Influence of cassette type on the DQE of CR systems

In our recent paper by Monnin et al. [Med. Phys. 33, 411-420 (2006)], an objective analysis of the relative performance of a computed radiography (CR) system using both standard single-side (ST-VI) and prototype dual-side read (ST-BD) plates was reported. The presampled modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) for the systems were determined at three different beam qualities representative of paediatric chest radiography, at an entrance detector air kerma of 5 microGy. Experiments demonstrated that, compared to the standard single-side read system, the MTF for the dual-side read system was slightly reduced, but a significant decrease in image noise resulted in a marked increase in DQE (+40%) in the low spatial frequency range. However, the DQE improvement for the ST-BD plate decreased with increasing spatial frequency, and, at spatial frequencies above 2.2 mm(-1), the DQE of the dual-side read system was lower than that of the single-side one.     

Performance evaluation of a "dual-side read" dedicated mammography computed radiography system

The image quality of a dedicated mammography computed radiography (CR) system was characterized. A unique feature of this system is that it collects image signals from both sides of the storage phosphor. Measurements of the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) were made. This work included improvements in our measurement methods to specifically account for the detrimental effects of system glare on the MTF and to accurately characterize the low-frequency NPS components. Image quality measurements were performed using a 25 kVp beam filtered with 2 mm Al and an exposure range of 1 to 100 mR (87 to 870 microGy). The DQE was found to decrease with increasing exposure due to an increased contribution of storage phosphor structure noise. The DQE of this system was compared to similar measurements made using a standard CR system. The dual-side read system demonstrated superior DQE compared to the standard system. The decrease in DQE with increasing exposure was more severe for the standard system than the dual-side read system. This finding suggests that the CR system noise was reduced for the dual-side read system compared to the standard system.     

Comparison between a built-in "dual side" chest imaging device and a standard "single side" CR

An integrated readout computed radiography system (Fuji XU-D1) incorporating dual-side imaging plates (ST-55BD) was analyzed in terms of modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) for standard beam qualities RQA 9 and RQA 5. NPS and DQE were assessed using a detector entrance air kerma consistent with clinical practice for chest radiography. Similar investigation was performed on a standard reader (Fuji FCR 5000) using single-side imaging plates (ST-VI). Negligible differences were found between the MTFs of the two imaging systems for RQA 9, whereas for RQA 5 the single-side system exhibited slightly superior MTF. Regarding noise response, the dual-side system turned out to be better performing for both beam qualities over a wide range of frequencies. For RQA 9, at 8 microGy, the DQE of the dual-side system was moderately higher over the whole frequency range, whereas for RQA 5, at 10 microGy, significant improvement was found at low- and midrange frequencies. As an example, at 1 cycle/mm, the following improvements in the DQE of the dual-side system were observed: +22% (RQA 9, at 8 microGy), +50% (RQA 9, at 30 microGy), and +45% (RQA 5, at 10 microGy).     

Image quality evaluation of a desktop computed radiography system

The modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of the Lumisys ACR-2000 desktop computed radiography (CR) reader were measured and compared to equivalent measurements acquired from a Fuji AC-3 CR system. The one-dimensional (1D) MTF was measured from an image of a sharp edge and the 1D NPS was derived from a 2D NPS measured from a uniform field exposure. The energy dependent ideal input signal to noise ratio of the incident x-ray beams was estimated using published x-ray spectra and attenuation coefficients. Measurements were acquired using Agfa, Fuji, and Kodak storage phosphor plates and it was concluded that use of the Fuji plates resulted in the highest system DQE for the ACR-2000. The DQE was measured using exposures of 0.10, 1.0, and 10.0 mR from 70 and 120 kVp x-ray beams filtered with aluminum. The DQE of the Lumisys ACR-2000 was lower than that of the Fuji AC-3.     

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.     

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.     

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.     

Experimental comparison of noise and resolution for 2k and 4k storage phosphor radiography systems.

