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Instrumentation problems in ophthalmic ultrasonography
Instrumentation ultrasonography
problems
in ophthalmic
G. Baum*
This paper describes the shortcomings of current ophthalmological ultrasonographic equipment and correlates the effects of these shortcomings upon the appearance of the u~trasonogram. The techniques used to improve transducer performance are described. Available compression methods and their effects upon the appearance of the ultrasonogram are analyzed.
Ultrasonographic diagnosis is dependent upon high resolution and an extended grey scale in the presented data. High resolution is required because the structures and disease processes of the eye are a millimetre or less in size and an extended grey scale is required simultaneously to display the 60-80dB difference in the intensity of echoes from the front surface of the eye and those coming from the apex of the orbital fat. The visualization potential of ultrasonographic examination is illustrated by the compound scanned ultrasonogram of the eye shown in Fig 1. Although the structural components of the eye are well mapped on this ultrasonogram, there is * Dr Baum is Assistant Clinical Professor of Ophthalmology, Albert Einstein College of Medicine, Yeshiva University, Bronx, New York
Fig 1 A compound sector scan ultrasonogram of the human eye recorded at 15MHz. This ultrasonogram displays excellent mapping but there is a loss of cornea1 resolution
loss of resolution as indicated by the varying degrees of corned resolution shown in Fig 2. The cornea always appears as a single line in Fig 1, but in Fig 2 it is, at times, resolved as a two layered structure. The transducer and the cathode ray tube are the prime sources for this loss in resolution. The limitations in these elements produce receiver problems. TRANSDUCER PROBLEMS Depth resolution is determined by duration of the acoustic pulse and azimuth resolution by the aperture of the transducer lens, An ultrasonic pulse is obtained by applying a very brief electrical pulse (0. lpsec) to a piezoelectric material which
Fig 2 A compound sector scan ultrasonogram of the human eye recorded at 15MHz. This ultrasonogram exhibits varying degrees of resolution as exemplified by the resolving of the cornea as a double-layered structure (arrow) ULTRASONICS January 1968
43
converts the electrical pulse.
pulse into an acoustic (mechanical)
Work at this laboratory has disclosed that the application of the voltage pulse shown in Fig 3a results in the optimal response illustrated in Fig.3b. Piezoelectric materials respond to a steep slope or rapid change in voltage and exhibit no response when the rate of voltage change is slow compared to the response time of the material. This results in an acoustic pulse of minimal duration.
Despite an ideal electrical pulse, Fig 3, the vibration of the piezo element of the transducer decays exponentially (Fig 4a) because of the acoustic impedance mismatch between the piezoelectric element and the coupling agent into which the acoustic pulse is transmitted. Three methods have been used to shorten the pulse width. Initially, metal-loaded epoxy backing blocks with a high acoustic impedance were used (Fig 4b). McSkiminl (1959) suggested the use of multiple layers for impedance matching of the crystal to the coupling agent. A trial with McSkimin’s transducers showed that, at lSMHz, ringing occurred within these layers. In 1965, Kossoff2 evolved a method of using a single quarter wave matching layer in place of the multiple layers suggested by McSkimin. When a single quarter wave matching layer is used, ringing does not occur in this layer. Combining the quarter wave metal layer and a metal-loaded epoxy backing block reduces the pulse duration from 0.4~~ to O.l5ns, 6dB down, at 15MHz and from 0.4~s to 0.2~s at lOMHz, 6dB down (Fig 4~). The second major problem in ultrasonographic instrumentation is the cathode ray tube. Recognition and identification of tissue pathology is frequently dependent upon changes in the tonal scale.3-7
