Richard G. P. Lopata

Three-dimensional cardiac strain imaging in healthy children using RF-data.

In this study, a new radio-frequency (RF)-based, three-dimensional (3-D) strain imaging technique is introduced and applied to 3-D full volume ultrasound data of the heart of healthy children. Continuing advances in performance of transducers for 3-D ultrasound imaging have boosted research on 3-D strain imaging. In general, speckle tracking techniques are used for strain imaging. RF-based strain imaging has the potential to yield better performance than speckle- based methods because of the availability of phase information but such a system output is commercially not available. Furthermore, the relatively low frame rate of 3-D ultrasound data has limited broad application of RF-based cardiac strain imaging. In this study, the previously reported two-dimensional (2-D) strain methodology was extended to the third dimension. Three-dimensional RF-data were acquired in 13 healthy children, in the age range of 6-15 years, at a relatively low frame rate of 38-51 Hz. A 3-D, free-shape, coarse-to-fine displacement and strain estimation algorithm was applied to the RF-data. The heart was segmented using 3-D ellipsoid fitting. Strain was estimated in the radial (R), circumferential (C) and longitudinal directions (L). Our preliminary results reveal the applicability of the 3-D strain estimation technique on full volume 3-D RF-data. The technique enabled 3-D strain imaging of all three strain components. The average strains for all children were in the lateral wall R = 37 ± 10% (infero-lateral) and R = 32% ± 10% (antero-lateral), C = -9% ± 4% (antero-lateral) and C = -9% ± 4% (infero-lateral), L = -18% ± 6 % (antero-lateral) and L = -15% ± 4% (infero-lateral). In the septum, strains were found to be R = 24% ± 10% (antero-septal) and R = 13% ± 5% (infero-septal), C = -13% ± 5% (antero-septal) and -13% ± 5% (infero-septal) and L = -13% ± 3% (antero-septal) and L = -16% ± 5% (infero-septal). Strain in the anterior and inferior walls seemed underestimated, probably caused by the low (in-plane) resolution and poor image quality. The field-of-view as well as image quality were not always sufficient to image the entire left ventricle. It is concluded that 3-D strain imaging using RF-data is feasible, but validation with other modalities and with conventional 3-D speckle tracking techniques will be necessary.

Correlation based 3-D segmentation of the left ventricle in pediatric echocardiographic images using radio-frequency data

Clinical diagnosis of heart disease might be substantially supported by automated segmentation of the endocardial surface in three-dimensional (3-D) echographic images. Because of the poor echogenicity contrast between blood and myocardial tissue in some regions and the inherent speckle noise, automated analysis of these images is challenging. A priori knowledge on the shape of the heart cannot always be relied on, e.g., in children with congenital heart disease, segmentation should be based on the echo features solely. The objective of this study was to investigate the merit of using temporal cross-correlation of radio-frequency (RF) data for automated segmentation of 3-D echocardiographic images. Maximum temporal cross-correlation (MCC) values were determined locally from the RF-data using an iterative 3-D technique. MCC values as well as a combination of MCC values and adaptive filtered, demodulated RF-data were used as an additional, external force in a deformable model approach to segment the endocardial surface and were tested against manually segmented surfaces. Results on 3-D full volume images (Philips, iE33) of 10 healthy children demonstrate that MCC values derived from the RF signal yield a useful parameter to distinguish between blood and myocardium in regions with low echogenicity contrast and incorporation of MCC improves the segmentation results significantly. Further investigation of the MCC over the whole cardiac cycle is required to exploit the full benefit of it for automated segmentation.

Feasibility of wall stress analysis of abdominal aortic aneurysms using three-dimensional ultrasound

OBJECTIVE Abdominal aortic aneurysms (AAAs) are local dilations that can lead to a fatal hemorrhage when ruptured. Wall stress analysis of AAAs is a novel tool that has proven high potential to improve risk stratification. Currently, wall stress analysis of AAAs is based on computed tomography (CT) and magnetic resonance imaging; however, three-dimensional (3D) ultrasound (US) has great advantages over CT and magnetic resonance imaging in terms of costs, speed, and lack of radiation. In this study, the feasibility of 3D US as input for wall stress analysis is investigated. Second, 3D US-based wall stress analysis was compared with CT-based results. METHODS The 3D US and CT data were acquired in 12 patients (diameter, 35-90 mm). US data were segmented manually and compared with automatically acquired CT geometries by calculating the similarity index and Hausdorff distance. Wall stresses were simulated at P = 140 mm Hg and compared between both modalities. RESULTS The similarity index of US vs CT was 0.75 to 0.91 (n = 12), with a median Hausdorff distance ranging from 4.8 to 13.9 mm, with the higher values found at the proximal and distal sides of the AAA. Wall stresses were in accordance with literature, and a good agreement was found between US- and CT-based median stresses and interquartile stresses, which was confirmed by Bland-Altman and regression analysis (n = 8). Wall stresses based on US were typically higher (+23%), caused by geometric irregularities due to the registration of several 3D volumes and manual segmentation. In future work, an automated US registration and segmentation approach is the essential point of improvement before pursuing large-scale patient studies. CONCLUSIONS This study is a first step toward US-based wall stress analysis, which would be the modality of choice to monitor wall stress development over time because no ionizing radiation and contrast material are involved.

