Hui Yao

Dual-Mode Ultrasound Phased Arrays for Image-Guided Surgery

A 64-element, 1 MHz prototype dual-mode array (DMUA) with therapeutic and imaging capabilities is described. Simulation and experimental results for the characterization of the therapeutic operating field (ThxOF) and imaging field-of-view (IxFOV) for a DMUA are given. In addition, some of the special considerations for imaging with DMUAs are given and illustrated experimentally using wire-target arrays and commercial, quality-assurance phantoms. These results demonstrate what is potentially the most powerful advantage of the use of DMUAs in image-guided surgery; namely, inherent registration between the imaging and therapeutic coordinate systems. We also present imaging results before and after discrete and volumetric HIFU-induced lesions in freshly-excised tissues. DMUA images consistently show changes in echogenicity after lesion formation with shape and extent reflecting the actual shape of the lesion. While changes in echogenicity cannot be used as an indicator of irreversible HIFU-induced tissue damage, they provide important feedback on the location and extent of the expected lesion. Thus, together with the self-registration property of DMUAs, lesion images can be expected to provide immediate and spatially-accurate feedback on the tissue response to the therapeutic HIFU beams. Based on the results provided here, the imaging capabilities of DMUAs can add unique features to other forms of image guidance, e.g. MRI, CT and diagnostic ultrasound.

Enhanced lesion visualization in image-guided noninvasive surgery with ultrasound phased arrays

We describe dual-mode ultrasound phased arrays for noninvasive image-guided surgical applications. In particular, we address the problem of real-time visualization of thermal lesion formation in the target (e.g., tumor) tissue using the therapeutic arrays. Post beamforming filter bank image reconstruction with nonlinear compounding is utilized to improve the lesion contrast in the (typically) very low-contrast ultrasound images. It is shown that the new image reconstruction algorithm leads to measurable improvement in lesion contrast on the order of 6-15 dB. This leads to significant improvement in lesion detectability and size estimation by standard segmentation techniques for speckle imagery. Experimental results strongly suggest that 2/sup nd/ harmonic imaging could play an important role in the enhancement of real-time lesion visualization.

Dual-mode ultrasound phased arrays for imaging and therapy

We have recently demonstrated the imaging capabilities of a prototype 64-element 1 MHz concave array with 100 mm radius of curvature. This array was optimized for therapeutic applications using high-intensity focused ultrasound (HIFU). We have shown that this dual-mode ultrasound array (DMUA) has a therapeutic operating field (ThxOF) that extends by /spl plusmn/3 cm and /spl plusmn/2 cm around its geometric center in the axial and lateral directions, respectively. We have also shown that appropriate apodization and accounting for element directivity along with conventional synthetic aperture beamforming produce a 50 dB imaging field of view (IxFOV) larger than the ThxOF. In addition, the spatial registration of imaging targets is as accurate as commercially available scanners. In this paper, we present results from an image-based refocusing algorithm whereby images formed by the DMUA are used to identify a refocusing target and a set of critical points where the incident power is to be minimized. The algorithm is validated experimentally in tissue mimicking phantom with strongly scattering ribs placed between the DMUA and the target. These results demonstrate what is potentially the most powerful advantage of the use of DMUAs in image-guided surgery. Namely, the inherent registration between the imaging and therapeutic coordinate systems. This allows for direct definition of targets and any surrounding critical structures to be avoided to minimize the collateral damage. With these capabilities, DMUAs may provide a most powerful paradigm for image-guided surgery.

Dual-mode ultrasound phased arrays for noninvasive surgery: post-beamforming image compounding algorithms for enhanced visualization of thermal lesions

A post-beamforming nonlinear compounding algorithm for ultrasonic imaging is presented. Fundamental and harmonic image components from beamformed radio frequency (RF) data are extracted, envelope detected and compounded using a spatial compounding functions (SCFs) derived from the transmit/receive beamforming topology used in obtaining the RF data. This is specially useful for applications where single-transmit focus (STF) imaging is used. In this paper, we present results from STF imaging experiments using a novel dual-mode phased array system for image-guided surgery. In particular, we address the enhancement of the echogenicity of thermal lesions formed in ex vivo tissue. It is shown that new nonlinear image compounding algorithm produces 25-30 dB enhancement in lesion echogenicity without loss in spatial resolution. This is to be compared with a typical enhancement of 5 dB achieved by standard echographic imaging and 20 dB achieved by second harmonic imaging alone. In addition, images resulting from the new algorithm are virtually free of beamforming artifacts that can severely degrade the performance of 2nd harmonic imaging.