50 ns/cm Fig 3 The upper wave form (a) displays the voltage pulse applied to the transducer and the lower portion (b) the current (power) response of the transducer to this type of excitation. The piezoelectric element of the transducer does not respond when the slope of the voltage gradient is slow compared to the response time of the material. Therefore, this type of excitation is optimum for a short acoustic pulse
Although many different disease processes yield identical ultrasonographic geometrical patterns, an analysis of the amplitude data may clearly differentiate two disease entities. Fig 5 vs Fig 6 illustrates this point. If ultrasonograms which are lacking in tonal scale are used, a combined vitreous haemorrhage and tumour yield the geometrical pattern shown in Fig 5. Under these conditions, it is difficult to differentiate vitreous haemorrhage and exudates from solid tumour tissue. On the other hand, ultrasonograms which simultaneously present both the geometrical and amplitude data in the form of an altered tonal scale clearly show a difference in structure. Fig 6 is such a semiquantitative ultrasonogram of a malignant melanoma of the choroid. When the amplitude of the echo is shown as change in tonal scale, the leading edge of the tumour yields an intense echo which appears as a dense line and the interior of the tumour is characterized by weaker echoes consisting of large dense echo areas against an amorphous background. An analysis of the amplitude data generated by a vitreous haemorrhage versus a malignant melanoma illustrates why the amplitude data is so important in differentiating between these two conditions. The leading edge of a vitreous haemorrhage has an echo which is 60dB below the reflection obtained from a standard glass plate (see Appendix 1) block under identical conditions but exhibits only 1OdB of attenuation over a 1. 5cm path. In contrast, the leading edge of a malignant melanoma has an echo which is only 40dB below the same standard glass test block but exhibits 7dB of attenuation over a 3mm path (Fig 7 vs Fig 8). Most cathode ray tubes can display a dynamic range of 30dB and maximally 40dB. The dynamic range of ocular tissue extends over a 60dB range because of the absorption of the overlying tissues. Compression must be used to display the 60dB dynamic range of tissue in the 30dB-40dB tonal scale of the cathode ray tube. Two different types of compression may be employed, logarithmic compression and signal time control (STC). The effects of these two characteristics on the output display are quite different.
Fig 4 (a)
44
The exponential decay of an air -backed transducer
(b)
The damping effect produced by a metal-loaded epoxy backing
(c)
The acoustic pulse produced by a transducer that has combined metal-loaded epoxy backing and the quarter wave matching layer. The pulse shown in Fig 3 was used to produce these wave forms ULTRASONICS January 1968
The characteristics of logarithmic compression are illustrated in Fig 9. Part (a) of this figure illustrates the transfer characteristics of an amplifier employing compression. For illustrative purposes, the gain of this amplifier is unity (the slope of the transfer characteristic = 1) for small signals and the gain decreases as the input signal level increases. Thus, a signal range at the input from 0-V,volts will be compressed into the smaller range from 0-V2volts at the output. The effect of this characteristic on a strong pulse is illustrated in part (b) of Fig 9. Here, the input pulse shape is
Fig 8 Electronic photodensitometric analysis of the ampli. tude data of a malignant melanoma of the choroid
Fig 5 An ultrasonogram of an eye containing a malignant melanoma and a vitreous haemorrhage. In the absence of a tonal scale, it is nearly impossible to distinguish between the areas of tumour and vitreous haemorrhage
Input voltage
a
Ll t-
Fig 9 (a) (b) Fig 6 A semiquantitative ultrasonogram of a malignant melanoma of the choroid. Differences in the interfacial characteristics, the internal texture of the tumour and the acoustic shadow cast by the tumour can readily be identified on ultrasonograms which possess an adequate tonal scale
with compression
The effect of logarithmic compression shape
on pulse
$
a
Fig 7 Electronic photodensitometric analysis of the amplitude data of a vitreous haemorrhage
Amplifier characteristic
?
