X-ray detectors increasingly utilize active pixel CMOS instead of amorphous silicon technology because of its superior noise, pixel lag, readout speed and offset stability. We already demonstrated [1] that adding an a additional pixel capacitance to a standard 3T CMOS pixel architecture allows operating the detector in either high sensitivity (HS) or high saturation dose (HD) mode. Since the charge capacity is limited in HS mode, a large signal will saturate the pixel, causing a loss of information. In HD mode, a very low exposure will lead to a loss of contrast-to-noise ratio (CNR) due to the inherently higher noise floor in this mode. Using the same 3T pixel architecture, we propose a dual readout method to combine the benefits of HS and HD modes. After a single exposure the pixel signal is read twice, respectively in HS and HD mode. The two linear signal values are then combined to create the pixel value of the final image. In this paper proof of concept is demonstrated using images acquired separately and combined offline. The benefits of this method are demonstrated for different x-ray imaging modalities such as mammography, extra-oral dental, interventional and non-destructive testing. Using different detector models, results show that extended dynamic range combined with low noise leads to better image quality without introducing artifacts. It is expected that implementing the fast CMOS-sensor dual readout and image synthesis inside the detector will preserve important application requirements such as frame rate, data bandwidth and power consumption.
KEYWORDS: Sensors, Digital breast tomosynthesis, Mammography, X-ray detectors, Image quality, Scintillators, X-ray imaging, Signal to noise ratio, High dynamic range imaging, X-rays, CMOS sensors
Digital Breast Tomosynthesis (DBT) requires excellent image quality in a dynamic mode at very low dose levels while Full Field Digital Mammography (FFDM) is a static imaging modality that requires high saturation dose levels. These opposing requirements can only be met by a dynamic detector with a high dynamic range. This paper will discuss a wafer-scale CMOS-based mammography detector with 49.5 μm pixels and a CsI scintillator. Excellent image quality is obtained for FFDM as well as DBT applications, comparing favorably with a-Se detectors that dominate the X-ray mammography market today. The typical dynamic range of a mammography detector is not high enough to accommodate both the low noise and the high saturation dose requirements for DBT and FFDM applications, respectively. An approach based on gain switching does not provide the signal-to-noise benefits in the low-dose DBT conditions. The solution to this is to add frame summing functionality to the detector. In one X-ray pulse several image frames will be acquired and summed. The requirements to implement this into a detector are low noise levels, high frame rates and low lag performance, all of which are unique characteristics of CMOS detectors. Results are presented to prove that excellent image quality is achieved, using a single detector for both DBT as well as FFDM dose conditions. This method of frame summing gave the opportunity to optimize the detector noise and saturation level for DBT applications, to achieve high DQE level at low dose, without compromising the FFDM performance.
Remote Hyperspectral and Multispectral sensors have been developed using modern CCD and CMOS fabrication
techniques combined with advanced dichroic filters. The resulting sensors are more cost effective while maintaining the
high performance needed in remote sensing applications. A single device can contain multiple imaging areas tailored to
different multispectral bandwidths in a highly cost effective and reliable package. This paper discusses a five band
visible to near IR scanning sensor. By bonding advanced dichroic filters onto the cover glass and directly in the imaging
path a highly efficient multispectral sensor is achieved. Up to 12,000 linear pixel arrays are possible1 with this advanced
filter technology approach. Individual imaging areas on the device are designed to have unique pixel sizes and clocking
to enable tailored imaging performance for the individual spectral bands. Individual elements are also based on high
resolution Time Delay and Integration technology2,3 (TDI) to maximize sensitivity and throughput. Additionally for
hyperspectral imagers, a split frame CCD design is discussed using high sensitivity back side illuminated (BSI)
processes that can achieve high quantum efficiency. As these sensors are used in remote sensing applications, device
robustness and radiation tolerance was required.
We have developed a 6032 element, 32 stage Tri-linear Time Delay and Integration Focal Plane Array for high resolution color imaging applications. The sensor offers an improvement of a factor of 10 over comparable line scan CCD sensors. The imager architecture utilizes three individual TDI arrays, with a new multi-layer dielectric interference film color filter that helps achieve accurate spectral separation in the three primary color bands centered at 450 nm, 550 nm and 650 nm respectively. We have also incorporated in the sensor a new programmable responsivity mechanism. This is achieved by an on-chip imaging area size selection mechanism. The sensors operate in the time-delay-and-integration mode. Depending on the level of illumination the user is able to dynamically select the number of TDI stages per sensor. The total number of TDI gain stages can be selected in blocks of 32, 16, 8, or 4. This property provides improvement in the dynamic range of the sensor by extending the response of the CCD over a wide range of illumination levels. Depending on the light energy incident on the sensor, the user can dynamically vary the number of TDI stages used for integrating an incident scene. The result is a programmable responsivity depending on the level of incident illumination. Such a technique optimizes the performance of the CCD color sensor. Given the resolution, speed, sensitivity and the dynamic range, this sensor is suitable for a variety of color imaging applications including multi-media, document scanning, pre-press scanning, medical and machine vision.
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