Crossed linear arrays for ultrasonic medical imaging

An improved piezoelectric transducer permitting linear scanning along two intersecting planes. The transducer is fabricated from a cross shaped piezoelectric plate covered on both faces with metallic electrodes. First and second linear arrays are formed by the partial dicing of the opposite faces of the electrodes and plate. The two segments of the orthogonal arrays are integrated in the crossed area. An aperture may be disposed in the central area so as to permit a surgical implement, such as a biopsy needle, to be inserted into the tissue under ultrasonic visualization. Scanning electronics are alternately connectable to each of the arrays by means of a multiple pole switch under the control of a clock.

BACKGROUND OF INVENTION 
This invention relates to ultrasonic transducers and more particularly to 
an ultrasonic transducer in the form of a crossed linear array for 
ultrasonic medical imaging and for biopsy needle guidance. 
In a linear array ultrasonic scanner the transducer consists of a series of 
individually addressable piezoelectric segments which emit ultrasonic 
waves when energized by the application of electricity. In a linear array 
the individual segments of the transducer are activated sequentially from 
one end to the other to generate a rectangular image. The ultrasonic waves 
emitted by the transducer are focused by the activating electronics and by 
an acoustic lens disposed between the transducer and the tissue to be 
imaged. The ultrasonic image generated by a linear array transducer is 
that of a rectangular "slice" of the tissue. 
However, the ultrasonic image generated is only two dimensional. That is, 
the image has no usable thickness information. Accordingly, the use of a 
linear array scan to place, for example, a biopsy needle is at best 
difficult because of the lack of usable thickness information. If the 
needle is not precisely in the plane of scan the needle will be invisible 
to the physician. Furthermore, under certain circumstances even if the 
needle is visible it may still be improperly placed. Accordingly, it is 
desirable that a physician be able to determine in three dimensions where 
a biopsy needle has been placed. 
In ultrasonic medical imaging another common type of scanning mode used 
with a segmented transducer is the so called "phased array" system. In a 
phased array the activating electronics are used to simulate the scanned 
motion of a transducer without actually physically moving the transducer. 
In a phased array system the segments of the transducer are activated such 
that the segments of certain sections of the transducer are delayed with 
respect to the others. Such a device produces a wedge shaped "sector scan" 
image. However in the sector scan the image formed is again only two 
dimensional with no useful thickness information. 
A transducer of the phased array typed suitable for ultrasonic imaging in 
more than one plane is shown in U.S. Pat. No. 4,671,293 which issued to 
the inventor herein and shows a phased array two plane transducer. 
However, the electronics needed to drive a phased array transducer are 
considerably more complex than that of a linear array. Accordingly, an 
improved two plane linear array device suitable for imaging in two planes 
to permit accurate biopsy needle placement is desired. The present 
invention is directed to providing such a device. 
U.S. Pat. No. 4,570,488 is directed to a various two dimensional 
arrangement of ultrasonic linear arrays. In one embodiment, two 
perpendicular linear arrays overlap each other. In another example, two 
perpendicular linear arrays are arranged across each other. However at the 
intersection of the linear arrays the elements of the transducer are 
divided into a large number of small elements which form a two dimensional 
array. Such an array is disadvantageous because a large number of 
electrical connections to the individual elements of the two dimensional 
array are required at the crossing area. Furthermore, the electronics 
required to drive the array at the center so as to provide two orthogonal 
linear scans are also quite complex. 
SUMMARY OF THE INVENTION 
The present invention is directed to providing a relatively simple and 
inexpensive piezoelectric transducer that includes two intersecting linear 
arrays without forming a two dimensional array in the intersection area. 
This is accomplished by exploiting the two opposite faces (front and back) 
of a piezoelectric plate to form the two intersecting arrays. Furthermore, 
the crossed linear array may include a guide hole in the center through 
which a biopsy needle may be inserted under ultrasonic visualization. 
The transducer constructed in accordance with the present invention is 
fabricated from a cross shaped piezoelectric plate covered on both faces 
with thin metallic electrodes. First and second linear arrays are formed 
by partially dicing the opposite faces the plate. In this configuration 
the two segments of the orthogonal arrays are integrated in the crossed 
area. Uniformity of the array elements can be assured by using a composite 
piezoelectric material or by cross dicing the entire area of the plate. 
