R.K. Panta

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.

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.

MARS-MD: Rejection based image domain material decomposition

This paper outlines image domain material decomposition algorithms that have been routinely used in MARS spectral CT systems. These algorithms (known collectively as MARS-MD) are based on a pragmatic heuristic for solving the under-determined problem where there are more materials than energy bins. This heuristic contains three parts: (1) splitting the problem into a number of possible sub-problems, each containing fewer materials; (2) solving each sub-problem; and (3) applying rejection criteria to eliminate all but one sub-problems solution. An advantage of this process is that different constraints can be applied to each sub-problem if necessary. In addition, the result of this process is that solutions will be sparse in the material domain, which reduces crossover of signal between material images. Two algorithms based on this process are presented: the Segmentation variant, which uses segmented material classes to define each sub-problem; and the Angular Rejection variant, which defines the rejection criteria using the angle between reconstructed attenuation vectors.

submitter : Semi-analytic off-axis X-ray source model

Spectral computed tomography (CT) systems are employed with energy-resolving photon counting detectors. Incorporation of a spectrally accurate x-ray beam model in image reconstruction helps to improve material identification and quantification by these systems. Using an inaccurate x-ray model in spectral reconstruction can lead to severe image artifacts, one of the extreme cases of this is the well-known beam-hardening artifacts. An often overlooked spectral feature of x-ray beams in spectral reconstruction models is the angular dependence of the spectrum with reference to the central beam axis. To address these factors, we have developed a parameterized semi-analytical x-ray source model in the diagnostic imaging range (30-120 kVp) by applying regression techniques to data obtained from Monte Carlo simulations (EGSnrc). This x-ray beam model is generalized to describe the off-axis spectral information within ±17o along θ (vertical direction), ±5o along (horizontal direction) of the central axis, and can be parameterized for specific x-ray tube models. Comparisons of our model with those generated by SpekCalc, TOPAS, and IPEM78 at central axis show good agreement (within 2 %). We have evaluated the model with experimental data collected with a small animal spectral scanner.

Oblique fluorescence in a MARS scanner with a CdTe-Medipix3RX

The latest version of the MARS small bore scanner makes use of the Medipix3RX ASIC, bonded to a CdTe or CZT semi-conductor layer, to count x-ray photons and create a spectroscopic CT data set. The MARS imaging chain uses the energy-resolved 2D transmission images to construct quantitative 3D spectral and material images. To improve the spectral performance of the imaging system it is important that the energy response of the detector is well calibrated. A common methodology for energy calibration is to use x-ray fluorescence (XRF), due to its effective monochromatic nature. Oblique (off-axis) XRF can be measured in situ in the MARS small bore scanner. A monoatomic foil is placed in front of the x-ray source and off-axis XRF is measured. A key issue is identifying near optimal measurement positions that maximize the XRF signal while minimizing transmitted and scattered x-rays from the primary beam. This work shows the development of a theoretical model that is able to identify where in the detector plane XRF is maximum. We present: (1) a theoretical model that calculates the XRF photon distribution across the detector plane produced from illuminated foils attached to the scanners filter bar; (2) preliminary experimental measurements of the XRF distribution outside of the main beam taken with a CdTe-Medipix3RX detector; and (3) a comparison between the model and experiment. The main motivation behind creating this model is to identify the region in the detector plane outside of the main beam where XRF is at a maximum. This provides the optimum detector location for measuring a monochromatic XRF source with minimal polychromatic contamination for its use in per-pixel energy calibration of Medipix3RX detectors in MARS scanners.

Dosimetry for spectral molecular imaging of small animals with MARS-CT

The Medipix All Resolution Scanner (MARS) spectral CT is intended for small animal, pre-clinical imaging and uses an x-ray detector (Medipix) operating in single photon counting mode. The MARS system provides spectrometric information to facilitate differentiation of tissue types and bio-markers. For longitudinal studies of disease models, it is desirable to characterise the system’s dosimetry. This dosimetry study is performed using three phantoms each consisting of a 30 mm diameter homogeneous PMMA cylinder simulating a mouse. The imaging parameters used for this study are derived from those used for gold nanoparticle identification in mouse kidneys. Dosimetry measurement are obtained with thermo-luminescent Lithium Fluoride (LiF:CuMgP) detectors, calibrated in terms of air kerma and placed at different depths and orientations in the phantoms. Central axis TLD air kerma rates of 17.2 (± 0.71) mGy/min and 18.2 (± 0.75) mGy/min were obtained for different phantoms and TLD orientations. Validation measurements were acquired with a pencil ionization chamber, giving an air-kerma rate of 20.3 (±1) mGy/min and an estimated total air kerma of 81.2 (± 4) mGy for a 720 projection acquisition. It is anticipated that scanner design improvements will significantly decrease future dose requirements. The procedures developed in this work will be used for further dosimetry calculations when optimizing image acquisition for the MARS system as it undergoes development towards human clinical applications.

Beam profile assessment in spectral CT scanners

Abstract In this paper, we present a method that uses a combination of experimental and modeled data to assess properties of x‐ray beam measured using a small‐animal spectral scanner. The spatial properties of the beam profile are characterized by beam profile shape, the angular offset along the rotational axis, and the photon count difference between experimental and modeled data at the central beam axis. Temporal stability of the beam profile is assessed by measuring intra‐ and interscan count variations. The beam profile assessment method was evaluated on several spectral CT scanners equipped with Medipix3RX‐based detectors. On a well‐calibrated spectral CT scanner, we measured an integral count error of 0.5%, intrascan count variation of 0.1%, and an interscan count variation of less than 1%. The angular offset of the beam center ranged from 0.8° to 1.6° for the studied spectral CT scanners. We also demonstrate the capability of this method to identify poor performance of the system through analyzing the deviation of the experimental beam profile from the model. This technique can, therefore, aid in monitoring the system performance to obtain a robust spectral CT; providing the reliable quantitative images. Furthermore, the accurate offset parameters of a spectral scanner provided by this method allow us to incorporate a more realistic form of the photon distribution in the polychromatic‐based image reconstruction models. Both improvements of the reliability of the system and accuracy of the volume reconstruction result in a better discrimination and quantification of the imaged materials.

Per-pixel energy calibration of photon counting detectors

Energy resolving performance of spectral CT systems is influenced by the accuracy of the detectors energy calibration. Global energy calibration maps a given threshold to the average energy response of all pixels of the detector. Variations arising from CMOS manufacturing processes and properties of the sensor cause different pixels to respond differently to photons of the same energy. Threshold dispersion adversely affects spectral imaging by degrading energy resolution, which contributes to blurring of the energy information. In this paper, we present a technique for per-pixel energy calibration of photon-counting x-ray detectors (PCXDs) that quantifies the energy response of individual pixels relative to the average response. This technique takes advantage of the measurements made by an equalized chip. It uses a known global energy map to quantify the effect of threshold dispersion on the energy response of the detector pixels across an energy range of interest. The proposed technique was assessed using a MARS scanner with an equalized Medipix3RX chip flip-bonded to 2 mm thick CdTe semiconductor crystal at a pitch of 110 μ m. Measurements were made of characteristic x-rays of a molybdenum foil. Results were compared between the case that the global calibration was used on its own and the case of using it in conjunction with our per-pixel calibration technique. The proposed technique quantified up to 1.87 keV error in energy response of 100 pixels of a selected region of interest (ROI). It made an improvement of 28.3% in average FWHM. The additional information provided by this per-pixel calibration technique can be used to improve spectral reconstruction.