A MONTE-CARLO SIMULATION FRAMEWORK FOR JOINT OPTIMISATION OF IMAGE QUALITY AND PATIENT DOSE IN DIGITAL PAEDIATRIC RADIOGRAPHY

In paediatric radiography, according to the as low as reasonably achievable (ALARA) principle, the imaging task should be performed with the lowest possible radiation dose. This paper describes a Monte-Carlo simulation framework for dose optimisation of imaging parameters in digital paediatric radiography. Patient models with high spatial resolution and organ segmentation enable the simultaneous evaluation of image quality and patient dose on the same simulated radiographic examination. The accuracy of the image simulation is analysed by comparing simulated and acquired images of technical phantoms. As a first application example, the framework is applied to optimise tube voltage and pre-filtration in newborn chest radiography. At equal patient dose, the highest CNR is obtained with low-kV settings in combination with copper filtration.

ITERATIVE SCATTER CORRECTION FOR GRID-LESS BEDSIDE CHEST RADIOGRAPHY: PERFORMANCE FOR A CHEST PHANTOM

The aim of this work was to experimentally compare the contrast improvement factors (CIFs) of a newly developed software-based scatter correction to the CIFs achieved by an antiscatter grid. To this end, three aluminium discs were placed in the lung, the retrocardial and the abdominal areas of a thorax phantom, and digital radiographs of the phantom were acquired both with and without a stationary grid. The contrast generated by the discs was measured in both images, and the CIFs achieved by grid usage were determined for each disc. Additionally, the non-grid images were processed with a scatter correction software. The contrasts generated by the discs were determined in the scatter-corrected images, and the corresponding CIFs were calculated. The CIFs obtained with the grid and with the software were in good agreement. In conclusion, the experiment demonstrates quantitatively that software-based scatter correction allows restoring the image contrast of a non-grid image in a manner comparable with an antiscatter grid.

Optimizing image quality and dose for digital radiography of distal pediatric extremities using the contrast-to-noise ratio

PURPOSE

To investigate the influence of X-ray tube voltage and filtration on image quality in terms of contrast-to-noise ratio (CNR) and dose for digital radiography of distal pediatric extremities and to determine conditions that give the best balance of CNR and patient dose.

MATERIALS AND METHODS

In a phantom study simulating the absorption properties of distal extremities, the CNR and the related patient dose were determined as a function of tube voltage in the range 40 - 66 kV, both with and without additional filtration of 0.1 mm Cu/1 mm Al. The measured CNR was used as an indicator of image quality, while the mean absorbed dose (MAD) - determined by a combination of measurement and simulation - was used as an indicator of the patient dose.

RESULTS

The most favorable relation of CNR and dose was found for the lowest tube voltage investigated (40 kV) without additional filtration. Compared to a situation with 50 kV or 60 kV, the mean absorbed dose could be lowered by 24 % and 50 %, respectively, while keeping the image quality (CNR) at the same level.

CONCLUSION

For digital radiography of distal pediatric extremities, further CNR and dose optimization appears to be possible using lower tube voltages. Further clinical investigation of the suggested parameters is necessary.

Digital chest radiography: an update on modern technology, dose containment and control of image quality

The introduction of digital radiography not only has revolutionized communication between radiologists and clinicians, but also has improved image quality and allowed for further reduction of patient exposure. However, digital radiography also poses risks, such as unnoticed increases in patient dose and suboptimum image processing that may lead to suppression of diagnostic information. Advanced processing techniques, such as temporal subtraction, dual-energy subtraction and computer-aided detection (CAD) will play an increasing role in the future and are all targeted to decrease the influence of distracting anatomic background structures and to ease the detection of focal and subtle lesions. This review summarizes the most recent technical developments with regard to new detector techniques, options for dose reduction and optimized image processing. It explains the meaning of the exposure indicator or the dose reference level as tools for the radiologist to control the dose. It also provides an overview over the multitude of studies conducted in recent years to evaluate the options of these new developments to realize the principle of ALARA. The focus of the review is hereby on adult applications, the relationship between dose and image quality and the differences between the various detector systems.

