<p>The work presented in this thesis is devoted to studying aberration in ultrasound medical imaging, and to provide methods for correcting aberration of ultrasound signals in order to obtain optimum image quality. The thesis is composed of five chapters. All chapters may be read individually. The presented results are generated from simulations.</p><p>Chapter 1 presents a description of the aberration phenomenon, and a brief discussion of its medical and practical implications. A mathematical description of aberration is introduced by modelling the Green's function for propagation in a heterogeneous medium.</p><p>In Ch. 2, aberration from a point scatterer in the focus of an array is studied. Aberration is generated by two body wall models, generating weak and strong aberration, emulating the human abdominal wall. The results show that if correctly estimated, aberration can be close to ideally characterized by arrival time and amplitude fluctuations measured across the receive array. Using the arrival time and amplitude fluctuations in a time-delay and amplitude transmit aberration correction filter, produce close to ideal correction of the retransmitted beam. A point source represents a situation which is rarely found in medical ultrasound imaging.</p><p>A method for estimating aberration from random scatterers is developed in Ch. 3. The method is based on a cross-correlation analysis, and may in general estimate aberration at each frequency component of the received ultrasound signal. Due to the results from Ch. 2, the method is only investigated for a time-delay and amplitude estimate at the center frequency of the signal. The same aberrators as in Ch. 2 are used. The results show that the method does not produce satisfactory estimates of the arrival time and amplitude fluctuations for both aberrators. The backscatter in ultrasound imaging is determined by the width of the focused transmit beam used to obtain the image. Aberration widens the transmit beam, and the back-scattering region may become quite large. Since the human body wall has a certain thickness, the body wall itself generates interference of the signals propagating from different scatterers to the array. This smoothens aberration parameters such as arrival time and amplitude fluctuations, making proper estimation of these unfeasible.</p><p>Aberration correction is performed as a filter process prior to transmit of the ultrasound beam. This means that aberration estimation/correction methods model aberration as a filter, that is, all effects of aberration are assumed to be fully described in an infinitely thin layer at the array surface. For a point source, this assumption is fulfilled since the signal received on different array elements originates from the same spatial point. For a large scattering region this is generally not true, and the aberration described on a specific array element is dependent of the sum of aberrations generated along different propagation paths from each contributing scatterer. It is then impossible to obtain ideal aberration correction for a specific point in space (usually the focus of the array).</p><p>A solution to this problem may be sought by iteration of transmit-beam aberration correction (transmit-beam iteration). Transmit-beam iteration is described as a process where an uncorrected transmit-beam is used for an initial estimate of aberration parameters. A new beam with correction is then transmitted, generating a new estimate of the parameters. This process is repeated until some convergence criterion is met. The goal of this process is to reduce the width of the transmit beam, in order for the aberration on a specific receive element to be independent of the scatterers spatial position.</p><p>Transmit-beam iteration is studied in Ch. 4. Now, eight different aberrators are used, all emulating the human abdominal wall. Here, the estimator developed in Ch. 3 is compared with a similar type of estimator. New insight into the equalities and differences between the estimation methods are provided through transmit-beam iteration considerations. The results show that using a time-delay and amplitude aberration correction filter, both algorithms provide close to ideal aberration correction after two to three transmit-beam iterations for all aberrators. In addition, an earlier developed focus criterion proves to give accurate description of the point of convergence, and the accuracy of the correction.</p><p>The aberration estimation method described in Chapter. 3, was developed in the frequency domain. In Ch. 5, a time domain implementation is introduced. Necessary assumptions made in the time domain implementation makes the algorithm different from the frequency domain implementation.</p><p>Since the receive signal in ultrasound imaging is a stochastic variable, estimation of arrival time-delays and amplitudes at the array, is connected with uncertainty. A variance analysis of both the time and frequency domain implementations is performed.</p><p>There exists only minor differences between the two implementations with respect to variance. The variance in the estimates proved to be highly dependent upon the aberrator. Results also indicate that a transmit-beam iteration process converges, even if the variance in the initial estimate for the iteration process is very high.</p><p>In appendix A, a brief discussion of aberration as a function of frequency is provided.</p>

