DE4428500C2 - Ultrasonic transducer array with a reduced number of transducer elements - Google Patents

Description

The invention relates to an ultrasonic transducer array.

In medical ultrasound diagnostics, space is provides the human body with ultrasound pulses and the reflected ultrasound echo pulses become from an ultrasound image is applied to a signal processing unit builds a two-dimensional (2-D) section through the Body corresponds. For sending and receiving the ultrasound So far, pulses have mainly been one-dimensional (1-D), especially linear arrays of piezoelectric transducers elements used by an electronic control unit can be controlled with predetermined phase delays. With such phase-controlled linear arrays can be in one of the normal to the array surface and the Longitudinal direction of the array spanned plane and Focusable ultrasound beams are sent and received the. The swivel angle measured for the In general, the smaller the ultrasonic beam, the larger the smaller are the transducer elements. The distance of the transducer elements is generally even across the entire array chosen equal to half the wavelength of the ultrasound, to avoid additional diffraction patterns (side lobes), and is, for example, at an examination frequency of 3.5 MHz about 0.2 mm. On the other hand, there is a certain minimum length of the linear array required to provide an adequate Sound amplitude and precise focusing of the beam to reach. From these two demands of the maximum Ab status of the transducer elements and the minimum length of the array a minimum number of typically 64 converter elements follows for the array, which should not be undercut.  

In addition to 1-D arrays, two-dimensional (2-D), in particular matrix-shaped ultrasound transducer arrays are also known, which are built up from individual, generally rectangular transducer elements. Matrix-shaped transducer arrays are known, for example, from DE-C-34 37 862 and the corresponding US Pat. No. 4,683,396 or from DE-A-37 33 776 and the corresponding US Pat. No. 4,801,835. If one now controls the transducer elements of the matrix array with correspondingly given phase delays, in contrast to the linear arrays one can generate and detect an ultrasound beam not only in one, but in two angular directions, which can be pivoted and focused. A higher image resolution is thus achieved. In order to be able to travel through a sufficiently large solid angle area with the ultrasound beam, the conditions of a maximum distance of typically about 0.2 mm of the transducer elements from one another and a minimum area (aperture) of the 2-D array of typically must again, in analogy to the linear arrays about 20 × 20 mm ² with a square array, ie N = M, and 3.5 MHz investigation frequency must be met. A minimum number of converter elements is therefore also required for the 2-D array, which can be, for example, 64 × 64 = 4096.

Problems arise with such a large number of converter elements elements and the required small dimensions Positioning and contacting the converter elements and also the for transmitting the control signals and the image signals the number of control and data lines. It is therefore Ways have been sought of how to count the number of transducer elements can decrease without the beam characteristics of the 2-D array to deteriorate significantly. In particular, the secondary side lobes of the ultrasound essentially below keep pressing.

From "IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control", Vol. 38, No. 4, July 1991, pages 320 to 333, an ultrasound transducer matrix with a square aperture of 10 × 10 mm ² typical for cardiography and square, equidistantly arranged transducer elements is known. Since the distance between the transducer elements is smaller than half the wavelength, side lobes in the beam characteristics of this transducer matrix are practically completely suppressed. Starting from this embodiment of an ultrasound converter matrix, two possibilities are known for how the number of converter elements can be reduced. In the first possibility, the transducer elements located in the corners of the matrix are removed, so that a transducer array with a circular aperture with a diameter corresponding to the side length of the original square is created. From the states of the transducer elements remain the same, so that the side lobes are still suppressed. However, the main lobe gets a little wider. The second option is to remove converter elements from the matrix array by statistical selection. This increases the average distance between the transducer elements, and the intensity of the side lobes increases with a decreasing number of transducer elements remaining in the array. The performance of the converter array also decreases.

A pressure wave converter is known from US Pat. No. 2,928,068 known with a solid ceramic body. Towards each other Overlying surfaces of the ceramic body are electrodes arranged that different pie between the electrodes zoelectrically activated areas arise. The Polarisa degree of these areas decreases from the center of the ceramic body pers outward.

