- Email: [email protected]
Elastography: A method for imaging the elasticity of biological tissues
ABSTRACTS, ULTRASONIC
IMAGING AND TISSUE CHARACTERIZATION
SYMPOSIUM
In past reports, we have shown that solid, micron-sized particles of iodipamide ethyl ester (IDE) can be accumulated by the Kupffer cells of the liver, resulting in enhanced backscatter. We have developed a procedure that greatly enhances the echogenicity of IDE by creating a highly stabilized, hybrid bubble-particle agent. In this new method, IDE particles are prepared as previously reported, but with modifications which result in gas voids, stabilized within 0.5 pm diameter particles. These provide a biocompatible, nondissolving vehicle for transport of the gas through Kupffer cells of the liver. In-vitro studies show high echogenicity of our agent at concentrations of 3 mg IDE/cc in bovine plasma with stable backscatter at room temperature over periods exceeding 5 hours. In-vivo tests in rabbits using slow (1 cclmin) infusions into the ear vein show enhancedleft heart and blood pool echogenicity over a period of 2-15 minutes, with subsequent enhancement of liver echogenicity. This product appears to have multiple uses for blood flow and liver imaging, using both x-ray and ultrasound imaging modalities, and may have greater stability and sustained echogenicity than do competing bubble contrast formulations.
3. Elasticity & Nonlinear Parameter Imaging 3.1 ELASTOGRAPHY: A METHOD FOR IMAGING THE ELASTICITY OF BIOLOGICAL TISSUES, J. Ophir, I. Cespedes, H. Ponnekanti, Y. Yaxdi and X. Li, Ultrasonics Laboratory, Department of Radiology, The University of Texas Medical School, Houston, TX 77030. We describe a new method for quantitative imaging of strain and elastic modulus distributions in soft tissues. The method is based on external tissue compression, with subsequent computation of the strain profile along the transducer axis, which is derived from cross-correlation analysis of pre- and postcompression A-line pairs. We show that the strain profiles along the A-line are dependent on the initial compression and on the interaction between elements of differing elasticities along the transducer axis which are generally unknown; the result is a strain image which is not quantitative. It is possible to convert a strain image into a quantitative elastic modulus image (elastogram) by measuring the local stress applied to the tissue by means of a known compressor standoff layer, which is interposed between the transducer and the tissue. Additionally, a correction for the nonuniform stress distribution is made. Supported in part by NIH grants #ROl-CA38515 and #ROl-CA44389. 3.2 ELASTOGRAPHY: EXPERIMENTAL RESULTS FROM PHANTOMS AND TISSUES, I. Cespedes, H. Ponnekanti and J. Ophir, Ultrasonics Laboratory, Department of Radiology, University of Texas Medical School, Houston, TX 77030. We report experimental results of elastography in phantoms and tissues in vitro. Phantoms were constructed from two grades of synthetic open cell foam materials. The first phantom was made by cutting a diagonal slit in a foam block and then rejoining the two resulting triangular pieces. An elastogram of this phantom clearly showed the diagonal seam between the blocks, while the B-scan from the same data did not show it; reduced speckle in the elastogram was also noted. Other phantoms were constructed from layers of one foam embedded in another. The elastograms show clear visibility of a 5 dB change in foam elasticity in these phantoms, as well as reasonable agreement with the absolute elasticities of the foams which were separately measured. Finally, a slab of bacon was scanned; the elastogram showed the existence of muscle and fat layers more clearly than the corresponding B-scan. We discuss these observations and the potential utility of elastography. Supported in part by NIH grants #ROl-CA38515 and #ROl-CA44389. 3.3 IMAGING OF VELOCITY AND NONLINEARITY DISTRIBUTIONS IN SOFT TISSUE FOR FORCED MECHANICAL VIBRATION, Masahiko Shinagawa,’ Takuso Sato,’ Hideyuki Ninoyu,’ Mikiya Sase,’ Stephen F. Levinson,’ Yoshiki Yamakosh? and Koichi Kobayashi4, ‘The Graduate School, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 227, Japan, *National Institutes of Health, Bethesda, MD 20892, 3Gunma University, Japan and @TokyoUniversity, Japan. The propagation characteristics of low frequency mechanical vibration in soft tissues are considered to be closely related to their dynamic properties. We have previously reported on the production of attenuation and velocity images for forced sinusoidal mechanical vibrations in the range from 10 Hz to 500 Hz [ 11. Others have observed wave distortions when the amplitude and frequency are increased [2].
