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Improved techniques in ultrasonic cross sectional echography
Improved techniques in ultrasonic cross sectional echography G. KOSSOFF
Methods to improve the grey scale, sensitivity and resolution of ultrasonic echograms are described. The grey scale is improved using film echography where the echogram is formed on a film using a conventional oscilloscope for display. The sensitivity is increased by writing the background level. Very small echoes are then displayed as a further increase in the illumination and are readily seen. The lateral resolution is improved by the use of medium focused transducers. Structures of greatest interest are then examined with the focal beam while choice of operating characteristics allows the display of out of focus areas at a diagnostically acceptable resolution. The axial resolution of large, long waveforms with small slowly varying changes is improved by adding the differential of the signal to the output. This emphasises the slowly varying portions while preserving the amplitude data. Typical echograms in obstetrics, ophthalmology and of the breast are used to illustrate the improvements obtained by the use of these techniques.
Introduction Ultrasonic cross sectional echography is a technique which provides accurate two-dimensional displays of acoustic impedance discontinuities in soft tissue. The principle of the method is well kn0wn.l A pulse of sound is transmitted in a known direction and the received echoes are displayed by intensity modulating the trace of an oscilloscope, the direction of the trace representing the direction of the sound pulse. A cross sectional display is obtained by moving the transducer in a plane and making the trace follow the movement. Small changes in the acoustic impedance are readily visualized and the technique has been used successfully in a variety of clinical investigations. Recently, improved methods of display, signal processing and transducer design, have given substantial improvements in the grey scale, sensitivity and resolution of echograms obtained with the Commonwealth Acoustic Laboratories (CAL) echoscopes and these are the subject of this paper. Film echoscopy The majority of CAL echograms published in the literature have been obtained using storage-tube oscilloscopes for display. Storage-tube oscilloscopes have the advantage that the echogram may be viewed as it is being formed and that only those echograms which contain the required Dr G. Kossoff is an international editor of ULTRASONICS and on the staff of the Commonwealth Acoustic Laboratories, Miller’s Point, Sydney, Australia. Paper received 8 January 1972.
ULTRASONICS.
SEPTEMBER
1972
clinical information are photographed. Unfortunately even the best storage-tube oscilloscopes have a limited dynamic range and grey scale. This means that it is not possible to display on storage-tube echograms any information about the size of the echo. This is not a limitation in cases where the diagnosis is based on the interpretation of geometrical relationships such as the position or size of an organ, but is not adequate in soft tissue differentiation analysis where the clinical information resides in small changes in the echo pattern. Also, because of the largerspot size and tendency of closely lying echoes to bloom into a single large echo, storage-tube echograms have a relatively coarse resolution. These disadvantages can be overcome to a large extent by using a conventional oscilloscope for display. The echogram is built up on a film which is exposed during the scanning period and will be referred to as the film echogram. The requirements for the display oscilloscope are that it should have a bright light output and a small spot size which does not defocus as its brightness is increased. A P-l 1 phosphor screen is preferable since the frequency spectrum of its light output is such that it can be readily matched to the absorption characteristics of many films. The oscilloscope should have a proper intensity modulation provision so that the light output is linearly proportional to the video voltage, and the tube should introduce minimum geometric distortion. Fortunately, some of the recently commercially available x-y display oscilloscopes satisfy these requirements. The optimum display system is obtained by using the conventional oscilloscope for film echography in parallel with a storage-tube oscilloscope for viewing.
