Photon-counting detector technology has enabled the first experimental investigations of energy-resolved computed tomography (CT) imaging and the potential use for K-edge imaging. However, ...limitations in regards to detecter technology have been imposing a limit to effective count rates. As a consequence, this has resulted in high noise levels in the obtained images given scan time limitations in CT imaging applications. It has been well recognized in the area of low-dose imaging with conventional CT that iterative image reconstruction provides a superior signal to noise ratio compared to traditional filtered backprojection techniques. Furthermore, iterative reconstruction methods also allow for incorporation of a roughness penalty function in order to make a trade-off between noise and spatial resolution in the reconstructed images. In this work, we investigate statistically-principled iterative image reconstruction from material-decomposed sinograms in spectral CT. The proposed reconstruction algorithm seeks to minimize a penalized likelihood-based cost functional, where the parameters of the likelihood function are estimated by computing the Fisher information matrix associated with the material decomposition step. The performance of the proposed reconstruction method is quantitatively investigated by use of computer-simulated and experimental phantom data. The potential for improved K-edge imaging is also demonstrated in an animal experiment.
In patients with stroke, IV cone-beam CTA in the angiography suite could be an alternative to CTA to shorten the door-to-thrombectomy time. However, image quality in cone-beam CTA is typically ...limited by artifacts. This study evaluated a prototype dual-layer detector cone-beam CT angiography versus CTA in patients with stroke.
A prospective, single-center trial enrolled consecutive patients with ischemic or hemorrhagic stroke on initial CT. Intracranial arterial segment vessel conspicuity and artifact presence were evaluated on dual-layer cone-beam CTA 70-keV virtual monoenergetic images and CTA. Eleven predetermined vessel segments were matched for every patient. Twelve patients were necessary to show noninferiority to CTA. Noninferiority was determined by the exact binomial test; the 1-sided lower performance boundary was prospectively set to 80% (98.75% CI).
Twenty-one patients had matched image sets (mean age, 72 years). After excluding examinations with movement or contrast media injection issues, all readers individually considered dual-layer cone-beam CT angiography noninferior to CTA (CI boundary, 93%, 84%, 80%, respectively) when evaluating arteries relevant in candidates for intracranial thrombectomy. Artifacts were more prevalent compared with CTA. The majority assessment rated each individual segment except M1 as having noninferior conspicuity compared with CTA.
In a single-center stroke setting, dual-layer detector cone-beam CTA virtual monoenergetic images are noninferior to CTA under certain conditions. Notably, the prototype is hampered by a long scan time and is not capable of contrast media bolus tracking. After excluding examinations with such scan issues, readers considered dual-layer detector cone-beam CTA noninferior to CTA, despite more artifacts.
To evaluate the capabilities of photon counting spectral CT to differentiate components of coronary atherosclerotic plaque based on differences in spectral attenuation and iodine-based contrast agent ...concentration.
10 calcified and 13 lipid-rich non-calcified histologically demonstrated atheromatous plaques from post-mortem human coronary arteries were scanned with a photon counting spectral CT scanner. Individual photons were counted and classified in one of six energy bins from 25 to 70 keV. Based on a maximum likelihood approach, maps of photoelectric absorption (PA), Compton scattering (CS) and iodine concentration (IC) were reconstructed. Intensity measurements were performed on each map in the vessel wall, the surrounding perivascular fat and the lipid-rich and the calcified plaques. PA and CS values are expressed relative to pure water values. A comparison between these different elements was performed using Kruskal-Wallis tests with pairwise post hoc Mann-Whitney U-tests and Sidak p-value adjustments.
RESULTS for vessel wall, surrounding perivascular fat and lipid-rich and calcified plaques were, respectively, 1.19 ± 0.09, 0.73 ± 0.05, 1.08 ± 0.14 and 17.79 ± 6.70 for PA; 0.96 ± 0.02, 0.83 ± 0.02, 0.91 ± 0.03 and 2.53 ± 0.63 for CS; and 83.3 ± 10.1, 37.6 ± 8.1, 55.2 ± 14.0 and 4.9 ± 20.0 mmol l(-1) for IC, with a significant difference between all tissues for PA, CS and IC (p < 0.012).
