Biomedical subjects

Alain Birer

Publications and source records attributed to Alain Birer.

7 recordsLinked to original sources

PubMed2007

High intensity ultrasound clamp for bloodless partial nephrectomy: In vitro and in vivo experiments.

In some patients at risk of disease recurrence of renal cancers, maximum conservation of the kidney is possible through partial nephrectomy. However, bloodless surgery is difficult to achieve. The article describes an ultrasonic clamp, which optimises energy deposition and monitors lesion development with an echo-based technique. Using this novel apparatus, coagulation necroses have been obtained in vitro on substantial thicknesses (23 to 38 mm) over exposure durations ranging from 10 s to 130 s, and with acoustic intensities of less than 15 W/cm(2) per transducer. When used for coagulation purposes, two transducers situated on opposite arms of the clamp are driven, while for monitoring, only one is used. Lesions are monitored in real time by analysing the echo signal returned by the opposite arm of the clamp. The presence of a lesion is evaluated on the basis of energy changes and echo phase as a function of time. Both kidneys of two pigs (30 to 36 mm thick) were treated in vivo with the clamp, and the partial nephrectomies performed proved to be bloodless.

Animals↗

PubMed2006

Numerical field intensity factor calculations for 1-3 piezocomposite structures.

The 1-3 piezocomposite transducers used in specific medical applications--as lithotripsy--must be excited by intense electrical impulses. By inference, the composite material is subjected to high-stress levels. To explain possible failure, a numerical tool for analyzing singular two- and three-dimensional stress fields in piezocomposite structures is proposed. Following a finite-element iterative method, singularity parameters values and intensity factors for the mechanical stresses and the electrical field are computed in several bimaterial configurations. The finite-element analysis uses commercial software.

Ceramics↗

PubMed2004

Comparison of two methods of treatment for intraluminal thermal ablation using an ultrasound cylindrical phased array.

Intraluminal (within the alimentary tract) thermal surgery has been shown to be a useful therapeutic option when extracorporeal focused ultrasound applicators cannot be used since their beam may not reach the target site. If plane transducers are used for the treatment of alimentary tract tumours, the applicator must be rotated in order to generate a cylindrical volume of necrosis. However, rotating these applicators and controlling their shooting direction presents technical difficulties. If tubular transducers are used it is difficult to treat arbitrary angles with a large therapeutic length. To solve these difficulties, the feasibility of an ultrasound phased array applicator has been evaluated using a cylindrical prototype (outer diameter 10.6 mm), which is composed of 16 elementary transducers working at 4.55 MHz and arranged on a quarter of the cylinder. Using this applicator it is possible to generate plane or cylindrical waves. Plane waves were generated by exciting eight successive elements of the array with appropriate delay times. The exposure direction was changed by exciting a different set of eight elements. In this way, the ultrasound beam was electronically rotated through the tissues. Cylindrical waves were generated by exciting several transducers without delay times. Imaging was provided using a miniature echographic probe. Ex vivo experiments were carried out in pig liver to compare two approaches of treatment. The first consisted of generating successive plane waves separated from each other by a 6 degrees angle. The second one consisted of exciting all the 16 elements without delay times. In the two cases, the lesions were well-defined and occupied a quarter of cylinder. In both sets of experiments, the sonication time and the intensity were 20 s and 17 W/cm(2), respectively. In the first case, the depth was up to 17 mm compared to 6 mm in the second case.

Animals↗

PubMed2004

Generation of higher pressure pulses at the surface of piezo-composite materials using electrical pre-strain.

New clinical concepts in lithotripsy demand smaller shock heads. Reducing the size of piezoelectric shock heads requires increasing the surface pressure of each transducer so that the total pressure at the focus remains the same. A new method allowing generation of large surface pressures is described. The hypothesis is that piezoelectric plots in piezo-composite material are more fragile in extension than in compression. For this reason, actuators are mechanically prestressed between two flasks. This method cannot be used for transducers working at high frequencies, such as 0.5 MHz. So we tried to electrically prestrain compressively the piezoelectric material by applying a high-electrical field in the opposite direction of polarization. Three protocols were tested and compared to classically driven transducers. In the first protocol, prestrain is permanently applied, in the second protocol prestrain is applied for 100 micros before the compressive impulse, and in the third protocol prestrain is applied for 100 micros and followed by a bipolar field that allows the material to be repoled between two successive pulses. With the two first protocols, rapid depoling and repoling in the opposite direction was noticed. Only with the last protocol was it possible to increase the maximum surface pressure. This increase was approximately the same whether the material was hard or soft. Using this protocol, aging tests were conducted on three samples of each kind of material, and a pressure of 4 MPa was obtained over 10(6) shocks. This value seems to be enough to develop a piezoelectric shock-wave generator with a diameter of approximately 20 cm instead of the 45-cm commercially available.

