• Sinkus, Ralph
• Annio, Giacomo
• Holm, Sverre
• Franck, Gregory

Abstract

<jats:p>The imaginary part of the complex shear modulus in tissue is not negligible. In liver the phase angle (ranging between 0 and 1) is about 0.2 while in kidney it is about 0.3. The presence of dispersion can have its origin either in a constitutive loss—i.e., absorption of energy—or in scattering of the wave and hence represents an apparent loss. Since dispersion in tissue follows over a wide range a frequency power-law, fractional order derivative models such as the springpot model are well suited to fit the data in the clinically accessible range of 30–200Hz. They, however, do not provide a fundamental understanding of whether loss is due to friction and hence conversion to heat, or due to material heterogeneities and thus scattering. Models such as ODA are able to relate the observable frequency power-law to the spatial distribution of scatterers. If loss in low-frequency elastography were solely due to scattering, this would render the method extremely powerful in characterizing for instance blood vessel architecture in oncology. To distinguish whether the measured prominant loss in porcine tissue (phase angle ∼0.4) is originating from absorption or scattering, we use MR-Spectroscopy to measure absolute temperature via the resonance-frequency-shift between water and methylene, at precisions better than 0.1 °C. Temperature should increase theoretically by about 1/2 °C at an amplitude of 10 μm, 500 Hz, and an exposure of 1000 s, which is not observed. This points towards scattering as the main mechanism for wave attenuation.</jats:p>

Topics

• impedance spectroscopy
• dispersion
• phase