Russell J. Fedewa

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.

Backscatter imaging and myocardial tissue characterization

The goal of myocardial ultrasonic tissue characterization is to complement two-dimensional and Doppler echocardiography by providing information (such as assessment of regional viability based on localized values of backscatter) beyond that derived from an assessment of myocardial dimensions and motion. Quantitative backscatter imaging can be subdivided into three broad areas: (1) direct applications, in which specific pathologies are identified and monitored, (2) indirect applications, in which quantitative techniques designed for use in tissue characterization serve to expand the role of echocardiography, and (3) contributions to the understanding of cardiac structure and function.

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.

The detection and exclusion of the prostate Neuro-Vascular Bundle (NVB) in automated HIFU treatment planning using a pulsed-wave doppler ultrasound system

Men with prostate cancer are likely to develop impotence after prostate cancer therapy if the treatment damages the neuro‐vascular bundles (NVB). The NVB are generally located at the periphery of the prostate gland. To preserve the NVB, a Doppler system is used to detect and localize the associated blood vessels. This information is used during the therapy planning procedure to avoid treatment surrounding the blood vessel areas. The Sonablate®500 (Focus Surgery, Inc.) image‐guided HIFU device is enhanced with a pulse‐wave multi‐gate Doppler system that uses the current imaging transducer and mechanical scanner to acquire Doppler data. Doppler detection is executed after the regular B‐mode images are acquired from the base to the apex of the prostate using parallel sector scans. The results are stored and rendered in 3‐D display, registered with additional models generated for the capsule, urethra, and rectal wall, and the B‐mode data and treatment plan itself. The display of the blood flow can be in 2‐D c...

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.

Impact of propagation through an aberrating medium on the linear effective apodization of a nonlinearly generated second harmonic field

Techniques based on the nonlinearly generated second harmonic signal (tissue harmonic imaging) have rapidly supplanted linear (fundamental) imaging methods as the standard in two-dimensional echocardiography. Enhancements to the compactness of the nonlinearly generated second harmonic (2f) field component with respect to the fundamental (1f) field component are widely considered to be among the factors contributing to the observed image quality improvements. The objective of this study was to measure the impact of phase and amplitude aberrations resulting from propagation through an inhomogeneous tissue, on the beamwidths associated with: the fundamental (1f); the nonlinearly generated second harmonic (2f); and the linearly propagated, effective apodization signal at the same (2f) frequency. Modifications to the transmit characteristics of a phased-array imaging system were validated with hydrophone measurements. Results demonstrate that the characteristics of the diffraction pattern associated with the linear-propagation effective apodization transmit case were found to be in good agreement with the detailed spatial characteristics of the nonlinearly generated second harmonic field. The effects of the abdominal wall tissue aberrators are apparent for all three of the beam profiles studied. Consistent with the improved image quality associated with harmonic imaging, the aberrated nonlinearly generated second harmonic beam was shown to remain more compact than the corresponding aberrated fundamental beam patterns in the presence of the interposed aberrator.

Automated treatment planning for prostate cancer HIFU therapy

A framework for computer-assisted treatment planning for prostate cancer high intensity focused ultrasound (HIFU) treatments using 3D ultrasound images, user tracing, and 3D models of the prostate, urethra, and rectal wall was presented previously. This framework provides the input for the current research: the development of a general-purpose HIFU treatment planner module. This module is capable of automatically specifying the prostate HIFU treatment sites using given prostate anatomical information from 3D ultrasound images combined with information on HIFU probes, transducers, and elementary lesion parameters that are stored in a lesion library file. The output of the automatic planner module is a complete treatment plan that is executed after interactive physician review. Additional inputs to this module include clinically relevant parameters, such as inter-lesion spacing and treatment margins. Advantages of this approach include a reduction in the overall treatment time, the ability to easily and accurately plan treatments for complex prostate shapes, and the ability to adapt the planner to other systems and geometries simply by providing a different lesion library specific to that system. The automatic planner module has been integrated into the treatment software of the Sonablate 500 image-guided HIFU device (Focus Surgery, Inc). The entire treatment planning process is presented, highlighting the usefulness of the automatic planner module.

Measurements from 22 to 105 MHz of the apparent anisotropy of ultrasonic backscatter from coronary arteries with atherosclerotic plaques identified by intravascular ultrasound

It is speculated that the anisotropic structure of coronary arteries may cause images of the coronary arteries produced by future oblique and forward-looking IVUS catheters to appear significantly different than the images produced by current side-looking IVUS catheters. In anticipation of these systems and to understand better the fundamental properties of the coronary arteries and associated plaques, we compare the measured anisotropy of ultrasonic backscatter in coronary arteries between side-looking and forward-looking directions over the bandwidth from 22 to 105 MHz. To do so, 44 segments from 19 human coronary arteries were each measured ultrasonically in 8 configurations. Each segment was imaged first with two clinical IVUS systems, and subsequently in two orthogonal orientations by an acoustic microscope operating with 3 interchangeable transducers of nominal center frequencies of 25, 50, and 100 MHz. The backscatter trend observed with side-looking IVUS was consistent with radial acoustic microscopy results, but axial acoustic microscopy exhibited a reversal of the typical pattern, with the media demonstrating greater apparent integrated backscatter than the intima/plaque. These results suggest that future IVUS catheters operating in new imaging planes may need to account for tissue anisotropy, and may be able to exploit this anisotropy for diagnostic advantage.

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.