Biomedical subjects

L Filipczyński

Publications and source records attributed to L Filipczyński.

At least 19 recordsLinked to original sources

PubMed2001

Nonlinear native propagation effects of diagnostic ultrasound computed and measured in blood.

Nonlinear propagation effects produced by focused pulses in blood were measured over a 20-cm range, being inspired by diagnostic applications in cardiology. The initial and maximum pressures applied during measurements in blood were equal to 0.40 MPa(pp) and 0.76 MPa(pp), while the pressure estimated at the patient body surface equalled 0.70 MPa(pp). Measurements of the frequency characteristic and the linearity of the ultrasonic probe used in experiments were performed in water. A numerical procedure developed previously was applied in blood to calculate the pressure distribution of its first and second harmonics along the beam axis. The comparison of numerical and measured distributions in blood at a temperature of 37 degrees C showed rather good agreement. Using numerical methods, a proportional growth of the second harmonic with the increased applied initial pressure was first observed, and finally the maximum limiting effect was found. In this way, much higher level of harmonics could be obtained. However, there arise the questions of the transmitting system construction and of the nonuniform resolution in the case of harmonic imaging when increasing the applied initial pressure.

Acoustics↗

PubMed1999

Temperature elevations computed for three-layer and four-layer obstetrical tissue models in nonlinear and linear ultrasonic propagation cases.

The authors computed temperature elevations in a three-layer and a four-layer tissue model, assuming the crucial obstetrical case when the ultrasonic pulse propagating through the abdominal wall and the fluid-filled bladder penetrates into soft fetal tissues. To consider nonlinear propagation, the authors applied a new theory of nonlinear increase of absorption recently developed by the first author. Computations were carried out for pulses with a carrier frequency of 3 MHz, duration time of 1.33 micros, and pulse repetition frequency of 3.3 kHz. Similar computations were carried out for a four-layer tissue model corresponding to the third trimester of gestation. The ceramic piezoelectric transducer 2 cm in diameter radiated the ultrasonic beam focused at a distance of 6.5 cm. The intensities at the radiating transducer (at the source) were I(SAPA) = 10 and 5 W/cm2. Temperature elevations and distributions were determined numerically for various values of low-amplitude absorption coefficients assumed to be the same as attenuation coefficients. It was shown in the three-layer tissue model that the maximum temperature elevation can be about 50% higher for nonlinear than for linear propagation. The maximum fetal temperature elevation in this case was 2.36 degrees C for nonlinear and 1.84 degrees C for linear propagation. The temperature elevation in the abdominal wall was lower than those temperatures when the attenuation of the abdominal wall was assumed to be a low value of 0.05 Np/cm.MHz (0.45 dB/cm.MHz). However, when it was increased to 0.16 Np/cm.MHz (1.4 dB/cm.MHz), the temperature elevation of the abdominal wall reached 3.2 degrees C and the maximum fetal elevation was 1.65 degrees C. In such cases, the abdominal wall became the principal source of heat production. In this case, the difference between fetal temperature elevations for nonlinear and linear propagation was only about 10%. The results obtained in the four-layer tissue model, in which the uterus tissue also was represented, show that temperature elevations in this case are about 3.6 times lower than in the three-layer tissue model, with comparable attenuation of the abdominal wall. Differences between nonlinear and linear propagation in the four-layer tissue model are negligible. The temperature elevations obtained were proportional to the pulse repetition frequency, without changing temperature distributions in the ultrasonic beam. In this manner, fetal temperature elevations can be reduced by reducing the repetition frequency.

Abdominal Muscles↗

PubMed1999

Nonlinear and linear propagation of diagnostic ultrasound pulses.

The effect of nonlinear propagation in fluid followed by soft tissue was studied both theoretically and experimentally for a most crucial case in obstetrical ultrasonography. For this purpose, short pressure pulses, with the duration time of 1.3 micros and a carrier frequency of 3 MHz, radiated by a concave transducer into water, with maximum intensities up to the value of 18 W/cm2, were computed and measured. The ultrasonic beam had the physical focus at the distance of 6.5 cm, where the highest focal intensity of I(SPPA) = 242 W/cm2 was obtained. In front of the transducer, at a distance of 7 cm, artificial tissue samples prepared on the basis of ground porcine kidney, with a thickness of 0.5, 1.5 and 3 cm, were placed in water. Pressure pulses and their spectral components were produced numerically and measured by means of a PVDF hydrophone in water before and after penetrating the tissue samples. The theoretical analysis and measurements were carried out, in every case, for two signal levels: for a high level assuring nonlinear propagation and for a low one where conditions of linear propagation were fulfilled. In this way, it was possible to compare directly the effects of nonlinear and linear propagation, in every case showing a good conformity of theoretical values with measured ones. A method of determination of the effective frequency response of the hydrophone was elaborated to enable quantitative comparisons of numerical and experimental results. The theoretical part of our study was based on a paper of Wójcik (1998), enabling us to compute the characteristic function of nonlinear increase of absorption. An agreement of up to 10% was obtained when comparing theoretical and measured values of these functions in the investigated beam in water and behind tissue samples. The results obtained showed that the recently given theory of nonlinear absorption, based on the spectral analysis and the elaborated numerical procedures, may be useful in various practical ultrasonic medical problems and also in technological applications.