221
The film echogram may be obtained either on the Polaroid or on a 35 mm film. Both have applications. With the Polaroid film the echogram may be viewed almost immediately and appropriate adjustments may be made to optimize the display. Because the absorption characteristics of the Polaroid 3000 film do not fully match those of the P-l 1 phosphor, and those of the larger size of print, the Polaroid film is approximately one stop slower than a 35 mm film such as the Ilford HP-4 film (ASA rating 400). Typically, echoes are written at a relatively fast scanning rate such as one second for a full sector scan. The oscilloscope must be driven fairly hard to record small echoes on the film, and this reduces the linearly proportional range of the voltage-brightness relationship used to portray the magnitude of echoes. For this reason the faster speed of the 35 mm film is a useful advantage. Also the 3.5 mm film has a wider grey range and because the echoes are written as black dots on a white background it is easier to see small changes in grey scale especially amongst strong signals.2 A further advantage of the 35 mm film method is that the process is more automatic, thus reducing the time required for the examination. The main disadvantages of the 35 mm film are that the film must be processed before it can be viewed and sometimes the examination must be repeated if the film is inadvertently damaged during processing. Normally both films are used, first the Polaroid film to ensure correct adjustments and then the 35 mm film for examination. Fig.1 Film echogram of an eye showing variations in background level due to hum pick-up and small changes in speed of sector scanning
Display of background level With film echography it is possible to display at least 10 different shades of grey, greatly increasing the diagnostic information content which can be displayed by the echogram. With the availability of the large grey scale it is possible to increase the sensitivity of the technique by operating the oscilloscope with a background level which is just visible when no echoes are present; ie at zero information. Then, even the smallest echoes are represented as a further illumination so that all echoes are displayed on the echogram. The display of the background level is a very sensitive method of checking the performance of the equipment. Saturation, ringing or mistriggering are readily seen on the display as a change in the background level. For instance, saturation is shown as a reduced background immediately behind a strong echo. Ringing is seen as a ripple of the background level, whereas mistriggering is displayed as a blank area. This check is best applied with simple scanning before the extra information obtained with compound scanning complicates the display. Ideally the background should be of uniform brightness. This method of display is so sensitive that a small regular variation in background is obtained on commercially available display oscilloscopes when a vertical line is swept horizontally by the time base across the screen at a time base speed of 0.1 s cm-l. This is due to hum pick-up from the main transformer, and on the CAL echoscopes this transformer is placed externally to reduce this variation to an acceptable level. Any nonuniformity in the scanning speed will also cause variations in background. In particular, if sector scanning is employed, it is necessary to switch off the background and echo signals before the transducer reaches the reversing position as otherwise bright lines are obtained at the extreme posi-
222
tions of the scan. An example of variations in background caused by variation in sector scanning speed and hum pickup is illustrated in Fig.1. This echogram of an eye was obtained with the CAL ophthalmological scanner.3 The clear space in the retrobulbar fat pattern represents the optic nerve. Display of internal echoes from organs Because no soft tissue is completely homogeneous, small internal echoes are obtained from soft tissue organs. With the background level of display, even the smallest echoes are represented at their appropriate intensity level and this type of information may be used to classify various types of soft tissue. With experience and standard operation of equipment it is likely that these low level internal echoes will be recognized by their signature; ie the sensitivity and the pattern with which they appear. In contrast, liquid filled structures do not give rise to internal echoes and are readily recognized by their lowest background level. A typical film echogram in obstetrics is shown in Fig.2. This was obtained with the CAL abdominal echoscope 4 and displays a cross section through the foetal trunk, heart and placenta. The echograms show a soft, detailed clear picture where it is possible to discern the detailed vascular structure of the placenta. The heart is also displayed in detail. The external boundaries of the heart return strong echoes and it is possible to distinguish three echo free liquid-filled areas, presumably the chambers of the heart and the major blood vessels. These chambers are enclosed by heart muscles which return small internal echoes from their structure.
ULTRASONICS.
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1972
Film echogram through foetal trunk, heart and placenta Fig.2 showing the soft fine detail end grey scale of the method
Film echogram with contact scanner showing foetal spine Fig.5 and ribs
A film echogram through the foetal lower abdomen, spine and kidneys is shown in Fig.3. The foetal spine, the kidneys and perhaps the adrenals are displayed in great detail. The echogram also shows three small liquid-filled structures. Because film echograms display variations in the magnitude of the echoes, the outline of major structures such as the foetal trunk, are not as clearly defined as in storage echograms. This is to be expected since the film echogram shows the variation in the magnitude of the echo as the outline is scanned by the ultrasonic beam. A film echogram of a hydatidiform mole is shown in Fig.4. Again a soft uniformly grey picture is obtained with darker spots from larger cysts in the mole.
