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A moving tissue-equivalent phantom for ultrasonic real-time scanning and Doppler techniques
L455
Letters to the Editor
DISCUSSION Although there are no clearly defined hazards associated with pulse-echo ultrasound as currently used in diagnostic medicine, there is now a gradual increase in interest in exposure parameters for clinical equipment due in part to experimental observations of effects of low-temporal-average-intensity, pulsed ultrasound like those described above. To encourage this trend, the American Institute of Ultrasound in Medicine awards commendations to manufactureres who provide specific information about the acoustical quantities which characterize their equipment. At present, the quantity which is recommended by AIUM/NEMA to characterize temporal peak intensity is the spatial peak, pulse average The performance standard for ultrasound therapy products administered by the intensity. National Center for Devices, and Radiological Health (NCDRH) requires that I, as well as the The NCDRH guide to be used by manufacturers of total power be indicated by the equipment. medical diagnostic ultrasound equipment in the preparation of product reports to be filed with NCDRH asks for I, as well as Ipa to characterize the peak intensity. The example chosen for these tests admittedly is contrived to distinguish clearly beHowever, the difference in the usefulness tween the pulse average and maximum intensities. In addition to of the two quantities as predictors of this biological effect is striking. its apparent biological relevance, the NCRP's maximum intensity has an advantage of simplicity of definition. With the advent of PVDF hydrophones which have uniform response over a wide range of frequencies and reasonably long term stability in calibration, it should be possible for measurements of maximum intensity to be made easily not only in the laboratory and factory but also in the clinical setting. ACKNOWLEDGEMENTS. The authors are indebted to Dr. Harold Stewart of the National Radiological Health and Professor Paul Carson, University of Michigan helpful discussions and information. This study was supported in part by U.S.P.S. Grant GMD9933.
Yours
Center for Devices and Medical Center for
etc.,
E.L. Carstensen, R.B. Berg, S.Z. Child. Dept. of Electrical Engineering, University of Rochester, Rochester, N.Y. References American Institute of Ultrasound in Medicine/National Electrical Manufacturers Association Safety standard for diagnostic ultrasound equipment. J. Ultrasound (AIUM/NEMA) (1983). Med. 2:Sl-S50. Ultrasonic power and intensities Carson, P.L., Fischella, R. and Oughton,'T..V. (1978). produced by diagnostic ultrasound equipment. Ultrasound Med. Biol. 3:341-350. Carson, P.L. (1983). In a personal communication, Prof. Carson reports that with the pulse shapes employed in pulse echo and pulse Doppler ultrasound the ratios of Im to I pa range from 1 to values as great as 6. Carstensen, E.L., Parker, K.J. and Barbee, D.B. (1983). Temporal peak intensity. J. Acoust. Sot. Am. (In press). Child, S.Z., Carstensen, E.L. and Lam, S.K. (1981). Effects of ultrasound on Drosophila: III. Exposure of larvae to low-temporal-average-intensity, pulsed irradiation. Ultrasound Med. Biol. 7:167-173. Biological effects of ultrasound: Mechanisms and clinical applications. N.C.R.P. (1983). Scientific Committee No. 66, National Council for Radiation Protection and Measurements. Bethesda, Maryland.
A Moving
Tissue-Equivalent
Phantom
For Ultrasonic
Real-Time
Scanning
And Doppler
Techniques.
Sir: Introduction Test phantoms which mimic the ultrasonic properties of tissue reasonably accurately have been described in the literature (Madsen et al, 1978; McCarty and Stewart, 1982). These phantoms are of value for comparing machines and for checking their constancy of performance. As tissue-equivalent phantoms improve, their value for predicting the performance of machines in clinical imaging also increases. However,the phantoms described in the literature have all
L456
Letters to the Editor
been static ones containing no moving parts. Since in practice virtually no tissues examined by ultrasound are static this is a significant limitation of present phantoms. It is worth bearing in mind that the echo signals resulting from scattering at closely spaced targets are altered by small motions of the targets. For example, with 5 MHz ultrasound the wavelength in tissue is about 0.3 mm. A change in the component of the separation of the targets along the ultrasound beam of as little as a tenth of a wave-length, i.e. 0.03 mm, could be expected to alter significantly the signal level from the targets. Tissue motions are also of importance since echoes from structures move in a similar way in an image whereas noise signals change in a random fashion. The eye is also particularly sensitive to moving edges. These factors contribute to the clarity of real-time images. A moving tissue phantom has been constructed for the assessment of ultrasonic scanners, The device has also been used to acquire an increased understanding of the factors which contribute to image quality. Moving tissue-equivalent phantom. To produce realistic images, the phantom was made from a tissue-equivalent material comprising a mixture of gelatin, water, propanol and graphite powder (Madsen et al 1978). Other synthetic material can also be used which is often easier to work with, e.g. Bulpren reticulated foam (Lerski et al 1982). The material is contained in a cylindrical drum whose wall is a thin polycarbonate sheet of thickness 0.25 mm. A few drums constructed with 1 mm thick plastic walls had large refraction artifacts at the sides of their images (Fig. l), The drum is interchangeable to aid development of test phantoms which simulate different Several have been constructed containing simulated cysts and tumours (Figs. applications. 1 and 2). Arrays of nylon fibres or holes have been included for resolution and registration The drum is rotated or oscillated about its axis by a variable-speed electric tests. motor coupled to the drum by reduction gearing and a magnetic drive. The container, surrounding the drum, is filled with oil to permit easy use with linear or sector scanners (Fig. 3).