Fig 10 (a),(b) and (c) The effects of pulse shape and dynamic range upon resolution ULTRASONICS January 1968
45
rithmic amplification causes the echoes to appear fused to the eye so that interpretation is most difficult (Fig 11). The characteristics of STC are illustrated in Fig 12. Part (a) of this figure illustrates the time varying gain characteristics which typically may be employed. Part (b) illustrates three typical echoes. Pulse 1 may correspond to the cornea, pulse 2 to the retina, and pulse 3 to a discontinuity in the orbital fat. Fig 12c illustrates the same three pulses after they have been passed through a stage in which STC has been employed. The overall dynamic range has been reduced as desired. Also, since the amplifier gain is essentially constant over a single pulse width, the pulse shapes are undistorted. Thus, from this standpoint, this is a more desirable form of compression than logarithmic compression. Note, however, that the STC does not make a significant change in the relative amplitudes of closely spaced pulses such as 2 and 3. Thus, if these pulses originally differ in amplitude by more than the range that can be displayed and recorded, STC alone will not provide the entire compression needed. In this case it will be necessary to obtain additional compression after the STC stage, through the use of logarithmic compression. Fig 11 Ultrasonogram displaying poor azimuth resolution and degradation of depth resolution due to pulse stretching and summation. Detection of the tumour (arrow), in this instance, is possible because of the difference in tonal scale
Although the use of linear amplifiers with signal time control would, under certain conditions, lessen pulse stretching, linear amplification, per se, would not compensate for the fall-off in echo strength which occurs when the tissue transducer angle varies from normal incidence (Fig 13). Owing to the limitations of the instrumentation in use at the moment, it has not been possible to determine which of the two methods is of most value clinically. Thus, both methods
similar to that indicated in Fig 9a. The result of the compression is to flatten the pulse and actually make it appear much broader. Thus, a sharp interface which results in an echo similar to the input pulse will appear as a much broader interface in the film record after compression. This type of amplification degrades depth resolution because the weak exponentially decaying portion of the pulse receives more amplification than the leading edge of the signal-thus stretching the pulse. This concept may be clarified by reference to Fig 10. Part (a) of this figure is a typical pulse shape after amplification and envelope detection is displayed. In this case, the pulse is seen to have a trailing edge which lasts significantly longer than the width of the pulse near its peak value. Part (b) of Fig 10 illustrates the concept of resolution for this pulse shape. Pulse 1 in this figure is a strong received pulse, and pulse 2 is an equally strong pulse occurring close to pulse 1. It can be seen that these pulses are distinguishable, so that in this case the resolution is essentially the width of the pulse as measured near its peak. Pulse 3 represents a very weak pulse which is separated by several pulse widths from pulse 2, but appears as a slight pip on the trailing edge of this pulse, and is essentially indistinguishable. Pulse 4 is a pulse of the same amplitude as pulse 3, but it is further separated from pulse 2, so that, in this case, it is distinguishable or resolvable. Thus, when there is a large difference in relative amplitudes between the pulses, the separation required for resolution of the weaker pulse may be several times the pulse width measured near its peak. This is illustrative of the relationship between resolution and the range of amplitudes encountered, or the dynamic range of the system. Part (c) of Fig 10 is illustrative of an idealized pulse which has no trailing edge. It is evident that, in this case, the resolution is a single pulse width, independent of the amplitudes of the pulses (so long as the weak pulse may be detected in the display). From this discussion, it is evident that, in a high resolution system, it is desirable to have both a received pulse which is narrow near its peak, and which has a trailing edge which falls off as rapidly as possible, approaching the case of Fig 10~. This situation is particularly desirable in the examination of orbital fat. Orbital fat is composed of closely spaced adjacent strong and weak echoes. Pulse stretching and summation produced by loga46
ULTRASONICS January 1968
.s 8 al
B
I
8
b'
Time
Time
Fig 12 Effects of sensitivity time control (a) STC characteristic (b) Input pulses (c)
Output pulses
REFERENCES
o-
McSkimin, H. J., Journal of the Acoustical Society of America, 31,1519 (1959)
loE b 20-
3
;
Rough ground
glass
30-
Baum, G. and Greenwood, I. ‘High resolution ultrasonography and its clinical application to clinical ophthalmology’, Proceedings of the 3rd International Conference on Medical Electronics, p 412 (1960)
8: 40$j 50f60-
Baum, G. ‘The ultrasonographic characteristics of malignant melanoma’, Archives of Ophthalmology ‘78, 12 (July 1967)
3 70.% z
60-
;
go-
Kossoff, G., Robinson, D. E. and Garrett, W. J., Commonwealth Acoustic Laboratory Report No. 31 (1965)
s 00
100
200
3o”
4o”
Angular deviation
Fig 13 A rapid fall off of echo amplitude occurs as the angle of deviation from the normal incidence is increased. Smooth surfaces exhibit greater loss than do rough surfaces. The ‘humps’ in the curves are believed to be produced by diffraction phenomena and side lobes.
