In vitro evaluation of an oscillating dual-mode ultrasound probe for sector imaging and directive therapy.

Interstitial probes have been shown as effective devices to deliver high-intensity ultrasound therapy. Here, cylindrically-focused dual-mode transducers with either one or 5-elements were characterized, and a monoelement probe was evaluated in vitro. In therapy mode, the transducers were maximally efficient (> or =70%) at 5.6 MHz with surface intensities up to 20 W/cm(2). In imaging mode, fractional bandwidths were 46% and 50+/-4% (ave+/-std) for the monoelement and 5-element transducers respectively. Axial and lateral resolutions were 0.5 mm and 1.0 mm, respectively, for both transducers as measured with a point scatterer in the focal plane. After characterization, the oscillating probe was used to image and apply therapy to porcine liver. B-mode images over a 140 degrees sector were formed before and after therapy, which was applied for 90 s at each of 5 angles separated by 20 degrees (e.g. -40 degrees , -20 degrees, 0 degrees, 20 degrees, 40 degrees) to form a composite lesion. Transducer surface intensity was 18 W/cm(2). Therapy was interrupted at 125 ms intervals to collect pulse/echo data along the therapy axes. Data were displayed in real-time as an M-mode image to monitor therapy. B-mode images adequately represented the liver tissue. M-mode image data agreed well with the formation of lesions in the liver.

[Ultrasound interstitial mini invasive probes for thermal ablation in liver: feasibility study in vivo]. / Sondes ultrasonores interstitielles mini-invasives pour l'ablation thermique dans le foie: étude de faisabilité in vivo.

High intensity ultrasounds are routinely used for thermal ablation of some cancers. However, for treating hepatic tumours with physical agents, RF applicators and cryoprobes are still preferred. The goal of the present study was to demonstrate the feasibility of using interstitial ultrasound probes in liver following two approaches: percutaneous and intra-tissular or endo vascular. In vivo trials on a porcine model demonstrated the minimally invasive nature of both procedures. Homogeneous and reproducible thermal lesions, up to 20 mm deep, were obtained. The work on these two original approaches deserves to be completed with more extended prospective studies. The association with an imaging method will have to be studied before proceeding to clinical trials.

Development of a compact self-focusing piezoelectric generator using electrical pre-strain piezocomposite material.

New clinical concepts in lithotripsy demand small shock heads. Reducing the size of piezoelectric shock heads will only be possible if the pressure generated at the surface of each transducer can be increased so that, the total pressure at the focus remains very high. So, we propose a new method allowing the generation of large surface pressures. It is well known that the piezoelectric rods in piezocomposite material are more fragile in the extension mode than in the compression mode. For this reason, actuators are mechanically pre-stressed between two flasks. This method cannot be used for transducers working at high frequencies, about 0.5 MHz. For this reason, we proposed to electrically pre-strain the piezoelectric material by applying high electric field in the opposite direction of the polarisation. In a first mode we proposed to pre-strain in continuous mode the transducer. Unfortunately we noticed a rapid de-poling and re-poling in the inverse direction. In a second mode to reduce depolarisation, this field was applied only during a short time just before the generation of the pulse which generate the compressive wave and in a third mode, the transducer was re-poled between two successive electrical pulses. Using this last method, it was possible to increase the maximum pressure at the surface of a 20 mm diameter plane piston to 20% and reach 4 MPa. According to this idea a very compact shock wave generator was designed. The generator made of a 1-3 piezocomposite material has a diameter of 120 mm and focused at 120 mm. The maximum pressure and the width of the compressive wave at the focus were, respectively, 60 MPa and 1.5 micros. The focal zone measured at -3 dB is an ellipsoid 6 mm high in the propagating axis and 3 mm width in the perpendicular direction. The efficacy of this generator was measured as the number of shocks necessary to totally disintegrate plaster balls 15 mm in diameter mimicking the kidney stones. At full power the number of shocks was only 150 which is rather the same number as the one obtained using electrohydraulic machine generally considered as the gold standard. This results show that piezoelectric material may be advantageously used for the manufacturing of shock wave generators.

1.5-D high intensity focused ultrasound array for non-invasive prostate cancer surgery.