The echoes Fig. 1. Frozen cross-sectional image of the tissue phantom. from the wire arrays for resolution tests are located centrally and are Three disc shaped structures of increased saturated in this image. One fluid filled cyst-like structure echogenicity represent tumours. The image of the cross section through the drum does not is also shown. appear circular due to a refraction artifact in the 1 mm thick plastic wall of the drum.
Letters
L457
to the Editor
Fj Lg. 2.
Cystic structures in the phantom material. These structures :e used to determine the resolving power of a scanner and how it varies as the structures move throughout the field of view.
al
Fig. 3. Use of a test-rig sector scanner.
with a rotating
transducer
real-time
L458
Letters
to the Editor
Uses of Moving Phantoms. The motion feature of the test phantom has been found to be relevant for a number of reasons. 1. Study of the speed of scan effects. When a moving structure is imaged with a frame rate which is too low, the structure appears to jump between positions rather than move in a smooth fashion. This is usually only a problem with fast moving structures if the true frame rate of the machine is relatively low e.g. less than 20 frames/set. Moving phantoms can be used to check the presentation of structures over a range of speeds. The unit can also be used to check on distortion in the presentation of heart valves due to the finite time taken for the ultrasound beam to sweep across the field of view. 2. Test of M-mode recording. The quality and accuracy of M-mode facilities can be checked with a moving tissue phantom. 3. Assessment of lag . In old systems lag may be due to the response of a TV camera or display screen, in new ones it is most likely due to signal processing techniques such as signal averaging or automatic TGC. 4. Resolution tests throughout field of view. With a moving tissue phantom, the presentation of structures can be observed as they move throughout the field of view. The resolving power of most ultrasonic imagers varies with position in the field of view. 5. Assessment of speckle pattern motion effects. Speckle pattern motion effects can either be detrimental or beneficial to image quality. The speckle pattern in ultrasonic images of organs is largely an interference pattern resulting from the summation of echo signals from scattering centres in the parenchyma and is not simply related to tissue structure. Motion of the organ is therefore not related in a simple way to motion of the speckle pattern. The moving tissue phantom has been used to compare the motion of the echo pattern and the motion of the scattering cenThe diversity of these motions is often quite large and depends on the type of tres. Observation of the speckle scanner and its beam characteristics (Morrison et al 1983). pattern motion is employed to see if a scanner is particularly susceptible to speckle pattern motion artifacts. Since the speckle pattern changes rapidly with motion of the scattering centres, some signal averaging occurs when the eye observes the image on a display screen. This averaging can improve the contrast resolution in the image. 6. Assessment of signal processing. Many real-time scanners now have electronic smoothing and signal averaging techniques This processing may take place over several image in their signal processing circuitry. It is therefore most effectively evaluated using a moving test phantom. frames. 7. Doppler tests. The moving test phantom has been found to be of value in setting up and testing A moving target consisting of a very large number of scattering pulsed Doppler units. A drum containing the scattering centres is a reasonable approximation to moving blood. (FPg. 4). The registration medium with a fluid-filled central core is used for the tests of the Doppler sample volume is checked by noting the Doppler signal as the position of The signal the sample volume is moved throughout the real-time image of the phantom. This test was level should drop quickly as the sample volume moves into the liquid core. also used to confirm that side-lobes were not contributing significantly to the output Reflectivity of muscle is much higher than that from a Doppler unit used in cardiology. of blood so muscle movements may contribute to the total output signal if they are intercepted by weak side-lobe beams. The Doppler output frequency can be calibrated by relating it to the velocity of the scattering material at different radii in the drum. One of the more difficult aspects of Doppler instruments to set up is the direction This circuitry can be set up with the moving phantom by noting the sensing circuitry. output signal when the sample volume is located in regions of forward or reverse motion. 8. Tests during research and development. Automatic TGC The moving phantom has been useful in research and development work. systems are being studied which often derive their compensating control signals from echoes Synthetic aperThey have therefore a finite response time. gathered from 4 or 5 frames. ture transducers for electronic focusing gather echo data over several transmit and receive They are therefore slower and more susceptible cycles to generate one line in the image. In both these areas of research and development, a moving to motion than standard arrays. phantom provides a more realistic test than a static one. Conclusion Several of the tests and assessments described are qualitative. To make them more quantitative it will be necessary to construct the phantom from materials of known attenuating, reflecting and scattering properties (McCarty and Stewart 1982). However,
Letters
to the Editor
L459
/
‘
Fig. 4. A phantom structure employed for testing Doppler instruments. The position of the sample volume of a pulsed Doppler unit is shown by the line in the image of the central cavity.