will have to be explored, and the final instrumentation may employ a combination of the two. These are but a few of the many problems this laboratory is attempting to solve so that ultrasonography can be transferred from the laboratory to the clinic. ACKNOWLEDGEMENTS This investigation was supported by the United States Health Service, Grant No 12460 of the National Institute of General Medical Sciences. Parts of this paper were presented at the 7th International Conference on Medical and Biological Engineering, Stockholm, Sweden, August 1967.
Baum, G. ‘A synopsis of ophthalmic ultrasonography’. Wissenschaftliche Zeitschrift der Humboldt University, Berlin, Mathematisch-Naturwissenschaftliche Reihe 14 pp 52-61 (1965) Boniuk, M. (Editor) ‘Use of ultrasonography in the differential diagnosis of ocular tumors’ p 308, Chapter 2 in ‘Ocular and adnexal tumors’, Mosby, St. Louis, Missouri (1964) Sorsby, A. (Editor) ‘Ultrasonography, clinical application’ p 19 in Modern trends in ophthalmology’, Vol 4, Butterworths, London (1967) APPENDIX 1 Standardization d the glass plate block Both the tissues and the test objects are examined under identical conditions using the same instrumentation, camera, film and film development. Calibration is accomplished by placing special test objects in the ultrasonic beam at the optimal geometric position before and after examination of tissues. Electrical attenuation is then introduced while the test objects are being examined. Similarly, upon completion of the regular serial examination of a patient, repeated undifferentiated scans are taken at a single level of the specimen with gradually increased attenuation, 5dB per scan, until echoes from tissue structure fall below detectible levels. The data is then analyzed by an electronic photodensitometer.
ULTRASONICS January 1968
47
problems
in ophthalmic
G. Baum*
This paper describes the shortcomings of current ophthalmological ultrasonographic equipment and correlates the effects of these shortcomings upon the appearance of the u~trasonogram. The techniques used to improve transducer performance are described. Available compression methods and their effects upon the appearance of the ultrasonogram are analyzed.
Ultrasonographic diagnosis is dependent upon high resolution and an extended grey scale in the presented data. High resolution is required because the structures and disease processes of the eye are a millimetre or less in size and an extended grey scale is required simultaneously to display the 60-80dB difference in the intensity of echoes from the front surface of the eye and those coming from the apex of the orbital fat. The visualization potential of ultrasonographic examination is illustrated by the compound scanned ultrasonogram of the eye shown in Fig 1. Although the structural components of the eye are well mapped on this ultrasonogram, there is * Dr Baum is Assistant Clinical Professor of Ophthalmology, Albert Einstein College of Medicine, Yeshiva University, Bronx, New York
Fig 1 A compound sector scan ultrasonogram of the human eye recorded at 15MHz. This ultrasonogram displays excellent mapping but there is a loss of cornea1 resolution
loss of resolution as indicated by the varying degrees of corned resolution shown in Fig 2. The cornea always appears as a single line in Fig 1, but in Fig 2 it is, at times, resolved as a two layered structure. The transducer and the cathode ray tube are the prime sources for this loss in resolution. The limitations in these elements produce receiver problems. TRANSDUCER PROBLEMS Depth resolution is determined by duration of the acoustic pulse and azimuth resolution by the aperture of the transducer lens, An ultrasonic pulse is obtained by applying a very brief electrical pulse (0. lpsec) to a piezoelectric material which
Fig 2 A compound sector scan ultrasonogram of the human eye recorded at 15MHz. This ultrasonogram exhibits varying degrees of resolution as exemplified by the resolving of the cornea as a double-layered structure (arrow) ULTRASONICS January 1968
43
converts the electrical pulse.
pulse into an acoustic (mechanical)
Work at this laboratory has disclosed that the application of the voltage pulse shown in Fig 3a results in the optimal response illustrated in Fig.3b. Piezoelectric materials respond to a steep slope or rapid change in voltage and exhibit no response when the rate of voltage change is slow compared to the response time of the material. This results in an acoustic pulse of minimal duration.
