Charge Summing Research Articles

Reducing beam hardening effects and metal artefacts in spectral CT using Medipix3RX

This paper discusses methods for reducing beam hardening effects and metal artefacts using spectral x-ray information in biomaterial samples. A small-animal spectral scanner was operated in the 15 to 80 keV x-ray energy range for this study. We use the photon-processing features of a CdTe-Medipix3RX ASIC in charge summing mode to reduce beam hardening and associated artefacts. We present spectral data collected for metal alloy samples, its analysis using algebraic 3D reconstruction software and volume visualisation using a custom volume rendering software. The cupping effect and streak artefacts are quantified in the spectral datasets. The results show reduction in beam hardening effects and metal artefacts in the narrow high energy range acquired using the spectroscopic detector. A post-reconstruction comparison between CdTe-Medipix3RX and Si-Medipix3.1 is discussed. The raw data and processed data are made available (http://hdl.handle.net/10092/8851) for testing with other software routines.

Charge Summing in Spectroscopic X-Ray Detectors With High-Z Sensors

The spectroscopic performance of photon counting detectors is limited by the effects of charge sharing between neighboring pixels and the emission of characteristic X-rays. For these reasons, an event can be either missed or counted more than once. These effects become more and more of a concern when pixel pitches are reduced, and for the technology available so far, this meant that there would always be a trade-off between a high spatial and a high spectral resolution. In this work, we present first measurements obtained with the new Medipix3RX ASIC, which features a network of charge summing circuits establishing a communication between pixels which helps to mitigate these effects. Combined with cadmium telluride sensors, we show that this new technology is successful at improving a detector's spectroscopic capabilities even at pixel pitches as small as 55 μm. At this pitch, we measure an energy response function similar to that observed for a pixel pitch of 165 μm in the absence of a charge summing circuitry. This amounts to an effective reduction of the pixel area by at least one order of magnitude at a comparable energy response. Additionally, we present synchrotron measurements at high X-ray fluxes, where significant pulse pile-up occurs, and provide first experimental evidence for a net benefit when balancing spectroscopic performance and high flux tolerance in charge summing mode.

The Medipix3RX: a high resolution, zero dead-time pixel detectorreadout chip allowing spectroscopic imaging

The Medipix3 chips have been designed to permit spectroscopic imaging in highly segmented hybrid pixel detectors. Spectral degradation due to charge sharing in the sensor has been addressed by means of an architecture in which adjacent pixels communicate in the analog and digital domains on an event-by-event basis to reconstruct the deposited charge in a neighbourhood prior to the assignation of the hit to a single pixel. The Medipix3RX chip architecture is presented. The first results for the characterization of the chip with 300 μm thick Si sensors are given. ∼ 72e− r.m.s. noise and ∼ 40e− r.m.s. of threshold dispersion after chip equalization have been measured in Single Pixel Mode of operation. The homogeneity of the image in Charge Summing mode is comparable to the Single Pixel Mode image. This demonstrates both modes are suitable for X-ray imaging applications.

Spectrum measurement using Medipix3 in Charge Summing Mode

We have obtained first spectrum measurements on a Medipix3 detector with a cadmium telluride (CdTe) sensor using Charge Summing Mode (CSM). It will be shown that CSM in Medipix3 is capable of reducing theadverse effects of charge sharing and fluorescent x-rays of CdTe onthe spectra recorded. The development of the Medipix All ResolutionSystem (MARS) x-ray camera has allowed us to explore this novel pixelcommunication feature in Medipix3. Spectrum measurements in this workwere carried out using a MARS camera consisting of a Medipix3 chipbump-bonded to a 1mm thick CdTe sensor layer. The characteristicpeaks of the Am-241 source as well as the spectroscopic properties ofthe CdTe sensor material were depicted at a spatial resolution of 55μm. Furthermore, a "connected component" algorithm shows a siliconbased Medipix3 is effective in reallocating spread charge into asingle pixel.

The detector simulation toolkit HORUS

In recent years, X-ray detectors used and developed at synchrotron sources and Free Electron Lasers (FELs) have become increasing powerful and versatile. However, as the capabilities of modern X-ray cameras grew so did their complexity and therefore their response functions are far from trivial. Since understanding the detecting system and its behavior is vital for any physical experiment, the need for dedicated powerful simulation tools arose.The HPAD Output Response fUnction Simulator (HORUS) was originally developed to analyze the performance implications of certain design choices for the Adaptive Gain Integrating Pixel Detector (AGIPD) and over the years grew to a more universal detector simulation toolkit covering the relevant physics in the energy range from below 1 keV to a few hundred keV. HORUS has already been used to study possible improvements of the AGIPD for X-ray Photon Correlation Spectroscopy (XPCS) at the European XFEL and its performance at low beam energies. It is currently being used to study the optimum detector layout for Coherent Diffration Imaging (CDI) at the European XFEL. Simulations of the charge summing mode of the Medipix3 chip have been essential for the improvements of the charge summing mode in the Medipix3 RX chip. HORUS is universal enough to support arbitrary hybrid pixel detector systems (within limitations). To date, the following detector systems are predefined within HORUS: The AGIPD, the Large Pixel Detector (LPD), the Cornell-Stanford Pixel Array Detector (CSPAD), the Mixed-Mode (MMPAD) and KEKPAD, and the Medipix2, Medipix3 and Medipix3 RX chips.

