Threshold Contrast Detail Detectability Research Articles

Extended detail contrasts for TO.10 use on flat panel detector fluoroscopy systems

Leeds Test Object Ten (TO.10) is routinely used to subjectively estimate Threshold Contrast Detail Detectability (TCDD) as a measure of image quality in fluoroscopy. However, manufacturer guidance provides calibrated contrasts for only limited peak voltage and copper filtration thickness combinations. Prescribed testing conditions are often difficult to attain as modern flat panel fluoroscopic systems independently determine voltage and copper filtration thickness settings. This work aims to extend the range of TO.10 contrasts available for routine testing at peak voltage and copper thickness settings likely to be encountered. Two methods are described for generalising the calculation of target contrasts: a three-dimensional interpolation/extrapolation model in MATLAB®, and a multivariate log-polynomial function. Both methods utilise the available calibrated contrasts to estimate contrasts at voltage and copper thickness combinations routinely encountered. Results are presented as Threshold Detection Index [Formula: see text] curves fit by a second-order polynomial of log [Formula: see text] to log [Formula: see text] . Results are found to be more accurate at unprescribed conditions while also reproducible for relatively consistent input air kerma rate (IAKR) expected from automatic dose rate controls (ADRC). The calculation of TO.10 contrasts at non-standard conditions aids in the determination of an absolute estimate of image quality in fluoroscopy with greater accuracy, reproducibility and efficiency. TO.10 detail contrasts for TCDD testing of fluoroscopy units have been significantly extended beyond those previously available. The described methods will aid the clinical physicist in absolute assessments of fluoroscopic image quality and facilitate inter system comparisons.

A novel quantitative measure of image quality in fluoroscopy

Assessment of fluoroscopic image quality has not kept pace with technological developments in interventional imaging equipment. Access to ‘for presentation’ data on these systems has motivated this investigation into a novel quantitative method of measuring image quality. We have developed a statistical algorithm as an alternative to subjective assessment using threshold contrast detail detectability techniques. Using sets of uniformity exposed fluoroscopy frames, the algorithm estimates the minimum contrast necessary for conspicuity of a range of virtual target object areas A. Pixel mean value distributions in a central image region are Gaussian, with standard deviation σ Pixel binning produces background distributions with area A. For 95% confidence of conspicuity a target object must exhibit a minimum contrast of 3.29σ. A range of threshold contrasts are calculated for a range of virtual areas. Analysis on a few seconds of fluoroscopy data is performed remotely and no test object is required. In this study Threshold Index and Contrast Detail curves were calculated for different incident air kerma rates at the detector, different levels of electronic magnification and different types of image processing. A limited number of direct comparisons were made with subjective assessments using the Leeds TO.10 test object. Results obtained indicate that the statistical algorithm is not only more sensitive to changes in levels of detector dose rate and magnification, but also to levels of image processing, including edge-enhancement. Threshold Index curves thus produced could be used as an interventional system optimisation tool and to objectively compare image quality between vendor systems.

NPWE model observer as a validated alternative for contrast detail analysis of digital detectors in general radiography

To propose and validate a non-prewhitening with eye filter (NPWE) model observer as an alternative means of quantifying and specifying imaging performance for general radiography detectors, in a comparative study with contrast detail analysis and detective quantum efficiency (DQE).Five different x-ray detectors were assessed, covering a range of detector technologies including powder computed radiography (CR), needle CR, and three indirect conversion flat panel digital radiography detectors (DR). For each detector, threshold contrast detail (c–d) detectability was measured using the Leeds TO20 test object. A tube voltage of 70 kV and 1 mm Cu added filtration was used and five target detector air kerma (DAK) levels were set, ranging from 0.625 µGy to 10 µGy. Three c–d images were acquired at the same DAK levels and these were scored by two observers. Presampling modulation transfer function (MTF) was measured using an edge method while contrast was measured with a 2 mm Al square of dimension 10 × 10 mm. The normalized noise power spectrum (NNPS) was calculated at the target DAK values of the c–d images. The MTF, NNPS and contrast data were then used to calculate a detectability index (d′) with the NPWE model and compared to the human observer c–d results. The standard quantitative means of evaluating detector performance i.e. DQE, was then calculated for each detector.A linear correlation was found between the logarithm of threshold contrast and the logarithm of d’ for all detectors, as DAK was increased. Furthermore, the absolute value of d′ tracked threshold contrast between the five detectors, enabling the use of detectability to quantify image quality rather than the intrinsically subjective threshold contrast scored by human observers from c–d test object images. At 2.5 µGy target DAK, d’ followed the differences in DQE between the five detectors.The NPWE detectability index can be used an alternative parameter for the quantification and specification of the image quality of general radiography x-ray detectors.

