) ranged from 0.02 to 0.20 for the CR systems, while DQE at 5 mm(-1) for the DR group ranged from 0.04 to 0.41, indicating higher DQE for the DR detectors and needle CR system than for the powder CR phosphor systems. The technical evaluation section of the study showed that the digital mammography systems were well set up and exhibiting typical performance for the detector technology employed in the respective systems.     

Anisotropic imaging performance in indirect x-ray imaging detectors

We report on the variability in imaging system performance due to oblique x-ray incidence, and the associated transport of quanta (both x rays and optical photons) through the phosphor, in columnar indirect digital detectors. The analysis uses MANTIS, a combined x-ray, electron, and optical Monte Carlo transport code freely available. We describe the main features of the simulation method and provide some validation of the phosphor screen models considered in this work. We report x-ray and electron three-dimensional energy deposition distributions and point-response functions (PRFs), including optical spread in columnar phosphor screens of thickness 100 and 500 microm, for 19, 39, 59, and 79 keV monoenergetic x-ray beams incident at 0 degrees, 10 degrees, and 15 degrees. In addition, we present pulse-height spectra for the same phosphor thickness, x-ray energies, and angles of incidence. Our results suggest that the PRF due to the phosphor blur is highly nonsymmetrical, and that the resolution properties of a columnar screen in a tomographic, or tomosynthetic imaging system varies significantly with the angle of x-ray incidence. Moreover, we find that the noise due to the variability in the number of light photons detected per primary x-ray interaction, summarized in the information or Swank factor, is somewhat independent of thickness and incidence angle of the x-ray beam. Our results also suggest that the anisotropy in the PRF is not less in screens with absorptive backings, while the noise introduced by variations in the gain and optical transport is larger. Predictions from MANTIS, after additional validation, can provide the needed understanding of the extent of such variations, and eventually, lead to the incorporation of the changes in imaging performance with incidence angle into the reconstruction algorithms for volumetric x-ray imaging systems.     

Micro-angiography for neuro-vascular imaging. I. Experimental evaluation and feasibility

Minimally invasive image-guided neuro-vascular interventions require very high image-resolution and quality, specifically over regions-of-interest (ROI) crucial to the procedure. ROI imaging or micro-angiography, allows limited patient integral radiation dose while permitting rapid frame transfer of high-resolution images. The design and performance of a charge coupled device (CCD) based x-ray detector or micro-angiographic camera was assessed for neuro-vascular procedures. The detector consists of a 250-microm-thick CsI(Tl) phosphor fiber-optically coupled through a 1.8:1 taper to a CCD chip, with an effective image pixel size of 50 microm and a frame rate of 5 fps in the 2:1 pixel-binned mode. The characteristics of the camera including the modulation transfer function (MTF), the noise equivalent quanta, the detective quantum efficiency, observer studies, and the effect of geometric magnification were evaluated. The MTF was found to have nonzero (1.7%) value at the Nyquist frequency of 10 cycles/mm, while the DQE(0) had a value of approximately 55%. All values were measured using head equivalent attenuating material in the beam at 80 kVp. Human observer studies performed using the 2 Alternative Forced Choice method revealed that iodinated vessels with inner diameter of 100 microm and 2 cm in length can be seen with a confidence level greater than 75%. The observer studies included a comparison with ideal observer performance calculations based on the integral signal to noise ratio in the image. Probabilities of visualization of various objects of interest in a neuro-intervention, such as stents, were assessed. A geometric magnification of 1 was found to be best for imaging under neuro-angiographic conditions. The detector appeared to satisfy all the demands of neuro-angiography and showed promise as an improvement over existing angiographic detectors.     

Segmented crystalline scintillators: an initial investigation of high quantum efficiency detectors for megavoltage x-ray imaging

