Pieter Kruizinga

Plane-wave ultrasound beamforming using a nonuniform fast fourier transform

Beamforming of plane-wave ultrasound echo signals in the Fourier domain provides fast and accurate image reconstruction. Conventional implementations perform a k-space interpolation from the uniform sampled grid to a nonuniform acoustic dispersion grid. In this paper, we demonstrate that this step can be replaced by a nonuniform Fourier transform. We study the performance of the nonuniform fast Fourier transform (NUFFT) in terms of signal-to-noise ratio and computational cost, and show that the NUFFT offers an advantage in the trade-off between speed and accuracy, compared with other frequency-domain beamforming strategies.

High-Definition Imaging of Carotid Artery Wall Dynamics

The carotid artery (CA) is central to cardiovascular research, because of the clinical relevance of CA plaques as culprits of stroke and the accessibility of the CA for cardiovascular screening. The viscoelastic state of this artery, essential for clinical evaluation, can be assessed by observing arterial deformation in response to the pressure changes throughout the cardiac cycle. Ultrasound imaging has proven to be an excellent tool to monitor these dynamic deformation processes. We describe how a new technique called high-frame-rate ultrasound imaging captures the tissue deformation dynamics throughout the cardiac cycle in unprecedented detail. Local tissue motion exhibits distinct features of sub-micrometer displacements on a sub-millisecond time scale. We present a high-definition motion analysis technique based on plane wave ultrasound imaging able to capture these features. We validated this method by screening a group of healthy volunteers and compared the results with those for two patients known to have atherosclerosis to illustrate the potential utility of this technique.

Photoacoustic imaging of carotid artery atherosclerosis

Abstract. We introduce a method for photoacoustic imaging of the carotid artery, tailored toward detection of lipid-rich atherosclerotic lesions. A common human carotid artery was obtained at autopsy, embedded in a neck mimicking phantom and imaged with a multimodality imaging system using interstitial illumination. Light was delivered through a 1.25-mm-diameter optical probe that can be placed in the pharynx, allowing the carotid artery to be illuminated from within the body. Ultrasound imaging and photoacoustic signal detection is achieved by an external 8-MHz linear array coupled to an ultrasound imaging system. Spectroscopic analysis of photoacoustic images obtained in the wavelength range from 1130 to 1250 nm revealed plaque-specific lipid accumulation in the collagen structure of the artery wall. These spectroscopic findings were confirmed by histology.

Measuring submicrometer displacement vectors using high-frame-rate ultrasound imaging

Measuring the magnitude and direction of tissue displacement provides the basis for the assessment of tissue motion or tissue stiffness. Using conventional displacement tracking by ultrasound delay estimation, only one direction of tissue displacement can be estimated reliably. In this paper, we describe a new technique for estimating the complete two-dimensional displacement vector using high-frame-rate ultrasound imaging. We compute the displacement vector using phase delays that can be measured between pairs of elements within an array. By combining multiple element-pair solutions, we find a new robust estimate for the displacement vector. In this paper, we provide experimental proof that this method permits measurement of the displacement vector for isolated scatterers and diffuse scatterers with high (submicrometer) precision, without the need for beam steering. We also show that we can measure the axial and lateral distension of a carotid artery in a transverse view.

High frame rate ultrasound imaging of human carotid artery dynamics

The carotid artery (CA) is one of the main arteries of interest to cardiovascular research, because of the clinical relevance of CA plaques as culprits of stroke and the accessibility of the CA for cardiovascular health screening. Viscoelastic properties of the arterial wall and of possible lesions within are key components for clinical evaluation of the CA. These viscoelastic properties can be assessed by monitoring the interaction of the wall with a passing arterial pulse wave using ultrasound (US) imaging. The local wall motion (position, velocity, and acceleration) may contain high frequency components and hence a full understanding of tissue mechanics in the CA requires very high frame rate imaging. In this paper we present a high density motion analysis of the CA based on an arterial pulse wave imaged with 2.5 up to 35 kHz frame rate US imaging.

