Accurately estimating the risk of Abdominal Aortic Aneurysm (AAA) rupture is key to improving the treatment of patients suffering from this disease. While conventional risk assessment is based on the geometrical properties of the vessel, acquired via ultrasound imaging, it is expected that acquiring data on more of the vessel's properties will lead to an improved prediction of the risk of rupture, and thus improved treatment of patients. These properties include 3-dimensional (3-D) blood flow through the vessel and its 3-D elasticity, which require an ultrasound probe consisting of thousands of transducer elements capable of imaging a large volume at very high volume-rates. To address this challenge, this dissertation presents the design, fabrication and characterization of a 4096-element ultrasound probe for high-volume-rate (HVR) cardiovascular imaging.

The probe consists of two custom-designed application-specific integrated circuits (ASICs), each of which interfaces with a 2048-element transducer array, which in turn can consist of bulk-fabricated piezo-electric transducers, or monolithically integrated capacitive micro-machined ultrasound transducers (CMUTs). The probe can image a 60◦×60◦×10-cm volume at 2000 volumes/s, the highest volume-rate with in-probe channel-count reduction reported to date. It uses a 2×2 delay-and-sum micro-beamformer (µBF) and 2× time-division multiplexing (TDM) to achieve an 8× receive (RX) channel count reduction, and is the first to scale this combination of techniques to an array of thousands of elements. Equalization, trained using a pseudorandom bit-sequence generated on the chip, reduces TDM-induced crosstalk by 10 dB, enabling power-efficient scaling of the cable drivers. The ASICs also implement a novel transmit (TX) beamformer (BF) that operates as a programmable digital pipeline, which enables steering of arbitrary pulse-density modulated waveforms. The TX BF drives element-level 65 V unipolar pulsers, which in turn drive the transducer elements. Both the TX BF and RX µBF are programmed with shift-registers that can either be programmed in a row-column fashion for fast upload.

As the ASICs in the probe can accommodate multiple transducer technologies, two variants of the probe were developed and acoustically validated to compare the performance of a CMUT and bulk PZT transducer array, demonstrating the potential of using the probe as a prototyping platform for further research activities, enabling validation of ultrasound arrays of up to thousands of elements. The CMUT variant probe was also used to characterize and compare the performance of multiple imaging schemes for the intended application. The scheme for imaging a 60◦×60◦×10-cm volume at 2000 volumes/s with half of the array achieves a median resolution of 4.0◦x1.9◦x660 µm, which accurately matches simulation results. Using this imaging scheme, Doppler images were reconstructed without aliasing artefacts of a blood-mimicking fluid flowing through a flow phantom with an average velocity up to 400 mm/s, which represents the peak velocity of blood in AAAs. Overall, the dissertation demonstrates that the developed system is a promising solution for ultrasound research and the improved treatment of AAAs.
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Medical ultrasound is crucial in modern healthcare, with high-frame-rate imaging expanding its applications to both high blood velocities over large fields-of-view and slow-moving blood. Accurate flow measurements require high temporal and spatial resolution, necessitating the use of matrix transducers, which face several challenges depending on the application. For monitoring the brains of preterm babies, challenges include designing ASIC-integrated matrices that meet clinical needs and minimizing cables to ensure unrestricted mother-child contact. For atherosclerosis screening, challenges involve sensor-ASIC integration, developing high-frame-rate data acquisition, and addressing high side-lobe levels in sparse matrices. The objective of this thesis is to address these challenges and develop the necessary technology to enable the use of ultrasound matrix transducers for quantifying blood flow and detecting physiologically abnormal flow patterns and velocities in the two mentioned applications. ... Read more

For over 50 years, medical ultrasound has been a pioneering force in healthcare, seamlessly blending diagnostic and therapeutic applications. In diagnostics, ultrasound reveals intricate internal structures based on the generation of acoustic pressure waves in the human body and capturing reflections from tissue and body structures, enabling precise anomaly identification. Simultaneously, in therapy, ultrasound utilizes its acoustic prowess for targeted interventions and submillimeter localization in unconventional cancer treatments like proton therapy. The field has shifted significantly with the advent of nonlinear acoustics, exploring wave propagation phenomena. Contrast agents marked a revolutionary leap, enhancing the specificity, sensitivity, and efficiency of diagnostic and therapeutic ultrasound. Microbubbles, as the most conventionally used agents, exhibit strong nonlinear scattering under ultrasonic excitation, making them suitable for CEUS applications. ... Read more

