A set of X-ray test objects for image quality control in digital subtraction fluorography. I: Design considerations

Seven general radiography x-ray detectors with pixel sizes ranging from 175 to 76μm: technical evaluation with the focus on orthopaedic imaging

Aim. Flat panel detectors with small pixel sizes general can potentially improve imaging performance in radiography applications requiring fine detail resolution. This study evaluated the imaging performance of seven detectors, covering a wide range of pixel sizes, in the frame of orthopaedic applications.Material and methods. Pixel sizes ranged from 175 (detector A175) to 76μm (detector G76). Modulation transfer function (MTF) and detective quantum efficiency (DQE) were measured using International Electrotechnical Commission (IEC) RQA3 beam quality. Threshold contrast (CT) and a detectability index (d') were measured at three air kerma/image levels. Rabbit shoulder images acquired at 60 kV, over five air kerma levels, were evaluated in a visual grading study for anatomical sharpness, image noise and overall diagnostic image quality by four radiologists. The detectors were compared to detector E124.Results. The 10% point of the MTF ranged from 3.21 to 4.80 mm-1, in going from detector A175to detector G76. DQE(0.5 mm-1) measured at 2.38μGy/image was 0.50 ± 0.05 for six detectors, but was higher for F100at 0.62. High frequency DQE was superior for the smaller pixel detectors, howeverCTfor 0.25 mm discs correlated best with DQE(0.5 mm-1). Correlation betweenCTand the detectability model was good (R2= 0.964).CTfor 0.25 mm diameter discs was significantly higher for D150and F100compared to E124. The visual grading data revealed higher image quality ratings for detectors D125and F100compared to E124. An increase in air kerma was associated with improved perceived sharpness and overall quality score, independent of detector. Detectors B150, D125, F100and G76, performed well in specific tests, however only F100consistently outperformed the reference detector.Conclusion. Pixel size alone was not a reliable predictor of small detail detectability or even perceived sharpness in a visual grading analysis study.

A methodology to estimate the patient diameter and thickness from thoracic and abdominal projection radiographs of adult patients

Patient dose management systems can be part of a department's quality management tools to estimate items such as the radiation burden in specific groups or list dose outliers for further follow up. Patient size information could improve both aspects, but is not generally available. A new metric is proposed to estimate patient size for thorax and abdominal projection radiography from parameters available in thedicomheader and therefore accessible by dose management software. The tested hypothesis was that an attenuation metric, related to the ratio of detector air-kerma to incident air-kerma, inversely correlates with patient size. Such a metric was defined and then worked out for thorax and abdomen projection radiography. Input material consisted of the thorax or abdominal radiographs of 137 cases, completed with a recent CT scan as ground truth size. From the CT, the water equivalent diameter (WED) and water equivalent thickness (WET) were calculated. The correlation between the attenuation metric and the patient size was established separately for thorax and abdomen. Validation of the attenuation metric predicting the patient size was performed using extra sets of examinations on three more radiographic x-ray devices, with available CT scan. The attenuation metric had a good correlation (R²) of 0.91 and 0.84 with the WED for thorax and abdomen respectively. The corresponding values for the WET were 0.89 and 0.78. Validation of the methodology on the devices with standardized exposure index in thedicomheaders showed that the WED could be estimated within ±15% and the WET within ±30% for thorax and abdomen examinations. The ground truth and estimated size were found statistically equivalent. An attenuation metric based ondicomtags allows to estimate the patient size in projection radiography. This could now be implemented in patient dose management systems.

Objective criteria for acceptability and constancy tests of digital subtraction angiography

PURPOSE

Demonstrate an objective procedure to quantify image quality in digital subtraction angiography (DSA) and suggest thresholds for acceptability and constancy tests.

METHODS

Series of images were obtained in a DSA system simulating a small (paediatric) and a large patient using the dynamic phantom described in the IEC and DIN standards for acceptance tests of DSA equipment. Image quality was quantified using measurements of contrast-to-noise ratio (CNR). Overall scores combining the CNR of 10-100 mg/ml Iodine at a vascular diameter of 1-4 mm in a homogeneous background were defined. Phantom entrance surface air kerma (Ka,e) was measured with an ionisation chamber.