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.

An image quality comparison of standard and dual-side read CR systems for paediatric radiology

An objective analysis of image quality parameters was performed for a computed radiography (CR) system using both standard single-side and prototype dual-side read plates. The pre-sampled 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 pediatric chest radiography, at an entrance detector air kerma of 5 microGy. The NPS and DQE measurements were realized under clinically relevant x-ray spectra for pediatric radiology, including x-ray scatter radiations. Compared to the standard single-side read system, the MTF for the dual-side read system is reduced, but this is offset by a significant decrease in image noise, resulting in a marked increase in DQE (+40%) in the low spatial frequency range. Thus, for the same image quality, the new technology permits the CR system to be used at a reduced dose level.

Flat-panel-detector chest radiography: effect of tube voltage on image quality

PURPOSE

To compare the visibility of anatomic structures in direct-detector chest radiographs acquired with different tube voltages at equal effective doses to the patient.

MATERIALS AND METHODS

The study protocol was approved by the institutional internal review board, and written informed consent was obtained from all patients. Posteroanterior chest radiographs of 48 consecutively selected patients were obtained at 90, 121, and 150 kVp by using a flat-panel-detector unit that was based on cesium iodide technology and automated exposure control. Monte Carlo simulations were used to verify that the effective dose for all kilovoltage settings was equal. Five radiologists subjectively and independently rated the delineation of anatomic structures on hard-copy images by using a five-point scale. They also ranked image quality in a blinded side-by-side comparison. Average ranking scores were compared by using one-way analysis of variance with repeated measures. Data were analyzed for the entire patient group and for two patient subgroups that were formed according to body mass index (BMI).

RESULTS

The visibility scores of most anatomic structures were significantly superior with the 90-kVp images (mean score, 3.11), followed by the 121-kVp (mean score, 2.95) and 150-kVp images (mean score, 2.80). Differences did not reach significance (P > .05) only for the delineation of the peripheral vessels, the heart contours, and the carina. This was also true for the subgroup of patients (n = 24) with a BMI greater than and the subgroup of patients (n = 24) with a BMI less than the mean BMI (26.9 kg/m(2)). At side-by-side comparison, the readers rated 90-kVp images as having superior image quality in the majority of image triplets; the percentage of 90-kVp images rated as "first choice" ranged from 60% (29 of 48 patients) to 90% (43 of 48 patients), with a median of 88% (42 of 48 patients), among the readers.

CONCLUSION

Delineation of most anatomic structures and overall image quality were ranked superior in digital radiographs acquired with lower kilovoltage at a constant effective patient dose.

Status and prospects of digital detector technology for CR and DR

Projection radiography is in the middle of the transition from conventional screen-film imaging to digital image acquisition modalities, mainly based on imaging plates (computed radiography, CR) and flat-panel detectors (direct radiography, DR). Cassette-based CR has been available for the past 20 y, and constitutes the major part of direct radiography installations in hospitals today. direct radiography systems based on large-area amorphous silicon active matrix arrays are commercially available for the last 5 y and exist basically in two different types: with scintillators or photoconductors as X-ray conversion material ('indirect' or 'direct' type). direct radiography systems allow for improved image quality and/or dose reduction due to their high detective quantum efficiency and enable faster workflow because of instant image availability. However, new technology developments are improving the performance for CR systems as well, rendering it competitive to direct radiography in many practical aspects. Therefore, it is assumed that the CR and direct radiography systems will coexist for many years to come. This paper reviews the digital detector technologies and the possible future directions of development.

Measurement of the modulation transfer function of digital X-ray detectors with an opaque edge-test device

A variant of the edge method for the determination of the pre-sampled modulation transfer function (MTF) of digital X-ray imaging devices has been developed and accepted as the standard method in the novel DQE measurement standard IEC 62220-1. An opaque tungsten edge-test device accomplishes the ideal step-like profile of the incident X rays. The edge spread function is measured over a large region across the edge transition that enables an accurate MTF measurement including the 'low-frequency drop'. The method has been applied to different state-of-the-art X-ray imaging detectors, a computed radiography, a CsI-based indirect and an Se-based direct flat-panel detector. The MTF measurement results will be presented. In contrast to the opaque edge device, the commonly used semi-transparent edge-test devices produce scatter radiation that deteriorates the incident X-ray profile, which leads to a systematic overestimation of the MTF.

