Rejection and redistribution of scattered radiation in Scan Equalization Digital Radiography (SEDR): simulation with spot images

摘要

The anti-scatter grid has been widely used to reject scatter and increase the perceptibility of low contrast object in chest radiography; however it also attenuate the primary x-rays, resulting in a substantial loss of information and an increased relative noise level in heavily attenuated regions. A more dose efficient approach to scatter rejection is the slot-scan imaging technique. Another problem in chest radiography is the low transmitted x-ray intensity in heavily attenuating regions. It results in higher relative noise level and subject the contrast sensitivity to limitation by the system noises. A solution to this problem is through the exposure equalization technique with which the incident x-ray intensity is regionally modulated to compensate for the differences of x-ray attenuation due to the anatomic variation. We are in the process of implementing the scan equalization digital radiography (SEDR) technique which combines the advantages of slot-scan imaging and exposure equalization. However, associated with the use of exposure equalization is a re-distribution of scattered radiation at the detector which may impact on the benefit of using exposure equalization in conjunction with the slot-scan imaging geometry. In order to understand the scatter properties and their impact in SEDR, we have used spot collimated aSi FP images to synthesize simulated SEDR images with which scatter components, primary signals, and scatter-to-primary ratios (SPRs) were measured. Using a simple model for scatter effects, we have also estimated and compared the contrast-to-noise ratio degradation factors (CNRDFs). It was found that for quantum limited situations the slot-scan technique has resulted in a substantial improvement of the image quality as indicated by higher estimated CNRDFs. Compared to slot-scan imaging, SEDR resulted in higher SPRs in the lungs and lower SPRs in the mediastinum. In the sub-diaphragmatic regions, the SPRs remain about the same. This correspond to lower CNRDFs in the lungs, higher CNRDFs in the mediastinum, and about the same CNRDFs in the sub-diaphragmatic regions. It was shown that although SEDR has resulted in minimum improvement over slot-scan imaging in reducing the SPRs, it could improve the contrast sensitivity by raising the primary signal levels in heavily attenuating regions. This advantage needs to be further investigated in our continuing study of the SEDR technique.