Ultrasonic sound speed microscope for biological tissue characterization driven by nanosecond pulse

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1 Ultrasonic sound speed microscope for biological tissue characterization driven by nanosecond pulse Naohiro Hozumi 1;, Ritsuko Yamashita 1, Cheol-Kyou Lee 1, Masayuki Nagao 1, Kazuto Kobayashi 2, Yoshifumi Saijo 3, Motonao Tanaka 3, Naohiko Tanaka 4 and Shigeo Ohtsuki 5 1 Toyohashi University of Technology, 1 1 Tempaku, Toyohashi, Japan 2 Honda Electronics Co., Ltd., 20 Koyamazuka, Oiwa-cho, Toyohashi, Japan 3 Tohoku University, 4 1 Seiryomachi, Aoba-ku, Sendai, Japan 4 Sibaura Institute of Technology, 307 Tameihara, Fukasaku, Saitama, Japan 5 Tokyo Institute of Technology, 4259 Nagatsuda-cho, Midori-ku, Yokohama, Japan ( Received 4 March 2003, Accepted for publication 17 March 2003 ) Keywords: Tissue characterization, Ultrasonic microscopy, Sound speed, Fourier transform, Spectrum analysis PACS number: Cs [DOI: /ast ] 1. Introduction Ultrasonic microscopy is expected as a powerful tool for local tissue characterization. Its clinical application to histopathological examination during surgery is being considered. The data are also important to assess the origin of clinical echographic imaging. In clinical point of view, the microscopic observation should be as quick as possible to carry out the surgery in a short period. A tissue slice of 3 10 m in thickness placed on a glass substrate is often employed as a specimen to be observed. Sound speed is considered to be a good parameter to characterize abnormality of the tissue. In order to estimate the sound speed, reflections from the front and rear surfaces of the tissue slice are compared. Time lag between these two reflections is not easy to be estimated, because the thickness is less than the wavelength of the acoustic wave, of which frequency range is normally less than several hundreds of megahertz. Therefore, phase analysis in frequency domain is often carried out [1,2]. A monotonic burst wave is irradiated in most cases, and the attenuation and phase of the reflection are analyzed. A frequency scan is needed in order to perform this measurement, however, it needs a long time to switch the frequency. As the result, recent ultrasonic microscope needs more than one hour for obtaining one microscopy. In addition, the accuracy of measurement strongly depends on the frequency stability. The device to maintain the stability tends to complicate the system and bring a high instrumentation cost. In stead of the above frequency domain measurement, the authors proposed a time domain measurement using a sharp pulse wave that has a widely spreading spectrum. The reflection was directly recorded by a digital oscilloscope and then Fourier transformed into frequency domain. Both sound speed and thickness at each point were simultaneously calculated by phase analysis. A two dimensional microscopy was obtained by mechanically scanning the transducer. This report describes the result of a preliminary experiment to prove the feasibility of the pulse driven sound speed microscope for tissue characterization. hozumi@eee.tut.ac.jp 2. Specimen A rat cardiac allograft model was investigated. The rat was killed after 7-day of transplantation without anti-immune therapy. The histological specimen was prepared as to make a cross-section of the heart, and was sliced at approximately 10 m-thick. The specimen was placed on a flat glass slide. 3. Experimental setup Figure 1 illustrates the conception of the ultrasonic microscope for tissue characterization. An acoustic wave is irradiated and received by the same transducer. Distilled water is used for the coupling medium between the specimen and the transducer. Reflections at both sides of the tissue are compared to measure the sound speed and thickness. Twodimensional profiles of reflection intensity, thickness and sound speed can be obtained by mechanically scanning the transducer. Figure 2 shows the schematic diagram of the measurement system employed for the experiment. A 0.6 m long coaxial cable with 50 ohms in characteristic impedance was charged at 200 V through a high resister. A pulse voltage was generated when the mechanical switch was closed. The repetition rate of the pulse was fixed at 50 times per second. In order to avoid multiple reflections of voltage pulse being measured with the target signal, a delay line of 17 m in length was inserted between the pulse generator and the transducer. The transducer was 1.2 mm in aperture diameter, and 1.5 mm in focal length. Its nominal frequency range was MHz ( 6 db), the central frequency being 80 MHz. An acoustic wave with a wide frequency component was generated by applying the voltage pulse, and was irradiated to the substrate. The reflection was detected by the same transducer, and was introduced into the digital oscilloscope (Tektronics TDS3032). The band limit and sampling rate were 300 MHz and 2.5 GS/s, respectively. In order to reduce random noise, four times of responses at the same point were averaged in the oscilloscope before being introduced into the computer. The transducer was mounted on an X-Y stage which was driven by the computer through GP-IB. Considering focal distance and the sectional area of the transducer, the diameter of focal spot was estimated as 20 m at 80 MHz. 386

2 N. HOZUMI et al.: ULTRASONIC MICROSCOPE DRIVEN BY NANOSECOND PULSE Fig. 1 Conception of the ultrasonic microscopy for tissue characterization. Therefore, distance between the nearest two points was set at 20 m. 4. Analysis The analysis for the pulse driven type microscopy is illustrated in Fig. 3 in comparison with the conventional burst driven type. The reflected wave is composed of the reflection at the front and rear surfaces of the tissue slice. In the pulse driven type, the reflected wave in time domain is Fourier transformed into frequency domain. Then the attenuation and phase spectra are compared with these of the direct reflection at the glass surface where no tissue is attached. As the signal Fig. 2 Schematic diagram of the measurement system employed for the experiment. is the result of the interference of two reflections, the attenuation spectrum has both maximum and minimum points along the frequency. Assuming f m as one of minimum and maximum points, and m as the corresponding phase angle, the phase lag between the above two reflections at the minimum point is ð2n 1Þ, giving 2f m 2d ¼ m þð2n 1Þ; ð1þ c 0 where d, c 0, and n are the tissue thickness, sound speed of the water, and a non-negative integer, respectively. The phase lag at the maximum point is 2n, giving 2f m 2d ¼ m þ 2n: ð2þ c 0 Fig. 3 Illustration of the pulse driven type microscopy in comparison with the conventional burst driven type. 387

