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Liver backscatter: A fractal model
ABSTRACTS, ULTRASONIC
IMAGING AND TISSUE CHARACTERIZATION
SYMPOSIUM
plastic standoff with ultrasonic transducer (10 cm focal length, broadband, 5 MHz center frequency) was applied to the anterior face of the upper arm over the belly of the biceps, perpendicular to the surface of the skin. The backscatter spectrum from a 3.75 mm portion of muscle about 1.5 cm deep from the skin was recorded at different sites. (Freq. range: 2-8 MHz in 1 MHz steps). The spectra were reduced to decibels relative to a planar, stainless steel reflector and the frequency average, or integrated, backscatter was calculated. Average integrated backscatter was obtained by averaging at each site, then all sites within a subject, then over all subjects. The value obtained for normal volunteers was -56.7 + 1.7 dB (mean &- sem, n = 5), while the level obtained in patients with DMD was -40.0 + 2.8 dB (mean f sem, n = 5). Integrated backscatter from afflicted patients was significantly higher than that of normal volunteers, with an average difference of 16.7 + 3.28 dB (mean + sd, p < 0.01). The ultrasonic method reported in this work may prove to be very useful as a new application of a noninvasive technique in the diagnosis and monitoring of muscle diseases in substitution of current invasive techniques. 1.6 LIVER BACKSCATTER: A FRACTAL MODEL, T.A. Tutbill and K.J. Parker, Rochester Center for Biomedical Ultrasound, University of Rochester, Rochester, NY 14627. Backscatter from the liver has been attributed to a variety of structures including collagen, portal triads, hepatocytes, fat globules, and other sized “scatterers” and small geometries. Some researchers have approximated these scattering structures as small, uniform scatterers in a semiregular architecture. However, a realistic assessment of the liver shows that the increased impedance mismatch should correspond to the collagen-sheathed, parallel branches of the portal vein and hepatic artery, which terminate in the portal triads. Plastic vascular casts of the liver show an arborization of blood vessels with branches of various sizes. In our analysis, the vasculature is modeled as a fractal structure of discrete branching scales with an exponential probability density function. Assuming a fractal dimension consistent with physiological data, a Gaussian autocorrelation for each branch size, and a linear superpositioning of spectra, the resulting backscatter from the fractal vascular tree displays a frequency dependence with a power law factor ranging between 1 and 2, depending on branching features. The spectral slope shows good agreement with experimental data using calf and rabbit livers, with and without enhancementof the portal vein and hepatic artery branches. This work illustrates the potential for tissue characterization based on backscatter from a fractal vasculature. 1.7 IN VIVO ULTRASONIC MEASUREMENT OF MICROSCOPIC ANATOMICAL CHANGES IN OBSTRUCTED KIDNEYS, M.F. Insana,’ T.J. Hall,’ J.G. Wood2 and L.Y. Cheung2, Departments of *Radiology and ‘Surgery, University of Kansas Medical Center, Kansas City, KS 66103. Properties derived from acoustic backscatter measurements are examined for dog kidneys undergoing changes in hydrostatic pressures induced intraoperatively. These experiments are a part of our investigations into the sources of acoustic scattering in normal renal tissues. The objective of the study was to verify in vitro experimental results reported at this meeting last year, which suggest that glomeruli are the principal scatterers at ultrasonic frequencies below 5 MHz and renal tubules and/or blood vessels above 5 MHz. Following unrelated experiments, normally-perfused dog kidneys were exposed and scanned, in situ, to measure the average scatterer size and several other acoustic properties at frequencies below and above 5 MHz. Diuresis was induced using marmitol prior to complete temporary obstruction of the left ureter. Arterial pressure, ureteral pressure, and urine flow was measured along with the acoustic measurements, throughout the experiment. Unilateral ureteral obstruction was maintained for 10 minutes before being released for 5 minutes; this pattern was repeated several times. Ureteral obstruction increased the pressure in the ureter from less than 10 mm Hg to values between 70 and 120 mm Hg, depending on the animal. We found that changes in the scatterer size estimates for frequencies below 5 MHz varied in proportion to the pressure in the ureter, where the magnitude of the change was between 12 and 30%. Subsequently, we found that the acoustically-determined scatterer size corresponded to the average glomerular diameter, further supporting the theory that glomeruli are the principal scattering structure below 5 MHz. At frequencies greater than 5 MHz, very little change in scatterer size was observed, suggesting that arterioles, rather than tubules, may be the principal scatterers at high diagnostic frequencies. Experiments using vasodilators and vasoconstrictors are expected to further delineate the contributions of renal blood vessels and tubules in ultrasonic backscatter. This work was supported in part by the Whitaker Foundation.
