1. Field of the Invention
This invention relates to an apparatus for focusing ultrasonic waves in a focal line to obtain good focusing of the sound field including good lateral resolution over a considerable depth.
2. Description of the Prior Art
Ultrasonic methods are becoming increasingly important in technology and medicine, for example, in material testing or in medical diagnosis. One known method is echo sounding, in which a sound head emits an ultrasonic pulse and the pulse reflected from an obstacle is received by a different sound head or the same sound head. The distance between the transmitter/receiver and a reflecting object can be determined from the time elapsing before the echo is received.
A measure of the accuracy of an echo-sounding method is the longitudinal resolution, i.e. in the direction of the sound waves, and the lateral resolution, i.e. at right angles to the longitudinal direction. When an ordinary conventional transducer is used, a relatively good longitudinal resolution can be obtained, but the lateral resolution is inadequate because of the relatively large diameter of the ultrasonic beam. If the diameter of the sound head is reduced, the beam has a wide divergence owing to diffraction phenomena. An improvement can be obtained by weakly focusing the beam so that its diameter becomes a minimum at the center of the object under observation. The minimum diameter must not be made too small, however, since otherwise the beam divergence again becomes too large. Typically, a beam diameter of 1-2 cm is obtained if a frequency of 2 MHz is to be used for observation over a depth of 20 cm.
A much better lateral resolution can be obtained if the ultrasonic beam is focused with a wide-aperture system. The lateral resolution may be in the order of magnitude of the wavelength, i.e. about 0.75 mm in the case of 2 MHz. This good lateral resolution is obtained, however, over only a very small depth, i.e. also approximately one wavelength.
It will therefore be clear that the disadvantage of the prior-art-echo-sounding methods is due to the fact that an improvement in lateral resolution is always accompanied by a reduction of the depth over which it can be obtained.