Sung-Jae Kwon

Ultrasound second harmonic imaging with a weighted chirp signal

In this paper, a new harmonic imaging technique is proposed and evaluated experimentally. In the proposed method, a weighted chirp signal with a Hanning window is transmitted. The RF samples obtained on each array element are individually compressed by correlating with the reference signal defined as the 2nd harmonic component ( 2f/sub 0/) of a transmitted chirp signal generated in a square-law system. The proposed method uses the compressed 2f/sub 0/ component to form an image, for which the crosscorrelation term should be suppressed below at least -50 dB. Another advantage of the proposed method is that the SNR of 2nd harmonic imaging can be improved without limitation by increasing the duration of the chirp signal.

Orthogonal Golay code based ultrasonic imaging without reducing frame rate

Orthogonal Golay codes feature perfect sidelobe cancellation properties, and are simple to implement because they have elements of +1 and -1 only. However, the frame rate is reduced by one half due to the requirement of two consecutive transmissions per scanline. This paper presents an ultrasonic B-mode imaging method using orthogonal complementary Golay codes without reducing the frame rate while achieving good SNR. In the method, the member codes applied to transducer elements are alternated from one transmission to another, with the focal points switched as well. The sidelobe and grating lobe levels from combining four consecutive transmit events are found to be as much as about 10 dB less than those from combining two transmit events.

Realization of Sinc Waves in Ultrasound Imaging Systems

It is known that the transmit pulse waveforms of a limited-diffraction beam in a linear array transducer should be varied according to transducer element location, dictating the use of sophisticated hardware. In order to overcome this disadvantage while achieving the same field response, we propose a method of synthesizing limited-diffraction beams by combined signal processing of pulsed plane waves propagating in distinct directions over several consecutive insonification time intervals. The method is capable of achieving both higher transmit power and better lateral resolution over a larger depth of field. Although its field response is not uniform throughout the imaging points, this is not a major problem since the response is quite uniform within a region of interest. The proposed method requires the use of multiple insonifications for transmit focusing, and, therefore, can be applied in imaging slowly-moving or still objects. Both simulation and experimental results corroborate its superiority in terms of the lateral resolution at all imaging depths.

Ultrasound inverse scattering determination of speed of sound, density, and absorption

This paper presents a method of determining three intrinsic mechanical parameters of an unknown object in the framework of ultrasound inverse scattering tomography. Those parameters are the speed of sound, density, and absorption whose values are given as the solution of an inhomogeneous Helmholtz wave equation. Computer simulations are carried out using the sinc basis moment method and Newton-Raphson method in an measurement configuration where the object is insonified by an incident plane wave over 360/spl deg/ and the scattered field is collected by detectors arranged in a rectangular fashion around it. Two distinct frequencies are used to separate each parameter of three Gaussian objects that are either overlapped at the same location or separately positioned from each other. The density turns out to be rather difficult to reconstruct as compared with the other two parameters.

Short-lag spatial coherence combined with synthetic aperture imaging

Recently, short-lag spatial coherence (SLSC) imaging has drawn much attention from researchers due to benefits such as improved speckle signal-to-noise ratio and contrast-to-noise ratio compared to conventional B-mode imaging based on delay-and-sum beamforming. However, SLSC imaging has good performance just around the transmit focal point, which is quite a drawback. In this paper, we propose a new method that combines SLSC imaging and synthetic aperture imaging to address such a shortcoming. The efficacy of the proposed method is verified through simulations and experiments using Field II echo data and phantom data acquired with an actual ultrasound system.

Multimode ultrasound breast imaging using a new array transducer configuration.

