Aili Kerttu Bloomquist, Abstract Co-Author: Nothing to Disclose

David Hunter MS, Abstract Co-Author: Nothing to Disclose

Daniel J Beideck MS, Abstract Co-Author: Nothing to Disclose

Martin Joel Yaffe PhD, Presenter: Nothing to Disclose

The purpose of this work is to evaluate the magnitude and temporal characteristics of ghosting and lag in selenium x-ray detectors used for full-field digital mammography. Both phenomena cause sensitivity variations in the detector related to previous exposure history. Ghosting refers to the apparent afterimage caused by a temporary (duration - tens of minutes) decrease in detector sensitivity in areas that have received increased exposure. Lag is a shorter-term effect resulting in the delay of signal reaching the readout circuitry. Lag causes part of the signal generated in a prior image to be superimposed on the signal generated during the current image acquisition. These effects are thought to be due to residual trapped charges in selenium. Ghosting or lag effects were noticed on quality control images acquired on early selenium-based systems. These effects have been reported by others in the laboratory setting. Here we evaluate their characteristics in clinical systems.

Experiments were devised to determine how the magnitude of lag/ghosting effects are influenced by phantom thickness, time between images and cumulative exposure to the detector. Phantoms employed included the Mammography Accreditation Phantom, slabs of PMMA, aluminum step wedges and lead sheets. Measurements were carried out on five selenium detector systems used in the Digital Mammography Imaging Screening Trial (DMIST), conducted by ACRIN. Measurements were also carried out on equipment using other detector materials.

The magnitude of the ghosting artifact varied from 0.4 to 1.4 % of the mean image signal, following a single exposure to un-attenuated x-rays at clinical techniques. The cumulative effect of ghosting could result in a signal reduction of up to 15 %. Lag was seen as a signal increase varying between 0 and 5.5% of the mean signal for a second image acquired within 100 seconds of the initial exposure.

Ghosting and lag appear to be most evident in areas where the unattenuated x-ray beam is incident on the detector for a number of images. Measurements made on more recent versions of this system show less evidence of these effects.

A.K.B.,D.H.,M.J.Y.: Martin Yaffe's laboratory has research collaborations on topics in digital mammography with GE Healthcare and Fischer Imaging. Dr. Yaffe is a member of the Scientific Advisory Board of XCounter.

Bloomquist, A, Hunter, D, Beideck, D, Yaffe, M, Measurement of Ghosting and Lag in a Selenium Detector Digital Mammography System: The ACRIN DMIST Experience.  Radiological Society of North America 2005 Scientific Assembly and Annual Meeting, November 27 - December 2, 2005 ,Chicago IL. http://archive.rsna.org/2005/4418182.html