DE4428500A1 - Ultrasonic transducer array with a reduced number of transducer elements - Google Patents

Abstract

In an ultrasonic transducer array, the spacings of its transducer elements (Tij) decrease towards the outside in the row direction (x direction) and column direction (y direction) from a centre of symmetry (S) in accordance with the rule that the integral over a function f(x) and, respectively, g(y) falling off linearly and strictly monotonically towards both sides with respect to the centre of symmetry (S) between the centre points (Mij) of adjacent transducer elements (Tij) is constant for each row and column, respectively. This reduces the number of transducer elements (Tij) without significantly degrading the beam characteristic of the array.

Description

The invention relates to an ultrasound 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 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 and pivotable plane Focusable ultrasound beams are sent and received the. The swing angle measured for the Generally, the smaller the ultrasonic beam, the larger the smaller are the transducer elements. The distance of the transducer elements is generally about 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 there follows a minimum number of typically 64 converter elements for the array, which should not be undercut.  

In addition to 1-D arrays are also two-dimensional (2-D), in particular dere matrix-shaped ultrasonic transducer arrays known from individual, generally rectangular transducer elements are built. Matrix-shaped transducer arrays are for example from DE-C-34 37 862 and the corresponding US patent document 4 683 396 or from DE-A-37 33 776 and the correspon US Patent 4,801,835. Now you control the converter elements of the matrix array are pre-selected accordingly given phase delays, in contrast to the linear arrays not only in one, but in two Ultra swiveling and focusable angular directions generate and detect sound beam. You get one higher image resolution. To a sufficiently large solid angle to be able to travel through the area with the ultrasound beam, in analogy to the linear arrays, the Bedin a maximum distance of typically about 0.2 mm the transducer elements from each other and a minimum area (Aperture) of the 2-D array, typically about 20 × 20 m² for a square array, i.e. H. N = M, and 3.5 MHz sub search frequency must be fulfilled. This is a minimum number of transducer elements also required for the 2-D array that 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 stay pressed.

From "IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control ", vol. 38, No. 4, July 1991, pages 320 to 333  is an ultrasound transducer matrix with one for the car diography typical square aperture of 10 × 10 mm² and square, equidistant transducer elements known. Since the distance between the transducer elements is smaller than half the wavelength, are side lobes in the beam character teristics of this converter matrix practically completely below presses. Based on this embodiment of an Ultra sound converter matrix, two options are known, such as the Number of converter elements can be reduced. In the The first possibility will be in the corners of the matrix removed the transducer elements, so that a transducer array with circular aperture with a side length of the original Liche square corresponding diameter arises. The Ab levels of the converter elements remain the same, so that the Side lobes are still suppressed. However, the Main lobe a little wider. The second option is there in it, from the matrix array by statistical selection Remove converter elements. This increases the mean distance of the transducer elements, and the intensity of the Sidelobes increase with decreasing number of in the array remaining converter elements. Performance also drops of the transducer array.

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 ultra  sound beam with maximum in the center 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 is known from US Pat. No. 2,837,728 lerarray is known with a variety of equal and even electrically excited transducer elements. The distances of the In the case of a matrix-shaped array, converter elements take from a central line as an axis of symmetry and with a circle shaped array towards the outside from a central point the mathematical rule

Distance = K · sec (n · R)

where K is a constant, e 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 Merk paint the claim 1. The distances of the transducer elements in of each line (x direction) decrease monotonically outwards 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 _{ÿ of} the transducer elements T _ÿ in each line are chosen such that the specific integral of the function f (x) over x between the center points M _ÿ and M _{ÿ + 1 of} adjacent transducer elements T _ÿ and T _{ÿ + 1 is} at least approximately constant.

This can be expressed in a formula:

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

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

The invention is based on the knowledge that the Sensitivity of the array on 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 widen.

In the embodiment of a two-dimensional array grows preferably also the distance between the adjacent transducer elements  te in each column (y-direction) monotonically outwards after the same rule as in the lines with a the function g (y) for the columns, d. H.

when y _{ÿ is} the y coordinate of the center M _{ÿ of} the transducer element T _ÿ 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, Doph 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. 1 978, 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 _vorzugsweise and preferably have a square shape. With the converter elements T _ÿ , rows i and columns j of the M × N matrix are formed with 1iM and 1jN, which are in an orthogonal (x, y) coordinate system with the origin S = (0,0) and an x and lie on 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 _ÿ and M _{ÿ + 1} M _{ÿ + 1} and M _{ÿ + 2} and M _{ÿ + 2} and M _{ÿ + 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. Thus, these functions are f (x) and g (y) 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.

The center of symmetry S coincides in the embodiment shown with the center M _{ÿ of} the transducer element T _ÿ , but can also lie outside the individual transducer element surfaces.

From the specified regulation for the distances between the center points M _{ÿ of} the transducer elements T _{ÿ 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 _{ÿ of} the transducer elements T _ÿ is only shown schematically and can also be corrected somewhat experimentally afterwards.

Claims (4)

1. Ultrasonic transducer array with at least one row (i) in an x direction and with columns (j) in the y direction of transducer elements (T _ÿ ), in which the distances between the center points (M _ÿ and M _{ÿ + 1} ) of adjacent transducer elements (T _ÿ and T _{ÿ + 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 _ÿ ) of the transducer elements (T _ÿ ) in each line (i) are chosen such that the particular integral of the function f (x) over x between the center points (M _ÿ and M _{ÿ +1} ) adjacent transducer elements (T _ÿ and T _{ÿ + 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 tions: triangular function, Hanning, Hamming, Riesz, De la Vall-Puissin, Tukey, Bohman, Poisson, Hanning-Poisson, Cauchy, Gauss, Doph 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 one of claims 1 or 2, in which the distances between the centers (M _ÿ and M _{i + 1j} ) of adjacent transducer elements (T _ÿ or T _{i + 1j} ) also increase monotonically outwards in each column (j) according to the regulation:

• a) a function g (y) is provided 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 _ÿ ) of the transducer elements (T _ÿ ) in each line (i) are chosen such that the determined integral of the function g (y) over y between the center points (M _ÿ and M _{i + 1j} ) of adjacent transducer elements (T _ÿ 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, Doph Chebyshev, Kaiser Bessel, Barilon - Femes, Exact Blackman, Blackman, Minimum 3-Sample Blackman-Harris or Minimum 4-Sample Blackman-Harris function.