Harrell G. Chotas

DQE of direct and indirect digital radiography systems

Current flat-panel detectors either directly convert x-ray energy to electronic charge or use indirect conversion with an intermediate optical process. The purpose of this work was to compare direct and indirect detectors in terms of their modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Measurements were made on three flat-panel detectors, Philips Digital Diagnost, GE Revolution XQ/i, and Hologic Direct-Ray DR1000 using the IEC-defined RQA5 (approximately 75 kVp, 21 mm Al) and RQA9 (approximately 120 kVp, 40 mm Al) radiographic techniques. The presampled MTF of the systems was measured using an edge method (Samei et al., Med Phys 25:102, 1998). The NPS of the systems was determined for a range of exposure levels by 2D Fourier analysis of uniformly exposed radiographs (Flynn and Samei, Med Phys 26:1612, 1999). The ideal signal-to-noise ratio per exposure for each system was estimated using a semi-empirical x-ray model. The DQE, reported only at the RQA5 technique, was assessed from the measured MTF, NPS, exposure, and the ideal signal-to-noise ratio. For the direct system, the MTF was found to be significantly higher than that for the indirect systems and very close to an ideal function associated with the detector pixel size. The NPS for the direct system was found to be constant in relation to frequency. The DQE results reflected expected differences based on the absorption efficiency of the different detector materials. Using RQA5 and 0.3 mR exposure, the measured DQE values at spatial frequencies of 0.15 mm-1 and 2.5 mm-1 were 64% and 14% for the XQ/i system and 35% and 19% for DR-1000. Using RQA5 and the averages at all exposures, the corresponding values were 58% and 13% for the XQ/i system and 36% and 19% for DR-1000.

Quantitative scatter measurement in digital radiography using a photostimulable phosphor imaging system

X-ray scatter fractions measured with two detectors are compared: a photostimulable phosphor system (PSP) and a conventional film-screen technique. For both detection methods, a beam-stop technique was used to estimate the scatter fraction in polystyrene phantoms. These scatter fraction measurements are compared to previously reported film-based measurements. Scatter fractions obtained with the PSP were in good agreement both with measurements using film as well as with most previously reported measurements. For the PSP measurements, repeatability was better than 1%. It was found that the PSP provides a precise x-ray detector for quantitative scatter measurement in digital radiography.

Quantitative radiographic imaging using a photostimulable phosphor system

We have evaluated a photostimulable phosphor x-ray imaging system [Philips Computed Radiography (PCR) system] for use in quantification of x-ray exposure in diagnostic radiography. An exponential function was fitted to data yielding quantitative x-ray exposure values as a function of digital pixel values. We investigated several factors that affect the accuracy of exposure measurement using the PCR including repeatability, background noise as a function of time delay between plate erasure and use, sensitivity variation between different plates, nonuniformity of sensitivity within a plate, decay of the latent image between time of exposure and readout (observed as a change in sensitivity), and the accuracy with which the (exponential) calibration function yields exposure values as a function of digital pixel values. The calibration was performed over the exposure range from 5.1 X 10(-9) to 2.0 X 10(-5) C/kg (0.02-75 mR). The accuracy of exposure measurements made with a single imaging plate is between 1.6% and 4.2%. If measurements from several plates are involved, the uncertainty in the final measurement will be between 5% and 5.9%.

Image optimization in a computed-radiography/photostimulable-phosphor system

Photostimulable phosphor imaging is an exciting new technology that has several advantages over film/screen radiography, the most important of which is the linearity of the photostimulable phosphor system over a wide exposure latitude. The photostimulable phosphor image is digital, and as such, provides options of how the image is viewed by radiologists. This report discusses the various image-processing parameters available for a photostimulable phosphor system and describes a rational approach for selecting these parameters in portable chest radiography. As photostimulable phosphor imaging becomes more widely implemented, an understanding of the processing parameters will facilitate the production of images that take full advantage of the benefits of these systems.

Digital Radiography with Photostimulable Storage Phosphors: Control of Detector Latitude in Chest Imaging

RATIONALE AND OBJECTIVES.A widely used digital radiography system based on a photostimulable storage phosphor (PSP) detector was analyzed with regard to radiographic contrast changes that result from the adjustment of detector latitude (x-ray sensitivity range) in the normal processing of chest images. METHODS.Images of an acrylic step wedge were acquired using the digital system in a mode that permitted direct control of effective detector latitude. The images were post-processed in conditions duplicating those used for portable chest examinations, and contrast was measured. RESULTS.Increases in effective detector latitude provided only marginal radiographic contrast gains in the subdiaphragm- equivalent areas of the laser-printed digital film image, while causing large reductions in radiographic contrast in the lung-equivalent region. CONCLUSION.Detector latitude is an important variable that should be monitored or controlled in investigations that compare reader performance using conventional and digital systems.

