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This chapter describes ultrasonic surgical devices for medical therapy. The chapter begins with a discussion of device requirements and goals, including a brief historical review and a discussion of different tissue types. This is followed by a review of the general characteristics of ultrasonic surgical devices. The next section summarizes the various methods of action that have been identified, as well as intended and unwanted biological effects. The various devices in common use are discussed, and an overview of the regulations that govern ultrasonic surgical devices is given. Finally, potential future developments within the field are described.

In the past few years, High Intensity Focused Ultrasound (HIFU) has developed from a scientific curiosity to an accepted therapeutic modality. Concomitant with HIFU's growing clinical use, there has been a need for reliable, economical and reproducible measurements of HIFU acoustic fields. A number of approaches have been proposed and investigated, most notably by Kaczkowski et al (Proc. 2003 IEEE Ultrasonics Symposium, 982-985). We are developing a similar reflective scatterer approach, incorporating several novel features which improve the hydrophone's bandwidth, reliability, and reproducibility. For the scattering element, we have used a fused silica optical fiber with a polyamide protective coating. The fused silica core is 73 microns in diameter with a 5 micron thick polyamide coating for a total diameter of 83 microns. The fiber was prepared by cleaving to yield a perpendicular/flat cut. The fiber is maintained in position using a capillary tube arrangement which provides structural rigidity with minimal acoustic interference. The receiver is designed as a segmented, truncated spherical structure with a 10cm radius; the scattering element is positioned at the center of the sphere. Each segment is approximately 6.3 cm square. The receiver is made from 25 micron thick, biaxially stretched PVDF, with a Pt-Au electrode on the front surface. Each segment has its own high impedance, wideband preamplifier, and the signals from multiple segments are summed coherently. As an additional feature, the system is designed to pulse the PVDF elements so that the pulse-echo response can be used to align the fiber at the center. This is important when the need arises to change the fiber due to, for instance, cavitation damage. The hydrophone can also be designed with a membrane structure to allow the region around the scatterer to be filled with a fluid which suppresses cavitation. Initial tests of the system have demonstrated a receiver array sensitivity of -279 dB re 1 microVolt/Pa (before preamplification), with a scattering loss at the fiber of approximately 39dB, producing an effective sensitivity of -318 dB re 1 microVolt/Pa. The addition of the closely coupled wideband preamplifiers boosts the signal to a range which is sufficient for the measurement of HIFU transducers. The effective bandwidth of the system exceeds 15MHz, through careful design and the use of PVDF as a sensor material. In order to test the system, a HIFU transducer in the 4.0MHz frequency range was tested at low output settings using a conventional PVDF membrane hydrophone. The prototype system was then used to characterize the same HIFU transducer at full power. The results showed good correlation between waveforms and cross-axis beam measurements, taking into account the additional shock losses at higher output settings.

Over the last 15 years, ultrasonic surgical devices have become the preferred instruments for cataract surgery within the ophthalmologic community. This approach, called phacoemulsification, uses needle tips which vibrate longitudinally at frequencies between 28 and 50 kHz (depending upon the manufacturer). Mechanical impact and inertial cavitation at the tip both act to erode and liquefy the lens material, which is then aspirated out through the needle core. Surgeons have reported that despite the aspiration flow through the needle, which would tend to bring the lens material into contact with the needle tip, lens material seemed to be repulsed from the tip region. It was theorized that acoustic streaming caused by sound radiation from the tip was the reason for this effect. One recent advance in phaco technology was the introduction of microbursts, or very short (less than 10ms) pulse lengths. These short bursts appeared to reduce the repulsion effect, while at the same time retaining the cutting effectiveness of continuous wave (CW) excitation. The purpose of this study was to investigate both the potential acoustic streaming and cavitation effects as a function of burst length. Measurements of cavitation energy were made using PVDF membrane hydrophones and custom designed analysis software; visualization of flow patterns in the region of the needle were made using a high resolution diagnostic ultrasound imaging system in color Doppler mode. The results show that cavitation levels from these short burst modes are similar to CW excitation at the same excursion levels, while the streaming patterns in the tip region were markedly reduced. This matches with clinical results, which indicate superior cutting effectiveness when compared to CW operation, with better "hold" on the cataract.

