GB2045434A - An ultrasonic imaging system - Google Patents

Abstract

A scene is irradiated with ultrasound by a transducer 2 and the reflected sound received by an image converter tube 4, and an image displayed on a TV display 10. Programmable stepped frequency oscillators 1 and 6 are controlled so that the beam current of the tube 4 is modulated at a frequency which is varied in step with, but differs by a set amount from, the ultrasound frequency so that the scene is irradiated with different frequencies from frame to frame (or line to line) of the TV display. Mixing of the ultrasound frequency with the beam modulation frequency occurs and an amplifier and band-pass filter 7 selects the sum or difference frequency component. Thus the signal fed from the amplifier 7 to the display has a constant bandwidth. A delay 13 may be provided to compensate for the transit time of ultrasound from the transducer 2 to the scene. An averager 13 may be provided to average several frames before display. <IMAGE>

Description

SPECIFICATION An ultrasonic imaging system The present invention relates to an ultrasonic imaging system including an image converter tube.

An image converter tube comprises an envelope containing a sensor plate of piezoelectric material for sensing ultrasound, a conductive signal plate capacitively coupled to the sensor plate and means for scanning the sensor plate with an electron beam, to obtain from the signal plate an electrical signal representing the amplitude and phase of mechanical vibrations induced in the sensor plate by the ultrasound.

Such a tube is described in British Patent 1 512 269 (EMI Limited). That patent also describes how "whiteout" can be reduced by modulating the electron beam current and selecting frequency components of the electrical signal resultant from mixing of the frequency of modulation and the ultrasound frequency whilst rejecting frequency components at the frequency of the ultrasound. The patent mentions that the frequency of the ultrasound may be varied in time.

An object of the present invention is to provide a modification to the system described in British Patent 1 512269.

According to the present invention, there is provided an ultrasonic imaging system comprising; a transducer for producing ultrasound for irradiating a scene, a first oscillator for energising the transducer at ultrasonic frequencies, an image converter tube for receiving the ultrasound from the scene and producing therefrom an electrical signal representing the scene, a second oscillator for modulating the current of the scanning beam of the tube, control means for causing the first oscillator to vary with time, in a predetermined manner, its frequency of oscillation over a first predetermined range of the ultrasonic frequencies and for causing the second oscillator to vary its frequency of oscillation, in the same manner in time relationship with the variation of the first oscillator, over a second range of frequencies differing by a preset amount from the frequencies in the first range, and means for selecting frequency components of the said electical signal resulting from the mixing of the frequency of the ultrasound with the frequency of modulation of the beam current, whilst rejecting frequency components at the frequency of the ultrasound.

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawing, the single figure of which is a schematic diagram of an ultrasonic imaging system according to the invention, for producing a picture of a scene underwater.

A first programmable stepped frequency, oscillator 1 energises a transducer arrangement 2 at each of a plurality of discrete frequencies in a predetermined sequence, within a first range of frequencies. The oscillator 1 changes frequency each time it receives a pulse from a pulse generator 3. The transducer arrangement 2 irradiates a scene including an object with ultrasound having the frequency produced by the oscillator 1.

An image converter tube 4 receives the ultrasound reflected from the object via an acoustic lens 5 and produces therefrom an electrical signal representing the scene. The construction of the tube 4 is described in British Patent 1 512269. As described in that patent the current of the electron beam for scanning the piezo-electric sensor plate of the tube is modulated at a fixed frequency, (different by a preset amount from that of the ultrasound) produced by a second oscillator 6. An amp!ifier 7 has a band-pass filter characteristic designed to select one of the sum and difference frequency components of the electrical signal resulting from the mixing of the modulation frequency and the frequency of the ultrasound, whilst rejecting the frequency component at the frequency of the ultrasound.The selected component is demodulated in a demodulator 8 and a picture of the scene is displayed on a cathode ray tube display device 10.

In the present example of the invention, the sensor plate of the tube 4 and the display screen of the display device 10 are scanned in the same raster pattern, synchronism being maintained by the pulse generator 3, which supplies synchronising pulses to a scan control 11 of the tube 4 and to the display device 10. The pulse generator 3 supplies a pulse to the oscillator 1 at the end of every field, during the field suppression period, so that successive fields of the displayed picture are produced with different ultrasound frequencies. At normal field scan rates persistence in the display tube phosphor and persistence of vision will not be effective beyond about five fields, so a sequence of five frequencies may be preferred, although a longer sequence may be used if desired; in a modification described ;hereinafter a sequence of nine frequencies is used.