Film echogram through lower foetal abdomen Fig.3 foetal spine, kidneys and three liquid-filled structures
showing the
A film echogram obtained with the CAL contact scanner is shown in Fig.5. The echogram shows an anterior placenta as well as a section through the foetal head and trunk. The section through the trunk shows the foetal spine and ribs. Because the internal echoes originate from small structures, the echoes are diffusely reflected in many directions. This is in contrast to the echoes obtained from the boundaries of the organs which are specularly reflected in a specific direction. This can be put to advantage to display the position of the organ even when it is not possible to show the echoes from the boundaries due to their inclination to the beam. This approach has been used to study the structure of the liver 5 and could also be useful in cardiology. In the latter investigations it is often difficult to display all the muscle-blood interfaces because of their inclination and changes with the heart cycle. By displaying the internal muscle echoes it should be possible to display the position of all muscles throughout the heart cycle and this would facilitate the interpretaton of heart echograms. Medium focused transducers
Fig.4
Hydatidiform
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mole seen with film echography
SEPTEMBER
1972
Spherically focusing transducers have been used on all CAL echoscopes to reduce the width of the examining beam and thus improve the lateral resolution. The degree
223
of focusing by a spherical transducer
is defined by the ratio as shown in Fig.6 and X the wavelength. For most transducers used, the echoscopy d
h
d2
X
8hA
It is convenient to define the degree of the focusing as weak if h/h < 3, medium if 3 10. Weakly focused transducers are used to improve the lateral resolution over the whole depth examined by the ultrasonic beam. With weak focusing the lateral resolution is approximately five times worse than the depth resolution limiting the resolution of the echogram. The lateral resolution may be improved with medium focusing by reducing the beam over a distance which extends approximately one quarter to one third of the examined depth. A good resolution is then obtained over this distance or focal zone while a diagnostically acceptable resolution is obtained elsewhere. Medium focusing allows the examination, with good resolution, of anatomical features of greatest interest, much in the same way a microscope is focused on a particular feature of a slide in histological analysis. Medium focused transducers have a number of distinct characteristics. As shown in Fig.7, medium focused transducers pick up a stronger echo from a flat surface when this surface is at the focus; ie they show effective focusing of echoes from flat surfaces. This is in contrast to weak focusing where the echo from a flat surface is independent of distance.6 Because of their larger aperture, medium focused transducers more readily pick up echoes from inclined surfaces. Whereas with weakly focused transducers an inclination of five degrees is sufficient to drop the amplitude of the echo by 20 dB, a medium focused transducer, as shown in Fig.7, can tolerate at the focus an inclination of eight degrees. At pre- and post-focal depths the inclination dependence curve is more complex showing a number of maxima and minima at angles other than normal incidence. At pre-focal depths a minimum is obtained at normal incidence, the nature of the function becoming more complex as one moves away from the focus.
and causes single flat targets to be displayed as two closely spaced structures. This effect does not usually develop in the focal zone which, for the medium focused ophthalmological transducer shown in Figs 7,8 and 9, extends approximately 2 cm about the focus. This transducer is used for depth investigations of 6 cm so that the focal zone corresponds to 30% of the examined depth. The axial echo from a l/32 in (0.79 mm) diameter spherical steel ball as a function of distance is shown in Fig.8. As expected, this echo illustrates the concentration of energy obtained with focusing. The American Institute of Ultrasonics in Medicine defines the experimental echo beamwidth as the distance between two lateral off-sets where the echo is 20 dB down on the axial value. If one extends this definition so that the beamwidth is defined as the distance between the two last lateral offsets where the echo is 20 dB down on the maximum value, one can readily examine the narrowing of the beam obtained with focusing. The beamwidth of the medium focused transducer is compared with the weak focusing transducer in Fig.9 and shows that, at the
h
~ Fig.6
Co-ordinates
of a spherically
focused transducer
The inclination dependence, however, is less critical than at the focus. As shown in Fig.7, an inclination of over 10 degrees is required to drop the size of the echo by 20 dB from the peak value, while the first off axis maximum occurs at an inclination of approximately five degrees. At post-focal depths, a maximum is obtained at normal incidence and again the inclination dependence becomes less critical so that an inclination of 10 degrees is required to drop the echo by 20 dB. Because of this less critical dependence on inclination more of the inclined structures are displayed on the echogram. Away from focus, the typical single maximum waveform from a flat surface target is changed into a double waveform. This change occurs usually at several angles of incidence, the two maximum being usually several cycles apart. This effect is caused by the phase distribution of received energy across the curved surface of the transducer
224
Depth
[cm]
Fig.7 Echo from a flat surface with a medium focused ophthalmological transducer as a function of depth and inclination (d = 3.5 cm, A = 10 cm, frequency = 8 MHz, h/h = 8)
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curves become more complex. Initially an axial minimum is developed which is about 50% or 6 dB down on the maximum obtained at the lateral offset. This changes into a sharp central peak with two broad secondary maxima which are between 10 and 6 dB below the axial maximum. Finally at large distances a broad curve is obtained with a series of maxima and minima. In general the dynamic range of echoes is much larger than the dynamic range of the oscilloscope and some compression is required to display the echoes. The CAL echoscopes are designed to display approximately a 40 dB range after time-gain compensation. As shown in Figs 7,8 and 9 the echoes in the focal zone from large flat targets and small spherical targets are approximately 20 dB higher than those outside the focal zone. Thus, only a 20 dB range or less is displayed outside the focal zone. As shown in these figures this range is concentrated near the axis of the beam so that only this area is displayed on the echogram. Thus, despite the wide beamwidth in the region outside the focal zone, this region is displayed with a diagnostically acceptable resolution though at a reduced sensitivity. The changes in the resolution and sensitivity with which the nipple and other breast tissues are displayed as the focus is moved 3 cm deeper into the breast tissue are shown on the 35 mm film echogram in Fig.10. These echograms were obtained with the CAL breast echoscope 7 and show the detail obtained from the fat and glandular tissues in the breast and the pectoralis muscles. The position of the focus is displayed by the arcs external to both sides of the breast.