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 mm2 (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.

Parameters of Digital Detectors and Image Quality

The input dose for an ideal digital detector depends quadratically on the threshold contrast detail detectability (TCDD). The TCDD at the level of image quality sufficiency should be minimized for diagnosis; initial parts of detector amplitude response, as well as pulse fluoroscopy adapted to organ mobility, should be used.

Optimisation strategies introduced for CR at health care centres in Estonia

In recent years, starting from 2001, conventional film-screen radiography has been rapidly replaced by digital (mostly by computed radiography, CR) techniques in Estonia. New strategies for optimisation and quality assurance for digital radiography have been introduced by the DIMOND III and SENTINEL partners recently. It includes consideration to diagnostic requirements of a given clinical situation, but also objectivation and standardisation of image quality, e.g. using CDRAD test phantom, and constancy testing. The aim of this work is to evaluate the performance of an automatic exposure control (AEC) at different sensitivities when used with the Fujifilm CR system. Image quality, using threshold contrast-detail detectability (TCDD) and signal-to-noise ratio squared related to dose (SNR2/D), was also investigated for different sensitivities and tube potential values for this combination. Based on the image evaluation data, optimum speed modes for the organ programme settings have been proposed.

Threshold contrast detail detectability curves for fluoroscopy and digital acquisition using modern image intensifier systems.

Threshold contrast detail detectability (TCDD) test objects are a commonly used tool to assess image quality of imaging systems. FAXIL (The Facility for the Assessment of X-ray imaging, Leeds) produced updated standard TCDD curves, for fluoroscopy systems in good adjustment, in 1992. Fluoroscopy curves can be corrected to account for the effect of image intensifier input air kerma rate and field size. This paper presents updated TCDD curves for fluoroscopy and new curves for digital acquisition. The results for digital acquisition suggest that the TCDD curves should not be corrected for input air kerma, as the quantum noise is not dominant and system noise is significant. These curves will prove useful for accepting new equipment, to give an indication of the expected image quality for a new image intensifier system.

Use of a quality index in threshold contrast detail detection measurements in television fluoroscopy

The use of a single index to assist in quality control procedures of X-ray television fluoroscopy systems was investigated. A single quality index was devised incorporating a measure of threshold contrast detail detectability (TCDD) performance and taking into account image intensifier input kerma rate, field size, differences in radiation beam quality, and pulsed fluoroscopy. This was applied to a number of clinical systems to investigate changes in image quality index quantified over time. Accepted measurement protocols were used to obtain these measurements. The results show system performance for different systems and can establish the decline in performance parameters over time or assess non-optimal image quality with clinical systems in field measurements. The systems studied were assessed with a variety of performance parameters including TCDD results, low contrast sensitivity, limiting resolution, and image intensifier input kerma rate under clinical modes of operation. The TCDD quality index, and dose normalized quality index, were found to be useful image quality assessment parameters for serial testing of systems, which augment the use of graphical methods for the display of TCDD curves.

The Use of Dynamic Phantoms in Interventional Radiology

The authors have constructed a 2D motor-controlled test object phantom holder to simulate clinical situations in which patient movement could be a cause of image degradation. The PAtient MOvement SImulation Test Object (PAMOSITO) has been constructed with modular parts to use different mobile test objects and static structures. The system allows the programming of different cycles of movement along two axes. PAMOSITO has been used in X ray equipment dedicated to interventional radiology. Those systems usually allow for different values for frame rate, pulse width or weighted frame averaging methods. The influence of selecting different values of the parameters, patient movement and its relation to patient dose and image quality has been studied. Image blurring due to motion has been evaluated with Leeds test objects TO.10 and 18FG. Spatial resolution limits and the threshold contrast detail detectability performance have been studied.

