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Biomedical subjects

F A Duck

Publications and source records attributed to F A Duck.

At least 37 records · Page 2Linked to original sources
PubMed1997

Measurement of radical production as a result of cavitation in medical ultrasound fields.

This paper describes the use of the terephthalate dosimeter to measure the threshold and extent of cavitation arising from medical ultrasound sources at high frequencies. Significant hydroxyl radical production was noted from a physiotherapy source and low level activity was also detected when using a pulsed, diagnostic type transducer system. The effect of sound intensity and the type of field is described. The possibility of using polymer degradation to monitor the cavitation is also discussed.

Calibration↗
PubMed1995

Technical note: measurement of collapse cavitation in ultrasound fields.

This note describes a method for the measurement of hydroxyl free radical concentration due to collapse acoustic cavitation in medical ultrasound fields using aqueous terephthalic acid (TA) solution. An enclosed cylindrical chamber with acoustically transparent membranes at either end was used. Control of cavitation activity was achieved by seeding the solution with polystyrene microspheres to provide cavitation centres. Insonation experiments using unseeded TA previously exposed to air gave very variable results, sometimes detecting cavitation activity and at other times detecting nothing, under the same exposure conditions. Introduction of polystyrene microsphere seeds into the TA enabled it to detect reproducibly levels of cavitation activity at least one order of magnitude higher than in unseeded solutions. An experiment using the seeded TA in a standing wave ultrasound field, set up using a brass reflecting plate, demonstrated that the presence of a standing wave inhibited the measured cavitation yield.

Free Radical Scavengers↗
PubMed1994

A study of the heating capabilities of diagnostic ultrasound beams.

A simple device for the experimental study of the heating capabilities of diagnostic ultrasound beams is described. Some results are reported that demonstrate the manner in which the device may be used to explore the heating potential of any particular commercial transducer, operating over the full range of output conditions. The heat generated in the base of a polyethylene container, filled with water, was measured using a fine-wire thermocouple, attached externally. The majority of measurements were carried out in beams generated by a curved array operating with a modern commercial scanner (Doppler, 2.5 MHz: imaging 3 MHz). A temperature rise in excess of 30 degrees C was generated by a pulsed Doppler beam, when the water path and scanner controls were set appropriately. Comparable temperatures were measured at comparable intensities generated by Doppler beams of other scanners. Of the imaging beams studied, the greatest temperature rise observed was less than 2 degrees C, when the highest frame rate and line density were selected. The greatest temperature rise in colour Doppler mode was 7.8 degrees C. It was observed that the position of the fixed (nonelectronic) focus was significant in controlling the heating profile with depth, for scanned beams. As expected, there was a strong dependence of temperature rise on axial time-average intensity. A weak dependence on -6 dB beam area was observed over a range of beam area of about 7 to 70 mm2. A strong dependence on finite amplitude effects was observed, resulting from energy loss associated with acoustic shock propagation.

Hot Temperature↗
PubMed1992

Bioeffects in echocardiography.

Two mechanisms have been identified through which ultrasound as it is used clinically could produce biologically significant effects. One is heating that results from the absorption of ultrasonic energy by tissues. The other is cavitation, the ultrasonic activation of gas bodies including the potentially violent collapse of small gas bodies in or near tissue that is sometimes called transient or inertial cavitation. The heart, itself, is well perfused and the likelihood of significant heating of the heart tissues in the most extreme conditions known today is negligible. Lung also appears to be relatively immune to heating under diagnostic exposure conditions. In normal echocardiographic procedures, the only tissues that need serious consideration are the ribs. Under extreme conditions, ultrasonic heating of the bone might be as great as 6 degrees C. Nonthermal action of ultrasound has been demonstrated to cause lung hemorrhage at pressure levels on the order of 1 MPa. Although many diagnostic devices produce focal pressures greater than this amount, it appears unlikely that hemorrhage will occur in normal echocardiographic applications. Under certain conditions, pulsed ultrasound can either stimulate or modify the contraction of the heart but the exposures required are not used in normal echocardiographic applications. Since specific devices have been identified who's outputs approach levels required to produce thermal and nonthermal effects, the user should be aware of potential biological effects, particularly in pediatric or obstetric applications, as output levels increase.

Animals↗
PubMed1992

A comparison of ultrasound exposure in therapy and pulsed Doppler fields.