Influence of limited field-of-view on wall stress analysis in abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) are local dilations of the aorta which can lead to a fatal hemorrhage when ruptured. Wall stress analysis of AAAs has been widely reported in literature to predict the risk of rupture. Usually, the complete AAA geometry including the aortic bifurcation is obtained by computed tomography (CT). However, performing wall stress analysis based on 3D ultrasound (3D US) has many advantages over CT, although, the field-of-view (FOV) of 3D US is limited and the aortic bifurcation is not easily imaged. In this study, the influence of a limited FOV is examined by performing wall stress analysis on CT-based (total) AAA geometries in 10 patients, and observing the changes in 99th percentile stresses and median stresses while systematically limiting the FOV. Results reveal that changes in the 99th percentile wall stresses are less than 10% when the proximal and distal shoulders of the aneurysm are in the shortened FOV. Wall stress results show that the presence of the aortic bifurcation in the FOV does not influence the wall stresses in high stress regions. Hence, the necessity of assessing the complete FOV, including the aortic bifurcation, is of minor importance. When the proximal and distal shoulders of the AAA are in the FOV, peak wall stresses can be detected adequately.

A novel experimental approach for three-dimensional geometry assessment of calcified human stenotic arteries in vitro

To improve diagnosis and understanding of the risk of rupture of atherosclerotic plaque, new strategies to realistically determine mechanical properties of atherosclerotic plaque need to be developed. In this study, an in vitro experimental method is proposed for accurate 3-D assessment of (diseased) vessel geometry using ultrasound. The method was applied to a vascular phantom, a healthy porcine carotid artery and human carotid endarterectomy specimens (n = 6). Vessel segments were pressure fixed and rotated in 10 ° steps. Longitudinal cross sections were imaged over 360 °. Findings were validated using micro-computed tomography (μCT). Results show good agreement between ultrasound and μCT-based geometries of the different segment types (ISI phantom = 0.94, ISI healthy = 0.79, ISI diseased = 0.75-0.80). The method does not suffer from acoustic shadowing effects present when imaging stenotic segments and allows future dynamic measurements to determine mechanical properties of atherosclerotic plaque in an in vitro setting.

Fast strain tensor imaging using beam steered plane wave ultrasound transmissions

Ultrasound strain imaging can be used to assess local mechanical properties of tissue. From conventional non-steered 2D ultrasound data, the axial (along the beam) displacements and strains can be estimated precisely, whereas lateral (perpendicular to the ultrasound beam) displacements and corresponding strains are more complicated to estimate. The lateral displacements/ strains can be estimated more precisely by adding data from acquisitions at various large beam steering angles, although frame rates are reduced. Plane wave ultrasound transmission enables ultrasound acquisition at high frame rates. This study investigates beam steered plane wave ultrasound transmission for full strain tensor estimation at high frame rates. Using finite element modeling (FEM) and Field II, ultrasound radio frequency data of a vessel with a vulnerable plaque were generated before and after the vessel underwent an intraluminal pressure increase of 4 mmHg. RF data were simulated for a linear array transducer (3–11 MHz, fs = 39 MHz, pitch = 135 μm) that either transmitted focused pulses or plane waves at beam steering angles of −30°, 0° and 30°. In receive dynamic focusing was applied. Band limited noise was added to obtain a signal-to-noise ratio of 20 dB. Displacements were iteratively estimated using 2D cross-correlation. Next, principal strains were derived using 1D least squares strain estimators. The absolute differences between the estimated principal strains and the FEM principal strains were determined to compare the transmission methods. It was found that plane wave beam steering enabled a fast and more precise estimation (Wilcoxon, P<0.001) of the full strain tensor than conventional 0° strain imaging. Although focused beam steering provided slightly more precise estimates, the main advantage of the plane wave approach is that it suffers less from motion artifacts when imaging tissue in vivo, due to its at least 50 times higher frame rate.

Assessment of mechanical properties of porcine aortas under physiological loading conditions using vascular elastography

