Computational Simulation And Implementation Of Ultrasound Source For Limited-Diffracting Acoustic Field

Medical ultrasound imaging is widely used in clinical medical diagnosis because ofits advantages such as real-time imaging, non-invasive, no radiation and low cost etc. Thetraditional ultrasonic imaging utilizes dynamic-focusing to improve the quality of image;however both emitting-focusing and receiving-focusing can only be focused in a smallarea. Therefore, it requires multi-emitting and receiving focusing which greatly reducesthe frame rate of imaging. Limited diffracting waves are a kind of special ultrasonic wavewhich can curb energy in limited space and/or period time. Theoretically, it can propagateto an infinite distance in theory; experiment showed that they have a large depth of fieldeven produced with finite aperture and energy. The natural focus characteristic of Limiteddiffracting waves means they can avoid the multi-focusing of traditional ultrasound, whichgreatly increase the speed of imaging and frame rate. Theoretically, the high frame rateimaging system based on limited diffracting waves can be as high as3750frames persecond.The dissertation studies on the theory characteristics of X-wave which is consideredas the most typical kind of limited diffracting waves，in order to employ limited diffractingwaves theory into practice application. The generation of X-wave ultrasonic fields needs acomplex and expensive technology. In order to decrease the high cost, a new approach wasproposed by approximating the excitation driving pulse with rectangular or triangulardriving pulse and chose the pulses according to the optimization result from L2curvecriterion. The advantages of this method include simple, efficient and reducing thecomplexity of hardware implementation. The main challenge of the method is to increasethe accuracy of X-wave approximation.Secondly, in order to increase the accuracy of fitting driving signal，a new methodcalled multiple simple driving fitting (MSDF) is proposed, which is based on the approximation of emitting signal on a ring. It uses a combination of multiple rectangularand triangle to produce the original signal. The parameters of each simple driving waveare determined through genetic algorithm. Simulation results showed that our method cangreatly improve the precision of fitting and establishe a solid foundation for commercialapplication of the technology.Thirdly, acoustic field simulation is an important step to valid the feasibility of thealgorithm. The disseration introduced the theory of linear acoustic system and simulationsoftware-Field II. Field II is the most widely used field simulation software in ultrasonicsociety. However, it cannot be directly used in X-wave simulation for the complexity oftraducer combination. To solve this problem, an algorithm to calculate acoustic fieldaccording to the linear system theory is developped. The field pressure can be computedby the convolution between the driving velocity and spatial impulse response between thefield point and transducer. In the simulation, the MSDF method with others methods werecompared，simulation results showed that the MSDF is closest to theoretical values invarious aspects and suitable for its application.Finally, a signal generator of X-wave driving pulse based on the principle of DDS(Direct Digital Synthesizer) technology were designed，and the signal generator produceprocesses from the aspect of hardware and software were discussed. Hardware systembased on FPGA (Field Programmable Gate Array) chip mainly realizes the frequency,amplitude adjustable function waveform output. Software system combines themulti-signals by adjusting time and amplitude of different rectangular and triangle drivingpulse. Analysis shows that the increasing of numbers of simple driving waves caneffectively enhance the fitting precision of the results. That means the hardware systemonly need to increase the number of D/A converter which is not expensive to improve theperformance.In conclusion, in this work we focused on the theory, simulation, and implementationof X-wave which is considered as most prospective high frame imaging technology. The author proposes an algorithm to simply the implementation of X-wave by using simplewaves to fit the original complex excitation signal，and it can greatly reduce the cost ofgenerating limited diffracting waves. In order to verify the feasibility of the new algorithm,we designed a new the sound field calculation method based on spatial impulse response.The new simulation algorithm can be used to calculate the annular sensor array acousticfield simulation. Further, this dissertation studies the hardware implementation ofultrasonic system which be expected generate the real X-wave emitting signal by usingDDS technology. All above works lay a valuable theoretical and practical foundation forthe future application of limited diffracting waves used in high frame rate imagingultrasound system.