Scatter rejection and low‐contrast performance of a slot‐scan digital chest radiography system with electronic aft‐collimation: A chest phantom study

Abstract

Anti-scatter grids have been widely used to reject scatter and increase the perceptibility of lowcontrast object in chest radiography; however they also attenuate the primary x-rays, resulting in a substantial degradation of primary information. Compensation for this degradation requires the use of higher exposure technique hence higher dose to the patient. A more efficient approach to reject scatter is the slot-scan imaging technique which employs a narrow scanning x-ray fan beam in conjunction with a slit or slot shaped solid state detector or an area detector used with an aftcollimator. With this approach, scatter can be rejected effectively without the need to attenuate primary x-rays. This paper demonstrates an electronic aft-collimation method, referred to as the alternate line erasure and readout ͑ALER͒ technique, for implementing the slot-scan digital radiography with a modern flat-panel detector. With this technique, instead of first exposing the detector and then reading the image line by line, the image line on the leading edge of the scanning fan beam is reset to erase the scatter accumulated prior to the arrival of the fan beam x-rays, while the image line on the trailing edge of the scanning fan beam is read out to acquire the image signals following the fan-beam exposure. These reset and readout processes are alternated and repeated as the x-ray fan beam scans across the detector. An anthropomorphic chest phantom was imaged to evaluate the scatter rejection ability and the low-contrast performance for the ALER technique and compare them with those for the anti-scatter grid method in full-field chest imaging. With a projected beam width of 16 mm, the slot-scan/ALER technique resulted in an average reduction of the scatter-toprimary ratios by 81%, 84%, 82%, and 86% versus 65%, 73%, 74%, and 73% with the anti-scatter grid method in the lungs, mediastinum, retrocardium, and subdiaphragm, respectively. The average CNR for the slot-scan/ALER technique was found to improve by 135%, 133%, 176%, and 87% versus 15%, 15%, 38%, and −11% with the anti-scatter grid method in the mediastinum, retrocardium, subdiaphragm, and lungs, respectively. These results demonstrated that the slot-scan/ALER technique can be used to achieve equally effective scatter rejection but substantially higher lowcontrast performance than the anti-scatter grid method.

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 readout scheme of the FP detector has been modified in our laboratory to allow "electronic aft-collimation" ͑referred to as the alternate line erasure and readout or ALER technique͒ to be achieved and to eliminate the need of using a bulky, heavy aft-collimator for slot-scan imaging. 31,32 With the ALER technique, each image line is reset to erase the scatter signals accumulated prior to the arrival of the moving fan-beam and image signals are read out immediately after the fan-beam passes. To achieve electronic aft-collimation during the slot-scanning, the control signals from a specially designed controller board are used to alternatively reset the image line on the leading edge of the moving fan-beam and to read out the image line on the trailing edge of the moving fan-beam.…”

“…The process to correct for gain nonuniformity, dark current offset, and dead pixels/lines externally was done in a similar manner as was done internally by the detector controller. 32 The "image correction" function of the detector controller was disabled so that only the uncorrected raw image data were transferred and saved in the acquisition workstation for the slot-scan or SEDR imaging mode.…”

“…This grid attenuates about 30% primary intensities at 120 kVp. 32 The grid was removed from the x-ray path for slot-scan or SEDR imaging.…”

“…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.…”

“…Thus, the slotscan technique has the advantage of dose efficiency and can notably enhance the effective detection efficiency of imaging system. [20][21][22][23][24][25][26][27][28][29][30][31][32] Second, a large variation in x-ray opacity of anatomical structures in chest radiography imposes a fundamental limit on the visualization of subtle abnormalities in the heavily attenuating regions. The regional variation in x-ray transmission through the chest could be as high as two orders of magnitude in typical clinical procedures.…”