Retrospective patient dose analysis of a digital radiography system in routine clinical use

The log files generated in the flat-panel detector based digital radiography systems (Philips Digital Diagnost) at 11 different hospitals were used to acquire data regarding the dose-area product (DAP) and the entrance air kerma (EAK) for all X-ray examinations performed in routine clinical use. A retrospective statistical analysis of the data with respect to the dose levels was performed for several examination types. The resulting average dose levels were compared with the diagnostic reference levels (DRLs). While for the same type of examination the median dose level at different hospitals could differ by up to a factor of 5, almost all the dose levels were found to be much below the published DRL. Only in one hospital for one type of examination (Thorax PA) the DRL for the DAP was slightly exceeded, while for the same site and examination the EAK was still below the DRL. In conclusion, examination log files can be used for retrospective dose analysis and correlation with DRLs.

Determination of the modulation transfer function using the edge method: Influence of scattered radiation

The edge method for measuring the modulation transfer function (MTF) has recently gained popularity due to its simplicity and appropriateness particularly for digital imaging systems. Often edge test devices made of rather thin metal sheets are used, which are semitransparent to x rays and may generate scattered radiation. The effect of this scattered radiation on the determined MTF was investigated both theoretically (assuming an ideal detector) and experimentally using a CsI-based digital detector. It was found that the MTF increases due to the scattered radiation for all spatial frequencies larger than 0 mm(-1). The theoretical model developed in this study predicts that the maximum error compared to the true detector MTF is given by S/A, where A is the attenuated fraction and S is the scattered fraction reaching the detector, relative to the incident radiation. Theoretical and experimental results are in good agreement for radiation qualities corresponding to general radiography (RQA3, RQA5, and RQA7), whereas for chest beam quality (RQA9) the experimentally observed MTF error is larger than predicted by the simple model, possibly because the energy response of the CsI-based detector differs from that of an ideal one. The theoretical MTF error reaches a value of 18% for a 0.25 mm thick lead edge of RQA9. Since the MTF enters squared into the determination of the detective quantum efficiency (DQE), an error of at least 36% in DQE may result when using this edge test device. In conclusion, the use of fully absorbing edge material is advised for MTF determination with the edge method.

Management of pediatric radiation dose using Philips digital radiography

Digital radiography systems based on flat-panel detectors have been introduced into clinical practice in the past few years. The high detective quantum efficiency of these detectors allows the radiation dose to be reduced while maintaining image quality, an issue particularly significant for pediatric radiography. Another important aspect for dose optimization and monitoring is the integration of the detector into a computer-controlled x-ray examination system. This enables full control and complete reporting of all dose-relevant exposure parameters, including the determination of the exposure indicator and the patient dose (kerma-area product). In this paper the implementation of these principles in the Philips Digital Diagnost DR system is described and their relevance for pediatric applications is discussed.

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.

Comparison of an Automatic Versus a Semiautomatic Mode for Gray-Scale Adaptation for Digital Chest Radiography

PURPOSE

To compare image quality of digital chest radiographs using 2 modes of gradation adjustment.

METHODS

We compared image quality and visualization of anatomic landmarks of 50 chest radiographs after digital processing using a semiautomatic mode with a fixed gamma of 2.6 and an automatic mode with individual gamma adaptation.

RESULTS

The mean gamma was significantly higher (P < 0.001) with the automatic mode (3.0 vs. 2.6, respectively). Patient constitution had no impact on the automatically adapted gamma for PA but showed a significant correlation (P < 0.001) for lateral images. For PA, there was a preference (P < 0.016) of the semiautomatic mode in heavier patients whereas no difference was seen in slim patients. For the lateral projection, there was a general preference (P = 0.001) of the automatic mode.