From "Journal of the Acoustical Society of America", Vol. 49, No. 5 (Part 2), May 1971, pages 1668 to 1669 is an Ultra sound transducer with a solid quartz body known. By a special electrode arrangement is placed in the quartz body a Gaussian distribution of the amplitude of the emitted ultrasound beam  with maximum in the middle of the quartz body testifies.

From DE-C-33 34 090 and the corresponding US patent Font 4 518 889 is an ultrasonic transducer array with rod shaped, parallel transducer elements known. The Distances of the transducer elements take one on both sides Center point or a central line to the outside in such a way that the acoustic response of the effective surface of the Arrays and thus the polarization with uniform electri excitation with increasing distance from the central point or decreases from the central line after a Gaussian function.

An ultrasonic wall lerarray is known from US Pat. No. 2,837,728 with a large number of identical and uniformly electrically excited transducer elements. The distances between the transducer elements increase in the case of a matrix-shaped array from a central line as an axis of symmetry and in the case of a circular array from a central point to the outside in accordance with the mathematical regulation

Distance = K.sec (n.θ),

where K is a constant, θ is a constant angle of approximately 10 ° and n the number of transducer elements calculated from the Zen tralline or the central point.

The invention is based on the object, an Ultra sound transducer array specifying the number of its wall ler elements is reduced compared to having an array same area and equidistant assignment with converter elements and at the same time not the beam characteristics is significantly deteriorated.

This object is achieved according to the invention with the characteristics of claim 1. The distances between the transducer elements in each line (x direction) decrease monotonically towards the outside according to the regulation:

• a) it is with respect to a center of symmetry (S) with the Coordinates x = O and y = O straight and on both sides monotonically falling function f (x) provided in the x direction;
• b) the x coordinates of the center points M _{ij of} the transducer elements T _ij in each line are chosen such that the specific integral of the function f (x) over x between the center points M _ij and M _{ij + 1 of} adjacent transducer elements T _ij and T _{ij + 1 is} at least approximately constant.

This can be expressed in a formula:

when x _{ij is} the x coordinate of the center M _{ij of} the transducer element T _ij and x _{ij + 1 is} the x coordinate of the center M _{ij + 1 of} the transducer element T _{ij + 1} . The definite integral

corresponds to the area between the abscissa (x-axis) and the graph of the function f (x) and the two straight lines defined by x = x _ij and x = x _{ij + 1} . The ultrasonic wall lerarray can be a linear array with only one line or also a two-dimensional, in particular matrix-shaped, array with several lines.

The invention is based on the knowledge that the Sensitivity of the array at its edge due to the variation the center distances of the transducer elements according to the Erfin dung can be reduced without the beam characteristic deteriorate significantly, d. H. without sidelobes considerably to reinforce or to ver the main beam significantly breitern.

In the embodiment of a two-dimensional array, the distance between the adjacent transducer elements in each column (y direction) preferably also increases monotonically outwards according to the same rule as in the rows with a corresponding function g (y) for the columns, ie

when y _{ij is} the y coordinate of the center M _{ij of} the transducer element T _ij and y _{i + 1j is} the y coordinate of the center M _{i + 1j of} the transducer element T _{i + 1j} .

The function f (x) and / or g (y) is preferably a three corner function, Hanning, Hamming, Riesz, De la Vall Puissin, Tukey, Bohman, Poisson, Hanning-Poisson, Cauchy, Gauss, Dolph Chebyshev, Kaiser Bessel, Barilon Femes, Exact Blackman, Blackman, Minimum 3-Sample Black man-Harris or minimum 4-sample Blackman-Harris function intended. These functions are part of a theoreti work for harmonic spectral analysis using dis creter Fourier transform in applications for the signal recognition known ("Proceedings of the IEEE", Vol. 66, No. 1, Jan. 1978, pages 51 to 83). The Fourier transform of these functions have pronounced main lobes and ver equally small side lobes. This property will in this advantageous development according to the invention for the beam characteristic exploited.

