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Ultrasonic computed tomography utilizing waves with wide frequency spectra
ABSTRACTS,
ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION
SYMPOSIUM
A CALIBRATION TECHNIQUE FOR ULTRASONIC SCATTERING MEASUREMENTS, S. K. Numrich, Laurence J. Frank, and Louis R. Dragonette, Naval Laboratory, Washington, DC 20375.
Research
Tungsten carbide sizing spheres can be used as calibration targets for ultrasonic measurements where the scattering body must be immersed in the rigid body returns over almost the entire frequency spectrum; however, the rigid body and elastic returns can be separated in a short pulse, broadband acoustic experiment. The rigid return, corrected for the target strength of a rigid sphere, can be used as a replica of the incident pulse for purposes of calibration. Alignment problems attendant upon the use of the traditional plate references are eliminated when a sphere is used as a calibration target. Furthermore, each individual sphere calibration can be checked for accuracy by taking the entire return from the sphere, including the elastic response, and normalizing by the rigid return. The result is the form function for the elastic sphere and can be compared with analytic computations. SESSION 4:
TRANSMISSION TOMOGRAPHY
CONSEQUENCES OF ANISOTROPY ON COMPUTED ULTRASONIC TOMOGRAPHY, J. R. Klepper, D. L. Snyder, and J. G. H. Brandenburger, Biomedical Computer Laboratory and Department of Physics, University, St. Louis, MO 63130.
G. Miller, Washington
We investigated the consequences of anisotropy in the ultrasonic attenuation on quantitative imaging. Using a phenomenological model for anisotropy, effects on the tomographic reconstructions of simple objects were demonstrated analytically. Numerical simulation was employed for more complicated object geometries. The predicted effects were compared with tomograms of tissue studied in vitro. Sources of apparent anisotropy in the attenuation in intrinsicallyisotropic media were also considered. The magnitude of these extrinsic sources of apparent anisotropy was demonstrated to be small compared with the magnitude of the intrinsic anisotropy exhibited by muscle. The attenuation reconstructed in areas adjacent to intrinsically anisotropic regions were found to exhibit either overestimation or underestimation, depending upon the magnitude and geometry of the anisotropic regions. Theoretical predictions of the consequences of anisotropy were in excellent agreement with experimentally obtained tomographic reconstructions of the slope of the ultrasonic attenuation coefficient as a function of frequency of excised dog hearts. The results of this study suggest that anisotropy in the attenuation of tissue has a significant impact on quantitative images and has the potential for substantially degrading quantitative ultrasonic imaging based upon multiple angular views. Anisotropy in ultrasonic backscatter should likewise result in degradation in compound B-scan images. Although anisotropy represents a challenge to the design of quantitative imaging systems, it may also offer the potential for novel methods of tissue characterization. This work was supported in part by NIH Grants RR00396 and HL17646. ULTRASONIC COMPUTED TOMOGRAPHY UTILIZING WAVES WITH WIDE FREQUENCY SPECTRA, M. Onoe, H. Yamada, and S. Inoue, Institute of Industrial Science, University of Tokyo, Roppongi, Minato, Tokyo, Japan. The remarkable success has stimulated the application ties, such as radio-isotope, waves, and ultrasonics. The true cross section from data section thus obtained yields
of
computed tomography (CT) in x-ray imaging of CT principles to other imaging modaliparticle, nuclear magnetic resonance, microCT principle allows the reconstruction of a projected from many directions. The cross not only a display of geometry and topograph-
197
ABSTRACTS,
ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION
SYMPOSIUM
ical features but also a quantitative mapping of a measured parameter, the absorption coefficient in x-ray imaging. The last feature is e.g., more important than the first in ultrasonic imaging for tissue characterization. Most previous ultrasonic CT methods yield a mapping at a certain frequency at one time and time-consuming scanning has to be repeated in order to obtain a mapping at another frequency. The frequency dependence of absorption also considerably deforms the output waveform. This makes the measurement of projected absorption rather arbitrary in conventional methods. In this paper, we describe a method which yields 20 attenuation distributions and its frequency dependence from only one series of measurements. The input waveform is a pulse with a wide frequency spectrum. The output waveform is digitized and then Fourier transformed. The projection of attenuation at any frequency can be obtained from the amplitude of the corresponding frequency spectrum component. Experiments at 2 and 5 MHz yielded good reconstructions of phantoms and excised organs of a bird. MULTIPATH CORRECTION FOR ULTRASONI& TOMOGRAPHY, C. R. Crawford and A. C. Kak, School of Electrical Engineering, Purdue University, Lafayette, IN 47907.
