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Medical Physics / Volume 39 / Issue 1 / RADIATION IMAGING PHYSICS

Med. Phys. 39, 18 (2012); http://dx.doi.org/10.1118/1.3664004 (10 pages)

A forward bias method for lag correction of an a-Si flat panel detector

Jared Starman

Department of Electrical Engineering, Stanford University, Stanford, California 94305 and Department of Radiology, Stanford University, Stanford, California 94305

Carlo Tognina and Larry Partain

Varian Medical Systems, Palo Alto, California 94304

Rebecca Fahrig

Department of Radiology, Stanford University, Stanford, California 94305

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(Received 4 June 2011; accepted 4 November 2011; revised 18 October 2011; published online 9 December 2011)

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Purpose: Digital a-Si flat panel (FP) x-ray detectors can exhibit detector lag, or residual signal, of several percent that can cause ghosting in projection images or severe shading artifacts, known as the radar artifact, in cone-beam computed tomography (CBCT) reconstructions. A major contributor to detector lag is believed to be defect states, or traps, in the a-Si layer of the FP. Software methods to characterize and correct for the detector lag exist, but they may make assumptions such as system linearity and time invariance, which may not be true. The purpose of this work is to investigate a new hardware based method to reduce lag in an a-Si FP and to evaluate its effectiveness at removing shading artifacts in CBCT reconstructions. The feasibility of a novel, partially hardware based solution is also examined.
Methods: The proposed hardware solution for lag reduction requires only a minor change to the FP. For pulsed irradiation, the proposed method inserts a new operation step between the readout and data collection stages. During this new stage the photodiode is operated in a forward bias mode, which fills the defect states with charge. A Varian 4030CB panel was modified to allow for operation in the forward bias mode. The contrast of residual lag ghosts was measured for lag frames 2 and 100 after irradiation ceased for standard and forward bias modes. Detector step response, lag, SNR, modulation transfer function (MTF), and detective quantum efficiency (DQE) measurements were made with standard and forward bias firmware. CBCT data of pelvic and head phantoms were also collected.
Results: Overall, the 2nd and 100th detector lag frame residual signals were reduced 70%–88% using the new method. SNR, MTF, and DQE measurements show a small decrease in collected signal and a small increase in noise. The forward bias hardware successfully reduced the radar artifact in the CBCT reconstruction of the pelvic and head phantoms by 48%–81%.
Conclusions: Overall, the forward bias method has been found to greatly reduce detector lag ghosts in projection data and the radar artifact in CBCT reconstructions. The method is limited to improvements of the a-Si photodiode response only. A future hybrid mode may overcome any limitations of this method.

© 2012 American Association of Physicists in Medicine

ACKNOWLEDGMENTS

This work was supported in part by Varian Medical Systems, the Stanford-NIH Biotechnology Traineeship, NIH Grant Nos. EB003524 and HL087917, and the Lucas Foundation.

Article Outline

  1. INTRODUCTION
  2. METHODS AND MATERIALS
    1. Lag removal measurements
    2. SNR, MTF, and DQE
    3. Detector mode switching
    4. CBCT reconstructions
  3. RESULTS
    1. Lag removal measurements
    2. SNR, MTF, and DQE
    3. Detector mode switching
    4. CBCT reconstructions
  4. DISCUSSION AND CONCLUSIONS
Digital Object Identifier
http://dx.doi.org/10.1118/1.3664004

KEYWORDS and PACS

Keywords

biomedical equipment, computerised tomography, photodetectors, photodiodes, semiconductor device noise, semiconductor thin films, thin film devices, X-ray apparatus, X-ray detection, x-ray detector, flat-panel, lag, after-glow

PACS

  • 87.57.Q-

    Computed tomography

  • 87.85.Ox

    Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)

  • 85.30.Kk

    Junction diodes

  • 85.60.Dw

    Photodiodes; phototransistors; photoresistors

  • 85.60.Gz

    Photodetectors (including infrared and CCD detectors)

  • 87.59.-e

    X-ray imaging

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

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