A Delayed-excitation Method For High-frequency Ultrasound Imaging

In recent years,high-frequency ultrasound(>20 MHz)provides high-resolution images and fast imaging frame rates,making high-frequency ultrasound widely used in biomedical aspects.More and more people use high-frequency ultrasound imaging to research cardiovascular and cancer diseases of small animals.And high-frequency ultrasound has been used in many parts of the human body imaging,while providing a number of new methods of disease treatment.These high-resolution images have an important role in the diagnosis and treatment of human diseases.Because the center frequency of high-frequency ultrasonic echoes is usually higher than 20 MHz,the sampling rate of high-frequency ultrasound systems needs to be higher than 120 MHz to satisfy the Nyquist theorem.While the traditional ultrasound system in the sampling rate range is 40-60 MHz.In order to achieve compatibility between traditional ultrasound systems and high-frequency ultrasound imaging,it is necessary to improve the effective sampling rate of the system.In order to solve the problem of insufficient sampling rate,we can choose to have higher sampling rate of analog-digital conversion chip.However,commercially available ultrasound systems typically have up to hundreds of data acquisition channels and if all channels are implemented with high sampling rate analog-to-digital conversion chips,the cost of the device will increase significantly.In order to achieve high-frequency ultrasound imaging of traditional ultrasound systems,while reducing system cost,the existing solution of insufficient sampling rate is to use interleaved sampling technique.The technology enhances the effective sampling rate by combining multiple analog-to-digital conversion chips.However,there are many problems in the implementation of the interleaved sampling method.In the traditional ultrasonic system data acquisition framework,this thesis presents a way of ultrasonic delay excitation that can improve the system effective sampling rate to achieve high-frequency ultrasound imaging.That is,the ultrasonic echo data is rearranged by delaying the transmission pulse for a certain number of times,so as to achieve the goal of high sampling rate.In order to verify the implementation of the above methods,this thesis mainly studies two aspects.On the one hand,the delayed excitation method was used in ultrasonic pulse echo testing and field II software simulation of delayed excitation high-frequency ultrasound imaging.The feasibility of ultrasonic delayed excitation imaging is illustrated by the comparison of echo results and imaging.On the other hand,we designed a single-element high-frequency ultrasound imaging system.Wire and tissue-mimicking phantoms were imaged to evaluate the performance of the proposed method,while a porcine small-intestine specimen and an excised rabbit eyeball were used for in vitro imaging evaluations.Experimental results show that the method of delayed excitation can improve the effective sampling rate of the system,making the traditional ultrasound system to achieve high-frequency ultrasound imaging.