In this thesis two topics are discussed. The covariant formulation ofMaxwell’s equations of electromagnetism and the formulation of said equations in the context of a rotating frame of reference. Through the development of the necessary theories of Differential Geometry and Special Relativity we show how to formulateMaxwell’s equations in terms of the covariant derivative and in terms of the hodge operator and differential operator. Using this formulation we study the transformation properties of these equations under Lorentz transformations and we conclude that they remain invariant under said transformations. Secondly,we discuss literature concerning the problem of electromagnetism in the context of a rotating frame of reference. We show that the method of direct transformation to a rotating frame results in an unobservable frame of reference and we show that it is impossible to reconstruct Coriolis like forces in an observable frame of reference. ... Read more

This essay shows a two dimensional implementation of the finite element method for the Westervelt equation. To do this the finite element method is first applied to the linear wave equation, then to non-linear diffusion and finally to the Westervelt equation. Both an element by element and a faster vectorized implementation are given for the finite element method. To verify the numerical solution two analytical solutions are used. The first is a one dimensional wave and the second a circularly symmetric wave.

We found that the two-dimensional implementation was successful in computing the Westervelt equation. The error of the solution scales with the mesh size with a power of around 1.7. It was also found that the time step used to compute the solution needs to be small enough for the implementation to converge. ... Read more

Ultrasonic flow meters are widely applied to measure flow in a variety of applications. The vast majority of ultrasonic flow meters are based on the measurement of the transit time of an acoustic pulse through the fluid. This can either be done in-line, by inserting a spool piece with ultrasonic transducers into the pipe carrying the fluid, or by clamping the transducers on an existing pipe. Clamp-on meters are attractive as they can be installed without cutting the pipe or shutting down the flow, but their stability is limited, and they are unable to measure flow profiles (in contrast with expensive multi-path in-line meters), which limits their linearity at low flow speeds. Moreover, their installation requires complex manual alignment of the transducers and input of a variety of setup parameters (e.g. pipe dimensions and material properties, speed of sound in the fluid) by the user. In this thesis, clamp-on meters based on matrix ultrasonic transducers are developed to address these drawbacks. These matrix transducers consist of a two-dimensional array of 100+ elements that enables beam steering in two directions by programming the timing of the electrical pulses applied to the elements. This allows to develop three innovative measurement techniques: (1) automatic beam alignment by adjusting the steering angles so as to optimize the signal-to-noise ratio and the path of the received pulse, thus simplifying installation and improving stability; (2) multi-path measurement by steering the beam at different angles, realizing the measurement of multiple paths through the fluid with a single pair of matrix transducers, and thus providing information about the flow profile; (3) self-calibration by using pulse-echo measurements between the elements of the matrix transducer to characterize the pipe wall and fluid, thus reducing the dependence on a-priori knowledge of their properties. The most significant steps to realize these kind of sensors were taken in this thesis. The mentioned measurement techniques were elaborated, and the relevant wave-propagation phenomena, beamforming schemes and transducer design were performed. Based on this, a prototype sensor was fabricated and successfully tested. Moreover, application-specific integrated circuits (ASICs) were developed with dedicated transmit and receive electronics to realize a cost-effective and accurate implementation of the beam forming and transit-time measurement. ... Read more