A detailed comparison of the ultrasound exposure in water from a therapeutic beam and a pulsed Doppler beam was carried out. A significant overlap in acoustic power was found between therapy intensity levels used clinically and the upper end of the diagnostic range, between approximately 100 mW and 200 mW. In addition, pulse pressure amplitudes in the range 0.5-1.0 MPa were measured close to the transducer on both units. It is common to use physiotherapy equipment at pulse average intensities of 0.5 W/cm2 or less, and at these levels exposures of similar magnitude may be obtained with beams currently defined as therapeutic and those available from pulsed Doppler equipment.

Acoustics↗
PubMed1991

Trends in diagnostic ultrasound exposure.

Trends in exposure for diagnostic ultrasonic equipment and a contemporary survey of exposure values are presented. It is demonstrated that both time-averaged intensities and peak pressures have increased steadily over the last two decades. Currently available diagnostic equipment generates pulses for which the average worst-case rarefaction pressure, p-, is about 2 MPa in water, irrespective of the mode of operation. Overall the highest values of p- are found in short-focus beams. Average spatial-peak time-averaged intensities, I(SPTA), are greatest for pulsed Doppler operation at a little over 1 W cm-2. M-mode and colour Doppler operation have average values of I(SPTA) of about 100 mW cm-2 and imaging operates at about an order of magnitude lower again. Vaginal transducers differ little in operational exposure from other transducers. Exposure measurement on colour Doppler systems demands a knowledge of the effect of scanner controls on the field, and some details are given.

Humans↗
PubMed1991

Forces acting in the direction of propagation in pulsed ultrasound fields.

This paper considers some non-thermal effects resulting from absorption of acoustic energy from an ultrasound beam. An experimental investigation of the location of the 'source pump', responsible for the generation of streaming in high amplitude diagnostic fields in water, is reported. Acoustically transparent membranes were inserted in the ultrasound field in order to restrict the streaming volume. It is shown that the major contribution to an acoustic stream is generated in the region near to the focus of a transducer where the intensity in the beam and the degree of non-linear distortion are both high. In the second part of the paper a simple model of non-linear propagation is used to predict the magnitude of the maximum pressure gradient induced in a medium by the absorption of acoustic energy from a beam. Propagation in water, in tissue and in amniotic fluid are considered. Within the limitations of this model it is shown that the pressure gradients induced in pulsed acoustic fields do not result in the ultimate shear stress of tissue being exceeded.

Amniotic Fluid↗
PubMed1989

An experimental investigation of streaming in pulsed diagnostic ultrasound beams.

Streaming is shown to occur in water in the focused beams produced by a number of medical pulse-echo devices. The use of hot film anemometry to measure the streaming velocity is described and velocities measured in water using commercial equipment are quoted. The highest velocities occur in pulsed Doppler mode with a maximum velocity of 14 cm s-1 being observed. An experimental set-up was used to investigate the parameters affecting streaming and it was found that the harmonic content of the pulse waveform had a major effect on the streaming velocity. The time taken for a stream to become established at the focus of the acoustic beams studied was typically approximately 0.5 s.

Calibration↗
PubMed1989

Surface heating of diagnostic ultrasound transducers.

Surface temperatures of a variety of transducers used with common commercial ultrasonic diagnostic equipment have been measured. Transducers operating in imaging mode, in both continuous and pulsed Doppler modes, and in mixed modes were investigated. A total of 30 transducers and scan-heads used with equipment from 10 manufacturers were examined, including a range of array types, mechanical sectors and continuous-wave Doppler transducers. Measurements were made using an infrared radiometer, or a thermocouple probe, with the transducers operating in air. Surface temperatures of 13 transducers operating in imaging mode were found to be in the range 0.0-13.1 degrees C above ambient after 5 min operation. Some transducers operating in pulsed Doppler mode reached considerably higher temperatures. The most extreme example increased the surface temperature by 36.5 degrees C after 1 min and reached a steady-state temperature of almost 80 degrees C. Transducers operating at these temperatures cannot be retained on the skin of a conscious subject without pain, and will cause skin burns within a brief period of time. A linear relationship has been demonstrated between temperature increase and spatial-average acoustic intensity. The rate of increase in air was found to be about 10 times greater for pulsed arrays than for continuous-wave Doppler transducers.

Hot Temperature↗
PubMed1987

A survey of the acoustic output of ultrasonic Doppler equipment.

