1. Field of the Invention
The present invention relates to an ultrasonic probe, and, more particularly, to an ultrasonic probe which has a weighted piezoelectric unit for generating ultrasonic waves.
2. Description of the Related Art
An ultrasonic probe is utilized as an ultrasonic transducer, which transmits and receives ultrasonic waves, in a medical ultrasonic diagnosis apparatus for providing information on the interior of a disease in a living body, by way of example. The ultrasonic probes are classified into several types by their functions and shape, one of which is an array type ultrasonic probe. The array type ultrasonic probe comprises a plurality of strip-shaped piezoelectric units arranged in their width direction, to which an electronic scanning method such as a sector driving method or the like is applied.
FIG. 1 illustrates the configuration of a conventional array type ultrasonic probe. The probe comprises a plurality of strip-shaped piezoelectric units 2, each of which has electrodes 1a and 1b formed on both main surfaces, respectively. Piezoelectric units 2 are arranged on backing material 3 in the width direction of piezoelectric units 2. The width direction of piezoelectric units 2 is defined in a left-to-right direction in FIG. 1, while the longitudinal direction of piezoelectric units 2 in a direction from the plane of the drawing sheet to the back. Acoustic matching layer 4 is provided to cover transmitting and receiving surfaces of respective piezoelectric units 2. In addition, filler 5 is filled in gaps between respective piezoelectric units 2. Actually, acoustic matching layer 4 is provided on a piezoelectric plate before it is separated into respective piezoelectric units 2, and then filler 5 is filled, so that filler 5 also extends through acoustic matching layer 4.
In regard to the array type ultrasonic probe as described above, JP, 5-23331, A and JP, 7-274292, A propose that each piezoelectric unit 2 is weighted in the longitudinal direction of piezoelectric unit 2 to enhance the vibration amplitude in a central region in the longitudinal direction and to reduce the vibration amplitude in both end regions, thereby providing ultrasonic beams with smaller side lobes.
FIGS. 2 and 3 are side cross-sectional views each illustrating an example of a weighted ultrasonic probe. FIGS. 2 and 3 illustrate cross-sectional structures in a plane perpendicular to the cross-section in FIG. 1, so that the left-to-right direction in the figures corresponds to the longitudinal direction of piezoelectric unit 2.
In the ultrasonic probe illustrated in FIG. 2, piezoelectric unit 2 is divided into a plurality of piezoelectric elements in the longitudinal direction. Here, piezoelectric unit 2 is divided into five piezoelectric elements 2d, 2b, 2a, 2c, and 2e, as illustrated in left to right, each of which is weighted. These piezoelectric elements 2a to 2e are electrically connected in parallel with one another. Piezoelectric elements 2a to 2e are weighted such that the degree of polarization is maximized in central piezoelectric element 2a, and becomes gradually smaller toward piezoelectric elements 2d and 2e, respectively, upon the poling process for piezoelectric unit 2. By thus controlling the degree of polarization in piezoelectric elements 2a to 2e, the vibration amplitude is maximized in central piezoelectric element 2a and minimized in piezoelectric elements 2d and 2e at both ends, even if respective piezoelectric elements 2a are applied with the same driving voltage, resulting in a smaller beam width of ultrasonic waves in the longitudinal direction of piezoelectric unit 2 to provide an ultrasonic beam with suppressed side lobes.
Likewise, in the ultrasonic probe illustrated in FIG. 3, piezoelectric unit 2 is divided into a plurality of piezoelectric elements, here, 2d, 2b, 2a, 2c, and 2e. Central piezoelectric element 2a is directly applied with a driving voltage, while the remaining piezoelectric elements 2b to 2e are applied with the driving voltage through respective resistors 6. Here, the resistances of resistors 6 are controlled to supply a maximum current to central piezoelectric element 2a and a minimum current to piezoelectric elements 2d and 2e at both ends. In this way, the vibration amplitude is maximized in central piezoelectric element 2a and minimized in both end piezoelectric elements 2d and 2e, thereby providing an ultrasonic beam with suppressed side lobes.
In the foregoing conventional ultrasonic probes, however, the piezoelectric unit must be mechanically divided into a plurality of piezoelectric elements in the longitudinal direction for weighting the piezoelectric unit, giving rise to a problem of complicated manufacturing process. Since a signal line is routed for each piezoelectric element to apply the driving voltage, the resulting ultrasonic probe also has a complicated structure. The complexity of the structure as well as manufacturing process causes a reduction in productivity of the ultrasonic probe. Also, the weighting as mentioned above can only change the vibration amplitude in steps, i.e., on an element-by-element basis, thus failing to sufficiently suppress side lobes. Furthermore, if the piezoelectric elements are weighted by changing the degree of polarization for each piezoelectric element, the ultrasonic probe will experience difficulties in controlling even the vibration amplitude.