Raz Carmi

Material separation with dual-layer CT

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.

FDG PET/CT Early Dynamic Blood Flow and Late Standardized Uptake Value Determination in Hepatocellular Carcinoma

PURPOSE To prospectively determine whether fluorine 18 fluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT) early dynamic blood flow estimates could be used to discriminate hepatocellular carcinoma (HCC) from background liver and to characterize HCC in patients with and those without angioinvasion; and to evaluate the association between blood flow measures at FDG PET/CT with metabolism in HCCs. MATERIALS AND METHODS Institutional review board approval and written informed consent were obtained for this prospective study. Twenty-one consecutive patients (mean age, 65 years) with 30 established HCCs (mean size, 5.5 cm; seven lesions in five patients with angioinvasion) underwent a blood flow study with an FDG dynamic scan divided into 18 sequences of 5 seconds each and a standard PET/CT scan. On the dynamic study, three independent operators obtained volumes of interest (VOIs) for which three blood flow estimates were calculated (hepatic perfusion index [HPI], time to peak [TTP], and peak intensity [PI]). On the late study, a VOI was placed on the fused scan for each HCC, and maximum standardized uptake value (SUV(max)) was obtained. By using a mixed-effects model analysis, comparison of blood flow estimates between HCC with and that without angioinvasion and background liver was performed. The association between blood flow estimates and SUV(max) was also assessed. RESULTS HPI and TTP showed better performance than did SUV(max) for discriminating HCC and background liver (areas under receiver operating characteristic curve: 0.96, 0.95, and 0.83, respectively; P < .05). HPI was higher in HCC in patients with angioinvasion (0.91 ± 0.15 [standard deviation]) than in those without angioinvasion (0.80 ± 0.18; P = .03). There was no difference in SUV(max) between HCC in patients with and those without angioinvasion (7.8 ± 2.9 vs 6.3 ± 3.4; P = .85). No clear association was found between HPI, PI, or TTP and SUV(max) (P = .49, .77, and .91, respectively). CONCLUSION Early dynamic blood flow FDG PET/CT may be used to help discriminate and characterize HCC tumors.

Dual-Energy Based Spectral Electronic Cleansing in Non-Cathartic Computed Tomography Colonography: An Emerging Novel Technique

Computed tomography colonography (CTC) with reduced or without bowel catharsis and with fecal tagging has emerged to improve CTC tolerability in patients and their subsequent compliance with colorectal cancer screening. With fecal tagging, electronic cleansing is performed by postprocessing software that removes remnants of contrast material. However, because the technique is threshold based, artifacts that lower the image quality and accuracy of the examination may be noted. Spectral electronic cleansing, based on dual-energy CT and on material-specific cleansing, decreases the number of artifacts and improves image quality. In this review we describe spectral cleansing with reduced catharsis CTC and illustrate its potential benefits.

An iodine-calcium separation analysis and virtually non-contrasted image generation obtained with single source dual energy MDCT

Dual energy CT enables the differentiation between various materials by analyzing their unique attenuation spectral response. These responses are represented as vectors on an image-based energy map. Practically, this spectral information can be very noisy requiring further analysis. Furthermore, the material response vectors may be affected by beam-hardening and varied between images.

Resolution enhancement of X-ray CT by spatial and temporal MLEM deconvolution correction

In multi-slice CT, there is an ongoing trend to use smaller slice thickness and faster rotation time, leading to smaller detector pixel size and shorter sampling time. In general, cross talk between adjacent detector pixels and scintillation short-term afterglow can cause resolution degradation and artifacts. Crosstalk effect becomes significant while using detector arrays with small pixels. Short-term afterglow becomes significant with high rotation speed, especially while using focal spot modulation between successive frames. In these cases of small pixels and high rotation speed, the signals of detector pixels arc relatively noisy. Thus, attempt to correct cross talk and afterglow by using techniques based on inverse filters or on subtraction of constant fraction of the adjacent pixel signals, may give poor results. We developed cross talk and afterglow deconvolution correction algorithm, applied on the scanner raw data. The algorithm is based on the maximum likelihood expectation maximization method (MLEM) and is similar to the Lucy-Richardson deconvolution in image restoration. We show that the results significantly improve the image resolution of fine bone structures

A unique noncathartic CT colonography approach by using two-layer dual-energy MDCT and a special algorithmic colon cleansing method

