Attenuation Correction and Normalisation for Quantification of Contrast Enhancement in Ultrasound Images of Carotid Arteries

Adaptive attenuation correction during H-scan ultrasound imaging using K-means clustering

H-scan ultrasound (US) imaging (where the 'H' stands for Hermite) is a novel non-invasive, low cost and real-time technology. Like traditional US, H-scan US suffers from frequency-dependent attenuation that must be corrected to have acceptable image quality for tissue characterization. The goal of this research was to develop a novel attenuation correction method based on adaptive K-means clustering. To properly isolate these signals, a lateral moving window approach applied to adaptively adjust GH filters based on the changing of RF vector spectrums. Then the signal isolated via the same filter will be combined together via overlap-add technology to keep the information loss minimum. Experimental data was collected using a Verasonics 256 US scanner equipped with a L11-4v linear array transducer. In vivo data indicates that H-scan US imaging after adaptive attenuation correction can optimally re-scale the GH kernels and match to the changing spectrum undergoing attenuation (i.e. high frequency shift). This approach produces H-scan US images with more uniform spatial intensity and outperforms global attenuation correction strategies. Overall, this approach will improve the ability of H-scan US imaging to estimate acoustic scatterer size and will improve its clinical use for tissue characterization when imaging complex tissues.

Contrast-Enhanced Ultrasound to Assess Carotid Intraplaque Neovascularization

Contrast-enhanced ultrasound (CEUS) is increasingly being used to identify patients with carotid plaques that are vulnerable to rupture, so-called vulnerable atherosclerotic plaques, by assessment of intraplaque neovascularization. A complete overview of the strengths and limitations of carotid CEUS is currently not available. The aim of this systematic review was to provide a complete overview of existing publications on the role of CEUS in assessment of carotid intraplaque neovascularization. The systematic review of the literature yielded 52 studies including a total of 4660 patients (mean age: 66 y, 71% male) who underwent CEUS for the assessment of intraplaque neovascularization. The majority of the patients (76%) were asymptomatic and had no history of transient ischemic attack (TIA) or stroke. The assessment of intraplaque neovascularization was mostly performed using a visual scoring system; several studies used time-intensity curves or dedicated quantification software to optimize analysis. In 17 studies CEUS was performed in patients before carotid surgery (endarterectomy), allowing a comparison of pre-operative CEUS findings with histologic analysis of the tissue sample that is removed from the carotid artery. In a total of 576 patients, the CEUS findings were compared with histopathological analysis of the plaque after surgery. In 16 of the 17 studies, contrast enhancement was found to correlate with the presence and degree of intraplaque neovascularization on histology. Plaques with a larger amount of contrast enhancement had significantly increased density of microvessels in the corresponding region on histology. In conclusion, CEUS is a readily available imaging modality for the assessment of patients with carotid atherosclerosis, providing information on atherosclerotic plaques, such as ulceration and intraplaque neovascularization, which may be clinically relevant. The ultimate clinical goal is the early identification of carotid atherosclerosis to start early preventive therapy and prevent clinical complications such as TIA and stroke.

A Novel Microflow Phantom Dedicated to Ultrasound Microvascular Measurements

Tumor microvascularization is a biomarker of response to antiangiogenic treatments and is accurately assessed by ultrasound imaging. Imaging modes used to visualize slow flows include Power Doppler imaging, dynamic contrast-enhanced ultrasonography, and more recently, microvascular Doppler. Flow phantoms are used to evaluate the performance of Doppler imaging techniques, but they do not have a steady flow and sufficiently small channels. We report a novel device for robust and stable microflow measurements and the study of the microvascularization. Based on microfluidics technology, the prototype features wall-less cylindrical channels of diameters ranging from as small as 147 up to 436 µm, cast in a soft silicone polymer and perfused via a microfluidic flow pressure controller. The device was assessed using flow rates from 49 to 146 µL/min, with less than 1% coefficient of variation over three minutes, corresponding to velocities of 6 to 142 mm/s. This enabled us to evaluate and confirm the reliability of the Superb Microvascular Imaging Doppler mode compared with the Power Doppler mode at these flow rates in the presence of vibrations mimicking physiological motion.

