ELSEVIER Ultrasonics 34 (1996) 567-570

Characterization of optoacoustic transducers

Rajan Bhatia, Peter A. Lewin*, Qian Zhang

Department of Electrical and Computer Engineering and Biomedical Engineering and Science Institute, Drexel University, Philadelphia, PA 19104. USA

Abstract

This paper describes a measurement technique specially developed to characterize optoacoustic sources in terms of their acoustic output. Attention is focused on laser assisted devices in which appropriately delivered light energy is converted into acoustic shock waves. A meaningful comparison of such devices with other therapeutic equipment designed for a direct interaction with tissue

requires knowledge of the energy needed for a successful surgical treatment. It is demonstrated that knowledge of the key shock wave parameters allows the total acoustic energy associated with the shock wave to be determined. The procedure developed to calculate this energy is discussed and it is shown that the value of this energy can be conveniently used as an indicator of efficacy of an optoacoustic converter in a clinical environment. The influence of the performance of the PVDF hydrophone probes on the measurement results was also analyzed. It was determined that when appropriately selected, the wideband PVDF probes are well suited for characterization of the optoacoustic devices in the frequency range l-100 MHz. The characterization procedure developed is applicable to surgical ultrasonic systems including conventional and laser assisted phacoemulsifiers.

Keywords: Ultrasonic exposimetry; PVDF hydrophone probes; Ultrasonic and shock wave phacoemulsifiers.

1. Introduction

The field of acoustic characterization of piezoelectric ultrasonic transducers has matured in the past decade

and today quantitative ultrasonic measurements of piezoelectric acoustic or imaging transducers in the frequency range l-10 MHz are carried out on a daily basis. In contrast, it appears that little work has been done in the field of characterization of optoacoustic sources, which are finding an increasing use in therapeu- tic applications. These applications include comminution of stones, cornea1 surgery, capsulotomy, and removal of cataracts. In addition, there is a growing interest in using optoacoustic sources for tumour therapy. It is clear that in order to optimize these surgical procedures, characterization of the sources is highly desirable. The

primary goal of this work was to develop a quantitative measurement method that would allow immediate com- parison of two therapeutic devices, namely a conven- tional, low frequency CW ultrasonic phacoemulsifier and a new, Laser Assisted Shock Wave Probe. An immediate application of both devices is in cataract

* Corresponding author. Fax: + 1-215-895-4983; e-mail: lewin@ece.drexel.edu

0041-624X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SOO41-624X(96)00042-X

removal [ 1 J. In the following, materials and methods

are briefly described and the measurement results of these two surgical devices are presented.

2. Materials and methods

To facilitate interpretation of the experimental data

presented here, it may be useful to briefly outline the principles of operation of the conventional ultrasonic phacoemulsifier probe and the laser assisted shock wave probe.

2.1. Conventional ultrasonic phacoemulsijier probe

A fairly detailed description of the principles of opera- tion of a conventional phacoemulsifier can be found in [ 11. In the context of the results presented here it is

appropriate to mention that an ultrasonic phacoemulsi- fier is used for disruption and removal of cataracteous tissue and that the probe acts as a transducer or energy converter. More specifically, the probe transforms electri- cal energy into mechanical energy interacting with a cataract tissue. This mechanical energy is generated in the form of mechanical vibrations and the effectiveness

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of the phacoemulsifier depends on the displacement amplitude of the vibrating tip and the number of cavita- tion events produced [l-3]. A typical conventional phacoemulsifier operates at approximately 55 kHz (con- tinuous wave) frequency.

For maximum efficiency of energy conversion, the drive frequency matches the resonance frequency of the acoustic vibrator. Also, to maximize the acoustic energy produced by the device, a velocity transformer is included in the design. This transformer amplifies the displacement amplitude of mechanical vibrations. The displacement amplitude at the distal end of the surgical tip is of the order of 100 urn [l-4] and is directly related to the acoustic energy generated by the device.

Although conventional ultrasonic phacoemulsifier devices have been used in clinical practice in the past 25 years, the exact mechanism that causes the tissue frag- mentation is not known at present. What is known is that disruption only occurs if the vibration tip is in direct contact with the cataractous tissue. However, it is conceivable that both direct mechanical tearing due to the movement of the needle tip, and cavitation phen- omena contribute to the cataract disruption. Collapsing cavitation bubbles produce cavitation jets and these jets cause microscopic dents at the cataract surface, thus accelerating the cataract disruption [ 11. A typical pha- coemulsification time is about 120 s.