191
IMAGING AND TISSUE CHARACTERIZATION
SYMPOSIUM
plastic standoff with ultrasonic transducer (10 cm focal length, broadband, 5 MHz center frequency) was applied to the anterior face of the upper arm over the belly of the biceps, perpendicular to the surface of the skin. The backscatter spectrum from a 3.75 mm portion of muscle about 1.5 cm deep from the skin was recorded at different sites. (Freq. range: 2-8 MHz in 1 MHz steps). The spectra were reduced to decibels relative to a planar, stainless steel reflector and the frequency average, or integrated, backscatter was calculated. Average integrated backscatter was obtained by averaging at each site, then all sites within a subject, then over all subjects. The value obtained for normal volunteers was -56.7 + 1.7 dB (mean &- sem, n = 5), while the level obtained in patients with DMD was -40.0 + 2.8 dB (mean f sem, n = 5). Integrated backscatter from afflicted patients was significantly higher than that of normal volunteers, with an average difference of 16.7 + 3.28 dB (mean + sd, p < 0.01). The ultrasonic method reported in this work may prove to be very useful as a new application of a noninvasive technique in the diagnosis and monitoring of muscle diseases in substitution of current invasive techniques. 1.6 LIVER BACKSCATTER: A FRACTAL MODEL, T.A. Tutbill and K.J. Parker, Rochester Center for Biomedical Ultrasound, University of Rochester, Rochester, NY 14627. Backscatter from the liver has been attributed to a variety of structures including collagen, portal triads, hepatocytes, fat globules, and other sized “scatterers” and small geometries. Some researchers have approximated these scattering structures as small, uniform scatterers in a semiregular architecture. However, a realistic assessment of the liver shows that the increased impedance mismatch should correspond to the collagen-sheathed, parallel branches of the portal vein and hepatic artery, which terminate in the portal triads. Plastic vascular casts of the liver show an arborization of blood vessels with branches of various sizes. In our analysis, the vasculature is modeled as a fractal structure of discrete branching scales with an exponential probability density function. Assuming a fractal dimension consistent with physiological data, a Gaussian autocorrelation for each branch size, and a linear superpositioning of spectra, the resulting backscatter from the fractal vascular tree displays a frequency dependence with a power law factor ranging between 1 and 2, depending on branching features. The spectral slope shows good agreement with experimental data using calf and rabbit livers, with and without enhancementof the portal vein and hepatic artery branches. This work illustrates the potential for tissue characterization based on backscatter from a fractal vasculature. 1.7 IN VIVO ULTRASONIC MEASUREMENT OF MICROSCOPIC ANATOMICAL CHANGES IN OBSTRUCTED KIDNEYS, M.F. Insana,’ T.J. Hall,’ J.G. Wood2 and L.Y. Cheung2, Departments of *Radiology and ‘Surgery, University of Kansas Medical Center, Kansas City, KS 66103. Properties derived from acoustic backscatter measurements are examined for dog kidneys undergoing changes in hydrostatic pressures induced intraoperatively. These experiments are a part of our investigations into the sources of acoustic scattering in normal renal tissues. The objective of the study was to verify in vitro experimental results reported at this meeting last year, which suggest that glomeruli are the principal scatterers at ultrasonic frequencies below 5 MHz and renal tubules and/or blood vessels above 5 MHz. Following unrelated experiments, normally-perfused dog kidneys were exposed and scanned, in situ, to measure the average scatterer size and several other acoustic properties at frequencies below and above 5 MHz. Diuresis was induced using marmitol prior to complete temporary obstruction of the left ureter. Arterial pressure, ureteral pressure, and urine flow was measured along with the acoustic measurements, throughout the experiment. Unilateral ureteral obstruction was maintained for 10 minutes before being released for 5 minutes; this pattern was repeated several times. Ureteral obstruction increased the pressure in the ureter from less than 10 mm Hg to values between 70 and 120 mm Hg, depending on the animal. We found that changes in the scatterer size estimates for frequencies below 5 MHz varied in proportion to the pressure in the ureter, where the magnitude of the change was between 12 and 30%. Subsequently, we found that the acoustically-determined scatterer size corresponded to the average glomerular diameter, further supporting the theory that glomeruli are the principal scattering structure below 5 MHz. At frequencies greater than 5 MHz, very little change in scatterer size was observed, suggesting that arterioles, rather than tubules, may be the principal scatterers at high diagnostic frequencies. Experiments using vasodilators and vasoconstrictors are expected to further delineate the contributions of renal blood vessels and tubules in ultrasonic backscatter. This work was supported in part by the Whitaker Foundation.
191