A Temporal and Spatial Analysis Approach to Automated Segmentation of Microbubble Signals in Contrast-Enhanced Ultrasound Images: Application to Quantification of Active Vascular Density in Human Lower Limbs

Contrast-enhanced ultrasound (CEUS) using microbubble contrast agents has shown great promise in visualising and quantifying active vascular density. Most existing approaches for vascular density quantification using CEUS are calculated based on image intensity and are susceptible to confounding factors and imaging artefact. Poor reproducibility is a key challenge to clinical translation. In this study, a new automated temporal and spatial signal analysis approach is developed for reproducible microbubble segmentation and quantification of contrast enhancement in human lower limbs. The approach is evaluated in vitro on phantoms and in vivo in lower limbs of healthy volunteers before and after physical exercise. In this approach, vascular density is quantified based on the relative areas microbubbles occupy instead of their image intensity. Temporal features of the CEUS image sequences are used to identify pixels that contain microbubble signals. A microbubble track density (MTD) measure, the ratio of the segmented microbubble area to the whole tissue area, is calculated as a surrogate for active capillary density. In vitro results reveal a good correlation (r² = 0.89) between the calculated MTD measure and the known bubble concentration. For in vivo results, a significant increase (129% in average) in the MTD measure is found in lower limbs of healthy volunteers after exercise, with excellent repeatability over a series of days (intra-class correlation coefficient = 0.96). This compares to the existing state-of-the-art approach of destruction and replenishment analysis on the same patients (intra-class correlation coefficient ≤0.78). The proposed new approach shows great potential as an accurate and highly reproducible clinical tool for quantification of active vascular density.

Differential Intensity Projection for Visualisation and Quantification of Plaque Neovascularisation in Contrast-Enhanced Ultrasound Images of Carotid Arteries

Studies have reported that intraplaque neovascularisation (IPN) is closely correlated with plaque vulnerability. In this study, a new image processing approach, differential intensity projection (DIP), was developed to visualise and quantify IPN in contrast-enhanced non-linear ultrasound image sequences of carotid arteries. DIP used the difference between the local temporal maximum and the local temporal average signals to identify bubbles against tissue non-linear artefact and noise. The total absolute and relative areas occupied by bubbles within each plaque were calculated to quantify IPN. In vitro measurements on a laboratory phantom were made, followed by in vivo measurements in which 24 contrast-enhanced non-linear ultrasound image sequences of carotid arteries from 48 patients were selected and motion corrected. The results using DIP were compared with those obtained by maximum intensity projection (MIP) and visual assessment. The results indicated that DIP can significantly reduce non-linear propagation tissue artefacts and is much more specific in detecting bubble signals than MIP, being able to reveal microbubble signals that are buried in tissue artefacts in the corresponding MIP image. A good correlation was found between microvascular area (MVA) (r = 0.83, p < 0.001)/microvascular density (r = 0.77, p < 0.001) obtained using DIP and the corresponding expert visual grades, comparing favourably to r = 0.26 and 0.23 obtained using MIP on the same data. In conclusion, the proposed method exhibits great potential in quantification of IPN in contrast-enhanced ultrasound images of carotid arteries.

Contrast enhancement of carotid adventitial vasa vasorum as a biomarker of radiation-induced atherosclerosis

PURPOSE

Abnormal proliferation of adventitial vasa vasorum (vv) occurs early at sites of atherosclerosis and is thought to be an early biomarker of vascular damage. Contrast-enhanced ultrasound (CEUS) can detect this process. Its usefulness in irradiated arteries as a measure of accelerated atherosclerosis is unknown. This study investigates contrast intensity in carotid adventitia as an early marker of radiation-induced damage in head and neck cancer (HNC) patients.