The aim of this study is to demonstrate the feasibility of a new spherically curved 1.5-D phased array for the treatment of localized prostatic cancer. The device is designed to conform to the Ablatherm machine (EDAP-Technomed, France), a commercially available machine in which high intensity focused ultrasound (HIFU) treatment for prostate cancer is administered transrectally. It uses high intensity electronically focused ultrasound to steer a beam along two axes, allowing enough depth to be reached to treat large prostates and eliminating two degrees of mechanical movement. Through computer simulation, it was determined that a curved 1.5-D configuration offered the optimal design. Two configurations were then proposed, and their ability to steer a beam within a target volume centered on the geometric focus of the transducer was simulated. An eight-element prototype was constructed to test the piezo-composite material and its electro-acoustical efficiency. Then, an array was constructed, and a multi-channel amplifier and control system were added, to permit remote operation. Acoustical and electrical measurements were made to verify performance. Finally, the 1.5-D array was tested in vitro on samples of pig liver to confirm the ability to induce lesions.

Generation of very high pressure pulses at the surface of a sandwiched piezoelectric material.

New clinical concepts in lithotripsy demand small shock heads. Reducing the size of piezoelectric shock heads will only be possible if the pressure generated at the surface of each transducer can be increased so that the total pressure at the focus remains very high. We propose for the first time to increase the pressure without increasing the transducer voltage by using sandwiched transducers, which are a combination of several stacked transducers. When excited at appropriate time intervals, the pressure waves generated by each one reinforce when they reach the load. This new technique has been successfully tested. A pressure of 2.5 MPa was generated with two stacked, 5 mm-thick 1-3 piezocomposite transducers operating at an excitation voltage of 8 kV. No transducer damage was detected after 10(6) shocks, which corresponds approximately to the treatment of 500 patients.

The feasibility of constructing a cylindrical array with a plane rotating beam for interstitial thermal surgery.

Thermal surgery has been shown to be a useful therapeutic option when external ultrasound applicators cannot be used as their beam will not reach the target site. If plane transducers are used, the ultrasound beam has to be rotated in order to generate a sufficiently large volume of necrosis. However, rotating deep-seated interstitial applicators and controlling their shooting direction presents major technical problems. The purpose of this study is to evaluate the feasibility of constructing a cylindrical array with a plane rotating beam for ablating esophageal tumors by interstitial hyperthermia. The feasibility of such an array has been initially evaluated using a plane array (which is easier to make from a technical point of view). This array was made with a new piezoelectric material because its mechanical properties make it ideal for the construction of a cylindrical array in the future. We showed that the beam of each array element is sufficiently divergent and that cross-coupling is small enough to generate a plane wave from a cylindrical array. In addition, power tests and electro-acoustic efficiency measurements demonstrated that the output was sufficient to induce tissue necrosis in the relevant conditions.

Comparison between the effects of cavitation induced by two different pressure-time shock waveform pulses.

Acoustic cavitation generates very large localized pressures and temperatures, and thus provides a mechanism whereby physical and biological effects are produced in a high-intensity acoustic field. In this work, we studied the influence of the temporal form of a pressure pulse waveform on the destructive effects of transient cavitation. Two different shock pressure-time waveforms with nearly the same acoustic energy content were used. The first pressure waveform starts with a tensile wave followed by a compressive one, and the second pressure waveform starts with a compressive wave followed by a tensile one. These two pressure waveforms are called direct and inverse-mode pulses respectively. Based on the measurements presented in this work, we can state that, between the two types of shock pressure pulses studied, the direct-mode pulse amplifies systematically tile cavitation effect. This conclusion was achieved from a series of several quantitative and qualitative experiments: cavitation bubble collapse time, disintegration efficacy of plaster balls (a kidney stone-mimicking material), macroscopic study of lesions in agar gel and in vitro isolated rabbit liver tissue destruction. Considering these results and those obtained by other research groups, we can express that the temporal form of a shock pressure pulse has a major role on the cavitation effects.

A piezocomposite shock wave generator with electronic focusing capability: application for producing cavitation-induced lesions in rabbit liver.