The single photon sensitivity of the Adaptive Gain Integrating Pixel Detector

Single photon sensitivity is an important property of certain detection systems. This work investigated the single photon sensitivity of the Adaptive Gain Integrating Pixel Detector (AGIPD) and its dependence on possible detector noise values. Due to special requirements at the European X-ray Free Electron Laser (XFEL) the AGIPD finds the number of photons absorbed in each pixel by integrating the total signal. Photon counting is done off line on a thresholded data set.It was shown that AGIPD will be sensitive to single photons of 8keV energy or more (detection efficiency ⪢50%, less than 1 count due to noise per 106 pixels). Should the final noise be at the lower end of the possible range (200–400 electrons) single photon sensitivity can also be achieved at 5keV beam energy.It was shown that charge summing schemes are beneficial when the noise is sufficiently low. The total detection rate of events is increased and the probability to count a single event multiple times in adjacent pixels is reduced by a factor of up to 40.The entry window of AGIPD allows 3keV photons to reach the sensitive volume with approximately 70% probability. Therefore the low energy performance of AGIPD was explored, finding a maximum noise floor below 0.035 hits/pixel/frame at 3keV beam energy. Depending on the noise level and selected threshold this value can be reduced by a factor of approximately 10. Even though single photon sensitivity, as defined in this work, is not given, imaging at this energy is still possible, allowing Poisson noise limited performance for signals significantly above the noise floor.

Simulated performance of high-Z detectors with Medipix3 readout

The Medipix3 hybrid pixel readout chip incorporates “charge summing” circuitry, which sums together signals detected in adjacent pixels to reconstruct individual hits. This should compensate for fluorescence and charge sharing effects which degrade the performance of high-Z semiconductor detectors such as germanium, gallium arsenide and cadmium telluride. The behaviour of different high-Z sensors read out by Medipix3 is simulated, to compare their performance and to test the effectiveness of a proposed new inter-pixel communication scheme.The effects of charge sharing are eliminated in all these sensors. In Ge and GaAs sensors with 55 μm pixels, fluorescence effects only cause a small reduction in detector performance, immediately above the k-edge energy. The improvement in spectral performance makes it easier to deal with charge trapping effects in GaAs; with an electron lifetime of 10 ns, the main result of charge trapping is a tolerable reduction in energy resolution. In CdTe, fluorescence effects strongly degrade the spatial resolution above the k-edge energy of 26.7 keV, but with appropriate threshold settings the resolution can be recovered at energies above 35 keV. When CdTe is used in colour mode with 110 μm pixels, charge summing greatly reduces the escape and fluorescence peaks in the spectral response. Finally, since the charge summing circuitry cannot correct parallax effects, parallax will become the main source of blurring in the detector in many X-ray scattering experiments.

Simulations of charge summing and threshold dispersion effects in Medipix3

A novel feature of the Medipix3 photon-counting pixel readout chip is inter-pixel communication. By summing together the signals from neighbouring pixels at a series of “summing nodes”, and assigning each hit to the node with the highest signal, the chip can compensate for charge-sharing effects. However, previous experimental tests have demonstrated that the node-to-node variation in the detector's response is very large. Using computer simulations, it is shown that this variation is due to threshold dispersion, which results in many hits being assigned to whichever summing node in the vicinity has the lowest threshold level. A reduction in threshold variation would attenuate but not solve this issue. A new charge summing and hit assignment process is proposed, where the signals in individual pixels are used to determine the hit location, and then signals from neighbouring pixels are summed to determine whether the total photon energy is above threshold. In simulation, this new mode accurately assigns each hit to the pixel with the highest pulse height without any losses or double counting.