CONCLUSION

For PA radiographs, the semiautomatic mode provides superior results for heavier patients without compromising the quality in slim patients. For lateral radiographs, the automatic mode provides generally superior results.

Skeletal Applications for Flat-Panel versus Storage-Phosphor Radiography: Effect of Exposure on Detection of Low-Contrast Details

PURPOSE

To compare exposure requirements for similar detection performance with flat-panel detectors and the most recent generation of storage-phosphor plates in the simulated scatter of typical skeletal radiographic examinations.

MATERIALS AND METHODS

A contrast-detail test object was covered with varying thicknesses of acrylic to simulate skeletal exposure conditions in the wrist, knee, and pelvis. Three series were obtained with increasing thicknesses of a simulated soft-tissue layer (5, 10, and 20 cm) and increasing tube voltage (50, 70, and 90 kVp). A fourth series was obtained with exposure conditions adapted to the phantom instructions (75 kVp). Images were acquired with a flat-panel detector (cesium iodide scintillator) and storage-phosphor plates at five exposure levels (speed class range, 100-1,600). Five readers evaluated 84 images to determine the threshold contrast of 12 lesion diameters (range, 0.25-11.1 mm). Statistical significance of differences between the two digital systems was assessed with two-way analysis of variance.

RESULTS

A linear relationship was found between the number of detected lesions and the logarithm of exposure (R(2) > 0.98 for all series). On average, the flat-panel system required 45% less exposure than did the phosphor plates when 20-cm-thick acrylic was superimposed on the test object. Differences in exposure requirements were smaller with decreasing thicknesses of simulated soft-tissue layers and lower tube voltages (39% at 10 cm and 70 kVp, and 17% at 5 cm and 50 kVp). All differences were statistically significant.

CONCLUSION

Flat-panel radiography provides improved contrast detectability and a potential for exposure reduction compared with those with storage-phosphor radiography. The best performance was achieved with conditions comparable to those for radiography of the trunk and lowest for conditions that simulate radiography of the extremities.

[Spatial resolution of digital systems: is the visual determination with a lead bar pattern meaningful?]

The factors influencing the spatial resolution of digital detectors are discussed and demonstrated on simulated bar pattern images. Lead bar patterns are often used in quality control procedures to test the spatial resolution of image detectors. In digital imaging systems the pixelization of the detector interferes with the periodic bar pattern and makes it difficult to determine the resolution limit. Bar pattern images seem to have only limited value for characterizing the imaging performance of digital detector systems.

Accuracy of a simple method for deriving the presampled modulation transfer function of a digital radiographic system from an edge image

Several methods for accurately deriving the presampled modulation transfer function (MTF) of a pixelated detector from the image of a slightly slanted edge have been described in the literature. In this paper we report on a simple variant of the edge method that produces sufficiently accurate MTF values for frequencies up to the Nyquist frequency limit of the detector with little effort in edge alignment and computation. The oversampled ESF is constructed in a very simple manner by rearranging the pixel data of N consecutive lines corresponding to a lateral shift of the edge by one pixel. A regular subsampling pitch is assumed for the oversampled ESF, which is given by the original pixel sampling distance divided by the integer number N. This allows the original data to be used for further computational analysis (differentiation and Fourier transform) without data preprocessing. Since the number of lines leading to an edge shift by one pixel generally is a fractional number rather than an integer, a systematic error may be introduced into the presampled MTF. Simulations and theoretical investigations show that this error is proportional to 1/N and increases with spatial frequency. For all frequencies up to the Nyquist limit, the relative error delta MTF/MTF is smaller than 1/(2N). It can thus be kept below a given threshold by suitably selecting N, which furnishes a certain maximum edge angle. The method is especially useful for applications where the presampled MTF is needed only for frequencies up to the Nyquist frequency limit, such as the determination of the detective quantum efficiency (DQE).

