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.
We have developed a simulation tool for modeling the performance of CMOS-based medical x-ray detectors, based on the Monte Carlo toolkit GEANT4. Following the Fujita-Lubberts-Swank approach recently reported by Star-Lack et al., we calculate modulation transfer function MTF(<i>f</i>), noise power spectrum NPS(<i>f</i>) and detective quantum efficiency DQE(<i>f</i>) curves. The complete optical stack is modeled, including scintillator, fiber optic plate (FOP), optical adhesive and CMOS image sensor. For critical parts of the stack, detailed models have been developed, taking into account their respective microstructure. This includes two different scintillator types: Gd<sub>2</sub>O<sub>2</sub>S:Tb (GOS) and CsI:Tl. The granular structure of the former is modeled using anisotropic Mie scattering. The columnar structure of the latter is introduced into calculations directly, using the parameterization capabilities of GEANT4. The underlying homogeneous CsI layer is also incorporated into the model as well as the optional reflective layer on top of the scintillator screen or the protective polymer top coat. The FOP is modeled as an array of hexagonal bundles of fibers. The simulated CMOS stack consists of layers of Si<sub>3</sub>N<sub>4</sub> and SiO<sub>2</sub> on top of a silicon pixel array. The model is validated against measurements of various test detector structures, using different x-ray spectra (RQA5 and RQA-M2), showing good match between calculated and measured MTF(<i>f</i>) and DQE(<i>f</i>) curves.
Compared to published amorphous-silicon (TFT) based X-ray detectors, crystalline silicon CMOS-based active-pixel detectors exploit the benefits of low noise, high speed, on-chip integration and featuring offered by CMOS technology. This presentation focuses on the specific advantage of high image quality at very low dose levels. The measurement of very low dose performance parameters like Detective Quantum Efficiency (DQE) and Noise Equivalent Dose (NED) is a challenge by itself. Second-order effects like defect pixel behavior, temporal and quantization noise effects, dose measurement accuracy and limitation of the x-ray source settings will influence the measurements at very low dose conditions. Using an analytical model to predict the low dose behavior of a detector from parameters extracted from shot-noise limited dose levels is presented. These models can also provide input for a simulation environment for optimizing the performance of future detectors. In this paper, models for predicting NED and the DQE at very low dose are compared to measurements on different CMOS detectors. Their validity for different sensor and optical stack combinations as well as for different x-ray beam conditions was validated.
We have built an all-solid-state camera that is directly modulated at the pixel level for frequency-domain fluorescence lifetime imaging microscopy (FLIM) measurements. This novel camera eliminates the need for an image intensifier through the use of an application-specific charge coupled device design in a frequency-domain FLIM system. The first stage of evaluation for the camera has been carried out. Camera characteristics such as noise distribution, dark current influence, camera gain, sampling density, sensitivity, linearity of photometric response, and optical transfer function have been studied through experiments. We are able to do lifetime measurement using our modulated, electron-multiplied fluorescence lifetime imaging microscope (MEM-FLIM) camera for various objects, e.g., fluorescein solution, fixed green fluorescent protein (GFP) cells, and GFP-actin stained live cells. A detailed comparison of a conventional microchannel plate (MCP)-based FLIM system and the MEM-FLIM system is presented. The MEM-FLIM camera shows higher resolution and a better image quality. The MEM-FLIM camera provides a new opportunity for performing frequency-domain FLIM.
We have built an all-solid-state camera which is directly modulated at the pixel level for frequency
domain fluorescence lifetime imaging microscopy (FLIM) measurement. This novel camera eliminates
the need for an image intensifier through the use of an application-specific CCD design,
which is being used in a frequency domain FLIM system. The first stage of evaluation for the
camera has been carried out. Camera characteristics such as noise distribution, dark current influence,
camera gain, sampling density, sensitivity, linearity of photometric response, and contrast
modulation transfer function have been studied through experiments. We are able to do lifetime
measurement using MEM-FLIM cameras for various objects, e.g. fluorescence plastic test slides,
fluorescein solution, fixed GFP cells, and GFP - Actin stained live cells.
A new-generation full-frame 36x48 mm<sup>2</sup> 48Mp CCD image sensor with vertical anti-blooming for professional digital
still camera applications is developed by means of the so-called building block concept. The 48Mp devices are formed
by stitching 1kx1k building blocks with 6.0 µm pixel pitch in 6x8 (hxv) format. This concept allows us to design four
large-area (48Mp) and sixty-two basic (1Mp) devices per 6" wafer. The basic image sensor is relatively small in order to
obtain data from many devices. Evaluation of the basic parameters such as the image pixel and on-chip amplifier
provides us statistical data using a limited number of wafers. Whereas the large-area devices are evaluated for aspects
typical to large-sensor operation and performance, such as the charge transport efficiency. Combined with the usability
of multi-layer reticles, the sensor development is cost effective for prototyping.
Optimisation of the sensor design and technology has resulted in a pixel charge capacity of 58 ke- and significantly
reduced readout noise (12 electrons at 25 MHz pixel rate, after CDS). Hence, a dynamic range of 73 dB is obtained.
Microlens and stack optimisation resulted in an excellent angular response that meets with the wide-angle photography
This paper presents an overview of the specific challenges that need to be overcome to make very-large CCD and CMOS
imagers, and presents some recent innovations in this area. The complete development chain is described: research,
production and industrialization. It will be shown that by innovative design and technology concepts, high-quality very large
area CCD and CMOS imagers can be made, even up to wafer size (6" for CCD, 8" for CMOS).
This paper gives an overview of the requirements for, and current state-of-the-art of, CCD and CMOS imagers for use in digital still photography. Four market segments will be reviewed: mobile imaging, consumer "point-and-shoot cameras", consumer digital SLR cameras and high-end professional camera systems. The paper will also present some challenges and innovations with respect to packaging, testing, and system integration.
This paper gives an overview of featuring possibilities in CCD imagers. By careful manipulation of charge packets in CCD imagers, a CCD can often be read out in different modes by simply modifying the applied pulse patterns. Since featuring is done in the charge domain and not in the voltage domain, it offers the best possible performance with respect to noise, dynamic range and signal-to-noise ratio.
A 28-M pixel, full-frame CCD imager with 7.2×7.2 μm<sup>2</sup> pixel size and Bayer RGB color pattern was developed for use in professional applications. As unique option a RGB compatible binning feature was designed into this sensor. This gives the possibility to exchange resolution for sensitivity, read-out speed and signal-to-noise ratio. This paper presents the device architecture, RGB binning principle and evaluation results of the overall sensor performance. The performed device simulations and the evaluation results of the RGB binning feature are described in detail.