3 The phase angle m can be expressed by 2f m 2d 1 1 ¼ m ; ð3þ c 0 c since m is the phase lag between the wave passed through the distance 2d with sound speed c and that passed though the corresponding distance with sound speed c 0. By solving the simultaneous equation of (1) and (3), d ¼f m þð2n 1Þgc 0 =4f m ð4þ is obtained for the minimum point. For the maximum point, eqs. (2) and (3) gives d ¼f m þ 2ngc 0 =4f m : Sound speed is then calculated by c ¼ 1 1 m : ð6þ c 0 4f m d ð5þ however, these reflections interfere so significantly that it is hard to determine the time lag between the two. As shown in Fig. 5(d), it is also difficult to see clearly the minimum and maximum points defined by Eqs. (1) and (2) in frequency domain. Therefore, we normalized the intensity and phase spectra by the reference signal. Figure 6 shows the result at the same point on the tissue. Periodical change in intensity is clearly seen. It is also clear from the plot on the complex plane that two reflections interfere together. The second minimum in the intensity spectrum that appears at 127 MHz is appropriate for the analysis, since it is close to the spectrum center in Fig. 5(b) and (d). The corresponding phase angle is 61 degrees. Applying Eqs. (1) and (3) with 1,480 m/s as the sound speed in water and n ¼ 2, c ¼ 1; 647 m/s and d ¼ 9:7 m are derived. Figure 7 shows two dimensional profiles of the specimen. It took ca. 30 minutes to acquire all data for points. 5. Results Figure 4 shows the voltage pulse as an output of the pulse generator. It has 5 ns of pulse width and a widely spreading spectrum up to 160 MHz or more. Figure 5(a) shows the reflection from the glass surface where the tissue is not attached. Although the pulse voltage applied to the transducer was impulse like, the reflection has an oscillating component, mainly due to the transfer function of the transducer. Its spectrum is shown in Fig. 5(b). The spectrum is widely spreading from 20 MHz to 140 MHz, its center being 100 MHz. This signal was employed as a reference waveform; the point being defined as the reference point. The decline of the glass surface was compensated by considering the time lags at different three points without the tissue, the points including the reference point. The detail of the decline compensation will be published on this journal by N. Tanaka et al. Figure 5(c) shows an example of the reflection from where the tissue was attached. The time domain signal shows that a small reflection appears prior to the major reflection, Fig. 4 Voltage pulse as an output of the pulse generator and its intensity spectrum. Fig. 5 Reflected acoustic waves from the specimen and their intensity spectra. 388

4 N. HOZUMI et al.: ULTRASONIC MICROSCOPE DRIVEN BY NANOSECOND PULSE Fig. 6 An example of the analysis. Optical microscopic inspection of the very next slice of the same specimen showed massive hyalinization in the endocardial side (left side of the figure), and that was classified as severe allograft rejection. The sound speed in the hyalinized lesion is 1,530 1,590 m/s, and it is significantly lower than that of normal myocardium (1,600 m/s or faster). This suggests that the lesioned part can be grasped properly. 6. Discussion Data acquisition and processing time are important factors for practical application of the microscope to tissue characterization. As was mentioned above, the time domain measurement at one point needs only one shot of acoustic pulse in principle. This brings a practical advantage of reducing time for measurement. Conventionally proposed frequency domain measurement using 20 or more different frequencies takes more than one hour for a sufficient resolution. This is mainly due to the multiple scan by changing the frequency. At this moment our prototype system needs as long as 30 minutes to finish the scan, however, we believe this will be reduced to less than several minutes by applying faster Fig. 7 Two dimensional profiles of the specimen with the resolution of pixcels. (a) Reflection intensity. (b) Sound speed. scanning system. The faster measurement brings the more precise analysis, because it can reduce the influence of environmental change such as temperature during the scan. The mechanical switch to generate the pulse voltage may be replaced by a semiconductor device to upgrade the repetition rate. As for the time for analysis, our new proposal needs a number of Fourier transforms. The preliminary analysis on LabView application program with resolution took ca. 20 seconds. It will therefore takes long time for a practical resolution. However, the time for measurement would be shortened a lot by using a faster computer and improving calculation algorithms. 389

5 7. Conclusion A sound speed microscope for tissue characterization driven by nanosecond pulse was proposed. A rough sound speed microscopy was obtained as the result of preliminary experiment. As a principle, this new method spends only one shot of acoustic pulse for one point, so that the time for measurement can be extremely reduced in comparison with the conventional burst method that needs a frequency scan. The pulse generator can be designed easily and complicated waveform adjustment is not necessary as long as the waveform contains the required frequency component. This can make the analog system simple. In order to develop an upgraded system for practical application, improvement of the speed of mechanical scan and repetition rate of the pulse is required. Acknowledgement A part of this work was supported by The 21st Century COE Program Intelligent Human Sensing of the Ministry of Education, Culture, Sports, Science and Technology. References [1] Y. Saijo, M. Tanaka, H. Okawai, H. Sasaki, S. Nitta and F. Dunn, Ultrasonic tissue characterization of infarcted myocadium by scanning acoustic microscopy, Ultrasound Med. Biol., 23, (1997). [2] H. Okawai, K. Kobayashi and S. Nitta, An approach to acoustic properties of biological tissues using acoustic microgfraphs of attenuation constant and sound speed, J. Ultrasound Med., 20, (2001). 390