even in its present form the moving tissue-equivalent phantom has proved to be very usefu As equipment becomes more complex, it is more in several areas of medical ultrasonics. necessary to include motion in test pieces which seek to simulate tissue.
Yours
etc.,
W.N. McDicken, D.C. Morrison, D.S.A. Smith, Dept. of Medical Physics The Royal Infirmary, Edinburgh, Scotland.
and Medical
Engineering,
References. A simple tissue-like ultrasound Lerski, R.A., Duggan, T.C. and Christie, J. (1982). Br. J. Radiol. 55:156-157. phantom material. Tissue mimicking Madsen, E.L., Zagzebski, J.A., Banjavic, R.A. and Jutila, R.E. (1978). Med. Phys. 5:391-394. materials for ultrasound phantoms. A simple calibration and evaluation phantom for McCarty, K. and Stewart, M. (1982). Ultrasound Med. Biol. 8:393-401. ultrasound scanners. Motion artifact in real-time Morrison, D.C., McDicken, W.N. and Smith, D.S.A. (1983). Ultrasound Med. Biol. (In Press). ultrasound images.
Letters to the Editor
DISCUSSION Although there are no clearly defined hazards associated with pulse-echo ultrasound as currently used in diagnostic medicine, there is now a gradual increase in interest in exposure parameters for clinical equipment due in part to experimental observations of effects of low-temporal-average-intensity, pulsed ultrasound like those described above. To encourage this trend, the American Institute of Ultrasound in Medicine awards commendations to manufactureres who provide specific information about the acoustical quantities which characterize their equipment. At present, the quantity which is recommended by AIUM/NEMA to characterize temporal peak intensity is the spatial peak, pulse average The performance standard for ultrasound therapy products administered by the intensity. National Center for Devices, and Radiological Health (NCDRH) requires that I, as well as the The NCDRH guide to be used by manufacturers of total power be indicated by the equipment. medical diagnostic ultrasound equipment in the preparation of product reports to be filed with NCDRH asks for I, as well as Ipa to characterize the peak intensity. The example chosen for these tests admittedly is contrived to distinguish clearly beHowever, the difference in the usefulness tween the pulse average and maximum intensities. In addition to of the two quantities as predictors of this biological effect is striking. its apparent biological relevance, the NCRP's maximum intensity has an advantage of simplicity of definition. With the advent of PVDF hydrophones which have uniform response over a wide range of frequencies and reasonably long term stability in calibration, it should be possible for measurements of maximum intensity to be made easily not only in the laboratory and factory but also in the clinical setting. ACKNOWLEDGEMENTS. The authors are indebted to Dr. Harold Stewart of the National Radiological Health and Professor Paul Carson, University of Michigan helpful discussions and information. This study was supported in part by U.S.P.S. Grant GMD9933.