Animals↗

PubMed1997

Electromagnetic hydrophone for pressure determination of shock wave pulses.

An electromagnetic hydrophone has been designed and tested to determine its ability to measure shock wave pulses similar to those produced by lithotripter machines. The principle of operation of the hydrophone, its design and performance are described. The hydrophone was exposed to 4000 shots and peak compressional pressures on the order of 30 MPa without any deleterious effects of its performance and operation. The hydrophone can be calibrated directly by measurement of the magnetic field of the permanent magnet and voltage induced in the electrical conductor. While the spatial resolution of the electromagnetic hydrophone is limited by the length of the vibrating conductor and was determined to be 5 mm, it can be improved. The overall bandwidth of the hydrophone, including its integral preamplifier, had to be limited to 17 MHz; however, the hydrophone appears to reproduce correctly the general shape of the propagating shock wave pulse. The influence of the hydrophone's bandwidth on the measured pulse shape and its amplitude is analysed, and it is shown that it affects rise time and peak compressional pressure. However, no deteriorating influence was observed in reproduction of peak rarefactional pressure.

Acoustics↗

PubMed1996

Amplitude, isobar and grey-scale imaging of ultrasonic shadows behind rigid, elastic and gaseous spheres.

The theory of wave reflection from spherical obstacles was applied for determination of the cause of the shadow created by plane wave pulses incident on rigid, steel, gaseous spheres and on spheres made of kidney stones. The spheres were immersed in water which was assumed to be a tissue-like medium. Acoustic pressure distributions behind the spheres with the radii of 1 mm, 2.5 mm and 3.5 mm were determined at the frequency of 5 MHz. The use of the exact wave theory enabled us to take into account the diffraction effects. The computed pressure distributions were verified experimentally at the frequency of 5 MHz for a steel sphere with a 2.5-mm radius. The experimental and theoretical pulses were composed of about three ultrasonic frequency periods. Acoustic pressure distributions in the shadow zone of all spheres were shown in the amplitude axonometric projection, in the grey scale and also as acoustic isobar patterns. Our analysis confirmed existing simpler descriptions of the shadow from the point of view of reflection and refraction effects; however, our approach is more general, also including diffraction effects and assuming the pulse mode. The analysis has shown that gaseous spherical inclusions caused shadows with very high dynamics of acoustic pressures that were about 15 dB higher in relation to all the other spheres. The shadow length, determined as the length at which one observes a 6-dB drop of the acoustic pressure, followed the relation r-6dB = 3.7a2/lambda with the accuracy of about 20% independent of the sphere type. lambda denotes the wavelength and a the sphere radius. Thus, a theoretical possibility of differentiating between gaseous and other inclusions and of estimation of the inclusion size in the millimeter range from the shadow was shown. The influence of the frequency-dependent attenuation on the shadow will be considered in the next study.

Acoustics↗

PubMed1993

Temperature elevation in focused Gaussian ultrasonic beams at various insonation times.

Transient solution of the thermal conductivity equation for the three-dimensional case of the Gaussian ultrasonic focused beam was derived and applied for cases relevant to medical ultrasonography. Quantitative results for the case of a homogeneous medium with constant values of thermal coefficients and constant absorption as well as for the two-layer tissue model used in obstetrics were presented for various diagnostic probes used in ultrasonography. The possible effects of perfusion and nonlinear propagation were neglected. The results obtained are in agreement with results of other authors when considering the steady-state and the infinitely short insonation time. The computations show the influence of the insonation time on the temperature elevation, thus making it possible to introduce its value as a factor in limiting the possible harmful effects in ultrasonography. This has been shown in diagrams presenting the temperature distribution along the beam axis of 6 different diagnostic probes for various insonation times and demonstrating the corresponding temperature decrease when limiting the insonation time to 5 and 1 min. For instance, the highest temperature elevation (for probe number 1, see Table 1) decreases 2.6 and 5 times with respect to the steady-state temperature when the insonation time equals 5 and 1 min, respectively.