West
Tomograms obtained from integrated phase and integrated frequency derivatives of attenuation suffer degradations caused by phase cancellation at the receiving transducer. Within the framework of geometrical acoustics, one type of phase cancellation occurs when there exist multiple paths between the transmitting and receiving transducers. Computer simulations will be presented that show that the effect of multipath on attenuation tomograms is worse than that for time-of-flight tomograms. This result is consistent with the fact that ultrasonic time-of-flight tomograms have a resolution greater than that for attenuation images. New estimators and corrections for existing estimators will be presented that improve the quality of attenuation tomograms. These will be demonstrated on data generated with computer simulation and on tissue equivalent phantoms. SOME SIESMIC METHODS APPLIED TO ULTRASONIC TRANSMISSION TOMOGRAPHY, J. F. Greenleaf, P. J. Thomas, and B. Rajagopalan, Mayo Foundation, Rochester, MN 55901. Siesmology methods which attempt to decrease the effects of diffraction are the "common midpoint gather" and "migration." The common midpoint gather is the grouping of received signals together to synthesize an aperture centered on the line of wave propagation of interest and is no more than using several transducers in a linear array to form a beam. Migration is merely backward propagation of the received signals to the point of interest within the object. Application of these methods in the transmission tomography geometry requires some modification. We insonified a finger cot (filled with saline) with a plane wave produced with a PVF2 transducer and scanned the scattered signal on the far side of the finger cot with a small hydrophone simulating the data gathered by siesmology but Common midpoint gathers were backward propagated in the transmission mode. or migrated by computer to the center of the rotation of the object. The resulting ultrasonic waveform was virtually free of aberrations caused by diffraction of the wave as it traveled in the immersing fluid from the The resulting reconstructions from time-ofobject to the receiver locus. flight and attenuation values measured from the backward propagated wave were extremely accurate. An analog method of accomplishing the colon midpoint gather and the migration was implemented and successfully used to obtain reconstructions.
198
ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION
SYMPOSIUM
A CALIBRATION TECHNIQUE FOR ULTRASONIC SCATTERING MEASUREMENTS, S. K. Numrich, Laurence J. Frank, and Louis R. Dragonette, Naval Laboratory, Washington, DC 20375.
Research
Tungsten carbide sizing spheres can be used as calibration targets for ultrasonic measurements where the scattering body must be immersed in the rigid body returns over almost the entire frequency spectrum; however, the rigid body and elastic returns can be separated in a short pulse, broadband acoustic experiment. The rigid return, corrected for the target strength of a rigid sphere, can be used as a replica of the incident pulse for purposes of calibration. Alignment problems attendant upon the use of the traditional plate references are eliminated when a sphere is used as a calibration target. Furthermore, each individual sphere calibration can be checked for accuracy by taking the entire return from the sphere, including the elastic response, and normalizing by the rigid return. The result is the form function for the elastic sphere and can be compared with analytic computations. SESSION 4:
TRANSMISSION TOMOGRAPHY
CONSEQUENCES OF ANISOTROPY ON COMPUTED ULTRASONIC TOMOGRAPHY, J. R. Klepper, D. L. Snyder, and J. G. H. Brandenburger, Biomedical Computer Laboratory and Department of Physics, University, St. Louis, MO 63130.
