The primary objective of antenatal assessment and monitoring is to ensure wellbeing of the fetus and the mother. There are different methods of assessment during pregnancy and in labour. Doppler ultrasound is one of the tests widely used in clinical practice in the evaluation of pregnancies that are at a greater risk of developing maternal or fetal complications due to uteroplacental insufficiency. Doppler ultrasound enables evaluation of sequential changes in circulatory haemodynamics in the fetus by evaluation of the fetus for signs of brain sparing and severity of redistribution of circulation. Recognition of abnormal Doppler flow patterns helps the clinician to optimise the appropriate timing of delivery. Identification of the ‘high risk’ fetus, before any changes of fetal compromise become evident, still remains one of the major dilemmas in contemporary clinical practice. This thesis seeks to explore the role of Doppler monitoring fetal intrauterine central shunts as a method of identifying the ‘high-risk’ fetus before any other established parameters, such as, fetal biometry, fetal weight or flow waveforms in umbilical artery become abnormal. This thesis also evaluates the role of serial Doppler monitoring of fetal central shunts in those fetuses where IUGR has been established. This is based on the premise that the intrauterine shunts are present in fetal circulation to work closely with the placenta to ensure appropriate nutrition and oxygenation of the fetus, bypassing the lungs. Four prospective longitudinal studies were designed to evaluate the role of fetal intrauterine shunts in adaptive response mechanisms in cardiovascular stress. Two models were taken into consideration: an ‘acute cardiovascular stress’ model and a ‘chronic cardiovascular stress’ model. To study the ‘response to acute cardiovascular stress’ in high-risk fetuses, a cohort of mothers undergoing fetal intrauterine transfusion for fetal anaemia were selected. These fetuses were scanned immediately before and after transfusion, and Doppler flows through all the intrauterine shunts were documented and compared with fetoplacental and cerebral circulation. To study the ‘response to chronic cardiovascular stress’, a prospective longitudinal observational study was designed and the sequence of changes in Doppler ultrasound of the fetal central shunts studied and compared with the Doppler flow waveforms of normal pregnancies with a group of pregnancies complicated by uteroplacental insufficiency. Normograms were designed for all the Doppler parameters and flows from adverse pregnancy outcomes were compared to the normogram. The pregnancy outcomes in the longitudinal study were correlated with placental pathology. Our study showed that although changes were demonstrated in the flow patterns within central shunts, these changes were not statistically significant in the ‘acute cardiovascular stress model’, suggesting that there may be other haemodynamic alterations in acute cardiovascular stress. However, in the ‘chronic cardiovascular stress model’, the results suggest that the intrauterine cardiac shunts may play an important role in redistribution of fetal flows in early stages of growth restriction, suggesting that Doppler ultrasound monitoring of foramen ovale can be potentially used as a screening tool to identify high-risk fetuses as early as 16 weeks. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1349883 / Thesis (Ph.D.) - University of Adelaide, School of Paediatrics and Reproductive Health, 2009

Doppler ultrasound imaging modalities arguably represent one of the most complex task performed (usually in real time) by ultrasound scanners. At the heart of these techniques lies the ability to detect and estimate soft tissues or blood motion within the human body. As they have become an invaluable tool in a wide range of clinical applications, these techniques have fostered an intensive effort of research in the field of signal processing for more than thirty years, with a push towards more accurate velocity or displacement estimation. Coded excitation has recently received a growing interest in the medical ultrasound community. The use of these techniques, originally developed in the radar field, makes it possible to increase the depth of penetration in B-mode imaging, while complying with safety standards. These standards impose strict limits on the peak acoustic intensity which can be transmitted into the body. Similar solutions were proposed in the early developments of Doppler flow-meters to improve the resolution / sensitivity trade-off from which typical pulsed Doppler systems suffer. This work discusses the potential improvements in resolution, sensitivity and accuracy achievable in the context of modern Doppler ultrasound imaging modalities (taken in its broadest sense, that is, all the techniques involving the estimation of displacements, or velocities). A theoretical framework is provided for discussing this potential improvements, along with simulations for a more quantitative assessment. Colour Flow Imaging (CFI) modalities are taken as the main reference technique for discussion, due to their historical importance, and their relevance in many clinical applications. The potential achievable improvement in accuracy is studied in the context of modern velocity estimation strategies, which can be broadly classified into narrowband estimators (such as the “Kasai” estimator still widely used in CFI) and time shift based wideband strategies (normalised crosscorrelation estimator used, for instance, in applications like strain or strain rate estimation, elastography, etc.). Finally, simulations and theoretical results are compared to experimental data obtained with a simple custom-designed experimental set-up, using a single-element transducer.

