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

Area of Science:

• Medical Imaging Physics
• Digital Radiography
• Cone-Beam Computed Tomography

Background:

• Amorphous silicon (a-Si) flat panel (FP) x-ray detectors can suffer from detector lag, causing ghosting and radar artifacts in images.
• Detector lag is primarily attributed to defect states (traps) within the a-Si layer.
• Existing software correction methods may rely on assumptions like system linearity, which may not always hold true.

Purpose of the Study:

• To investigate a novel hardware-based method for reducing detector lag in a-Si FP x-ray detectors.
• To evaluate the effectiveness of this hardware method in mitigating shading artifacts in CBCT reconstructions.
• To examine the feasibility of a partially hardware-based solution for lag reduction.

Main Methods:

• A minor modification to the FP detector was implemented, introducing a forward bias operation step.
• This forward bias step is applied between readout and data collection for pulsed irradiation, filling defect states with charge.
• Measurements included residual lag contrast, detector step response, signal-to-noise ratio (SNR), modulation transfer function (MTF), and detective quantum efficiency (DQE).
• CBCT data were acquired using pelvic and head phantoms with both standard and forward bias modes.

Main Results:

• The forward bias method reduced residual lag signals by 70%-88% in lag frames 2 and 100.
• A slight decrease in collected signal and a minor increase in noise were observed, with small impacts on SNR, MTF, and DQE.
• The hardware successfully reduced radar artifacts in CBCT reconstructions by 48%-81% for both phantom types.

Conclusions:

• The forward bias hardware method effectively reduces detector lag ghosts in projection data and radar artifacts in CBCT.
• The current method's improvements are limited to the a-Si photodiode response.
• Future hybrid approaches may address the limitations of this hardware-based solution.