NO862342L - PROCEDURE AND APPARATUS FOR IMPROVING A PICTURE. - Google Patents

Description

The present invention relates to a method for obtaining an image of an area being examined, in which method wave pulses that are produced by a pulse source for coherent waves are introduced into the area, the echo waves from reflectors in the area being received and a corresponding electrical echo signal is provided in each case. The invention also relates to an apparatus for carrying out the method.

Among methods of the above-mentioned kind, ultrasound imaging of organs, for example, is increasingly used in medical diagnoses. The advantages of ultrasound imaging compared to X-ray methods are that non-ionizing radiation is used, that soft tissue can be imaged without contrast agents, that real-time imaging is possible, and that ultrasound equipment is available at a relatively low price.

Medical ultrasound diagnosis has today reached a level that enables widespread clinical use. However, the image quality is degraded, among other things by speckle noise which is marked by a granular appearance of the ultrasound images, and which constitutes an obstacle with regard to tissue differentiation. Small structures or interfaces encountered at an unfavorable angle cannot be recognized due to the image spots,

i.e. a granular structure in an ultrasound image. This is an artifact produced by the coherence of ultrasound. Smudge formation is today the main obstacle with regard to recognizing details on ultrasound images. Because of the speckles, the signal-to-noise ratio of an ultrasound image has a value of only 1.91 (C.B. Burckhardt), "Speckle in Ultrasound B-Mode Sessions", IEEE Trans, on Sonics and Ultrasonics, vol. SU-25, 1978, pages 1-6). Staining thus has an unfavorable effect on the recognition of small and/or low-contrast structures, e.g. metastases. Various methods have therefore already been proposed with the aim of reducing spotting, but they are

all fraught with certain disadvantages. The methods used so far and their disadvantages are as follows:

1. Low-pass filtering of image signals:

In this method, with regard to the amplitude of the image signals, an average is taken over a number of spot grains, so that the fluctuations for such amplitudes are reduced. This method is rarely used because the result is reduced resolution.

2. Averaging the image signals over a plurality of images recorded at different frequencies: This is known as the frequency compounding method (P.A. Magnin, O.T. von Ramm, F.L. Thurstone, "Frequency Compounding for Speckle Contrast Reductions in Phased

Array Images", Ultrasonic Imaging, vol. 4, 1982, pages 267-281). Because the images are recorded at different frequencies, the speckle formations in the different images will be essentially uncorrelated with this method (depending on the overlap of the frequency bands) and it re- the average image shows less speckle However, this method suffers from the following disadvantages: A broadband system is required for image recording is at a plurality of frequencies. Because ultrasound absorption in the tissue increases with increasing frequency, the number of usable frequencies is theoretically limited. - The broadband system could be used to provide better longitudinal resolution. Division into a plurality of frequency bands provides a longitudinal resolution that is less than the maximum possible resolution. 3. Averaging the image signals over a plurality of images recorded from different directions: This is the principle of the composite scanning method (C.B. Burckhardt, "Specie in Ultrasound B-Mode Sessions", IEEE Trans, on Sonics and Ultrasonics, vol. SU-25, 19 78, pages 1-6; D. P. Shattuck and O. T. von Ramm, Ultrasonic Imaging 4, 1982, pages 93-107). Although this method provides a significant reduction with regard to spotting and thus a corresponding improvement in image quality, it is subject to the following disadvantages: The complexity of a device for using this forward procedure is much greater than the complexity with respect to a normal ultrasound device.

More time is required for recording images in different directions, and thus the maximum possible image speed will drop.

The structure to be displayed must be "visible" to ultrasound over a relatively large angular range, i.e. a larger "ultrasound window" is required than compared to conventional methods.

Ultrasound imaging is not the only imaging method that faces the speckle problem. Similar imaging methods that operate with coherent radiation, i.e. radar, are also affected by this problem. Special mention should be made of "Synthetic Aperture Radar" (W.M. Brown, J.L. Porcello, "An Introduction to Synthetic Aperture Radar", IEEE Spectrum vol. 6, 1969, pages 52-62).

The purpose of the present invention is to provide a method and apparatus of the type indicated above, by means of which image spotting can be reduced without the disadvantages that previously known techniques are afflicted with.

In this connection, according to the invention, a method is used which is characterized by: a) a first output signal representing the amplitude information for the echo signal is obtained by an amplitude demodulation of a signal derived from the echo signal, b) another signal represents the frequency or phase information for the echo signal, is provided by a frequency or phase demodulation of the signal derived from the echo signal, and c) the first and second output signals are combined to provide an image signal that can be displayed using

of display bodies to obtain the image of the area.