This study demonstrates the capability of energy-sensitive photon counting spectral CT to differentiate between calcifications and iodine-infused regions of human coronary artery atherosclerotic plaque samples by analysing differences in spectral attenuation and iodine-based contrast agent concentration.
Photon counting spectral CT is a promising technique to identify plaque components by analysing differences in iodine-based contrast agent concentration, photoelectric attenuation and Compton scattering.
Purpose To investigate the feasibility of using spectral photon-counting computed tomography (CT) to differentiate between gadolinium-based and nonionic iodine-based contrast material in a colon ...phantom by using the characteristic k edge of gadolinium. Materials and Methods A custom-made colon phantom was filled with nonionic iodine-based contrast material, and a gadolinium-filled capsule representing a contrast material-enhanced polyp was positioned on the colon wall. The colon phantom was scanned with a preclinical spectral photon-counting CT system to obtain spectral and conventional data. By fully using the multibin spectral information, material decomposition was performed to generate iodine and gadolinium maps. Quantitative measurements were performed within the lumen and polyp to quantitatively determine the absolute content of iodine and gadolinium. Results In a conventional CT section, absorption values of both contrast agents were similar at approximately 110 HU. Contrast material maps clearly differentiated the distributions, with gadolinium solely in the polyp and iodine in the lumen of the colon. Quantitative measurements of contrast material concentrations in the colon and polyp matched well with those of actual prepared mixtures. Conclusion Dual-contrast spectral photon-counting CT colonography with iodine-filled lumen and gadolinium-tagged polyps may enable ready differentiation between polyps and tagged fecal material.
RSNA, 2016.
Recent advances in the development of direct‐conversion, energy‐sensitive x‐ray detectors stimulate research in the domain of pre‐clinical and clinical photon‐counting x‐ray computed tomography (CT). ...The ability to quantify the energy of individual X‐ray photons allows for novel approaches to improve the soft tissue differentiation, material decomposition and labeling techniques, the suppression of beam‐hardening artifacts as well as the potential reduction of radiation dose. Moreover, spectral data acquisition enables the selective and quantitative imaging of certain contrast media on top of the conventional anatomy, by tuning an energy threshold in the detector system to the K‐edge discontinuity of the contrast generating element in the agent.
The present lecture will provide an overview of both, the opportunities and the technological challenges arising in the context of clinical, energy‐resolved, photon‐counting CT. Some examples of potential future applications will be given together with the most challenging technical difficulties encountered in the use of photon counting detectors in CT. The problem of counting photons at the high flux conditions of clinical CT will be discussed as well as the degradation of energy resolution by effects like pulse‐pileup or charge sharing.
All authors are employees of Philips Research.
Learning Objectives:
1. To understand the physical basics of photon counting detectors and the implications on their use in x‐ray computed tomography
2. To learn about a possible future application of energy‐sensitive CT in connection with the identification and quantification of contrast‐agents by means of the K‐edge discontinuity in the attenuation.
3. To obtain an overview of the technological challenges to be overcome in order to realize photon‐counting CT in clinical practice.
The initial stages of growth of noble metals deposited onto untreated as well as Ar
+ beam-treated polymer surfaces were investigated by means of X-ray photoelectron spectroscopy (XPS), transmission ...electron microscopy (TEM), and radiotracer methods. Condensation coefficients
C of the metals on the polymer surfaces were determined by comparison of the XPS signals of the metals with the nominal thicknesses measured by a quartz microbalance during deposition. These measurements were combined with radiotracer measurements, which allow to determine
C independently with high accuracy and sensitivity.
C was found to depend strongly on the chemical composition of the polymer surface, e.g., at room temperature,
C varies between 0.95 for PMDA-ODA polyimide and 0.002 for Teflon AF. By ion beam treatment of the latter, C was raised considerably. During nucleation of the metal on the polymer surface,
C increases strongly with metal coverage.
C also depends on the deposition rate of the metal and decreases strongly at elevated temperatures.