A pseudo random sequence of frequencies fn such as f1 , f3, f5, f2, f4 is preferred to a sequence such as fl , f2, f3, f4, f5 where 1 Of n > fn+ 1 . An odd number of frequencies is chosen so that the same two frequencies fn and fn+1 are used in the same frame of two interlaced fields only once every two sequences. The changing of frequencies should take place at the beginning of the field suppression period to allow for the time delay between transmission of ultrasound from the transducer 2 and its arrival at the tube 4. The length of the field suppression period limits the niaximum range from the transducer to the scene to the image converter tube for which the transient effects of changing frequency are not seen on the display.In some circumstances these effects may not be unacceptable when shown on the display, and furthermore, a modification described hereinbelow may reduce them.

In accordance with the present invention, the second oscillator 6, which in this example is also a programmable stepped frequency oscillator, is able to vary its frequency of oscillation, in the same manner as the first oscillator, in time relationship (in this example in synchronism) with the variation of the first oscillator, over a second range of frequencies differing by the said preset amount from the first range of frequencies. Thus the frequency of modulation of the beam current of the tube 4 is able to be varied with the frequency of the ultrasound produced by the transducer 4 to produce a constant sum or difference frequency component resulting from the mixing of the beam current modulation frequency and the ultrasound frequency.

The change of frequency of modulation of the beam current takes place, under the control of the pulse generator 3, at the beginning of the field suppression period so that the transient effects of the change, due to the transit time of ultrasound from the transducer 2 to the tube 5 via the scene, are not seen on the display. Clearly, the length of that period limits the maximum range from transducer 2 to scene to tube 4, for which those effects are not displayed. In some circumstances the effects may not be unacceptable if displayed, but in other circumstances they may be, so a switch 12 is provided to disconnect the oscillator 6 from the pulse generator 3 to cause it to operate at a fixed frequency if desired.At fixed frequency of modulation of the beam current, the transient effects of changes in frequency of the ultrasound, at a range to the scene so great that the transit time of the ultrasound cannot be accommodated by the field suppression period, may be less than the effects of changing both the modulation frequency and the ultrasound frequency.

At the very long ranges, where the transit time cannot be accommodated during the field supression period, it may be possible to produce an acceptable picture on the display 10, with changes of frequency taking place anywhere within the raster scan of the tube 4.

Although the invention has been described by way of example with reference to "fields" of the raster scan, it will be understood that references to "fields" may be replaced by references to "frames" a "frame" comprising more than one field.

A modification may be made which may reduce the transient effects mentioned hereinbefore. This modification comprises the use of an averaging device 1 2 which causes the display to display on each frame the average of n preceding frames.

Such an averager is described in British Patent Application 7900111 which describes an example in which n = 9 frames. (It will be appreciated that reference to "frames" may be replaced by "fields".) The averager receives pulses from the pulse generator 3 to keep its operation in synchronism with the operation of the rest of the system.

The system described hereinbefore operates with the ultrasound frequency changing at the end of each field (or frame) of the raster scan.

However, the frequency could change at the end of every line of the raster scan. In this case, the pulse generator 3 supplies pulses at the line frequency to the oscillators 1 and 6. Furthermore a longer pseudo-random sequence of frequencies would be used. The length of the sequence would be chosen so that a given frequency would be repeated on a given line only after an interval of several fields.

The problem of the transit time of the ultrasound from the transducer 2 to the tube 4 is more severe where the frequency changes at the end of every line. To reduce this problem a delay device 13 may be useful. The device delays the pulses produced by the pulse generator 3 and fed to the oscillator 6, scan control 11, display 10 relative to the pulses fed to the oscillator 1 r The delay is controlled in dependence upon the range from the system to the scene, for example in dependence upon the adjustment of the lens 5 by a focus adjuster 14.The delay is of such magnitude that when the frequency of the oscillator is changed to a new frequency, ultrasound of that new frequency initially reaches the tube 4 from the target at the end of a line of the raster scan and just as the frequency of modulation of the beam current changes to the corresponding new frequency.

The delay device 13 could be used with the example of the system in which the frequencies are changed at the end of every field, in a similar manner to its use in the line frequency system.

The examples of the invention described hereinbefore use many different frequencies of ultrasound for irradiating the scene, variable frequency of modulating the scanning beam current, and in some cases averaging of frames or fields.

