Increasing average power in medical ultrasonic endoscope imaging system by coded excitation

Medical ultrasonic endoscope is the combination of electronic endoscope and ultrasonic sensor technology. Ultrasonic endoscope sends the ultrasonic probe into coelom through biopsy channel of electronic endoscope and rotates it by a micro pre-motor, which requires that the length of ultrasonic probe is no more than 14mm and the diameter is no more than 2.2mm. As a result, the ultrasonic excitation power is very low and it is difficult to obtain a sharp image. In order to increase the energy and SNR of ultrasonic signal, we introduce coded excitation into the ultrasonic imaging system, which is widely used in radar system. Coded excitation uses a long coded pulse to drive ultrasonic transducer, which can increase the average transmitting power accordingly. In this paper, in order to avoid the overlapping between adjacent echo, we used a four-figure Barker code to drive the ultrasonic transducer, which is modulated at the operating frequency of transducer to improve the emission efficiency. The implementation of coded excitation is closely associated with the transient operating characteristic of ultrasonic transducer. In this paper, the transient operating characteristic of ultrasonic transducer excited by a shock pulse δ(t) is firstly analyzed, and then the exciting pulse generated by special ultrasonic transmitting circuit composing of MD1211 and TC6320. In the final part of the paper, we designed an experiment to validate the coded excitation with transducer operating at 5MHz and a glass filled with ultrasonic coupling liquid as the object. Driven by a FPGA, the ultrasonic transmitting circuit output a four-figure Barker excitation pulse modulated at 5MHz, ±20 voltage and is consistent with the transient operating characteristic of ultrasonic transducer after matched by matching circuit. The reflected echo from glass possesses coded character, which is identical with the simulating result by Matlab. Furthermore, the signal's amplitude is higher.