ULTRASONIC DIAGNOSTIC APPARATUS

Abstract: The present invention embodies an ultrasonic diagnostic apparatus for displaying a high-quality image with less speckle noise, wherein reception signals of a plurality of sound rays having mutually different directivities within the range of bearing resolution of an ultrasonic beam are subjected to correlation processing to obtain a reception signal of a single sound ray and an image is formed on the basis of the reception signal thus obtained to thereby eliminate speckle noise. The correlation processing may be an arithmetic means processing in the simplest case.

Description: The present invention relates to an ultrasonic diagnosis apparatus for displaying a high-quality image having a smaller amount of speckle noise. Background Art An ultrasonic diagnosis apparatus is an apparatus in which ultrasonic pulses are applied to the interior of a subject's body, and a sectional image formed on the basis of reflected waves returning from the interior of the subject's body is displayed on an image display unit such as a cathode ray tube (CRT) display. In the image of such an ultrasonic diagnosis apparatus, noise called speckle noise appears in the pattern of a fabric or pear skin and thus deteriorates the picture quality. This noise occurs unavoidably due to the phasewise interference with reflected waves from scattering objects that are smaller than the wavelength of ultrasonic waves applied to the interior of the subject's body. Prior art document DE-A-34 13 074 discloses an ultrasonic diagnosis apparatus which scans the interior of a subject's body with a beam of ultrasonic pulses, receives reflected waves from the subject's body, and forms a sectional image of the subject's body on the basis of the received signals. This apparatus comprises means for determining a receiving signal by effecting correlation processing and means for forming the sectional image of the subject's body on the basis of the received signals. Further, prior art document M.I. Skolnik, "Introduction to Radar Systems", 2nd Edition, McGraw-Hill, pp. 517-528, is concerned with the reduction of speckle in a SAR (synthetic aperture radar), and discloses this reduction of speckle by looking at the subject from slightly different angles and then non-coherently combining the results. Finally, it is known from document US-A-4 488 435 that a phase insensitive signal combination can be carried out using power law compression. Disclosure of Invention An object of the present invention is to realize an ultrasonic diagnosis apparatus for displaying a high-quality image having a smaller amount of speckle noise. To solve this object the present invention provides an ultrasonic diagnosis apparatus as specified in claim 1. In this apparatus receiving signals of a plurality of receiving beams each having a different directionality within a range of bearing resolution of an ultrasonic scanning beam are subjected to correlation processing to determine a receiving signal for one receiving beam, and an image is formed on the basis of receiving signals thus determined, thereby eliminating speckle noise. Correlation processing in its simplest form may be addition averaging processing. Brief Description of Drawings Fig. 1 is a block diagram of a configuration of an embodiment in accordance with the present invention; Figs. 2, 3, 4A to 4E are explanatory diagrams of received signal processing in accordance with the embodiment of the invention; Fig. 5 is a block diagram of a configuration of another embodiment in accordance with the present invention; Figs. 6A, 6B and 7 are explanatory diagrams of received signal processing in-accordance with the another embodiment of the present invention; and Figs. 8 and 9 are diagrams illustrating sequences of received-signal processing in accordance with the another embodiment of the present invention. Best Mode for Carrying Out the Invention First, referring now to Fig. 1, a description will be given of a first embodiment of the present invention. In Fig. 1, a high-frequency signal generated in a oscillator 1 is subjected to pulse modulation by a transmitting signal generator 2 and is input to a transmitting beam former 3. The transmitting beam former 3 generates a group of electric signals for forming a transmitting ultrasonic beam on the basis of the input high-frequency pulses. These electric signals are subjected to power amplification by a transmit/receive circuit 4 and is then sent to the interior of a subject's body. Ultrasonic waves reflected from a reflecting object located in the subject's body are received by a transducer array 5, are converted to a group of electric signals, and are simultaneously input to receiving beam formers 6A, 6B, 6C via the transmit/receive circuit 4. The receiving beam formers 6A, 6B, 6C are receiving beam formers which respectively synthesize three receiving beams on the basis of a group of receiving signals imparted from the transducer array 5. These three receiving beams are formed in such a manner that the difference between their directionality varies within a range of bearing resolution of the ultrasonic scanning beam. Reference numerals 7A, 7B, 7C denote logarithmic amplifiers for amplifying output signals of the receiving beam formers 6A, 6B, 6C by subjecting them to logarithmic compression. Output signals of these logarithmic amplifiers 7A, 7B, 7C are multiplied by weighting coefficients by weighting circuits 21A, 21B, 21C, respectively, are then subjected to addition by an adder 22, and are converted to an output having an appropriate amplitude by an attenuator 23. The output signal of the attenuator 23 is subjected to envelope detection by a detector 8, is then converted to a signal of a television format by a digital scan converter 9, and is displayed on a CRT 10. The directions of transmission and reception of