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Diffraction correction: The lateral point of view
ABSTRACTS, UL,TRASONIC IMAGING AND TISSUE CHARACTERIZATION SYMPOSIUM
THE ROTATING RANDOM PHASE PROBE: SIMULATIONS AND EXPERIMENTS, P. Rua,’ M. Fink,lZ R. Mallar? and D. Beudon, ’ ‘Laboratoires d’Electronique et de Physique appliquee, 3, avenue Descartes, 94451 Limeil-Brevamres Cedex and ‘University of Paris VII, Groupe de Physique des Solides, 2, place Jussieu, 75251 Paris Cedex 05, France. Echographic images from soft tissues suffer from interference noise (speckle) due to the coherent nature of this imaging modality. At the last Symposium, we showed how coherence can be reduced with a random phase screen. A translative screen displacement involves large probes. In order to reduce the probe size, we have developed a rotating random phase probe. In this paper, we will present simulations of the efficiency of the probe as well as experimental results. Different configurations will be discussed and the influence of random phase statistics will be studied. Signal-to-noise ratio improvements of more than 5 have been achieved with these probes. Images will be presented. SPECKLE REDUCTION WITH THE RANDOM PHASE SCREEN: EXPERIMENTAL RESULTS, P. Laugier,’ M. Fink: S. Aboueklaran? and G. Berger,’ ‘Laboratory of Biophysics, UA CNRS 593, CHU Co&in, 24, rue du faubourg St. Jacques, 75674 Paris Cedex 14 and ‘GPS, University Paris VII, 2 Place Jussieu, 75005 Paris, France. An incoherent processing approach for speckle reduction has been proposed by M. Fink [I]: the spatial coherence of the transducer is controlled by moving a random phase screen (RI-S) across the ultrasonic beam. The RPS is a rough surface generating phase shifts that are randomly distributed between 0 and 2~. For a better understanding of the ability of the RPS to obtain uncorrelated speckle patterns, we designed two different RPS. Statistical properties of the rough surface are controlled by the standard deviation of the surface fluctuations o, and the We measured these parameters and RPS coherence length of the surface roughness. transmittance-autocorrelation functions were computed. We present here experimental results on the directivity pattern. Measurements (RPS moved by translation) were performed with unfocused and focused transducers. This data is used to give an interpretation of the RPS in terms of superposition of sine phase gratings. According to theoretical predictions, uncorrelated speckle patterns are obtained only if the transmittance-autocorrelation function vanishes when the RPS shift increases. This can be achieved with high ratio aX1. The influence of ah1 was experimentally pointed out by echo-autocorrelation measurements at different frequencies and with two different RPS. We discuss the performance of the RPS in SNR improvement and the tradeoff between degradation of the resolution and SNR improvement. Experimental results are With a focused transducer, SNR improvement in good agreement with theoretical predictions. reaches 1.94 when there is no loss of resolution in transmit mode. Better improvement can be reached only at the price of a loss of resolution. Under these conditions, SNR improvement is similar with focused or unfocused transducers. We show that an RPS used together with unfocused transducers yieIds significant SNR improvement. For a given RPS, the SNR increases with the center frequency of the transducer, the size of the illuminated scattering surface and the distance between the RPS and the transducer. We report here the detection of low contrast specular reflectors with RPS. [l] Fink, M. and Beudon, D., Ultrasonic Imaging 9, p. 70, 1987 (abstract). DIFFRACTION CORRECTION: THE LATERAL POINT OF VIEW, J.M. Thijssen, B.J. Oosterveld and R. Fabel, Biophysics Laboratory of the Institute of Ophthalmology, University Hospital, 6500 HB Nijmegen, The Netherlands. The effects of beam diffraction and focusing on speckle in B-mode echograms have been investigated [l]. In addition, attenuation in intervening tissue layers and within the tissue itself has to be considered. Until recently, the diffraction correction was applied to the rf signals prior to the estimation of the attenuation coefficient [2, 31 and to the calculation of the speckle characteristics in the axial direction [4]. By this means, the B-mode image is considered as an ensemble of A-mode lines. Since it can be shown that the lateral speckle dimensions depend on the volume density of the scattering sites to a larger extent than the axial dimension [ 11, it may be worthwhile to perform a lateral diffraction correction as well. The intrinsic problems of this procedure are related to the application of deconvolution methods and to the limited density and available number of scan lines in the B-mode image. The results of a study on the feasibility of lateral correction, as well as experimental results, will be presented. (1985).