Non-invasive assessment of the elastic properties of the arterial wall is often performed with ultrasound (US) imaging. The purpose of this study is to estimate mechanical properties of the vascular wall using in vitro inflation testing on biological tissue and two-dimensional (2-D) US elastography, and investigate the performance of the proposed methodology for physiological conditions. An inflation experiment was performed on 12 porcine aortas for (a) a large pressure range (0-140mmHg); and (b) physiological pressures (70-130mmHg) to mimic in vivo hemodynamic conditions. Two-dimensional radiofrequency (RF) data were acquired for one longitudinal and two transverse cross-sections for both experiments, and were analyzed to obtain the geometry and diameter-time behavior. The shear modulus (G) was estimated from these data for each pressure range applied. In addition, an incremental study based on the static data was performed to (1) investigate the changes in G for increasing mean arterial pressure (MAP) for a certain pressure difference (30, 40, 50 and 60mmHg); (2) compare the results with those from the dynamic experiment, for the same pressure range. The resulting stress-strain curves and shear moduli G (94±16kPa) for the static experimentare in agreement with literature and previous work. A linear dependency on MAP was found for G, yet the effect of the pulse pressure difference was negligible. The dynamic data revealed a G of 250±20kPa, whereas the incremental shear modulus (Ginc) was 240±39kPa. For all experiments, no significant differences in the values of G were found between different image planes. This study shows that 2-D US elastography of aortas during inflation testing is feasible and reproducible under controlled and physiological circumstances. In future studies, the in vivo, dynamic experiment should be repeated for a range of MAPs, and pathological vessels should be examined.

Ex vivo photoacoustic imaging of atherosclerotic carotid plaques

Vulnerability assessment of carotid plaques is vital to prevent atherosclerosis-related mortality and disability. Photoacoustic imaging (PAI) in combination with plane-wave ultrasound (PUS) may have the ability to reveal the composition and the anatomical structure of the plaque, which infers its mechanical properties and vulnerability. In this study, we used PAI and PUS imaging to scan endarterectomy samples ex vivo, targeting intraplaque hemorrhage, and compared the results with those obtained in healthy (porcine) carotids and histology. A fully integrated hand-held photoacoustic probe was used, consisting of a pulsed diode laser (tp = 130 ns, Ep = 1 mJ, λ = 808 nm) and a linear array transducer (fc = 7.5 MHz). Three porcine carotid arteries and six carotid plaque samples were obtained from a local slaughterhouse and hospital respectively, and were mounted to the imaging setup. Data of endarterectomy samples revealed that PAI of carotid plaques at 808 nm wavelength is capable of detecting blood clots, which can be extensions of vasculature in the plaque, intra-plaque hemorrhage, or the result of trauma inflicted on the medial vascularization. Due to calcification and the limited optical penetration, imaging depth was mostly limited to the proximal wall of the samples. The porcine carotids revealed no hemorrhaging, which was corroborated by the lack of PAI contrast.

New developments in paediatric cardiac functional ultrasound imaging

Ultrasound imaging can be used to estimate the morphology as well as the motion and deformation of tissues. If the interrogated tissue is actively deforming, this deformation is directly related to its function and quantification of this deformation is normally referred as ‘strain imaging’. Tissue can also be deformed by applying an internal or external force and the resulting, induced deformation is a function of the mechanical tissue characteristics. In combination with the load applied, these strain maps can be used to estimate or reconstruct the mechanical properties of tissue. This technique was named ‘elastography’ by Ophir et al. in 1991. Elastography can be used for atherosclerotic plaque characterisation, while the contractility of the heart or skeletal muscles can be assessed with strain imaging. Rather than using the conventional video format (DICOM) image information, radio frequency (RF)-based ultrasound methods enable estimation of the deformation at higher resolution and with higher precision than commercial methods using Doppler (tissue Doppler imaging) or video image data (2D speckle tracking methods). However, the improvement in accuracy is mainly achieved when measuring strain along the ultrasound beam direction, so it has to be considered a 1D technique. Recently, this method has been extended to multiple directions and precision further improved by using spatial compounding of data acquired at multiple beam steered angles. Using similar techniques, the blood velocity and flow can be determined. RF-based techniques are also beneficial for automated segmentation of the ventricular cavities. In this paper, new developments in different techniques of quantifying cardiac function by strain imaging, automated segmentation, and methods of performing blood flow imaging are reviewed and their application in paediatric cardiology is discussed.

Toward the detection of intraplaque hemorrhage in carotid artery lesions using photoacoustic imaging

Abstract. Photoacoustic imaging (PAI) may have the ability to reveal the composition and the anatomical structure of carotid plaques, which determines its mechanical properties and vulnerability. We used PAI and plane wave ultrasound (PUS) imaging to obtain three-dimensional (3-D) images of endarterectomy samples ex vivo and compared the results with histology to investigate the potential of PAI-based identification of intraplaque hemorrhage. Seven carotid plaque samples were obtained from patients undergoing carotid endarterectomy and imaged with a fully integrated hand-held photoacoustic (PA) probe, consisting of a pulsed diode laser (tpulse=130  ns, Epulse=1  mJ, λ=808  nm) and a linear array transducer (fc=7.5  MHz). The samples were rotated 360 deg with 10 deg steps, and data were spatially compounded to obtain complete 3-D images of the plaques. Areas of high absorption in the 3-D datasets were identified and compared to histological data of the plaques. Data in six out of seven endarterectomy samples revealed the presence of intraplaque hemorrhages that were not visible in the PUS images. Due to the noninvasive nature of PAI, this ex vivo study may elucidate preclinical studies toward the in vivo, noninvasive, vulnerability assessment of the atherosclerotic carotid plaque.