This thesis produces a pre-characterization numerical model capable of handling and calculating electromagnetic fields within a rectangular reverberation chamber near its lowest usablefrequency at 200 MHz. As reverberation chambers strive to have high electric field uniformityto meet field uniformity standards, high electric fields, having the property of being naturallymore uniform, are out of the scope of interest. Additionally high frequencies require more computational memory and CPU time.The model is made using a finite element method based modelling software called Comsol Multiphysics. The modelled reverberation chamber consisting of an antenna, a reflectiveshielded chamber and a Z-fold mode-stirrer is gradually build up. This means that first ananalysis of only the antennas will be made, thereafter the antennas will be put into a reflec-tive shielded chamber environment and finally the antennas are put into a reflective shieldedchamber environment with a z-fold mode stirrer. Furthermore, an extra situation will be considered in which a dielectric object will be added to a shielded chamber environment excited by an antenna without mode stirrer. The effect of an added dielectric object will be studied because of physical interest and completeness and not as added intermediate step to build a reverberation chamber environment. As all situations have similar difficulties in modelling and measuring, the gradual development of the reverberation chamber will allow for the best error analysis of the model. The model replicates the setup of a real reverberation chamber located at Comtest, Zoeterwoude. The reliability and accuracy of the model is studied by comparing the modelled electric field to the measured one. It was found that all models showed a good resemblance between simulated and measured electric fields above 60 MHz except the most complex reverberation chamber model. The simulated field uniformity expressed as standard deviationis twice as high as measurements suggest. The model can therefore only be used as worst-case scenario prediction for the field uniformity. Two kinds of antennas are used during the modelling and measuring phase, a 3104c biconical antenna used in the frequency range 25-200 MHz and the 3146a log-periodic dipole array antenna in the frequency range 200-1000 MHz. The electromagnetic radiation pattern in the far field domain was modelled and corresponded to the expected omni-directional and directional field pattern respectively. Next the antennas were placed in a highly reflective shielded chamber with dimensions (4.05 m×2.55 m×2.925 m). By taking data in specific slices from the model and comparing these to the measurements at the same points it was concluded that the model does not ac-curately predict the electric fields below 60 MHz. Above 60 MHz the model does predict the general electric field pattern in the chamber. It however does not predict local maxima or minima of the electric field.The same comparison was done for the situation in which a dielectric object was added to the setup. The dielectric object was chosen to be a container filled with water (0.27 m×0.565 m×0.269 m). This container is placed in the formerly empty shielded chamber to change the inner electric field. Water was used due to its favourable properties for reflectivity. The same behaviour of the model was observed with this added dielectric object. Finally the electric field in the reverberation chamber at Comtest (5.03 m×3.97 m×2.85 m) was modelled and solved using a GMRES algorithm with geometric multigrid preconditioning.The preconditioning in the GMRES allows for faster convergence. The field uniformity wascomputed for both the model and the measurements as outlined by IEC 61000-4-21. This wasdone for 4 and 12 stirrer rotation positions. Both showed that the model had a less uniform field compared to the Comtest reverberation chamber. The Comtest reverberation chamber complied with the electromagnetic compatibility requirements for measurements using 12 stirrer positions whereas the model did not. However the 4 stirrer position model, which made use of perfect electric conductor boundary conditions, showed a maximum of 13% increase in field uniformity when steel walls were used instead. To show a glimpse of an innovation that put the Delft University of Technology on the map, the shifted laplacian preconditioner is briefly discussed. As an intermediate step in solving a problem with little to no damping, a complex preconditioning matrix is used. It is shown that for an increasing imaginary shift in the Helmholtz problem, expressed as an increasing electrical conductivity σ, the number of iterations needed to reach a relative tolerance smaller than 0.01 decreases. At last, the effect of a shifted laplacian contribution to a multigrid preconditioning on the convergence speed is studied for an non-damped pressure acoustic Helmholtz problem. It is shown that the added contribution slows convergence in a simple geometry, while a more complex geometry cannot be solved without this contribution. ... Read more

In this research, a Shive wave machine is used to study (a) the velocity of waves throughout different media and (b) the transition of waves between two different media. The Shive wave machine used in this research consists of 32 parallel aluminum bars attached perpendicularly to three parallel central wires. When a perturbation is applied to one of the bars, a torsional wave is initiated in the Shive wave machine, which is mapped to a transverse wave at the extremities of the bars. The velocity of a wave in the Shive wave machine is theoretically determined by four variables: (i) The distance between the two outside wires and the central wire; (ii) the tension in the two outside wires; (iii) the distance between the bars; (iv) the moment of inertia of the bars. The transition of waves between two different media is theoretically determined by the wave velocities in the two media. The theory states that a part of the wave is reflected and a part is transmitted at the intersection between the two media. The results for (a) show that the measured velocity is higher than theoretically expected, which may be caused by an incorrect measurement of (ii). The results for (b) show that a change in (i) throughout the system does not comply with the theoretically expected ratios for reflection and transmission, whereas a change in (iv) does comply with the theoretically expected ratios. ... Read more

In this thesis, we investigate a sparse basis for ultrasound images, so that we can use sparse regularization in imaging. Actually, there are few previous researches explicitly demonstrating that medical ultrasound images can be sparsified for some dictionary. We consider various orthogonal transforms such as wavelet transforms, cosine transforms and wave atom transforms. Then, we perform those transforms on various ultrasound images and analyzes their sparsity. These ultrasound images include the images of two computer ultrasound phantoms and beamformed ultrasound images with good quality from real people. We looked at sparsity of the true pre-beamformed images, as well as beamformed images. We also consider constructing a specific ultrasound image dictionary using the K-SVD algorithm. We observed that, the pre-beamformed images hardly haVe no sparse basis, and the sparsity of beamformed images will only increase slightly if we use different 1D-DWT in each direction. We also found that the wide overdetermined dictionary generated by K-SVD significantly increases sparsity. After this, we simulate the ultrasound image reconstruction from the ultrasound RF measurements, and we analyze the effects of the different sparse spaces on the reconstruction performance. We observed that, the L1-regularization can work for ultrasound imaging better than L2-regularization, but the orthogonal transforms as well as the dictionary do not improve the reconstruction image quality much. ... Read more