111 Oosterveld, B.J., Thijssen, J.M. and Verhoef, W.A., Ultrasonic
129
Imaging 1, 142-160
AELSTRACE,
ULTRASONIC
IMAGING
AND
TISSUE
CHARACIERlZATION
SYh4POSIUM
[2] Cloostermans, M.J.T.M. and Thijssen, J.M., UltrasonicImagine: 5, 136-147 (1983). [3] Verhoef, W.A., Cloostennans, M.J.T.M. and Thijssen, J.M., IEEE Trans. Biomed l&&g. BME-32, 521-529 (1985). [4] Oosterveld, B.J., Thijssen, J.M., Hattman, PC. and Rosenbusch, G., in Ultrasonic bgand Vol. 7, J.M. Thijssen, ed., pp. 43-54, (Office for Official Publ. EC., Luxembourg, 1988). ULTRASOUND IMAGE SEGMENTATION THROUGH MORPHOLOGICAL FILTERING TECHNIQUES, R.H. Sperry and K.J. Parker, Department of Electrical Engineering, University of Rochester, Rochester, NY 14627. Most tissue characterization techniques require that a region of interest be chosen in which it is assumed that the speckle pattern represents a single, homogeneous tissue type. Selecting such regions visually is hazardous due to the high signal-to-noise ratio of speckle and the relative insensitivity of the visual system to second order statistical properties. The inclusions of small specular reflectors, such as hepatic arteries and portal vein branches, may adversely affect the estimation of the parameters of the speckle process, and hence, a robust technique is needed to partition B-scan images. We present a method for segmenting entire abdominal B-scan images based on the first order statistics of the envelope process, and of the level crossing process derived from the thresholded speckle patterns. The latter properties are related to the second order statistics of speckle. Our method has proven effective in differentiating between specular reflectors, blood-filled voids, and regions of differing inherent reflectivity. Initially, two regions are differentiated on the basis of thresholding at a level determined by a maximum likelihood decision criterion. This gives rise to a new process, filtering operations are then applied referred to as the level-crossing process. Morphological based on the expected size and shape of excursions above and below me threshold level. Results on phantoms and clinical scans show that segmentation and lesion detection can be successfully applied, with improvements over performance of conventional algorithms, which make use of only first order statistics of the envelope process.
BACKSCATTERING COMBINING NARROWBAND IMAGES TO YIELD OPTIMAL ESTIMATES OF ACOUSTIC SPECTRAL PARAMETERS, Daniel Rachlin and F. Graham Sommer, Department of Diagnostic Radiology, Stanford University Medical Center, Stanford, CA 94305. Previous efforts in utilizing narrowband images to detect differences in frequency dependence of backscatter generally have utilized one narrowband filter to create a single image. Under such circumstances, the information available in the entire signal and width is not fully exploited. In addition, ambiguity is introduced by the fact that the intensity in such images is due to a combination of both frequency and nonfrequency dependent backscatter variability. We address these problems by using a parametric model to describe spectral differences among adjacent structures in a region of interest, solving a maximum likelihood equation for the spectral parameter, and implementing the solution as a combination of narrowband and wideband image intensities. The result is an image whose intensity at each point is proportional Simulations and images of phantom targets to a local estimate of the spectral parameter. demonstrate the effectiveness of the procedure. Dr. Sommer was partially supported by This work funded by NIH grant CA38109. NIH RCOA award CA00960. INTERPRETATION OF SCATTERER SIZE ESTIMATES IN RANDOM MEDIA WITH BROAD PARTICLE SIZE DISTRIBUTIONS, Michael F. Insana and Timothy J. Hall, Department of Diagnostic Radiology, University of Kansas Medical Center, Kansas City, KS 66103. Several investigators have shown that it is possible to directly image structural properties of scattering media. For example, the average size of scattering particles may be determined directly from the frequency dependence of backscatter when the medium contains a narrow distribution of particle sizes. In media with a broad size distribution, interpretation of the results is less straightforward. Where incoherent scattering can be assumed, the measured size is a weighted average that depends of the interrogating frequencies as well as the distribution of sizes.
