Arc scan transducer array having a diverging lens

An ultrasonic transducer array for arc scan imaging systems comprises a plurality of elongated piezoelectric transducers arranged successively to define a convexed energy radiating surface. A plano-concave acoustic diverging lens is attached to the convexed surface to diverge the acoustic energy transmitted from the transducers in an increased steering angle. The transducers are assembled on an impedance matching layer which defines the convexed radiating surface. The acoustic impedance of the diverging lens is substantially equal to the acoustic impedance of the human body, while the acoustic impedance of the impedance matching layer is greater than that of the human body.

BACKGROUND OF THE INVENTION 
The present invention relates to a piezoelectric transducer array for use 
in an ultrasonic imaging system particularly for medical diagnostic 
applications, and in particular to an arc scan type piezoelectric 
transducer array. 
Ultrasonic transducer arrays are broadly classified under the categories of 
linear scan type and sector scan type. Conventional linear scan type 
arrays comprise piezoelectric transducers, 256 in number, which are 
successively linearly arranged side by side. A group of 16 transducers is 
selectively activated by delayed burst pulses generated by a commonly 
shared transmit circuitry so that a focused ultrasonic beam is 
transmitted. The selected group is successively shifted to the next by one 
transducer element to shift the beam linearly to the next, so that the 
ultrasonic energy is scanned in a rectangular format. Advantages of the 
liner scan imaging system are that it can hold the device size to a 
minimum due to the common sharing of transmit and receive circuitry among 
the transducers and that it can provide detailed near-field tomographic 
information. However, the linear scan system has a disadvantage in that it 
is incapable of scanning areas behind ribs and in that the transducer 
array is relatively bulky for manipulation. 
Conventional transducer arrays of the sector scan type, on the other hand, 
usually comprise 32 transducer elements each of which is associated with 
its own transmit and receive circuitry. The transmit circuitry steers the 
ultrasonic beam in a sector format by applying successively delayed burst 
pulses to the transducers. Although the sector scan system is capable of 
obtaining tomographic information from behind ribs, the control circuitry 
is expensive due to the large number of circuit elements associated with 
the transducers and the near-field image is not satisfactory for 
diagnostic purposes. 
SUMMARY OF THE INVENTION 
An object of the present invention is therefore to provide a piezoelectric 
transducer array which combines the advantages of the linear and sector 
scan type transducer arrays. 
This object is achieved by forming an array of piezoelectric transducers on 
a frame structure which is convexed in the direction of propagation of 
ultrasonic energy. The number of transducers is greater than the 
conventional sector scan type array but smaller than the conventional 
linear scan type array. A plano-concave acoustic lens is attached to the 
curved transducer array so that a plane entry surface is defined with a 
human subject. The acoustic lens is formed of a material having 
substantially the same acoustic impedance as that of the human body but 
having a characteristic so that the acoustic energy travels in the lens at 
a speed lower than it travels in the human body. The acoustic lens thus 
serves to diverge the transmitted ultrasonic energy to thereby 
successfully scan behind-the-rib areas hitherto inaccessible to by 
conventional linear scan imaging systems, while retaining its advantage in 
near-field details. The transducer array is driven by a steering circuit 
which incorporates the advantages of the linear scan type steering circuit 
by activating a successively selected group of transducers. 
Preferably, the transducers are assembled on an impedance matching layer 
having a higher acoustic impedance than the acoustic impedance of the 
diverging lens. The impedance matching layer is attached to the diverging 
lens to transmit the acoustic energy with a minimum of loss to the human 
body.