INTRODUÇÃO: O tratamento estrogênico isolado e contínuo por seis meses é uma opção no tratamento de sintomas climatéricos. A monitorização endometrial deve ser realizada rotineiramente; nela, a ultra-sonografia (US) e a biópsia uterina têm papel importante. A US e a Dopplervelocimetria também avaliam as mudanças circulatórias uterinas. OBJETIVO: Estudar o comportamento da vascularização uterina e do endométrio em mulheres na pós-menopausa tratadas com estrógeno contínuo por seis meses, seguido de progestógeno isolado por 14 dias, e estabelecer suas relações com a proliferação endometrial. MÉTODO: Estudo clínico, prospectivo e controlado, onde quarenta mulheres na pós-menopausa, sem contraindicações para tratamento hormonal (TH). Foram divididas em dois grupos: Estrógeno e Controle. As do Grupo Estrógeno (GE), n= 24, receberam 50 mcg de estradiol-17 beta (E2) transdérmico, duas vezes por semana, durante seis meses. As mulheres do Grupo Controle (GC), n=16, não receberam TH. Todas realizaram FSH, E2 e glicemia de jejum; US transvaginal; Dopplervelocimetria das artérias uterinas, miometriais e endometriais e biópsia aspirativa de endométrio. O GE repetiu os mesmos exames, com exceção de FSH, E2 e glicemia, no terceiro e no sexto mês de tratamento. No GC, a biópsia do endométrio foi repetida apenas no sexto mês de tratamento. As mulheres do GE utilizaram, ao fim de seis meses, 10 mg de acetato de medroxiprogesterona por dia, durante 14 dias. RESULTADOS: No GE, a resistência vascular das artérias uterinas diminuiu no terceiro e no sexto mês de tratamento. O fluxo miometrial das artérias arqueadas aumentou significantemente no sexto mês de tratamento. O aumento da espessura do endométrio ocorreu de forma significante no terceiro mês. No GE houve hiperplasia endometrial simples e sem atipias em 20,8 % das mulheres. No GE, comparando as mulheres que tiveram proliferação com aquelas que mantiveram a atrofia endometrial, observou-se que, no sexto mês de tratamento, o grupo que apresentou proliferação teve diminuição significante da resistência vascular da artéria uterina esquerda, enquanto no grupo que manteve a atrofia, a resistência vascular aumentou na artéria uterina direita. No GC não ocorreu variação da resistência vascular das artérias uterinas bilaterais; o fluxo miometrial das artérias arqueadas não se modificou e não houve proliferação endometrial. CONCLUSÃO: A terapia estrogênica isolada por seis meses diminuiu a resistência vascular das artérias uterinas bilateralmente. A proliferação endometrial precedeu o aumento de vascularização miometrial. Houve associação entre a proliferação endometrial e a diminuição da resistência vascular na artéria uterina esquerda, no final do sexto mês de tratamento estrogênico / INTRODUCTION: Isolated continuous estrogen therapy for 6 months is an option to manage climacteric symptoms. Endometrial monitoring should be performed as a routine, in which ultrasound and uterine biopsy have an important role. Ultrasound with Dopplervelocimetry also assesses uterine circulatory changes. OBJECTIVE: To study the uterine circulatory changes of women in continuous estrogen therapy for 6 months using Doppler velocimetry and to define correlations with endometrial proliferation. METHOD: Clinical prospective controlled study. Forty menopause women were studied, without contraindications to hormone therapy (HT). They were divided into 2 groups: Estrogen and Control. In the Estrogen Group (EG) n = 24, they were treated with transdermal 50mcg estradiol-17 beta (E2), changed twice a week for 6 months. Women in the Control Group (CG) n=16, were not treated with hormones. They all underwent FSH, E2, fast glucose, transvaginal ultrasound , uterine, myometrial and endometrial artery Dopplervelocimetry and aspiration biopsy of endometrium. The EG repeated the same procedures in months 3 and 6 of treatment. In CG, endometrial biopsy was repeated only in the 6th month of treatment. At the end of treatment, EG women received 10 mg of medroxyprogesterone acetate per day for 14 days. RESULTS: In EG, vascular resistance of uterine arteries reduced in the 3rd and 6th months of treatment. Myometrial flow of arcuate arteries was significantly increased in the 6th month of treatment. Increased endometrial thickness was significant in the 3rd month. In EG, the authors detected simple endometrial hyperplasia without atypias in 20.8% of the subjects. In EG, in the 6th month of treatment, upon comparing women who had proliferation and those who maintained the endometrial