正向偏置法用于 a-Si 平板探测器的滞后校正。

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

机构信息

Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA.

出版信息

Med Phys. 2012 Jan;39(1):18-27. doi: 10.1118/1.3664004.

DOI:10.1118/1.3664004

PMID:22225271

原文链接:https://pmc.ncbi.nlm.nih.gov/articles/PMC3257750/

Abstract

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.

摘要

目的

数字非晶硅平板（FP）X 射线探测器可能会出现百分之几的探测器滞后，或者剩余信号，这可能会导致投影图像中的重影，或者在锥形束计算机断层扫描（CBCT）重建中出现严重的阴影伪影，称为雷达伪影。据信，探测器滞后的一个主要原因是非晶硅层中的缺陷状态或陷阱。存在用于描述和校正探测器滞后的软件方法，但它们可能会做出系统线性和时间不变性等假设，而这些假设可能并不正确。这项工作的目的是研究一种新的基于硬件的方法来减少非晶硅 FP 中的滞后，并评估其在 CBCT 重建中消除阴影伪影的效果。还研究了一种新颖的部分基于硬件的解决方案的可行性。

方法

用于减少滞后的拟议硬件解决方案仅需要对 FP 进行微小更改。对于脉冲辐射，所提出的方法在读出和数据收集阶段之间插入一个新的操作步骤。在这个新的阶段，光电二极管以正向偏置模式工作，这会使电荷填满缺陷状态。对瓦里安 4030CB 面板进行了修改，以允许在正向偏置模式下运行。对于标准和正向偏置模式，在停止辐照后，测量滞后帧 2 和 100 的剩余滞后鬼影的对比度。使用标准和正向偏置固件进行探测器阶跃响应、滞后、信噪比、调制传递函数（MTF）和探测量子效率（DQE）测量。还收集了骨盆和头部体模的 CBCT 数据。