DETAILED DESCRIPTION 
An array of piezoelectric transducers embodying the present invention is 
generally indicated at 10 in FIG. 1. The transducer array 10 comprises a 
conductive frame 11 which is convexed in the direction of propagation of 
ultrasonic energy. A plurality of elongated piezoelectric transducers 12 
is successively arranged on the convexed frame structure 11 as seen from 
FIG. 2. As illustrated in detail in FIGS. 3 and 4, each transducer 12 
comprises a piezoelectric element 20 which extends transverse to the frame 
11 to bridge its parallel side members and connected thereto by a suitable 
adhesive material. On the upper and lower side faces of the piezoelectric 
element 20 are electrodes 21 and 22, respectively. The lower electrodes 22 
are electrically connected to the side members of the frame 11 by 
conductive adhesive 25 so that the frame 11 serves as a common electrode 
of the transducer array 10. In a preferred embodiment, each piezoelectric 
element 20 is so dimensioned that its width-to-thickness ratio imparts a 
transverse expansion vibrational mode to the array 10. With this 
vibrational mode a high sensitivity and excellent bandwidth 
characteristics are obtained. In a further preferred embodiment, each 
transducer 12 includes a first impedance matching element 23 which is 
attached to the lower electrode 22. The transducers 12 are secured to a 
second, or common impedance matching layer 24 which extends along the 
length of the frame 11 in contact with the first impedance matching 
elements 23. Suitable material for the first impedance matching elements 
23 is rock crystal, glass or fused quartz and suitable material for the 
second impedance matching layer 24 is epoxy resin. The acoustic impedance 
of the first impedance matching elements 23 is preferably 2.5 to 9.5 times 
greater than the acoustic impedance of the human body and the acoustic 
impedance values of the common impedance matching layer 24 is preferably 
1.6 to 2.7 times greater than that of the human body. 
According to the present invention, a diverging acoustic lens 30 generally 
of a plano-concave construction is secured to the common impedance 
matching layer 24 with its plane surface facing toward the human body to 
define an entry surface for the generated ultrasonic energy. The acoustic 
lens 30 is formed of silicone rubber having substantially the same 
acoustic impedance as the human body but having such an acoustic property 
that in the lens 30 the acoustic energy propagates at a speed lower than 
it propagates in the human body. Because of the increase in sound velocity 
in the human body, the incident ultrasonic beam is deflected in a 
direction away from the normal to the array 10 as it impinges on the plane 
entry surface at an angle thereto as illustrated in FIG. 5, and therefore 
the scanned beam propagates as if it originates from a point 31 closer to 
the array 10 rather than from a point 32 from which it would originate if 
the acoustic lens 30 is not provided. The amount of tomographic 
information available from the arc scan transducer array of the invention 
is thus greater than that available with conventional linear scan type 
arrays. The plane entry surface defined by the acoustic lens 30 assures an 
intimate contact with the human subject, so that acoustic energy 
encounters no loss upon entry into and return from the human body. It is 
seen from FIG. 3 that the acoustic lens 30 preferably has a convexed 
surface as viewed in the longitudinal direction of the array to provide 
beam focusing in a direction normal to the direction of scan. 
FIG. 6 is an illustration of a control circuit for driving the transducer 
array 10 of the invention. For purposes of illustration analog 
multiplexers 41-1 through 41-16 are provided for the array 10 which 
includes transducers #1 through #128. These transducers are divided into 
16 subgroups of eight transducers each. Each analog multiplexer 41 is 
provided with eight output terminals for connection to those transducers 
which are spaced by sixteen elements, with the corresponding output 
terminals of the multiplexers being connected respectively to adjacent 
transducers of each transducer group. For example, the #1 output terminals 
of multiplexers 41-1 to 41-16 are connected respectively to the #1 to #16 
transducers, the #2 output terminals being connected respectively to the 
#17 to #32 transducers, and the #16 output terminals being connected 
respectively to the #113 to #128 transducers. Counters 42-1 to 42-16 are 
connected to the inputs of the multiplexers 41-1 to 41-16 respectively to 
select one of the eight output terminals of the associated multiplexers in 
response to output signals supplied individually from a shift register 43 
which in turn is connected to receive a clock signal from a clock source 
44. The counters 42-1 to 42-16 are counted up in response to every 16th 
clock pulse and cleared by a reset counter 45 in response to every 128th 
clock pulse. In response to the #1 clock pulse all the counters are 
conditioned so that the #1 output terminals of all the multiplexers are 
activated to couple their inputs to the transducers #1 to #16. This 
condition is retained for a clock interval so that upon the occurrence of 
a #2 clock pulse the transducers #2 to #17 are selected. Therefore, a 
group of 16 successive transducers is shifted to the next by one 
transducer element in response to each clock pulse. 
To the inputs of the muliplexers 41-1 to 41-16 are connected a compensator 
unit 46 and a receiver unit 49. The compensator unit 46 receives its input 
signals from a focusing delay multiplexer 47 which essentially comprises a 
plurality of successively connected variable delay elements. These delay 
elements introduce a delay interval in succession into a transmit burst 
signal supplied from a pulse generator 48. The transducers of a given 
selected group are then energized by successively delayed burst signals so 
that focused ultrasonic energy is angulated on one side of the normal to 
the array 10. The amounts of delay are varied in response to the clock 
pulse from source 44 to successively deflect the transmitted ultrasonic 
beam on either side of the normal to the array. The amplitude of the 
delayed burst signals are amplified with different gain by the compensator 
unit 46 to compensate for the different attenuation of the ultrasonic 
energy due to different path lengths of the acoustic lens 30, so that the 
ultrasonic energy is transmitted at a constant strength regardless of 
steering angles. The connections to the selected transducers from the 
multiplexers are retained until the occurrence of the next clock pulse to 
allow echo signals returning from different tissues of the human subject 
to be received by the same transducers for conversion to electrical 
signals which are applied to the receiver unit 49. The received echo 
signals are processed to give a tomographical representation in an arc 
format on a display unit, not shown.