Fast strain tensor imaging using beam steered plane wave ultrasoundtransmissions

\u3cp\u3eUltrasound strain imaging can be used to assess local mechanical propertiesof tissue. From conventional non-steered 2D ultrasound data, the axial (alongthe beam) displacements and strains can be estimated precisely, whereas lateral(perpendicular to the ultrasound beam) displacements and corresponding strainsare more complicated to estimate. The lateral displacements/strains can beestimated more precisely by adding data from acquisitions at various large beamsteering angles, although frame rates are reduced. Plane wave ultrasoundtransmission enables ultrasound acquisition at high frame rates. This studyinvestigates beam steered plane wave ultrasound transmission for full straintensor estimation at high frame rates. Using finite element modeling (FEM) andField II, ultrasound radio frequency data of a vessel with a vulnerable plaquewere generated before and after the vessel underwent an intraluminal pressureincrease of 4 mmHg. RF data were simulated for a linear array transducer (3-11MHz, fs = 39 MHz, pitch = 135 μm) that either transmitted focused pulses orplane waves at beam steering angles of 30°, 0° and 30°. In receivedynamic focusing was applied. Band limited noise was added to obtain asignal-to-noise ratio of 20 dB. Displacements were iteratively estimated using2D cross-correlation. Next, principal strains were derived using 1D leastsquares strain estimators. The absolute differences between the estimatedprincipal strains and the FEM principal strains were determined to compare thetransmission methods. It was found that plane wave beam steering enabled a fastand more precise estimation (Wilcoxon, P>0.001) of the full strain tensorthan conventional 0° strain imaging. Although focused beam steering providedslightly more precise estimates, the main advantage of the plane wave approachis that it suffers less from motion artifacts when imaging tissue in vivo, dueto its at least 50 times higher frame rate.\u3c/p\u3

Optical and acoustic characterization of freeze-thawed polyvinyl alcohol gels

\u3cp\u3ePreclinical validation of non-invasive photoacoustic imaging of carotid artery atherosclerosis requires vessel phantoms that imitate optical, acoustic and mechanical properties of vascular tissue. Polyvinyl alcohol (PVA) phantoms that are widely used as ultrasound phantoms due to their elastic properties are also promising for photoacoustics. This study contributes to the field by quantifying the optical and acoustic properties of PVA gel, and aims at the characterization of realistic phantoms for future studies. In this study, we investigated the relation between acoustic scatterers and optical absorbers to quantify optical and acoustic properties of the PVA phantoms. Four different concentrations of orgasol acoustic scatterers, and varying concentration of Indian ink and molecular dye absorbers were added to a 15 wt% PVA solution. Samples were subjected to 1 to 5 freeze-thaw cycles and were examined after each cycle to quantify the effect on the optical and the acoustic properties. Optical attenuation was measured between 400 nm and 990 nm using a plate reader. Additionally, pulse-echo plane wave ultrasound was used for acoustic characterization. Changing the concentration of orgasol between 0.5 wt% and 4 wt% increased the mean optical attenuation of PVA by 35% after the first freeze thaw cycle. Likewise, each freeze-thaw cycle increased the optical attenuation due to scattering of light by the microstructure of PVA. The absorbance of pure PVA increased 40% between the first and second cycle and 3% between the fourth and fifth cycle. While the orgasol concentration and the freeze-thaw cycles altered the acoustic speed and attenuation, the ink and the dye inclusions did not significantly affect the acoustic properties of PVA.\u3c/p\u3

Mechanical characterization of vascular tissue using ultrasound

\u3cp\u3eRupture of carotid plaques accounts for 15 to 20% of ischemic strokes. To prevent a stroke patients undergo surgery (carotid endarterectomy). However, only one out of six patients benefits from this intervention, so there is a significant overtreatment of patients. To develop better treatment criteria, and to improve patient diagnosis and clinical decision making, patient specific information of the plaque's composition and mechanical properties is needed. In this study, a method was developed to estimate global material properties of the vascular wall using ultrasound imaging. Here, a finite element model (FEM) is matched with the displacement field measured by US, by updating the material properties of the FEM. As a result, a measure for arterial wall stiffness is obtained non-invasively.\u3c/p\u3