RESULTS

The visibility of a low-contrast vessel in DSA images has been identified with a CNR value of 0.50 ± 0.03. Despite using 14 times more Ka,e (8.85 vs 0.63 mGy/image), the protocol for large patients showed a decrease in the overall score CNRsum of 67% (4.21 ± 0.06 vs 2.10 ± 0.05). The uncertainty in the results of the objective method was below 5%.

CONCLUSION

Objective evaluation of DSA images using CNR is feasible with dedicated phantom measurements. An objective methodology has been suggested for acceptance tests compliant with the IEC/DIN standards. The defined overall scores can serve to fix a reproducible baseline for constancy tests, as well as to study the device stability within one acquisition series and compare different imaging protocols. This work provides aspects that have not been included in the recent European guidelines on Criteria for Acceptability of Medical Radiological Equipment.

Quality control measurements for digital x-ray detectors

This paper describes a digital radiography (DR) quality control protocol for DR detectors from the forthcoming report from the Institute of Physics and Engineering in Medicine (IPEM). The protocol was applied to a group of six identical caesium iodide (CsI) digital x-ray detectors to assess reproducibility of methods, while four further detectors were assessed to examine the wider applicability. Twelve images with minimal spatial frequency processing are required, from which the detector response, lag, modulation transfer function (MTF), normalized noise power spectrum (NNPS) and threshold contrast-detail (c-d) detectability are calculated. The x-ray spectrum used was 70 kV and 1 mm added copper filtration, with a target detector air kerma of 2.5 µGy for the NNPS and c-d results. In order to compare detector performance with previous imaging technology, c-d data from four screen/film systems were also acquired, at a target optical density of 1.5 and an average detector air kerma of 2.56 µGy. The DR detector images were typically acquired in 20 min, with a further 45 min required for image transfer and analysis. The average spatial frequency for the 50% point of the MTF for six identical detectors was 1.29 mm(-1) ± 0.05 (3.9% coefficient of variation (cov)). The air kerma set for the six systems was 2.57 µGy ± 0.13 (5.0% cov) and the NNPS at this air kerma was 1.42 × 10(-5) mm(2) (6.5% cov). The detective quantum efficiency (DQE) measured for the six identical detectors was 0.60 at 0.5 mm(-1), with a maximum cov of 10% at 2.9 mm(-1), while the average DQE was 0.56 at 0.5 mm(-1) for three CsI detectors from three different manufacturers. Comparable c-d performance was found for these detectors (5.9% cov) with an average threshold contrast of 0.46% for 11 mm circular discs. The average threshold contrast for the S/F systems was 0.70% at 11 mm, indicating superior imaging performance for the digital systems. The protocol was found to be quick, reproducible and gave an in-depth assessment of performance for a range of digital x-ray detectors.

A comparison between objective and subjective image quality measurements for a full field digital mammography system

This paper presents pre-sampling modulation transfer function (MTF), normalized noise power spectrum (NNPS) and detective quantum efficiency (DQE) results for an amorphous selenium (a-Se) full field digital mammography system. MTF was calculated from the image of an angled 0.5 mm thick Cu edge, acquired without additional beam filtration. NNPS data were acquired at detector air-kerma levels ranging from 9.1 microGy to 331 microGy, using a standard mammography x-ray spectrum of 28 kV, Mo/Mo target/filter combination and 4 cm of PMMA additional filtration. Prior to NNPS estimation, the image statistics were assessed using a variance image. This method was able to easily identify a detector artefact and should prove useful in routine quality assurance (QA) measurements. Detector DQE, calculated from the NNPS and MTF data, dropped to 0.3 for low detector air-kerma settings but reached an approximately constant value of 0.6 above 50 microGy at the detector. Subjective image quality data were also obtained at these detector air-kerma settings using the CDMAM contrast-detail (c-d) test object. The c-d data reflected the trend seen in DQE, with threshold contrast increasing at low detector air-kerma values. The c-d data were then compared against predictions made using two established models, the Rose model and a standard signal detection theory model. Using DQE(0), the Rose model gave results within approximately 15% on average for all the detector air-kerma values studied and for detail diameters down to 0.2 mm. Similar agreement was also found between the measured c-d data and the signal detection theory results, which were calculated using an ideal human visual response function and a system magnification of unity. The use of full spatial frequency DQE improved the agreement between the calculated and observer results for detail sizes below 0.13 mm.

