Michael F. Walsh

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.

Energy Calibration of the Pixels of Spectral X-ray Detectors

The energy information acquired using spectral X-ray detectors allows noninvasive identification and characterization of chemical components of a material. To achieve this, it is important that the energy response of the detector is calibrated. The established techniques for energy calibration are not practical for routine use in pre-clinical or clinical research environment. This is due to the requirements of using monochromatic radiation sources such as synchrotron, radio-isotopes, and prohibitively long time needed to set up the equipment and make measurements. To address these limitations, we have developed an automated technique for calibrating the energy response of the pixels in a spectral X-ray detector that runs with minimal user intervention. This technique uses the X-ray tube voltage (kVp) as a reference energy, which is stepped through an energy range of interest. This technique locates the energy threshold where a pixel transitions from not-counting (off) to counting (on). Similarly, we have developed a technique for calibrating the energy response of individual pixels using X-ray fluorescence generated by metallic targets directly irradiated with polychromatic X-rays, and additionally γ-rays from 241Am. This technique was used to measure the energy response of individual pixels in CdTe-Medipix3RX by characterizing noise performance, threshold dispersion, gain variation and spectral resolution. The comparison of these two techniques shows the energy difference of 1 keV at 59.5 keV which is less than the spectral resolution of the detector (full-width at half-maximum of 8 keV at 59.5 keV). Both techniques can be used as quality control tools in a pre-clinical multi-energy CT scanner using spectral X-ray detectors.

Characterization of Medipix3 with the MARS readout and software

The Medipix3 x-ray imaging detector has been characterized using the MARS camera. This x-ray camera comprises custom built readout electronics and software libraries designed for the Medipix family of detectors. The performance of the Medipix3 and MARS camera system is being studied prior to use in real-world applications such as the recently developed MARS-CT3 spectroscopic micro-CT scanner. We present the results of characterization measurements, describe methods for optimizing performance and give examples of spectroscopic images acquired with Medipix3 and the MARS camera system. A limited number of operating modes of the Medipix3 chip have been characterized and single-pixel mode has been found to give acceptable performance in terms of energy response, image quality and stability over time. Spectroscopic performance is significantly better in charge-summing mode than single-pixel mode however image quality and stability over time are compromised. There are more modes of operation to be tested and further work is required to optimize the performance of the chip.

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.This paper discusses methods for reducing beam hardening effects using spectral data for biomaterial applications. A small-animal spectral scanner operating in the diagnostic energy range was used. We investigate the use of photon-processing features of the Medipix3RX ASIC in reducing beam hardening and associated artefacts. A fully operational charge summing mode was used during the imaging routine. 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. Narrow high energy acquisition using the photon-processing detector revealed substantial reduction in beam hardening effects and metal artefacts.

MARS spectral molecular imaging of lamb tissue: data collection and image analysis

Spectral molecular imaging is a new imaging technique able to discriminate and quantify different components of tissue simultaneously at high spatial and high energy resolution. Our MARS scanner is an x-ray based small animal CT system designed to be used in the diagnostic energy range (20–140 keV). In this paper, we demonstrate the use of the MARS scanner, equipped with the Medipix3RX spectroscopic photon-processing detector, to discriminate fat, calcium, and water in tissue. We present data collected from a sample of lamb meat including bone as an illustrative example of human tissue imaging. The data is analyzed using our 3D Algebraic Reconstruction Algorithm (MARS-ART) and by material decomposition based on a constrained linear least squares algorithm. The results presented here clearly show the quantification of lipid-like, water-like and bone-like components of tissue. However, it is also clear to us that better algorithms could extract more information of clinical interest from our data. Because we are one of the first to present data from multi-energy photon-processing small animal CT systems, we make the raw, partial and fully processed data available with the intention that others can analyze it using their familiar routines. The raw, partially processed and fully processed data of lamb tissue along with the phantom calibration data can be found at http://hdl.handle.net/10092/8531.