The work presented in this thesis is devoted to studying aberration in ultrasound medical imaging, and to provide methods for correcting aberration of ultrasound signals in order to obtain optimum image quality. The thesis is composed of five chapters. All chapters may be read individually. The presented results are generated from simulations. Chapter 1 presents a description of the aberration phenomenon, and a brief discussion of its medical and practical implications. A mathematical description of aberration is introduced by modelling the Green's function for propagation in a heterogeneous medium. In Ch. 2, aberration from a point scatterer in the focus of an array is studied. Aberration is generated by two body wall models, generating weak and strong aberration, emulating the human abdominal wall. The results show that if correctly estimated, aberration can be close to ideally characterized by arrival time and amplitude fluctuations measured across the receive array. Using the arrival time and amplitude fluctuations in a time-delay and amplitude transmit aberration correction filter, produce close to ideal correction of the retransmitted beam. A point source represents a situation which is rarely found in medical ultrasound imaging. A method for estimating aberration from random scatterers is developed in Ch. 3. The method is based on a cross-correlation analysis, and may in general estimate aberration at each frequency component of the received ultrasound signal. Due to the results from Ch. 2, the method is only investigated for a time-delay and amplitude estimate at the center frequency of the signal. The same aberrators as in Ch. 2 are used. The results show that the method does not produce satisfactory estimates of the arrival time and amplitude fluctuations for both aberrators. The backscatter in ultrasound imaging is determined by the width of the focused transmit beam used to obtain the image. Aberration widens the transmit beam, and the back-scattering region may become quite large. Since the human body wall has a certain thickness, the body wall itself generates interference of the signals propagating from different scatterers to the array. This smoothens aberration parameters such as arrival time and amplitude fluctuations, making proper estimation of these unfeasible. Aberration correction is performed as a filter process prior to transmit of the ultrasound beam. This means that aberration estimation/correction methods model aberration as a filter, that is, all effects of aberration are assumed to be fully described in an infinitely thin layer at the array surface. For a point source, this assumption is fulfilled since the signal received on different array elements originates from the same spatial point. For a large scattering region this is generally not true, and the aberration described on a specific array element is dependent of the sum of aberrations generated along different propagation paths from each contributing scatterer. It is then impossible to obtain ideal aberration correction for a specific point in space (usually the focus of the array). A solution to this problem may be sought by iteration of transmit-beam aberration correction (transmit-beam iteration). Transmit-beam iteration is described as a process where an uncorrected transmit-beam is used for an initial estimate of aberration parameters. A new beam with correction is then transmitted, generating a new estimate of the parameters. This process is repeated until some convergence criterion is met. The goal of this process is to reduce the width of the transmit beam, in order for the aberration on a specific receive element to be independent of the scatterers spatial position. Transmit-beam iteration is studied in Ch. 4. Now, eight different aberrators are used, all emulating the human abdominal wall. Here, the estimator developed in Ch. 3 is compared with a similar type of estimator. New insight into the equalities and differences between the estimation methods are provided through transmit-beam iteration considerations. The results show that using a time-delay and amplitude aberration correction filter, both algorithms provide close to ideal aberration correction after two to three transmit-beam iterations for all aberrators. In addition, an earlier developed focus criterion proves to give accurate description of the point of convergence, and the accuracy of the correction. The aberration estimation method described in Chapter. 3, was developed in the frequency domain. In Ch. 5, a time domain implementation is introduced. Necessary assumptions made in the time domain implementation makes the algorithm different from the frequency domain implementation. Since the receive signal in ultrasound imaging is a stochastic variable, estimation of arrival time-delays and amplitudes at the array, is connected with uncertainty. A variance analysis of both the time and frequency domain implementations is performed. There exists only minor differences between the two implementations with respect to variance. The variance in the estimates proved to be highly dependent upon the aberrator. Results also indicate that a transmit-beam iteration process converges, even if the variance in the initial estimate for the iteration process is very high. In appendix A, a brief discussion of aberration as a function of frequency is provided.

<p>Healthy infants in the age group of 9 to 20 weeks post term have a distinct movement pattern called fidgety movements. The abscense of, or anomalies in these movements are indications that the infant may suffer from Cerebral Palsy. Data from video clips of infants have been extracted and used as features for classifying these movements as normal or abnormal. An earlier project used the concept of motiongrams for extracting the centroid of motion and quantity of motion of the image from each frame of the video clips. This project attempts to improve the dataset by extracting the same features using the concept of optical flow. Using the classification methods that yielded the best results from the motiongram project, the new dataset based on optical flow was constructed and tested for comparison. The image was as before divided into four quadrants with a circle of varying radius in the middle. Tracking how the centroid of motion moved between the separated areas of the image generated a transition probability matrix. By classifying on the entropy and variance of this matrix, a sensitivity of 90,0% and a specificity of 94,6% was reached, which was an improvement compared to the best classification result based on motiongrams with a sensitivity of 90,0% and specificity of 86,6%. The overall performance of both methods are around a sensitivity of 90% and a specificity of 70%. Using motiongrams and optical flow to extract features from video data for classification has turned out to be a promising approach for a simple and non-invasive objective tool for diagnosing infants with Celebral Palsy. The movement information in an optical flow field does however have a much higher potential for generating discriminative features than just as the simple representation a centroid of motion actually is.</p>

