M. Prydderch

Empirical electro-optical and x-ray performance evaluation of CMOS active pixels sensor for low dose, high resolution x-ray medical imaging

Monolithic complementary metal oxide semiconductor (CMOS) active pixel sensors with high performance have gained attention in the last few years in many scientific and space applications. In order to evaluate the increasing capabilities of this technology, in particular where low dose high resolution x-ray medical imaging is required, critical electro-optical and physical x-ray performance evaluation was determined. The electro-optical performance includes read noise, full well capacity, interacting quantum efficiency, and pixels cross talk. The x-ray performance, including x-ray sensitivity, modulation transfer function, noise power spectrum, and detection quantum efficiency, has been evaluated in the mammographic energy range. The sensor is a 525 x 525 standard three transistor CMOS active pixel sensor array with more than 75% fill factor and 25 x 25 microm pixel pitch. Reading at 10 f/s, it is found that the sensor has 114 electrons total additive noise, 10(5) electrons full well capacity with shot noise limited operation, and 34% interacting quantum efficiency at 530 nm. Two different structured CsI:Tl phosphors with thickness 95 and 115 microm, respectively, have been optically coupled via a fiber optic plate to the array resulting in two different system configurations. The sensitivity of the two different system configurations was 43 and 47 electrons per x-ray incident on the sensor. The MTF at 10% of the two different system configurations was 9.5 and 9 cycles/mm with detective quantum efficiency of 0.45 and 0.48, respectively, close to zero frequency at approximately 0.44 microC/kg (1.72 mR) detector entrance exposure. The detector was quantum limited at low spatial frequencies and its performance was comparable with high resolution a: Si and charge coupled device based x-ray imagers. The detector also demonstrates almost an order of magnitude lower noise than active matrix flat panel imagers. The results suggest that CMOS active pixel sensors when coupled to structured CsI:Tl can be used for conventional and advanced digital mammography due to their low noise, high resolution performance.

Charge sharing in silicon pixel detectors

We used a pixellated hybrid silicon X-ray detector to study the effect of the sharing of generated charge between neighbouring pixels over a range of incident X-ray energies, 13–36 keV. The system is a room temperature, energy resolving detector with a Gaussian FWHM of 265 eV at 5.9 keV. Each pixel is 300 μm square, 300 μm deep and is bump bonded to matching read out electronics. The modelling packages MEDICI and MCNP were used to model the complete X-ray interaction and the subsequent charge transport. Using this software a model is developed which reproduces well the experimental results. The simulations are then altered to explore smaller pixel sizes and different X-ray energies. Charge sharing was observed experimentally to be 2% at 13 keV rising to 4.5% at 36 keV, for an energy threshold of 4 keV. The models predict that up to 50% of charge may be lost to the neighbouring pixels, for an X-ray energy of 36 keV, when the pixel size is reduced to 55 μm.

Digital autoradiography using room temperature CCD and CMOS imaging technology

CCD (charged coupled device) and CMOS imaging technologies can be applied to thin tissue autoradiography as potential imaging alternatives to using conventional film. In this work, we compare two particular devices: a CCD operating in slow scan mode and a CMOS-based active pixel sensor, operating at near video rates. Both imaging sensors have been operated at room temperature using direct irradiation with images produced from calibrated microscales and radiolabelled tissue samples. We also compare these digital image sensor technologies with the use of conventional film. We show comparative results obtained with (14)C calibrated microscales and (35)S radiolabelled tissue sections. We also present the first results of (3)H images produced under direct irradiation of a CCD sensor operating at room temperature. Compared to film, silicon-based imaging technologies exhibit enhanced sensitivity, dynamic range and linearity.

A CMOS active pixel sensor system for laboratory- based x-ray diffraction studies of biological tissue.

X-ray diffraction studies give material-specific information about biological tissue. Ideally, a large area, low noise, wide dynamic range digital x-ray detector is required for laboratory-based x-ray diffraction studies. The goal of this work is to introduce a novel imaging technology, the CMOS active pixel sensor (APS) that has the potential to fulfil all these requirements, and demonstrate its feasibility for coherent scatter imaging. A prototype CMOS APS has been included in an x-ray diffraction demonstration system. An industrial x-ray source with appropriate beam filtration is used to perform angle dispersive x-ray diffraction (ADXRD). Optimization of the experimental set-up is detailed including collimator options and detector operating parameters. Scatter signatures are measured for 11 different materials, covering three medical applications: breast cancer diagnosis, kidney stone identification and bone mineral density calculations. Scatter signatures are also recorded for three mixed samples of known composition. Results are verified using two independent models for predicting the APS scatter signature: (1) a linear systems model of the APS and (2) a linear superposition integral combining known monochromatic scatter signatures with the input polychromatic spectrum used in this case. Cross validation of experimental, modelled and literature results proves that APS are able to record biologically relevant scatter signatures. Coherent scatter signatures are sensitive to multiple materials present in a sample and provide a means to quantify composition. In the future, production of a bespoke APS imager for x-ray diffraction studies could enable simultaneous collection of the transmitted beam and scattered radiation in a laboratory-based coherent scatter system, making clinical transfer of the technique attainable.