In medical ultrasound imaging phased array transducers are used for non-invasive imaging. For 3D intra-cardiac-echography technical borders are reached since blood vessel size limits the dimensions of the array. This dimensional limitation lowers lateral resolution. To solve part of the problem, non conventional ways of lateral resolution improvements are needed. For linear arrays R. Reeg [1] proposed null subtraction imaging (NSI), which resulted at best in 30 dB lower side lobes and a width reduction of up to 25 times for the main lobe. This is a non-linear image processing technique based on implementing multiple apodizations in post-processing. Three images are made from which one has a sharp drop to zero in the middle which can be exploited by subtracting it from the other images. This results in a improvement over a regular apodization scheme.
To verify the technique for 2D phased array imaging, simulations are done using Field-II [2, 3] while taking array dimension into account. Measurements take place on an artificial phantom with evenly spaced line scatter targets. For the experiment a P4-l phased array transducer was used in combination with a Verasonics Vantage 256TMwhere a sub-aperture of 30 elements is used. The experiment is done on a CIRS 040GSE Phantom, where evenly spaced line scatter targets and lateral resolution targets are looked at. To form an image a simple delay-and-sum (DAS) algorithm was used. The effect of NSI on the speckle in an image is also measured, where for normal developed speckle a signal to noise ratio (SNR) of 1.91 is expected. Results for simulation were as expected with an average beamwidth reduction of 22.5 times and on average higher main lobe to side lobe ratio (MSR) of 33.4 dB for NSI imaging opposed to using a rectangular apodization. Experimentally an average beamwidth reduction of 5 times and an on average higher MSR of 20.5 dB are realised. The beamwidth reduction is lower then expected, this is due to rounding errors in the DAS algorithm, where data can only be delayed by integer elements. For the lateral scatter targets no extra targets can be distinguished when using NSI imaging opposed to rectangular apodization imaging, only the targets that where already visible show reduction in beamwidth and side lobe levels. When using NSI speckle SNR is reduced from 1.78 to on average 0.50 and seems to scale with energy present in the image. An hybrid image between the NSI image and rectangular apodization image is made to solve this. It results in reduced beamwidth, restored speckle SNR but the MSR is reduced. It can be concluded that the method works for phased array imaging under perfect conditions, the simulation. To get the full potential of the technique more research is needed towards the effects of improper sampling. Which in this case resulted in less then ideal ’sharp’ drops, lowering the reduction in beamwidth. Also the reduced speckle needs further research to understand it well. Once this is all achieved the next step to 3D imaging can be made. ... Read more

Currently a lot of effort is put into developing matrix arrays which allow for volumetric imaging and new applications. There are however multiple problems. Compared to arrays currently in use which have in the order of 128 elements, matrix arrays can easily contain 1000 to 10000 elements. If all elements would be connected independently, the cable would become very thick. Therefore, beamforming methods are required that can operate with fewer transmit and receive channels. Furthermore, the room for electronics on the chip is limited. So, the required electronics for the beamforming methods should be kept simple.

In this thesis we will propose beamforming methods that are able to operate with fewer channels. This will be done separately for the transmit and receive part, but they do in no way exclude each other. To be able to focus pulsed waves in transmit we propose a method based on Fresnel zone plates which are used in optics to focus continuous wave light. Our method only requires a single continuous-wave excitation signal to be present, which is connected and disconnected on demand to each element. We have evaluated our method with measurements and simulations. As compared to the conventional focusing method, the spatial resolution is not affected by our method, but the Contrast-to-Noise ratio is 5\% lower for shallow depths and up to 20\% lower deeper into the medium. Overall though, the differences were relatively small and so it is clear that our new focusing method works very well. If needed, better results can be obtained by trading in frame rate. In this case the results are almost indistinguishable from the conventional focusing method.

To solve the problem with image formation, we have developed a frequency domain two stage beamforming method for use with matrix arrays, which does not require all element data to be present. This has been done for two matrix types. For the first method we have confirmed with simulations that it performs similar to the respective results obtained with two 2D frequency domain two stage beamforming method that have already been experimentally verified. For the second method we have evaluated the performance with simulations and measurements. Our method was able to obtain a 25\% better spatial resolution as compared to Dynamic Receive Focusing(DRF), without additional artefacts. As an alternative to the last method, we have also developed a frequency domain beamforming method that does require all element data, but only requires a single insonification by a spherical wave. This method did perform worse than the method discussed before in both simulations and measurements, but it does outperform DRF applied to spherical wave data.
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