In this work, a piezocomposite shock wave generator with electronic focusing capability is presented. The system is composed of a bidimensional array and its electronic hardware. The array is composed of 274 independent piezocomposite transducers arranged in a spherical shell of 280 mm in diameter and focused at 190 mm from its surface. The electronic hardware includes 274 x 6.6 kV distinct impulse generators. For the purpose of performing the electronic steering of shock waves, the delay time of each channel can be adjusted from 100 ns to 100 microseconds in steps of 100 ns. In order to enhance the effect of cavitation at the focus for the purpose of tissue destruction, the pressure-time waveform starts with a half cycle of negative pressure with a peak amplitude of about -150 x 10(5) Pa, followed by a very steep shock front with a positive peak pressure > 1000 x 10(5) Pa and a rise time of about 10 ns. Using this generator, the cavitation-induced lesions in rabbit liver were studied. To obtain a predefined lesion volume, two methods of scanning were used: mechanical and electronic. Comparison of the lesions obtained by these two methods shows that they have identical macroscopic and histological characteristics, which justify the feasibility of electronic beam steering of shock waves in tissue destruction applications.

Electronic beam steering of shock waves.

Shock-wave generators are now currently used for the treatment of renal stones. In all these generators the focal zone is determined by their geometrical parameters. We propose, for the first time, a piezocomposite shock-wave generator with electronic focusing. The system is composed of a two-dimensional array and its electronic hardware. The array is composed of 121 independent piezocomposite transducers arranged in a spherical shell 20 cm in diameter and focused at 190 mm. The electronic hardware includes 121 x 6 kV-impulse generators. The interdelay of each channel can be adjusted between 10 ns to 100 microseconds by steps of 10 ns. The results show: the use of composite material is possible for the generation of high amplitude pressure waves; the pressure-voltage relationship is linear up to a pressure of about 28 x 10(5) Pa at the transducer front face; the material can be used for a long period of time; i.e., after one million shocks, no decrease in sensitivity, no alteration in its electrical behaviour and no time wave form distortion were observed. Electronic focusing is efficient in an ellipsoidal region of about 4 cm in diameter and 6 cm in length. The pressure in the focal zone is about 600 x 10(5) Pa.

Assessment of a potentially noninvasive method for monitoring aortic blood flow in children.

Ultrasonic methods have rendered possible noninvasive quantitative blood flow measurement. In this work, a method is proposed to measure aortic blood flow in children by means of a specially designed miniaturized esophageal probe and an autonomous apparatus combining an M-mode imaging system and a pulsed Doppler. In vivo experimental results in animals are presented and demonstrate the interest of simultaneous and continuous measurement of aortic diameter and blood flow velocity giving accurate measurements. A 0.94 correlation coefficient is found when comparing blood flow in the descending aorta measured using this method and using an electromagnetic flowmeter.

A novel method to control P+/P- ratio of the shock wave pulses used in the extracorporeal piezoelectric lithotripsy (EPL).

There is growing evidence that acoustic cavitation plays an important role in stone fragmentation during extracorporeal shock wave lithotripsy (ESL) treatment. In addition, side effects of the treatment, such as the hemorrhage and destruction of the tissue in the vicinity of the stone are also ascribed to cavitation phenomenon. Since cavitation is associated with the maximum negative pressure in the shock pulse, it would thus appear that possibility of controlling this pressure would be desirable in ESL applications. This paper describes a novel technique developed to control the ratio of compressional peak (P+) to rarefactional peak pressure (P-) of the shock wave for use in lithotripsy treatment. The procedure is based on the finite amplitude wave generation by focused piezoelectric transducers and subsequent interaction of the shocked waves in the common focal region. The highly asymmetrical shock wave is produced in the focal region by providing an appropriate time delay to each of the high voltage electrical excitation signals which drive the transducers. The degree of relative reduction of negative halfcycles and the corresponding positive halfcycles amplification increases with the number of the acoustic sources used. The practical implementation of the shock wave generator was obtained by using 5 cm diameter, focused 1 MHz transmitter, and additional transducers of identical construction having frequencies corresponding to the harmonics and subharmonics of the 1 MHz frequency. The importance of the results for the future development of lithotripters, and stone treatment efficiency is pointed out.