J 9
8
II
IO Depth
12
13
14
[cm]
Axial echo from a l/32 in (0.79 mm) diameter spherical Fig.8 steel ball with a medium focused ophthalmological transducer
focus, the beam is narrowed by a factor better than two and that an improvement is obtained throughout the focal zone. Fig.9 also shows the size of the echo as the steel ball is moved laterally off axis. The voltage scale at each distance is marked on the figure. In the focal zone the lateral offset curve shows the normal behaviour with a maximum echo obtained when the target is on axis. Closer, the curve begins to exhibit a small axial minimum which is approximately 70% or 3 dB down on the maximum obtained at a lateral offset of approximately 1 mm. Past the focal zone the
Examined
Differentiate
and add method of signal processing
As already mentioned, the dynamic range of echoes is compressed by some method such as time-gain compensation,
depth
2v
8
9
II
IO Depth
Fig.9 Beamwidth of a medium and weakly focused transducer medium focused ophthalmological transducer
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[cm]
and echo from
l/32
in (0.79
mm) ball as a function
of lateral offset with a
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logarithmic amplification, compression amplifiers, etc. Apart from this type of signal processing, either the basic echo waveform or the differential of this waveform is used to intensity modulate the display oscilloscope. The effects of differentiation of the signal have been described.* Essentially the method emphasises changes in the waveform at the expense of removing from the display the information content in the constant portions of the waveform. Because differentiation removes some information and is potentially more likely to display artifacts, it is not commonly used for signal processing in the CAL echoscope. In many examinations a situation is encountered in which a large and relatively long echo waveform showing small slowly varying changes is received. In obstetrics, the echo from the foetal surface has this characteristic. In these circumstances it is desirable to emphasize changes in the echo waveform so that the smaller changes become more apparent to the investigator while preserving the amplitude information content of the waveform. A circuit which achieves this is shown schematically in Fig. 11. The waveform from the detector is broken into two paths. The waveform is unchanged along one path, but is subject to a
Add
4
To disploy oscilloscope
Circuit schematic of the differentiate Fig.1 1 signal processing
and add method of
Echo
Differential
I
Glandular tissue
Echo
Long
Short
waveform
Principle of the differentiate Fig.12 processing
Fig.10 Changes in display of the nipple as the focal zone is moved 3 cm deeper into the breast tissue. The position of the focus is displayed by the arcs external to both sides of the breast fd = 4 cm, A = 8 cm, frequency = 2 MHz, h/A = 3.5)
226
+ differential
waveform
and add method of signal
differentiation along the other path. The two waveforms are then combined in the correct proportion by the adder and the resultant waveform is used to intensity modulate the oscilloscope. The principle of the method is shown in Fig. 12. The long echo waveform shows a small variation in amplitude and it is required to emphasize this variation. The differentiation changes the waveform into two short pulses of appropriate height and when these are added in suitable proportion, usually between 20 and 30%, to the original waveform, the resultant emphasizes the leading edges and shows more clearly the small changes in the original waveform while retaining the amplitude information. The time constant of the differentiating circuit is chosen to have no effect on the short echo waveform of the type obtained from single interfaces. Since the majority of the echoes are of this nature, this method of signal processing does not affect the general character of the echogram.
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1972
Conclusion The described methods of display, signal processing and transducer design have given substantial improvement to the grey scale, sensitivity and resolution of ultrasonic echography. All of these improvements are considered as necessary pre-requisites for soft tissue differentiation studies where it is necessary to display small subtle changes in echo patterns. These improvements allow the display of a wealth of previously unseen anatomical detail and these have significantly increased the clinical usefulness of the technique in investigations of the abdomen, breast and eye.
Acknowledgements I would like to acknowledge the contributions of Mr M. Dadd and Mr J. Jellins who helped put into practice on the CAL echoscopes the techniques described in this paper. In this regard Mr G. Radanovich has provided us with valuable technical assistance. I would also like to acknowledge Dr W. J. Garrett and Dr H. Hughes for their contributions to the interpretations of the echograms.
ULTRASONICS.
