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

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

OBJECTIVES

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.

METHODS

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

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).

CONCLUSIONS

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.

ADVANCES IN KNOWLEDGE

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.

IPEM Topical Report: An evidence and risk assessment based analysis of the efficacy of quality assurance tests on fluoroscopy units-part II; image quality

This work aims to assess the efficacy of x-ray quality assurance tests undertaken on fluoroscopy units in the UK. Information was gathered on the results of image quality tests recommended by the reports of the Institute of Physics and Engineering in Medicine, and those additionally undertaken by medical physics departments. The assessment of efficacy considers the frequency with which a test result breaches the remedial level or other relevant threshold where applicable. The third quartile of those results exceeding the remedial level or threshold is used to estimate the severity of such a breach in terms of potential impact on patient dose and image quality. A risk assessment approach is then used to recommend to what degree, if any, the test should be included in an on-going test regimen. Data was analysed from 469 testing sessions to 337 unique fluoroscopy units throughout the UK. Across all tests, the rate with which the remedial level was exceeded varied from 0-10.6%, with severity ranging from little or none to major degradation to image quality or significant increase on population dose. Where possible, the data has also been used to produce representative ranges for the results of image quality tests. These could be useful as an up to date comparator for those sites considering the purchase of or commissioning new equipment. Overall the results indicate a wide range for the efficacy of those tests undertaken at present; this can be used to review local test protocols and to inform future changes to national guidance in the UK. The results also highlight some tests where measurement technique varies significantly throughout the UK, making any valid comparison difficult. This may indicate a need for further guidance on how best to undertake these tests.

Performance evaluation of two computed radiography systems and patient dose in pelvic examination

This study was carried out to evaluate the performance of two computed radiography (CR) units. These evaluations became necessary following the introduction of CR systems in Sudan. Evaluation of the CR systems was performed using physical image quality parameters: signal transfer property, modulation transfer function, normalised noise power spectrum, detective quantum efficiency and the subjective contrast detail detectability. Patient dose was measured in terms of entrance surface air kerma estimated from tube output and exposure factors for 100 patients who had undergone pelvic X-ray examinations. Fuji computed radiography velocity system with columnar screen dose results was much lower than those using CR975 system with granular screen. Patient doses delivered by both systems were within the international diagnostic reference levels.

Image quality assessment using the CD-DISC phantom for vascular radiology and vascular surgery

The purpose of the study was to evaluate image quality (IQ) associated with vascular radiology and vascular surgery procedures in Belgium and to determine reference values for future image quality assessment. IQ was evaluated with the CD-DISC contrast-detail phantom. This circular PMMA phantom contains 225 holes with different diameter and depth, to quantify resolution and contrast. Images of the phantom were acquired for both fluoroscopy and subtraction images on 21 systems. Three observers evaluated the images by determining the threshold contrast visible for every diameter. This results in contrast-detail curves and image quality figures. We observed a large difference in IQ between the centres. No straightforward correlation could be found with radiation dose or other exposure settings. A comparison was made with the image quality evaluation of the systems performed with the TOR[18FG] phantom for fluoroscopy. There is no clear correlation observed between the results of the CD-DISC phantom and the TOR phantom. However, systems with very poor or very good image quality could be detected by both phantoms. An important result is that a 75th percentile reference contrast-detail curve could be proposed to separate the best centres from these with poorer quality. Some centres had also a significantly better image quality than others. Therefore, we introduced also a 25th percentile. Centres with IQ above this value are recommended to lower the dose and work with acceptable rather than excellent image quality. The CD-DISC phantom thus allows to guide the image quality setting.

Quality assurance of computed and digital radiography systems

Computed radiography (CR) and digital radiography (DR) are replacing traditional film screen radiography as hospitals move towards digital imaging and picture archiving and communication systems (PACS). Both IPEM and KCARE have recently published quality assurance and acceptance testing guidelines for DR. In this paper, the performance of a range of CR and DR systems is compared. Six different manufacturers are included. Particular attention is paid to the performance of the systems under automatic exposure control (AEC). The patient is simulated using a range of thicknesses of tissue equivalent material. Image quality assessment was based on detector assessment protocols and includes pixel value measures as well as subjective assessment using Leeds Test Objects. The protocols for detector assessment cover a broad range of tests and in general detectors (whether DR or CR) performed satisfactorily. The chief limitation in performing these tests was that not all systems provided ready access to pixel values. Subjective tests include the use of the Leeds TO20. As part of this work, suggested reference values are provided to calculate the TO20 image quality factor. One consequence of moving from film screen to digital technologies is that the dynamic range of digital detectors is much wider, and increased exposures are no longer evident from changes in image quality. As such, AEC is a key parameter for CR and DR. Dose was measured using a standard phantom as a basic means of comparing systems. In order to assess the AEC performance, exit doses were also measured while varying phantom thickness. Signal-to-noise ratios (SNRs) were calculated on a number of systems where pixel values were available. SNR was affected by the selection of acquisition protocol. Comparisons between different technologies and collation of data will help refine acceptance thresholds and contribute to optimising dose and image quality.

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.

Investigation of optimum energies for chest imaging using film-screen and computed radiography

The purpose of the study was to compare the image quality of film-screen (FS) and computed radiography (CR) for adult chest examinations across a range of beam energies. A series of images of the CDRAD threshold contrast detail detection phantom were acquired for a range of tube potential and exposure levels with both CR and FS. The phantom was placed within 9 cm of Perspex to provide attenuation and realistic levels of scatter in the image. Hardcopy images of the phantom were scored from a masked light-box by two scorers. Threshold contrast indices were used to calculate a visibility index (VI). The relationships between dose and image quality for CR and for FS are fundamentally different. The improvements in VIs obtained using CR at 75 kVp and 90 kVp were found to be statistically significant compared with 125 kVp at matched effective dose levels. The relative performance of FS and CR varies as a function of energy owing to the different k-edges of each system. When changing from FS to CR, the use of lower tube potentials may allow image quality to be maintained whilst reducing effective dose. A tube voltage of 90 kVp is indicated by this work, but may require clinical verification.

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.