Matthew R. Rielly

Spatial coherence of the nonlinearly generated second harmonic portion of backscatter for a clinical imaging system

Correlation-based approaches to phase aberration correction rely on the spatial coherence of backscattered signals. The spatial coherence of backscatter from speckle-producing targets is predicted by the auto correlation of the transmit apodization (Van Cittert-Zernike theorem). Work by others indicates that the second harmonic beam has a wider mainlobe with lower sidelobes than a beam transmitted at 2f. The purpose of this paper is to demonstrate that the spatial coherence of backscatter for the second harmonic is different from that of the fundamental, as would be anticipated from applying the Van Cittert-Zernike theorem to the reported measurements of the second harmonic field. Another objective of this work is to introduce the concept of the effective apodization and to verify that the effective apodization of the second harmonic is narrower than the transmit apodization. The spatial coherence of backscatter was measured using three clinical arrays with a modified clinical imaging system. The spatial coherence results were verified using a pseudo-array scan in a transverse plane of the transmitted field with a hydrophone. An effective apodization was determined by backpropagating these values using a linear angular spectrum approach. The spatial coherence for the harmonic portion of backscatter differed systematically and significantly from the auto correlation of the transmit apodization.

A Low-Cost Ultrasound Program Leads to Increased Antenatal Clinic Visits and Attended Deliveries at a Health Care Clinic in Rural Uganda

Background In June of 2010, an antenatal ultrasound program to perform basic screening for high-risk pregnancies was introduced at a community health care center in rural Uganda. Whether the addition of ultrasound scanning to antenatal visits at the health center would encourage or discourage potential patients was unknown. Our study sought to evaluate trends in the numbers of antenatal visits and deliveries at the clinic, pre- and post-introduction of antenatal ultrasound to determine what effect the presence of ultrasound at the clinic had on these metrics. Methods and Findings Records at Nawanyago clinic were reviewed to obtain the number of antenatal visits and deliveries for the 42 months preceding the introduction of ultrasound and the 23 months following. The monthly mean deliveries and antenatal visits by category (first visit through fourth return visit) were compared pre- and post- ultrasound using a Kruskal-Wallis one-way ANOVA. Following the introduction of ultrasound, significant increases were seen in the number of mean monthly deliveries and antenatal visits. The mean number of monthly deliveries at the clinic increased by 17.0 (13.3–20.6, 95% CI) from a pre-ultrasound average of 28.4 to a post-ultrasound monthly average of 45.4. The number of deliveries at a comparison clinic remained flat over this same time period. The monthly mean number of antenatal visits increased by 97.4 (83.3–111.5, 95% CI) from a baseline monthly average of 133.5 to a post-ultrasound monthly mean of 231.0, with increases seen in all categories of antenatal visits. Conclusions The availability of a low-cost antenatal ultrasound program may assist progress towards Millennium Development Goal 5 by encouraging women in a rural environment to come to a health care facility for skilled antenatal care and delivery assistance instead of utilizing more traditional methods.

Low-Cost Teleradiology for Rural Ultrasound

In under-resourced communities, there are several preventable pregnancy complications that can lead to significant maternal or perinatal morbidity and mortality when left untreated. The lack of access to prenatal imaging is one of many factors that contribute to the greater mortality rates in such populations. Analysis of the most common complications suggest that even limited access to obstetric ultrasound imaging could have a positive impact, particularly in a rural context in which imaging can prompt transport of the patient to a capable facility. The authors describe their work to establish and validate an obstetric care model in Uganda, and the design and for a clinical study to measure this models efficacy in terms of improving outcomes at delivery. The central objective of this care model is to reduce mortality in remote locations by providing expectant mothers and their primary care-givers with advance notice of complications so that women at risk can be referred to appropriate care centers in time. The components of this model are described, including portable ultrasound machine, scanning protocols for clinical data acquisition by local operators, custom clinical data compression and transmission capabilities, and internet-based infrastructure for remote reading and reporting. The objectives, hypotheses, and design of the associated clinical outcomes study and progress to date are also presented.

Spatial coherence of backscatter for the nonlinearly produced second harmonic for specific transmit apodizations

To be successful, correlation-based, phase-aberration correction requires a high correlation among backscattered signals. For harmonic imaging, the spatial coherence of backscatter for the second harmonic component is different than the spatial coherence of backscatter for the fundamental component. The purpose of this work was to determine the effect of changing the transmit apodization on the spatial coherence of backscatter for the nonlinearly generated second harmonic. Our approach was to determine the effective apodizations for the fundamental and second harmonic using both experimental measurements and simulations. Two-dimensional measurements of the transverse cross sections of the finite-amplitude ultrasonic fields generated by rectangular and circular apertures were acquired with a hydrophone. Three different one-dimensional transmit apodization functions were investigated: uniform, Riesz, and trapezoidal. An effective apodization was obtained for each transmit apodization by backpropagating the values measured from within the transmit focal zone using a linear angular spectrum approach. Predictions of the spatial coherence of backscatter were obtained using the pulse-echo Van Cittert-Zernike theorem. In all cases the effective apodization at 2f was narrower than the transmit apodization. We demonstrate that certain transmit apodizations result in a greater spatial coherence of backscatter at the second harmonic than at the fundamental.