The purpose of this study was to compare the image quality for a digital storage phosphor system using 1760 x 2140 (2k) and 3520 x 4280 (4k) image arrays. Measurements were made on a chest radiography system (Fuji FCR-9501) with special provisions to be operated in both 2k (standard) and 4k (HQ) modes. Presampled modulation transfer functions (MTF) were measured using an edge method. Noise power spectra (NPS) were determined for different input exposures by two-dimensional Fourier analysis. These measures along with exposure measurements and an x-ray spectral model were used to determine the frequency-dependent detective quantum efficiency DQE (f) of the system for the 4k and the 2k modes. The magnitude of the NPS for the 4k mode was about 1/2 that of the 2k mode. A MTF value of 0.5 was found at 1.25 cycles/mm for the 4k system and 1.50 cycles/mm for the 2k system. The 4k images had an extended MTF of 0.1 at 4.5 cycles/mm in the plate-scan direction. Overall, the DQE (f) of the 4k mode was slightly better than that for the 2k mode by about 0.02 due primarily to its better noise characteristics.     

Characterization of noise sources for two generations of computed radiography systems using powder and crystalline photostimulable phosphors

The performances of two generations of computed radiography (CR) were tested and compared in terms of resolution and noise characteristics. The main aim was to characterize and quantify the noise sources in the images. The systems tested were (1) Agfa CR 25.0, a flying spot reader with powder phosphor image plates (MD 40.0); and (2) the Agfa DX-S, a line-scanning CR reader with needle crystal phosphor image plates (HD 5.0). For both systems, the standard metrics of presampled modulation transfer function (MTF), normalized noise power spectra (NNPS) and detective quantum efficiency (DQE) were measured using standard radiation quality RQA5 as defined by the International Electrotechnical Commission. The various noise sources contributing to the NNPS were separated by using knowledge of their relationship with air kerma, MTF, absorption efficiency and antialiasing filters. The DX-S MTF was superior compared with the CR 25.0. The maximum difference in MTF between the DX-S scan and CR 25.0 subscan directions was 0.13 at 1.3 mm(-1). For a nominal detector air kerma of 4 microGy, the peak DQE of the DX-S was 43 (+/-3)%, which was over double that of the CR 25.0 of 18 (+/-2)%. The additive electronic noise was negligible on the CR 25.0 but calculated to be constant 3.4 x 10(-7) (+/-0.4 x 10(-7)) mm2 at 3.9 microGy on the DX-S. The DX-S has improved image quality compared with a traditional flying spot reader. The separation of the noise sources indicates that the improvements in DQE of the DX-S are due not only to the higher quantum, efficiency and MTF, but also the lower structure, secondary quantum, and excess noise.     

Imaging performance of amorphous selenium based flat-panel detectors for digital mammography: characterization of a small area prototype detector

Our work is to investigate and understand the factors affecting the imaging performance of amorphous selenium (a-Se) flat-panel detectors for digital mammography. Both theoretical and experimental methods were developed to investigate the spatial frequency dependent detective quantum efficiency [DQE(f)] of a-Se flat-panel detectors for digital mammography. Since the K edge of a-Se is 12.66 keV and within the energy range of a mammographic spectrum, a theoretical model was developed based on cascaded linear system analysis with parallel processes to take into account the effect of K fluorescence on the modulation transfer function (MTF), noise power spectrum (NPS), and DQE(f) of the detector. This model was used to understand the performance of a small-area prototype detector with 85 microm pixel size. The presampling MTF, NPS, and DQE(f) of the prototype were measured, and compared to the theoretical calculation of the model. The calculation showed that K fluorescence accounted for a 15% reduction in the MTF at the Nyquist frequency (fNy) of the prototype detector, and the NPS at fNy was reduced to 89% of that at zero spatial frequency. The measurement of presampling MTF of the prototype detector revealed an additional source of blurring, which was attributed to charge trapping in the blocking layer at the interface between a-Se and the active matrix. This introduced a drop in both presampling MTF and NPS at high spatial frequency, and reduced aliasing in the NPS. As a result, the DQE(f) of the prototype detector at fNy approached 40% of that at zero spatial frequency. The measured and calculated DQE(f) using the linear system model have reasonable agreement, indicating that the factors controlling image quality in a-Se based mammographic detectors are fully understood, and the model can be used to further optimize detector imaging performance.     