Absorption↗

PubMed1992

An attempt to reconstruct the lithotriptor shock wave pulse in kidney: possible temperature effects.

Based on measurements carried out in water in two lithotriptor systems, the authors have made an attempt to reconstruct numerically amplitudes and shapes of shock wave pulses penetrating into kidney which differ from those in water. The difference between these pulses and those observed in water was analyzed and was also demonstrated experimentally. The amplitude and the steepness of the reconstructed pulse front were shown to be much lower than in water depending on the distance of the kidney stone from the patient's body surface. For a distance equal to 4 cm, the shock wave pulse amplitude of 40 MPa in water was estimated to decrease in the kidney by a factor of about two and the steepness of the positive shock pulse front to decrease several times. The analysis was carried out by considering the possible changes of absorption and attenuation in tissues which increase in an unknown way with the wave amplitude. It was shown that the temperature elevation caused by the increase of nonlinear high amplitude absorption is limited due to a corresponding increase in attenuation of the shock wave penetrating soft tissues. The temperature elevation was estimated on the basis of this work to be at most 1.8 times that one estimated in the case of two considered lithotripsy systems when assuming small amplitude absorption and attenuation coefficients.

Animals↗

PubMed1991

Estimation of transient temperature elevation in lithotripsy and in ultrasonography.

Transient solutions of the thermal conductivity equation for the two-dimensional case of an elongated cylindrical focus in the ultrasonic beam were derived and applied for lithotripsy and obstetrical ultrasonography. Assuming uniform and Gaussian distributions in the focus of the beam cross section, it was possible to estimate the temperature elevation arising in lithotripsy for various repetition frequencies of shock-wave pulses and for various radii of the beam. In obstetrical ultrasonography where the blood perfusion is difficult to determine, the authors suggested that the insonation time be used as the decisive factor for the temperature determination. Values of focal intensities were found necessary to increase the tissue temperature by 1 degree C as a function of the insonation time and the beam radius which exclude the possibility of any hazardous effect caused by temperature elevation.

Body Temperature↗

PubMed1990

Estimation of the temperature increase in the focus of a lithotripter for the case of high rate administration.

In a recent study Delius et al. demonstrated an increased amount of kidney damage when applying shock waves with a high pulse repetition frequency equal to 100 Hz. The authors interpreted this effect by means in terms of a cavitational mechanism although they could not exclude thermal effects. The present paper estimates the temperature increase for the conditions of the pre-cited study. Taking into account spectral properties of the pressure in the form of cosine, triangle pulses, including their overshoot, as well as an experimental shock wave pulse shape it was possible to compute the temperature increase within the focus of the lithotripter. The maximum estimated temperature increase was equal to 1.8 degrees C for 3000 shock wave pulses of 80 MPa peak pressure. The results of those computations exclude the possibility of significant thermal effects in the damage of kidney tissues even if the administration rate equals 100 shock wave pulses per second.

Animals↗

PubMed1990

Capacitance hydrophones for pressure determination in lithotripsy.

Capacitance hydrophones have been designed and applied for pressure measurements of shock waves in lithotripsy. Their working principle, properties, and design are discussed. Formulae are given, permitting the determination of pressures and calibration of hydrophones from electric measurements. Examples of comparative measurements with electrodynamic and PVDF hydrophones are presented, confirming the correctness of obtained peak pressure values. The hydrophones are characterized by high durability, small controlled nonlinearity, and a wide frequency band. However, they cause distortions in the shape of the overshoot of the measured shock wave pulses and their application is limited to measurements in the focal region.

Calibration↗

PubMed1986

Absorption of longitudinal and shear waves and generation of heat in soft tissues.

The author analysed the oblique incidence of a longitudinal plane wave and the generation of a shear wave at a boundary between a soft tissue and a gas, considering a viscoelastic model (Voigt solid) of the medium. From the data measured by Frizzell et al. (J. Acoust. Soc. Am. 60, 1409-1411) for shear waves, the average velocity 30 m/s and the average absorption coefficient 7.7 X 10(3) cm-1 at the frequency of 5 MHz were assumed for calculations. The amplitude and the intensity of the generated shear wave were obtained and hence the rate of heat production per unit volume was determined. At the boundary, this quantity was found to be of the same order of magnitude for the generated shear wave as for the incident longitudinal wave. In the case of the shear wave, which propagates almost perpendicularly to the boundary, it decreases rapidly with the distance. Therefore, the temperature increase caused by shear waves was negligible in respect to longitudinal waves in spite of the extremely high absorption coefficient. This conclusion could be confirmed by solving the inhomogeneous equation of heat conductivity for the case under consideration.

Absorption↗

PubMed1980