the ultrasonic beam are changed consecutively, and appropriate ultrasonic beam scanning, such as linear scanning and sector scanning, is effected. The three receiving beams received by the receiving beam formers 6A, 6B, 6C are formed as shown in Fig. 2, for example. In Fig. 2, reference numeral 15 denotes a pin target having a diameter corresponding to the bearing resolution of the ultrasonic scanning beam, i.e., a minimum beam width; 16, a micro scattering object having a diameter smaller than the wavelength of the ultrasonic wave, and corresponds to a parenchymal part of an organ's cell, for instance. Reference characters a , b, c denote the three receiving beams each having a difference in directionality smaller than the diameter of the pin target 15. The receiving signals of the receiving beams a , b, c are subjected to logarithmic compression and amplification by the logarithmic amplifiers 7A, 7B, 7C, and are weighted by weighting circuits 21A, 21B, 21C so as to be converted to high-frequency signals such as a , b, c shown in Fig. 3. In these signals, signal parts 17a, 17b, 17c are reflection signals from the pin target 15, while signal parts 18a, 18b. 18c are reflection signals from the micro scattering objects 16. Since the pin target 15 has a diameter corresponding to the bearing resolution of the ultrasonic beam, there are substantially large correlations in amplitude and phase between the signal parts 17a, 17b, 17c that are reflected signals thereof. In contrast, the signal parts 18a, 18b, 18c obtained by the micro scattering objects 16 display various speckle patterns due to interference with waves, and the traveling path of the receiving beam only deviates slightly and undergoes substantial change, so that there are no correlations. These receiving signals a , b, c are added by the adder 22, and since this addition is that of an output of the logarithmic amplifier, it follows that, with respect to the receiving signals, the signals of the respective receiving beams are multiplied, so that this process is equivalent to establishing a correlation between the high-frequency signals of the three receiving beams. Consequently, a receiving signal reflected from the micro scattering object 16 and having a very low correlation, i.e., a receiving signal such as d in which speckle noise is substantially eliminated, is obtained. This signal d is attenuated by the attenuator 23 so as to be converted to an output signal equivalent to an average value. In consequence, the signal from the pin target 15 is amplified by means of addition, and mutually irrelevant signals such as speckle noise and white noise are suppressed, so that a signal-to-noise ratio improves. In addition, since the speckle noise substantially disappears, the picture quality is improved. In addition to the above-described embodiment, the receiving beams may be formed as shown in Figs. 4A to 4E. Fig. 4A shows a case in which receiving signals of parallel three receiving beams are utilized, and Fig. 4B shows a case in which receiving signals of three receiving beams which intersect each other at the position of the pin target 15. Figs. 4C to 4E show cases in which receiving signals based on two receiving beams 31 are made used of, in which Fig. 4C illustrates the case of mutually parallel two receiving beams; Fig. 4D, the case of two receiving beams which diverge at very small angles; and Fig. 4E, the case of two receiving beams which intersect each other at very small angles. The number of the receiving beams is arbitrary insofar as it is two or more. However, it is essential that the difference in their directionality be smaller than the bearing resolution of the ultrasonic beam. The number of the transducer elements of the transducer array for supplying a group of receiving signals to the respective receiving beam formers need not be strictly identical among the receiving beam formers. Although a greater effect is obtained if the added average is obtained after logarithmic compression, it is still possible to obtain a meritorious effect if the added average is obtained by means of the signal prior to the logarithmic compression. Fig. 5 is a block diagram of another embodiment of the present invention, in which only one set of receiving beam formers are used to simplify the circuitry. In the drawing, components that are identical to those of Fig. 1 are denoted by the same reference numerals. In the drawing, reference numeral 11 denotes an AD converter for converting an analog signal to a digital signal, and its digital output signal is input to a one line buffer memory 12 for temporarily storing one-line data of the receiving beam and is input to a correlator 13 as well. The correlator 13 determines a correlation between the immediately preceding receiving beam data stored in the one line buffer memory 12 and the present receiving beam data which is input from the AD converter 11. Here, it is possible to adopt an added average as the simplest operation for determining the correlation. In the apparatus having the above-described configuration, the high-frequency signal generated by the oscillator 1 is subjected to pulse modulation by the transmitting signal generator 2, and an electric signal for a transmitting beam formed by the transmitting beam former 3 is imparted to the transducer array 5 via the transmit/receive circuit 4. In a first cycle of transmission and reception, the transducer array 5 transmits and receives an ultrasonic beam of, for example, the direction a , and in an ensuing cycle of transmission and reception transmits and receives an ultrasonic beam of direction b having directionality different from that of direction a within the range of bearing resolution of the ultrasonic scanning beam. Subsequently, the transducer array 5 consecutively scans the interior of the subject's body by changing the