195
IMAGING AND TISSUE CHARACTERIZATION
SYMPOSIUM
In past reports, we have shown that solid, micron-sized particles of iodipamide ethyl ester (IDE) can be accumulated by the Kupffer cells of the liver, resulting in enhanced backscatter. We have developed a procedure that greatly enhances the echogenicity of IDE by creating a highly stabilized, hybrid bubble-particle agent. In this new method, IDE particles are prepared as previously reported, but with modifications which result in gas voids, stabilized within 0.5 pm diameter particles. These provide a biocompatible, nondissolving vehicle for transport of the gas through Kupffer cells of the liver. In-vitro studies show high echogenicity of our agent at concentrations of 3 mg IDE/cc in bovine plasma with stable backscatter at room temperature over periods exceeding 5 hours. In-vivo tests in rabbits using slow (1 cclmin) infusions into the ear vein show enhancedleft heart and blood pool echogenicity over a period of 2-15 minutes, with subsequent enhancement of liver echogenicity. This product appears to have multiple uses for blood flow and liver imaging, using both x-ray and ultrasound imaging modalities, and may have greater stability and sustained echogenicity than do competing bubble contrast formulations.
3. Elasticity & Nonlinear Parameter Imaging 3.1 ELASTOGRAPHY: A METHOD FOR IMAGING THE ELASTICITY OF BIOLOGICAL TISSUES, J. Ophir, I. Cespedes, H. Ponnekanti, Y. Yaxdi and X. Li, Ultrasonics Laboratory, Department of Radiology, The University of Texas Medical School, Houston, TX 77030. We describe a new method for quantitative imaging of strain and elastic modulus distributions in soft tissues. The method is based on external tissue compression, with subsequent computation of the strain profile along the transducer axis, which is derived from cross-correlation analysis of pre- and postcompression A-line pairs. We show that the strain profiles along the A-line are dependent on the initial compression and on the interaction between elements of differing elasticities along the transducer axis which are generally unknown; the result is a strain image which is not quantitative. It is possible to convert a strain image into a quantitative elastic modulus image (elastogram) by measuring the local stress applied to the tissue by means of a known compressor standoff layer, which is interposed between the transducer and the tissue. Additionally, a correction for the nonuniform stress distribution is made. Supported in part by NIH grants #ROl-CA38515 and #ROl-CA44389. 3.2 ELASTOGRAPHY: EXPERIMENTAL RESULTS FROM PHANTOMS AND TISSUES, I. Cespedes, H. Ponnekanti and J. Ophir, Ultrasonics Laboratory, Department of Radiology, University of Texas Medical School, Houston, TX 77030. We report experimental results of elastography in phantoms and tissues in vitro. Phantoms were constructed from two grades of synthetic open cell foam materials. The first phantom was made by cutting a diagonal slit in a foam block and then rejoining the two resulting triangular pieces. An elastogram of this phantom clearly showed the diagonal seam between the blocks, while the B-scan from the same data did not show it; reduced speckle in the elastogram was also noted. Other phantoms were constructed from layers of one foam embedded in another. The elastograms show clear visibility of a 5 dB change in foam elasticity in these phantoms, as well as reasonable agreement with the absolute elasticities of the foams which were separately measured. Finally, a slab of bacon was scanned; the elastogram showed the existence of muscle and fat layers more clearly than the corresponding B-scan. We discuss these observations and the potential utility of elastography. Supported in part by NIH grants #ROl-CA38515 and #ROl-CA44389. 3.3 IMAGING OF VELOCITY AND NONLINEARITY DISTRIBUTIONS IN SOFT TISSUE FOR FORCED MECHANICAL VIBRATION, Masahiko Shinagawa,’ Takuso Sato,’ Hideyuki Ninoyu,’ Mikiya Sase,’ Stephen F. Levinson,’ Yoshiki Yamakosh? and Koichi Kobayashi4, ‘The Graduate School, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 227, Japan, *National Institutes of Health, Bethesda, MD 20892, 3Gunma University, Japan and @TokyoUniversity, Japan. The propagation characteristics of low frequency mechanical vibration in soft tissues are considered to be closely related to their dynamic properties. We have previously reported on the production of attenuation and velocity images for forced sinusoidal mechanical vibrations in the range from 10 Hz to 500 Hz [ 11. Others have observed wave distortions when the amplitude and frequency are increased [2].
195