Electronic portal imaging devices (EPIDs) based on indirect detection, active matrix flat panel imagers (AMFPIs) have become the technology of choice for geometric verification of patient localization and dose delivery in external beam radiotherapy. However, current AMFPI EPIDs, which are based on powdered-phosphor screens, make use of only approximately 2% of the incident radiation, thus severely limiting their imaging performance as quantified by the detective quantum efficiency (DQE) (approximately 1%, compared to approximately 75% for kilovoltage AMFPIs). With the rapidly increasing adoption of image-guided techniques in virtually every aspect of radiotherapy, there exist strong incentives to develop high-DQE megavoltage x-ray imagers, capable of providing soft-tissue contrast at very low doses in megavoltage tomographic and, potentially, projection imaging. In this work we present a systematic theoretical and preliminary empirical evaluation of a promising, high-quantum-efficiency, megavoltage x-ray detector design based on a two-dimensional matrix of thick, optically isolated, crystalline scintillator elements. The detector is coupled with an indirect detection-based active matrix array, with the center-to-center spacing of the crystalline elements chosen to match the pitch of the underlying array pixels. Such a design enables the utilization of a significantly larger fraction of the incident radiation (up to 80% for a 6 MV beam), through increases in the thickness of the crystalline elements, without loss of spatial resolution due to the spread of optical photons. Radiation damage studies were performed on test samples of two candidate scintillator materials, CsI(Tl) and BGO, under conditions relevant to radiotherapy imaging. A detailed Monte Carlo-based study was performed in order to examine the signal, spatial spreading, and noise properties of the absorbed energy for several segmented detector configurations. Parameters studied included scintillator material, septal wall material, detector thickness, and the thickness of the septal walls. The results of the Monte Carlo simulations were used to estimate the upper limits of the modulation transfer function, noise power spectrum and the DQE for a select number of configurations. An exploratory, small-area prototype segmented detector was fabricated by infusing crystalline CsI(Tl) in a 2 mm thick tungsten matrix, and the signal response was measured under radiotherapy imaging conditions. Results from the radiation damage studies showed that both CsI(Tl) and BGO exhibited less than approximately 15% reduction in light output after 2500 cGy equivalent dose. The prototype CsI(Tl) segmented detector exhibited high uniformity, but a lower-than-expected magnitude of signal response. Finally, results from Monte Carlo studies strongly indicate that high scintillator-fill-factor configurations, incorporating high-density scintillator and septal wall materials, could achieve up to 50 times higher DQE compared to current AMFPI EPIDs.     

A comparison of the performance of digital mammography systems

An objective analysis of image quality parameters was performed for six digital mammography systems. The presampled modulation transfer function (MTF), normalized noise power spectrum (NNPS), and detective quantum efficiency (DQE) for the systems were determined at different doses, for 28 kVp with a Mo/Mo or W/Al target/filter combination and 2 mm of additional aluminium filtration. The flat-panel units have higher MTF and DQE in the mid to high frequency range than standard CR systems. The highest DQE, over the whole dose range, is for the slit-scanning direct photon counting system. Dual-side read CR can overcome the inherent x-ray absorption and signal collection limitations of standard CR mammography, improving the low-frequency DQE by 40%, to the same level as full-field systems, but it does not improve the poor spatial resolution of phosphor.     

Image quality in two phosphor-based flat panel digital radiographic detectors

Two general types of phosphor screens are currently used in indirect digital radiographic systems: structured phosphor screens and turbid phosphor screens. The purpose of the study was to experimentally compare the image quality characteristics of two flat-panel digital radiography detectors with similar electronics and pixel sizes (0.127 mm), but otherwise equipped with the two types of screens (0.6-mm-thick structured CsI and Lanex Regular). The presampled modulation transfer functions (MTFs) of the detectors were assessed using an edge method. The noise power spectra (NPS) were measured by two-dimensional Fourier analysis of uniformly-exposed radiographs at 50-100 kVp with 19 mm added Al filtration. The detective quantum efficiencies (DQEs) were assessed from the MTF, the NPS, and estimates of the ideal signal-to-noise ratio. The MTF measures of the two detectors were generally similar above a spatial frequency of 2 mm(-1), with approximately 2.5 and approximately 3.8 mm(-1) spatial frequencies corresponding to 0.2 MTF and 0.1 MTF, respectively. Below 2 mm(-1), the MTF for the CsI-based detector was slightly higher by an average of 0.07. At 70 kVp, the measured DQE values in the diagonal (and axial) direction(s) at spatial frequencies of 0.15 mm(-1) and 2.5 mm(-1) were 78% (78%) and 26% (20%) for the CsI-based detector, and 20% (20%) and 7% (6%) for the Lanex-based detector, respectively. The comparative findings experimentally confirm that in indirect flat-panel detectors, structured phosphor screens provide a more favorable tradeoff between resolution and noise compared to turbid-phosphor screens, effectively increasing the detection efficiency of the detector without a negative impact on the detector's spatial resolution response.     

