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Assessing ultrasonic arrays for imaging tissue properties
ABSTRACTS, ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION SYMPOSIUM
ASSESSING ULTRASONIC ARRAYS FOR IMAGING TISSUE PROPERTIES, R. Martin Arthur, Michael L. Sieger and Donald W. Stein, Jr., Biomedical Computer Laboratory and Department of Electrical Engineering, Washington University, St. Louis, MO 63130. The field of view of an array can be extended beyond its lateral dimensions by phasing the elements of the array so that constructive interference occurs off the axis of the array. Clearly, the content of the resultant beam is dependent on the beam profiles of the individual elements in the array. Typically, beam profiles are determined by measuring the peak pressure received as a function of range and angle. Unfortunately, this approach does not describe the frequency content of the ultrasound delivered to a particular region in the field of view, when the excitation is a pulse. One measure of the usable field of an array for pulse-echo tissue characterization is the energy available within appropriate frequency bands at a given range and angle. We mapped the frequency response of individual elements of an array by rotating the array within its image-plane. The axis of rotation passed through the center of the element face. The rotation was performed by a stepper motor under computer control. Another motor fixed the range between the array element under study and a reflector or hydrophone. Rece.ived signals were sampled at rates up to 100 MHz with 8 bits of precision. The energy in various frequency bands was calculated to find the energy present at a given range and viewing angle. For example, the 2-7 MHz energy scattered to an element near the center of a 3.5 MHz, 32-element linear array from a 2.4 mm radius steel rod on axis at a range of 5 cm was ,89 percent of the The 2-7 MHz energy measured total. It was <13 of the total 30" off axis. with a 0.6 mm diameter Dapco ceramic probe was >79 percent of the total on axis at 5 cm. It was 13 percent of the total energy at 30". In both cases the 3 dB bandwidth was 2 MHz centered at about 1.5 MHz. This work was supported in part by the NIH under Grants ~1~01362 and RR01380 from the Division of Research Resources and by Washington University. COARSE FINE ARRAY AND MAXIMUM ENTROPY METHOD: A COMPARISON, Tat-Jin Teo, Biomedical Engineering and Science Institute, Drexel University, Philadelphia, PA 19104. The present work is a comparison of two spectral analysis techniques as applied to array processing. The first technique is a refinement of the conventional delay and sum beamformer except that multiplicative processing is employed rather than additive. The second technique is the maximum entropy method which has 'super-resolution' property. The first method employs a coarse sub-array and a fine sub-array together. Two beams are formed and utilized simultaneously as compared with a single beam processing in the conventional beamformer. The second method has been found to be a one step predictive error filter as well as an autoregressive modeling of the signal. These two methods are compared against each other. Their advantages and disadvantages over the conventional beamformer are discussed. Resolution in terms of beam width and performance under various signal-to-noise ratios are used in the comparison. 1-D simulation results will be presented and extension to 2-D image processing will be discussed. IMAGING THROUGH INHOMOGENEOUSMEDIA WITH A LINEAR PHASED ARRAY: RESULTS AND IMPLEMENTATION, A. Gamboa-Aldeco' A. Macovskil and G. Sommer', Departments of Electrical Engineering' and Radiology2, Stanford University, Stanford, CA 94305. Time-of-flight with a Linear Phased Array (LPA) assumes a constant and known speed of sound throughout the imaged field to do the proper beam
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ASSESSING ULTRASONIC ARRAYS FOR IMAGING TISSUE PROPERTIES, R. Martin Arthur, Michael L. Sieger and Donald W. Stein, Jr., Biomedical Computer Laboratory and Department of Electrical Engineering, Washington University, St. Louis, MO 63130. The field of view of an array can be extended beyond its lateral dimensions by phasing the elements of the array so that constructive interference occurs off the axis of the array. Clearly, the content of the resultant beam is dependent on the beam profiles of the individual elements in the array. Typically, beam profiles are determined by measuring the peak pressure received as a function of range and angle. Unfortunately, this approach does not describe the frequency content of the ultrasound delivered to a particular region in the field of view, when the excitation is a pulse. One measure of the usable field of an array for pulse-echo tissue characterization is the energy available within appropriate frequency bands at a given range and angle. We mapped the frequency response of individual elements of an array by rotating the array within its image-plane. The axis of rotation passed through the center of the element face. The rotation was performed by a stepper motor under computer control. Another motor fixed the range between the array element under study and a reflector or hydrophone. Rece.ived signals were sampled at rates up to 100 MHz with 8 bits of precision. The energy in various frequency bands was calculated to find the energy present at a given range and viewing angle. For example, the 2-7 MHz energy scattered to an element near the center of a 3.5 MHz, 32-element linear array from a 2.4 mm radius steel rod on axis at a range of 5 cm was ,89 percent of the The 2-7 MHz energy measured total. It was <13 of the total 30" off axis. with a 0.6 mm diameter Dapco ceramic probe was >79 percent of the total on axis at 5 cm. It was 13 percent of the total energy at 30". In both cases the 3 dB bandwidth was 2 MHz centered at about 1.5 MHz. This work was supported in part by the NIH under Grants ~1~01362 and RR01380 from the Division of Research Resources and by Washington University. COARSE FINE ARRAY AND MAXIMUM ENTROPY METHOD: A COMPARISON, Tat-Jin Teo, Biomedical Engineering and Science Institute, Drexel University, Philadelphia, PA 19104. The present work is a comparison of two spectral analysis techniques as applied to array processing. The first technique is a refinement of the conventional delay and sum beamformer except that multiplicative processing is employed rather than additive. The second technique is the maximum entropy method which has 'super-resolution' property. The first method employs a coarse sub-array and a fine sub-array together. Two beams are formed and utilized simultaneously as compared with a single beam processing in the conventional beamformer. The second method has been found to be a one step predictive error filter as well as an autoregressive modeling of the signal. These two methods are compared against each other. Their advantages and disadvantages over the conventional beamformer are discussed. Resolution in terms of beam width and performance under various signal-to-noise ratios are used in the comparison. 1-D simulation results will be presented and extension to 2-D image processing will be discussed. IMAGING THROUGH INHOMOGENEOUSMEDIA WITH A LINEAR PHASED ARRAY: RESULTS AND IMPLEMENTATION, A. Gamboa-Aldeco' A. Macovskil and G. Sommer', Departments of Electrical Engineering' and Radiology2, Stanford University, Stanford, CA 94305. Time-of-flight with a Linear Phased Array (LPA) assumes a constant and known speed of sound throughout the imaged field to do the proper beam
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