Photon counting spectral CT component analysis of coronary artery atherosclerotic plaque samples

OBJECTIVE

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.

METHODS

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

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).

CONCLUSION

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.

ADVANCES IN KNOWLEDGE

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.

Abstract P4-03-06: Non-invasive classification of microcalcifications by the use of X-ray phase contrast mammography as a novel tool in breast diagnostics

Purpose: Phase-contrast and scattering-based x-ray imaging are known to provide additional and complementary information to conventional, absorption-based methods. The goal of this study is to evaluate this method to distinguish between benign and malignant microcalcifications with the benefit to increase accuracy of early breast cancer diagnosis. Phase-contrast mammography has been shown to increase image quality of native breast samples when compared to conventional mammography.

Two major types of microcalcifications are found within breast tissue. Type I consist of calcium oxalate dehydrate and type II microcalcifications are composed of calcium phosphates. Type I is seen most frequently in benign lesions whereas type II is indicative for proliferative lesions, including carcinomas. Phase contrast mammography is shown here to distinguish between the two types of microcalcifications and therefore indicates a step forward to improve early breast cancer diagnosis.

Material and Methods: Freshly dissected breast specimens (n = 50) were imaged using a Talbot-Lau interferometer equipped with a conventional x-ray tube; the interferometer was operated at the fifth Talbot distance, tube voltage of 40 kVp with mean energy of 28 keV, and current of 25 mA. The device simultaneously recorded absorption, differential phase and small-angle scattering signals. These quantities were combined into novel, high-frequency-enhanced radiographic images. Histopathological analysis was performed and regions of interests correlated with the findings of phase contrast mammography.

Results: Our novel imaging approach yields complementary and otherwise inaccessible information on electron density distribution and small-angle scattering power of the sample at microscopic scale. Recently we generated the world's first phase contrast mammograms of native, not-fixed human whole breast samples. Our results indicate the superiority of the new technique with respect to image quality and lesion conspicuity. A clinical reader study is currently carried out. Exploiting the multiple, complementary information obtained by grating-based interferometry, we are able to classify microcalcifications by a non-invasive technique within the clinical environment.

By considering the small-angle scattering signal as a complement to the absorption signal, our method can analyze the differences in the attenuation coefficient as well as in the crystal structure of the microcalcifications. Further, the scattering signal is used to decouple the thickness parameter. We demonstrate that type I and type II microcalcifications give opposite absorption and scattering signals and in addition the small-angle scattering signal helps to determine the type of microcalcification.

Conclusions: The potential clinical significance of phase-contrast enhanced mammography has been evaluated by our team. This technique yields improved diagnostic capabilities when compared with conventional mammography, can provide superior contrast, inaccessible and complementary information, and potentially also reduce dose deposition. The non-invasive classification of microcalcifications is an important step toward early diagnosis and differentiation of breast lesions.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-03-06.

Cramér-Rao lower bound of basis image noise in multiple-energy x-ray imaging

We present an analytical method to compute the basis image noise in the context of multi-energy x-ray imaging based on the Cramér-Rao lower bound (CRLB). The proposed formalism extends the original idea of Alvarez and Macovski (1976 Phys. Med. Biol. 21 733) to estimate the noise in the photo-effect and Compton-effect basis images in the case of dual-energy imaging. It includes an arbitrary number of independent, spectrally distinct attenuation measurements and also goes beyond the two-dimensional decomposition of the attenuation, including, e.g., a contrast agent as a third basis material. To illustrate our method, we consider three simple applications. The first application is to study the influence of the exact values for the energy thresholds on the basis image noise for a binned photon-counting system. The second application relates to the same detector system as the first and is an investigation of the dependence of the basis image noise on the energy resolution of the detector system. Finally, the third application provides an example for the case of an energy-integrating detector: the aim is to optimize the front-scintillator layer thickness of a dual-crystal detector for dual-energy imaging. The CRLB is used to minimize the noise of a photo-effect/Compton-effect basis material decomposition.

K-edge imaging in x-ray computed tomography using multi-bin photon counting detectors

After passage through matter, the energy spectrum of a polychromatic beam of x-rays contains valuable information about the elemental composition of the absorber. Conventional x-ray systems or x-ray computed tomography (CT) systems, equipped with scintillator detectors operated in the integrating mode, are largely insensitive to this type of spectral information, since the detector output is proportional to the energy fluence integrated over the whole spectrum. The main purpose of this paper is to investigate to which extent energy-sensitive photon counting devices, operated in the pulse-mode, are capable of revealing quantitative information about the elemental composition of the absorber. We focus on the detection of element-specific, K-edge discontinuities of the photo-electric cross-section. To be specific, we address the question of measuring and imaging the local density of a gadolinium-based contrast agent, in the framework of a generalized dual-energy pre-processing. Our results are very promising and seem to open up new possibilities for the imaging of the distribution of elements with a high atomic number Z in the human body using x-ray attenuation measurements. To demonstrate the usefulness of the detection and the appropriate processing of the spectral information, we present simulated images of an artherosclerotic coronary vessel filled with gadolinium-based contrast agent. While conventional systems, equipped with integrating detectors, often fail to differentiate between contrast filled lumen and artherosclerotic plaque, the use of an energy-selective detection system based on the counting of individual photons reveals a strong contrast between plaque and contrast agent.