130
THE ROTATING RANDOM PHASE PROBE: SIMULATIONS AND EXPERIMENTS, P. Rua,’ M. Fink,lZ R. Mallar? and D. Beudon, ’ ‘Laboratoires d’Electronique et de Physique appliquee, 3, avenue Descartes, 94451 Limeil-Brevamres Cedex and ‘University of Paris VII, Groupe de Physique des Solides, 2, place Jussieu, 75251 Paris Cedex 05, France. Echographic images from soft tissues suffer from interference noise (speckle) due to the coherent nature of this imaging modality. At the last Symposium, we showed how coherence can be reduced with a random phase screen. A translative screen displacement involves large probes. In order to reduce the probe size, we have developed a rotating random phase probe. In this paper, we will present simulations of the efficiency of the probe as well as experimental results. Different configurations will be discussed and the influence of random phase statistics will be studied. Signal-to-noise ratio improvements of more than 5 have been achieved with these probes. Images will be presented. SPECKLE REDUCTION WITH THE RANDOM PHASE SCREEN: EXPERIMENTAL RESULTS, P. Laugier,’ M. Fink: S. Aboueklaran? and G. Berger,’ ‘Laboratory of Biophysics, UA CNRS 593, CHU Co&in, 24, rue du faubourg St. Jacques, 75674 Paris Cedex 14 and ‘GPS, University Paris VII, 2 Place Jussieu, 75005 Paris, France. An incoherent processing approach for speckle reduction has been proposed by M. Fink [I]: the spatial coherence of the transducer is controlled by moving a random phase screen (RI-S) across the ultrasonic beam. The RPS is a rough surface generating phase shifts that are randomly distributed between 0 and 2~. For a better understanding of the ability of the RPS to obtain uncorrelated speckle patterns, we designed two different RPS. Statistical properties of the rough surface are controlled by the standard deviation of the surface fluctuations o, and the We measured these parameters and RPS coherence length of the surface roughness. transmittance-autocorrelation functions were computed. We present here experimental results on the directivity pattern. Measurements (RPS moved by translation) were performed with unfocused and focused transducers. This data is used to give an interpretation of the RPS in terms of superposition of sine phase gratings. According to theoretical predictions, uncorrelated speckle patterns are obtained only if the transmittance-autocorrelation function vanishes when the RPS shift increases. This can be achieved with high ratio aX1. The influence of ah1 was experimentally pointed out by echo-autocorrelation measurements at different frequencies and with two different RPS. We discuss the performance of the RPS in SNR improvement and the tradeoff between degradation of the resolution and SNR improvement. Experimental results are With a focused transducer, SNR improvement in good agreement with theoretical predictions. reaches 1.94 when there is no loss of resolution in transmit mode. Better improvement can be reached only at the price of a loss of resolution. Under these conditions, SNR improvement is similar with focused or unfocused transducers. We show that an RPS used together with unfocused transducers yieIds significant SNR improvement. For a given RPS, the SNR increases with the center frequency of the transducer, the size of the illuminated scattering surface and the distance between the RPS and the transducer. We report here the detection of low contrast specular reflectors with RPS. [l] Fink, M. and Beudon, D., Ultrasonic Imaging 9, p. 70, 1987 (abstract). DIFFRACTION CORRECTION: THE LATERAL POINT OF VIEW, J.M. Thijssen, B.J. Oosterveld and R. Fabel, Biophysics Laboratory of the Institute of Ophthalmology, University Hospital, 6500 HB Nijmegen, The Netherlands. The effects of beam diffraction and focusing on speckle in B-mode echograms have been investigated [l]. In addition, attenuation in intervening tissue layers and within the tissue itself has to be considered. Until recently, the diffraction correction was applied to the rf signals prior to the estimation of the attenuation coefficient [2, 31 and to the calculation of the speckle characteristics in the axial direction [4]. By this means, the B-mode image is considered as an ensemble of A-mode lines. Since it can be shown that the lateral speckle dimensions depend on the volume density of the scattering sites to a larger extent than the axial dimension [ 11, it may be worthwhile to perform a lateral diffraction correction as well. The intrinsic problems of this procedure are related to the application of deconvolution methods and to the limited density and available number of scan lines in the B-mode image. The results of a study on the feasibility of lateral correction, as well as experimental results, will be presented. (1985).
