The Study Of Coded Signals Used In Medical Ultrasound Imaging

Conventional medical ultrasound systems use single-carrier and short pulses. There are some problems such as poor signal-to-noise ratio (SNR). One has to make compromise between the resolution and penetration of the transmitted signal due to the low SNR. Recent study of using coded signals can resolve these problems.Conventional code signals used in medical ultrasound imaging have some short-comings and there is less study of code signals currently. It is important to search for codes with better property and practicability. The study faces several challenges. First task is to search for the optimal code sequences. Secondly, it is required to find the coded signal to be suitable for the different non-linear environment. Thirdly, it is re-quired that the codes should have both good auto-correlation and cross-correlation.The aim of this dissertation is to investigate systematically coded signals used in medical ultrasound imaging and to solve the three difficult problems mentioned above. Focusing on the third problem, we try to introduce some advanced research results from the field of telecommunication.As only binary codes and linear frequency modulation signal is used in medical ultrasound imaging at present, some new ideas of the combination of multiple modu-lations or multiple coded signals are given. Costas signal is introduced into our study and it's adaptation to ultrasound imaging is discussed in detail.As frequency-dependent attenuation effect makes trouble for the implementation of coded signals for medical ultrasound imaging, especially about signals with thumbtack ambiguity function, the signals with frequency shift tolerance are investi-gated. Subsequently, staggered code that is a new coding scheme used in ultrasound imaging is proposed. As there is not practical construction algorithm, in-depth study for that code can not be conducted.A new idea of secondary coding is proposed to meet both good auto-correlation and cross-correlation:auto-correlation and cross-correlation properties of coded sig- nals will be achieved by two mechanisms respectively. This idea provides a new way to the following investigation.According to the idea of secondary coding staggered technique used for reducing cross-talk in medical ultrasound imaging is proposed. This technique is investigated deeply since it practically increases the frame rate significantly or improves the im-aging quality. And two schemes are proposed using the staggered idea.First, the increase of frame rate is generally realized by transmitting multiple beams simultaneously, which leads to increase the cross-talks between different aper-tures. In view of this, a new coding scheme using staggered technique is proposed. The theoretical analysis and simulations showed that, the cross-correlation reduction is proportional to the size of repetitions number, and for an arbitrary coded signal a good cross-talk reduction will be achieved if the staggered delay is kept in an appro-priate range. Secondly, a scheme is introduced by using staggered technique to reduce cross-talks between different signals caused by increasing pulse repeat frequency.The performance analyse of coded signals should always be integrated with spe-cific application. Therefore at the last of the dissertation a scheme using Golay codes in time-shift blood velocity estimator is proposed. The scheme is evaluated through simulations and implementation. The advantages of Golay codes are maximized and better performance is achieved compared with system using single-carrier pulses in this scheme.From investigations we come to the following conclusion:good code property for the application of ultrasound imaging is very important. Auto-correlation and cross-correlation properties of coded signals can be achieved by two mechanisms re-spectively. Cross-correlation property can be achieved in various ways, and staggered technique is a feasible one.