Measuring Fluoresence of Intralipid in Tissue Simulating Phantoms

Abstract

V. N. Du Le1*, Zhaojun Nie2, Joseph E. Hayward1, Thomas J. Farrell1 and Qiyin Fang2,3 1 Medical Physics and Applied Radiation Sciences, McMaster University, Hamilton, Ontario, Canada 2 School of Biomedical Engineering, McMaster University, Hamilton, Ontario, Canada 3 Department of Engineering Physics, McMaster University, Hamilton, Ontario, Canada *ledvn@mcmaster.ca Intralipid with lipid concentration of 1% to 10% v/v has been widely used to simulate background scattering which enables mapping of gold nanoparticle inclusions in spectroscopy [1], ultrasound imaging [2], and photoacoustic tomography [3]. These studies have shown that gold nanoparticles embedded in Intralipid are potential tools for cancer delineation. While optical properties and fluorescence of gold nanoparticles (GNS) were well established [1,3,4], the fluorescence characteristics of Intralipid at the lipid concentration 1% to 10% v/v has not been a subject of extensive study and remain controversy. In the present study, we construct Intralipid phantoms at concentrations that mimic tissue scattering, and measure their autoflouresence in broadband spectral region 350-650 nm using laser excitation at 355 nm with power of 2.8 µJ. Methods Two Intralipid phantoms with lipid concentrations of 1.5% and 2% v/v were created for fluorescence measurements. These phantoms were prepared by diluting the concentrated Intralipid 20% v/v solution in de-ionized water. The selected lipid concentrations produce best simulations for scattering of stromal layer in mucosal tissue which was reported previously [5]. Fluorescence measurements of Intralipid phantoms were performed using an excitation pulsed laser at 355 nm, a single optical fiber with core diameter of 600 µm, and a calibrated spectrometer to record fluorescence at a broadband wavelength range 350-650 nm. Results The µ s values of Intralipid 10% v/v was extrapolated from the transmission measurement of low lipid concentration using linear regression method and was compared to previous data. As shown in Fig. 1, an agreement (within 10% error) in µ s values between current extrapolated data and van Staveren et al.’s data was obtained [6]. Therefore, we apply similar anisotropy expression to back-calculate reduced scattering coefficients (µ s′) values for Intralipid phantoms. We demonstrated that phantoms with lipid concentration of 1.5% and 2% were best simulations for scattering of stromal layer in the mucosa (Fig. 2). The µ s′ value of lipid 1.5% and 2% at wavelength 355 nm is approximately 28.4 cm-1 and 37.9 cm-1, respectively. The fluorescence intensity of two phantoms was shown in Fig. 3. As illustrated, autofluoresence of Intralipid increases gradually from 350 nm to 500 nm (with primary peak at 500 nm and secondary peak at 450 nm), and decreases rapidly from 500 nm to 650 nm. It has been shown that GNS have fluorescence emission at wavelength range 330-440 nm when illuminated with laser at 300 nm [4]. Compounds of GNS also have fluorescence emission in visible wavelengths [4]. Therefore, it is very likely for fluorescence of Intralipid to interfere with fluorescence of GNS. Conclusions It has been shown that Intralipid phantoms have a primary emission peak at 500 nm and a secondary emission peak at 450 nm. These measurements proved that fluorescence of Intralipid with lipid concentration at stromal scattering level is significant and should not be ignored in studies of gold nanoparticles embedded in Intralipid. Acknowledgements This project is supported in part by the Natural Sciences and Engineering Research Council (NSERC) of Canada, Canada Foundation for Innovation (CFI), Ontario Ministry of Research and Innovation (MRI), and Canada Canadian Cancer Society Research Institute (CCSRI).