Particle size distribution and concentration of Intralipid^® 20

Intralipid^® is widely used as a tissue-mimicking phantom due to its optical similarity to biological tissues, i.e., high scattering and low absorption of light. Accurate modeling of the optical behavior of phantoms requires detailed knowledge of the properties of its scatterers.

We aim to determine the particle size distribution (PSD), concentration, and refractive index (RI) of particles in Intralipid^® 20%.

Cryo-electron microscopy, three different flow cytometers, and microfluidic resistive pulse sensing were used to measure the size distribution and concentration of particles in Intralipid^® 20%. To select the most accurate technique, the measured volume fraction of lipid particles was calculated and compared with the expected volume fraction (0.227). The measured size distribution and RI of soybean oil and Dulbecco's phosphate-buffered saline were used to calculate key optical parameters for the independent, single scattering regime. Calculations were performed with Mie theory across the 400- to 700-nm-wavelength range. Analytical expressions were then fitted to describe the wavelength dependence of each optical parameter.

The PSD, obtained from measurements of more than 10 million particles, spanned from 67 nm to over 2000 nm, with concentration decreasing continuously as diameter increased. The particle volume fraction calculated from the PSD deviated by less than 2% from the expected value, confirming the accuracy of the measurements. Optical parameters calculated from the PSD and RIs, including the scattering coefficient, phase function, anisotropy factor, and reduced scattering coefficient, showed good agreement with values reported in the literature. These parameters were also well described by empirical fits ( $R>0.97$ ).

The accurate measurement of particle size, concentration, and RI of Intralipid^® 20% supports the use of Intralipid^® as a reliable tissue-mimicking phantom and calibration sample in optical studies.