Broadband ultrasonic attenuation: are current measurement techniques inherently inaccurate?

Measurements of broadband ultrasonic attenuation (BUA) are currently made using large aperture, piezoelectric transducers. The use of such a receiver is known to lead to the possibility of an overestimate of ultrasonic attenuation due to phase cancellation and it is shown theoretically that this same effect can also lead to an overestimate of BUA. Using a new scanning approach, BUA was measured using two methods, one sensitive to the phase of the acoustic wave, the other not. The phase sensitive BUA measurements were found to be of significantly higher value (p < 0.0001) than the phase insensitive measurements with a mean difference of 31.2 dB MHz-1. These findings are compatible with the hypothesis that BUA measurement systems with large aperture, piezo-electric receivers are inherently inaccurate.

Ultrasonic wave fluctuations through tissue: an experimental pilot study.

The fundamental role of the assumption of plane wave ultrasonic propagation in attenuation and scattering measurements on soft tissues is discussed. The evidence for and against the validity of the assumption is shown to be inconclusive. An experimental procedure for direct assessment, by measurement of the ultrasonic amplitude and phase fluctuations occurring after an interrogating plane wave has passed through a tissue specimen, is described. The phase measurements impose significant demands on the mechanical precision, temperature stability and electronic timing capabilities of the measurement system. Measurements are reported on two specimens of fresh beef liver. One exhibited significant fluctuations of amplitude and phase which were reduced by 50% when the effects of thickness variations in the specimen were removed. The other specimen exhibited relatively small fluctuations. The feasibility of making such measurements has been demonstrated and the detailed features of the experimental procedure-which requires considerable care-have been outlined. Although more measurements are needed for definitive conclusions, these will require the development of a reliable technique for determining the presence of gaseous microbubbles in tissue in vitro. The preliminary conclusion that some samples of liver may exhibit low levels of wavefront disruption is consistent with recent evidence in the literature.

Study of pressure of the normal anterior tibial compartment in different age groups using the slit-catheter method.

Using the slit-catheter method, the pressure of the anterior tibial compartment was studied in thirty volunteers, whose ages ranged from seventeen to eighty-five years, at rest and during and after exercise. The average resting pressure was +5.1 millimeters of mercury. There was a wide variation between subjects, although 95 per cent of them had a resting pressure of less than twelve millimeters of mercury. This pressure was not consistently or significantly affected by age. The pressure increased with exercise, but in an average of 3.4 minutes after the exercise was stopped it returned to within one millimeter of mercury of the resting pressure. The resting pressure was also found to be affected by the positions of the lower extremity and the trunk during measurement. To reduce this effect, it is recommended that for measurement of pressure in the lower extremity the patient should be lying supine and the big toe should be pointing vertically upward.

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.

Application of ultrasound in assessing burn depth.

Conventional pulse echo ultrasound equipment was modified to provide resolution capable of distinguishing the interfaces in burnt skin. The identification of these interfaces allowed a quantitative assessment of the depth of a burn. Ultrasound is non-invasive and accurate, and so is highly acceptable for clinical use.