Initial technical and clinical evaluation of a new universal image receptor system.

The aim of this study was to evaluate the physical performance of an experimental flat-panel digital X-ray detector plate (FDXD), and to assess its clinical potential in radiographic and fluoroscopic mode. The efficiency of the detector was assessed by calculating the low-frequency detective quantum efficiency (DQE(0)), and a measure of image quality was obtained using a threshold contrast detail detectability (TCDD) test object. A range of clinical examinations were also carried out, and the results reviewed by members of the radiology staff. The DQE(0) of the system was calculated to be almost 75%, compared with a value of approximately 20 % for modern computed radiography equipment, offering the potential for increased image quality or significant dose reduction. Measurements using the TCDD test object demonstrated a corresponding advantage for the FDXD in image quality and dose efficiency. Clinical studies are producing radiographic results which are at least the equal of the best currently available digital technology, and a limited number of examinations using fluoroscopic mode at 25 frames per second have been equally encouraging. Equipment using FDXD technology could potentially fulfill all the radiographic and fluoroscopic requirements of the digital department, with improved image quality and dose efficiency.

A comparison of the threshold detail detectability of a screen-film combination and computed radiology under conditions relevant to high-kVp chest radiography

There is conflicting evidence regarding the viability of photostimulable phosphor computed radiology (CR) when used with high X-ray beam energies. In this study a standard screen-film combination was compared with CR using identical exposure conditions, which simulated those of a high-energy chest examination. The comparison was made using a psychophysical technique in which the threshold contrast detail detectability is assessed by human observers. Threshold contrast determination is an efficient way with which to assess the overall image quality. The CR images were processed and displayed using two independent image processing algorithms and hard-copy formats. The results showed, at the 5% level of statistical significance, that there was no difference in the overall image quality between any of the modalities being compared. It may be concluded therefore that CR is a potentially viable option for use in high-energy radiography of the chest.

Update on the recommended viewing protocol for FAXIL threshold contrast detail detectability test objects used in television fluoroscopy.

The significance of varying the viewing conditions that may affect the perceived threshold contrast of X-ray television fluoroscopy systems has been investigated. Factors investigated include the ambient room lighting and the viewing distance. The purpose of this study is to find the optimum viewing protocol with which to measure the threshold detection index. This is a particular problem when trying to compare the image quality of television fluoroscopy systems in different input field sizes. The results show that the viewing distance makes a significant difference to the perceived threshold contrast, whereas the ambient light conditions make no significant difference. Experienced observers were found to be capable of finding the optimum viewing distance for detecting details of each size, in effect using a flexible viewing distance. This allows the results from different field sizes to be normalized to account for both the magnification and the entrance air kerma rate differences, which in turn allow for a direct comparison of performance in different field sizes.

Signal, noise and SNR transfer properties of computed radiography

An investigation of the signal, noise and signal-to-noise transfer properties of a Fuji 7000 series computed radiography (CR) system using third-generation storage plate technology has been undertaken. The results show a significant improvement in signal-to-noise performance of the system compared to an earlier generation of CR technology. Results of the CR evaluation compare favourably to those obtained from a selection of rare-earth screen-film systems evaluated under identical exposure conditions. These results were further substantiated by results from threshold contrast-detail detectability tests.

A physical image quality evaluation of a digital spot fluorography system

A physical evaluation of the imaging performance of the first digital spot fluorography system installed in the UK is presented. Measurements have been made of dynamic range, unsharpness (MTF), noise (Wiener spectrum) and threshold contrast detail detectability (TCDD) and the results compared with those produced by conventional spot-film radiography systems. Noise-equivalent parameters are derived from the objective measurements and the results related to the findings of the TCDD tests.