This article presents a diagnostic ultrasound imaging technique that can be used in imaging protruding objects such as a human breast using two opposing array transducers. Because two B-mode images obtained from each of the two linear array transducers facing each other represent the same imaging area viewed in different directions, the image quality can be improved using a compounding technique. Using one array as a transmitter and the other as a receiver, the speed of sound distribution in a medium interposed between them is also reconstructed. In addition, because the spacing between the two arrays can be finely controlled, strain image can also be obtained. This new method can be used to produce a compound B-mode image, a speed of sound image, and a strain image of the same region-of-interest, making it possible to obtain more information leading to better diagnosis. Experimental results on a phantom containing a cylinder of different speed of sound and elasticity confirm that the proposed method is useful in obtaining compound and speed of sound images as well as strain images.

Experimental study of sidelobe reduction filters in ultrasound imaging

The experimental results of two types of sidelobe reduction filters are presented. These filters, named sidelobe filter and null filter, improve the resolution by scaling the received signal according to the ratio of the mainlobe and the sidelobe levels. In the sidelobe filter, sidelobe echoes can be obtained in the sidelobe direction of transmit beam by applying beam steering in receive focusing. Also some echoes can be obtained in null directions of the transmit field response, and this information, which can be obtained by taking Fourier transform, can also be used for purposes of sidelobe reduction. We can control each filter response by adjusting the filter order and scale factor. Computer simulation results show that the two types of filters can achieve more than 10 dB of the sidelobe level reduction, as well as the mainlobe width narrowing, which helps overcome the diffraction limited resolution of conventional focusing schemes. Experiments are also performed using a prototype system based on a modified commercial diagnostic ultrasound scanner (SA8800, Medison Co.). Experimental results with phantom and in-vivo data demonstrate that the proposed scheme is highly effective in enhancing the image resolution and contrast.

A digital scan conversion algorithm using fourier transform

In clinical ultrasound scanners, the echo data from phased or convex arrays are acquired in the polar coordinates but need to be displayed in the Cartesian coordinates. This requires a coordinate conversion process. In this paper, we present a method of performing scan conversion in the frequency domain using Fourier transform. Performance comparison of spatial and frequency domain scan conversion algorithms is carried out by computer simulation. We show that the frequency domain method produces B-scan images comparable to the bilinear interpolation method.

Clutter reduction in plane wave synthetic aperture imaging

A synthetic aperture imaging method which uses a steered plane wave is advantageous in that the resolution can be improved by covering a wide area with a small number of transmissions, and that the calculation of focusing delay for transmit focus synthesis is relatively simple. Through simulation studies, we found, however, that the clutter level of this method is higher than that of SAI using virtual sources positioned in the imaging plane, and the main reason is considered to be that the area covered by plane wave (PW) is wider than that of the virtual source case so that the scattering occurs over a large area, and the received signal basically has higher clutter levels than the virtual source case. A straightforward solution to this problem would be to use narrower PWs. In this case, however, the number of transmissions needs to be increased to cover the same area, resulting in a lowered frame rate. In order to maintain the same frame rate while reducing the clutter level, we divide the aperture into several sub-apertures, and simultaneously transmit two PWs in spatially separated sub-apertures, and furthermore, we use an up-chirp signal for one PW and a down-chirp signal for the other PW to minimize the crosstalk between the two PWs. In addition, transmit apodization is employed to reduce the fringing effect of the transmit aperture, and a further reduction of clutter level is achieved.

Measurement of the Shear Modulus of an Ultrasound Tissue Phantom

In this paper we propose a method for measuring the shear modulus of an ultrasound soft tissue phantom using an acoustic radiation force. The proposed method quantitatively determines the shear modulus based on the rise time of a displacement induced by an acoustic radiation force at the focal point of a focused ultrasound beam. The shear wave speed and shear modulus obtained from the proposed method and a shear wave propagation method were compared to verify the validity of the proposed method. In the shear wave propagation method, the shear modulus is first computed by measuring the propagating speed of a shear wave induced in a phantom by a limited-diffraction transmit field, and then was compared to that obtained with the proposed method in an ultrasound data acquisition system calibrated based on the first computed shear modulus. The relative errors between the two methods were found to be 4% for shear wave speed and less than 9% for shear modulus, confirming the usefulness of the proposed method.