Measurements of the acoustic output generated by a variety of clinical ultrasonic Doppler instruments have been carried out. The instruments surveyed include continuous wave Doppler units for cardiovascular investigations, fetal monitors, stand-alone pulsed Doppler equipment and 'duplex' scanners working in Doppler mode. Acoustic measurements have been made using a calibrated PVDF membrane hydrophone, and a milliwatt radiation force balance. Almost all the pulsed Doppler and duplex systems investigated could generate spatial-peak, temporal-average intensities in water which exceeded 100 mW cm-2, with a maximum of 825 mW cm-2 measured. These intensities were also reached by some continuous-wave Doppler systems, particularly those incorporated into duplex scanning systems. Fetal monitoring equipment was found to operate typically at lower intensities. Doppler units have been found to show a very wide variation in pulse length, repetition frequency and spatial peak pressure, and a wider range of pulse average intensities than previously reported. Units were found to vary considerably in their control of total acoustic power whilst operating parameters such as gate width and range were altered.

Acoustics↗
PubMed1986

The locations of peak pressures and peak intensities in finite amplitude beams from a pulsed focused transducer.

The effect of finite-amplitude distortion on the positions of peak pressures and peak intensities in beams generated in water by pulsed focused transducers has been investigated experimentally. The pulses generated by three single-element, circular, focused transducers with nominal frequencies of 2.25, 3.5, and 5.0 MHz have been investigated with pressures at the transducer P0 being varied over the range 10 kPa to 1.4 MPa. Measurements were made using a 9 micron thick polyvinylidene difluoride membrane hydrophone. In all cases the locations of peak pressures were not stationary as the field strength was altered. As P0 increased, the distance from the transducer to the positive peak initially increased, and then decreased. The distance to the negative, or decompression peak decreased monotonically with increasing P0. The location of peak pulse intensity integral was found to alter slightly with power, taking a position between the peak positive and negative pressures.

Calibration↗
PubMed1986

The development of harmonic distortion in pulsed finite-amplitude ultrasound passing through liver.

The progressive development of finite-amplitude distortion of ultrasonic pulses has been investigated in excised bovine liver using pulsed focused ultrasonic beams at nominal frequencies of 2.5 and 3.5 MHz. Both the transducers and the powers used were those which may be encountered with clinical imaging equipment. Significant distortion of the waveform was observed to occur, particularly at higher powers. For example, at 2.5 MHz, with a mean input pressure (p0) of 0.58 MPa, the second harmonic in the pulse spectrum showed a maximum value of 10.5 dB below the fundamental and the highest third harmonic component was 19 dB below the fundamental. These particular observations illustrate that finite-amplitude distortion may be of considerable significance in the transmission through tissue of ultrasonic pulses during diagnostic scanning.

Animals↗
PubMed1985

Evidence for ultrasonic finite-amplitude distortion in muscle using medical equipment.

Finite-amplitude distortion of ultrasonic waves from medical equipment has been observed to occur following transmission through calf muscle in human volunteers. Measurements were made using both dynamic pulse-echo imaging equipment and physiotherapy equipment. In both cases irradiation was carried out under operating conditions commonly used clinically. Pressure waveforms were measured at the skin surface using a broadband polyvinylidene difluoride membrane hydrophone. Using a pulsed, weakly focused 2.5-MHz beam with input peak pressure of 0.8 MPa and a pressure gain of 5.3 at the focus, the mean second harmonic peak magnitude (16 measurements) was 17 dB below the fundamental peak. A 1.1-MHz continuous wave therapy set with input peak pressure of 0.5 MPa showed mean second harmonic magnitude 23 dB below the fundamental.

Adult↗
PubMed1985

The output of pulse-echo ultrasound equipment: a survey of powers, pressures and intensities.

A survey of the powers, pressures and intensities generated by ultrasonic pulse-echo equipment in clinical use has been carried out. Three conventional B-scanners, four linear-array scanners and four mechanically sectored scanners were included in the study. Measurements were made on a total of 22 transducers covering the nominal frequency range 2.25-7.5 MHz. On those instruments where an output power control was provided, two measurements were made: one at the maximum available power and a second at a lower power. On arrays with a variable transmit focus control, measurements were made at all available focus settings. In all, measurements were made on 38 separate focused pulsed ultrasonic fields. The measurements were carried out using a calibrated ultrasonic force balance, and a calibrated polyvinylidene difluoride (PVdF) membrane hydrophone. A very wide range of maximum powers, pressures and intensities were found. Powers from 0.5-80 mW were measured; spatial-average temporal-peak positive pressures at the transducer varied between 30 kPa and 1.15 MPa, and spatial-peak pulse-average intensities were in the range 3.6 X 10(3)-1.1 X 10(7) Wm-2.

Equipment Design↗