Characterization of Medipix3 With Synchrotron Radiation

Medipix3 is the latest generation of photon counting readout chips of the Medipix family. With the same dimensions as Medipix2 (256 × 256 pixels of 55 μm × 55 μm pitch each), Medipix3 is however implemented in an 8-layer metallization 0.13 μm CMOS technology which leads to an increase in the functionality associated with each pixel over Medipix2. One of the new operational modes implemented in the front-end architecture is the Charge Summing Mode (CSM). This mode consists of a charge reconstruction and hit allocation algorithm which eliminates event-by-event the low energy counts produced by charge-shared events between adjacent pixels. The present work focuses on the study of the CSM mode and compares it to the Single Pixel Mode (SPM) which is the conventional readout method for these kind of detectors and it is also implemented in Medipix3. Tests of a Medipix3 chip bump-bonded to a 300 μm thick silicon photodiode sensor were performed at the Diamond Light Source synchrotron to evaluate the performance of the new Medipix chip. Studies showed that when Medipix3 is operated in CSM mode, it generates a single count per detected event and consequently the charge sharing effect between adjacent pixels is eliminated. However in CSM mode, it was also observed that an incorrect allocation of X-rays counts in the pixels occurred due to an unexpectedly high pixel-to-pixel threshold variation. The present experiment helped to better understand the CSM operating mode and to redesign the Medipix3 to overcome this pixel-to-pixel mismatch.

Characterization of the Medipix3 pixel readout chip

The Medipix3 chip is a hybrid pixel detector readout chip working in Single Photon Counting Mode. It has been developed with a new front-end architecture aimed at eliminating the spectral distortion produced by charge diffusion in highly segmented semiconductor detectors. In the new architecture charge deposited in overlapping clusters of four pixels is summed event-by-event and the incoming quantum is assigned as a single hit to the summing circuit with the biggest charge deposit (this mode of operation is called Charge Summing Mode (CSM)). In Single Pixel Mode (SPM) the charge reconstruction and the communication between neighbouring pixels is disabled. This is the operating mode in traditional detector systems.This paper presents the results of the characterization of the chip with electrical stimuli and radioactive sources.

Study of charge-sharing in MEDIPIX3 using a micro-focused synchrotron beam

X-ray photon-counting detectors consisting of a silicon pixel array sensor bump-bonded to a CMOS electronic readout chip offer several advantages over traditional X-ray detection technologies used for synchrotron applications. They offer high frame rate, dynamic range, count rate capability and signal-to-noise ratio. A survey of the requirements for future synchrotron detectors carried out at the Diamond Light Source synchrotron highlighted the needs for detectors with a pixel size of the order of 50μm. Reducing the pixel size leads to an increase of charge-sharing events between adjacent pixels and, therefore, to a degradation of the energy resolution and image quality of the detector. This effect was observed with MEDIPIX2, a photon-counting readout chip with a pixel size of 55μm. The lastest generation of the MEDIPIX family, MEDIPIX3, is designed to overcome this charge-sharing effect in an implemented readout operating mode referred to as Charge Summing Mode. MEDIPIX3 has the same pixel size as MEDIPIX2, but it is implemented in an 8-metal 0.13μm CMOS technology which enables increased functionality per pixel. The present work focuses on the study of the charge-sharing effect when the MEDIPIX3 is operated in Charge Summing Mode compared to the conventional readout mode, referred to as Single Pixel Mode. Tests of a standard silicon photodiode array bump-bonded to MEDIPIX3 were performed in beamline B16 at the Diamond Light Source synchrotron. A monochromatic micro-focused beam of 2.9μm x 2.2μm size at 15keV was used to scan a cluster of nine pixels in order to study the charge collection and X-ray count allocation process for each readout mode, Single Pixel Mode and Charge Summing Mode. The study showed that charge-shared events were eliminated when Medipix3 was operated in Charge Summing Mode.

First CT using Medipix3 and the MARS-CT-3 spectral scanner

The MARS research group has created a new version of their scanner for taking improved spectral CT datasets. This version of the scanner (MARS-CT-3) has taken the first Medipix3 CT images of a phantom. MARS-CT-3 incorporates a new gantry, new x-ray sources and the new MARS readout board, as well as the ability to connect gas lines to the specimen. The new gantry has improved mechanical rigidity and can perform scans faster. Magnification can be controlled by moving the detector and the x-ray source independently. The brighter x-ray source means images can be taken six times faster. Gas lines allow the user to control various environmental factors inside the scanner, such as temperature, or deliver oxygen and anaesthetics, providing the ability to do a full spectroscopic CT scan of a live sedated biological specimen, such as a mouse. The new MARS readout is able to read from all current chips from the Medipix family, has faster image downloading, and the use of up to six Medipix detectors in parallel on the same chip carrier. The use of Medipix3 chips allows for compensation of charge sharing via Charge Summing Mode.

A modified spectrum reconstruction method for the Charge Summing Mode of Medipix3

Because of its spectral capabilities, the MEDIPIX2 [2] detector has been used to determine the spectrum of an X-ray tube [1]. The measured spectrum is smeared out in the detector due to charge sharing among pixels. Therefore, a deconvolution method has been applied to reconstruct the real spectrum. The Medipix3 [3], as successor of the Medipix2, will be able reduce the influence of charge sharing in real time. Based on simulations, the aim of this paper is to investigate how good one will be able to determine an incoming X-ray spectrum with the Medipix3.