Imaging properties of digital magnification radiography

Flat panel detectors exhibit improved signal-to-noise ratio (SNR) and display capabilities compared to film. This improvement necessitates a new evaluation of optimal geometry for conventional projection imaging applications such as digital projection mammography as well as for advanced x-ray imaging applications including cone-beam computed tomography (CT), tomosynthesis, and mammotomography. Such an evaluation was undertaken in this study to examine the effects of x-ray source distribution, inherent detector resolution, magnification, scatter rejection, and noise characteristics including noise aliasing. A model for x-ray image acquisition was used to develop generic results applicable to flat panel detectors with similar x-ray absorption characteristics. The model assumed a Gaussian distribution for the focal spot and a rectangular distribution for a pixel. A generic model for the modulated transfer function (MTF) of indirect flat panel detectors was derived by a nonlinear fit of empirical receptor data to the Burgess model for phosphor MTFs. Noise characteristics were investigated using a generic noise power spectrum (NPS) model for indirect phosphor-based detectors. The detective quantum efficiency (DQE) was then calculated from the MTF and NPS models. The results were examined as a function of focal spot size (0.1, 0.3, and 0.6 mm) and pixel size (50, 100, 150, and 200 microm) for magnification ranges 1 to 3. Mammography, general radiography (also applicable to mammotomography), and chest radiography applications were explored using x-ray energies of 28, 74, and 120 kVp, respectively. Nodule detection was examined using the effective point source scatter model, effective DQE, and the Hotelling SNR2 efficiency. Results indicate that magnification can potentially improve the signal and noise performance of digital images. Results also show that a cross over point occurs in the spatial frequency above and below which the effects of magnification differ indicating that there are task dependent tradeoffs associated with magnification. The cross over point varies depending upon focal spot size, pixel size, x-ray energy, and source-to-image-distance (SID). For mammography, the cross over point occurs for a 0.3 mm focal spot while a 0.6 mm focal spot indicates that magnification does not improve image quality due to focal spot blurring. Thus, the benefit of magnification may be limited. For general radiography (as well as mammotomography), and chest radiography, the cross over point changes with SID. For a system with a 0.3 mm focal spot, 100 microm pixel size, a 2 m SID, and the applicable tissue thickness and scatter components, optimal magnification improved SNR2 by approximately 1.2 times for mammography and 1.5 times for general radiography (and mammotomography). These results indicate that the optimal geometry can improve image quality without changing patient dose or otherwise reduce dose without compromising image quality.     

Proportionality between Wiener spectra of quantum mottle and the squares of modulation transfer functions.

Rossmann proposed that the Wiener spectra of the quantum mottle of radiographs made using screen-film systems were proportional to the squares of the modulation transfer functions (MTFs) of the screen-film systems. On the other hand, Lubberts theoretically pointed out that the shape of the Wiener spectrum of the quantum mottle depended on the sum of the squares of the MTFs for different depths in the screen phosphor layer, rather than the square of the sum of the MTFs for the different depths, i.e. the square of the MTF of the screen-film systems. The purpose of this study is to experimentally investigate the proportionality between the Wiener spectra of the quantum mottle and the squares of the MTFs of screen-film systems using two screen-film systems having different screen thicknesses. For this purpose, we determined correction factors for the square of the MTF of the screen-film system in the Wiener spectrum of the quantum mottle at each spatial frequency when the Wiener spectral values of the screen mottle were separated into those of the quantum mottle and structure mottle. The correction factor is the ratio of the normalized Wiener spectrum of the quantum mottle to the square of the MTF of the screen-film system. As a result, for a thin screen, the correction factors were unity for all spatial frequencies; on the contrary, for a thick screen, the factor increased with spatial frequency. By calculating the theoretical correction factors using the models for the MTF and Wiener spectrum of the quantum mottle of Nishikawa and Yaffe based on Lubberts' theory, we verified that our experimental results agreed with Lubberts' theory. Furthermore, by obtaining the screen thickness dependence of the theoretical correction factors for the two screens, we showed that, for screens thinner than 0.02 mm, Rossmann's theory can be applied to the relationship between the Wiener spectrum of the quantum mottle and the MTF of the screen-film system, whereas for screens thicker than 0.02 mm, Lubberts' theory should be applied.     

Experimental investigation of the dose and image quality characteristics of a digital mammography imaging system

Our purpose in this study was to investigate the image quality and absorbed dose characteristics of a digital mammography imaging system with a CsI scintillator, and to identify an optimal x-ray tube voltage for imaging simulated masses in an average size breast with 50% glandularity. Images were taken of an ACR accreditation phantom using a LORAD digital mammography system with a Mo target and a Mo filter. In one experiment, exposures were performed at 80 mAs with x-ray tube voltages varying between 24 and 34 kVp. In a second experiment, the x-ray tube voltage was kept constant at 28 kVp and the technique factor was varied between 5 and 500 mAs. The average glandular dose at each x-ray tube voltage was determined from measurements of entrance skin exposure and x-ray beam half-value layer. Image contrast was measured as the fractional digital signal intensity difference for the image of a 4 mm thick acrylic disk. Image noise was obtained from the standard deviation in a uniformly exposed region of interest expressed as a fraction of the background intensity. The measured digital signal intensity was proportional to the mAs and to the kVp5.8. Image contrast was independent of mAs, and dropped by 21% when the x-ray tube voltage increased from 24 to 34 kVp. At a constant x-ray tube voltage, image noise was shown to be approximately proportional to (mAs)(-05), which permits the image contrast to noise ratio (CNR) to be modified by changing the mAs. At 80 mAs, increasing the x-ray tube voltage from 24 to 34 kVp increased the CNR by 78%, and increased the average glandular dose by 285%. At a constant lesion CNR, the lowest average glandular dose value occurred at 27.3 kVp. Increasing or decreasing the x-ray tube voltage by 2.3 kVp from the optimum kVp increased the average glandular dose values by 5%. These results show that imaging simulated masses in a 4.2 cm compressed breast at approximately 27 kVp with a Mo/Mo target/filter results in the lowest average glandular dose.