For biopsy applications a guiding hole for the needle is disposed at the 
center of the crossed arrays.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a transducer constructed in accordance with the present 
invention. Transducer 5 is cross shaped in plan view and comprises a first 
linear array 10 and second linear array 20 disposed orthogonally to each 
other each array extending along an "arm" of transducer 5. Transducer 5 
consists of a plate 30 of piezoelectric material. Disposed on the upper 
face of plate 30 is conductive electrode 32 and on the lower face of the 
plate 30 is a conductive electrode 34. The electrical connection to 
activate piezoelectric plate 30 is by means of electrodes 32, 34 which are 
in turn connected to the activating and scanning circuitry discussed 
below. The piezoelectric material of plate 30 is preferably a composite 
constructed from a matrix of parallel rods of a piezoelectric ceramic 
material distributed in an electrically insulating binding material such 
that each of the rods is completely surrounded by the insulating and 
damping material. The rods extend from the upper surface of plate 30 to 
the lower surface of plate 30 and are in electrical contact with the 
respective electrodes 32, 34. Alternatively, any piezoelectric material 
suitable for ultrasonic imaging applications is also usable in connection 
with this device. Plate 30 may either be formed as cross shaped or the 
appropriate shape may be cut from a rectangular or square blank. 
Furthermore, arrays 10, 20 need not intersect at 90.degree. as other 
crossing angles may also be constructed. 
Array 10 is defined by a series of traverse grooves 36 cut into electrode 
32 and the upper surface of plate 30 which do not extend to its lower 
surface. Grooves 36 divide electrode 32 into a series of independently 
addressable electrodes 32.sub.1, 32.sub.2, . . . 32.sub.n. Grooves 36 
serve to electrically isolate the segments of array 10 for independent 
addressing by the scanning electronics. The lower electrode 34 remains 
uncut along array 10, except at its center section, and serves as its 
ground. The center section of lower electrode 34 is cut only for the 
grooves defining array 20. Grooves 38, 40 are disposed along the central 
section of array 10 in line with its edges to isolate its segments from 
those of array 20. 
Linear array 20 is defined by a series of transverse grooves 42 cut into 
electrode 34 and the lower surface of plate 30 which do not extend to the 
upper surface of plate 30. Thus, electrode 34 is divided into a series of 
independently addressable electrodes 34.sub.1, 34.sub.2, . . . 34.sub.n 
along the lower surface of array 20 and they and plate 30 form an 
independent linear array which is orthogonally disposed with respect to 
array 10. In linear array 20 electrode 32 on the upper surface of plate 30 
serves as the ground electrode. Disposed at the center of arrays 10 and 20 
is an aperture 44 which extends completely through plate 30 and electrodes 
32, 34. Aperture 44 permits the insertion of a biopsy needle so it can be 
guided by ultrasonic visualization in two orthogonal planes. 
To accomplish the out of plane focusing for the two arrays (10, 20), a 
spherical lens is attached to the intersection area and conventional 
cylindrical acoustic lenses, of the same radius of curvature, are attached 
to the portions of the arrays out of the intersection area. The spherical 
lens acts as a combination of two orthogonal cylindrical lenses. For each 
array, one of these lenses performs the out of plane focusing, while the 
other introduces a phase aberration in the lateral direction . This 
aberration can be compensated electronically by adjusting the delay times 
for the electronic focusing in the scanning plane. 
The crossed linear arrays can be operated sequentially to scan the 
ultrasonic beam in two orthogonal planes in the following way. A scan 
along array 10 is obtained by connecting the set of electrodes 32 to the 
scanning circuit and electrodes 34 to ground. In a similar manner, a scan 
along array 20 is obtained by connecting the set of electrodes 34 to the 
scanning circuit and electrode 32 to ground. 
Integration of the crossed linear arrays into a linear array system can be 
accomplished by means of a switch 50, as shown in FIG. 2. Switch 50 is 
disposed between transducer 5 and scanning electronics 52 and is 
controlled by a clock 54. Switch 50 is comprised of a multiple pole switch 
which switches electrodes 32 of array 10 and electrodes 34 of array 20 for 
alternate connection to scanning electronics 52. As shown, the electrodes 
of each array is either switched to scanner electronics 52 or to ground. 
As shown in FIG. 2 electrodes 34.sub.1, 34.sub.2, . . . 34.sub.n of array 
20 are connected to the scanner electronics 52 with electrodes 32.sub.1, 
32.sub.2, . . . 32.sub.n of array 10 shorted to ground. Switch 50 swings 
between the two positions in synchrony with the scanner frame rate under 
the control of clock 54. In the position shown, array 20 is connected to 
scanning electronics 52 to provide one field of view and the electrodes of 
array 10 are connected to ground. Subsequent switching to the other 
position activates array 10 to provide the orthogonal field of view. 
Higher data rates, and hence increased frame rates, can be achieved by 
incorporating parallel processing techniques into the linear array system. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations may be resorted to without departing from the spirit of the 
invention as those skilled in the art will readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the invention and the appended claims to follow.