1.  Field of the Invention
The present invention relates to an ultrasonic probe for transmitting ultrasonic waves into a subject to be examined and receiving the reflection waves from the subject to be examined and an apparatus for obtaining an ultrasonic image comprising said ultrasonic probe for obtaining diagnostic information inside the subject to be examined. 
2.  Description of the Related Art
An apparatus for obtaining an ultrasonic image is known for producing an image inside a subject to be examined based on the reflection waves from the subject to be examined by scanning inside the subject to be examined with ultrasonic waves.  Such an apparatus for obtaining an ultrasonic image transmits ultrasonic waves inside a subject to be examined by an ultrasonic probe with ultrasonic transducers, and receives the reflection waves generated by non-matching with the acoustic impedance in the subject to be examined. 
The ultrasonic probe is provided with a plural of ultrasonic transducers arranged in the scanning direction.  The ultrasonic transducers generate ultrasonic waves by oscillation based on the transmission signal and produce receiving signals by receiving the reflection waves.  As explained, the transmission/reception of ultrasonic waves is done by ultrasonic transducers; however, the acoustic impedance of the ultrasonic transducer and that of the subject to be examined differs greatly.  As a result of ultrasonic waves being directly transmitted into the subject to be examined, the reflection is excessively large.  For this reason, an acoustically matching layer is installed between the ultrasonic waves and the subject to be examined to make the acoustic agreement favorable by changing the acoustic impedance gradually with the acoustic adjusting layer.  In addition, the ultrasonic waves generated from flat ultrasonic transducers are transferred in the shape of a plain wave, resulting in the ultrasonic waves being dispersed naturally.  For this, an acoustic lens is used to converge the ultrasonic waves at a focal point at a certain depth, as taught in Japanese Unexamined Patent Application 2000-201929 and Japanese Unexamined Patent Application H3-128048, the disclosure of which are hereby incorporated by reference. 
Here the ultrasonic probe relating to the prior art is explained in reference to FIG.  1.  FIG.  1 represents a perspective view showing the outline constitution of the head of the ultrasonic probe relating to the prior art.  The ultrasonic probe consists of a head side and cable side, but only the head side is shown in FIG.  1. 
As shown in FIG.  1, for the ultrasonic probe relating to the prior art, an ultrasonic transducer section 3 is installed on a backing material 2, and an acoustic matching layer 4 is installed on the ultrasonic transducer section 3.  Here the acoustic matching layer 4 is constituted by a first acoustic matching layer 4a and a second acoustic matching layer 4b.  In addition, an acoustic lens 5 is installed on the acoustic matching layer 4.  Further, the ultrasonic transducer section 3 and acoustic matching layer 4 are multiply divided and configured in the scanning direction. 
However, with the ultrasonic probe relating to the prior art having the above constitution, the ultrasonic waves are not sufficiently converged immediately after being transferred from the acoustic lens 5 to the subject to be examined, providing an unstable sound field.  The image obtained based on the unstable sound field is unclear compared to an image in a region where ultrasonic waves are converged. 
Here the sound field to be formed by the ultrasonic probe is explained in reference to FIG.  2A and FIG.  2B. FIG.  2A shows the sound field distribution of ultrasonic waves to be transmitted or received by the ultrasonic probe relating to the prior art.  FIG.  2B shows the sound pressure at distance x from the surface of the ultrasonic probe relating to the prior art.  At the same time, the sound field distribution shown in FIG.  2A and FIG.  2B is that formed by the ultrasonic transducer of a disk (diameter a), and is extracted from the handbook of ultrasonic wave equipment for medical application (Electric Industrial Association of Japan: EIAJ), the disclosure of which are hereby incorporated by reference. 
FIG.  2A shows the sound field distribution of ultrasonic waves radiated from the surface of the ultrasonic probe relating to the prior art, in which the intensity of sound pressure is expressed by the shade of color.  In FIG.  2A, the thicker portion indicates higher sound pressure and the thin portion lower sound pressure of ultrasonic waves. 
An area in which the distance x from the surface of an ultrasonic probe is less than D1 is referred to as a short distance sound field whereas an area exceeding the distance D1 is referred to as a long distance sound field.  The position of distance D1 corresponds to the position of the focus of ultrasonic waves in the slice direction (direction perpendicular to the scanning direction).  Namely, the position of distance D1 corresponds to the position at which an ultrasonic wave beam is focused in the slice direction.  The width of the beam pattern of ultrasonic waves relates to the resolution of ultrasonic wave images at which a narrower width provides higher resolution. 
With the ultrasonic probe relating to the prior art, as shown in FIG.  2A, the sound field distribution near the surface of the ultrasonic probe (near the surface of acoustic lens 5) provides a sound field distribution in the stripes of complicated intensity.  The ultrasonic wave image produced based on the signal obtained from the region with sound field distribution in such stripes does not provide a clear image.  In the prior art, since the surface of the acoustic lens 5 forms a contact surface to a subject to be examined (living body), the sound pressure distribution near the subject to be examined (living body) is uneven, thus preventing a clear image from being obtained near the surface of the living body. 