Over the last 15 years, ultrasonic surgical devices have become the preferred instruments for cataract surgery within the ophthalmologic community. This approach, called phacoemulsification, uses needle tips which vibrate longitudinally at frequencies between 28 and 50 kHz (depending upon the manufacturer). Mechanical impact and inertial cavitation at the tip both act to erode and liquefy the lens material, which is then aspirated out through the needle core. This study attempted to determine the total acoustic exposure of the eye during a typical cataract removal procedure. This paper reviews the methods of determining the acoustic exposure from phaco devices, provides a clinical example, and demonstrates the utility of providing this information to the surgeon. A series of nearly 200 surgeries were performed by a single surgeon, using either of two different phacoemulsification systems. Each cataract was graded for relative density and hardness. After each surgery, system settings and surgical times were recorded. The acoustic energy radiated into the eye was calculated assuming a monopole model, consistent with IEC standard 61847, with inputs of frequency, tip excursion, tip dimensions, and exposure time. Power was determined as (/spl rho//sub 0/ck/sup 2/Q/sub s//sup 2/)/8/spl pi/, where Q/sub s/ is the source strength (radiating area times the normal surface velocity). Simplifying assumptions were made to account for factors such as excursion ramp up, ramp down, and pulsing sequences. In addition, one or the two systems used a unique microburst ultrasound pulsing scheme using bursts as short as 6 milliseconds. The results indicated that the total acoustic energy ranged from 0.01 joules for a grade 1 cataract to up to 0.3 joules for a grade 5 cataract, with an exponential increase in ultrasound exposure with cataract grade (e.g. y = 0.0033e/sup 0.66x/, R/sup 2/ = 0.974). Further, after an initial run of experiments, the surgeon was able to modify the system control settings to reduce the patient exposure by 20 percent without compromising clinical effectiveness. The conclusions from this work are: (1) the novel pulsing approach using very short bursts appears to significantly reduce the total acoustic exposure; (2) if surgeons are given dosimetric information, they can reduce exposure without reducing clinical effectiveness; (3) the data currently available from manufacturers are very difficult for clinicians to interpret and compare between devices; (4) based on the latter two conclusions, manufacturers are encouraged to provide on-screen labeling of acoustic output.

In the context of diagnostic ultrasound coupling fluid a technique is presented to measure fluid attenuation over a wide frequency range using time-delay spectrometry (TDS). With the TDS technique, a swept sinusoidal signal drives a wideband transmitter facing a wideband receiver at a known separation distance. By accounting for the time delay between the transmitter and receiver, it is possible to isolate the direct acoustic path from any reflected paths, through narrowband filtering. The attenuation measurement was conducted as a comparison of a water-only path and one with a sample of liquid interposed between the transmitter and receiver. Specially designed test cells were constructed to contain various liquids as thin planar slabs. The results of tests on several liquids, such as castor oil and polyethylene glycol, are given, and the advantages and limitations of the technique are pointed out. A significant advantage of the TDS-based approach is that it provides data as a virtually continuous function of frequency. >

A technique for analyzing the surface velocity patterns of ultrasonic transmitters based on the angular spectrum method of wave-field analysis is presented. In this approach, acoustic propagation between parallel planar surfaces is modeled using the two-dimensional Fourier transform of the wave field, with each element in the spatial frequency domain multiplied by the appropriate phase factor. The technique was extended from the basic monochromatic model to the wideband pulsed case. An experimental system was built to measure the acoustic fields from various transducers, including the single-element and multi-element phased arrays. Backpropagation results are shown for circular planar, circular focused, and rectangular phase-steered transducers. The results demonstrate the ability of the extended angular spectrum method to reconstruct the surface velocity distribution of complex acoustic radiators. >

Extracorporeal shock wave lithotripsy (ESWL) has recently become the preferred procedure for the treatment of urinary and gall bladder stones. The device used in this treatment generates acoustic shock waves which are focused onto the stone, causing it to disintegrate. Interest in studying the properties of the shock waves quantitatively has intensified, since it is believed that the acoustic shock wave parameters, such as rise time, peak positive and peak negative pressure amplitudes, and frequency content, may influence the treatment's efficiency. In the present work the authors describe the evaluation of several piezoelectric materials and design configurations for their applicability to quantitative shock wave measurement. Preliminary experimental results indicate that PVDF (polyvinylidene fluoride) polymer sensors with a low acoustic impedance backing have the greatest potential as shock wave sensors. >

The authors' objective is to develop a flexible, noninvasive 3-D wound surface reconstruction system which can be used to evaluate physical characteristics, such as perimeter, area, depth and volume of wounds. The authors describe here their system which relies on a new temporal structured light (SL) coding method using a LCD projection system. In SL, the scene is illuminated with a pattern of light and is observed from another vantage point. Analysis of this image gives the 3-D surface points after establishing correspondence and triangulation. The authors report on initial performance tests of this system on phantoms and compared the results to physical measurements and saline infusion method. A sample pig skin wound reconstruction is also given.

Therapeutic ultrasound has been widely used to treat various diseases including chronic wounds. However, the mechanisms of the ultrasound effects in wound healing and the best regimen of treatment is still unclear. In this study, the authors used skin electrical impedance measurement during ultrasound treatment and detected a fall in local skin impedance under ultrasound while simultaneous temperature measurement did not show any rise. This result supports the non-thermal effects of the ultrasound and indicates that ultrasound may cause changes in permeability, intracellular electrolytes etc.