<p>The presence of clustered micro-calcifications is an important indicator of early stage breast cancers, and an efficient real-time ultrasound imaging support is needed for conducting needle biopsies. SURF is a dual-frequency band ultrasound technique capable of imaging nonlinear scattering using the superposition of a low-frequency pulse to manipulate the tissue characteristics and an imaging pulse to extract the informations. This new technique has already demonstrated its superiority in imaging high intensity nonlinear scatterers like ultrasound contrast agents compared to the classical methods. However, imaging lower intensity nonlinear scatterers like micro-calcifications requires the use of higher manipulation pressures, causing the apparition of complex distortions of the imaging pulse neglected hitherto. In order to achieve a sufficient level of tissue suppression and detect the micro-calcifications using SURF imaging, these nonlinear forward propagation effects must be characterized and corrected for. In this thesis, simulations have been conducted based on the geometry of the first manufactured prototypes of SURF probes, named Viglen and Okla. The nonlinear propagation effects were described as the regroupment of a nonlinear propagation delay due the average level of LF manipulation pressure experienced by the HF pulse, a mix of compressions/expansions of the imaging pulse resulting from the fluctuations of the LF manipulation pressure over the HF pulse, and a phenomenon of SURF aberration due to the difference in the focusing and diffraction pattern of the HF and LF beams. The build-up of these nonlinear propagation effects were shown to be fully characterized by the combination of the profiles of the LF beam and the phase-relation between the two pulses. The different beamforming strategies, with the superposition of a focused HF beam on a plane or co-focused LF beam, were also discussed: a net trade-off appears between the minimization of delays and SURF aberrations ensured by the co-focal configurations, and the minimization of compressions/expansions effects by using plane LF beams. A strategy to limit the nonlinear propagation effects would be the use of an especially designed transducer providing an excellent HF focusing in the study range and a sufficiently large LF aperture. A filter was then designed and tested to ensure a correction of the pulse-form distortions in the received signals. The filter improved the level of suppression of the linear scattering from tissue by 15 dB in the numerical simulations, allowing a theoretical detection of the micro-calcifications and raising the resolution of the imaging system up to 1 dB close to the noise floor. The same filter demonstrated a smaller suppression of only 7 dB with the experiments conducted on phantoms, 2 dB below the limit required for detecting the smallest micro-calcifications. Further work studying the pulse-form correction filter should be conducted to understand this difference in performances, along with a real-time implementation on the scanner, before achieving a correct detection of micro-calcification using SURF imaging.</p>

<p>High quality images with strong contrast, good resolution and geometrical consistency are of crucial importance in magnetic resonance imaging, where the relatively low intrinsic sensitivity of MR methods places high demands on the imaging hardware. One of many key components in the imaging system is the radiofrequency coil, responsible for transmitting excitation signals and/or listening for response from the object. In this project a number of such coils are developed for specific applications (namely proton imaging of rat and fish brains), then evaluated against phantoms and empirically in simulated imaging situations. Evaluation of the produced coils shows promising initial results, with various opportunities for further refinement into a device suitable for regular use.</p>

Healthy infants in the age group of 9 to 20 weeks post term have a distinct movement pattern called fidgety movements. The abscense of, or anomalies in these movements are indications that the infant may suffer from Cerebral Palsy. Data from video clips of infants have been extracted and used as features for classifying these movements as normal or abnormal. An earlier project used the concept of motiongrams for extracting the centroid of motion and quantity of motion of the image from each frame of the video clips. This project attempts to improve the dataset by extracting the same features using the concept of optical flow. Using the classification methods that yielded the best results from the motiongram project, the new dataset based on optical flow was constructed and tested for comparison. The image was as before divided into four quadrants with a circle of varying radius in the middle. Tracking how the centroid of motion moved between the separated areas of the image generated a transition probability matrix. By classifying on the entropy and variance of this matrix, a sensitivity of 90,0% and a specificity of 94,6% was reached, which was an improvement compared to the best classification result based on motiongrams with a sensitivity of 90,0% and specificity of 86,6%. The overall performance of both methods are around a sensitivity of 90% and a specificity of 70%. Using motiongrams and optical flow to extract features from video data for classification has turned out to be a promising approach for a simple and non-invasive objective tool for diagnosing infants with Celebral Palsy. The movement information in an optical flow field does however have a much higher potential for generating discriminative features than just as the simple representation a centroid of motion actually is.