Single-exposure conventional and computed radiography image acquisition.

A technique for simultaneously acquiring a conventional film-screen radiographic image and a digital computed radiography (CR) image with a single x-ray exposure is described. Measurements of image contrast, spatial resolution, and signal-to-noise ratios demonstrate that a modified film cassette in which the first intensifier screen has been replaced with a CR imaging plate permits dual-image, single-exposure imaging with only nominal degradation in film and CR image quality relative to the two standard image counterparts. This technique may be used to acquire matched image pairs for research or as a way to provide full-size conventional film images in the clinical environment, while retaining the advantages offered by computed radiography systems.

Scatter-reduction characteristics of an infinity-focused gridded radiographic cassette.

RATIONALE AND OBJECTIVES.The scatter-reduction properties of a new infinity-focused grid incorporated into a radiographic imaging cassette were analyzed. METHODS.A polystyrene chest phantom was imaged using the cassette and bedside radiographic procedure. Scatter fractions were measured using a beam-stop technique at several anatomically equivalent locations. The performance of this cassette also was evaluated as a function of the orientation angle. RESULTS.The gridded cassette provided a decrease in scatter fraction from 61% to 49% in the lung and from 87% to 76% in the mediastinum. To obtain equivalent film density when the grid was used, the exposure was increased by a factor of 3.0. While there was a decrease in scatter clean-up as the film cassette was tilted from the perpendicular, the grid performed well out to an angle of 10 degrees. CONCLUSION.The gridded cassette for portable radiography provides scatter reduction with little sensitivity to alignment. The availability of this device could improve scatter rejection, and therefore, contrast in portable radiographic imaging.

Evaluation of a selenium-based digital chest radiography system

Preliminary results are presented from an examination of a selenium based thoracic radiography system (Philips Thoravision). The system has been evaluated with four criteria: (1) systems repeatability, (2) digital linearity, (3) spatial resolution, and (4) scattered photon detection fractions. Phantoms were imaged at 120 kV. Repeatability was measured using a polystyrene phantom and evaluating the mean exposure value detected in a region of interest over a period of time. Digital linearity was examined by plotting the output digital value as a function of input exposure. Resolution was evaluated using a line pair phantom. To measure scatter fractions, an anthropomorphic phantom was exposed with a superimposed array of lead beam stops. Three configurations were examined: (1) the selenium system (including an air gap), (2) the system with an added 12:1 antiscatter grid, and (3) a photostimulable phosphor system (Philips PCR) for reference. For a 4 day interval, output varied less than 1%. Digital output of the system was linear with exposure (regression Rvalue of 0.998) over the range from 0.2 mR to 10 mR. The system resolved 2.5 line pair per mm. Resolution was comparable to phosphor plate systems. Scatter fractions were improved when a grid was included.

Direct digitization of optical images using a photostimulable phosphor system

The authors describe a method for directly digitizing optical images with a photostimulable phosphor (PSP) system. A PSP plate is initially charged with an exposure to a uniform x-ray field, and is then exposed to an optical image which discharges the plate in relation to the amount of incident light. Two applications were investigated: a contact-print technique for digitizing film radiographs, and a projection technique for digitizing transparent objects such as histology slides. Spatial uniformity was found to be adequate, and linearity of optical density response was excellent from 0.0-2.9 o.d. after look-up table correction. Spatial frequency response was degraded with the optical technique relative to the x-ray imaging properties of the plates, but was restorable by Fourier filtering. Image noise following spatial enhancement was satisfactory at intermediate to high optical densities using a high-resolution PSP plate, but was somewhat degraded at low densities.

Digital chest radiography with storage phosphor systems: Potential masking of bilateral pleural effusions

A photostimulable storage phosphor (PSP) computed radiography imaging system was analyzed for its potential to mask pleural effusion during normal image processing. This phenomenon has been observed in several clinical cases in our hospital. To better understand the relationship between pleural effusion and the PSP radiograph appearance, portable radiographs of an anthropomorphic chest phantom were acquired with the PSP system in conditions simulating various quantities and distributions of pleural fluid. It was observed that the optical density of the film in one hemithorax was significantly influenced by whether or not fluid was present in the opposite hemithorax. This optical density dependence was determined to be a system-induced effect that results from the image processing (histogram analysis) technique used by the PSP system during image plate readout. It is important to recognize that the PSP system’s normal optical density (sensitivity) adjustment can obscure the presence of bilateral pleural fluid accumulation, particularly if the opposite hemithorax contains fluid in an equal or greater amount.