Numerical observer for atherosclerotic plaque classification in spectral computed tomography

Abstract

Abstract. Spectral computed tomography (SCT) generates better image quality than conventional computed tomography (CT). It has overcome several limitations for imaging atherosclerotic plaque. However, the literature evaluating the performance of SCT based on objective image assessment is very limited for the task of discriminating plaques. We developed a numerical-observer method and used it to assess performance on discrimination vulnerable-plaque features and compared the performance among multienergy CT (MECT), dualenergy CT (DECT), and conventional CT methods. Our numerical observer was designed to incorporate all spectral information and comprised two-processing stages. First, each energy-window domain was preprocessed by a set of localized channelized Hotelling observers (CHO). In this step, the spectral image in each energy bin was decorrelated using localized prewhitening and matched filtering with a set of Laguerre-Gaussian channel functions. Second, the series of the intermediate scores computed from all the CHOs were integrated by a Hotelling observer with an additional prewhitening and matched filter. The overall signal-to-noise ratio (SNR) and the area under the receiver operating characteristic curve (AUC) were obtained, yielding an overall discrimination performance metric. The performance of our new observer was evaluated for the particular binary classification task of differentiating between alternative plaque characterizations in carotid arteries. A clinically realistic model of signal variability was also included in our simulation of the discrimination tasks. The inclusion of signal variation is a key to applying the proposed observer method to spectral CT data. Hence, the task-based approaches based on the signal-known-exactly/background-known-exactly (SKE/BKE) framework and the clinical-relevant signal-known-statistically/background-known-exactly (SKS/BKE) framework were applied for analytical computation of figures of merit (FOM). Simulated data of a carotid-atherosclerosis patient were used to validate our methods. We used an extended cardiac-torso anthropomorphic digital phantom and three simulated plaque types (i.e., calcified plaque, fatty-mixed plaque, and iodine-mixed blood). The images were reconstructed using a standard filtered backprojection (FBP) algorithm for all the acquisition methods and were applied to perform two different discrimination tasks of: (1) calcified plaque versus fatty-mixed plaque and (2) calcified plaque versus iodine-mixed blood. MECT outperformed DECT and conventional CT systems for all cases of the SKE/BKE and SKS/BKE tasks (all p < 0.01). On average of signal variability, MECT yielded the SNR improvements over other acquisition methods in the range of 46.8% to 65.3% (all p < 0.01) for FBP-Ramp images and 53.2% to 67.7% (all p < 0.01) for FBP-Hanning images for both identification tasks. This proposed numerical observer combined with our signal variability framework is promising for assessing material characterization obtained through the additional energ...

The role of extra-foveal processing in 3D imaging

“…Second, the findings are relevant to the development of model observers. Many groups are currently working the development of model observers for 3D image modalities [26] including digital breast tomosynthesis [27]–[29] and computed tomography [30], [31]. Our results suggest that model observers for 3D images require taking into account visual processing in the periphery.…”

Preliminary study on the differentiation of vulnerable carotid plaques via analysis of calcium content and spectral curve slope by using gemstone spectral imaging

“…To the best of our knowledge, the present study is the first time the GSI technique has been adopted to analyze the calcium content in carotid atherosclerotic plaque. In the present study, this novel technique was selected because, inheriting the detection sensitivity of CT towards calcium, GSI uses X-rays and expresses the absorption of the energy spectrum based on tissue composition and lesions, performing quantitative analysis via the MD technique, where the calcium map displays only calcium density and enables measurement of calcium content in plaques ( 7 , 8 ). Of note, the present study also indicated that the spectral curve slope of the CT value of the plaque in patients with stable plaques was significantly lower than that of patients with unstable plaques, which is in agreement with a previous study by Karçaaltıncaba and Aktaş ( 18 ), who reported that vulnerable plaques were rich in lipid cores and their energy spectrum curve exhibited a bow-up curve with a positive slope, whereas stable plaques had a bow-down curve with a negative slope.…”

“…Hence, GSI holds great potential to better characterize and quantify the concentration of plaque components, such as lipids, calcification and fibrous tissues ( 8 ), in order to accurately distinguish vulnerable plaques from stable plaques in carotid arteries. A recent study by Shinohara et al ( 9 ) compared the applicability of GSI to that of 3D time-of-flight magnetic resonance angiography (MRA) in patients with carotid artery stenosis, revealing that the effective Z-value of noncalcified carotid plaques was markedly lower in the group with high signal intensity than that in the group with low signal intensity on MRA.…”

Differentiation of Vulnerable Carotid Plaques by Analysis of Calcium Content and Spectral Curve Slope using Gemstone Spectral Imaging

“…Among these neuroimaging technologies, a novel gemstone spectral imaging (GSI) that incorporates computed tomographic angiography (CTA) with a gemstone detector, received increasing attention in recent years, as it allow for both monochromatic energy imaging and material basis decomposition. More importantly, GSI holds great potential to better characterize and quantify the concentration of the plaque components such as lipid, calci cation and brous tissue [12], so as to accurately distinguish vulnerable plaques from stable plaques in carotid arteries. However, fewer studies have been conducted to investigate the role of GSI in assessment of vulnerable carotid plaque.…”