Scan equalization digital radiography (SEDR) implemented with an amorphous selenium flat-panel detector: initial experience

Abstract

It is well recognized in projection radiography that low-contrast detectability suffered in heavily attenuating regions due to excessively low x-ray fluence to the image receptor and higher noise levels. Exposure equalization can improve image quality by increasing the x-ray exposure to heavily attenuating regions, resulting in a more uniform distribution of exposure to the detector. Image quality is also expected to be improved by using the slot-scan geometry to reject scattered radiation effectively without degrading primary x-rays. This paper describes the design of prototype scan equalization digital radiography (SEDR) system implemented with an amorphous silicon (a-Si) thin-film transistor (TFT) array based flat-panel detector. With this system, the slot-scan geometry with the alternate line erasure and readout (ALER) technique was used to achieve scatter rejection. A seven-segment beam height modulator assembly was mounted onto the fore-collimator to regulate exposure regionally for chest radiography. The beam modulator assembly, consisting of micro linear motors, lead screw cartridge with lead beam blocks attached, position feedback sensors, and motor driver circuitry, has been tested and found to have an acceptable response for exposure equalization in chest radiography. An anthropomorphic chest phantom was imaged in the posterior-anterior (PA) view under clinical conditions. Scatter component, primary x-rays, scatter-to-primary ratios (SPRs), and primary signal-to-noise ratios (PSNRs) were measured in the SEDR images to evaluate the rejection and redistribution of scattered radiation, and compared with those for conventional full-field imaging with and without anti-scatter grid methods. SPR reduction ratios (SPRRRs, defined as the differences between the non-grid full-field SPRs and the reduced SPRs divided by the former) yielded approximately 59% for the full-field imaging with grid and 82% for SEDR technique in the lungs; and 77% for the full-field imaging with grid and 95% for SEDR technique in the subdiaphragm. The SEDR technique demonstrated a substantial improvement in PSNRs over the anti-scatter grid technique. The improvements of PSNRs are varied with the regions and are more pronounced in the heavily attenuating regions.

Effects of exposure equalization on image signal‐to‐noise ratios in digital mammography: A simulation study with an anthropomorphic breast phantom

“…To determine the effectiveness of scatter reduction with the SEDM technique, the scatter-to-primary ratio (SPR) has to be measured with different slot widths and compared with that of the anti-scatter grid method in conventional mammography. We used an edge device technique 38 to measure the SPR along a line parallel to one of the detector sides for both slot-scanning and FFDM. With this method, a rectangular tungsten plate (0.4 mm thick and 30 mm wide, long enough to cover the long side of the FP detector) with a straight and sharp edge was placed directly on top of the breast phantom.…”

“…22 Recently, we developed a scan equalization digital radiography (SEDR) system that combines slot-scan geometry, regional beam width modulation, and digital detection technology to reduce scattered radiation and to improve SNRs in heavily attenuating regions of the chest. 38,39 An electronic aft-collimation technique, referred to as the alternate line erasure and readout (ALER) technique, was implemented with a flat-panel (FP) detector to achieve scatter reduction without using a large and heavy aft-collimator. 40,41 The same technique can be applied to digital mammography to improve SNRs of soft tissue masses and the visibility of micro-calcifications in dense areas of the breast.…”

Comparison of scatter rejection and low‐contrast performance of scan equalization digital radiography (SEDR), slot‐scan digital radiography, and full‐field digital radiography systems for chest phantom imaging

“…The beam modulation transfer functions were computed from the flatfield images and stored as a look up table in the image acquisition/processing workstation for SEDR image acquisition. 48 An Intel Pentium VI based computer ͑Precision 340, Dell Inc., Austin, TX͒ with a CPU frequency of 2.80 GHz and 512 MB RAM was used to control image acquisition, motion of fore-collimator ͑for both slot-scan and SEDR imaging͒, and beam width modulation.…”

“…Image data were averaged horizontally line-by-line within each segment to yield 18 mean signal profiles in the beam scanning ͑vertical͒ direction. These signal profiles were further convolved with a low-pass filtering kernel ͑kernel size 10 was used in this study as the result of previous study 48 to smooth out their shape and reduce their fluctuations. The exposure equalization pattern used to regulate the entrance exposure for individual segment can be computed from the prescan image as follows:…”

“…The system combines slot-scanning geometry with regional beam width modulation to reject scatter radiation effectively while regulating x-ray exposure regionally. 48 In this paper, we will present the scatter rejection/redistribution and low-contrast performances of the prototype SEDR technique measured with an anthropomorphic chest phantom and compare the results to those measured for the antiscatter grid method and slot-scan technique. The main differences between the current work and the previous study 48 are as follows: ͑1͒ The SEDR system has been redesigned/built with a 24-segment beam width modulation assembly instead of a seven-segment module presented in the previous publication to improve the equalization accuracy and resolution for chest imaging; ͑2͒ the beam aperture array method 32 was used to separate the primary intensities from the total image signals and to estimate scatter-to-primary ratios ͑SPRs͒ in various anatomical regions of the chest phantom image instead of measuring image lines across the chest phantom image with edge device technique; ͑3͒ the low-contrast performance was directly estimated by measuring the contrast signals and noise levels in this study instead of using primary signal-to-noise ratio ͑PSNR͒ to indirectly estimate the CNR improvement in the chest phantom image; ͑4͒ and, finally, the effects of effective exposures on the CNR were studied to examine the potential dose reduction for SEDR technique.…”