The presence of clustered micro-calcifications is an important indicator of early stage breast cancers, and an efficient real-time ultrasound imaging support is needed for conducting needle biopsies. SURF is a dual-frequency band ultrasound technique capable of imaging nonlinear scattering using the superposition of a low-frequency pulse to manipulate the tissue characteristics and an imaging pulse to extract the informations. This new technique has already demonstrated its superiority in imaging high intensity nonlinear scatterers like ultrasound contrast agents compared to the classical methods. However, imaging lower intensity nonlinear scatterers like micro-calcifications requires the use of higher manipulation pressures, causing the apparition of complex distortions of the imaging pulse neglected hitherto. In order to achieve a sufficient level of tissue suppression and detect the micro-calcifications using SURF imaging, these nonlinear forward propagation effects must be characterized and corrected for. In this thesis, simulations have been conducted based on the geometry of the first manufactured prototypes of SURF probes, named Viglen and Okla. The nonlinear propagation effects were described as the regroupment of a nonlinear propagation delay due the average level of LF manipulation pressure experienced by the HF pulse, a mix of compressions/expansions of the imaging pulse resulting from the fluctuations of the LF manipulation pressure over the HF pulse, and a phenomenon of SURF aberration due to the difference in the focusing and diffraction pattern of the HF and LF beams. The build-up of these nonlinear propagation effects were shown to be fully characterized by the combination of the profiles of the LF beam and the phase-relation between the two pulses. The different beamforming strategies, with the superposition of a focused HF beam on a plane or co-focused LF beam, were also discussed: a net trade-off appears between the minimization of delays and SURF aberrations ensured by the co-focal configurations, and the minimization of compressions/expansions effects by using plane LF beams. A strategy to limit the nonlinear propagation effects would be the use of an especially designed transducer providing an excellent HF focusing in the study range and a sufficiently large LF aperture. A filter was then designed and tested to ensure a correction of the pulse-form distortions in the received signals. The filter improved the level of suppression of the linear scattering from tissue by 15 dB in the numerical simulations, allowing a theoretical detection of the micro-calcifications and raising the resolution of the imaging system up to 1 dB close to the noise floor. The same filter demonstrated a smaller suppression of only 7 dB with the experiments conducted on phantoms, 2 dB below the limit required for detecting the smallest micro-calcifications. Further work studying the pulse-form correction filter should be conducted to understand this difference in performances, along with a real-time implementation on the scanner, before achieving a correct detection of micro-calcification using SURF imaging.

High quality images with strong contrast, good resolution and geometrical consistency are of crucial importance in magnetic resonance imaging, where the relatively low intrinsic sensitivity of MR methods places high demands on the imaging hardware. One of many key components in the imaging system is the radiofrequency coil, responsible for transmitting excitation signals and/or listening for response from the object. In this project a number of such coils are developed for specific applications (namely proton imaging of rat and fish brains), then evaluated against phantoms and empirically in simulated imaging situations. Evaluation of the produced coils shows promising initial results, with various opportunities for further refinement into a device suitable for regular use.

Et av de mer opplagte problemene som møter en person hvis arm har blitt erstattet med en protese, er reduksjonen i antall mulige bevegelser som en protese har i forhold til en frisk arm. Studier foretatt på friske armer, viser et større repertoar av bevegelser når håndleddets rotasjonsakse er stilt på skrå i forhold til underarmens lengdeakse. Et pågående forskningsprosjekt har som mål å undersøke om disse resultatene kan brukes til å hjelpe protesebrukere, og i den anledning har det blitt laget et håndledd med mulighet for vinkling av rotasjonsaksen.Gjennom bruk av en utviklingsmetodikk med navn V-modellen har denne oppgaven, som fortsettelse på tidligere arbeid, forsøkt å lage et fungerende styresystem for håndleddet. Underveis i arbeidet har utviklingsmetodikkens faser blitt fulgt, og all nødvendig dokumentasjon og planer for testing har blitt testet.Maskinvaren til håndleddets styresystem har blitt modularisert og designet for å håndtere sine tilordnede oppgaver. Kretsskjema for hver modul ble tegnet og lagt ut på flere kretskort som deretter kunne stables for å realiserte en prototyp av hele systemet. Programvare for hver av maskinvarens moduler har også blitt skrevet.Av testingen som har blitt gjennomført, vises det at den realiserte prototypen tilfredsstiller kravene fra funksjons- og teknisk spesifikasjon.

Invasive measurements of cardiac functioning allows for more accurate measures of cardiac functioning than non-invasive measurements. However, invasive measurements is often not available in clinical settings. By comparing invasive and non-invasive measurements collected in an experimental context, better relationships between non-invasive measurements and cardiac functioning may be found. This master thesis describes the development of two computer applications for simultaneous acquisition, calibration and synchronization of these measurements. The developed applications were tested out during operations on pigs with all measurement sources connected. The results shows that all the desired measurements were successfully acquired by the system. Calibration of the different measurement were also achieved. Different methods for synchronization were tested out during the experiments. It was possible to achieve synchronization of all clocks present in the system. Finally, all of the desired parameters were calculated.