The invention also relates to an apparatus for procurement

of an image of an area being examined, the apparatus comprising the following elements:

A pulsed source and coherent waves,

and a transducer device which is connected to the source, and by means of which wave pulses are sent into the area, the echo waves being reflected from the reflectors in the area being received, and a corresponding electrical echo signal being produced in each case,

a receiver connected to the transducer device to provide an image signal by processing the echo signal, and an image display device connected to the receiver.

According to the invention, this device is characterized in that the receiver comprises a circuit with the following structure: a) A first signal path with an input connected to trans- . the transducer device, an amplitude demodulator, and an output where a first output signal representing the amplitude information of the echo signal is provided, b) another signal path with an input connected to the input of the first signal path and thus with the transducer device, a frequency or phase demodulator and an output at which where another output signal is delivered representing frequency or phase information for the echo signal, and

c) a combination circuit with a first input connected to the output of the first signal path, a second input

connected to the output from the second signal path, and an output from which an image signal obtained by logic operations on the output signals from the first and second signal path can be tapped.

The advantage of the invention is mainly that image spot formation is reduced by means of relatively simple means, while avoiding the disadvantages of previously known methods.

Some exemplified embodiments of the invention will be discussed below with reference to the attached drawings. Figure 1 is a block diagram of an ultrasound device. Figure 2 is a typical characteristic for the logarithmic amplifier 18 in Figure 1.

Figure 3 is a block diagram of a radar device.

Figure 4 is a block diagram of a first embodiment of the invention used in the detector 19 in Figures 1 and 3. Figure 5 is a block diagram of another embodiment of the invention used in the detector 19 in connection with variations of the devices shown in Figures 1 and 3. Figure 6 is a block diagram of preferred additions to the detector 19 shown in Figures 4 and 5. Figure 1 is a block diagram of an ultrasound device, e.g. a mechanical sector scanner. A pulsed transmitter 15 triggers an ultrasound transducer 11 via a duplexer 14. The received signal is fed via the duplexer to a preamplifier 16. An amplifier 17 with swept gain, which follows the preamplifier 16, has a gain that varies with time, and is used for compensation of the amplitude drop caused by the tissue damping. The amplifier 17 is followed by a logarithmic amplifier 18 which delivers an output signal which is proportional to the logarithm of the input signal over a given range.

The properties of the logarithmic amplifier 18 are shown in figure 2. From this it will be seen that at zero input amplitude the output amplitude is also zero, in contrast to the actual logarithm of the input amplitude, which then assumes the value infinity .

As shown in figure 2, the properties for negative amplitudes are point-symmetric with respect to the properties for positive in-amplitudes.

The output signal from the logarithmic amplifier 18 is fed via a line 29 to the input of a detector 19. In conventional ultrasound equipment, this detector forms an absolute value and performs low-pass filtering of the output signal from the logarithmic amplifier 18. The output signal from the detector is delivered via a line 91. This signal is subjected to analog/digital conversion in an analog/digital converter 21. The output signal from the converter 21 is temporarily stored in an image storage/standard converter 22. The latter is simultaneously read out. The read signal is subjected to digital/analog conversion in a digital/analog converter 23. The output signal from the converter 23 is fed to a TV monitor 24 as a standard television signal.

The transducer 11 is moved by means of a motor 12 which is controlled by a motor control unit 13. The electronic control unit 25 which is shown provides command and synchronization signals to all the blocks shown, via connections not shown.

Figure 3 is a block diagram of a radar device. Because this is very similar to the ultrasound device shown in Figure 1, only the differences between these devices will be described. The ultrasound device according to figure 1 is operated at a frequency in the megahertz range, while the radar device shown in figure 3 operates in the microwave range. Instead of an ultrasound transducer, the radar device has an antenna 41 by means of which electromagnetic wave pulses are emitted and echo waves are received. The echo signal which is amplified by means of an HF amplifier 46 is transformed into a signal in the intermediate frequency range in a mixer 47. For this purpose, the mixer receives a carrier frequency signal from a local oscillator 56. The mixer 47 is followed by a logarithmic IF amplifier 48. The characteristics of this amplifier are similar to those of the amplifier 18 in Figure 1. The IF is typically in the mega-hertz range, so that the output of the logarithmic amplifier 48 delivers a signal similar to that of the output of the amplifier 18 in Figure 1.

In the radar device shown in Figure 3, a video amplifier 51 is arranged between the detector 19 and the analogue/digital converter 21. An amplifier with swept gain (known as a sensitivity-time control in radar)

can be arranged between the HF amplifier 46 and the mixer 47. Figure 3 does not show an amplifier of this kind.

The invention relates more particularly to the construction of the detector 19 in the device shown in Figures 1 and 3.