atrophy, we observed that the group that presented proliferation had significant reduction of vascular resistance of left uterine artery, whereas the group that maintained atrophy had increase in vascular resistance of right uterine artery. In CG, there was no vascular resistance modification, no myometrial flow diference and no endometrial proliferation. CONCLUSION: Isolated estrogen therapy for 6 months reduced vascular resistance of bilateral uterine arteries. Morphological affections to the endometrium preceded myometrial vascular abnormalities. There was association of endometrial proliferation and reduction of vascular resistance of the left uterine artery in the 6th month of estrogen treatment

INTRODUÇÃO: O tratamento estrogênico isolado e contínuo por seis meses é uma opção no tratamento de sintomas climatéricos. A monitorização endometrial deve ser realizada rotineiramente; nela, a ultra-sonografia (US) e a biópsia uterina têm papel importante. A US e a Dopplervelocimetria também avaliam as mudanças circulatórias uterinas. OBJETIVO: Estudar o comportamento da vascularização uterina e do endométrio em mulheres na pós-menopausa tratadas com estrógeno contínuo por seis meses, seguido de progestógeno isolado por 14 dias, e estabelecer suas relações com a proliferação endometrial. MÉTODO: Estudo clínico, prospectivo e controlado, onde quarenta mulheres na pós-menopausa, sem contraindicações para tratamento hormonal (TH). Foram divididas em dois grupos: Estrógeno e Controle. As do Grupo Estrógeno (GE), n= 24, receberam 50 mcg de estradiol-17 beta (E2) transdérmico, duas vezes por semana, durante seis meses. As mulheres do Grupo Controle (GC), n=16, não receberam TH. Todas realizaram FSH, E2 e glicemia de jejum; US transvaginal; Dopplervelocimetria das artérias uterinas, miometriais e endometriais e biópsia aspirativa de endométrio. O GE repetiu os mesmos exames, com exceção de FSH, E2 e glicemia, no terceiro e no sexto mês de tratamento. No GC, a biópsia do endométrio foi repetida apenas no sexto mês de tratamento. As mulheres do GE utilizaram, ao fim de seis meses, 10 mg de acetato de medroxiprogesterona por dia, durante 14 dias. RESULTADOS: No GE, a resistência vascular das artérias uterinas diminuiu no terceiro e no sexto mês de tratamento. O fluxo miometrial das artérias arqueadas aumentou significantemente no sexto mês de tratamento. O aumento da espessura do endométrio ocorreu de forma significante no terceiro mês. No GE houve hiperplasia endometrial simples e sem atipias em 20,8 % das mulheres. No GE, comparando as mulheres que tiveram proliferação com aquelas que mantiveram a atrofia endometrial, observou-se que, no sexto mês de tratamento, o grupo que apresentou proliferação teve diminuição significante da resistência vascular da artéria uterina esquerda, enquanto no grupo que manteve a atrofia, a resistência vascular aumentou na artéria uterina direita. No GC não ocorreu variação da resistência vascular das artérias uterinas bilaterais; o fluxo miometrial das artérias arqueadas não se modificou e não houve proliferação endometrial. CONCLUSÃO: A terapia estrogênica isolada por seis meses diminuiu a resistência vascular das artérias uterinas bilateralmente. A proliferação endometrial precedeu o aumento de vascularização miometrial. Houve associação entre a proliferação endometrial e a diminuição da resistência vascular na artéria uterina esquerda, no final do sexto mês de tratamento estrogênico / INTRODUCTION: Isolated continuous estrogen therapy for 6 months is an option to manage climacteric symptoms. Endometrial monitoring should be performed as a routine, in which ultrasound and uterine biopsy have an important role. Ultrasound with Dopplervelocimetry also assesses uterine circulatory changes. OBJECTIVE: To study the uterine circulatory changes of women in continuous estrogen therapy for 6 months using Doppler velocimetry and to define correlations with endometrial proliferation. METHOD: Clinical prospective controlled study. Forty menopause women were studied, without contraindications to hormone therapy (HT). They were divided into 2 groups: Estrogen and Control. In the Estrogen Group (EG) n = 24, they were treated with transdermal 50mcg estradiol-17 beta (E2), changed twice a week for 6 months. Women in the Control Group (CG) n=16, were not treated with hormones. They all underwent FSH, E2, fast glucose, transvaginal ultrasound , uterine, myometrial and endometrial artery Dopplervelocimetry and aspiration biopsy of endometrium. The EG repeated the same