Richard Yung Chiao

Subharmonic Imaging with Microbubble Contrast Agents: Initial Results

The subharmonic emission from insonified contrast microbubbles was used to create a new imaging modality called Subharmonic Imaging. The subharmonic response of contrast microbubbles to ultrasound pulses was first investigated for determining adequate acoustic transmit parameters. Subharmonic A-lines and gray scale images were then obtained using a laboratory pulse-echo system in vitro and a modified ultrasound scanner in vivo. Excellent suppression of all backscattered signals other than from contrast microbubbles was achieved for subharmonic A-lines in vitro while further optimization is required for in vivo gray scale subharmonic images.

Analytic evaluation of sampled aperture ultrasonic imaging techniques for NDE

This paper presents a theoretical comparison of three generic sampled aperture ultrasonic imaging systems for nondestructive evaluation. The imaging systems are categorized according to their source-receiver combination for data acquisition: common-source, back-scatter, and full-array imaging. First, forward modeling is performed for a point source and a point receiver. This is then used to model the received data set for each of the imaging categories. Subsequently, the inversion algorithm for each category is derived, and their performance is evaluated in terms of resolution, noise, and computation. We show that in terms of resolution, back-scatter imaging is the best, followed by full-array and common-source imaging. However, in terms of material noise, full-array imaging is the best, with back-scatter and common-source imaging having the same material noise response. Full-array imaging is the only system with inherent redundancy to reduce electronic noise, but at the expense of significantly more computation. The physical transducer is in the full-array category, allowing mechanical scanning to be traded for dynamic focusing and computational power.<<ETX>>

Synthetic transmit aperture imaging using orthogonal Golay coded excitation

The frame rate in medical ultrasound imaging may be increased significantly by reducing the number of transmits per image frame. Cooley et al. (1994) and Lockwood et al. (1995) have described synthetic transmit aperture (STA) systems where each frame is imaged using data obtained from a small number of point sources fired in succession. These systems have potential for very high frame rates, but they also suffer from low SNR. In this paper we present a computationally efficient method to increase the SNR of STA systems by using spatio-temporal encoding which increases SNR by 101og(ML) dB, where M is the number of active phase centers or transmits and L is the temporal code length. By using an orthogonal Golay set for the spatio-temporal encoding, the received data can be sorted by each transmit phase center and pulse-compressed for subsequent synthetic aperture beamforming. Computer simulations are used to demonstrate the method.

B-mode blood flow (B-flow) imaging

B-flow is a new technique that extends the resolution, frame rate, and dynamic range of B-mode to simultaneously image blood flow and tissue. B-flow relies on coded excitation to boost weak signals from blood scatterers and on tissue equalization to simultaneously display flowing blood and tissue without threshold decision and overlay. Various classes of codes such as Barker and Golay may be used. Clinical B-flow cineloops demonstrate 3/spl times/ resolution and frame rate improvement over color flow, which, together with over 60 dB of display dynamic range, are able to image hemodynamics and vessel walls with unprecedented clarity.

Aperture formation on reduced-channel arrays using the transmit-receive apodization matrix

We present the transmit-receive apodization matrix (T/R matrix) as a tool for aperture synthesis on reduced-channel arrays. Reduced-channel arrays have a small number of transmit/receive channels multiplexed to a conventional dense array. For a 1D array of N elements, the T/R matrix is the N/spl times/N matrix of apodization values, where the rows correspond to transmit element positions and the columns correspond to receive element positions. We show that the round-trip beam pattern may be obtained from this matrix simply as the Fourier transform of its cross-diagonal sum. The aperture synthesis process consists of choosing the T/R matrix under certain constraints. If the T/R matrix has rank one, then a single transmit with parallel receive forms the beam (conventional case), and the round-trip beam can be separated into the product of the transmit beam and the receive beam. As the rank of the T/R matrix increases, greater beamforming flexibility is achieved, allowing for tradeoffs in SNR, frame-rate, and system complexity.

Higher order nonlinear ultrasonic imaging

The processing of second harmonic echoes from both biological tissue and contrast agents has generated new diagnostic methods in medical ultrasound. The work presented here demonstrates the extraction of higher order nonlinearities. The underlying idea is to model the nonlinear wave propagation or reflection from a contrast bubble by a polynomial expansion of some basis waveform. When this model is excited by a number of transmit pulses which only differ in their amplitude and phase then the coefficients of this polynomial model can be extracted through least squares inversion. The coefficients correspond to the individual nonlinear components. An important feature of the method is the evaluation of nonlinear components whose spectra are folded back into the transmission band. All odd order nonlinearities can create such echo components. The reception of these components eliminates the high bandwidth requirements encountered in second harmonic imaging. Higher‐order even harmonics may also be detected by t...

Implementation of subharmonic imaging

Ultrasound contrast agents promise to improve the sensitivity and specificity of diagnostic ultrasound imaging. It is of great importance to adapt ultrasound equipment for optimal use with contrast agents e.g., by exploiting the nonlinear properties of the contrast microbubbles. Harmonic Imaging is known to be associated with problems, due to second harmonic generation and accumulation within the tissue itself. Given the lack of subharmonic generation in tissue, one alternative is the creation of subharmonic images with better blood-to-tissue contrast. Here, the implementation of grayscale subharmonic imaging is presented.

Optimization of 1.5D arrays

1.5D arrays have been proposed to improve elevational beam focus over 1D arrays (D.G. Wildes et al., 1997). The design of 1.5D arrays is complicated by the potential for large elevational row dimensions which results in many possible design tradeoffs (P. Tournois et al., 1995). Here, the authors presents method for optimizing 1.5D arrays. The method is a global search by simulated annealing over the array parameters of row sizes, lens/electronic focus, and apodization in order to optimize a given beam criterion. The authors illustrate the method by showing how to minimize the peak sidelobe level by searching over row sizes and apodization in the farfield, narrowband case. The more realistic nearfield, broadband case is demonstrated by showing an example of 1.5D array optimization using this method.

ESTIMATION AND CORRECTION OF QUADRATURE SIGNAL ERROR FOR HIGH-RESOLUTION ACOUSTIC MICROSCOPY

In this letter, an approach to the estimation and correction of quadrature data‐acquisition error in acoustic microscopy is presented. Prior methods modeled the signal error mainly as phase perturbations. Because the amplitude error was not included in the model, the prior approach was not as effective as expected for some experiments of which amplitude error was significant. Here the technique is extended and modified by including the amplitude error as a component of the perturbation, and subsequently a matrix operation is formulated for the error correction process. This improvement enables one to perform estimation and correction of the signal errors effectively without additional iterations. An experimental data set from the scanning laser acoustic microscope is utilized to demonstrate the effectiveness of the algorithm.

Holographic acoustic microscopy for quantitative velocity profile imaging

In this letter, a new approach is reported for the quantitative measurement of the acoustic velocity profile with the scanning laser acoustic microscope operating in the holographic mode. Due to the addition of a quadrature detector, the acoustic microscope is now capable of holographic recording of the complex wave field. As a result, it enables one to estimate the velocity profile with superior resolving capability directly from the wave field distribution instead of from the interferometry fringe patterns of the conventional methods.