JPH01250226A - Ultrasonic diagnostic apparatus - Google Patents

Abstract

PURPOSE:To enhance the image quality of the tomographic image of the shallow part in the vicinity of the surface of the body by relatively reducing the intensity of multiple reflection with respect to the echo of the shallow part by filtering, by constituting a filter circuit of a variable zone film having center frequency almost same to the resonance frequency of an ultrasonic vibrator. CONSTITUTION:Noticing that the shape of the frequency spectrum of multiple reflection is different from that of the spectrum of the original reflected wave from a shallow part, the characteristics 1, 2, 3 of a BPF are respectively provided to the shallow part, a deep part and an intermediate depth part. In circuit constitution an ultrasonic probe 11 equipped with a piezoelectric vibrator converting an electric signal to sound is connected to a rate pulse generator 13 through a pulser 12. The ultrasonic wave reflected from the boundary of the tissue in a living body is received by the probe and the signal thereof is amplified by a preamplifier 14 in a receiver and, after the pair echo signal ratio of multiple reflection is reduced by a variable band pass filter 15, said signal is converted to a digital signal by an A/D converter 18 through a logarithmic amplifier 16 and an envelope detection circuit 17 to be sent to an image memory 19 and said digital signal is converted to a TV format to be displayed on a TV monitor 20.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to an ultrasonic diagnostic apparatus that obtains tomographic images of a living body using ultrasound, and particularly relates to multiplexing of sound waves generated inside a living body. The present invention relates to an ultrasonic diagnostic device that reduces reflection and improves image quality.

(Prior technology) Ultrasonic pulses are emitted into the living body and biological information is obtained from reflected waves from each tissue.Ultrasonic pulses do not have radiation problems like X-rays and can diagnose soft tissues without the need for contrast materials. It has the advantage of being able to Figure 2 shows the principle of ultrasound diagnosis. Ultrasonic waves emitted from an ultrasound probe 4 consisting of a piezoelectric vibrator 5 are transmitted through tissues with different acoustic impedances (the product of sound speed and density of biological tissue). A portion of it is reflected at the boundary surface and received by the same oscillator used during transmission. The reflected intensity and distance to the reflector can be determined from the amplitude and reception time of this received signal, but in order to display it as a two-dimensional image, move the ultrasound probe 4 in the direction of the arrow in the figure.
move along the body surface. Initially, this probe 4 was scanned manually, but the recent development of electronic scanning devices has made it possible to perform high-speed scanning, and the performance has significantly improved in terms of resolution and operability. The configuration of the ultrasonic probe is shown in Figure 3.

The piezoelectric vibrator 31 currently in use is made of ceramic material, and its acoustic impedance (determined by the product of the sound speed and density of the material) is significantly different from the impedance of the living body 34. Many ultrasonic waves are reflected at the interface, and the transmission and reception efficiency is significantly reduced. Next, an impedance matching layer (matching layer) 32 is provided to allow the throat ultrasound to efficiently enter the living body or to efficiently receive reflected waves from the living body.

Further, an acoustic lens 33 is provided between the matching layer 32 and the living body to converge the ultrasonic beam, and the lens material is silicon rubber or the like having approximately the same acoustic impedance as the living body.

Note that the back loading material 30 on the rear surface of the vibrator absorbs unnecessary ultrasonic waves emitted in the direction opposite to the living body, and also serves as a support base.

The thickness of the matching layer 32 is 1/4 of the ultrasonic wavelength in this material, and the impedance material used is the average value of the respective impedances of the living body and the vibrator.

In FIG. 3, a living body 34, a matching layer 32, a vibrator 3
The acoustic impedance of 1 is Zl respectively.

If Z2 and Z3 and the thickness of the matching layer 32 is t, then the reflection coefficient R(f) of the ultrasonic wave incident on the transducer from the living body side on the transducer surface is as follows. However, as mentioned above, KZ2=y'21*Za and t
=λ. /4 (λ is the wavelength of the ultrasonic center frequency component (fo) in the matching layer), so as already mentioned, the impedance matching layer allows the υ oscillator to be
The reflection of ultrasonic waves between living bodies becomes zero at a specific frequency r==r, but reflection still occurs at a frequency f0. On the other hand, the pulse waves used in ultrasonic diagnostic equipment are short pulses with a wave number of about 3 to 5, and their spectrum spreads around fo. Therefore, even if an impedance matching layer is provided, reflection cannot be completely suppressed. That is, in contrast to the spectrum (echo spectrum) before reflection on the vibrator surface shown by the solid line in FIG. 5, the wave after reflection has a frequency spectrum shown by the broken line.

Incidentally, in the ultrasonic diagnostic apparatus, scattering that occurs inside the living body 34 is regarded as weak scattering. That is, since the scattering coefficient is small in living tissue, it is considered that only one scattered wave needs to be considered.

However, since the reflection and scattering intensity from tissue layers 15 close to the body surface (for example, muscle layers and fat layers) is not necessarily small, multiple reflections within the body 34 cannot be ignored, and this causes artifacts (virtual images) and noise on tomographic images. It causes. In particular, when reflections on the transducer surface are not sufficiently suppressed, multiple reflections occurring between the body surface tissue layer 35 and the ultrasound transducer 31 surface are displayed as being superimposed on the shallow tomographic image of the body 34. This makes it difficult to continue displaying images. FIG. 4 explains the occurrence of multiple reflections. However, only one layer of the body surface tissue layer 35 is shown to simplify the explanation. If the distance between the transducer 45 and the tissue j@35 is t, multiple reflections that make two trips back and forth between the two will be displayed at a depth of 2t, and those that make three trips will be displayed at a depth of 3t on the tomographic image. (3 in the diagram)
Since the body surface tissue layer actually consists of several layers, such multiple reflections appear almost continuously in the shallow part of the tomogram, and the original shallow part tomogram It appears as an artifact on the top.