procedures in months 3 and 6 of treatment. In CG, endometrial biopsy was repeated only in the 6th month of treatment. At the end of treatment, EG women received 10 mg of medroxyprogesterone acetate per day for 14 days. RESULTS: In EG, vascular resistance of uterine arteries reduced in the 3rd and 6th months of treatment. Myometrial flow of arcuate arteries was significantly increased in the 6th month of treatment. Increased endometrial thickness was significant in the 3rd month. In EG, the authors detected simple endometrial hyperplasia without atypias in 20.8% of the subjects. In EG, in the 6th month of treatment, upon comparing women who had proliferation and those who maintained the endometrial atrophy, we observed that the group that presented proliferation had significant reduction of vascular resistance of left uterine artery, whereas the group that maintained atrophy had increase in vascular resistance of right uterine artery. In CG, there was no vascular resistance modification, no myometrial flow diference and no endometrial proliferation. CONCLUSION: Isolated estrogen therapy for 6 months reduced vascular resistance of bilateral uterine arteries. Morphological affections to the endometrium preceded myometrial vascular abnormalities. There was association of endometrial proliferation and reduction of vascular resistance of the left uterine artery in the 6th month of estrogen treatment

The incidence of disease of the cardiovascular system is very high and increasing worldwide, especially in the developing world. The radial and ulnar arteries are implicated in some important ailments where blood flow related parameters such as flow rate (FR), wall shear rate (WSR), arterial wall motion (AWM) and pressure, all of which can be measured using ultrasound techniques, are useful in diagnosis and patient management. However these measurements are prone to error due to the manner of image formation and the complex flow conditions within the vessels. In this thesis, the errors in ultrasound-measured parameters in the radial and ulnar arteries are investigated using experimental phantoms, computer simulation and on volunteers. Using the Womersley theory, FR and WSR were estimated using a clinical ultrasound scanner with the pulsed wave (PW) mode and B mode. Experimental flow phantoms were designed to evaluate those measurements under different circumstances. A simulation technique which combined image-based computational fluid dynamics and ultrasound simulation was also used to evaluate ultrasound estimation of these parameters. A case study was then conducted on healthy volunteers to evaluate the method of measuring FR and WSR in-vivo. For the AWM in the radial artery, an auto-correlation method was used based on the radio-frequency (RF) data and validations were done by a flow phantom, simulation, and in-vivo trial. The blood pressure waveform in a volunteer’s radial artery was derived from the ultrasound measured AWM and compared with the waveform from a tonometry. FR and WSR were both found to be overestimated by up to 50%, mainly due to the beam-vessel angle in the PW Doppler ultrasound. Measurement of the vessel diameter and assumption of the blood flow direction can also influence the estimations. Other factors, such as flow amplitude, vessel size, imaging depth and flow waveforms, do not seem to affect the estimation of these two parameters. Results taken from the flow phantoms agree with those from simulation and the estimations from the in-vivo case study also agree with the published data. The auto-correlation method for the AWM was validated from the phantom and simulation. It is able to detect motion amplitude of about tens of micrometres. The trial on volunteers proved the feasibility of this motion detection method. Blood pressure waveforms at the radial artery of a volunteer, derived from this ultrasound-measured wall motion and from the tonometry, were very similar. The Womersley-based method is able to estimate the FR and WSR in the radial and ulnar arteries with high accuracy. Sources of the error and their magnitudes in estimation of the two parameters by ultrasound pointed out in this thesis are beam-vessel angle, vessel diameter measurement and flow direction assumption. Researchers and clinicians using these measurements in practice and research should be aware. The capability of ultrasound imaging to measure arterial AWM in the radial artery is demonstrated and it is found that the blood pressure waveform can also be derived from the arterial AWM.