Yours
Center for Devices and Medical Center for
etc.,
E.L. Carstensen, R.B. Berg, S.Z. Child. Dept. of Electrical Engineering, University of Rochester, Rochester, N.Y. References American Institute of Ultrasound in Medicine/National Electrical Manufacturers Association Safety standard for diagnostic ultrasound equipment. J. Ultrasound (AIUM/NEMA) (1983). Med. 2:Sl-S50. Ultrasonic power and intensities Carson, P.L., Fischella, R. and Oughton,'T..V. (1978). produced by diagnostic ultrasound equipment. Ultrasound Med. Biol. 3:341-350. Carson, P.L. (1983). In a personal communication, Prof. Carson reports that with the pulse shapes employed in pulse echo and pulse Doppler ultrasound the ratios of Im to I pa range from 1 to values as great as 6. Carstensen, E.L., Parker, K.J. and Barbee, D.B. (1983). Temporal peak intensity. J. Acoust. Sot. Am. (In press). Child, S.Z., Carstensen, E.L. and Lam, S.K. (1981). Effects of ultrasound on Drosophila: III. Exposure of larvae to low-temporal-average-intensity, pulsed irradiation. Ultrasound Med. Biol. 7:167-173. Biological effects of ultrasound: Mechanisms and clinical applications. N.C.R.P. (1983). Scientific Committee No. 66, National Council for Radiation Protection and Measurements. Bethesda, Maryland.
A Moving
Tissue-Equivalent
Phantom
For Ultrasonic
Real-Time
Scanning
And Doppler
Techniques.
Sir: Introduction Test phantoms which mimic the ultrasonic properties of tissue reasonably accurately have been described in the literature (Madsen et al, 1978; McCarty and Stewart, 1982). These phantoms are of value for comparing machines and for checking their constancy of performance. As tissue-equivalent phantoms improve, their value for predicting the performance of machines in clinical imaging also increases. However,the phantoms described in the literature have all
L456
Letters to the Editor
been static ones containing no moving parts. Since in practice virtually no tissues examined by ultrasound are static this is a significant limitation of present phantoms. It is worth bearing in mind that the echo signals resulting from scattering at closely spaced targets are altered by small motions of the targets. For example, with 5 MHz ultrasound the wavelength in tissue is about 0.3 mm. A change in the component of the separation of the targets along the ultrasound beam of as little as a tenth of a wave-length, i.e. 0.03 mm, could be expected to alter significantly the signal level from the targets. Tissue motions are also of importance since echoes from structures move in a similar way in an image whereas noise signals change in a random fashion. The eye is also particularly sensitive to moving edges. These factors contribute to the clarity of real-time images. A moving tissue phantom has been constructed for the assessment of ultrasonic scanners, The device has also been used to acquire an increased understanding of the factors which contribute to image quality. Moving tissue-equivalent phantom. To produce realistic images, the phantom was made from a tissue-equivalent material comprising a mixture of gelatin, water, propanol and graphite powder (Madsen et al 1978). Other synthetic material can also be used which is often easier to work with, e.g. Bulpren reticulated foam (Lerski et al 1982). The material is contained in a cylindrical drum whose wall is a thin polycarbonate sheet of thickness 0.25 mm. A few drums constructed with 1 mm thick plastic walls had large refraction artifacts at the sides of their images (Fig. l), The drum is interchangeable to aid development of test phantoms which simulate different Several have been constructed containing simulated cysts and tumours (Figs. applications. 1 and 2). Arrays of nylon fibres or holes have been included for resolution and registration The drum is rotated or oscillated about its axis by a variable-speed electric tests. motor coupled to the drum by reduction gearing and a magnetic drive. The container, surrounding the drum, is filled with oil to permit easy use with linear or sector scanners (Fig. 3).
The echoes Fig. 1. Frozen cross-sectional image of the tissue phantom. from the wire arrays for resolution tests are located centrally and are Three disc shaped structures of increased saturated in this image. One fluid filled cyst-like structure echogenicity represent tumours. The image of the cross section through the drum does not is also shown. appear circular due to a refraction artifact in the 1 mm thick plastic wall of the drum.
Letters
L457
to the Editor
Fj Lg. 2.
Cystic structures in the phantom material. These structures :e used to determine the resolving power of a scanner and how it varies as the structures move throughout the field of view.
al
Fig. 3. Use of a test-rig sector scanner.