Principles of image processing in digital chest radiography

Image processing has a major impact on image quality and diagnostic performance of digital chest radiographs. Goals of processing are to reduce the dynamic range of the image data to capture the full range of attenuation differences between lungs and mediastinum, to improve the modulation transfer function to optimize spatial resolution, to enhance structural contrast, and to suppress image noise. Image processing comprises look-up table operations and spatial filtering. Look-up table operations allow for automated signal normalization and arbitrary choice of image gradation. The most simple and still widely applied spatial filtering algorithms are based on unsharp masking. Various modifications were introduced for dynamic range reduction and MTF restoration. More elaborate and more effective are multi-scale frequency processing algorithms. They are based on the subdivision of an image in multiple frequency bands according to its structural composition. This allows for a wide range of image manipulations including a size-independent enhancement of low-contrast structures. Principles of the various algorithms will be explained and their impact on image appearance will be illustrated by clinical examples. Optimum and sub-optimum parameter settings are discussed and pitfalls will be explained.

Direct digital radiography versus storage phosphor radiography in the detection of wrist fractures

AIM

To define the value of digital radiography with a clinical flat panel detector system for evaluation of wrist fractures in comparison with state of the art storage phosphor radiography.

MATERIAL AND METHODS

Hard copy images of 26 fractured wrist specimens were acquired with the same exposure dose on a state of the art storage phosphor radiography system and a clinical flat panel detector. Image features like cortical bone surface, trabecular bone, soft tissues and fracture delineation were independently analysed by 4 observers using a standardised protocol. Image quality ratings were evaluated with an analysis of variance (ANOVA).

RESULTS

Flat panel detector radiographs were rated superior with respect to cortical and trabecular bone representation as well as fracture evaluation, while storage phosphor radiographs produced better soft tissue detail.

CONCLUSION

In some of the observed image quality aspects, the performance of caesium iodide/amorphous silicon flat panel detector exceeds state of the art storage phosphor radiography. This makes it well suited for skeletal imaging particularly in trauma as seen in the detection of wrist fractures.

Digital radiography with a large-scale electronic flat-panel detector vs. screen-film radiography: observer preference in clinical skeletal diagnostics

The imaging performance of a recently developed digital flat-panel detector system was compared with conventional screen-film imaging in an observer preference study. In total, 34 image pairs of various regions of the skeleton were obtained in 24 patients; 30 image pairs were included in the study. The conventional images were acquired with 250- and 400-speed screen-film combinations, using the standard technique of our department. Within hours, the digital images were obtained using identical exposure parameters. The digital system employed a large-area (43x43 cm) flat-panel detector based on amorphous silicon (Trixell Pixium 4600), integrated in a Bucky table. Six radiologists independently evaluated the image pairs with respect to image latitude, soft tissue rendition, rendition of the periosteal and enosteal border of cortical bone, rendition of cancellous bone and the visibility of potentially present pathological changes, using a subjective five-point scale. The digital images were rated significantly (p=0.001) better than the screen-film images with respect to soft tissue rendition and image latitude. Also the rendition of the cancellous bone and the periosteal and enosteal border of the cortical bone was rated significantly (p=0.05) better for the flat-panel detector. The visibility of pathological lesions was equivalent; only large-area sclerotic lesions (n=2) were seen superiorly on screen-film images. The new digital flat-panel detector based on amorphous silicon appears to be at least equivalent to conventional screen-film combinations for skeletal examinations, and in most respects even superior.

Comparison of low-contrast detail perception on storage phosphor radiographs and digital flat panel detector images

A contrast detail analysis was performed to compare perception of low-contrast details on X-ray images derived from digital storage phosphor radiography and from a flat panel detector system based on a cesium iodide/amorphous silicon matrix. The CDRAD 2.0 phantom was used to perform a comparative contrast detail analysis of a clinical storage phosphor radiography system and an indirect type digital flat panel detector unit. Images were acquired at exposure levels comparable to film speeds of 50/100/200/400 and 800. Four observers evaluated a total of 50 films with respect to the threshold contrast for each detail size. The numbers of correctly identified objects were determined for all image subsets. The overall results show that low-contrast detail perception with digital flat panel detector images is better than with state of the art storage phosphor screens. This is especially true for the low-exposure setting, where a nearly 10% higher correct observation ratio is reached. Given its high detective quantum efficiency the digital flat panel technology based on the cesium iodide scintillator/amorphous silicon matrix is best suited for detection of low-contrast detail structures, which shows its high potential for clinical imaging.