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1972
This paper is published with the permission of the Australian Director-General of Health. References Kossoff, G., Robinson, D. E., Garrett, W. J. Ultrasonic twodimensional visualization for medical diagnosis, Journal of the Acoustical Society ofAmerica 5 (1968) 1310 Baum, G. Problems in ultrasonographic localization and attempts at their solution, Ophthalmic Ultrasound: Edited by K. A. Gilter et al (C. V. Mosby, St Louis 1969) Hughes, H., Dadd, M. J. The application of ultrasonography to the examination of the eye, The Medical Journal of Australia 2 (1969) 848 Robinson, D. E., Garrett, W. J., Kossoff, G. Foetal anatomy displayed by ultrasound, Investigative Radiology 6 (1968) 442 Kelly, E., Oknyama, D., Fry, W. Comparison of ultrasonotomographs of pig livers, Japanese Society of Medical Ultrasonics (1967) 125 Kossoff, G., Robinson, D. E., Garrett, W. J. Ultrasonic twodimensional visualization techniques, IEEE Transactions on Sonics and Ultrasonics 2 (1965) 31 Jellins. J.. Kossoff. G.. Buddee. F. W.. Reeve. T. S. Ultrasonic visualization of the brkast, The’Medichl Jourkl of Australia 6 (1971) 305 Robinson, D. E., Kossoff, G., Garrett, W. J. Artifacts in ultrasonic echoscopic visualization, Ultrasonics 4 (1966) 186
227
Methods to improve the grey scale, sensitivity and resolution of ultrasonic echograms are described. The grey scale is improved using film echography where the echogram is formed on a film using a conventional oscilloscope for display. The sensitivity is increased by writing the background level. Very small echoes are then displayed as a further increase in the illumination and are readily seen. The lateral resolution is improved by the use of medium focused transducers. Structures of greatest interest are then examined with the focal beam while choice of operating characteristics allows the display of out of focus areas at a diagnostically acceptable resolution. The axial resolution of large, long waveforms with small slowly varying changes is improved by adding the differential of the signal to the output. This emphasises the slowly varying portions while preserving the amplitude data. Typical echograms in obstetrics, ophthalmology and of the breast are used to illustrate the improvements obtained by the use of these techniques.
Introduction Ultrasonic cross sectional echography is a technique which provides accurate two-dimensional displays of acoustic impedance discontinuities in soft tissue. The principle of the method is well kn0wn.l A pulse of sound is transmitted in a known direction and the received echoes are displayed by intensity modulating the trace of an oscilloscope, the direction of the trace representing the direction of the sound pulse. A cross sectional display is obtained by moving the transducer in a plane and making the trace follow the movement. Small changes in the acoustic impedance are readily visualized and the technique has been used successfully in a variety of clinical investigations. Recently, improved methods of display, signal processing and transducer design, have given substantial improvements in the grey scale, sensitivity and resolution of echograms obtained with the Commonwealth Acoustic Laboratories (CAL) echoscopes and these are the subject of this paper. Film echoscopy The majority of CAL echograms published in the literature have been obtained using storage-tube oscilloscopes for display. Storage-tube oscilloscopes have the advantage that the echogram may be viewed as it is being formed and that only those echograms which contain the required Dr G. Kossoff is an international editor of ULTRASONICS and on the staff of the Commonwealth Acoustic Laboratories, Miller’s Point, Sydney, Australia. Paper received 8 January 1972.
ULTRASONICS.
SEPTEMBER
1972
clinical information are photographed. Unfortunately even the best storage-tube oscilloscopes have a limited dynamic range and grey scale. This means that it is not possible to display on storage-tube echograms any information about the size of the echo. This is not a limitation in cases where the diagnosis is based on the interpretation of geometrical relationships such as the position or size of an organ, but is not adequate in soft tissue differentiation analysis where the clinical information resides in small changes in the echo pattern. Also, because of the largerspot size and tendency of closely lying echoes to bloom into a single large echo, storage-tube echograms have a relatively coarse resolution. These disadvantages can be overcome to a large extent by using a conventional oscilloscope for display. The echogram is built up on a film which is exposed during the scanning period and will be referred to as the film echogram. The requirements for the display oscilloscope are that it should have a bright light output and a small spot size which does not defocus as its brightness is increased. A P-l 1 phosphor screen is preferable since the frequency spectrum of its light output is such that it can be readily matched to the absorption characteristics of many films. The oscilloscope should have a proper intensity modulation provision so that the light output is linearly proportional to the video voltage, and the tube should introduce minimum geometric distortion. Fortunately, some of the recently commercially available x-y display oscilloscopes satisfy these requirements. The optimum display system is obtained by using the conventional oscilloscope for film echography in parallel with a storage-tube oscilloscope for viewing.