Evaluation and testing of computed radiography systems

The implementation of film replacement digital radiographic imaging systems throughout Europe is now gathering momentum. Such systems create the foundations for totally digital departments of radiology, since radiographic examinations constitute the most prevalent modality. Although this type of development will lead to improvements in the delivery and management of radiological service, such widespread implementation of new technology must be carefully monitored. The implementation of effective QA tests on installation, at periodic intervals and as part of a routine programme will aid this process. This paper presents the results of commissioning tests undertaken on a number of computed radiography imaging systems provided by different manufacturers. The aim of these tests was not only to provide baseline performance measurements against which subsequent measurements can be compared but also to explore any differences in performance, which might exist between different units. Results of measurements will be presented for (1) monitor and laser printer set-up; (2) imaging plates, including sensitivity, consistency and uniformity; (3) resolution and contrast detectability; and (4) signal and noise performance. Results from the latter are analysed in relationship with both system and quantum noise components.

Evaluation of image quality in terahertz pulsed imaging using test objects

As with other imaging modalities, the performance of terahertz (THz) imaging systems is limited by factors of spatial resolution, contrast and noise. The purpose of this paper is to introduce test objects and image analysis methods to evaluate and compare THz image quality in a quantitative and objective way, so that alternative terahertz imaging system configurations and acquisition techniques can be compared, and the range of image parameters can be assessed. Two test objects were designed and manufactured, one to determine the modulation transfer functions (MTF) and the other to derive image signal to noise ratio (SNR) at a range of contrasts. As expected the higher THz frequencies had larger MTFs, and better spatial resolution as determined by the spatial frequency at which the MTF dropped below the 20% threshold. Image SNR was compared for time domain and frequency domain image parameters and time delay based images consistently demonstrated higher SNR than intensity based parameters such as relative transmittance because the latter are more strongly affected by the sources of noise in the THz system such as laser fluctuations and detector shot noise.

A practical demonstration of improved technique factors in paediatric fluoroscopy

Dose incurred with fluoroscopic procedures accounts for a significant proportion of medically induced diagnostic exposure. Children are particularly vulnerable and it is therefore important to minimize exposure where practicable. A recent theoretical study has highlighted the potential for X-ray equipment to produce significant dose savings during paediatric fluoroscopy without incurring loss of diagnostic image quality. This is achieved by hardening the beam with additional copper (Cu) filtration (approximately 0.2 mm Cu) and biasing exposure factors towards low tube potential, high tube current output. In practice, this method will have limited applicability because the high powered and programmable generator characteristics required are not commonly available in installations used for paediatric imaging. However, we describe a simple experiment in which our clinical equipment was modified to approximate desired low dose performance by altering the filtration and automatic exposure control characteristics of ordinary clinical equipment in the Sheffield Children's Hospital. This enabled us to obtain significant savings in dose. We performed a comparative study (normal dose vs low dose) using water phantoms to simulate patient attenuation in the age range 0-15 years. The Leeds N2 contrast sensitivity phantom was used to provide a measure of image quality. Dosimetric measurements recorded up to 40% reduction in dose rate with only marginal loss of image quality when 0.1-0.2 mm Cu filtration was used with the modified settings. This is a strong indication that significant dose reduction is achievable on routine clinical equipment without compromising image quality. Such simple and cost effective methods of dose reduction should be considered for wider implementation.

Receptor dose in digital fluorography: a comparison between theory and practice

A method of identifying the dose per image when quantum mottle no longer dominates the image statistics is presented as a first step towards quantitative optimization in native and subtracted digital fluorography. The method is based on measurements of threshold contrast over a range of receptor doses and the application of a simple model of the threshold contrast detection task to estimate the magnitude of system noise sources. The point at which system and quantum noise sources are equal in magnitude is proposed as the practical upper limit for dose per image. The method is applied to a typical digital fluorography system and the results are placed into the context of the range of dose per image values found from a regional survey of digital fluorography units. While there is broad agreement between the dose per image values in the survey with values predicted from the experimental method, the considerable spread in survey doses suggests there are instances where the use of a high dose per image is unjustified.