Despite an ideal electrical pulse, Fig 3, the vibration of the piezo element of the transducer decays exponentially (Fig 4a) because of the acoustic impedance mismatch between the piezoelectric element and the coupling agent into which the acoustic pulse is transmitted. Three methods have been used to shorten the pulse width. Initially, metal-loaded epoxy backing blocks with a high acoustic impedance were used (Fig 4b). McSkiminl (1959) suggested the use of multiple layers for impedance matching of the crystal to the coupling agent. A trial with McSkimin’s transducers showed that, at lSMHz, ringing occurred within these layers. In 1965, Kossoff2 evolved a method of using a single quarter wave matching layer in place of the multiple layers suggested by McSkimin. When a single quarter wave matching layer is used, ringing does not occur in this layer. Combining the quarter wave metal layer and a metal-loaded epoxy backing block reduces the pulse duration from 0.4~~ to O.l5ns, 6dB down, at 15MHz and from 0.4~s to 0.2~s at lOMHz, 6dB down (Fig 4~). The second major problem in ultrasonographic instrumentation is the cathode ray tube. Recognition and identification of tissue pathology is frequently dependent upon changes in the tonal scale.3-7
50 ns/cm Fig 3 The upper wave form (a) displays the voltage pulse applied to the transducer and the lower portion (b) the current (power) response of the transducer to this type of excitation. The piezoelectric element of the transducer does not respond when the slope of the voltage gradient is slow compared to the response time of the material. Therefore, this type of excitation is optimum for a short acoustic pulse
Although many different disease processes yield identical ultrasonographic geometrical patterns, an analysis of the amplitude data may clearly differentiate two disease entities. Fig 5 vs Fig 6 illustrates this point. If ultrasonograms which are lacking in tonal scale are used, a combined vitreous haemorrhage and tumour yield the geometrical pattern shown in Fig 5. Under these conditions, it is difficult to differentiate vitreous haemorrhage and exudates from solid tumour tissue. On the other hand, ultrasonograms which simultaneously present both the geometrical and amplitude data in the form of an altered tonal scale clearly show a difference in structure. Fig 6 is such a semiquantitative ultrasonogram of a malignant melanoma of the choroid. When the amplitude of the echo is shown as change in tonal scale, the leading edge of the tumour yields an intense echo which appears as a dense line and the interior of the tumour is characterized by weaker echoes consisting of large dense echo areas against an amorphous background. An analysis of the amplitude data generated by a vitreous haemorrhage versus a malignant melanoma illustrates why the amplitude data is so important in differentiating between these two conditions. The leading edge of a vitreous haemorrhage has an echo which is 60dB below the reflection obtained from a standard glass plate (see Appendix 1) block under identical conditions but exhibits only 1OdB of attenuation over a 1. 5cm path. In contrast, the leading edge of a malignant melanoma has an echo which is only 40dB below the same standard glass test block but exhibits 7dB of attenuation over a 3mm path (Fig 7 vs Fig 8). Most cathode ray tubes can display a dynamic range of 30dB and maximally 40dB. The dynamic range of ocular tissue extends over a 60dB range because of the absorption of the overlying tissues. Compression must be used to display the 60dB dynamic range of tissue in the 30dB-40dB tonal scale of the cathode ray tube. Two different types of compression may be employed, logarithmic compression and signal time control (STC). The effects of these two characteristics on the output display are quite different.
