Detection of sub-micron lipid droplets using transmission-mode attenuation measurements in emulsion phantoms and liver

In liver transplantation, donor liver viability is assessed by the both the amount and type of fat present in the organ. General guidelines dictate that livers with more than 30% fat should not be transplanted; however, a lack of available donor organs has led to the consideration of livers with more fat. As a part of this process, it is desirable to distinguish micro-vesicular fat (< 1 µm droplets) from macro-vesicular fat (~10 µm droplets). A method of evaluating the relative amounts of micro- and macro-fat is proposed based on transmission-mode ultrasound attenuation measurements. For an emulsion of one liquid in another, attenuation comprises both intrinsic losses in each medium and excess attenuation associated with interactions between media. Using an established coupled-phase model, the excess attenuation associated with a monodisperse population of lipid droplets was calculated with physical properties representative of both liver tissue and dairy products. Calculations predict that excess attenuation can exceed intrinsic attenuation and that a well-defined peak in excess attenuation at 1 MHz should occur for droplets around 0.8 µm in diameter. Such predictions are consistent with preliminary transmission-mode measurements in dairy products. [Work supported by NIH grants EB017857, EB007643, EB016118, and T32 DK007779.]