Development of Novel Methods to Reduce Ghosting and Lag in a-Se X-ray Detectors

Direct X-Ray Digital Imaging Technology, West Chester PA

Investigators

Abstract

DESCRIPTION (provided by applicant): Directxray Digital Imaging Technology (DXT) was formed in April of 2006, with a goal to introduce advanced technology developments for selenium detectors in x-ray imaging. We seek creative solutions to overcome the physical limitations of existing selenium detector designs and thus facilitate the broader use of such detectors. Selenium x-ray detectors are widely used in radiography, mammography, tomosynthesis, fluoroscopy, and cone-beam computed tomography. There are two basic forms of selenium detectors: the PIN and the dielectric detectors. While both have benefits, dielectric detectors are now widely used in many hospitals. Due to their low leakage, these detectors are also important for some biomedical applications where the exposure window can be as long as one minute. We have identified a number of limitations of dielectric-type selenium detectors. Ghosting, lag, and temporal response are the greatest limitations. In this application, we propose to address the root cause: to effectively remove the trapped charge after the x-ray exposure. Our first goal is to investigate residual charge removal using alternating current (AC) and light. In this Phase I proposal, we seek to show that AC-erase with light can significantly reduce lag and ghosting, and avoid the related local loss of gain. Our ultimate goal is to restore the detector condition every time after x-ray exposure. We propose two specific aims: (SA.1): Measure DC- properties on a large-area test fixture. We will use large-area a-Se sister samples to investigate the electrical properties of the selenium: (1) dark current and leakage current under electrical bias;(2) photocurrent stimulate by visible light;(3) photocurrent stimulate by x-radiation;(4) charge trapping in Se as measured by time-of- flight;and (5) de-trapping with infrared radiation. (SA.2): Test AC-Erase on a large-area test fixture. Using the sister samples produced in SA.1, we will characterize the photocurrent stimulated by visible light with an applied AC voltage. We will quantify the effect of this AC-erase strategy in terms of the change in charge trapping in Se from x-ray exposures as measured by time-of-flight;and change in sensitivity (sensitivity recovery) arising from the AC-erase. The verification and validation of the resulted theory on residual charge removal in amorphous selenium using light, infrared radiation and AC will mark the endpoint of Phase I. New image acquisition sequences with the potential addition of the deeply penetrating infrared radiation to the convention methods of visible light and single reversal of bias field can be rationally derived from these experimental results and new theory. Phase II of this project will modify imaging panels from commercial products and measure the effectiveness of these new methods. Faster erase procedures will also be developed for high speed other imaging modalities where rapid image acquisition are required. PUBLIC HEALTH RELEVANCE: The Selenium detectors with dielectric blocking layer is now widely used in clinical use. In this Phase I proposal, we seek to show that AC-erase with light can significantly reduce lag and ghosting artifacts in dielectric Selenium detectors, and avoid the related local loss of gain. Our ultimate goal is produce ghost-free images and to restore the detector condition every time after x-ray exposure, which would allow us to introduce new image acquisition sequences for more advanced clinical applications such as tomosynthesis or cone beam tomography.