Development of a Bipolar Pulse Generator for High-Frequency Ultrasound Imaging Systems

The pulse generator is the critical component in high-frequency ultrasound imaging systems. Currently, there still exist some shortcomings about commercial ultrasound image systems and devices such as 5900 PR ...etc.. However, to achieve a higher sensitivity and high performance, a programmable pulser generator is desired to match the requirements of high frequency ultrasound image system. In this study, we utilized a novel bipolar pulse generator method to overcome those issues in developed high resolution image system. As the needs of application in clinic, a real –time of high-frequency ultrasound imaging systems, including pulse generator, high speed scanning mechanism, high voltage protection and FPGA trigger control circuit, was developed in our Laboratory. This paper will focus to compare the performance of the bipolar generator and unipolar generator as used in pulse generator device respectively. Finally, the developed bipolar pulse generator show the better performance than the unipolar generator does. The bipolar system can generate clean N-cycle bipolar pulses and improve the center frequency of transducer up to 60 MHz. The axial resolution of scan system has been improved to around 60 μm. In addition, the result indicated that the proposed bipolar pulser could increase 4 dB for pulse/echo waveforms amplitude. Introduction Recent development of high frequency ultrasound transducer has led to a vast range of clinical applications. With the rapidly development of high frequency ultrasound in recent years because of noninvasive and real-time imaging, the research of medical ultrasonic image focuses on superficial tissue. The pulse generator is the critical component in all ultrasound systems. However, the commercial available pulser such as 5900PR (Panametrics Inc, Waltham, MA, USA) can not be used to excite the transducer with frequency over 60 MHz. Several recent papers have discussed both the importance and the design of unipolar and bipolar pulsers for high frequency (>20 MHz) ultrasound applications [1]-[3]. In order to improve the image quality in special depth of tissue, increasing the trigger frequency of pulse generator and designing a fast imaging method are important in ultrasound imaging system .The structure of purposed system including the high frequency ultrasonic data acquisition unit combined with the excitation/reception circuit, mechanics for linear scanning, high frequency transducer and electronics for scanning control has been discussed [4]-[8]. This study attempts to build the high frequency driving circuit and uses high speed N-P channel MOSFETs technique to increase the trigger frequency and the image resolution. Experrmental This study is to use the coding pulse in exciting transducer for high performance. Consideration of matching the physical characteristic of the transducer, we use bipolar pulse to excite transducer. We use microprocessor (FPGA) to produce two pulses with precise timing control. One is invert and with a delay from the other, and they will turn on N-channel and P-channel MOSFET respectively. The two pulses pass though isolator (ADuM3400, Analog Device Inc.) [2], rail-to-rail comparator (TLV3501, Texas Instrument Inc.), MOSFET driver (MD1213, Supertex Inc.), enhancement-mode MOSFET pair (TC6320, Supertex Inc.) and two diodes package (BAV99, NXP Semiconductors) [9]. Finally, we can get the bipolar pulse at BAV99 output. The Schematic of pulser circuit block diagram is shown in Figure1. In order to obtain precise timing control to generate the pulses, we choose FPGA with Cyclone II be the microprocessor of the system. We implement the PLL module to promote the clock to 200 MHz, then using divider module to divide 200 MHz into the desirable center frequency of output function. Likewise, a divider module is used to divide 200 MHz into several PRF. The PRF is always smaller than center frequency of output function. Figure 2 shows the results of simulation for 200 MHz PLL waveform and two PWM signals in Quartus II 9.0. For the function of adjustable pulse cycle, we use finite-state machine (FSM) to implement. It will alternately generate two pulses until FSM runs into final state. Figure 3 shows that the pulses are generated alternately according to FSM. In this case, pulse cycle is two, and the final state of FSM will be four. While FSM runs into state “four”, next state will be state “zero” and wait next trigger signal. In addition, we should give the FPGA the external trigger function. It is important because B-mode images get the information of correct position and scale by external trigger. After system is triggered and two pulses are generated completely, system will wait trigger signal to be low (voltage < 0.8 V). When trigger signal is low, the FSM turns its state into state “four” and a counter for delay will start count. An adequate delay time will be set to prevent system from being triggered by jitter voltage wave of trigger signal. Results and Discussions In Figure 4 was shows photographs of the prototype bipolar pulser generator board, the output of the bipolar pulser drove a 50 Ω load. It is used to excite a 50 MHz transducer (SEUT-506, Acoustic Sensor Co., Ltd.). We try to use different cycle numbers and center frequency bipolar pulses to excite transducer. The difference between echo signals was observed. Figure 5 shows a monocycle 60 MHz pulse with amplitude of approximately 100 Vpp and its spectrum, Figure 6 shows a two-cycle 60 MHz bipolar pulse with amplitude of approximately 100 Vpp and its spectrum, Figure 7 shows a three-cycle 65 MHz bipolar pulse with amplitude of approximately 100 Vpp and its spectrum. Figure 8 plots the comparison the pulse/echo waveforms using unipolar pulse ( 5900 PR) and the bipolar pulse of prototyped pulse generator. The bipolar pulser generated a 33 ns 100 Vpp monocycle pulse, and the 5900PR generated a 35 ns 105 Vpp negative spike respectively. The resulted revealed the amplitude of received echo waveform was increased more 4 dB than the unipolar pulser used. Conclusions. A high-frequency ultrasound imaging board has been described in this paper. The experimental results show that the bipolar pulse frequency is over 60 MHz. This system can be used in dermatologic and small animal imaging etc. The clinical performance of the system will be further improved. The designed pulser generator is suitable for applications in home_made high-frequency ultrasound image systems and Doppler imaging system in the future. Acknowledgements This research is supported by the National Science Council, R.O.C. under contract Nos. NSC-101-2221-E-218-002 and NCKU-Delta Nos.D101-15105. References [1] A. Brown and G. R. Lockwood, “A low-cost, high-performance pulse generator for ultrasound imaging,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 49(6), pp. 848–851, 2002. [2] Xiaochen Xu, Jesse T. Yen, and K. Kirk Shung, “A low-cost bipolar pulse generator for high-frequency ultrasound applications,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 54(2), pp. 443–447, 2007. 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