Diffraction limited 3D cell volume derivation for scattering data analysis
Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in December 2013.
Ultrasound speckle carries information about the interrogated scattering microstructure. To effectively analyze this scattering microstructure using a monochromatic signal, a separate transmit and receive crossbeam geometry is implemented. The complex echo signal is represented as a superposition of signals due to all scatterers within an effective resolution cell volume, VE, in the crossbeam geometry. An estimate of such an effective resolution cell volume is required in order to remove system effects when attempting to characterize material using moment analysis. The effective resolution cell volume, VE is defined in terms of the overlapping diffraction beam patterns for the transmit-receive transducers. Given the focused piston transducer's radius and geometrical focal distance, a Lommel diffraction formulation suitable for monochromatic excitation is used to calculate VE as a function of frequency and angle. This formulation amounts to a Fresnel approximation to the diffraction problem valid in both the near and far field. Theoretically, VE is numerically integrated within the overlapping region of the product of the transmit-receive velocity-potential transfer functions. Experimentally, VE is calculated from pressure amplitude field patterns from three focused transducers, two 2.5 MHz and one 3.5 MHz, excited by a monochromatic signal and detected by a 0.5 mm diameter PVDF membrane hydrophone. We present theoretical and experimental evaluations of the effective resolution cell volume for the crossbeam geometry at frequencies within the transducers' bandwidths along with its application to material microstructure characterization.