In vivo three-dimensional spectroscopic photoacoustic imaging for monitoring nanoparticle delivery

Abstract

In vivo monitoring of nanoparticle delivery is essential to better understand cellular and molecular interactions of nanoparticles with tissue and to better plan nanoparticle-mediated therapies. We developed a three-dimensional ultrasound and photoacoustic (PA) imaging system and a spectroscopic PA imaging algorithm to identify and quantify the presence of nanoparticles and other tissue constituents. Using the developed system and approach, three-dimensional in vivo imaging of a mouse with tumor was performed before and after intravenous injection of gold nanorods. The developed spectroscopic PA imaging algorithm estimated distribution of nanoparticle as well as oxygen saturation of blood. Moreover, silver staining of excised tumor tissue confirmed nanoparticle deposition, and showed good correlation with spectroscopic PA images. The results of our study suggest that three-dimensional ultrasound-guided spectroscopic PA imaging can monitor nanoparticle delivery in vivo.

Keywords: (110.7170) Ultrasound; (170.3880) Medical and biological imaging; (170.5120) Photoacoustic imaging.

Fig. 1

The absorption spectra of deoxyhemoglobin (Hb, blue dashed line) and oxyhemoglobin (HbO₂, red dash-dotted line) [17] compared to the absorbance spectrum of the PEGylated gold nanorod (PEG-Au NR, green solid line) measured by UV-Vis spectroscopy. Dots on each line correspond to wavelengths used for spectroscopic photoacoustic imaging. The transmission electron microscopy (TEM) image of PEG-Au NRs is also shown.

Fig. 2

Schematic diagram of the developed in vivo three-dimensional ultrasound and photoacoustic imaging system.

Fig. 3

Schematic diagram of the developed spectroscopic photoacoustic imaging algorithm.

Fig. 4

Comparison between linear least square (LLS) and minimum mean square error (MMSE) methods. Ultrasound image (a) was used for the skin-tissue segmentation. The fluence compensated photoacoustic image (b) at 800 nm is shown as a reference. Spectral analysis based on LLS method can produce negative concentrations (c) of optical absorbers due to imperfect fluence compensations, noisy measurements, etc. The regions producing a negative NP concentration by LLS method get removed from the NP image. (d). However, the developed MMSE method can reliably reconstruct spatial distribution and concentration of NP: white arrows in panel (e) indicate locations where NP concentrations were recovered using MMSE method.

Fig. 5

Three-dimensional ultrasound images (panels a,g), photoacoustic images (panels b,h), and PA-derived oxygen saturation images (SO₂, panels c,i), and images of oxyhemoglobin (C[HbO₂], panels d,j), deoxyhemoglobin (C[Hb], panels e,k), and nanoparticle (C[NP], panels f,l) concentration before and 31 hours after tail vein injection of PEGylated gold nanorods into tumor bearing mouse. PA and PA-derived images are shown with ultrasound images in the background. Oxygen saturation was calculated using the concentrations of oxyhemoglobin and deoxyhemoglobin. Furthermore, anatomical features of the tumor (i.e., tumor size and location) are better shown in rotating 3-D images: ultrasound (panel g, Media 1), 3-D photoacoustics (panel h, Media 2), oxygen saturation (panel i, Media 3), and nanoparticle concentration (panel l, Media 4).

Fig. 6

(a) Photograph of the mouse with dotted lines indicating the approximate location of imaging planes and corresponding histological slides. (b) PA-derived maps of NP concentration displayed over the ultrasound images of the tumor. (c) Silver stained tissue slides where gray to black colors indicate presence of NPs. The enlarged areas of silver stained slides are shown in panels (d) and (e).