with a rotating
transducer
real-time
L458
Letters
to the Editor
Uses of Moving Phantoms. The motion feature of the test phantom has been found to be relevant for a number of reasons. 1. Study of the speed of scan effects. When a moving structure is imaged with a frame rate which is too low, the structure appears to jump between positions rather than move in a smooth fashion. This is usually only a problem with fast moving structures if the true frame rate of the machine is relatively low e.g. less than 20 frames/set. Moving phantoms can be used to check the presentation of structures over a range of speeds. The unit can also be used to check on distortion in the presentation of heart valves due to the finite time taken for the ultrasound beam to sweep across the field of view. 2. Test of M-mode recording. The quality and accuracy of M-mode facilities can be checked with a moving tissue phantom. 3. Assessment of lag . In old systems lag may be due to the response of a TV camera or display screen, in new ones it is most likely due to signal processing techniques such as signal averaging or automatic TGC. 4. Resolution tests throughout field of view. With a moving tissue phantom, the presentation of structures can be observed as they move throughout the field of view. The resolving power of most ultrasonic imagers varies with position in the field of view. 5. Assessment of speckle pattern motion effects. Speckle pattern motion effects can either be detrimental or beneficial to image quality. The speckle pattern in ultrasonic images of organs is largely an interference pattern resulting from the summation of echo signals from scattering centres in the parenchyma and is not simply related to tissue structure. Motion of the organ is therefore not related in a simple way to motion of the speckle pattern. The moving tissue phantom has been used to compare the motion of the echo pattern and the motion of the scattering cenThe diversity of these motions is often quite large and depends on the type of tres. Observation of the speckle scanner and its beam characteristics (Morrison et al 1983). pattern motion is employed to see if a scanner is particularly susceptible to speckle pattern motion artifacts. Since the speckle pattern changes rapidly with motion of the scattering centres, some signal averaging occurs when the eye observes the image on a display screen. This averaging can improve the contrast resolution in the image. 6. Assessment of signal processing. Many real-time scanners now have electronic smoothing and signal averaging techniques This processing may take place over several image in their signal processing circuitry. It is therefore most effectively evaluated using a moving test phantom. frames. 7. Doppler tests. The moving test phantom has been found to be of value in setting up and testing A moving target consisting of a very large number of scattering pulsed Doppler units. A drum containing the scattering centres is a reasonable approximation to moving blood. (FPg. 4). The registration medium with a fluid-filled central core is used for the tests of the Doppler sample volume is checked by noting the Doppler signal as the position of The signal the sample volume is moved throughout the real-time image of the phantom. This test was level should drop quickly as the sample volume moves into the liquid core. also used to confirm that side-lobes were not contributing significantly to the output Reflectivity of muscle is much higher than that from a Doppler unit used in cardiology. of blood so muscle movements may contribute to the total output signal if they are intercepted by weak side-lobe beams. The Doppler output frequency can be calibrated by relating it to the velocity of the scattering material at different radii in the drum. One of the more difficult aspects of Doppler instruments to set up is the direction This circuitry can be set up with the moving phantom by noting the sensing circuitry. output signal when the sample volume is located in regions of forward or reverse motion. 8. Tests during research and development. Automatic TGC The moving phantom has been useful in research and development work. systems are being studied which often derive their compensating control signals from echoes Synthetic aperThey have therefore a finite response time. gathered from 4 or 5 frames. ture transducers for electronic focusing gather echo data over several transmit and receive They are therefore slower and more susceptible cycles to generate one line in the image. In both these areas of research and development, a moving to motion than standard arrays. phantom provides a more realistic test than a static one. Conclusion Several of the tests and assessments described are qualitative. To make them more quantitative it will be necessary to construct the phantom from materials of known attenuating, reflecting and scattering properties (McCarty and Stewart 1982). However,
Letters
to the Editor
L459
/
‘
Fig. 4. A phantom structure employed for testing Doppler instruments. The position of the sample volume of a pulsed Doppler unit is shown by the line in the image of the central cavity.
even in its present form the moving tissue-equivalent phantom has proved to be very usefu As equipment becomes more complex, it is more in several areas of medical ultrasonics. necessary to include motion in test pieces which seek to simulate tissue.
Yours
etc.,
W.N. McDicken, D.C. Morrison, D.S.A. Smith, Dept. of Medical Physics The Royal Infirmary, Edinburgh, Scotland.
and Medical
Engineering,
References. A simple tissue-like ultrasound Lerski, R.A., Duggan, T.C. and Christie, J. (1982). Br. J. Radiol. 55:156-157. phantom material. Tissue mimicking Madsen, E.L., Zagzebski, J.A., Banjavic, R.A. and Jutila, R.E. (1978). Med. Phys. 5:391-394. materials for ultrasound phantoms. A simple calibration and evaluation phantom for McCarty, K. and Stewart, M. (1982). Ultrasound Med. Biol. 8:393-401. ultrasound scanners. Motion artifact in real-time Morrison, D.C., McDicken, W.N. and Smith, D.S.A. (1983). Ultrasound Med. Biol. (In Press). ultrasound images.