Mixing is a crucial process for many large scale and small scale applications from food industry to cosmetics, from drug industry to petrochemical processes, etc. Changes in parameters (temperature, viscosity, velocity distribution, etc.) during the mixing affect the production process and the end product quality and the cost. Thus, these parameters, mostly the hydrodynamic parameters, should be monitored closely during the process. In order to ensure good and efficient mixing in the solution, high degree of turbulence is maintained while dead zones in the tank should be avoided. In chemical industry, the mixing processes generally involve complex solutions that exhibit non-Newtonian flow behavior that merits a study on the agitation hydrodynamics and mixing time. Thus, in this study agitation of carboxymethyl cellulose (CMC) solution in a laboratory scale mixing tank is investigated. The effects of CMC concentration and agitation speed on the hydrodynamics of the solution and mixing time are studied in detail. CMC concentrations studied are 0.5 wt%, 1 wt% and 2 wt%. Impeller speeds, on the other hand, are set as 150 rpm, 300 rpm and 600 rpm. The hydrodynamics of mixing can be studied easily by Ultrasound Doppler Velocimetry (UDV) which is a fast, non-invasive measuring technique in fluid dynamics. Also, the mixing time measurements were carried out through electrical conductivity of the agitated solution. UDV results show that the flow field has a typical pattern produced by the Rushton turbine. The main characteristics of the flow are that, in the impeller region radial components of the flow dominate. Near the wall flow occurs mainly in the axial direction towards the top and bottom of the tank. Mixing time measurements reveal that mixing time increases with decreasing impeller speed and with increasing solution concentration (i.e. viscosity). Typical mixing time values are in the range of 250-2600 seconds for different impeller speeds and CMC concentrations.

Since the discovery of the drag reduction effects of even small amount of macromolecules in solutions in turbulent pipe flows, there have been many experimental and theoretical studies in order to understand mechanisms behind this phenomenon. Theories have been proposed based on the observations on the change in the characteristics of the turbulent flow near the pipe wall where friction of the momentum transfer between the flow and the conduit takes place. In this study drag reduction in fully developed turbulent pipe flow with four concentrations (200 to 500 wppm) of low molecular weight Sodium Carboxymethylcellulose (CMC) in aqueous solutions was investigated experimentally. Drag reduction was determined by pressure drop measurements. In order to observe the impact of the presence of CMC on the flow, Ultrasound Doppler Velocimetry (UDV) was employed to monitor the instantaneous velocity distributions. UDV is a non-invasive technique allowing one to obtain quick velocity profiles. Experimental measurements were used to calculate Fanning friction factor and radial distributions of the axial time-averaged velocity, velocity fluctuation (turbulent intensity) and eddy viscosity. The drag reduction level was determined through the Fanning friction factor versus Reynolds number data. Velocity data could be obtained as close as 3 mm to the wall by UDV. Two impacts of increasing CMC concentration on the flow field, hence pressure drop, were observed. The first effect was the decrease of the mean velocity gradient especially near the wall with increasing polymer amount which in turn gave rise to lower friction factor or pressure drop. In addition smaller eddy viscosities were obtained in the flow. The second impact of the polymer addition was on the velocity fluctuation or turbulent intensity variation along the radial distribution. An increasing trend in turbulence intensity in the turbulent core with polymer addition was observed. This was in agreement with the earlier studies in which similar turbulence behavior was observed in addition to the suppression of the turbulent intensities near the wall

This thesis presents the investigations conducted in the use of ultrasonic technology to measure two-phase flow in both horizontal and vertical pipe flows which is important for the petroleum industry. However, there are still key challenges to measure parameters of the multiphase flow accurately. Four methods of ultrasonic technologies were explored. The Hilbert-Huang transform (HHT) was first applied to the ultrasound signals of air-water flow on horizontal flow for measurement of the parameters of the two- phase slug flow. The use of the HHT technique is sensitive enough to detect the hydrodynamics of the slug flow. The results of the experiments are compared with correlations in the literature and are in good agreement. Next, experimental data of air-water two-phase flow under slug, elongated bubble, stratified-wavy and stratified flow regimes were used to develop an objective flow regime classification of two-phase flow using the ultrasonic Doppler sensor and artificial neural network (ANN). The classifications using the power spectral density (PSD) and discrete wavelet transform (DWT) features have accuracies of 87% and 95.6% respectively. This is considerably more promising as it uses non-invasive and non-radioactive sensors. Moreover, ultrasonic pulse wave transducers with centre frequencies of 1MHz and 7.5MHz were used to measure two-phase flow both in horizontal and vertical flow pipes. The liquid level measurement was compared with the conductivity probes technique and agreed qualitatively. However, in the vertical with a gas volume fraction (GVF) higher than 20%, the ultrasound signals were attenuated. Furthermore, gas-liquid and oil-water two-phase flow rates in a vertical upward flow were measured using a combination of an ultrasound Doppler sensor and gamma densitometer. The results showed that the flow gas and liquid flow rates measured are within ±10% for low void fraction tests, water-cut measurements are within ±10%, densities within ±5%, and void fractions within ±10%. These findings are good results for a relatively fast flowing multiphase flow.