2.2. The laser assisted optoacoustical phacoemulsijier probe

The laser assisted phacoemulsifier (LAP) probe offers an alternative to conventional phacoemulsification tech- niques. In contrast to the continuous operation of the conventional ultrasonic probe, the LAP can operate in a single or repetitive pulse mode [S]. The probe acts as an energy converter which transforms light energy sup- plied by a laser source into mechanical energy interacting with a cataract tissue. More specifically, the probe uses a metal target which, upon illumination at appropriate light intensity levels evaporates and generates an acous- tic shock wave, which disrupts the cataractous tissue. The comminuted cataract tissue is removed from the eye by continuous aspiration. The effectiveness of the LAP probe depends on the characteristics of the acoustic shock wave pulse generated. These characteristics include such parameters as peak compressional and rarefactional pressure amplitude, rise time of the wave and its duration, and the overall shape of the pressure - time dependence.

Preliminary clinical tests indicate that a typical cata- ract removal procedure requires the use of approximately 600 pulses. In this context, it is appropriate to note that knowledge of the energy contained in a single shock wave pulse is sufficient to determine the total acoustic energy associated with successful cataract removal.

Preliminary in vitro experiments ruled out the exis- tence of cavitation and indicated that the mechanism of cataract disruption when using the LAP can be classified as a mechanical one. This is because the plasma gener- ated by the impact of light on the target extends to a maximum distance of 300-500 urn only. Hence, the most likely mechanism of interaction between the tissue and the LAP is associated with mechanical stresses produced by a shock wave in the cataract. This wave is initially propagating into the irrigation liquid (saline solution) and enters the cataract where it undergoes multiple reflections consistent with acoustic energy transmission and reflection laws. These reflections lead to a creation of tiny cracks within the cataract and generate forces that assist in a gradual fracture of the cataract.

2.3. Measurement arrangement

All measurements were carried out in a water tank lined out with Sorbothane@ to minimize the influence of the standing waves produced by a conventional ultrasonic phacoemulsifier. The tank was filled with distilled, deionized and degassed water. Both needle (Force Institutes, Copenhagen-Brondby, Denmark) and membrane type PVDF hydrophone probes (Sonic Technologies, Hatboro, PA) were used to record the pressure - time waveforms generated by the laser assisted probe. A detailed description of these wideband probes can be found in [ 63.

A PZT hydrophone (Type B&K 8103, Bruel & Kjaer, Copenhagen, Denmark) was used for acoustic character- ization of a conventional 60 kHz phacoemulsifier. The hydrophone has a uniform sensitivity versus frequency in the range from 0.1 Hz to 100 kHz. Also, at the frequency at which the phacoemulsifier operates, this hydrophone exhibits an omnidirectional directivity pattern, to within + 1 dB. That is an important feature as it minimizes the requirement for a mechanical align- ment between the hydrophone and the tested probe.

2.4. Conventional ultrasonic phacoemulsijier measurements

The conventional, CW ultrasonic phacoemulsifier was measured at the 100% of the output power setting; this was done to obtain the maximum value of acoustic output power. The calibrated PZT hydrophone was placed at a distance of about 30 mm from the applicator tip to reduce the effects of nonlinear propagation [2-41. The pressure amplitudes were measured as a function of the radial distance (30 to 80 mm) from the applicator’s tip and displayed on the Tektronix TDS 520 oscilloscope.

2.5. Measurement of laser assisted optoacoustic probe

The laser assisted probe (LAP) was measured at the 95% laser energy setting. This was done to prevent any

R. Bhatia et al. I Ultrasonics 34 (1996) 567-570 569

uncontrolled damage of the fiber optic which was used to transfer the light energy from the laser output to the vicinity of the metal target. The shock wave measure- ments were performed according to the FDA Guidelines on Measurements of Shock Waves [7], The LAP was placed on the axis connecting the center of the orifice with the center of an active element of a PVDF polymer hydrophone. The PVDF hydrophones were moved along the radial distance from the LAP orifice in order to determine the pressure amplitude versus distance depen- dence. The radial distance varied from 1.5 to 100 mm. The pressure - time waveforms recorded by a calibrated PVDF polymer hydrophone were captured by the Tektronix TDS 520 oscilloscope and transferred to a printer via an IEEE 488/GPIB interface.