In conventional ultrasonic probes, this arch 7 act has been reduced by using an acoustic lens made of a material with high attenuation. In other words, multiple reflections mean that the number of times the waves pass through this acoustic lens is greater than the number of waves reflected from the original living body (
(hereinafter referred to as an echo signal), the artifact level can be relatively reduced by making the ultrasonic attenuation coefficient of the lens material larger than the attenuation coefficient of the living body. However, according to this conventional method, the echo signal itself is attenuated within the lens during transmission and reception. This had the disadvantage that it was not possible to obtain sufficient amounts of

The present invention focuses on the fact that the shape of the frequency spectrum of multiple reflections is different from the shape of the spectrum of the original reflected waves from shallow areas, and uses filtering processing to reduce the magnitude of multiple reflections relative to shallow echoes. The aim is to improve the image quality of shallow tomographic images near the body surface.

〔Example〕

Before showing specific examples of the present invention, the principle of a method for reducing multiple reflected waves will be explained. The present invention focuses on the fact that the shape of the multiple reflection spectrum decreases significantly near its center as shown in FIG. 5, and attempts to further improve Sc through optimal filtering. The filter here refers to a band-pass filter, and its center frequency is made to coincide with the ultrasonic center frequency.

In other words, it can be seen that the narrower the BPF band, the more the S/'N improves.

However, if this band is made extremely narrow, the amplitude of the echo wave will be too small, and the pulse width will also be long, resulting in a decrease in resolution. Therefore, it is necessary to avoid making the band extremely narrow. Ultrasonic diagnostic equipment has sufficient receiving sensitivity for echoes at short distances. Therefore, it is acceptable for the echo amplitude to be reduced to some extent by the filter. However, unless the attenuation of the echoes is reduced as the distance from the vibrator increases, the echoes may be buried in system noise. Therefore, in the present invention, the echo t-t etc. from a part far from the vibrator
It is desirable to widen the band of PF'. That is, as shown in Figure 6, the characteristics of the BPF corresponding to the shallow part are (1), the characteristics in the deep part are (3), and (2) for intermediate depths.
).

FIG. 1 shows a specific circuit configuration of an embodiment of the present invention.

11 is an ultrasonic probe equipped with a piezoelectric vibrator that converts electrical signals into sound, and this probe is connected to a transmitter, such as a pulser 1.
2 to a rate pulse generator 13 which determines the repetition frequency of the transmitted pulses. That is,
The pulser 12 is actuated by the trigger pulse from this rate pulse generator, a high voltage pulse is applied to the piezoelectric vibrator, and an ultrasonic pulse is emitted into the living body. On the other hand, the ultrasound waves reflected at the boundaries of tissues within the living body are received by the probe 11 .

This received signal is amplified by the preamplifier 14 in the receiver, and is sent to the logarithmic amplifier 16 and envelope detection circuit 17 after reducing the echo signal ratio of multiple reflections by the variable bandpass filter 15 as described above. Further, the output of the detection circuit 17 is converted into a digital signal by an A/D converter 18, and then sent to an image memory 19 where it is temporarily stored.
It is converted into a TV format and displayed on the TV monitor 20. In order to obtain a two-dimensional ultrasound image, it is necessary to move the probe 11 mechanically or electronically over the body surface, but since these techniques are already widely known,
The explanation here will be omitted. As already mentioned, the variable bandpass filter 15 must have a function of widening its band over time, but it is well known that this can be achieved by using a filter composed of voltage variable capacitance diodes, for example. It's like this.

The bandwidth of this bandpass filter is determined by the bandwidth control circuit 21.

Although this example describes the case in which a single piezoelectric vibrator is used as an ultrasonic probe, the same holds true for array-type vibrators for electronic scanning devices that have become popular in recent years. It goes without saying that the present invention is effective.

[Brief explanation of the drawing]

Fig. 1 shows an embodiment of the present invention, Fig. 2 shows the principle of ultrasound diagnosis, Fig. 3 shows the configuration of an ultrasound probe, and Fig. 4 shows an ultrasound probe. An explanatory diagram of multiple reflections that occur between the probe and the body surface tissue layer of a living body. Figure 5 is a diagram showing the frequency spectrum of waves reflected on the ultrasonic probe surface. Figure 6 is a diagram according to the present invention. FIG. 3 is a diagram showing changes in the spectrum of multiple reflected waves. 11... Ultrasonic probe, 12... Pulsa, 13...
・Rate pulse generator, 14... Preamplifier, 15.
... variable band pass filter, 16... logarithmic amplifier,
17... Envelope detection circuit, 18... VD converter, 1
9... Image memory, 20... TV monitor, 21...
・Bandwidth control circuit. Agent Patent Attorney Nori Ken Chika Yudo Matsuyama Hikaru 2 Figure 1 Figure 2 Ruto Otsuo Figure 4 Figure 5 Figure 6

Claims (2)

[Claims] (1) An ultrasonic transducer for transmitting and receiving ultrasonic waves, a transmission circuit for driving this transducer, a filter circuit for limiting the band of the received signal from the transducer, and an output of this filter circuit. In an ultrasonic diagnostic apparatus equipped with a display circuit for detecting and displaying waves, the filter circuit has a center frequency that is approximately the same as the resonance frequency of the ultrasonic transducer, and is controlled so that its bandwidth widens with the reception time of the sound waves. An ultrasonic diagnostic device characterized by being a variable band filter. (2) The ultrasonic diagnostic apparatus according to claim 1, characterized in that a matching layer is provided on the surface of the vibrator to match the acoustic impedance between the vibrator and the living body.