Figure 4 is a block diagram of a first embodiment of the detector 19 according to the present invention. This comprises a first signal path 86 and a second signal path 78. The output signals from these two signal paths are added using an adding circuit 85. The output signal from this circuit is also the output signal from the detector 19, which is delivered via line 91.

The first signal path 86 is essentially a conventional amplitude demodulator. It comprises a series connection of a rectifier 81, a low-pass filter 82, and a delay network 83 intended to equalize the delays for the signals through the first and second signal paths.

The second signal path 78 is a frequency or phase demodulator comprising a series circuit of the following blocks: An amplitude limiter 71, a delay line 72 with a delay of TQ/4, where TQ is the period of the transmit frequency, an integrated circuit (e.g. National Semiconductor Integrated Circuit LM1496) which is used as a multiplier 73, by means of which the output signal from the limiter 71 and the output signal from the delay line 72 are multiplied, a low-pass filter 74 to eliminate the higher harmonics formed by the multiplication, a circuit 75 for obtaining an output signal representing the absolute value of the amplitude of the output signal from the low-pass filter 74, and a multiplier 76 by means of which the output signal from the circuit 75 is multiplied by a factor K. For any echo signal, the amplitude of the output signal from the multiplier 76 will be proportional to the absolute value of the frequency deviation of the echo signal with respect to a reference frequency.

Frequency-dependent attenuation of the tissue causes the echo medium frequency to drop with increasing depth for the reflectors. The transmission frequency fQ should therefore preferably be used as a reference frequency only when there is a short time interval between the transmitter pulse and echo. With an increasing time interval between the transmitter pulse and the echo, the reference frequency should preferably be able to be reduced accordingly. This can be done by increasing the delay on the delay line 72 in Figure 4 with increasing time. In this respect, it is unfair to use e.g. voltage-controlled capacitors (varicaps) in a suitable circuit.

Figure 5 is a block diagram of another embodiment of a detector 19 according to the invention as shown in Figures 1 and 3. This other embodiment is designed as a variant of the ultrasound device shown in

figure 1, and for the radar device shown in figure 3. The variant differs from the devices shown in figures 1 and 3 only in that a linear amplifier is used instead of the logarithmic amplifier 18 or 84, i.e. an amplifier where the amplitude of the output signal is proportional to the amplitude of the input signal. The differences between the detector shown in Figure 5 and the detector shown in Figure 4 are as follows: Instead of the multiplier 76 in Figure 4, a circuit 101 is used which produces an output signal with an amplitude which has a non-linear function of the absolute value |Afl for the frequency deviation of the echo signal in relation to a reference frequency. A function of this kind is e.g. an exponentiation with the exponent K. |Af|.

Instead of the addition circuit 85 in Figure 4, an integrated circuit 102 is used as a multiplier (e.g.

Motorola integrated circuit MC 1495 L). In this to-.'. error, the output signal from the detector 19 is therefore delivered via the line 91, obtained by multiplying the output signal from the first signal path 86 and the output signal from the second signal path 78.

According to the invention, it is possible to replace the circuit 75 in Figures 4 and 5 with a circuit whose property represents a smooth function, i.e. a function of the form g(-x) = g(x), e.g. g(x) = x^.

Figure 6 shows preferred additions to the embodiment of the detector 19, shown in Figures 4 and 5. In Figure 6, the combination circuit 111 represents the addition circuit 85 in Figure 4, or the multiplier 102 in Figure 5.

According to a first addition shown in Figure 6, the detector 19 comprises a circuit 112 which, in response to the output signal from a threshold circuit 113, connects either the output from the combination circuit 111 or the output 84 from the first signal path 86 with the output line 91 from the detector 19, via which the the latter is connected to the TV monitor 24. An input 116 of the threshold circuit 113 is connected to the output of the first signal path 86. A reference signal corresponding to a first predetermined threshold value is fed to another input 117 of the threshold circuit 113. When the amplitude of the output signal for the first signal path 86 exceeds the first predetermined threshold value, the corresponding output signal from the threshold circuit 113 will cause the switch 112 to connect the output 84 of the first signal path 86 with the line 91. With the addition just described above, only the first signal path 86 from the detector 19 is used for detector function in the case of echo signals with amplitude exceeding e n predetermined threshold value. This is advantageous because larger echo signals generally arise from image-reflecting objects, and such echoes generally cause no spotting on the image, so that the function for the detector 19 according to the invention, as shown in figures 4 and 5, is unnecessary for such echoes, and can result in deterioration of the longitudinal resolution.

