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ULTRASONIC ASSESSMENT OF TISSUE ANISOTROPY

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ULTRASONIC ASSESSMENT OF TISSUE ANISOTROPY WILLIAM D. O'BRLEN JR.' and JOHN E. OLERUD' *Department of Electrical and Computer Engineering, University of Illinois, 1406 West Green Street, Urbana, IL 61801 USA and Dlvlslon of Dermatology, Department of Medicine, University of q . .. Washington, Seattle, WA 98195 USA ABSTRACT: Two completely separate ex- periments using the Scanning Laser Acoustic Microscope, operating at an ultrasonic fre- quency of la0 MHz, assessed the attenuation coefficient and propagation speed of two ani- Achilles tendon samples and bovine longis- sotropic biological materials, viz., human simus striated muscle samples. For the tendon samples, three separate locations were also evaluated (superior, medial and inferior). Ani- sotropy was evaluated by measuring the sam- ples with the ulaasonic beam propagating ei- ther parallel or perpendicular to the tendon muscle fibers. Achilles tendons generally ex- hibited a greater anisompy than did skeletal muscle, and for both materials, the parallel ori- entation of both attenuation coefficient and propagation speed were consistently greater that those of the perpendicular orientation. I. INTRODUCTION Ultrasonic propagation properties in biological tissues are not well understood [l] but are thought to be determined at the level of large molecules [2]. In muscle and connective tis- the oriented structures contribute to sound sues, for example, it is not clear to what degree propagation. These considerations are of in- terest in the clinical use of ultrasound since the composition and configuration of tissues changes during disease states (e.g., cirrhosis of the liver, certain muscular pathologies and degenerative joint diseases) as well as in nor- mal physiologic processes (e.g., wound heal- ing and fetal development). Anisotropy has been reported in a number of oriented biological tissues in the low megahertz frequency range [3-91. No work has been published characterizing muscle or tendon fiber anisotropy at higher frequencies. Therefore, in bovine skeletal muscle and human Achilles this study investigates the acoustic anisotropy tendon at high frequency (100 MHz) using the Scanning Laser Acoustic Microscope. 0-7803-2940-6/95/$4.00 0 1995 IEEE 11. METHODS Thirty-six longissimus striated muscle speci- mens were obtained from eighteen individual beef carcasses 24 to 48 hours postmortem which had been placed in a 4'C refigerator proximately 8 x 3 x 3 mm were placed on within 2 hours postmortem. Specimens ap- either parallel or perpendicular to the cork sur- separate circular cork disks with muscle fibers face and covered with O.C.T. All 36 corks were wrapped and labeled with specimen number and muscle fiber orientation, and stored in a -70°C freezer. Twenty-eight human Achilles tendon speci- mens were obtained from three locations of the lotendonous junction (superior), eight speci- same tendon, 12 specimens near the muscu- mens midway down the tendon (medial) and eight specimens from near the calcaneal at- don was removed from a surgical specimen at tachment (inferior). The human Achilles ten- the time of amputation and frozen at -70°C un- til thawed for removing samples. Specimens approximately 8 x 3 x 3 mm were placed on separate circular cork disks with collagen fi- face and covered with O.C.T. All 28 corks ben parallel or perpendicular to the cork sur- were wrapped and labeled with specimen number and collagen fiber orientation, and dry ice to UIUC. stored in a -70°C freezer after being shipped on At the time of ultrasonic analysis, the cork to which the frozen specimen was attached was mounted in a microtome chuck for sectioning by a Lipshaw Cryostat Microtome (Lipshaw Manufacturing Company, Detroit, MI) at -10°C. The frozen section tissue thicknesses ranged from 70 to 140 pm for muscle and from 25 to 150 pm for tendon. The sectioned sample was trimmed into a rectangular piece of about 5 x 1.5 mm and then transferred to the acoustic microscope stage for measurement of the ultrasonic propagation properties. The 1995 IEEE ULTRASONICS SYMPOSIUM - 1145evaluations were conducted at mm tempera- ture (=22 .C). The two types of sample orientation are de- fined according to the relationship between muscle or tendon fiber orientation and the the sound beam traveling parallel to the fibers sound beam direction wherein parallel refers to traveling perpendicular to the fibers. and perpendicular refers m the sound beam The Scanning Laser Acoustic Microscope (SLAM, Sonomicroscope. 100, Sonoscan Inc., Bensenville, E), operating at a frequency of men’s attenuation coefficient and propagation 100 MHz, was used to detennine the speci- speed. The operational details of the SLAM have been published previously [lo-121. An insertion loss procedure was used to esti- mate the attenuation coefficient [IO]. In prin- ciple, this procedure compares the received signal amplitude of the specimen of known erence medium, nofinal saline. The signals thickness in the sound path with that of the ref- received from the subimage area (400 x 250 value (V). Six V values were recorded for pm) are digitized to yield an average amplitude normal saline, the reference medium, and six V values were recorded at each of six separate value, in dB, was estimated using specimen locations. An insertion loss (L) IL = v, -(vr) where (Vr) is the average of V recorded from normal saline and V, represents individual V values from the specimen. This process yields six IL values for each specimen thickness. Typically, three or four specimen thicknesses ranging from 70 pm to 140 pm for muscle and from 25 pm to 150 pm for tendon were used, via a linear least-squares fit (utilizing all IL and the slope of IL versus specimen thickness, ficient. thickness values) yielded the attenuation coef- The spatial frequency domain technique was used to yield speed from the interference image men thicknesses were analyzed, with each [l l]. For each specimen, three or four speci- thickness yielding a mean speed value from the speed profile region within the specimen. Anisotropy is defined as a percentage by .. xpdlel xperpendicular orientation of the


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