221
The film echogram may be obtained either on the Polaroid or on a 35 mm film. Both have applications. With the Polaroid film the echogram may be viewed almost immediately and appropriate adjustments may be made to optimize the display. Because the absorption characteristics of the Polaroid 3000 film do not fully match those of the P-l 1 phosphor, and those of the larger size of print, the Polaroid film is approximately one stop slower than a 35 mm film such as the Ilford HP-4 film (ASA rating 400). Typically, echoes are written at a relatively fast scanning rate such as one second for a full sector scan. The oscilloscope must be driven fairly hard to record small echoes on the film, and this reduces the linearly proportional range of the voltage-brightness relationship used to portray the magnitude of echoes. For this reason the faster speed of the 35 mm film is a useful advantage. Also the 3.5 mm film has a wider grey range and because the echoes are written as black dots on a white background it is easier to see small changes in grey scale especially amongst strong signals.2 A further advantage of the 35 mm film method is that the process is more automatic, thus reducing the time required for the examination. The main disadvantages of the 35 mm film are that the film must be processed before it can be viewed and sometimes the examination must be repeated if the film is inadvertently damaged during processing. Normally both films are used, first the Polaroid film to ensure correct adjustments and then the 35 mm film for examination. Fig.1 Film echogram of an eye showing variations in background level due to hum pick-up and small changes in speed of sector scanning
Display of background level With film echography it is possible to display at least 10 different shades of grey, greatly increasing the diagnostic information content which can be displayed by the echogram. With the availability of the large grey scale it is possible to increase the sensitivity of the technique by operating the oscilloscope with a background level which is just visible when no echoes are present; ie at zero information. Then, even the smallest echoes are represented as a further illumination so that all echoes are displayed on the echogram. The display of the background level is a very sensitive method of checking the performance of the equipment. Saturation, ringing or mistriggering are readily seen on the display as a change in the background level. For instance, saturation is shown as a reduced background immediately behind a strong echo. Ringing is seen as a ripple of the background level, whereas mistriggering is displayed as a blank area. This check is best applied with simple scanning before the extra information obtained with compound scanning complicates the display. Ideally the background should be of uniform brightness. This method of display is so sensitive that a small regular variation in background is obtained on commercially available display oscilloscopes when a vertical line is swept horizontally by the time base across the screen at a time base speed of 0.1 s cm-l. This is due to hum pick-up from the main transformer, and on the CAL echoscopes this transformer is placed externally to reduce this variation to an acceptable level. Any nonuniformity in the scanning speed will also cause variations in background. In particular, if sector scanning is employed, it is necessary to switch off the background and echo signals before the transducer reaches the reversing position as otherwise bright lines are obtained at the extreme posi-
222
tions of the scan. An example of variations in background caused by variation in sector scanning speed and hum pickup is illustrated in Fig.1. This echogram of an eye was obtained with the CAL ophthalmological scanner.3 The clear space in the retrobulbar fat pattern represents the optic nerve. Display of internal echoes from organs Because no soft tissue is completely homogeneous, small internal echoes are obtained from soft tissue organs. With the background level of display, even the smallest echoes are represented at their appropriate intensity level and this type of information may be used to classify various types of soft tissue. With experience and standard operation of equipment it is likely that these low level internal echoes will be recognized by their signature; ie the sensitivity and the pattern with which they appear. In contrast, liquid filled structures do not give rise to internal echoes and are readily recognized by their lowest background level. A typical film echogram in obstetrics is shown in Fig.2. This was obtained with the CAL abdominal echoscope 4 and displays a cross section through the foetal trunk, heart and placenta. The echograms show a soft, detailed clear picture where it is possible to discern the detailed vascular structure of the placenta. The heart is also displayed in detail. The external boundaries of the heart return strong echoes and it is possible to distinguish three echo free liquid-filled areas, presumably the chambers of the heart and the major blood vessels. These chambers are enclosed by heart muscles which return small internal echoes from their structure.
ULTRASONICS.
SEPTEMBER
1972
Film echogram through foetal trunk, heart and placenta Fig.2 showing the soft fine detail end grey scale of the method
Film echogram with contact scanner showing foetal spine Fig.5 and ribs
A film echogram through the foetal lower abdomen, spine and kidneys is shown in Fig.3. The foetal spine, the kidneys and perhaps the adrenals are displayed in great detail. The echogram also shows three small liquid-filled structures. Because film echograms display variations in the magnitude of the echoes, the outline of major structures such as the foetal trunk, are not as clearly defined as in storage echograms. This is to be expected since the film echogram shows the variation in the magnitude of the echo as the outline is scanned by the ultrasonic beam. A film echogram of a hydatidiform mole is shown in Fig.4. Again a soft uniformly grey picture is obtained with darker spots from larger cysts in the mole.