The only figure in the drawing shows an exemplary embodiment of a matrix-shaped ultrasound transducer array. A section of the central region of such a converter matrix around a center of symmetry S is shown schematically. The transducer elements are denoted by T _ij and preferably have a square shape. With the converter elements T _ij , rows i and columns j of the M × N matrix are formed with 1 ≦ i ≦ M and 1 ≦ j ≦ N, which are in an orthogonal (x, y) coordinate system with the origin S = (0 , 0) and an x and a y axis. The rows i run in the x direction and the columns j in the y direction. The ultrasound transducer matrix can be square, ie the number M of rows is equal to the number N of columns, or it can also be rectangular, ie the number M of rows is different from the number N of columns. The distances between the centers of adjacent transducer elements, for example M _ij and M _{ij + 1} , M _{ij + 1} and M _{ij + 2} as well as M _{ij + 2} and M _{ij + 3} , continue to increase from the inside to the outside. For this purpose, a function f (x) is selected for the rows i and a function g (y) for the columns j, which are straight functions with respect to the symmetry center S, ie f (x) = f (-x) or g (y) = g (-y) and for increasing amounts | x | or | y | lose weight monotonously. These functions f (x) and g (y) are window functions which, in this exemplary embodiment, disappear at an adjustable maximum value ± x _max or ± y _max , ie f (x _max ) = f (-x _max ) = 0 or g (y _max ) = g (-y _max ) = 0. The edges of the window functions ± x _max or ± y _max can also lie with positive or negative function values.

Preferably, both functions f and g are the same, i. H. f (z) = g (z) for a real argument z.

In the embodiment shown, the center of symmetry S coincides with the center M _{ij of} the transducer element T _ij , but can also lie outside the individual transducer elements.

From the specified rule for the distances between the center points M _{ij of} the transducer elements T _{ij it} follows that at least the number N of the columns j is greater than 3, and preferably also the number M of the rows i.

The variation of the distances between the center points M _{ij of} the transducer elements T _ij is shown only schematically and can also be corrected somewhat experimentally afterwards.

Claims (4)

1. Ultrasonic transducer array with at least one row (i) extending in an x direction and with columns (j) of transducer elements (T _ij ) extending in a y direction, in which the distances between the center points (M _ij and M _{ij + 1} ) of adjacent transducer elements (T _ij and T _{ij + 1} ) increase monotonically outwards in each line (i) according to the following rule:

• a) a function f (x) falling in the x direction with respect to a center of symmetry (S) with the coordinates x = O and y = O is straight and monotonically falling on both sides;
• b) the x coordinates of the center points (M _ij ) of the transducer elements (T _ij ) in each line (i) are chosen such that the determined integral of the function f (x) over x between the center points (M _ij and M _{ij +1} ) adjacent transducer elements (T _ij and T _{ij + 1} ) is at least approximately constant.

2. Ultrasonic transducer array according to claim 1, wherein the Function f (x) is selected from the group of the following radio tion: triangular function, Hanning, Hamming, Riesz, De la Vall-Puissin, Tukey, Bohman, Poisson, Hanning-Poisson, Cauchy, Gauss, Dolph Chebyshev, Kaiser Bessel, Barilon Femes, Exact Blackman, Blackman, Minimum 3-Sample Blackman-Harris or minimum 4-sample Blackman-Harris Function. 3. Ultrasonic transducer array according to claim 1 or 2, _wherein the distances between the centers (M _ij and M _{i + 1j} ) of adjacent transducer elements (T _ij and T _{i + 1j} ) also increase monotonically outwards in each column (j) according to Before writing:

• a) there is provided a function g (y) in the y direction which is straight and monotonically falling with respect to a center of symmetry (S);
• b) the y coordinates of the center points (M _ij ) of the transducer elements (T _ij ) in each line (i) are selected such that the determined integral of the function g (y) over y between the center points (M _ij and M _{ij +1} ) adjacent transducer elements (T _ij and T _{i + 1j} ) for each column (j) is at least approximately constant.

4. Ultrasonic transducer array according to claim 3, in which the function g (y) is selected from the group of the following functions:
Triangular function, Hanning, Hamming, Riesz, De la Vall Puissin, Tukey, Bohman, Poisson, Hanning Poisson, Cauchy, Gauss, Dolph Chebyshev, Kaiser Bessel, Barilon - Femes, Exact Blackman, Blackman, Minimum 3-Sample Blackman-Harris or Minimum 4-Sample Blackman-Harris function.