The CMS binary chip for microstrip tracker readout at the SLHC

A 130 nm CMOS chip has been designed for silicon microstrip readout at the SLHC. The CBC has 128 channels, and utilises a binary un-sparsified architecture for chip and system simplicity. It is designed to read out signals of either polarity from short strips (capacitances up to ~ 10 pF) and can sink or source sensor leakage currents up to 1 μA. Details of the design and measured performance are presented.

Dynamic position and force measurement for multiple optically trapped particles using a high-speed active pixel sensor

A high frame rate active pixel sensor designed to track the position of up to six optically trapped objects simultaneously within the field of view of a microscope is described. The sensor comprises 520 x 520 pixels from which a flexible arrangement of six independent regions of interest is accessed at a rate of up to 20 kHz, providing the capability to measure motion in multiple micron scale objects to nanometer accuracy. The combined control of both the sensor and optical traps is performed using unique, dedicated electronics (a field programmable gate array). The ability of the sensor to measure the dynamic position and the forces between six optically trapped spheres, down to femtonewton level, is demonstrated paving the way for application in the physical and life sciences.

Two approaches to hybrid x-ray pixel array readout

We have designed two different X-ray pixel array readout Integrated Circuits for silicon pixel detectors operating between 4 keV and 25 keV. The first allows full readout of the deposited charge for each X-ray photon and is intended for imaging X-ray spectroscopy. The second is a photon counting device capable of very high rates (1 MHz per pixel) but without energy resolution. This paper compares the architectures of these two detectors and presents experimental data from complete bump-bonded devices. These detectors have many applications from X-ray diffraction to material inspection and satellite based X-ray imaging.

Silicon pixel detector for X-ray spectroscopy.

We have built a back-illuminated, silicon x-ray pixel detector which is bump bonded to an array of readout electronics. The system is intended for x-ray spectroscopy measurement in the 1 keV-25keV range with a resolution of 250eV FWHM. The readout electronics consists of an array of 16 by 16 preamplifiers on the bump bonded integrated circuit, this unit is wire bonded to two 128 channel integrated circuits which have signal shaping, peak-hold and sparcification logic. This paper describes the construction of the silicon detector, the readout electronics and the performance of these components. The energy range of the detector system can be increased by using a GaAs or CdZnTe detector instead of the 300 micrometers -500 micrometers thick silicon pixel detector described here.

Performance of an energy resolving X-ray pixel detector

We have built a back-illuminated, silicon X-ray detector with 16×16 pixels. This is bump-bonded to an integrated circuit containing a corresponding array of pre-amplifiers. The bump-bonded unit is wire bonded to two 128 channel integrated circuits which have signal shaping, peak-hold and sparcification logic. These integrated circuits output the analogue value of the individual X-ray and the address of the 300 μm×300 μm pixel. The system has previously demonstrated X-ray spectroscopy measurement in the 5–40 keV range with a resolution of 1 keV FWHM. This paper describes the performance of the system used in an X-ray diffraction experiment performed on the Daresbury Synchrotron Radiation Source. The second part demonstrates the successful operation of this pixellated detector for spectroscopy. In this part, the variation among the pixel outputs is accounted for without significantly affecting the noise performance.

The CBC microstrip readout chip for CMS at the High Luminosity LHC

The CMS Binary Chip (CBC) is designed for readout of silicon microstrips in the CMS Tracker at the High Luminosity LHC (HL-LHC). Binary, unsparsified readout is well suited to the high luminosity environment, where particle fluences and data rates will be much higher than at the LHC. In September 2011, a module comprising a CBC bonded to a silicon microstrip sensor was tested with 400 GeV protons in the H8 beamline at CERN. Performance was in agreement with expectations. The spatial resolution of the sensor and CBC has been shown to be better than pitch/ p 12 due to spatial distribution of one and two strip clusters. Large cluster events show consistency with the production of delta rays. At operating thresholds, the hit efficiency has been shown to be approximately 98%, limited by the resolution of timing apparatus, while the noise occupancy is measured to be below 10 4 . The distribution of charge deposition in the sensor has been reconstructed by measurement of the hit efficiency as a function of comparator threshold; assuming the underlying distribution is a Landau.