Effect of changing the transmit aperture on the spatial coherence of backscatter for the nonlinearly generated second harmonic

The present work measures the effective apodizations for the fundamental and second harmonic and uses the Van Cittert-Zernike theorem to predict the spatial coherence of the second harmonic portion of backscatter. Two-dimensional pseudo-array scans of a transverse cross section of the finite amplitude ultrasonic fields generated by rectangular and circular apertures were performed with a hydrophone. Three transmit apodization functions were investigated: rectangular, Riesz, and trapezoidal. An effective apodization was obtained by backpropagating the values measured from within the transmit focal zone using a linear angular spectrum approach. In all cases the effective apodization at 2f was narrower than the transmit apodization. Our results demonstrate that choices of apodization can be identified that yield better spatial coherence at the second harmonic than at the fundamental.

Statistically significant differences in the spatial coherence of backscatter for fundamental and harmonic portions of a clinical beam

Correlation-based approaches to phase aberration correction rely on the spatial coherence of backscattered signals. The spatial coherence of backscatter was measured using a clinical linear array with a modified clinical imaging system (ATL HDI 5000). The spatial coherence results were verified using a 14 mm/spl times/14 mm pseudo-array scan in a transverse plane of the transmitted beam with a 0.6 mm hydrophone. An effective apodization was determined by backpropagating these values using a linear angular spectrum approach. The effective apodizations were compared with the spatial coherence measurements using the Van Cittert-Zernike theorem. The spatial coherence for the fundamental beam exhibited good agreement with the autocorrelation of the transmit apodization. The spatial coherence for the harmonic differed systematically and statistically from the autocorrelation of the transmit apodization. Additionally, our experimental results verify that the effective apodization of the nonlinearly-generated harmonic beam is more aggressive than the transmit apodization.

A theoretical and experimental investigation of nonlinear ultrasound propagation through tissue mimicking fluids

A numerical model is used to investigate finite amplitude ultrasound propagation through multiple layers of tissue-like media. This model uses a finite difference method to solve the nonlinear parabolic KZK wave equation. The code is modified to include an arbitrary frequency dependence of absorption and transmission effects for wave propagation across a plane interface at normal incidence. Measurements are taken of the axial nonlinear pressure field generated from a circular focused, 2.25 MHz source, through single- and multiple-layered tissue mimicking fluids. Two tissue mimicking fluids are developed to show acoustic properties similar to amniotic fluid and a typical soft tissue. The measured values of the nonlinearity parameter sound velocity and frequency dependence of absorption for both fluids are presented. These acoustic parameters, in addition to the measurement of the source conditions, are input to the numerical model allowing the experimental conditions to be simulated. Comparisons are made between the model predictions and the axial pressure field measurements. Results are presented in the frequency domain showing the fundamental and three subsequent harmonic amplitudes on axis, as a function of axial distance. The results show that significant nonlinear distortion can occur through media with characteristics typical of tissue. Excellent agreement is found between theory and experiment indicating that the model can be used to predict nonlinear ultrasound propagation through multiple layers of tissue-like media.

On the stability of the effective apodization of the nonlinearly generated second harmonic with respect to range

The concept of an effective apodization was introduced to describe the field pattern for the nonlinearly generated second harmonic (2f) within the focal zone using a linear propagation model. Our objective in this study was to investigate the validity of the concept of an effective apodization at 2f as an approach to approximating the field of the second harmonic over a wide range of depths. Two experimental setups were employed: a vascular imaging array with a water path and an adult cardiac imaging array with an attenuating liver path. In both cases the spatial dependencies of the ultrasonic fields were mapped by scanning a point-like hydrophone within a series of planes orthogonal to the propagation direction. The sampling distances were located before, within, and beyond the focal zone. The signals were Fourier transformed and the complex values at 2f were linearly backpropagated to the transmit plane in order to obtain an effective apodization. The measured results demonstrated a relatively constant effective apodization at 2f as a function of propagation distance. Finite amplitude computer simulations were found to be in agreement with these measurements. Thus the measure of the effective apodization at 2f provides an approximation to the second harmonic field outside the focal zone.

Measurements of the stability of the effective apodization for the nonlinearly generated second harmonic as a function of propagation distance

The concept of an effective apodization was introduced to approximate the nonlinearly generated second harmonic field pattern based solely on the linear propagation of a field transmitted at the frequency of the harmonic. We have previously demonstrated that transmitting with an effective apodization determined from measurements made only in the focal plane yields an accurate description of the nonlinearly generated field over a very wide range of depths. The goal of this work was to determine the stability of the effective apodizations obtained from measurements of the ultrasonic field in a series of planes before and beyond the focal zone. Transverse 2D scans of the transmitted fields were performed with a 0.6‐mm‐diam. hydrophone for both vascular and cardiac arrays. Linear angular spectrum backpropagation of the measured fields determined the effective apodizations. These measurements were compared with simulations based on a nonlinear Burgers equation enhanced angular spectrum approach. The resulting ...

The simulation of ultrasonic field measurements in a manufacturing environment

This paper describes how ultrasonic field models may be used as an alternative to hydrophone measurement techniques.