Fig 4 (a)
44
The exponential decay of an air -backed transducer
(b)
The damping effect produced by a metal-loaded epoxy backing
(c)
The acoustic pulse produced by a transducer that has combined metal-loaded epoxy backing and the quarter wave matching layer. The pulse shown in Fig 3 was used to produce these wave forms ULTRASONICS January 1968
The characteristics of logarithmic compression are illustrated in Fig 9. Part (a) of this figure illustrates the transfer characteristics of an amplifier employing compression. For illustrative purposes, the gain of this amplifier is unity (the slope of the transfer characteristic = 1) for small signals and the gain decreases as the input signal level increases. Thus, a signal range at the input from 0-V,volts will be compressed into the smaller range from 0-V2volts at the output. The effect of this characteristic on a strong pulse is illustrated in part (b) of Fig 9. Here, the input pulse shape is
Fig 8 Electronic photodensitometric analysis of the ampli. tude data of a malignant melanoma of the choroid
Fig 5 An ultrasonogram of an eye containing a malignant melanoma and a vitreous haemorrhage. In the absence of a tonal scale, it is nearly impossible to distinguish between the areas of tumour and vitreous haemorrhage
Input voltage
a
Ll t-
Fig 9 (a) (b) Fig 6 A semiquantitative ultrasonogram of a malignant melanoma of the choroid. Differences in the interfacial characteristics, the internal texture of the tumour and the acoustic shadow cast by the tumour can readily be identified on ultrasonograms which possess an adequate tonal scale
with compression
The effect of logarithmic compression shape
on pulse
$
a
Fig 7 Electronic photodensitometric analysis of the amplitude data of a vitreous haemorrhage
Amplifier characteristic
?
Fig 10 (a),(b) and (c) The effects of pulse shape and dynamic range upon resolution ULTRASONICS January 1968
45
rithmic amplification causes the echoes to appear fused to the eye so that interpretation is most difficult (Fig 11). The characteristics of STC are illustrated in Fig 12. Part (a) of this figure illustrates the time varying gain characteristics which typically may be employed. Part (b) illustrates three typical echoes. Pulse 1 may correspond to the cornea, pulse 2 to the retina, and pulse 3 to a discontinuity in the orbital fat. Fig 12c illustrates the same three pulses after they have been passed through a stage in which STC has been employed. The overall dynamic range has been reduced as desired. Also, since the amplifier gain is essentially constant over a single pulse width, the pulse shapes are undistorted. Thus, from this standpoint, this is a more desirable form of compression than logarithmic compression. Note, however, that the STC does not make a significant change in the relative amplitudes of closely spaced pulses such as 2 and 3. Thus, if these pulses originally differ in amplitude by more than the range that can be displayed and recorded, STC alone will not provide the entire compression needed. In this case it will be necessary to obtain additional compression after the STC stage, through the use of logarithmic compression. Fig 11 Ultrasonogram displaying poor azimuth resolution and degradation of depth resolution due to pulse stretching and summation. Detection of the tumour (arrow), in this instance, is possible because of the difference in tonal scale
Although the use of linear amplifiers with signal time control would, under certain conditions, lessen pulse stretching, linear amplification, per se, would not compensate for the fall-off in echo strength which occurs when the tissue transducer angle varies from normal incidence (Fig 13). Owing to the limitations of the instrumentation in use at the moment, it has not been possible to determine which of the two methods is of most value clinically. Thus, both methods
similar to that indicated in Fig 9a. The result of the compression is to flatten the pulse and actually make it appear much broader. Thus, a sharp interface which results in an echo similar to the input pulse will appear as a much broader interface in the film record after compression. This type of amplification degrades depth resolution because the weak exponentially decaying portion of the pulse receives more amplification than the leading edge of the signal-thus stretching the pulse. This concept may be clarified by reference to Fig 10. Part (a) of this figure is a typical pulse shape after amplification and envelope detection is displayed. In this case, the pulse is seen to have a trailing edge which lasts significantly longer than the width of the pulse near its