Satchi Panda

cMUTs and electronics for 2D and 3D imaging: monolithic integration, in-handle chip sets and system implications

Capacitive microfabricated ultrasound transducers (cMUTs) have been shown to be practical for medical imaging. Breakthrough performance requires combining these MEMS transducers with electronics. This paper explores synergies between cMUTs and electronics for 2D and 3D imaging. For example, low-noise receive signal conditioning improves tissue penetration, while transmitters capable of arbitrary waveforms minimize clutter. Bias control circuitry can create simple multi-row arrays for improved 2D contrast resolution. It also can enable 3D scanning with less complexity than current alternatives. Matrix transducer elements for 3D present challenges due to their high impedance, number and density. Monolithically integrated cMUTs can offer unique solutions to these problems, enabling isotropic 3D imaging from fully sampled arrays.

System and method of operating microfabricated ultrasonic transducers for harmonic imaging

A capacitive microfabricated ultrasonic transducer (cMUT) is operated to improve its performance during harmonic imaging in non-linear media, such as in contrast agents or in human tissue. The cMUT is operated by inverting the transmit waveform to adjacently spaced azimuth elements, and combining at least two additional firings without adjacent inversion, for each transmit vector, thereby canceling the second harmonic generation of the cMUT; and thus, the performance of harmonic imaging using the cMUTs can achieve improvement.

Sediment classification based on impedance and attenuation estimation

This paper presents a remote marine sediment classification model that can be implemented in real time while underway in a survey. The model is based on the estimation of impedance and attenuation of subbottom sediments from normal incident reflection seismograms. A robust impedance inversion model utilizing layer detection was developed which is implemented with significantly less computation when compared to full inverse methods, and hence runs in real time. A new ‘‘weighted least‐squares fitting’’ procedure is proposed for evaluating the impulse response of the sediment column. The acoustic attenuation in the sediment is determined by measuring the frequency shift of the pulse spectrum using an instantaneous frequency method. The impedance inversion model requires an input of the estimated attenuation to account for the loss in signal energy due to absorption. A recently developed model relating sediment acoustic properties to sediment physical properties for a given depositional environment is employed. The constants appearing in the classification model are evaluated using measurements from a few core samples. Impedance and attenuation estimates are used to predict sediment properties such as porosity, density, mean grain size, and sound speed. The reflection data for the present study were acquired by a linear wideband (full spectrum) sonar. It is used because of its linear system components, high resolution, and wide bandwidth. Analysis of acoustic data acquired by the full spectrum sonar demonstrates the feasibility of remote acoustic seafloor sediment classification.

Volumetric Elasticity Imaging with a 2-D CMUT Array

This article reports the use of a two-dimensional (2-D) capacitive micro-machined ultrasound transducer (CMUT) to acquire radio-frequency (RF) echo data from relatively large volumes of a simple ultrasound phantom to compare three-dimensional (3-D) elasticity imaging methods. Typical 2-D motion tracking for elasticity image formation was compared with three different methods of 3-D motion tracking, with sum-squared difference (SSD) used as the similarity measure. Differences among the algorithms were the degree to which they tracked elevational motion: not at all (2-D search), planar search, combination of multiple planes and plane independent guided search. The cross-correlation between the predeformation and motion-compensated postdeformation RF echo fields was used to quantify motion tracking accuracy. The lesion contrast-to-noise ratio was used to quantify image quality. Tracking accuracy and strain image quality generally improved with increased tracking sophistication. When used as input for a 3-D modulus reconstruction, high quality 3-D displacement estimates yielded accurate and low noise modulus reconstruction.

Elevation beam profile control with bias polarity patterns applied to microfabricated ultrasound transducers

In contrast to PZT probes, capacitive microfabricated ultrasonic transducers (cMUTs) require a DC bias. We investigate elevation beam profile shaping by spatially varying this bias polarity. Such a scheme introduces a 180/spl deg/ phase shift in the devices impulse response. A Fresnel zone plate is realized which is capable of generating tight elevation foci using a large aperture. This paper presents measurements from a prototype Fresnel cMUT probe, and matching simulation results. The Fresnel probe is capable of improved slice thickness compared with both a conventionally lensed probe and a multi-row design. Also, combinations of bias polarity control and multiple firings can enable harmonic imaging without pre-distortion of the transmit signal. Multiple firing schemes further optimize slice thickness, and allow beam steering in elevation.

5G-1 Two Approaches to Electronically Scanned 3D Imaging Using cMUTs

Capacitive micromachined ultrasonic transducers (cMUTs) introduce new degrees of freedom in transducer design. For example, it is easy to make elements of any size, and electronics may be integrated directly under the transducer element. These characteristics derive from lithographic manufacturing on a silicon substrate, and the use of a low-temperature surface micromachining process. They are particularly helpful in creating 2D arrays for electronically scanned volume imaging. This paper describes two contrasting ways to achieve volume data acquisition. In bias voltage scanning, we create a Fresnel elevation focus using a crossed electrode design with traditional azimuth beam formation. Then using the bias voltage dependence of kT in a cMUT, N2 element connections can be reduced to 2N. Such a scheme is especially useful in high frequency linear scanning, where N can exceed 200 elements. Measurements and images from a prototype probe are presented. We also describe a method to achieve fully sampled 2D receive apertures with autonomous elements using monolithically integrated electronics. This achieves spatial Nyquist sampling in both array dimensions, with arbitrary element delay and amplitude control. It allows much more of the information in the field returning to the probe to be retrieved by the imager, causing improvements in image quality and diagnostic confidence

SU‐FF‐I‐123: 3‐D Elasticity Imaging with a 2‐D Array

Purpose: Assess the performance of various motion tracking strategies applied to a 3‐D RF echo data set from an oil‐in‐gelatin phantom with spherical targets for a multi‐step deformation totaling about 15% axial strain. Discuss the prospects and preliminary experience of in vivo motion tracking. Method and Materials: A prototype 9‐MHz 2‐D CMUT array connected to a Siemens SONOLINE Antares was used to acquire RF echo data from a 100‐mm × 100‐mm × 70‐mm oil‐in‐gelatin phantom containing a 10‐mm diameter spherical inclusion that has a 5:1 elastic contrast with the background. This CMUT array images like a 1‐D linear array in generating a 2‐D image in the azimuthal plane, and it acquires a 3‐D volume by electronically stepping the 2‐D imaging plane in the elevational direction. A series of controlled compressions of 1.5–2% axial strain were applied. Phantom motion was tracked with off‐line data processing using different approaches including 2‐D, 2.5‐D and 3‐D axial guidance tracking. The method that performed the best was applied to a 3‐D in vivo data set obtained with the same transducer.Results: The contrast to noise ratios (CNR) and the cross correlation between the motion‐compensated RF and the reference RF for the four motion tracking approaches were used as metrics of performance. The CNR increased with increasingly sophisticated motion tracking with 3‐D axial guidance performing the best. The same trend was observed with the normalized cross correlation. Tracking in vivo data has proved more difficult. Conclusions: These results demonstrate the improvement in motion tracking available through 3‐D tracking. This work also demonstrates that volume data acquisition allows accurate motion tracking and axial strain image formation for an entire target (within the field of view). Volume data acquisition with 2‐D arrays will provide a major advancement in the capabilities of elasticity imaging systems.