The ability of dual- and multi-energy CT to differentiate materials of different effective atomic numbers makes possible several new and clinically relevant CT applications.

Dual- and Multi-Energy CT: Principles, Technical Approaches, and Clinical Applications

Theoretical considerations predicted the feasibility of K-edge x-ray computed tomography (CT) imaging using energy discriminating detectors with more than two energy bins. This technique enables material-specific imaging in CT, which in combination with high-Z element based contrast agents, opens up possibilities for new medical applications. In this paper, we present a CT system with energy detection capabilities, which was used to demonstrate the feasibility of quantitative K-edge CT imaging experimentally. A phantom was imaged containing PMMA, calcium-hydroxyapatite, water and two contrast agents based on iodine and gadolinium, respectively. Separate images of the attenuation by photoelectric absorption and Compton scattering were reconstructed from energy-resolved projection data using maximum-likelihood basis-component decomposition. The data analysis further enabled the display of images of the individual contrast agents and their concentrations, separated from the anatomical background. Measured concentrations of iodine and gadolinium were in good agreement with the actual concentrations. Prior to the tomographic measurements, the detector response functions for monochromatic illumination using synchrotron radiation were determined in the energy range 25 keV-60 keV. These data were used to calibrate the detector and derive a phenomenological model for the detector response and the energy bin sensitivities.

https://iopscience.iop.org/article/10.1088/0031-9155/53/15/002/pdf

Experimental feasibility of multi-energy photon-counting K-edge imaging in pre-clinical computed tomography.

ContextMultislice computed tomography (MSCT) has recently evolved as a modality
for noninvasive coronary imaging.ObjectiveTo assess the accuracy and robustness of MSCT vs the criterion standard
of invasive coronary angiography for detection of obstructive coronary artery
disease.Design, Setting, and PatientsProspective, single-center study conducted in a referral center setting
in Germany and enrolling 103 consecutive patients (mean age, 61.5 [SD, 9.7]
years) from November 2003–August 2004 who were undergoing both invasive
coronary angiography and MSCT using a scanner with 16 detector rows.Main Outcome MeasuresBlinded results for both modalities compared using the patient as the
primary unit of analysis, with supplementary segment- and vessel-based analyses.ResultsOne thousand three hundred eighty-four segments (≥1.5 mm diameter)
were identified by invasive coronary angiography; nondiagnostic image quality
of MSCT was identified for only 88 (6.4%) of these segments, mainly due to
faster heart rates. Compared with invasive coronary angiography for detection
of significant lesions (>50% stenosis), segment-based sensitivity, specificity,
and positive and negative predictive values of MSCT were 95%, 98%, 87%, and
99%, respectively. Quantitative comparison of MSCT and invasive coronary angiography
showed good correlation (r = 0.87, P<.001), with MSCT systematically measuring greater-percentage
stenoses (bias, +12%). In the patient-based analysis, the area under the receiver
operating characteristic curve was 0.97 (95% confidence interval, 0.90-1.00),
indicating high discriminative power to identify patients who might be candidates
for revascularization (>50% left main artery stenosis and/or >70% stenosis
in any other epicardial vessel). Threshold optimization allowed either detection
of these patients with 100% sensitivity at a reasonable false-positive rate
(specificity, 76.5%; MSCT stenosis, >66%) or optimization of both the sensitivity
and specificity (>90%; MSCT stenosis, >76%).ConclusionsMultislice computed tomography provides high accuracy for noninvasive
detection of suspected obstructive coronary artery disease. This promising
technology has potential to complement diagnostic invasive coronary angiography
in routine clinical care.

https://jamanetwork.com/journals/jama/articlepdf/200952/JOC50031.pdf

Noninvasive Coronary Angiography With Multislice Computed Tomography

Despite major advances in x-ray sources, detector arrays, gantry mechanical design and especially computer performance, one component of computed tomography (CT) scanners has remained virtually constant for the past 25 years—the reconstruction algorithm. Fundamental advances have been made in the solution of inverse problems, especially tomographic reconstruction, but these works have not been translated into clinical and related practice. The reasons are not obvious and seldom discussed. This review seeks to examine the reasons for this discrepancy and provides recommendations on how it can be resolved. We take the example of field of compressive sensing (CS), summarizing this new area of research from the eyes of practical medical physicists and explaining the disconnection between theoretical and application-oriented research. Using a few issues specific to CT, which engineers have addressed in very specific ways, we try to distill the mathematical problem underlying each of these issues with the hope of demonstrating that there are interesting mathematical problems of general importance that can result from in depth analysis of specific issues. We then sketch some unconventional CT-imaging designs that have the potential to impact on CT applications, if the link between applied mathematicians and engineers/physicists were stronger. Finally, we close with some observations on how the link could be strengthened. There is, we believe, an important opportunity to rapidly improve the performance of CT and related tomographic imaging techniques by addressing these issues.