Medipix3: A 64 k pixel detector readout chip working in single photon counting mode with improved spectrometric performance

Medipix3 is a 256×256 channel hybrid pixel detector readout chip working in a single photon counting mode with a new inter-pixel architecture, which aims to improve the energy resolution in pixelated detectors by mitigating the effects of charge sharing between channels. Charges are summed in all 2×2 pixel clusters on the chip and a given hit is allocated locally to the pixel summing circuit with the biggest total charge on an event-by-event basis. Each pixel contains also two 12-bit binary counters with programmable depth and overflow control. The chip is configurable such that either the dimensions of each detector pixel match those of one readout pixel or detector pixels are four times greater in area than the readout pixels. In the latter case, event-by-event summing is still possible between the larger pixels. Each pixel has around 1600 transistors and the analog static power consumption is below 15 μW in the charge summing mode and 9 μW in the single pixel mode. The chip has been built in an 8-metal 0.13 μm CMOS technology. This paper describes the chip from the pixel to the periphery and first electrical results are summarized.

A 12-bit 3.7-Msample/s Pipelined A/D Converter Based on the Novel Capacitor Mismatch Calibration Technique

This paper proposes a 12-bit 3.7-MS/s pipelined A/D Converter based on the novel capacitor mismatch calibration technique. The conventional stage is improved to an algorithmic circuit involving charge summing, capacitors' exchange and charge redistribution, simply through introducing some extra switches into the analog circuit. This proposed ADC obtains the linearity beyond the accuracy of the capacitor match and verifies the validity of reducing the nonlinear error from the capacitor mismatch to the second order without additional power dissipation through the novel capacitor mismatch calibration technique. It is processed in 0.5 μm CMOS technology. The transistor-level simulation results show that 72.6 dB SNDR, 78.5 dB SFDR are obtained for a 2 V Vpp 159.144 kHz sine input sampled at 3.7 MS/s. The whole power dissipation of this ADC is 33.4 mW at the power supply of 5 V.

The Medipix3 Prototype, a Pixel Readout Chip Working in Single Photon Counting Mode With Improved Spectrometric Performance

A prototype pixel detector readout chip has been developed with a new front-end architecture aimed at eliminating the spectral distortion produced by charge diffusion in highly segmented semiconductor detectors. In the new architecture neighbouring pixels communicate with one another. At the corner of each pixel summing circuits add the total charge deposited in each sub-group of 4 pixels. Arbitration logic assigns a hit to the summing circuit with the highest charge. In the case where incoming X-ray photons produce fluorescence-a particular issue in high-Z materials-the charge deposited by those fluorescent photons will be included in the charge sum provided that the deposition takes place within the volume of the pixels neighbouring the initial impact point. The chip is configurable such that either the dimensions of each detector pixel match those of one readout pixel or detector pixels are 4 times greater in area than the readout pixels. In the latter case event-by-event summing is still possible between the larger pixels. As well as this innovative analog front-end circuit, each pixel contains comparators, logic circuits and two 15-bit counters. When the larger detector pixels are used these counters can be configured to permit multiple thresholds in a pixel providing spectroscopic information. The prototype chip has been designed and manufactured in an 8-metal 0.13 mum CMOS technology. First measurements show an electronic pixel noise of ~ 72 e-rms (Single Pixel Mode) and ~ 140 e-rms (Charge Summing Mode).

Charge sharing suppression using pixel-to-pixel communication in photon counting X-ray imaging systems

In planar silicon detector structures, the charge sharing between pixels is one limiting factor for colour X-ray imaging using integrated photon counting pixel detectors. 3D detector structures have been proposed as one solution to this problem. However, there are also readout system solutions to the problem, i.e., introducing pixel-to-pixel communication and distributed charge summing in the readout electronics. In this work, different charge summing schemes are evaluated using Monte Carlo simulation techniques. The increase in electronic noise introduced by the charge summing is one of the most severe problems. A proper selection of summing scheme is necessary to obtain an efficient system.

A ratio-independent algorithmic analog-to-digital conversion technique

An algorithmic analog-to-digital conversion technique is described which is capable of achieving high-resolution conversion without the use of matched capacitors in an MOS technology. The exact integral multiplication of the signal required by the conversion is realized through an algorithmic circuit method which involves charge summing with an MOS integrator and exchange of capacitors. A first-order cancellation of the charge injection effect from MOS transistor switches is attained with a combination of differential circuit implementation and an optimum timing scheme. An experimental prototype has been fabricated with a standard 5-/spl mu/m n-well CMOS process. It achieves 12-bit resolution at a sampling rate of 8 kHz. The analog chip area measures 2400 mils/SUP 2/.