Noninvasive localized ultrasonic measurement of tissue properties

We present in vitro and in vivo results validating a new localized noninvasive ultrasonic measurement of both tissue absorption and perfusion. The method employs sub-second low-intensity focused ultrasonic beams to generate a brief temperature rise on the order of 1/spl deg/C. The RF data from an imaging transducer is processed to produce a noninvasive temperature estimate in the localized heated volume using published speckle tracking or frequency-domain algorithms (Seip, R., 1996; Simon, C. et al., 1998). Absorption can be obtained from the initial heating rate, while perfusion can be estimated from the initial decay. Due to the small size of the heated spot, the noninvasive temperature estimation can produce results that are virtually free of thermal lensing effects. Furthermore, since the measurements are based on heating and decay rates, only the parameters of the transient bioheat equation (density and heat capacity) are needed for the estimate. Both ex vivo and in vivo results show two-fold to three-fold increases in tissue absorption. For the ex vivo results, the change in absorption was estimated using direct fine wire thermocouple measurements at the treatment site in addition to the noninvasive temperature estimation. The decay rate of the in vivo estimated temperature increased by two-fold, indicating increased perfusion in the tumor surrounding the small lesion. This is very likely in the single-shot lesion formation experiment, while the opposite effect can be expected in volumetric lesion formation. The in vivo results show very clearly the feasibility of perfusion estimation based on decay rate.

Post-beamforming second-order Volterra filter for nonlinear pulse-echo imaging

In this paper, we demonstrate that a second-order Volterra filter (SVF) can adequately model pulse-echo imaging data acquired from tissue media. Furthermore, we demonstrate that the coefficients of both the linear impulse response and the second-order kernel can be robustly estimated from imaging data using pseudorandom binary input sequences. The validation of the SVF as an appropriate model for ultrasonic imaging will allow for more intelligent use of post-beamforming signal processing for enhancing image contrast and producing quantitative imaging.

Refocusing dual-mode ultrasound arrays in the presence of strongly scattering obstacles

This paper provides a first experimental verification of the use of image-based feedback for refocusing the therapeutic beam in the presence of strongly scattering objects. This capability may be critical in the use of dual mode ultrasound array (DMUA) systems for noninvasive targeting of liver tumors and noninvasive cardiac ablation. In both cases, the target is partially obstructed by the rib cage, which limits the access and distorts the geometrically-focused high-intensity focused ultrasound (HIFU) therapeutic beam. The optimization procedure is based on the use of single-transmit focus (STF) imaging, in which a single transmit imaging beam employing delays derived from phasing for the therapeutic beam is used. We present experimental results of a DMUA refocusing algorithm that selects the control points from the target(s) and the ribs visible in the STF image. In addition, STF images before and after refocusing provide similar feedback by consistently showing increased echogenicity of the target region while the echogenicity of the ribs is not increased (often reduced).

Imaging with large-aperture arrays with heterogeneous directive elements

We have developed a new imaging algorithm for optimization of the contrast resolution of large-aperture arrays utilizing directive elements with heterogeneous responses. The algorithm employs depth-dependant pre- and post-beamforming filtering to optimize the SNR of the echo data throughout the imaging field. Sub-aperture processing is also employed to maximize the coherence of the echo data, both with respect to tissue aberrations and element heterogeneity. Pre-beamforming and sub-aperture processing leads to significant reduction in imaging artifacts due to reverberations, a significant degradation factor due to the need for water standoff. Images of quality assurance phantoms as well as ex-vivo liver tissue were obtained using our concave (100 mm radius of curvature) 64-element 1 MHz dual-mode array (DMA) and a commercial scanner. The results show that the DMA has a 50 dB field-of-view (FOV) centered at its geometric focus. This FOV extends by 6 cm and 4 cm in the axial and lateral directions, respectively. In addition, the spatial and contrast resolutions of this DMA have been tested using wire targets and speckle cell size calculations and found to be consistent with the transducer bandwidth and aperture size.

Real-time monitoring of the transients of HIFU-induced lesions

RF data from standard B-mode and pulse inversion (PI) imaging of HIFU lesion formation of freshly excised tissue was collected before, during, and after lesion formation experiments in ex vivo tissue. Exposures at intensity levels of 1100 W/cm/sup 2/ to 2500 W/cm/sup 2/ for durations of 2, 3, and 5 seconds in a single shot were used. Also continuous raster scan of longer duration (10-20 seconds) to form slice or volumetric lesions were monitored. Monitoring was done with a diagnostic scanner and RF data was acquired at 1 frame/second for 60 seconds starting 5 seconds before each shot. Lesion maps from grayscale B-mode and PI images were obtained using level-set methods for each frame and compared with the actual lesion found by inspection. Lesion maps from PI imaging were consistently smaller in size and more in line with the actual lesion size. Transient analysis of harmonic content of lesion echoes show sustained harmonic activity for 10-15 seconds after the therapy pulse is turned off (in ex vivo liver tissue). A gradual drop in this activity follows with steady state reached within 50-60 seconds. It was also shown that the use of short microsecond pulses from the therapy transducer to expose the lesion location during real-time imaging significantly increased the scattering from lesion location.

Detection and mapping of thermal lesions using dual-mode ultrasound phased arrays

It has long been recognized that thermal lesions formed using high-intensity focused ultrasound (HIFU) exhibit nonlinear behavior that can be detected in pulse-echo ultrasound. Second harmonic imaging of freshly formed thermal lesions have consistently shown significant enhancement in their visualization confirming this nonlinear behavior. In this paper, we describe a post-beamforming nonlinear filtering algorithm based on a second-order Volterra filter (SVF) model that separates the linear and quadratic components of the echo signal leading to significant enhancement of lesion visualization. Images from ex vivo tissue samples are shown to demonstrate the level of contrast enhancement achieved with the SVF-based quadratic filter compared with standard echo and 2nd harmonic imaging results.