In the Bioacoustics Research Laboratory, spherically-focused, high-frequency transducers
are used in our investigations. Because the transducer is an integral part of any acoustic
data acquisition and imaging system, it is imperative to have accurate information on its
spatial and temporal acoustic field characteristics. These characteristics include:
Spatial acoustic field characteristics:
Focal length
-6 dB transmit-receive beamwidth in focal region
-6 dB transmit-receive depth of focus
Temporal acoustic field characteristics in the focal region:
Center frequency
-3 dB bandwidth
Fractional bandwidth
-20 dB pulse duration
To obtain the acoustic pulse-echo field characteristics, a 38 オm tungsten
wire target oriented normal to the sound beam direction is scanned in a
rectangular grid pattern in a tank filled with filtered, degassed water.
The grid spacings are a half or a quarter of the acoustic wavelength, depending
on the estimated center frequency of the transducer and the estimated total
length of the scan. The spatial field distribution is obtained by taking the
pulse intensity integral of each received waveform. The pulse intensity integral
is defined as the time integral of the intensity of a pulse taken over the time
in which the acoustic pressure is nonzero. Beamwidth, depth of focus and focal
length at true focus are determined from a spatial contour map of the pulse
intensity integral. Center frequency, bandwidth, fractional bandwidth, and pulse
duration are determined from the RF signal at the true focal point.
Why is this all necessary when the transducer comes with specifications from the
manufacturer? Most likely, the system used to test the transducer at the factory
is not the same system being used in the laboratory. The transmit-receive spatial
and temporal acoustic field characteristics have been found to vary with the
excitation system used. Also, over the life of the transducer, its characteristics
or performance may change, e.g. deterioration of electrical connections. Thus
the measurement of acoustic field characteristics should be done when the
transducer is new from the factory, and periodically thereafter to ensure accuracy.
For more information on the acoustic field measurement technique:
K. Raum and W. D. O'Brien, Jr. Pulse-Echo Field Distribution Measurement
Technique of High-Frequency Ultrasound Sources. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 44, 810-815, 1997.
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K. Raum and W. D. O達rien, Jr. Pulse-Echo Field Distribution Measurement
Technique for High-Frequency Ultrasound Sources. Proceedings of the 1997 IEEE Ultrasonics Symposium, pp 1747-1750, 1997.
Additional contributions that deal with ultrasound exposure are as follows:
F. Dunn, A. J. Averbuch and W. D. O達rien, Jr. A Primary Method for
the Determination of Ultrasonic Intensity with Elastic Sphere Radiometer. Acustica, 38, 58-61, 1977.
R. C. Preston, D. R. Bacon, S. S. Corbett III, G. R. Harris, P. A.
Lewin, J. A. MacGregor, W. D. O'Brien, Jr. and T. L. Szabo. Interlaboratory Comparison of
Hydrophone Calibrations. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 35, 206-213, 1988.
C. M. W. Daft, T. A. Siddiqi, D. W. Fitting, R. A. Meyer and W. D.
O'Brien, Jr. In-Vivo Fetal Exposimetry. IEEE Transaction on Ultrasonics, Ferroelectrics, and Frequency Control 37, 500-505, 1990.
N. B. Smith, C. V. Vorhees, R. A. Meyer and W. D. O'Brien, Jr. An
Automated Ultrasonic Exposure System to Assess the Effects of In Utero Diagnostic Ultrasound.
Proceedings of the 1990 IEEE Ultrasonics Symposium, pp. 1385-1388, 1990.
J. A. Jensen, D. R. Gandhi and W. D. O達rien, Jr. Ultrasound Fields
in an Attenuating Medium. Proceedings of the 1993 IEEE Ultrasonics Symposium, pp 943-946, 1993.
D. R. Gandhi and W. D. O達rien, Jr. Nonlinear Acoustic Wave Propagation
in Tissue. Proceedings of the 1993 IEEE Ultrasonics Symposium, pp 939-942, 1993.
A. Goldstein, D. R. Gandhi and W. D. O達rien, Jr. Diffraction
Phenomena with Co-Axial Plane Piston Transducers. Proceedings of the 1994 IEEE Ultrasonics Symposium, pp 1757-1760,1994.
D. Swiney and W. D. O達rien, Jr. Human Fetal Diagnostic Ultrasound
Exposimetry System. Proceedings of the 1996 IEEE Ultrasonics Symposium, pp 1167-1169, 1996.
A. Goldstein, D. R. Gandhi and W. D. O達rien, Jr. Diffraction Effects in
Hydrophone Measurements. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 45, 972-979, 1998.
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