G. Miller, Washington
We investigated the consequences of anisotropy in the ultrasonic attenuation on quantitative imaging. Using a phenomenological model for anisotropy, effects on the tomographic reconstructions of simple objects were demonstrated analytically. Numerical simulation was employed for more complicated object geometries. The predicted effects were compared with tomograms of tissue studied in vitro. Sources of apparent anisotropy in the attenuation in intrinsicallyisotropic media were also considered. The magnitude of these extrinsic sources of apparent anisotropy was demonstrated to be small compared with the magnitude of the intrinsic anisotropy exhibited by muscle. The attenuation reconstructed in areas adjacent to intrinsically anisotropic regions were found to exhibit either overestimation or underestimation, depending upon the magnitude and geometry of the anisotropic regions. Theoretical predictions of the consequences of anisotropy were in excellent agreement with experimentally obtained tomographic reconstructions of the slope of the ultrasonic attenuation coefficient as a function of frequency of excised dog hearts. The results of this study suggest that anisotropy in the attenuation of tissue has a significant impact on quantitative images and has the potential for substantially degrading quantitative ultrasonic imaging based upon multiple angular views. Anisotropy in ultrasonic backscatter should likewise result in degradation in compound B-scan images. Although anisotropy represents a challenge to the design of quantitative imaging systems, it may also offer the potential for novel methods of tissue characterization. This work was supported in part by NIH Grants RR00396 and HL17646. ULTRASONIC COMPUTED TOMOGRAPHY UTILIZING WAVES WITH WIDE FREQUENCY SPECTRA, M. Onoe, H. Yamada, and S. Inoue, Institute of Industrial Science, University of Tokyo, Roppongi, Minato, Tokyo, Japan. The remarkable success has stimulated the application ties, such as radio-isotope, waves, and ultrasonics. The true cross section from data section thus obtained yields
of
computed tomography (CT) in x-ray imaging of CT principles to other imaging modaliparticle, nuclear magnetic resonance, microCT principle allows the reconstruction of a projected from many directions. The cross not only a display of geometry and topograph-
197
ABSTRACTS,
ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION
SYMPOSIUM
ical features but also a quantitative mapping of a measured parameter, the absorption coefficient in x-ray imaging. The last feature is e.g., more important than the first in ultrasonic imaging for tissue characterization. Most previous ultrasonic CT methods yield a mapping at a certain frequency at one time and time-consuming scanning has to be repeated in order to obtain a mapping at another frequency. The frequency dependence of absorption also considerably deforms the output waveform. This makes the measurement of projected absorption rather arbitrary in conventional methods. In this paper, we describe a method which yields 20 attenuation distributions and its frequency dependence from only one series of measurements. The input waveform is a pulse with a wide frequency spectrum. The output waveform is digitized and then Fourier transformed. The projection of attenuation at any frequency can be obtained from the amplitude of the corresponding frequency spectrum component. Experiments at 2 and 5 MHz yielded good reconstructions of phantoms and excised organs of a bird. MULTIPATH CORRECTION FOR ULTRASONI& TOMOGRAPHY, C. R. Crawford and A. C. Kak, School of Electrical Engineering, Purdue University, Lafayette, IN 47907.
West
Tomograms obtained from integrated phase and integrated frequency derivatives of attenuation suffer degradations caused by phase cancellation at the receiving transducer. Within the framework of geometrical acoustics, one type of phase cancellation occurs when there exist multiple paths between the transmitting and receiving transducers. Computer simulations will be presented that show that the effect of multipath on attenuation tomograms is worse than that for time-of-flight tomograms. This result is consistent with the fact that ultrasonic time-of-flight tomograms have a resolution greater than that for attenuation images. New estimators and corrections for existing estimators will be presented that improve the quality of attenuation tomograms. These will be demonstrated on data generated with computer simulation and on tissue equivalent phantoms. SOME SIESMIC METHODS APPLIED TO ULTRASONIC TRANSMISSION TOMOGRAPHY, J. F. Greenleaf, P. J. Thomas, and B. Rajagopalan, Mayo Foundation, Rochester, MN 55901. Siesmology methods which attempt to decrease the effects of diffraction are the "common midpoint gather" and "migration." The common midpoint gather is the grouping of received signals together to synthesize an aperture centered on the line of wave propagation of interest and is no more than using several transducers in a linear array to form a beam. Migration is merely backward propagation of the received signals to the point of interest within the object. Application of these methods in the transmission tomography geometry requires some modification. We insonified a finger cot (filled with saline) with a plane wave produced with a PVF2 transducer and scanned the scattered signal on the far side of the finger cot with a small hydrophone simulating the data gathered by siesmology but Common midpoint gathers were backward propagated in the transmission mode. or migrated by computer to the center of the rotation of the object. The resulting ultrasonic waveform was virtually free of aberrations caused by diffraction of the wave as it traveled in the immersing fluid from the The resulting reconstructions from time-ofobject to the receiver locus. flight and attenuation values measured from the backward propagated wave were extremely accurate. An analog method of accomplishing the colon midpoint gather and the migration was implemented and successfully used to obtain reconstructions.
198