Fundamental imaging characteristics of a slot-scan digital chest radiographic system

Our purpose in this study was to evaluate the fundamental image quality characteristics of a new slot-scan digital chest radiography system (ThoraScan, Delft Imaging Systems/Nucletron, Veenendaal, The Netherlands). The linearity of the system was measured over a wide exposure range at 90, 117, and 140 kVp with added Al filtration. System uniformity and reproducibility were established with an analysis of images from repeated exposures. The modulation transfer function (MTF) was evaluated using an established edge method. The noise power spectrum (NPS) and the detective quantum efficiency (DQE) of the system were evaluated at the three kilo-voltages over a range of exposures. Scatter fraction (SF) measurements were made using a posterior beam stop method and a geometrical chest phantom. The system demonstrated excellent linearity, but some structured nonuniformities. The 0.1 MTF values occurred between 3.3-3.5 mm(-1). The DQE(0.15) and DQE(2.5) were 0.21 and 0.07 at 90 kVp, 0.18 and 0.05 at 117 kVp, and 0.16 and 0.03 at 140 kVp, respectively. The system exhibited remarkably lower SFs compared to conventional full-field systems with anti-scatter grid, measuring 0.13 in the lungs and 0.43 in the mediastinum. The findings indicated that the slot-scan design provides marked scatter reduction leading to high effective DQE (DQEeff) of the system and reduced patient dose required to achieve high image quality.     

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.     

A comparison of the performance of modern screen-film and digital mammography systems

This work compares the detector performance and image quality of the new Kodak Min-R EV mammography screen-film system with the Fuji CR Profect detector and with other current mammography screen-film systems from Agfa, Fuji and Kodak. Basic image quality parameters (MTF, NPS, NEQ and DQE) were evaluated for a 28 kV Mo/Mo (HVL = 0.646 mm Al) beam using different mAs exposure settings. Compared with other screen-film systems, the new Kodak Min-R EV detector has the highest contrast and a low intrinsic noise level, giving better NEQ and DQE results, especially at high optical density. Thus, the properties of the new mammography film approach those of a fine mammography detector, especially at low frequency range. Screen-film systems provide the best resolution. The presampling MTF of the digital detector has a value of 15% at the Nyquist frequency and, due to the spread size of the laser beam, the use of a smaller pixel size would not permit a significant improvement of the detector resolution. The dual collection reading technology increases significantly the low frequency DQE of the Fuji CR system that can at present compete with the most efficient mammography screen-film systems.     

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.     

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.     

An experimental comparison of detector performance for computed radiography systems

The intrinsic resolution, noise, and signal-to-noise transfer characteristics of five commercial digital computed radiography (CR) systems were compared using identical experimental methods. The reader/screen combinations evaluated were Agfa ADC-Compact/MD-10, Agfa ADC-Compact/MD-30, Agfa ADC-Solo/MD-10, Agfa ADC-Solo/MD-30, Lumisys CR-2000/MD-10, Fuji FCR-9501 (HQ)/ST-Va, Kodak CR-400/GP-25, and Kodak CR-400/HR. Measurements were made at 70 and 115 kVp with 19 mm added aluminum filtration. The presampled modulation transfer functions (MTFs) of the systems were measured using an edge method. The noise power spectra (NPS) were determined by 2D Fourier analysis of uniformly exposed radiographs. The frequency-dependent detective quantum efficiencies (DQEs) were computed from the MTF, NPS, exposure measurements, and computational estimates of the ideal signal-to-noise ratios. Using 70 kVp and 0.1-0.12 mm pixel sizes, spatial frequencies of 2.1, 2.0, 2.2, 1.9, 2.0, 2.0, 2.3, 2.3, and 3.5 cycles/mm were measured at 0.2 MTF for the eight reader/screen combinations, respectively. Using 70 kVp, 7.74 x 10(-8) C/kg (0.3 mR), and 0.1-0.12 mm pixel sizes, DQE(0.15) values of 20.3%, 22.9%, 24.6%, 28.6%, 22.2%, 30.0%, 29.5%, and 17.3% were obtained for the eight combinations, respectively. The corresponding values at 115 kVp were 15.9%, 18.5%, 21.5%, 21.8%, 15.3%, 23.1%, 22.3%, and 13.8%, respectively. The findings of the study demonstrate the pixel size, orientation, beam quality, screen, and reader dependencies of image quality in CR systems. The physical performance of the systems having standard-resolution screens demonstrated similar resolution performance but more notable variations in DQE. The one high-resolution screen tested had reduced DQE and increased MTF at high frequencies.     