3. Results

As already mentioned the primary goal of this work was to develop a quantitative measurement method which would allow immediate comparison of a conven- tional, low frequency CW ultrasound phacoemulsifier and a new, laser assisted shock wave probe. However, several additional aspects of the new probe design were tested. These included: the influence of the cavity volume and laser source stability. In addition, the pressure amplitude dependence on the radial distance was deter- mined for both devices. Also, the influence of the con- struction and frequency response of the wideband PVDF hydrophones was examined. Here, for brevity, only the results concerned with the acoustic output energy meas- urements will be discussed. The plots of Fig. 1 show the pressure - time waveforms generated by a conventional ultrasonic phacoemulsifier and by a laser assisted probe. These waveforms were used to estimate the total acoustic

energy needed by each device to perform a successful cataract surgery.

4. Discussion

As mentioned above the pressure - time waveforms of Fig. 1 were used to calculate the total acoustic energy generated by a conventional ultrasonic phacoemulsifier probe and by a laser assisted shock wave phacoemulsifier probe. Prior to the calculations a monopole source model behavior of the probes suggested in [4] was verified. The verification was done by measuring the pressure amplitudes as a function of radial distance. The measurements were taken at the distance varying from 1.5 to 100 mm and clearly indicated an l/r dependence.

The total acoustic energy needed for a successful surgical treatment when using the tested conventional ultrasonic phacoemulsifier probe was determined to be approximately 4.2 J and was calculated as a product of the intensity and the effective treatment time (typically 120 s). The corresponding energy produced by the tested laser assisted probe was approximately 400 mJ; this calculation assumed a typical number of shots needed to complete a surgery to be 600. It would thus appear that the cataract removal performed using a laser assisted probe requires significantly lower acoustic energy.

5. Conclusions

A method of characterization of a new design of optoacoustic therapeutic device has been described. It was shown that the wideband PVDF hydrophone probes are well suited for measurements of acoustic output of the laser assisted probes, which produce pulsed shock

(b)

Fig. 1. (a) Pressure - time waveform generated by a conventional ultrasonic phacoemulsifier probe and measured by the B&K 8103 hydrophone

at the radial distance of approximately 30 mm. The peak-to-peak amplitude was measured to be 120 mV and the operating frequency: 58 kHz.

The pressure sensitivity of the PZT hydrophone at this frequency was 26 pV Pa-‘. (b) Pressure - time waveform generated by a laser assisted

shock wave phacoemulsifier probe and measured by a wideband PVDF hydrophone at the radial distance of 7.5 mm. The peak compressional

voltage amplitude corresponds to 38.8 mV. The pressure sensitivity of the PVDF hydrophone was 13.5 mV MPa-‘.

570 R Bhatia et al. / Ultrasonics 34 (1996) 567-570

waves interacting with tissue. The results of the measure- ments indicated that the pressure amplitude generated by both devices decreases according to the l/r law. This confirms the validity of modeling these devices by using a monopole approach. In addition, it was demonstrated that acoustic output measurements allow comparison of the total energy needed by different therapeutic devices for a successful completion of the surgical procedure.

Overall, the method described provides a convenient and effective tool for the design and optimization of the performance of the ultrasonic and optoacoustic thera- peutic device prototypes.

Acknowledgments

The authors would like to thank Dr J. Dodick, (Department of Ophthalmology, Manhattan Eye, Ear

and Throat Hospital, New York), Dr C. Sherwood (IOLAB Corporation, Claremont, CA), Dr W. Collins (Ethicon Endo-Surgery, Cincinnati, OH) and Dr R. Thyzel for many valuable discussions and for their constant encouragement.

References

[l] H.G. Trier, J. d’Echographie et de Med., Ultrasonore 1 (1985) 17. [2] IEC Draft, TC 87, WG6, Ultrasonics - Surgical Systems (1995). [3] K. Beissner, Fortschrifften d. Akustik, DAGA (VDE Verlag,

Berlin, 1980) 567. [4] M.E. Schafer, A. Broadwin, IEEE Ultrasonics Symposium,

(1994) 1903. [S] J.M. Dodick, Surgical Instrument with Input Power, US Patent

no 5324282. [6] P.A. Lewin, Ultrasonic Exposimetry, Ziskin and Lewin, Eds.

(CRC Press, 1993) 185. [7] FDA Guidelines on Measurements of Shock Waves (1992).