MATERIALS/METHODS

Patients with HNC treated with a wedged-pair and matched neck technique or hemi-neck radiotherapy (RT) (unirradiated side as control) at least 2years previously were included. Patients had been prescribed a dose of at least 50Gy to the neck. CEUS was performed on both carotid arteries and a region of interest was selected in the adventitia of the far wall of both left and right distal common carotid arteries. Novel quantification software was used to compare the average intensity per pixel between irradiated and unirradiated arteries.

RESULTS

48 patients (34 males) with median age of 59.2years (interquartile range (IQR) 49.2-64.2) were included. The mean maximum point dose to the irradiated artery was 61.2Gy (IQR 52.6-61.8) and 1.1Gy (IQR 1.0-1.8Gy) to the unirradiated side. The median interval from RT was 59.4months (IQR 41-88.7). There was a significant difference in the mean (SD) contrast intensity per pixel on the irradiated side (1.1 (0.4)) versus 0.96 (0.34) on the unirradiated side (p=0.01). After attenuation correction, the difference in mean contrast intensity per pixel was still significant (1.4 (0.58) versus 1.2 (0.47) (p=0.02). Previous surgery or chemotherapy had no effect on the difference in contrast intensity between the 2 sides of the neck. Mean intensity per pixel did not correlate to traditional risk prediction models (carotid intima-medial thickness, QSTROKE score).

CONCLUSIONS

Proliferation of vv is demonstrated by increased contrast intensity in irradiated carotid arteries. This may be a useful, independent biomarker of radiation-induced carotid atherosclerosis when used as a tool to quantify neovascularization.

Detection of Carotid Atherosclerotic Plaque Neovascularization Using Contrast Enhanced Ultrasound: A Systematic Review and Meta-Analysis of Diagnostic Accuracy Studies

BACKGROUND

Intraplaque neovascularization is considered an important indicator of plaque vulnerability. Contrast-enhanced ultrasound (CEUS) of carotid arteries improves imaging of carotid intima-media thickness and permits real-time visualization of neovascularization of the atherosclerotic plaque. The authors conducted a systematic review and meta-analysis to evaluate the accuracy of CEUS-detected carotid atherosclerotic plaque.

METHODS

A systematic search was performed to identify studies published in the MEDLINE, Embase, Scopus, and Web of Science databases from 2004 to June 2015. Studies evaluating the accuracy of quantitative analysis and qualitative analysis (visual interpretation) for the diagnosis of intraplaque neovascularization compared with histologic specimens and/or clinical diagnosis of symptomatic plaque were included. Parameters evaluated were plaque quantitative CEUS intensity and the visual grading of plaque CEUS. A random-effects meta-analysis was used to pool the likelihood ratios (LRs), diagnostic odds ratios, and summary receiver operating characteristic curves. Corresponding areas under the curves were calculated.

RESULTS

The literature search identified 203 studies, 20 of which were selected for systematic review; the final meta-analysis included seven studies. For qualitative CEUS, pooled sensitivity was 0.80 (95% CI, 0.72-0.87), pooled specificity was 0.83 (95% CI, 0.76-0.89), the pooled positive LR was 3.22 (95% CI, 1.67-6.18), the pooled negative LR was 0.24 (95% CI, 0.09-0.64), the pooled diagnostic odds ratio was 15.57 (95% CI, 4.94-49.03), and area under the curve was 0.894. For quantitative CEUS, pooled sensitivity was 0.77 (95% CI, 0.71-0.83), pooled specificity was 0.68 (95% CI, 0.62-0.73), the pooled positive LR was 2.34 (95% CI, 1.69-3.23), the pooled negative LR was 0.34 (95% CI, 0.25-0.47), the pooled diagnostic odds ratio was 7.06 (95% CI, 3.6-13.82), and area under the curve was 0.888.

CONCLUSIONS

CEUS is a promising noninvasive diagnostic modality for detecting intraplaque neovascularization. Standardization of quantitative analysis and visual grading classification is needed to increase reliability and reduce technical heterogeneity.