Quantum-limited-dose (QLD) and noise-equivalent-dose (NED) are performance metrics often used interchangeably.
Although the metrics are related, they are not equivalent unless the treatment of electronic noise is carefully
considered. These metrics are increasingly important to properly characterize the low-dose performance of flat panel
detectors (FPDs). A system can be said to be quantum-limited when the Signal-to-noise-ratio (SNR) is proportional to
the square-root of x-ray exposure. Recent experiments utilizing three methods to determine the quantum-limited dose
range yielded inconsistent results. To investigate the deviation in results, generalized analytical equations are
developed to model the image processing and analysis of each method. We test the generalized expression for both
radiographic and fluoroscopic detectors. The resulting analysis shows that total noise content of the images processed
by each method are inherently different based on their readout scheme. Finally, it will be shown that the NED is
equivalent to the instrumentation-noise-equivalent-exposure (INEE) and furthermore that the NED is derived from the
quantum-noise-only method of determining QLD. Future investigations will measure quantum-limited performance of
radiographic panels with a modified readout scheme to allow for noise improvements similar to measurements
performed with fluoroscopic detectors.
Sensor fill factor is one of the key pixel design requirements for high performance imaging arrays. In our conventional imaging pixel architecture with a TFT and a photodiode deposited in the same plane, the maximum area that the photodiode can occupy is limited by the size of the TFT and the surrounding metal lines. A full fill factor array design was previously proposed using a continuous sensor layer1. Despite the benefits of 100% fill factor, when applied to large-area applications, this array design suffers from high parasitic line capacitances and, thus, high line noise. We have designed and fabricated an alternative pixel structure in which the photodiode is deposited and patterned over the TFT, but does not overlap with the lines underneath. Separating the diode from the TFT plane allows extra space for an additional TFT which can be used for pixel reset and clipping excessive charge in the photodiode developed under high illumination. This reduces memory effect by 250%. The yield and the reliability are expected to improve as well since the TFTs and lines are buried underneath the diode. With the increased fill factor, we collect 56% more electrons per pixel, thereby improving the signal to noise ratio. The maximum signal to noise ratio is achieved when the increased signal and the undesirable parasitic capacitance on the data line are best optimized. Linearity, sensitivity, leakage, and MTF characteristics of a prototype X-ray imager based on this architecture are presented.