Cathartic bowel preparation as part of CT colonography examination (CTC) can be uncomfortable or even dangerous for certain patient groups. Noncathartic CT colonography (i.e. without cathartic cleansing) including contrast-material fecal tagging can offer significant clinical advantages and can increase the screening compliance for colorectal cancer. Current techniques of conventional CTC with fecal tagging use “electronic cleansing” algorithms to remove the remaining tagged colonic contents from the images. In such cases these methods can give satisfactory results, but they face serious problems with the inferior tagging quality of noncathartic protocols. To overcome this fundamental problem, a completely new approach is proposed using both a two-layer dual-energy MDCT and a dedicated algorithmic cleansing method that utilizes the spectral information. Feasibility study was performed with a two-layer dual-energy MDCT which was utilized in clinical studies with oral intake of both iodine and barium contrast agents. The new method was compared to a conventional electronic cleansing technique. We show that the new approach is better at detecting highly dilute contrast agents in the colon, particularly where the tagged colonic content is mixed or adjacent to air regions. Therefore, the new technique may surmount the need for cathartic bowel preparation in CTC.

Arterial double-contrast dual-energy MDCT: in-vivo rabbit atherosclerosis with iodinated nanoparticles and gadolinium agents

An in-vivo feasibility study of potentially improved atherosclerosis CT imaging is presented. By administration of two different contrast agents to rabbits with induced atherosclerotic plaques we aim at identifying both soft plaque and vessel lumen simultaneously. Initial injection of iodinated nanoparticle (INP) contrast agent (N1177 - Nanoscan Imaging), two to four hours before scan, leads to its later accumulation in macrophage-rich soft plaque, while a second gadolinium contrast agent (Magnevist) injected immediately prior to the scan blends with the aortic blood. The distinction between the two agents in a single scan is achieved with a double-layer dual-energy MDCT (Philips Healthcare) following material separation analysis using the reconstructed images of the different x-ray spectra. A single contrast agent injection scan, where only INP was injected two hours prior to the scan, was compared to a double-contrast scan taken four hours after INP injection and immediately after gadolinium injection. On the single contrast agent scan we observed along the aorta walls, localized iodine accumulation which can point on INP uptake by atherosclerotic plaque. In the double-contrast scan the gadolinium contributes a clearer depiction of the vessel lumen in addition to the lasting INP presence. The material separation shows a good correlation to the pathologies inferred from the conventional CT images of the two different scans while performing only a single scan prevents miss-registration problems and reduces radiation dose. These results suggest that a double-contrast dual-energy CT may be used for advanced clinical diagnostic applications.

Complementary tumor vascularity imaging in a single PET-CT routine using FDG early dynamic blood flow and contrast-enhanced CT texture analysis

A feasibility study of improved PET-CT tumor imaging approach is presented. A single PET-CT routine includes three different techniques: 18F-FDG early dynamic blood flow intended for perfusion assessment; standard late 18F-FDG uptake; and high-resolution contrast-enhanced CT enabling tissue texture analysis. Both PET protocols utilize the same single standard radiotracer dose administration. Quantitative volumetric arterial perfusion maps are derived from the reconstructed dynamic PET images corresponding to successive acquisition time intervals of 3 seconds only. For achieving high accuracy, the analysis algorithm differentiates the first-pass arterial flow from other interfering dynamic effects, and a noise reduction scheme based on adaptive total-variation minimization aims to provide appreciable quantitative map in physical conditions of high noise and low spatial resolution. The CT texture analysis comprises a practical and robust method for generating volumetric tissue irregularity maps. A local map value is represented by the entropy function which is derived from a weighted co-occurrence matrix histogram of the corresponding image voxel three-dimensional vicinity. Unique entropy scaling scheme and parameter optimization process, as well as appropriate scaling for varying image noise levels and contrast agent concentrations, improve the results toward quantitative absolute measure with respect to diverse scanning conditions and key analysis parameters. Representative imaging results are demonstrated on several clinical cases involving different organs and cancer types. In these cases, significant tumor characterization relative to the normal surrounding tissues is seen on the quantitative maps of all three imaging techniques. This proof of concept can lead the way to a new practical diagnostic imaging application.

Experimental evaluation of image-based spectral analysis method for multi energy window photon counting x-ray CT

Photon counting spectral x-ray CT provides powerful techniques for analyzing various materials in-situ by utilizing multiple separate energy windows. In the recent few years, preliminary results of humans and animals which were scanned with such prototype systems have been shown. Although photon counting technology allows numerous separate energy windows, practical system design should consider and optimize the advantages of additional windows relative to the complexity, image quality and system’s cost. In this work we investigated experimentally intricate material differentiation with dedicated multi energy window spectral analysis method. Particular attention was given to the simultaneous presence of several mixtures and partial volume regions of different materials. These scenarios resemble complicated analyses in potential medical and pre-clinical applications. The results show that multi energy window setting has a great advantage over dual-energy setting especially where multiple material mixtures should be differentiated one from each other.