Film echogram through lower foetal abdomen Fig.3 foetal spine, kidneys and three liquid-filled structures
showing the
A film echogram obtained with the CAL contact scanner is shown in Fig.5. The echogram shows an anterior placenta as well as a section through the foetal head and trunk. The section through the trunk shows the foetal spine and ribs. Because the internal echoes originate from small structures, the echoes are diffusely reflected in many directions. This is in contrast to the echoes obtained from the boundaries of the organs which are specularly reflected in a specific direction. This can be put to advantage to display the position of the organ even when it is not possible to show the echoes from the boundaries due to their inclination to the beam. This approach has been used to study the structure of the liver 5 and could also be useful in cardiology. In the latter investigations it is often difficult to display all the muscle-blood interfaces because of their inclination and changes with the heart cycle. By displaying the internal muscle echoes it should be possible to display the position of all muscles throughout the heart cycle and this would facilitate the interpretaton of heart echograms. Medium focused transducers
Fig.4
Hydatidiform
ULTRASONICS.
mole seen with film echography
SEPTEMBER
1972
Spherically focusing transducers have been used on all CAL echoscopes to reduce the width of the examining beam and thus improve the lateral resolution. The degree
223
of focusing by a spherical transducer
is defined by the ratio as shown in Fig.6 and X the wavelength. For most transducers used, the echoscopy d
h
d2
X
8hA
It is convenient to define the degree of the focusing as weak if h/h < 3, medium if 3
and causes single flat targets to be displayed as two closely spaced structures. This effect does not usually develop in the focal zone which, for the medium focused ophthalmological transducer shown in Figs 7,8 and 9, extends approximately 2 cm about the focus. This transducer is used for depth investigations of 6 cm so that the focal zone corresponds to 30% of the examined depth. The axial echo from a l/32 in (0.79 mm) diameter spherical steel ball as a function of distance is shown in Fig.8. As expected, this echo illustrates the concentration of energy obtained with focusing. The American Institute of Ultrasonics in Medicine defines the experimental echo beamwidth as the distance between two lateral off-sets where the echo is 20 dB down on the axial value. If one extends this definition so that the beamwidth is defined as the distance between the two last lateral offsets where the echo is 20 dB down on the maximum value, one can readily examine the narrowing of the beam obtained with focusing. The beamwidth of the medium focused transducer is compared with the weak focusing transducer in Fig.9 and shows that, at the
h
~ Fig.6
Co-ordinates
of a spherically
focused transducer
The inclination dependence, however, is less critical than at the focus. As shown in Fig.7, an inclination of over 10 degrees is required to drop the size of the echo by 20 dB from the peak value, while the first off axis maximum occurs at an inclination of approximately five degrees. At post-focal depths, a maximum is obtained at normal incidence and again the inclination dependence becomes less critical so that an inclination of 10 degrees is required to drop the echo by 20 dB. Because of this less critical dependence on inclination more of the inclined structures are displayed on the echogram. Away from focus, the typical single maximum waveform from a flat surface target is changed into a double waveform. This change occurs usually at several angles of incidence, the two maximum being usually several cycles apart. This effect is caused by the phase distribution of received energy across the curved surface of the transducer
224
Depth
[cm]
Fig.7 Echo from a flat surface with a medium focused ophthalmological transducer as a function of depth and inclination (d = 3.5 cm, A = 10 cm, frequency = 8 MHz, h/h = 8)
ULTRASONICS.
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curves become more complex. Initially an axial minimum is developed which is about 50% or 6 dB down on the maximum obtained at the lateral offset. This changes into a sharp central peak with two broad secondary maxima which are between 10 and 6 dB below the axial maximum. Finally at large distances a broad curve is obtained with a series of maxima and minima. In general the dynamic range of echoes is much larger than the dynamic range of the oscilloscope and some compression is required to display the echoes. The CAL echoscopes are designed to display approximately a 40 dB range after time-gain compensation. As shown in Figs 7,8 and 9 the echoes in the focal zone from large flat targets and small spherical targets are approximately 20 dB higher than those outside the focal zone. Thus, only a 20 dB range or less is displayed outside the focal zone. As shown in these figures this range is concentrated near the axis of the beam so that only this area is displayed on the echogram. Thus, despite the wide beamwidth in the region outside the focal zone, this region is displayed with a diagnostically acceptable resolution though at a reduced sensitivity. The changes in the resolution and sensitivity with which the nipple and other breast tissues are displayed as the focus is moved 3 cm deeper into the breast tissue are shown on the 35 mm film echogram in Fig.10. These echograms were obtained with the CAL breast echoscope 7 and show the detail obtained from the fat and glandular tissues in the breast and the pectoralis muscles. The position of the focus is displayed by the arcs external to both sides of the breast.