Journal Article↗

PubMed2002

Ultrasound cylindrical phased array for transoesophageal thermal therapy: initial studies.

This work was undertaken to investigate the feasibility of constructing a cylindrical phased array composed of 64 elements spread around the periphery (OD 10.6 mm) for transoesophageal ultrasound thermotherapy. The underlying operating principle of this applicator is to rotate a plane ultrasound beam electronically. For this purpose, eight adjacent transducers were successively excited with appropriate delay times so as to generate a plane wave. The exposure direction was changed by exciting a different set of eight elements. For these feasibility studies, we used a cylindrical prototype (OD 10.6 mm) composed of 16 elementary transducers distributed over a quarter of the cylinder, all operating at 4.55 MHz. The active part was mechanically reinforced by a rigid damper structure behind the transducers. It was shown that an ultrasound field similar to that emitted by a plane transducer could be generated. Ex vivo experiments on pig's liver demonstrated that the ultrasound beam could be accurately rotated to generate sector-based lesions to a suitable depth (up to 19 mm). Throughout these experiments, exposures lasting 20 s were delivered at an acoustic intensity of 17 W cm(-2). By varying the power from exposure to exposure, the depth of the lesion at different angles could be controlled.

Esophageal Neoplasms↗

PubMed2002

Numerical simulation of the electro-acoustical response of a transducer excited by a time-varying electrical circuit.

Existing methods for the modeling of piezoelectric transducer response are generally frequency domain-based. The major disadvantage of this type of model is that they cannot take into account the electrical elements present in the emitting or receiving circuit whose values vary with respect to time. The need for a method that accounts for time-varying elements arises, for example, when the circuit comprises active electrical elements, such as diodes, or when the transducer is excited by capacitive discharge via a switch. Indeed, in this last example, it is known that the output impedance of the generator depends on the state of the switch: if it is off, its value is high; if it is on, its value is low. A time domain-based method is presented to compute the electro-acoustical response of a piezoelectric transducer and its electrical circuit, taking into account the presence of time-varying elements. An application to a current example makes it possible to show the influence of these elements on waveforms and the capacity of our model to account for them.

Journal Article↗

PubMed2002

Damping, amplitude, aging tests of stacked transducers for shock wave generation.

New clinical concepts in lithotripsy demand small shock heads. Reducing the size of piezoelectric shock heads will be possible only if the pressure generated at the surface of each transducer can be increased so that the total pressure at the focus remains the same. To solve this problem, different solutions were proposed. For example, it has been demonstrated that piezocomposite material, as opposed to piezoceramic material, allows the generation of a higher surface pressure before breaking, mainly because radial modes are dramatically reduced. In addition, in a previous paper, we showed the feasibility of generating high-pressure pulse waves without increasing the transducer voltage by using sandwiched transducers, which are a stack of two or more transducers. Some discrepancies appeared, however, between the pressure measured at the surface of the front transducer and the arithmetic sum of the pressures generated by each transducer constituting the stack. In fact, development of such stacked transducers capable of generating surface pressures in the range of 2 to 5 MPa is very complex, which may explain why no aging tests have been reported in the literature thus far. In the first part of this paper, we theoretically determine the importance of the electroacoustical coupling between the two transducers on the generated surface pressure. We show that pressure losses due to these electroacoustical couplings are less than 5%. Experimental measurements done on a stacked transducer assembled and tightened in a castor oil-filled tank are in excellent accordance with the theoretical measurements. Using this assembly technique, it was possible to obtain, on average, out of four elements, a pressure of 7.5 MPa for the duration of 4 million shocks, which would allow the treatment of approximately 1000 patients.

Computer Simulation↗