Currently there is no test to accurately predict stillbirth. It is proposed that better identification of placental disease in utero may aid stillbirth prediction and prevention. Pregnancies complicated by reduced fetal movement (RFM) have increased risk of stillbirth. We hypothesised that RFM is a symptom of placental dysfunction associated with adverse pregnancy outcome (APO) and that this placental abnormality can be detected antenatally and used to identify fetuses at highest-risk of APO. We tested this hypothesis by: 1) comparison of ex vivo placental structure and function between APO RFM pregnancies and their normal outcome RFM counterparts, 2) comparison of in utero estimates of placental size, vascularity, vascular and endocrine functions obtained from placental ultrasound, Doppler waveform analysis and maternal circulating placentally-derived hormone concentrations, to their ex vivo correlates and 3) examination of the predictive potential of placental biomarkers at the time of RFM.Ex vivo placentas from APO RFM pregnancies, compared to normal outcome RFM counterparts, were smaller (diameter, area, weight and volume, p<0.0001), less vascular (vessel number and density, p≤0.002), with arteries that were less responsive to sodium nitroprusside (p<0.05), and with aberrant endocrine function (reduced tissue content and/or release of human chorionic gonadotrophin (hCG), human placental lactogen (hPL) and soluble fms-like Tyrosine Kinase-1 (sFlt-1), p<0.03). Placental volume (PV) ex vivo correlated with sonographic estimated PV (p<0.004), hPL, hCG and placental growth factor (PlGF) concentrations in the maternal circulation (p<0.03). Ex vivo villous vessel number and density correlated with Doppler impedance at the umbilical artery free-loop (UAD-F, p=0.02) and intraplacental arteries (p<0.0001) respectively, whilst UAD-F impedance correlated with arterial thromboxane sensitivity (p<0.04). Examination of placental structure and function at the time of presentation with RFM identified 15 independently-predictive biomarkers. Three potential predictive models, incorporating measures of placental size (PlGF), endocrine function (sFlt-1), arterial thromboxane sensitivity and villous vascularity (UAD-F), were proposed. Using these models, sensitivity for APO was improved from 8.9% with baseline care (assessment of fetal size and gestation) to up to 37.5% at a fixed specificity of 99% (p<0.05). This series of studies shows that antenatal placental examination is possible and improves identification of pregnancies at highest risk of stillbirth in a high-risk population by up to 29%. Therefore such tests merit further development to prospectively assess their ability to predict and prevent stillbirth itself.

Pulsed ultrasound Doppler velocimetry proved to be capable of measuring velocities accurately (relative error less than 0.5 percent). In this research, the limitations of the method are investigated when measuring: in channels with a small thickness compared to the transducer diameter, at low velocities and in the presence of a flow reversal area. A review of the fundamentals of pulsed ultrasound Doppler velocimetry reveals that the accuracy of the measured velocity field mainly depends on the shape of the acoustic beam through the flow field and the intensity of the echo from the incident particles where the velocity is being measured. The ultrasonic transducer turned out to be most critical component of the system. Fundamental limitations of the method are identified. With ultrasonic beam measurements, the beam shape and echo intensity is further investigated. In general, the shape of the ultrasonic beam varies depending on the frequency and diameter of the emitter as well as the characteristics of the acoustic interface that the beam encounters. Moreover, the most promising transducer to measure velocity profiles in small channels is identified. Since the application of pulsed ultrasound Doppler velocimetry often involves the propagation of the ultrasonic burst through Plexiglas, the effect of Plexiglas walls on the measured velocity profile is analyzed and quantified in detail. The transducers ringing effect and the saturation region caused by highly absorbing acoustic interfaces are identified as limitations of the method. By comparing measurement results in the small rectangular channel to numerically calculated results, further limitations of the method are identified. It was not possible to determine velocities correctly throughout the whole channel at low flow rates, in small geometries and in the flow separation region. A discrepancy between the maximum measured velocity, velocity profile perturbations and incorrect velocity determination at the far channel wall were main shortcomings. Measurement results are improved by changes in the Doppler angle, the flow rate and the particle concentration. Suggestions to enhance the measurement system, especially its spatial resolution, and to further investigate acoustic wave interactions are made.