Noninvasive strain imaging in pulsating vessels using beam steered ultrasound acquisitions

\u3cp\u3eStrain in the arterial wall can be estimated by cross-correlation of radiofrequency ultrasound data recorded at various blood pressure levels. Intravascular studies revealed a high correlation between high radial strain regions and plaque vulnerability in coronary arteries. A noninvasive variant of the technique is desired, since it will enable early screening for rupture prone plaques, also in asymptomatic populations. Recently, we have shown that it is possible to obtain more precise radial (and circumferential) strain estimates by combining data from multiple beam steered ultrasound acquisitions in phantoms. However, the multi-angle method was tested using quasi-static data only, thus no motion artifacts were present. This study investigates the performance of the multi-angle method for pulsating vessels. Results are presented for a periodically pulsating vessel mimicking phantom and in vivo recordings of a healthy carotid artery. It is shown that the multi-angle technique also outperforms conventional single angle strain imaging for pulsating vessels.\u3c/p\u3

Intraplaque haemorrhage detection using single-wavelength PAI and singular value decomposition in the carotid artery

\u3cp\u3eThe rupture of a vulnerable carotid plaque featuring a lipid-rich necrotic core and intra-plaque haemorrhages is the major cause of stroke. Photoacoustic imaging (PAI) is a promising technique for assessing plaque vulnerability in the carotid artery due to its ability to assess the chemical composition in addition to its anatomy. However, assessment of chemical composition is usually based on the absorption differences of chromophores between multiple wavelengths, which heavily increase the complexity and cost of the imaging system. In this study, a new method based on single-wavelength PAI to detect intra-plaque haemorrhages, an important indicator of plaque vulnerability, is developed. The method uses wall filtering based on singular value decomposition. To test the method, a carotid plaque phantom mimicking intra-plaque haemorrhages, lumen and vasa vasorum is designed and imaged at 808nm in vitro. The phantom experiment shows wall filtering using singular value decomposition to be a viable method capable of discriminating signals originating from the lumen, vasa vasorum and intraplaque haemorrhages, allowing for the detection of intra-plaque haemorrhages with single wavelength PAI. This enables new opportunities for PAI of vulnerable carotid plaques with more cost effective and diverse laser sources.\u3c/p\u3

Hemorrhages detection in atherosclerotic plaques using ultrasound and photoacoustic, phantom study

\u3cp\u3eRupture of an atherosclerotic plaque in the carotid artery is one of the main causes of stroke and stroke-induced death. Currently, to prevent this risk, endarterectomy is performed based on the stenosis grade assessed with Duplex ultrasound (US). However, plaque composition, e.g. presence of lipids and hemorrhages, is a more important factor of rupture risk than stenosis. The optical absorption of hemorrhages (composed of coagulated, deoxygenated blood) is different from that of oxygenated blood and allows their distinction using different wavelengths. In this study, photoacoustic (PA) imaging is used to detect intraplaque hemorrhages using contrast in optical absorption in combination with the resolution of US.\u3c/p\u3

3D geometry assessment and wall stress analysis of carotid arteries using 2D US segmentation during a controlled sweep

\u3cp\u3ePatients with a 70-99% stenosed carotid artery will undergo surgery to prevent rupture of the plaque. However, the degree of stenosis does not determine the vulnerability of the plaque, since vulnerability depends on the plaque's morphology, mechanical properties and the force acting on the plaque. Mechanical models of the carotid artery can improve decision-making of the surgeon by quantifying and mapping wall stress and strain. In this study, a method was developed to obtain the 3D geometry of the carotid artery by a 'slow' sweep over the neck (5 cm) using 2D ultrasound. Rather than using a 3D ultrasound probe, the high axial and temporal resolution are maintained.\u3c/p\u3

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

\u3cp\u3eNon-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 (0-140. mmHg); and (b) physiological pressures (70-130. mmHg) 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 60. mmHg); (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 (G\u3csub\u3einc\u3c/sub\u3e) 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.\u3c/p\u3