The use of different frequencies of ultrasound reduces the visibility in the final picture of interference fringes but increases the bandwidth required in the amplifier, when used alone.

Furthermore, the impedance of the transducer arrangement varies with frequency and efficiency will vary, and the response of other components varies with frequency. This variation of response may cause flicker in the picture.

The variation of the modulating frequency of the scanning beam with the variation of the frequency of the ultrasound allows the possibility of reducing the bandwidth required for the amplifier in comparison with the system where the modulating frequency is not varied; this is more advantageous the higher the bandwidth of the ultrasound, and the higher the bandwidth of the transducer arrangement and the image converter tube. By its use, flicker due to the variation in the response of the amplifier with frequency is reduced.

The averaging of fields or frames reduces degradation of the final picture due to speckle and interference fringes, noise or other variable effects.

However, it also introduces a delay in the production of the picture.

Claims (16)

1. An ultrasonic imaging system comprising: a transducer for producing ultrasound for irradiating a scene, a first oscillator for energising the transducer at ultrasonic frequencies, an image converter tube for receiving the ultrasound from the scene and producing therefrom an electrical signal representing the scene, a second oscillator for modulating the current of the scanning beam of the tube, control means for causing the first oscillator to vary with time, in a predetermined manner, its frequency of oscillation over a first predetermined range of the ultrasonic frequencies and for causing the second oscillator to vary its frequency of oscillation, in the same manner in time relationship with the variation of the first oscillator, over a second range of frequencies differing by a preset amount from the frequencies in the first range, and means for selecting frequency components of the said electrical signal resulting from the mixing of the frequency of the ultrasound with the frequency of modulation of the beam current, whilst rejecting frequency components at the frequency of the ultrasound.

2. A system according to claim 1, wherein the scanning beam of the tube follows a periodically repeated scanning pattern, and the control means controls the said frequencies of oscillation of the first and second oscillators such that they are held constant over periods substantially equal in duration to the period of the scanning pattern and are changed between the said periods.

3. A system according to claim 2, wherein the frequency of oscillation of at least the first oscillator is changed between successive scanning patterns.

4. A system according to claim 2 or 3, wherein the frequency of oscillation of the second oscillator is changed substantially at the same time as that of the first oscillator.

5. A system according to claim 3, further comprising delay means for causing the frequency of oscillation of the second oscillator to change with a delay relative to the change in the frequency of oscillation of the first oscillator.

6. A system according to claim 2, 3, 4 or 5, wherein successive frequencies of oscillation of the oscillators form a pseudo random sequence of frequencies.

7. A system according to claim 2, 3, 4, 5 or 6 wherein the said scanning pattern comprises a frame formed of a plurality of interlaced fields.

8. A system according to claim 2, 3, 4, 5 or 6, wherein the said scanning pattern comprises a field, successive patterns being interlaced to form a frame.

9. A system according to claim 1, wherein the scanning beam of the tube follows a periodically repeated scanning pattern formed of a plurality of scanning lines, and the control means controls the said frequencies of oscillation of the first and second oscillators such that they are held constant over periods substantially equal in duration to the duration of one scanning line, and are changed between the said periods.

10. A system according to claim 9, wherein the frequency of oscillation of at least the first oscillator is changed between successive lines.

1 A system according to claim 9 or 10, wherein the frequency of oscillation of the second oscillator is changed substantially at the same time as that of the first oscillator.

12. A system according to claim 10, further comprising delay means for causing the frequency of oscillation of the second oscillator to change with a delay relative to the change in frequency of the first oscillator.

13. A system according to claim 9, 10, 11 or 12, wherein successive frequencies of oscillation of the oscillators form a pseudo random sequence of frequencies.

14. A system according to claim 2, 3, 4, 5, 6, 7 or 8, comprising means for averaging the electrical signals produced during successive repetitions of .the scanning pattern.

15. A system according to any preceding claim, wherein the selecting means comprises an amplifier having a band-pass filter characteristic designed to select one of the sum and difference frequency components of the electrical signal resulting from mixing of the modulation frequency and the frequency of the ultrasound, whilst rejecting the frequency component at the frequency of the ultrasound.

16. A system according to claim 5 or any one of claims 6 to 8 when appended thereto, or to claim 12 or any one of claims 13 to 15 when appended thereto, further comprising means for varying the said delay.

1 7. An ultrasonic imaging system substantially as herein before described with reference to the accompanying drawing.