/pdf/why-do-commercial-ct-scanners-still-employ-traditional-19rizoprup.pdf

Why do commercial CT scanners still employ traditional, filtered back-projection for image reconstruction?

Our results show that in addition to contrast-enhanced imaging, dual-energy multidetector CT can provide virtually unenhanced data sets,that are a reasonable approximation of true unenhanced multidetector CT data sets at a dose less than that required for two acquisitions performed separately.

Dual-energy CT in patients suspected of having renal masses: can virtual nonenhanced images replace true nonenhanced images?

Dual-energy CT is known to enable possible improvement of material separation over regular CT. However, in clinical implementation most of the dual-energy techniques show limited results mainly due to their sensitivity to noisy data. We simulate data acquisition by a dual-layer CT based on two scintillation layers one on top of the other with which the data is acquired simultaneously. We map the results of the reconstruction into a plane created from the Hounsfield units (HU) of the upper-layer image versus the HU of the lower-layer image. We find that different scanned materials end up in different definable regions in the HU-plane. Application of a special correction on the reconstructed images achieves stability on the HU-plane despite beam-hardening effects. In order to assess the practical material separation capabilities, part of the simulations were done with exact noise calculations. We analyze the material separation capabilities with such a configuration and conclude that the combination of the dual-layer CT with the classification analysis in the HU-plane is a practical and robust method that significantly improves clinical applications, in particular those involving iodine-calcium separation such as analysis and classification of coronary artery calcifications and soft plaques.

Material separation with dual-layer CT

The three CT components with the greatest impact on image quality are the X-ray source, detection system and reconstruction algorithms. In this paper, we focus on the first two. We describe the state-of-the-art of CT detection systems, their calibrations, software corrections and common performance metrics. The components of CT detection systems, such as scintillator materials, photodiodes, data acquisition electronics and anti-scatter grids, are discussed. Their impact on CT image quality, their most important characteristics, as well as emerging future technology trends for each, are reviewed. The use of detection for multi-energy CT imaging is described. An overview of current CT X-ray sources, their evolution to support major trends in CT imaging and future trends is provided.

/pdf/state-of-the-art-of-ct-detectors-and-sources-a-literature-29meitdi4u.pdf

State of the Art of CT Detectors and Sources: A Literature Review

To evaluate the feasibility of multicolour quantitative imaging with spectral photon-counting computed tomography (SPCCT) of different mixed contrast agents. Phantoms containing eleven tubes with mixtures of varying proportions of two contrast agents (i.e. two selected from gadolinium, iodine or gold nanoparticles) were prepared so that the attenuation of each tube was about 280 HU. Scans were acquired at 120 kVp and 100 mAs using a five-bin preclinical SPCCT prototype, generating conventional, water, iodine, gadolinium and gold images. The correlation between prepared and measured concentrations was assessed using linear regression. The cross-contamination was measured for each material as the root mean square error (RMSE) of its concentration in the other material images, where no signal was expected. The contrast-to-noise ratio (CNR) relative to a phosphate buffered saline tube was calculated for each contrast agent. The solutions had similar attenuations (279 ± 10 HU, mean ± standard deviation) and could not be differentiated on conventional images. However, a distinction was observed in the material images within the same samples, and the measured and prepared concentrations were strongly correlated (R2 ≥ 0.97, 0.81 ≤ slope ≤ 0.95, -0.68 ≤ offset ≤ 0.89 mg/mL). Cross-contamination in the iodine images for the mixture of gold and gadolinium contrast agents (RMSE = 0.34 mg/mL) was observed. CNR for 1 mg/mL of contrast agent was better for the mixture of iodine and gadolinium (CNRiodine = 3.20, CNRgadolinium = 2.80) than gold and gadolinium (CNRgadolinium = 1.67, CNRgold = 1.37). SPCCT enables multicolour quantitative imaging. As a result, it should be possible to perform imaging of multiple uptake phases of a given tissue/organ within a single scan by injecting different contrast agents sequentially.

