1. Field of the Invention
The present invention relates to an ultrasonic echograph comprising at least one set of n ultrasonic transducers, a stage for the emission of ultrasonic waves towards a medium to be explored, and a stage for the reception and for the processing of the ultrasonic signals returned by the obstacles encountered in the said medium. The n ultrasonic transducers may be disposed in a one-dimensional array, in a two-dimensional matrix, or in a plane or curved annular array with or without sectorial fragmentation.
This invention is applicable, in a particularly advantageous manner, in the medical field and in that of the non-destructive monitoring of materials.
2. Description of the Prior Art
In the medical field, the application may be qualitative, when it is principally desired to obtain the formation of images which are as precise as possible of the media explored and particularly to demonstrate the contours of organs and the specular walls such as veins, as well as quantitative, when it is principally desired to characterize in a precise manner the biological tissues explored with a view to their pathological investigation, for example by the prescise measurement of quantities such as the parameter of attenuation of the ultrasonic waves in the media explored. Such medical applications are described, for example, in the article "Electronique medicale : principe de l'imagerie a ultrasons et analyse quantitative en imagerie a ultrasons" (Medical Electronics: Principle of Ultrasonic Imagery and Quantitative Analysis in Ultrasonic Imagery) which appeared in the journal "Toute l'Electronique", No. 497, Oct. 1984, pp. 31-37.
In the field of the non-destructive monitoring of materials, it is likewise important either to undertake qualitative detections of fissures or of any defects within the materials or to effect any quantitative measurement relevant to the non-destructive monitoring, in this case again especially by the accurate measurement of parameters associated with the local behaviour of the ultrasonic waves (for example by the measurement of the ultrasonic attenuation). The materials concerned are then actually solid materials, such as materials incorporating grains or fibres, composite materials, ceramics, etc . . . , all multimaterials in general, as well as loose materials, such as cheeses, diary products, meats, and, in a general way, all those encountered in particular, in the field of food agriculture.
The article "Detection of spatially nonuniform ultrasonic radiation with phase sensitive piezoelectric and phase insensitive acoustoelectric receivers" by L. J. Busse and J. G. Miller, which appeared in the journal "Journal of Acoustical Society of America", 70(5), Nov. 1981, pp. 1377-1386, emphasizes the importance of the concept of phase in the matter of propagation and of detection of ultrasonic waves, and demonstrates that the use of transducers which are insensitive to the instantaneous phase of the waves may lead to more reliable observations, both in qualitative imaging and in the case of quantitative measurements. Such a finding is made, for example, in the course of measurements of attenuation as a function of the frequency, by investigating, for receiving transducers of specified aperture, the variations of the amplitude of the ultrasonic waves as a function of the frequency: this amplitude decreases greatly and/or is eliminated around certain frequencies. There is in fact, at certain frequencies and in certain situations, a partial or total compensation of the components of the complex signal investigated, while the local variations of the ultrasonic energy detected remain very limited with transducers which are insensitive to the phase.
The ultrasonic echographs which are in current use are not, however, equipped with transducer structures which are insensitive to the phase and therefore liable to mask the effects of the coherence of the ultrasonic waves emitted. The qualitative determination of contours of organs or of the presence of defects within materials (or, more generally, of the presence of fixed specular walls within the medium explored) is therefore disturbed by the interferences, which may be constructive or destructive, indicated hereinabove, which are due to the multidiffusers contained within the medium investigated, and which lead to images which are tainted by the defect referred to as "speckle", that is to say to the spotted or grainy appearance. Moreover, the measurement of acoustic parameters (such as the attenuation), which results from the analysis of echographic signals originating from the multidiffusers distributed in a random manner within the medium investigated, requires the recording of a large number of decorrelated echographic lines and thus the displacement of the transducer during the measurement; this significantly reduces the spatial resolution. Moreover, this analysis of the echographic signals is disturbed by the presence of possible specular reflectors within the medium, and this cannot be remedied except by a precise localization of such reflectors with a view to the elimination or to the reduction of the effects of their presence.