High frame rate ultrasound displacement vector imaging

The tissue displacement vector obtained with high frame-rate imaging may provide valuable diagnostic information. Measuring 2D displacements caused by rapid dynamic processes, such as arterial pulse waves or shear waves, requires inter-frame displacement precision in the micrometer range. Conventional vector estimation techniques do not satisfy this criterion. In this paper we describe a new robust technique for estimating the 2D displacement vector with the required precision. Echoes reflected from a moving scatterer will cause unique delays along the elements of an array. For each pair of elements the solution for the new scatterer position is exact. By averaging many pair-wise estimates we find a robust estimation of the new scatterer location and thereby the displacement vector. We successfully tested our method on rotating scatterers and on a pulsating carotid artery in transverse view.

3D functional ultrasound imaging of the visual system in the pigeon brain

Recently, a new functional neuroimaging method called fUS was proposed, which is based on high-frame-rate Power Doppler imaging. So far, fUS has only been performed on rodents, but major issues in neuroscience such as cerebral asymmetries and language learning are mainly studied in birds. Here, we show the first successful fUS measurements on a non-mammalian species without cortical brain architecture, such as pigeons. These measurements are based on a framerate enhanced fUS acquisition algorithm, which was necessary to suppress the signal variations originating from the slower heartrate of pigeons.

Laser-driven resonance of dye-doped oil-coated microbubbles: A theoretical and numerical study

Microbubbles are used to enhance the contrast in ultrasound imaging. When coated with an optically absorbing material, these bubbles can also provide contrast in photoacoustic imaging. This multimodal aspect is of pronounced interest to the field of medical imaging. The aim of this paper is to provide a theoretical framework to describe the physical phenomena underlying the photoacoustic response. This article presents a model for a spherical gas microbubble suspended in an aqueous environment and coated with an oil layer containing an optically absorbing dye. The model includes heat transfer between the gas core and the surrounding liquids. This framework is suitable for the investigation of both continuous wave and pulsed laser excitation. This work utilizes a combination of finite difference simulations and numerical integration to determine the dependancy on the physical properties, including composition and thickness of the oil layer on the microbubble response. A normalization scheme for a linearized version of the model was derived to facilitate comparison with experimental measurements. The results show that viscosity and thickness of the oil layer determine whether or not microbubble resonance can be excited. This work also examines the use of non-sinusoidal excitation to promote harmonic imaging techniques to further improve the imaging sensitivity.

Monodisperse Versus Polydisperse Ultrasound Contrast Agents: Non-Linear Response, Sensitivity, and Deep Tissue Imaging Potential

It has been proposed that monodisperse microbubble ultrasound contrast agents further increase the signal-to-noise ratio of contrast-enhanced ultrasound imaging. Here, the sensitivity of a polydisperse pre-clinical agent was compared experimentally with that of its size- and acoustically sorted derivatives by using narrowband pressure- and frequency-dependent scattering and attenuation measurements. The sorted monodisperse agents had up to a two-orders-of-magnitude increase in sensitivity, that is, in the average scattering cross section per bubble. Moreover, we found, for the first time, that the highly non-linear response of acoustically sorted microbubbles can be exploited to confine scattering and attenuation to the focal region of ultrasound fields used in clinical imaging. This property is a result of minimal pre-focal scattering and attenuation and can be used to minimize shadowing effects in deep tissue imaging. Moreover, it potentially allows for more localized therapy using microbubbles through the spatial control of resonant microbubble oscillations.

Optimizing the directivity of piezoelectric matrix transducer elements mounted on an ASIC

Over the last decade, clinical studies show a strong interest in real-time 3D imaging. This calls for ultrasound probes with high-element-count 2D matrix transducer arrays. These may be interfaced to an imaging system using an in-probe Application Specific Integrated Circuit (ASIC) that takes care of signal amplification, element switching, sub-array beamforming, etc. Since the ASIC is made from silicon and is mounted directly behind the transducer elements, it can acoustically be regarded as a rigid plate that can sustain traveling lateral waves. These waves lead to acoustical cross-talk between the elements, and results in extra peaks in the directivity pattern. We propose two solutions to this problem, based on numerical simulations. One approach is to decrease the phase velocity in the silicon by reducing the silicon thickness and absorbing the energy using a proper backing material. Another solution is to disturb the waves inside the silicon plate by sub-dicing the back-side of the ASIC. We conclude that both solutions can be used to improve the directivity pattern.