peak value. Part (b) of Fig 10 illustrates the concept of resolution for this pulse shape. Pulse 1 in this figure is a strong received pulse, and pulse 2 is an equally strong pulse occurring close to pulse 1. It can be seen that these pulses are distinguishable, so that in this case the resolution is essentially the width of the pulse as measured near its peak. Pulse 3 represents a very weak pulse which is separated by several pulse widths from pulse 2, but appears as a slight pip on the trailing edge of this pulse, and is essentially indistinguishable. Pulse 4 is a pulse of the same amplitude as pulse 3, but it is further separated from pulse 2, so that, in this case, it is distinguishable or resolvable. Thus, when there is a large difference in relative amplitudes between the pulses, the separation required for resolution of the weaker pulse may be several times the pulse width measured near its peak. This is illustrative of the relationship between resolution and the range of amplitudes encountered, or the dynamic range of the system. Part (c) of Fig 10 is illustrative of an idealized pulse which has no trailing edge. It is evident that, in this case, the resolution is a single pulse width, independent of the amplitudes of the pulses (so long as the weak pulse may be detected in the display). From this discussion, it is evident that, in a high resolution system, it is desirable to have both a received pulse which is narrow near its peak, and which has a trailing edge which falls off as rapidly as possible, approaching the case of Fig 10~. This situation is particularly desirable in the examination of orbital fat. Orbital fat is composed of closely spaced adjacent strong and weak echoes. Pulse stretching and summation produced by loga46
ULTRASONICS January 1968
.s 8 al
B
I
8
b'
Time
Time
Fig 12 Effects of sensitivity time control (a) STC characteristic (b) Input pulses (c)
Output pulses
REFERENCES
o-
McSkimin, H. J., Journal of the Acoustical Society of America, 31,1519 (1959)
loE b 20-
3
;
Rough ground
glass
30-
Baum, G. and Greenwood, I. ‘High resolution ultrasonography and its clinical application to clinical ophthalmology’, Proceedings of the 3rd International Conference on Medical Electronics, p 412 (1960)
8: 40$j 50f60-
Baum, G. ‘The ultrasonographic characteristics of malignant melanoma’, Archives of Ophthalmology ‘78, 12 (July 1967)
3 70.% z
60-
;
go-
Kossoff, G., Robinson, D. E. and Garrett, W. J., Commonwealth Acoustic Laboratory Report No. 31 (1965)
s 00
100
200
3o”
4o”
Angular deviation
Fig 13 A rapid fall off of echo amplitude occurs as the angle of deviation from the normal incidence is increased. Smooth surfaces exhibit greater loss than do rough surfaces. The ‘humps’ in the curves are believed to be produced by diffraction phenomena and side lobes.
will have to be explored, and the final instrumentation may employ a combination of the two. These are but a few of the many problems this laboratory is attempting to solve so that ultrasonography can be transferred from the laboratory to the clinic. ACKNOWLEDGEMENTS This investigation was supported by the United States Health Service, Grant No 12460 of the National Institute of General Medical Sciences. Parts of this paper were presented at the 7th International Conference on Medical and Biological Engineering, Stockholm, Sweden, August 1967.
Baum, G. ‘A synopsis of ophthalmic ultrasonography’. Wissenschaftliche Zeitschrift der Humboldt University, Berlin, Mathematisch-Naturwissenschaftliche Reihe 14 pp 52-61 (1965) Boniuk, M. (Editor) ‘Use of ultrasonography in the differential diagnosis of ocular tumors’ p 308, Chapter 2 in ‘Ocular and adnexal tumors’, Mosby, St. Louis, Missouri (1964) Sorsby, A. (Editor) ‘Ultrasonography, clinical application’ p 19 in Modern trends in ophthalmology’, Vol 4, Butterworths, London (1967) APPENDIX 1 Standardization d the glass plate block Both the tissues and the test objects are examined under identical conditions using the same instrumentation, camera, film and film development. Calibration is accomplished by placing special test objects in the ultrasonic beam at the optimal geometric position before and after examination of tissues. Electrical attenuation is then introduced while the test objects are being examined. Similarly, upon completion of the regular serial examination of a patient, repeated undifferentiated scans are taken at a single level of the specimen with gradually increased attenuation, 5dB per scan, until echoes from tissue structure fall below detectible levels. The data is then analyzed by an electronic photodensitometer.
ULTRASONICS January 1968
47