Selenium radiography versus storage phosphor and conventional radiography in the detection of simulated chest lesions

PURPOSE

To compare selenium detectors with three conventional and digital detector systems for the detection of simulated pulmonary lesions.

MATERIALS AND METHODS

Templates containing nodules, linear structures, and micronodular opacities were superimposed over an anthropomorphic chest phantom. The authors compared lesion detection with use of storage phosphor radiography (250 speed), selenium radiography (250 speed) with an antiscatter grid, selenium radiography (450 speed) without an antiscatter grid, an asymmetric screen-film system (400 speed), and a conventional screen-film system (250 speed). Detection performance of 10 radiologists was compared by using a multireader-multicase receiver operating characteristic analysis of variance.

RESULTS

For the detection of nodules, no statistically significant differences between imaging modes were seen. For the detection of micronodules and linear lesions, both selenium techniques were superior to all other modes (P < .05). In addition, the asymmetric screen-film radiographs were inferior (P < .05) to the conventional screen-film radiographs and to storage phosphor radiographs for the detection of micronodules.

CONCLUSION

The selenium detector improves detection of simulated fine linear and low-contrast micronodular details and appears to be superior to other detector systems for chest radiography.

[Quality assurance in roentgen diagnosis. Comparison of constancy test methods according to DIN and IEC and practical consequences in Germany]

Since 1987 constancy tests for X-ray imaging equipment are required by the German Röntgenordinance and are performed according to the DIN 6868 standards. On an international level, standards for these tests have been developed by the IEC (International Electrotechnical Commission, IEC 1223 series); however, these standards have so far not been applied in nation-wide quality programmes in any country. Typical aspects of both, DIN and IEC standards, are demonstrated for direct radiography and indirect radioscopy and indirect radiography as examples. DIN standards optimise test procedures for minimum effort and cost efficiency designed for execution even in small hospitals and practices by the users. For each type of x-ray equipment tests are performed with only one universal test device, which is precisely specified and independent of local conditions. Oplimisation is misation is missing for IEC test procedures; they are designed for more performance characteristics and need for each x-ray equipment more individual test devices, which are mostly open for local choice and need a considerably higher effort in respect of time consumption and test films compared to DIN. Adoption of the IEC standards for the constancy tests in Germany would involve considerable financial and organisational effort which is probably not counterbalanced by an increased benefit.

Digital chest imaging with a selenium detector: comparison with conventional radiography for visualization of specific anatomic regions of the chest

OBJECTIVE

The purpose of this study was to compare a new digital chest radiography system that uses amorphous selenium as the X-ray detector with conventional radiography for the visualization of various anatomic regions of the chest as a first phase of testing image quality.

MATERIALS AND METHODS

Six observers analyzed pairs of posteroanterior chest radiographs of 40 patients. One radiograph in each pair was obtained with a conventional chest film changer, and the other was obtained with the digital selenium chest radiography system. Each observer rated the visibility and the radiographic quality of 12 different anatomic regions.

RESULTS

The observers rated visualization obtained with the digital system as better than that obtained with the conventional system in four regions (right lower lobe, upper lobes, ribs, and soft tissue), as better than or equal to that obtained with the conventional system in four regions (retrocardiac, retrodiaphragmatic, hilum, and upper mediastinum), and as equal to that obtained with the conventional system in four regions (horizontal fissure, carina, azygoesophageal recess, and thoracic spine). Some observers had a strong preference for the digital images, whereas others showed no preference. The conventional system was not ranked high for any region (2880 observations, p < .01, sign test).

