The present invention pertains to the field of ultrasonic transducers. More specifically, the invention relates to an array of ultrasonic transducer elements comprising a plurality of transducer elements that are distributed over a distribution area.

Ultrasounds have been used for a long time in imaging, whether for medical or industrial applications. Ultrasonic transducers organised in an array are used to generate and/or to detect these ultrasounds. The most common type of array is the one-dimensional array, called 1D array, in which the transducer elements are aligned. However, a 1D array only enables scanning of the ultrasonic beam in one plane. Matrix arrays, called 2D arrays since the transducer elements are distributed over two dimensions, have been developed to enable volume scanning and better focusing.

For applications such as ultrasonic echography, it is sought to have a matrix array of transducer elements capable of deflecting the ultrasonic beam, that is to say displacing the ultrasonic beam by adjusting the delays in the emissions of ultrasounds by the different transducer elements constituting the array. The transducer elements are activated according to successive phases determined by computer calculation in order to direct the emitted beam. This is known as phase control array or “phased array”.

Conventionally, matrix arrays of transducer elements group together transducer elements of small dimension compared to the wavelength of the ultrasounds in water at the operating frequency of the transducer elements. These transducer elements being of small size, in terms of emitting area, the emitted beams are naturally divergent by diffraction, that is to say that the ultrasonic wave beam emitted by such a transducer element widens on moving away from the transducer element. The angular range that can be scanned by the ultrasounds by means of the phase command of the transducer elements is all the greater the more divergent the emitted beam. The divergence of these small transducer elements thus makes it possible to scan a wider angular range.

Furthermore, the ultrasonic beam emitted by an array may have array lobes when the pitch of the array, that is to say the average centre to centre distance between two neighbouring transducer elements, exceeds half the wavelength of the ultrasounds in water at the maximum frequency of the transducer elements. These array lobes are all the more important as the pitch increases. It is then sought to restrict the spacing between the small transducer elements, such that the pitch of the array is small compared to the wavelength of the ultrasounds in water at the operating frequency of the transducer elements. On account of the small dimension of the transducer elements, the small pitch of the array and the limited number of elements controllable by an ultrasonic imaging system commercially available at the present time (several hundred maximum), the array generally has a small size.

It may however be desired to obtain an array of large dimension, for example to increase sensitivity in the case of ultrasonic echography or to increase antenna gain in the case of high intensity focused ultrasounds, or instead to make it possible to produce images under different viewing angles without displacing the array. In this case, the number of transducer elements required so that the array covers a large area, while conserving the aforementioned dimensioning constraints, becomes very important.

As an example, for a frequency of 5 MHz, the wavelength λ in water is equal to 0.3 mm. A matrix array covering an area of 30 mm×30 mm must thus have at the least 40,000 elements while retaining an array pitch less than λ/2, that is to say 0.15 mm, in the two directions.

The command of as many transducer elements then becomes problematic, not just at the level of the connections required, but also the management of the calculations.

Using larger transducer elements in order to limit the number of elements to command is problematic. Indeed, a transducer element is all the more directive the larger its emission area. Transducer elements of large size limit the possible amplitude of deflection for ultrasounds. Moreover, on account of the large size of the transducer elements, the pitch of the array is large. Array lobes then appear, of which the interferences degrade the quality of the ultrasounds and thereby distort the image obtained by means of these ultrasounds.

DESCRIPTION OF THE INVENTION

The aim of the invention is to resolve at least a part and preferably all of these drawbacks, and notably aims to propose an array of ultrasonic transducer elements that enable a wide angular scanning of the emitted ultrasonic beam, while limiting the number of transducer elements to command and with a beam geometry in which the main lobe has a much greater amplitude than that of parasitic lobes created outside of the main direction.

To this end, the invention proposes a matrix array of ultrasonic transducer elements comprising a plurality of transducer elements that are distributed over a distribution area, each of the transducer elements being suitable for emitting, from an emission area, ultrasounds at a frequency comprised between 100 kHz and 100 MHz in the direction of a working volume,

remarkable in that:

each of the transducer elements is configured to emit ultrasounds in a divergent manner at least at the level of the working volume;said transducer elements each have a largest dimension of their emission area greater than 1.5 times the wavelength of the ultrasounds in water at 30° C.,the distribution of the transducer elements over the distribution area of the array is aperiodic.

The matrix array of ultrasonic transducer elements is advantageously completed by the following characteristics, taken alone or in any technically possible combinations thereof:the distribution of the transducer elements over the distribution area has an absence of symmetry;the array of transducers comprises at least 64 transducer elements and the largest dimension of the distribution area over which said transducer elements are distributed is greater than twelve times the wavelength of the ultrasounds in water at 30° C.;the distribution area has a concave geometry seen from the direction of emission of the ultrasonic waves;the emission area of the transducer elements has a convex shape seen from the direction of emission of the ultrasonic waves;each transducer element is provided with a divergent acoustic lens;the divergent acoustic lens comprises at least two layers of materials arranged in the direction of emission, a first layer of material close to the transducer element, in which the ultrasounds propagate at a higher velocity than in water, and a second layer of material, further away from the transducer element, in which the ultrasounds propagate at a lower velocity than in water;the interface between the two layers of materials of the divergent acoustic lens has a convex shape, such that the distance between said interface and the emission area is greater at the centre of the emission area compared to its periphery;each transducer element is composed of several emission zones each commanded in emission by a local emission signal, and a phase shifter receiving from a command channel an overall emission signal for the whole of a transducer element and emitting in the direction of each of the emission zones of said transducer element the local emission signal specific to said emission zone, said local emission signal being delayed with respect to said overall emission signal by a delay that is all the greater the further away the zone for which said local emission signal is destined is from the centre of the transducer element;the elementary focal length between the emission area of a transducer element and the elementary focal point of said transducer element is less than the overall focal length between the distribution area and an overall focal point of the matrix array of ultrasonic transducer elements;the transducer elements each have an elementary focal point distinct from the elementary focal points of the other transducer elements;each transducer element is provided with a convergent acoustic lens;each transducer element is composed of several emission zones each commanded in emission by a local emission signal, and a phase shifter receiving from a command channel an overall emission signal for the whole of a transducer element and emitting in the direction of each of the emission zones of said transducer element the local emission signal specific to said emission zone, said local emission signal being delayed with respect to said overall emission signal by a delay that is all the greater the closer the zone for which said local emission signal is destined is to the centre of the transducer element.