Purpose
Conventional cone‐beam computed tomography CT (CBCT) provides limited discrimination between low‐contrast tissues. Furthermore, it is limited to full‐spectrum energy integration. A ...dual‐energy CBCT system could be used to separate photon energy spectra with the potential to increase the visibility of clinically relevant features and acquire additional information relevant in a multitude of clinical imaging applications. In this work, the performance of a novel dual‐layer dual‐energy CBCT (DL‐DE‐CBCT) C‐arm system is characterized for the first time.
Methods
A prototype dual‐layer detector was fitted into a commercial interventional C‐arm CBCT system to enable DL‐DE‐CBCT acquisitions. DL‐DE reconstructions were derived from material‐decomposed Compton scatter and photoelectric base functions. The modulation transfer function (MTF) of the prototype DL‐DE‐CBCT was compared to that of a commercial CBCT. Noise and uniformity characteristics were evaluated using a cylindrical water phantom. Effective atomic numbers and electron densities were estimated in clinically relevant tissue substitutes. Iodine quantification was performed (for 0.5–15 mg/ml concentrations) and virtual noncontrast (VNC) images were evaluated. Finally, contrast‐to‐noise ratios (CNR) and CT number accuracies were estimated.
Results
The prototype and commercial CBCT showed similar spatial resolution, with a mean 10% MTF of 5.98 cycles/cm and 6.28 cycles/cm, respectively, using a commercial standard reconstruction. The lowest noise was seen in the 80 keV virtual monoenergetic images (VMI) (7.40 HU) and the most uniform images were seen at VMI 60 keV (4.74 HU) or VMI 80 keV (1.98 HU), depending on the uniformity measure used. For all the tissue substitutes measured, the mean accuracy in effective atomic number was 98.2% (SD 1.2%) and the mean accuracy in electron density was 100.3% (SD 0.9%). Iodine quantification images showed a mean difference of −0.1 (SD 0.5) mg/ml compared to the true iodine concentration for all blood and iodine‐containing objects. For VNC images, all blood substitutes containing iodine averaged a CT number of 43.2 HU, whereas a blood‐only substitute measured 44.8 HU. All water‐containing iodine substitutes measured a mean CT number of 2.6 in the VNC images. A noise‐suppressed dataset showed a CNR peak at VMI 40 keV and low at VMI 120 keV. In the same dataset without noise suppression applied, a peak in CNR was obtained at VMI 70 keV and a low at VMI 120 keV. The estimated CT numbers of various clinically relevant objects were generally very close to the calculated CT number.
Conclusions
The performance of a prototype dual‐layer dual‐energy C‐arm CBCT system was characterized. Spatial resolution and noise were comparable with a commercially available C‐arm CBCT system, while offering dual‐energy capability. Iodine quantifications, effective atomic numbers, and electron densities were in good agreement with expected values, indicating that the system can be used to reliably evaluate the material composition of clinically relevant tissues. The VNC and monoenergetic images indicate a consistent ability to separate clinically relevant tissues. The results presented indicate that the system could find utility in diagnostic, interventional, and radiotherapy planning settings.
X-ray emission spectra from thin targets show significant directional effects that have their origin in the polar dependence of the atomic bremsstrahlung cross section. This fact has led to the idea ...of enhancing the characteristic lines from an electron-impact X-ray source by observing X-rays from optically thin (foil) targets in a direction antiparallel to that of the incident electron beam.
Efficiency considerations dictate the use of foils of maximum possible thickness consistent with the desired degree of characteristic line enhancement. Foil thickness and electron energy are crucial parameters determining the angular divergence of the electron beam within the foil and hence the extent of characteristic line enhancement. The aim of this article is to find a relationship between electron energy and foil thickness that preserves a high characteristic to bremsstrahlung X-ray ratio while maximizing efficiency. This is achieved by relating the root mean square electron deflection angle, derived from the mass scattering power, to the angular width of the “valley”, centered on 180°, within which bremsstrahlung emission can be neglected.
Representative spectra are presented and conclusions for applications and for further work are briefly presented.
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