Denny L. Lee PhD, Presenter: Nothing to Disclose

Andrew D.A. Maidment PhD, Abstract Co-Author: Nothing to Disclose

In selenium (Se) detectors, image charges draft towards the pixel directly below the point of x-ray absorption forming images with high spatial definition. Given the high resistance of Se, detectors with very low dark noise can be designed. However, the high resistivity also leads to a long relaxation time for trapped charge. Trapped charge perturbs the bias field, alters the local sensitivity and it is a source of image lag. Thus, residual charge must be removed to ensure high diagnostic quality.

Current Se detectors consist of a thin layer of amorphous material sandwiched between a TFT and a high voltage plane. Electrically it is a planar capacitor in which the Se layer acts as the dielectric. While the DC resistance across the detector is high, the AC impedance depends on frequency. We have observed that by applying an AC potential to the high voltage electrode after image readout, trapped charge can be removed effectively.

A Se panel with 2560 x 3072 139 micron pixels was used for the investigation. The panel had 500 microns of Se operated at 3000 Volts. Trapped charges were induced by high intensity irradiation (e.g., 45 kVp, 32 mAs). An opaque metal object shadowing a saturated background yielded a typical ghosting of 2% after a standard DC erase was used; no measurable ghosting occurred when an AC erase was applied for a comparable period of time. Since the erase is applied only after readout is completed, no additional noise is injected into the image. We have shown that the AC erase restores the panel sensitivity and uniformity, resulting in superior NPS. The AC erase does not degrade resolution. The erase current has no adverse effect on the TFT transistor or the readout electronics.

The effectiveness of AC erase has been demonstrated for Se detectors. Erase parameters vary with detector geometry and application.

Ghosting/lag reduce image quality, particularly in rapid readout. Residual charge elimination is essential for certain applications including fluoroscopy, CBT, tomosynthesis, NDT and crystallography.

Lee, D, Maidment, A, Residual Charge removal for Direct-conversion Selenium Detectors Using Alternating Current.  Radiological Society of North America 2007 Scientific Assembly and Annual Meeting, November 25 - November 30, 2007 ,Chicago IL. http://archive.rsna.org/2007/5006905.html