J 9
8
II
IO Depth
12
13
14
[cm]
Axial echo from a l/32 in (0.79 mm) diameter spherical Fig.8 steel ball with a medium focused ophthalmological transducer
focus, the beam is narrowed by a factor better than two and that an improvement is obtained throughout the focal zone. Fig.9 also shows the size of the echo as the steel ball is moved laterally off axis. The voltage scale at each distance is marked on the figure. In the focal zone the lateral offset curve shows the normal behaviour with a maximum echo obtained when the target is on axis. Closer, the curve begins to exhibit a small axial minimum which is approximately 70% or 3 dB down on the maximum obtained at a lateral offset of approximately 1 mm. Past the focal zone the
Examined
Differentiate
and add method of signal processing
As already mentioned, the dynamic range of echoes is compressed by some method such as time-gain compensation,
depth
2v
8
9
II
IO Depth
Fig.9 Beamwidth of a medium and weakly focused transducer medium focused ophthalmological transducer
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[cm]
and echo from
l/32
in (0.79
mm) ball as a function
of lateral offset with a
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logarithmic amplification, compression amplifiers, etc. Apart from this type of signal processing, either the basic echo waveform or the differential of this waveform is used to intensity modulate the display oscilloscope. The effects of differentiation of the signal have been described.* Essentially the method emphasises changes in the waveform at the expense of removing from the display the information content in the constant portions of the waveform. Because differentiation removes some information and is potentially more likely to display artifacts, it is not commonly used for signal processing in the CAL echoscope. In many examinations a situation is encountered in which a large and relatively long echo waveform showing small slowly varying changes is received. In obstetrics, the echo from the foetal surface has this characteristic. In these circumstances it is desirable to emphasize changes in the echo waveform so that the smaller changes become more apparent to the investigator while preserving the amplitude information content of the waveform. A circuit which achieves this is shown schematically in Fig. 11. The waveform from the detector is broken into two paths. The waveform is unchanged along one path, but is subject to a
Add
4
To disploy oscilloscope
Circuit schematic of the differentiate Fig.1 1 signal processing
and add method of
Echo
Differential
I
Glandular tissue
Echo
Long
Short
waveform
Principle of the differentiate Fig.12 processing
Fig.10 Changes in display of the nipple as the focal zone is moved 3 cm deeper into the breast tissue. The position of the focus is displayed by the arcs external to both sides of the breast fd = 4 cm, A = 8 cm, frequency = 2 MHz, h/A = 3.5)
226
+ differential
waveform
and add method of signal
differentiation along the other path. The two waveforms are then combined in the correct proportion by the adder and the resultant waveform is used to intensity modulate the oscilloscope. The principle of the method is shown in Fig. 12. The long echo waveform shows a small variation in amplitude and it is required to emphasize this variation. The differentiation changes the waveform into two short pulses of appropriate height and when these are added in suitable proportion, usually between 20 and 30%, to the original waveform, the resultant emphasizes the leading edges and shows more clearly the small changes in the original waveform while retaining the amplitude information. The time constant of the differentiating circuit is chosen to have no effect on the short echo waveform of the type obtained from single interfaces. Since the majority of the echoes are of this nature, this method of signal processing does not affect the general character of the echogram.
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Conclusion The described methods of display, signal processing and transducer design have given substantial improvement to the grey scale, sensitivity and resolution of ultrasonic echography. All of these improvements are considered as necessary pre-requisites for soft tissue differentiation studies where it is necessary to display small subtle changes in echo patterns. These improvements allow the display of a wealth of previously unseen anatomical detail and these have significantly increased the clinical usefulness of the technique in investigations of the abdomen, breast and eye.
Acknowledgements I would like to acknowledge the contributions of Mr M. Dadd and Mr J. Jellins who helped put into practice on the CAL echoscopes the techniques described in this paper. In this regard Mr G. Radanovich has provided us with valuable technical assistance. I would also like to acknowledge Dr W. J. Garrett and Dr H. Hughes for their contributions to the interpretations of the echograms.
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This paper is published with the permission of the Australian Director-General of Health. References Kossoff, G., Robinson, D. E., Garrett, W. J. Ultrasonic twodimensional visualization for medical diagnosis, Journal of the Acoustical Society ofAmerica 5 (1968) 1310 Baum, G. Problems in ultrasonographic localization and attempts at their solution, Ophthalmic Ultrasound: Edited by K. A. Gilter et al (C. V. Mosby, St Louis 1969) Hughes, H., Dadd, M. J. The application of ultrasonography to the examination of the eye, The Medical Journal of Australia 2 (1969) 848 Robinson, D. E., Garrett, W. J., Kossoff, G. Foetal anatomy displayed by ultrasound, Investigative Radiology 6 (1968) 442 Kelly, E., Oknyama, D., Fry, W. Comparison of ultrasonotomographs of pig livers, Japanese Society of Medical Ultrasonics (1967) 125 Kossoff, G., Robinson, D. E., Garrett, W. J. Ultrasonic twodimensional visualization techniques, IEEE Transactions on Sonics and Ultrasonics 2 (1965) 31 Jellins. J.. Kossoff. G.. Buddee. F. W.. Reeve. T. S. Ultrasonic visualization of the brkast, The’Medichl Jourkl of Australia 6 (1971) 305 Robinson, D. E., Kossoff, G., Garrett, W. J. Artifacts in ultrasonic echoscopic visualization, Ultrasonics 4 (1966) 186
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