/pdf/multicolour-imaging-with-spectral-photon-counting-ct-a-2h6iqqzybm.pdf

Multicolour imaging with spectral photon-counting CT: a phantom study

A radiation detector (24) includes a two-dimensional array of upper scintillators (30τ) which is disposed facing an x-ray source (14) to convert lower energy radiation into visible light and transmit higher energy radiation. A two-dimensional array of lower scintillators (30B) is disposed adjacent the upper scintillators (30τ) distally from the x-ray source (14) to convert the transmitted higher energy radiation into visible light. Respective active areas (94, 96) of each upper and lower photodetector arrays (38τ, 38B) are optically coupled to the respective upper and lower scintillators (30τ, 30B) at an inner side (60) of the scintillators (30τ, 30B) which inner side (60) is generally perpendicular to an axial direction (Z). Interference filters (110, 112) may be deposited on the active areas (94, 96) of the associated upper and lower photodetectors (38τ, 38B) to restrict radiation wavelengths received by the upper and lower photodetectors (38τ, 38B) to wavelengths emitted by the respective upper and lower scintillators (30τ, 30B). The upper scintillators (30τ) may include at least one of ZnSe(Te) and YAG(Ce).

Double decker detector for spectral ct

The emerging of fast‐rotating MDCT scanners, and the recent approach of the, so called, “Slice War” to its saturation, opened new opportunities for CT evolution. New routes to extend CT applications beyond anatomical imaging have been pursued. Quantitative functional imaging seems to become one of the main trends in this process, with perfusion applications and multi‐energy CT methods as the leading techniques. To enable a simultaneous dual energy CT scanning, using a single X‐ray tube, without FOV, or sampling limitations, a special double‐layer detectorCT has been developed in PHILIPS Healthcare. The conventional CTdetection pixel has been reconfigured, to consist of 2 Scintillator layers, read simultaneously, by a double‐layer, side‐looking photodiode. The top‐layer scintillator has been chosen to contain, relatively, low atomic‐number elements, while having a very high light output, and very low afterglow. Its thickness has been optimized to achieve a maximum spectral separation between Iodine and Calcium. A 2‐mm layer of GOS has been used for the bottom Scintillator layer, to enable stopping of 99.8% of the X‐rays, transmitted through the top layer, without limiting the GOS light collection which is done sideways. The raw data of each of the detector layers is reconstructed separately, resulting in a Low‐Energy and a High‐Energy HU images. Thus, each slice‐pixel has two HU values assigned to it, the low and the high energy, respectively. In addition, a weighted sum of the two raw data is reconstructed to give the conventional CTimage, while the weighting factor can be modified to present the combined image at any desired mean energy, within the system energy range. The pixels of each slice are mapped on a 2D spectral scatter plot, HU_Low_E VS. HU_High_E. The physics nature of the radiation interaction in this energy range, determines that varied concentrations of the same material (same effective atomic number) are represented along a straight line on this map. Also, as will be shown, this method enables the separation and quantification of specific materials, like Iodine, from mixtures with other compounds. The system capability of material identification and quantification enables plaque characterization, non cathartic virtual colonoscopy, virtual non‐contrast imaging, and an accurate quantitative mapping of perfused Iodine for advanced functional CT applications. More than 2000 human patients have been scanned in a fully operated system, as well as plenty of research animals, at the Hadassah University Hospital in Jerusalem. The animal experiments (rabbits) aimed, mainly, at testing the system capability with new, targeted contrast agents, indicating a great opportunity for the use of multi‐energy CT, and full spectralCT in the future. Learning Objectives: 1. Understanding the clinical opportunities associated with a dual‐energy CT. 2. Understanding the unique advantages, the tradeoffs, and the limitations of the double‐layer detectorCT approach. 3. Understanding the principles of the material identification and analysis method in the image domain, chosen for this system.

Ami Altman

Papers

Material separation with dual-layer CT

State of the Art of CT Detectors and Sources: A Literature Review

Multicolour imaging with spectral photon-counting CT: a phantom study

Double decker detector for spectral ct

TU-E-210A-03: A Double-Layer Detector, Dual-Energy CT — Principles, Advantages and Applications