Modulated (or coded) excitation signals can potentially improve the quality and increase the frame rate in medical ultrasound scanners. The aim of this dissertation is to investigate systematically the applicability of modulated signals in medical ultrasound imaging and to suggest appropriate methods for coded imaging, with the goal of making better anatomic and flow images and threedimensional images. On the first stage, it investigates techniques for doing high-resolution coded imaging with improved signal-to-noise ratio compared to conventional imaging. Subsequently it investigates how coded excitation can be used for increasing the frame rate. The work includes both simulated results using Field II, and experimental results based on measurements on phantoms as well as clinical images. Initially a mathematical foundation of signal modulation is given. Pulse compression based on matched filtering is discussed. Correlation and compression properties of coded signals are shown to depend on a single parameter of the coded signals: the time-bandwidth product. It is shown that, due to attenuation in the tissues, the matched flter output is related to the ambiguity function of the excitation signal. Although a gain in signal-to-noise ratio of about 20 dB is theoretically possible for the time-bandwidth product available in ultrasound, it is shown that the effects of transducer weighting and tissue attenuation reduce the maximum gain at 10 dB for robust compression with low sidelobes. Frequency modulation and phase modulation are considered separately and their resolution, sidelobes, expected signal-to-noise gain and performance in tissue imaging are discussed in detail. A method to achieve low compression sidelobes by reducing the ripples of the amplitude spectrum of the FM signals is described. Application of coded excitation in array imaging is evaluated through simulations in Field II. The low degree of the orthogonality among coded signals for ultrasound systems is first discussed, and the effect of mismatched filtering in the cross-correlation properties of the signals is evaluated. In linear array imaging it is found that the frame rate can be doubled without any degradation in image quality, by using two coded sequences that have a cross-correlation of at least 11 dB. Other coding schemes that can increase the frame rate by nearly 5 times with a small compromise in resolution are discussed. Coded synthetic transmit aperture imaging with only 4 emissions is shown to yield the same signal-to-noise ratio as with conventional phased-array imaging which

Ultrasound imaging using coded signals

The problem of moving target indicator (MTI) filter design for radar systems with non-uniform (staggered) pulse repetition intervals is examined. The goal is to realise and then utilise a trade-off in the design of MTI filter between the conflicting requirements of high suppression of undesired signal (clutter echo) and minimal suppression of desired signal (target echo). To that aim, three design methodologies, namely, least squares, convex optimisation and min–max error, are studied. The numerical results indicate that the presented designs yield high-performance MTI filters which are easily applicable to a variety of operational scenarios. A ready-to-use source code for the design of suggested filters is also provided.

On the design of staggered moving target indicator filters

In this paper we propose orthogonal waveforms and equal gain combination algorithm in radar sensor network (RSN) to alleviate blind speed problem. We also design a fuzzy logic system (FLS) to optimize the number of radars in RSN. Simulation results show that our FLS-based RSN can not only balance the number of radars and QoS in terms of probability of miss detection (PMD), but to some extent achieve constant PMD with different system configuration.

Radar Sensor Network Design and Optimization for Blind Speed Alleviation

In this work, design of non-uniform MTI filter for staggered MTI Radar systems is presented in the sense of min-max filter design. Corresponding design is studied to obtain the nearest improvement factor to the optimum MTI filter improvement factor. The performance of the design is examined with simulations and a detailed comparison with the non-uniform MTI filters in the literature is given.

Min-max design of MTI filters with non-uniform pulse repetition intervals

Signal processing in radar and non-radar sensor networks

To avoid blind speed phenomena, staggered-p.r.f. m.t.i. systems are used in modern radars for detection of high-speed targets. In this paper a procedure is presented which considers both the effect of filter weights and the interpulse durations. The goal of this procedure is to yield an m.t.i. response with minimum variations in the pass-band and maximum attenuation in the stop-band.

https://ieeexplore.ieee.org/iel5/5266378/5269321/05269332.pdf

Design of a staggered-p.r.f. moving target indication filter

The problem of a two-frequency m.t.i. system is formulated for a general case. It is concluded that: (a) The two-frequency m.t.i. system sees an increased clutter Doppler variance as compared to that of a single frequency system. (From σ2 to σ2(1 + r2) where r is the ratio of the carrier frequencies in the two frequency channels.) (b) The mean clutter-Doppler frequency, at the same time, is reduced accordingly from f0 to f0(1 − r).

Analysis of a dual-frequency moving target indication system

: To avoid blind speed phenomena, staggered PRF MTI systems are used in modern radars for detection of high-speed targets. In this report a procedure is presented which considers both the effect of filter weights and the interpulse durations simultaneously. The design goal of this procedure is to yield a filter with minimum variations in the passband and maximum attenuation in the stopband.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0908181&Location=U2&doc=GetTRDoc.pdf

James K. Hsiao

Papers

Design of a staggered-p.r.f. moving target indication filter

Analysis of a dual-frequency moving target indication system

Design of a Staggered-PRF MTI Filter