A comparison of the imaging characteristics of the new Kodak Hyper Speed G film with the current T-MAT G/RA film and the CR 9000 system

Three standard radiation qualities (RQA 3, RQA 5 and RQA 9) and two screens, Kodak Lanex Regular and Insight Skeletal, were used to compare the imaging performance and dose requirements of the new Kodak Hyper Speed G and the current Kodak T-MAT G/RA medical x-ray films. The noise equivalent quanta (NEQ) and detective quantum efficiencies (DQE) of the four screen-film combinations were measured at three gross optical densities and compared with the characteristics for the Kodak CR 9000 system with GP (general purpose) and HR (high resolution) phosphor plates. The new Hyper Speed G film has double the intrinsic sensitivity of the T-MAT G/RA film and a higher contrast in the high optical density range for comparable exposure latitude. By providing both high sensitivity and high spatial resolution, the new film significantly improves the compromise between dose and image quality. As expected, the new film has a higher noise level and a lower signal-to-noise ratio than the standard film, although in the high frequency range this is compensated for by a better resolution, giving better DQE results--especially at high optical density. Both screen-film systems outperform the phosphor plates in terms of MTF and DQE for standard imaging conditions (Regular screen at RQA 5 and RQA 9 beam qualities). At low energy (RQA 3), the CR system has a comparable low-frequency DQE to screen-film systems when used with a fine screen at low and middle optical densities, and a superior low-frequency DQE at high optical density.     

Assessment of the effects of pixel loss on image quality in direct digital radiography

Modern digital radiographic 'flat panel' detectors can exhibit a progressive form of image degradation arising from non-functioning pixels. The effect of these 'dead pixels' on the quantitive image quality measures of modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) is investigated by a simulated degradation of images obtained from an Hologic EPEX system. The effects on the semi-quantitive measures obtained from contrast threshold test objects and resolution gratings are also investigated. Results suggest that the contrast-detail tests often employed in quality assurance measures are not sufficient to reveal the presence of dead pixels until well beyond the recommended replacement point for the flat panel detector. However, measurements of spatial resolution using a line pairs phantom were found to be more sensitive to pixel loss. Measurement of the MTF, NPS and DQE can reveal small changes in image quality with increasing pixel loss, with a distinctive pattern in the trend of the NPS.     

Improved computed radiography image quality from a BaFl:Eu photostimulable phosphor plate

Recent advances in the design of photostimulable phosphor (PSP) plates for computed radiography (CR) systems have made it possible to manufacture plates made of BaFI:Eu phosphor material in the cassette form. The image quality of this plate, six BaF(Br,I):Eu plates, and one BaFBr:Eu plate were evaluated in terms of presampling modulation transfer function (MTF), normalized Wiener spectra (WSN), and detective quantum efficiency (DQE). Compared with the best-performing BaF(Br,I):Eu plate, the BaFI:Eu plate provided DQE that was higher, at spatial frequencies of 0.5, 1.0, and 2.0 cycles/mm, by 12% (21.8 versus 19.4), 13% (18.8 versus 16.7), and 11% (12.0 versus 10.8), respectively. Since presampling MTF values of the two plates were comparable, the BaFI:Eu plate's higher DQE is attributable to total WSN conversely being lower by 17% [8.65 x 10(-5) (mm2) versus 10.38 x 10(-5) (mm2)], 17% [5.85 x 10(-5) (mm2) versus 7.03 x 10(-5) (mm2)], and 12% [2.82 x 10(-5) (mm2) versus 3.19 x 10(-5) (mm2)] at the specified frequencies, respectively, primarily due to the contribution of x-ray quantum WSN. This jibes with the high x-ray absorption provided by the 27%-higher x-ray attenuation coefficients (7.54 versus 6.07, at 60 KeV) that BaFI offers over BaF(Br(0.85),I(0.15)), a result of the high atomic number of BaFI's exclusively iodine halide content. The results were consistent with earlier studies of several of these same plates and indicate that BaFI:Eu is a promising avenue to lower image noise and higher overall CR system image quality.     

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.