Measuring and testing – Vibration – By mechanical waves
Patent
1983-08-18
1985-06-04
Ciarlante, Anthony V.
Measuring and testing
Vibration
By mechanical waves
73642, 73632, G01N 2904
Patent
active
045206708
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and an apparatus for generating short ultrasonic pulses.
Ultrasonic echo pulses are widely used nowadays in medical diagnosis. This diagnostic technique is based upon reflections of ultrasonic energy from boundary surfaces between media having different acoustic impedance. The time interval or delay between the emission of a pulse and the reception of an echo is a measure of the distance from the ultrasonic emitter/receiver (transducer) to the reflecting surface. Thus, the same transducer is usually employed for both emission and reception.
Sound is propagated in water and in soft human tissue at a velocity of about 1500 m/s, for which reason an echo delay of 1 .mu.s corresponds to a distance of about 0.75 mm. To be able to distinguish echo pulses from adjacent boundary surfaces, the echo pulses thus must be short and well-defined. The axial resolution in the ultrasonic echo method thus is strongly dependent on the duration of the impulse response of the ultrasonic transducer.
It has previously been endeavoured to improve the mechanical construction of the ultrasonic transducer, and also a special signal processing of the received echo pulse signals immediately before the detection proper was introduced in order to determine the distances to the boundary surfaces. For example, the signal processing utilized has comprised the analogue or digital inverse filtration of the electric signals emitted by the transducer in response to received echo pulse signals. However, the results obtained with this technique have not been entirely satisfactory, inter alia because the impulse response of the ultrasonic transducer has a narrow bandpass characteristic so that it is difficult to devise suitable inverse filters.
According to the invention, short ultrasonic pulses are generated by imparting to the method and the apparatus the special features which are stated in the appended claims.
The invention will be described in more detail in the following, reference being had to the accompanying drawings.
FIG. 1 illustrates the basic principle of this invention in an apparatus according to the invention directed towards a specific test object.
FIGS. 2a and 2b show the impulse response and the associated frequency function of an ultrasonic transducer.
FIGS. 3a and 3b show the impulse response and frequency characteristic of a pulse shaping filter of the weighted least squares type.
FIG. 4 is a block diagram of a test equipment incorporating the invention.
FIGS. 5a and 5b show examples of echo signals obtained respectively without and with preshaping of the excitation pulse of the ultrasonic transducer.
The apparatus according to the invention, as shown in FIG. 1, comprises a signal generator 1 which is coupled to an ultrasonic transducer 2 for the excitation thereof. In the example illustrated in FIG. 1, the ultrasonic emitter 2 emits its pulses in a water tank towards two blocks 3, 4 of plexiglass, each having a thickness of 15 mm and being spaced apart a distance of 2 mm. It should be mentioned in this connection that the velocity of sound in plexiglass is twice as high as in water, or approximately 3000 m/s.
According to the present invention, the signal generator 1 is adapted to generate an excitation signal which substantially constitutes a weighted least squares signal to the transducer proper. More particularly, this excitation signal is generated preferably by means of a pulse shaping filter which preshapes the excitation signal to the ultrasonic transducer 2.
FIG. 2a shows an Example of the impulse response s(t) for an ultrasonic transducer 2 having a centre frequency of about 2 MHz. The corresponding frequency spectrum is shown in FIG. 2b illustrating the narrow bandpass characteristic typical of an ultrasonic transducer.
Upon conventional excitation of the ultrasonic transducer 2 with a rectangular pulse, an echo signal of the type shown in FIG. 5a is obtained, and this signal can be observed for instance on an oscilloscope 5 connected to the ultrasonic transducer 2.
If the
REFERENCES:
patent: 4271705 (1981-06-01), Crostack
Borjesson Per O.
Holmer Nils-Gunnar
Lindstrom Kjell
Mandersson Bengt
Salomonsson Goran
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