The sound field distribution shown in FIG.  2A and FIG.  2B is that formed by ultrasonic transducers of a disk shape, but ultrasonic transducers of a rectangular shape show similar characteristics as that shown in FIG.  2A and FIG.  2B, although the distribution shape differs to some extent. 
The ultrasonic waves transmitted into a subject to be examined (living body) are attenuated inside the subject to be examined.  Particularly in the case of reflection waves from a deep position, the attenuation is large, thus making the intensity of the reflection wave small.  Therefore, in order to receive the reflection waves from a deeper spot while maintaining their intensity, the ultrasonic transducer is required to be large.  However, the larger the ultrasonic transducer is, the larger the top end of the ultrasonic probe (contact surface with the subject to be examined) becomes, resulting in the lack of the operability.  In order to upgrade the operability of ultrasonic probes, the contact surface to a living body must be reduced.  For this purpose, the ultrasonic transducer should be minimized.  Smaller ultrasonic transducers degrade the sensitivity, preventing a clear image from being obtained. 
On the other hand, making the ultrasonic transducer causes thickness in the direction perpendicular to the scanning direction, resulting in receiving unnecessary reflection waves in that direction.  This deteriorates the definition of the ultrasonic wave mage. 
Also, a puncture method is popularly conducted by injecting puncture needles such as injection needles into a living body to sample tissues including tumors or local injection of medicine.  Said puncture method is conducted by observing tomography image obtained by an ultrasonic wave image processing device in order to assure the correct puncturing of tissues such as target tumors. 
In order to inject the abovementioned puncture needle into a living body, a puncture adapter is used.  For the puncture adapter, an acoustic coupler to be mounted on the transmission/reception surface of an ultrasonic probe for use is known.  The acoustic coupler for external mounting has a puncture needle guide to inject the puncture needle into a targeted spot, and the puncture needle is injected into a living body through the puncture needle guide, as taught in Japanese Unexamined Patent Application 2005-144028 and Japanese Examined Patent Application H1-17693, the disclosure of which are hereby incorporated by reference. 
In addition, a method for injecting a puncture needle into a living body is known in which a notch on a part of an ultrasonic transducer and a puncture needle guide on the notch section are provided. 
Injecting a puncture needle into a living body at around the center of the transmission/reception plane of an ultrasonic probe by using an acoustic coupler for external fitting or providing a notch on a part of an ultrasonic transducer allows the puncture needle to be depicted at around the center of a tomography image.  In this way, a puncture needle is allowed to be injected into a living body while identifying the puncture needle from the near part of the body surface. 
When the acoustic coupler for external mounting relating to the prior art is used, the puncture needle can be checked visually by a tomography image before injecting the puncture needle into a living body.  However, in this case, the ultrasonic probe should be used under the state in which the acoustic coupler for external mounting is removed.  Therefore, the driving condition of the ultrasonic probe should be set under the state when the acoustic coupler for external fitting is removed.  This driving condition includes the driving voltage to be applied to the ultrasonic transducer, the focus position of ultrasonic waves, etc. 
In the case of imaging with mounting the acoustic coupler for external mounting to the ultrasonic probe after the driving condition was set when the acoustic coupler for external fitting is removed, the driving condition may not be appropriate when the acoustic coupler for external fitting is mounted.  For example, if the driving voltage of the focus position of ultrasonic waves is set to properly drive the ultrasonic probe when the acoustic coupler for external fitting is removed, there will be a problem of excessively low driving voltage or deviation in the focus position from a desired position with mounting the acoustic coupler for external mounting. 
On the other hand, in the case of imaging with removing the acoustic coupler for external mounting from the ultrasonic probe under the state while setting the driving condition under the state when the acoustic coupler for external mounting is mounted, the driving condition may not be appropriate under the state when the acoustic coupler for external fitting is removed.  For example, when setting the driving voltage or focus position of ultrasonic waves to properly drive the ultrasonic probe with the acoustic coupler for external mounting mounted, there may be a problem of excessively high driving voltage or deviation in the focus position from the desired position when the acoustic coupler for external mounting is removed. 
As explained above, the ultrasonic waves have not been sufficiently converged immediately after being transferred from the acoustic lens outwardly, resulting in an unstable sound field.  The ultrasonic wave image obtained from the unstable sound field will be unclear compared to the ultrasonic wave image in a region where ultrasonic waves have been converged.  In the ultrasonic probe with the acoustic coupler removed, the sound field distribution near the body surface is not uniform as the surface of the acoustic lens is the contact surface with the living body, preventing a clear image from being obtained near the surface of a living body. 
When a notch section is provided on a part of the ultrasonic transducer, which is a generating source of ultrasonic waves, there may be a problem of unstable luminance of the tomography image due to deteriorated sensitivity of the notch section.