Spectral CT data acquisition with Medipix3.1

This paper describes the acquisition of spectral CT images using the Medipix3.1 in spectroscopic mode, in which the chip combines 2 × 2 pixel clusters to increase the number of energy thresholds and counters from 2 to 8. During preliminary measurements, it was observed that the temperature, DAC and equalisation stability of the Medipix3.1 outperformed the Medipix3.0, while maintaining similar imaging quality. In this paper, the Medipix3.1 chips were assembled in a quad (2 × 2) layout, with the four ASICs bump-bonded to a silicon semiconductor doped as an np-junction diode. To demonstrate the biological imaging quality that is possible with the Medipix3.1, an image of a mouse injected with gold nano-particle contrast agent was obtained. CT acquisition in spectroscopic mode was enabled and examined by imaging a customised phantom containing multiple contrast agents and biological materials. These acquisitions showed a limitation of imaging performance depending on the counter used. Despite this, identification of multiple materials in the phantom was demonstrated using an in-house material decomposition algorithm. Furthermore, gold nano-particles were separated from biological tissues and bones within the mouse by means of image rendering.

Hybrid Spectral Micro-CT: System Design, Implementation, and Preliminary Results

Spectral CT has proven an important development in biomedical imaging, and there have been several publications in the past years demonstrating its merits in pre-clinical and clinical applications. In 2012, Xu reported that near-term implementation of spectral micro-CT could be enhanced by a hybrid architecture: a narrow-beam spectral “interior” imaging chain integrated with a traditional wide-beam “global” imaging chain. This hybrid integration coupled with compressive sensing (CS)-based interior tomography demonstrated promising results for improved contrast resolution, and decreased system cost and radiation dose. The motivation for the current study is implementation and evaluation of the hybrid architecture with a first-of-its-kind hybrid spectral micro-CT system. Preliminary results confirm improvements in both contrast and spatial resolution. This technology is shown to merit further investigation and potential application in future spectral CT scanner design.

Characterization of Si and CdTe sensor layers in Medipix assemblies using a microfocus x-ray source

Medipix2 assemblies with Si and CdTe sensors have been characterized using poly-energetic x-ray sources. This work reports the results of inhomogeneities within the sensors; individual pixel sensitivity response and their saturation effects at higher photon fluxes over one hundred frames. At higher tube currents saturation of both sensors is observed. We have performed correction for these inhomogeneities on both sensors. CT images with CdTe-Medipix2 are presented.

Characterization of CdTe X-Ray Sensor Layer on Medipix Detector Chips

We have been characterizing various sensor layers bump-bonded to Medipix detector chips. We report here characterizationthe leakage current variations of a cadmium telluride (CdTe) assembled-Medipix2 assemblyat different temperatures.Medipix detectors are being used in small animal computed tomography (CT) scanner systems known as MARS-CT. The spectroscopic imaging of a mouse and human atheroma using this system has previously been reported [1-2]. In those reports, Medipix2 detectors were used with Si and GaAs sensor layers, respectively. Cadmium telluride (CdTe) is expected to be a useful sensor layer for clinical CT imaging detectors because of its good detection efficiency for x-rays in the energy range up to 120 keV. However, Aan understanding of the detection characteristics of these sensor layers is vital to high-quality imaging. This work presents the results of inhomogeneities within the CdTe sensor, temperature variations and wrinkle pattern instability.We present leakage current variations with temperature and sensitivity inhomogeneity across the detector. This is an extension of the tests previously reported in Aamir R et al [1][3].

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 the adverse effects of charge sharing and fluorescent x-rays of CdTe on the spectra recorded. The development of the Medipix All Resolution System (MARS) x-ray camera has allowed us to explore this novel pixel communication feature in Medipix3. Spectrum measurements in this work were carried out using a MARS camera consisting of a Medipix3 chip bump-bonded to a 1mm thick CdTe sensor layer. The characteristic peaks of the Am-241 source as well as the spectroscopic properties of the CdTe sensor material were depicted at a spatial resolution of 55 ?m. Furthermore, a connected component algorithm shows a silicon based Medipix3 is effective in reallocating spread charge into a single pixel.