According to another addition shown in Figure 6, the detector 19 may also comprise another threshold circuit 114

and a device 115 for measuring the signal/noise ratio for the output signal from the first signal path 86. The output from this device is connected to an input 118 to the threshold circuit 114. A reference signal corresponding to a

another predetermined threshold value is applied to another input 119 of the threshold circuit 114. In response to the output signal from the threshold circuit 114, the switch 112 connects either the output of the combination circuit 111 or the output 84 of the first signal path with the line 91. When the amplitude of the output signal from the device 115 falls below the second predetermined threshold value, the corresponding output signal from the threshold circuit 114 will cause the switch 112 to connect the output of the first signal path 86 with the line 91. With the above-mentioned second addition, only the first signal path 86 will be used for echo signals if the signal/noise ratio falls below a predetermined threshold value. This is advantageous because the use of the detector 19, shown in figures 4 or 5, for echo signals of this nature can result in deterioration of the image quality.

Claims (11)

1. Method for obtaining an image of an area under observation, by which method wave pulses obtained from a pulsed source of coherent waves are sent into the area, the echo waves reflected from reflectors in the area are received, and a corresponding electrical echo signal is produced in each case, characterized by a) a first output signal representing the amplitude formation for the echo signal is obtained by an amplitude demodulation of a signal derived from the echo signal, b) another signal represents the frequency or phase information for the echo signal, is obtained by a frequency or phase demodulation of the signal derived from the echo signal, and c) the first and second output signals are combined to provide an image signal which can be displayed by means of display means to provide the image of the area. 2. Method as stated in claim 1, where the combination of the first and second output signal comprises a change in the amplitude of the first output signal, depending on the amplitude of the second output signal, and the first output signal that has been changed in this way constitutes the image signal . 3. Apparatus for producing an image of an area under observation, the apparatus comprising the following elements: A pulsed source of coherent waves and a transducer device which is connected to the source and by means of which wave pulses are sent into the area, the echo waves reflected from the reflectors in the area being countered and a corresponding electrical echo signal being produced in each case , a receiver connected to the transducer device to provide an image signal by processing the echo signal, and an image display device connected to the receiver, characterized in that the receiver comprises a circuit with the following structure: a) A first signal path (86) with an input connected to the transducer device (11, 41), an amplitude demodulator (81, 82) and an output (84) where a first output signal representing the amplitude information of the echo signal is provided, b) another signal path (78) with an input connected to the input of the first signal path and thus to the transducer device (11, 41), a frequency or phase demodulator (72, 73, 74) and an output (77) at which another output signal representing frequency or phase information for the echo signal, and c) a combination circuit (85, 102, 111) with a first input connected to the output (84) of the first signal path, a second input connected to the output (77) of the second signal path, and an output (91) from which taps an image signal which is produced by logic operations on the output signals from the first and second signal paths. 4. Apparatus as stated in claim 3, characterized in that it comprises a linear amplifier connected between the transducer device (11, 41) and the input to the first signal path (86), and that the combination circuit is a multiplier circuit (102). 5. Apparatus as stated in claim 3, characterized in that it comprises a logarithmic amplifier (18, 48) connected between the transducer device (11, 41) and the input to the first signal path (86), and that the combination circuit is an addition circuit (85) . 6. Apparatus as stated in claim 3 or 4, characterized in that the second signal path (78) is a circuit for the production of the second output signal, and arranged in such a way that the amplitude of the second output signal is a non-linear function of the absolute value of the frequency deviation of the echo signal in relation to a reference frequency. 7. Apparatus as specified in claim 3 or 5, characterized in that the second signal path (78) comprises a circuit for producing the second output signal, arranged so that the amplitude of the second output signal is proportional to the absolute value of the frequency deviation for the echo signal in relation to a reference frequency. 8. Apparatus as specified in claims 6 and 7, characterized in that the reference frequency is equal to the transmission frequency. 9. Apparatus as specified in claims 6 and 7, characterized in that the reference frequency is variable and is lower than the transmission frequency by a depth correction factor. 10. Apparatus as specified in one of claims 3-9, characterized in that it comprises a switch (112) which, in reaction to the output signal from a first threshold circuit (113), connects either the output from the combination circuit (111) or the output (84) from the first signal path (86) with the image display device (24), an input to the threshold circuit being connected to the output from the first signal path (86) and the switch connecting the output to the image display device when the amplitude for the first output signal exceeds a first predetermined threshold value. 11. Apparatus as stated in one of claims 3 - 10, characterized in that it comprises a switch (112) which, in response to the output signal from another threshold circuit (114), connects either the output from the combination circuit (111) or the output (84) ) from the first signal path with the image display device (24), an input to the second threshold circuit (114) being connected to the output of a device (115) for measuring the signal/noise ratio for the first output signal, and the switch (112) connecting the output (84) of the first signal path to the image display device when the amplitude of the output signal from the signal-to-noise ratio measurement device falls below another predetermined threshold value.