Contrast-detail Performance Research Articles

Fluoroscopy Quality Assurance Measurements: Automatic Exposure Rate Control and Image Quality

A fluoroscopy system is a relatively complicated piece of equipment made up of several sub-components. For the operator to achieve good image quality without the overhead of high patient and staff doses, it is essential that all of these components are set up correctly. The automatic exposure rate control (AEC) and image quality are important aspects of a fluoroscopy system. Two principal parameters are used to assess AEC function: air kerma rate at the input phosphor with 1.0 mm copper filtration added and entrance surface dose rate for a 20 cm Perspex phantom. Spatial resolution is measured using the Hüttner type 18 spatial frequency grating. The Leeds TO10 contrast-detail test object is used to measure threshold contrast-detail performance. Median limiting spatial resolution was 1.41 lp.mm-1 for image intensifier fields lying between 18 and 24 cm in diameter. Mean low contrast resolution at 63 kV and 0.40 µGy.s-1 was 2.5%.

Subjective evaluation of image quality based on images obtained with a breast tissue phantom: comparison with a conventional image quality phantom

A breast tissue phantom was used for assessment of mammographic image quality. Three images, exposed to give a reference area film density of 1.25 +/- 0.03, were obtained for each of five different film/kV combinations. Each of three radiologists carried out a series of three blind readings, in which the 15 films were ranked according to sharpness and contrast. In a similar experiment with a conventional image quality phantom, scores for the visualization of simulated calcifications and contrast-detail performance, and measures for resolution and contrast were obtained. In both experiments, the judgements of the readers were highly consistent, and both the intraobserver and interobserver variabilities were non-significant. No significant differences between the film/kV combinations were found with respect to high-contrast resolution and visualization of the simulated calcifications. The contrast measures obtained with the conventional phantom correlated well with the contrast scores obtained with the breast tissue phantom. The contrast-detail scores obtained with the conventional phantom showed no correlation with the scores obtained with the breast tissue phantom, nor with the contrast measures obtained with the conventional phantom.

Techniques for the assessment of the imaging characteristics of intravascular ultrasound scanners.

Methods of assessing the imaging characteristics of intravascular ultrasound systems are described and applied to an HP Sonos 3000 system imaging at 12, 20 and 30 MHz. The presence of geometrical image distortion associated with bending of the catheter was detected by imaging silk filaments positioned at 45 degrees around a circle. Distortion was clearly present indicating that it is likely to occur in clinical studies. Resolution measurements (-12 dB) were made from images of 30 microns Nylon. The axial resolution did not change with depth and was in the range 150-400 microns. The lateral resolution increased linearly with depth for all frequencies, with values in the range 0.9-1.7 mm at a depth of 5 mm. High frequency catheter transducers therefore generate ultrasound with beamshapes similar to those expected from the familiar performance of devices in the 2-10 MHz range. Penetration measurements were made with material known to be tissue equivalent at 3-7 MHz. The penetration depth varied from 4.5 to 14 mm depending on catheter and frequency. There is a need for the manufacture of tissue equivalent materials for use in the frequency range 20-30 MHz, and also for the design of phantoms based on these materials for the measurement of contrast-detail performance and penetration depth.

A Quantitative Test Object for Optimisation Studies in Image Intensifier-TV and Digital Imaging Systems

A contrast-detail test object has been designed for use in optimisation studies with digital imaging systems. The use of contrast-detail performance measurements as a non-invasive method of evaluating the complete imaging capability of an imaging system is widely recognised. The test objects were designed with a view to assessing the low contrast imaging capability of digital subtraction systems. As the thicknesses of the objects within the test object and measurement geometry are known, the relevant contrasts can be calculated for any given beam conditions. The test objects can therefore be used to evaluate and optimise a range of technique factors, thus enabling the optimisation of different examinations. Preliminary measurements resulting from the use of the test object with the Fuji Imaging Plate system and a standard digital subtraction fluorography unit are presented.

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.

Performance assessment of X-ray image intensified television fluoroscopy systems in New Zealand.

The protocol for image intensifier (II) quality assurance recommended by the UK Hospital Physicists' Association (HPA) and the UK Department of Health and Social Security has been adapted for use in New Zealand for assessment of such systems. Test objects were constructed at the National Radiation Laboratory to match the specifications of those recommended by the HPA. Over 100 individual surveys have been made and, in a few cases, machines have been surveyed several times. Results are summarized in terms of key parameters: II input dose rate, low-contrast detectability, contrast-detail performance and limiting resolution. These are given as mean and standard deviation, and also as good, average or poor. Video waveforms, conversion factors and modulation transfer functions have also been determined on a number of systems. In several cases, video voltages were found to be seriously maladjusted.

Frequency independent ultrasound contrast-detail analysis

An ultrasound contrast-detail phantom has been developed to evaluate the image quality of diagnostic ultrasound imaging systems. The phantom includes eight conical targets whose B-mode images show disk lesions such that the object contrast of each lesion relative to background is independent of the imaging device or transducer frequency/spectrum. By maintaining conditions for Rayleigh scattering and Rayleigh speckle statistics in the phantom gel, the object contrast of each lesion depends only on the scatterer concentration in the lesion relative to the scatterer concentration in the background. Experimental data confirmed this frequency independence. Results of contrast-detail performance of an ultrasound imaging system are shown, and a standard technique for error analysis of contrast-detail data is described.

Noise characteristics of a microchannel plate x-ray image intensifier.

The noise performance of an experimental microchannel plate x-ray image intensifier has been evaluated. The intensifier, constructed for use with photons of energies between 20 and 150 keV, uses an MCP as the photon-to-electron converter. The influence of noise was determined by analysis of the optical-density fluctuations of a photograph of the viewing screen of the intensifier when the conversion layer was exposed to between 1 and 60 mR. Additionally, the contrast-detail performance of the experimental device was determined. The influence of both stochastic noise, due to quantum mottle and pulse-height variations, and structural noise, due to fluctuations in inherent gain from point to point, have been considered by using a model that adds these components.