Comparison of film, hard copy computed radiography (CR) and soft copy picture archiving and communication (PACS) systems using a contrast detail test object

This paper describes two experiments where a widely available test object (FAXIL TO20) was used to compare film, hard copy computed radiography (CR) and soft copy picture archiving and communication systems (PACS) images. Baseline images were produced with a fixed mAs. All images were scored by four experienced medical physicists. Contrast detail curves for the three types of images were almost identical. A second series of images was produced with the mAs varying from 1 mAs to 250 mAs. The contrast detail curves were plotted for each mAs value and the wider exposure latitude of CR compared with film was demonstrated. Use of PACS provided no further increase in exposure latitude. The density of the film images increased with mAs but the density of the CR hard copy images remained constant. It is of concern that the wider latitude of the CR images extends to exposures that are much higher than those used for film with no noticeable change in CR image density but with better images at higher exposures, because the potential exists for patient doses to increase. Hard copy CR images provide information about the exposure index which relates to the input dose to the plate and hence approximately to the dose to the patient. However, since such information is currently not available on default soft copy images, the authors suggest that all manufacturers of PACS should provide an indication of dose as a mandatory default setting.

The contrast-detail behaviour of a photostimulable phosphor based computed radiography system

Contrast-detail measurements were performed on a computed radiography imaging system as a function of detector entrance air kerma over the dose range from 0.743 microGy (0.085 mR) to 277 microGy (31.8 mR). A theoretical model of contrast-detail behaviour for a photostimulable phosphor computed radiography system has been derived, which is based on a modified version of the Rose theory of threshold detection. Included in the model are both system and x-ray quantum noise terms, as well as the response of the eye. The zero-frequency noise power of the computed film images was measured with a double-beam scanning microdensitometer. For a given detector dose, good agreement was found between the predicted and measured data when this measurement of system noise was included in the model. The contrast-detail results obtained for the computed radiography system were also compared with contrast-detail results for an image intensifier-TV based digital imaging system and a conventional film-screen system.

A phantom and a procedure to obtain variable-object-contrast test images in gamma camera emission computed tomography (SPECT)

The value of test objects with a range of object contrast has been widely recognized for the testing of medical imaging equipment. A simple phantom is described, which, together with a data processing procedure, provides variable-contrast rod objects in gamma camera emission computed tomography (SPECT). There is only one compartment to fill with radioisotope, and data from hot rods, cold rods and a uniform section are available for analysis. The rod object contrast varies from 100% to 0%, with nine contrast steps used in this analysis. The measured image contrast in the transaxial slices is shown to be sensitive to variations in the system spatial resolution. With only one compartment to fill with radioisotope, this procedure is applicable for routine checking or optimization of gamma camera SPECT systems.

A phantom for the measurement of contrast detail performance in film-screen mammography

A National Breast Cancer Screening Programme, based on X-ray mammography, has been introduced in the UK. The success of this screening programme is dependent on the production of high quality radiographs. There are various methods of assessing imaging performance in mammography, but contrast detail detectability measurements have an advantage in that the observation of radiographs is taken into consideration. The design and construction of a contrast detail phantom for the assessment of image quality in mammography is described. This phantom can be used to assess the imaging performance of mammographic film-screen combinations. The results of some measurements to verify the accuracy of the contrast predictions are presented together with some initial results of an investigation into the contrast detail performance of film-screen combinations used in mammography.

Description and appraisal of a prototype digital fluorographic system

A prototype system for digital fluorography (DF) has been developed from commercially available DSA equipment coupled to a standard screening unit. We present our initial clinical experience of this system with particular reference to the ergonomics, the image quality and the effect of image processing. We have concentrated upon dynamic studies such as upper gastrointestinal tract barium studies where the logistic advantages of the system are considerable. We have found DF to be ergonomically more efficient than conventional techniques. Using careful image processing the 512*512 matrix images were considered diagnostic for most studies assessed.

Digital radiography

A quantitative comparison of the digital techniques reviewed in section 4.1-4.7 is difficult, for two reasons. Firstly, various authors have used slightly different techniques for assessing aspects of imaging performance (e.g. a variety of test objects for, and definitions of, spatial resolution). Secondly, with all imaging systems there exists an inter-relationship between spatial resolution, image acquisition time, image noise and dose. Some authors have chosen to emphasise one feature at the expense of others. Arnold (1982), in an overview of digital radiographic technology at that time, also noted the lack of standardisation of measurement techniques and exposure conditions, but nevertheless attempted a quantitative comparison of some aspects of digital radiographic systems with screen-film radiography and CT. The continuing developments in the field since then make a brief quantitative intercomparison of dubious value. Nevertheless, a qualitative summary of point, line and area exposure techniques is given in table 2 which incorporates many of the comments made by Arnold et al (1986) in a similar summary of digital radiographic systems.