directionality of the ultrasonic beam. Reflected waves concerning the transmitting ultrasonic beam in the first cycle are received by the transducer array 5, are amplified by the transmit/receive circuit 4, and are formed as a receiving signal of the receiving beam a by the receiving beam former 6. This receiving signal is subjected to logarithmic compression and amplification by the logarithmic amplifier 7, is converted to a digital signal by the AD converter 11, and is stored in the one line buffer memory 12 and is also input to the correlator 13. An ensuing reflected signal of the receiving beam b is similarly received and processed, and its receiving signal is converted to a digital signal by the AD converter 11, and is input to the one line buffer memory 12 and the correlator 13. At this time, the correlator 13 determines a correlation between the input data of the receiving beam b from the AD converter 11 and the receiving data of the receiving beam a in the preceding cycle stored in the one line buffer memory 12. The resultant signal of this correlation calculation is subjected to envelope detection by the detection circuit 8, is converted to a signal of a television format by the DSC 9, and it then displayed on the CRT 10. Processing of the receiving signal of the receiving beam a and the receiving signal of the receiving beam b at this juncture will be described with reference to Figs. 6A, 6B and 7. In 6A, reference numeral 15 denotes the pin target having a diameter corresponding to the bearing resolution of the transmitting beam, and an angle Δϑ between the receiving beam a and the receiving beam b is selected in such a manner as to become smaller than the angle of viewing the pin target 15. Fig. 6B illustrates a pattern of the ultrasonic beams of these receiving beams a and b. Fig. 7 is a waveform diagram of the receiving signal, in which numeral 16 denotes a waveform of the reflected signal from the pin target 15, and numeral 17 denotes a speckle noise waveform caused by reflection from the micro scattering object. By subjecting the receiving signal of the receiving beam a and the receiving signal of the receiving beam b to correlation processing (addition averaging), it is possible to obtain a receiving signal ( a + b)/2 in which the speckle noise 17 is reduced for the same reason as the one described in the preceding embodiment. Then, by subjecting this signal to envelope detection by the detection circuit 8, it is possible to obtain a receiving signal such as the one shown at the end in Fig. 7. It should be noted that the detection circuit 8 is not confined to one for effecting envelope detection. Figs. 8 and 9 illustrate sequences for signal processing accompanying the progress of scanning with the ultrasonic beam. Fig. 8 illustrates ultrasonic beams that are consecutively transmitted and received by the transducer 5. Fig. 9 is a diagram illustrating a state of processing reflected signals by means of the respective ultrasonic beams, and shows that the receiving signals of the respective receiving beams are consecutively subjected to correlation processing with the receiving signal of the immediately preceding cycle. If a plurality of one line buffer memories are used, it is possible to effect correlation processing and the like with respect to multiple receiving beams. The correlator is not confined to the one for effecting simple addition averaging, but a one for effecting weighted addition. If a correlator whose correlation function varies in correspondence with a depth of a reflecting point of the ultrasonic wave is used as this correlator, the speckle noise eliminating effect further improves.

Claims: An ultrasonic diagnosis apparatus which scans the interior of a subject's body with a beam of ultrasonic pulses, receives reflected waves returning from the subject's body along various receiving beams and forms a sectional image of the subject's body on the basis of a plurality of receiving signals obtained from said reflected waves, characterized in that: a plurality of receiving beam formers (6A-6C) respectively form the various ultrasonic beams, each having a difference in directionality within the range of bearing resolution of the ultrasonic scanning beam, and correlation means subject output signals from said receiving beam formers to correlation processing. An ultrasonic diagnosis apparatus according to claim 1, wherein said correlation means includes a plurality of logarithmic amplification means (7A-7C) for respectively subjecting the output signals from said plurality of receiving beam formers (6A-6C) to logarithmic amplification, and adding means (21A-21C, 22) for subjecting output signals from said logarithmic amplification means (7A-7C) to weighted addition. An ultrasonic diagnosis apparatus according to claim 1, wherein means for determining said receiving signals includes means for effecting correlation processing with respect to two receiving signals that are received consecutively thereby by means of two receiving ultrasonic beams each having a difference in directionality within the range of bearing resolution of the ultrasonic beam. An ultrasonic diagnosis apparatus according to claim 1, wherein means for determining said receiving signals includes a buffer memory (12) for temporarily storing two receiving signals that are consecutively received thereby by means of two receiving ultrasonic beams each having a difference in directionality within the range of bearing resolution of the ultrasonic scanning beam, and correlation means (13) for effecting correlation between the signal present being received and the previously received signal stored in said buffer memory (12).

Inventors: SHIMAZAKI TORU, SHIMAZAKI, TORU
Applicants: YOKOGAWA MEDICAL SYST, YOKOGAWA MEDICAL SYSTEMS, LTD
Application year: 1988
Main classification: G01S7
IPC codes: G01S7