111 Oosterveld, B.J., Thijssen, J.M. and Verhoef, W.A., Ultrasonic
129
Imaging 1, 142-160
AELSTRACE,
ULTRASONIC
IMAGING
AND
TISSUE
CHARACIERlZATION
SYh4POSIUM
[2] Cloostermans, M.J.T.M. and Thijssen, J.M., UltrasonicImagine: 5, 136-147 (1983). [3] Verhoef, W.A., Cloostennans, M.J.T.M. and Thijssen, J.M., IEEE Trans. Biomed l&&g. BME-32, 521-529 (1985). [4] Oosterveld, B.J., Thijssen, J.M., Hattman, PC. and Rosenbusch, G., in Ultrasonic bgand Vol. 7, J.M. Thijssen, ed., pp. 43-54, (Office for Official Publ. EC., Luxembourg, 1988). ULTRASOUND IMAGE SEGMENTATION THROUGH MORPHOLOGICAL FILTERING TECHNIQUES, R.H. Sperry and K.J. Parker, Department of Electrical Engineering, University of Rochester, Rochester, NY 14627. Most tissue characterization techniques require that a region of interest be chosen in which it is assumed that the speckle pattern represents a single, homogeneous tissue type. Selecting such regions visually is hazardous due to the high signal-to-noise ratio of speckle and the relative insensitivity of the visual system to second order statistical properties. The inclusions of small specular reflectors, such as hepatic arteries and portal vein branches, may adversely affect the estimation of the parameters of the speckle process, and hence, a robust technique is needed to partition B-scan images. We present a method for segmenting entire abdominal B-scan images based on the first order statistics of the envelope process, and of the level crossing process derived from the thresholded speckle patterns. The latter properties are related to the second order statistics of speckle. Our method has proven effective in differentiating between specular reflectors, blood-filled voids, and regions of differing inherent reflectivity. Initially, two regions are differentiated on the basis of thresholding at a level determined by a maximum likelihood decision criterion. This gives rise to a new process, filtering operations are then applied referred to as the level-crossing process. Morphological based on the expected size and shape of excursions above and below me threshold level. Results on phantoms and clinical scans show that segmentation and lesion detection can be successfully applied, with improvements over performance of conventional algorithms, which make use of only first order statistics of the envelope process.
BACKSCATTERING COMBINING NARROWBAND IMAGES TO YIELD OPTIMAL ESTIMATES OF ACOUSTIC SPECTRAL PARAMETERS, Daniel Rachlin and F. Graham Sommer, Department of Diagnostic Radiology, Stanford University Medical Center, Stanford, CA 94305. Previous efforts in utilizing narrowband images to detect differences in frequency dependence of backscatter generally have utilized one narrowband filter to create a single image. Under such circumstances, the information available in the entire signal and width is not fully exploited. In addition, ambiguity is introduced by the fact that the intensity in such images is due to a combination of both frequency and nonfrequency dependent backscatter variability. We address these problems by using a parametric model to describe spectral differences among adjacent structures in a region of interest, solving a maximum likelihood equation for the spectral parameter, and implementing the solution as a combination of narrowband and wideband image intensities. The result is an image whose intensity at each point is proportional Simulations and images of phantom targets to a local estimate of the spectral parameter. demonstrate the effectiveness of the procedure. Dr. Sommer was partially supported by This work funded by NIH grant CA38109. NIH RCOA award CA00960. INTERPRETATION OF SCATTERER SIZE ESTIMATES IN RANDOM MEDIA WITH BROAD PARTICLE SIZE DISTRIBUTIONS, Michael F. Insana and Timothy J. Hall, Department of Diagnostic Radiology, University of Kansas Medical Center, Kansas City, KS 66103. Several investigators have shown that it is possible to directly image structural properties of scattering media. For example, the average size of scattering particles may be determined directly from the frequency dependence of backscatter when the medium contains a narrow distribution of particle sizes. In media with a broad size distribution, interpretation of the results is less straightforward. Where incoherent scattering can be assumed, the measured size is a weighted average that depends of the interrogating frequencies as well as the distribution of sizes.
130