The authors discuss the development of a miniature precision acoustic sensor (hydrophone) which would survive in an in-vivo shock-wave field. The immediate application of the device is in improving the safety and efficacy of extracorporeal shock-wave lithotripsy (ESWL) treatment of kidney and gallbladder stones. The hydrophone is designed for insertion into the body using a catheter or biopsy needle, to provide reliable in-vivo acoustic pressure measurements of the ESWL. The research efforts include, (1) theoretical modeling to examine design trade-offs associated with material choice and sensor configuration; (2) prototype construction; and (3) acoustical performance characterization. The results of prototype testing are shown and compared to existing hydrophone designs. Performance differences involve bandwidth, directivity, and survivability under shock-wave exposure. The fundamental difficulty is improving survivability without sacrificing bandwidth or signal fidelity. >

Experimental results from a study of three alternative approaches to the estimation of in-situ or derated ultrasonic intensity parameters are presented. The existing regulatory approach uses the exponential derating factor e exp(-0.069F/sub c/d), where F/sub c/ is the center frequency of the ultrasonic pulse and d is the focal depth in centimeters. This derating is applied to the values of ultrasonic intensity as measured in water, which are significantly influenced by nonlinear propagation effects. This first alternative approach applies the same general formula to all the frequency components of the ultrasonic pulse spectrum. The second alternative involves reconstructing a time waveform from only the dominant spectral peak of the pulse spectrum, adjusted to account for the energy lost in the higher harmonics. In the third approach, the centroid of the frequency spectrum is used rather than the center frequency. Results of axial beam scans at low and high energy levels (where nonlinear effects dominate) show that the alternative methods may be appropriate for regulatory purposes. >

Ultrasonic inspection of solid wood and lumber has been investigated for over 20 years, with limited commercial impact. Potential applications run from logs to finished boards, with the goal of either characterizing internal defects (knots, splits, rot), or assessing overall strength. This paper reviews recent efforts to apply ultrasound technology to the Forest Products industry.

Ultrasonic surgical instruments operating from 20 to 60 kHz have typically been characterized only by their operating frequency, cutting tip excursion, and input electrical power. This paper presents acoustical measurement methodologies which more completely characterize device output. The methods are divided into two frequency regimes: frequencies below 100 kHz which correspond to the tip driving frequency, and frequencies above 1 MHz which corresponds to the cavitation developed at the tip. Low frequency radiation is modelled as an acoustic dipole; high frequency acoustic radiation is caused by cavitation bubble collapse at the tip, generating spherically diverging shock waves. This measurement method incorporates the energy in a single shock wave event, the number of events per unit time, and the spherical divergence of the wave. Combining the two measurement techniques shows the growth of the total radiated energy as a function of excursion, and a commensurate shift from low frequency energy to cavitational energy

The measurement arrangement, which complies with the requirements of the US Food and Drug Administration, consists of a positioning system with a full range of degrees-of-freedom and a digital oscilloscope, both under complete computer control. The acoustic pressure-time waveform is recorded using membrane-type and needle-type polyvinylidene fluoride (PVDF) hydrophone probes. The overall bandwidth of the system depends on the hydrophone probe used and can range up to 100 MHz. A complete description of the system and the measurement procedures is given, along with a brief discussion of the various factors which affect measurement uncertainty. The largest overall uncertainty of the same associated with acoustic intensity measurements was determined to be no greater than 20% for I/sub sppa/ and 25% for I/sub spta/ (spatial peak pulse average intensity and spatial-peak temporal-average intensity, respectively). Other applications of the system include transducer characterization and research work in ultrasound dosimetry and bioeffects. >

Les systemes d'imagerie ultrasonore dependent des transducteurs de l'electronique de controle pour la generation des impulsions d'irradiation pour la detection des echos reflechis et la conversion des donnees en forme numerique

This paper describes a wideband acoustic source made of piezoelectric polymer (PVDF) material. The source was developed for the characterization and absolute calibration of ultrasonic hydrophone probes. Construction details are described and performance characteristics of the wideband PVDF transmitter, including its transmitting voltage response and directivity patterns, are presented in the frequency range up to 40 MHz. The KLM model was used to examine the influence of the PVDF polymer film thickness, the backing acoustic impedance, the cable length, and the electrical source resistance on overall transmit transfer characteristics.

The increasing use of extracorporeal shock wave lithotripsy (ESWL) in the treatment of kidney and gall bladder stones has intensified interest in studying the properties of the pressure waveforms used for the treatment. It is expected that the acoustic shock wave parameters, such as risetime, peak positive and peak negative pressure amplitudes, and frequency content can all influence the treatment's efficiency. The authors evaluate three prototype piezoelectric sensors for their applicability to quantitative shock-wave measurements. The results of the evaluation indicate that PVDF (polyvinylidene fluoride) polymer transducers with a low acoustic backing have the greatest potential as shock wave-sensors. >

This paper briefly outlines a spectral analysis procedure for qualitative characterization of ultrasound transducers and presents applications of the procedure for the assessment of acoustic parameters. The parameters include the transmitting and receiving frequency response, directivity pattern and effective aperture of different shape piezoelectric tranceivers. The advantages and fundamental limitations of the spectral analysis technique are discussed in terms of frequency range, signal to noise ratio and dynamic range. Experimental data are presented which show that the technique is extremely useful in transducer performance evaluation as a continuous function of frequency. Future applications of the technique, including complex, i.e. both amplitude and phase, characterization of ultrasound piezoelectric transducers, are also pointed out.