CONCLUSION

The digital selenium chest radiography system performs well in a clinical setting, providing visualization of anatomic structures that is better than or at least equal to that provided by standard screen-film images.

Image quality of a digital chest radiography system based on a selenium detector

A digital chest radiography system has been developed, with a detector based on the photoelectric properties of amorphous selenium. The selenium layer is deposited on a cylindrical aluminium drum, large enough to cover the full field of view for chest imaging. The electrostatic charge image which is formed on the selenium surface after x-ray exposure is read out by electrometer probes using fast drum rotation. For a physical evaluation of the attainable image quality, the characteristic curve, the modulation transfer function, and the noise spectra were measured. From these measurements, the signal-to-noise properties of the detector in terms of detective quantum efficiency (DQE) and noise equivalent quanta (NEQ) were derived. The results show that the selenium-based detector has a wide dynamic range and a significantly better DQE than screen-film and storage phosphor systems for spatial frequencies below the Nyquist limit (2.7 lp/mm). As a consequence, the detectability of small, low-contrast details is considerably improved.

Grids or air gaps for scatter reduction in digital radiography: a model calculation

The relative advantages of grids and air gaps for scatter reduction in a digital radiography system were investigated using a theoretical model. In this model the properties of the scatter reduction device are described by primary transmission and selectivity. The signal-to-noise (SNR) improvement factor for fixed exposure to the patient was used as a performance indicator. The results show that the SNR improvement depends strongly on the local scatter fraction; for all practical configurations, however, it stays below a factor of 2. For high scatter fractions, an air gap of 20 cm has about the same effect on SNR improvement as a highly selective grid; for low and medium scatter conditions the air gap performs better than any grid. Additive system noise reduces the SNR improvement factor compared to the case with quantum noise only, the reduction being more pronounced for the grids than for the air gap. The results suggest that the use of an air gap instead of a grid is advantageous in digital radiography systems.

X-ray diffraction computed tomography

Coherent scattering of x-ray photons leads to the phenomenon of x-ray diffraction, which is widely used for determining atomic structure in materials science. A technique [x-ray diffraction computed tomography (CT)] is described, analogous to conventional CT, in which the x-ray diffraction properties of a stack of two-dimensional object sections may be imaged. The technique has been investigated using a first generation (single pencil beam) CT scanner to measure small angle coherent scatter, in addition to the customary transmitted radiation. Diffraction data from a standard CT performance phantom obtained with this new technique and with an x-ray diffractometer are compared. The agreement is satisfactory bearing in mind the poor momentum resolution of our apparatus. The dose and sensitivity of x-ray diffraction CT are compared with those of conventional transmission CT. Diffraction patterns of some biological tissues and plastics presented in a companion paper indicate the potential of x-ray diffraction CT for tissue discrimination and material characterization. Finally, possibilities for refinement of the technique by improving the momentum resolution are discussed.

X-ray diffraction measurements of some plastic materials and body tissues

X-ray diffraction allows the investigation of the atomic or molecular structure of materials. The combination of diffractometry with computerized tomography enables spatially resolved imaging of the diffraction properties of extended objects as described in more detail in a companion article [Harding et al., Med. Phys. 14, 515 (1987)]. We present measured diffraction patterns of some plastics and several biological materials, which allow further optimization of our method and the selection of suitable application areas.

Coherent scatter in radiographic imaging: a Monte Carlo simulation study

The significance of coherently scattered radiation in radiographic imaging is investigated using the Monte Carlo simulation technique. Recent data on the form factor of liquid water, which take into account intermolecular interference effects, have been used for the calculation of the coherent differential cross section. The spatial distribution of scattered radiation in the detection plane was calculated separately for coherent and incoherent single and multiple scattering. In the pencil beam geometry, it is found that coherent scattering leaving the object, is almost exclusively single scattering, is concentrated near but not exactly at the transmitted primary beam and dominates over multiple incoherent scattering in this region even for thick objects and polyenergetic radiation. Some consequences concerning the performance of grids, the choice of phantom materials and a new imaging method are given.