KEYWORDS: Tissues, Monte Carlo methods, Photoacoustic spectroscopy, Diffusion, Optical simulations, Acoustics, Signal generators, Interfaces, Signal detection, Chemical elements
A finite element (FE)-based simulation model for photoacoustic (PA) has been developed incorporating light
propagation, PA signal generation, and sound wave propagation in soft tissues using a commercial FE simulation
package, COMSOL Multiphysics. The developed simulation model is evaluated by comparing with other known
simulation models such as Monte Carlo method and heat-pressure model. In this in silico simulation, FE model is
composed of three parts of 1) homogeneous background soft tissues submerged in water, 2) target tissue inclusion (or
PA contrast agents), and 3) short pulsed laser source (pulse length of 5-10 ns). The laser point source is placed right
above the tissues submerged in water. This laser source light propagation through the multi-layer tissues using the
diffusion equation is compared with Monte Carlo solution. Photoacoustic signal generation by the target tissue inclusion
is simulated using bioheat equation for temperature change, and resultant stress and strain. With stress-strain model, the
process of the PA signal generation can be simulated further in details step by step to understand and analyze the photothermal
properties of the target tissues or PA contrast agents. The created wide-band acoustic pressure (band width > 150
MHz) propagates through the background tissues to the ultrasound detector located at the tissue surface, governed by
sound wave equation. Acoustic scattering and absorption in soft tissues also have been considered. Accuracy and
computational time of the developed FE-based simulation model of photoacoustics have been quantitatively analyzed.
Spectral analysis of photoacoustic (PA) molecular imaging (PMI) of ferritin expressed in human melanoma cells
(SK-24) was performed in vitro. Ferritin is a ubiquitously expressed protein which stores iron that can be detected by PA
imaging, allowing ferritin to act as a reporter gene. To
over-express ferritin, SK-24 cells were co-transfected with plasmid
expressing Heavy chain ferritin (H-FT) and plasmid expressing enhanced green fluorescent protein (pEGFP-C1) using
LipofectamineTM 2000. Non-transfected SK-24 cells served as a negative control. Fluorescent imaging of EGFP
confirmed transfection and transgene expression in co-transfected cells. To detect iron accumulation in SK-24 cells, a
focused high frequency ultrasonic transducer (60 MHz, f/1.5), synchronized to a pulsed laser (<20mJ/cm2), was used to
scan the PA signal from 680 nm to 950 nm (in 10 nm increments) from the surface of the 6-well culturing plate. PA
signal intensity from H-FT transfected SK-24 cells was not different from that of non-transfected SK-24 cells at
wavelengths less than 770 nm, but was over 4 dB higher than
non-transfected SK-24 cells at 850 ~ 950 nm. Fluorescent
microscopy indicates significant accumulation of ferritin in H-FT transfected SK-24 cells, with little ferritin expression
in non-transfected SK-24 cells. The PA spectral analysis clearly differentiates transfected SK-24 cells from nontransfected
SK-24 cells with significantly increased iron signal at 850 ~ 950 nm, and these increased signals were
associated with transfection of H-FT plasmid. As such, the feasibility of ferritin as a reporter gene for PMI has been
demonstrated in vitro. The use of ferritin as a reporter gene represents a new concept for PA imaging, and may provide
various opportunities for molecular imaging and basic science research.
Multiple cardiovascular inflammatory biomarkers were simultaneously imaged in vivo using antibody conjugated
gold nanorods (GNRs) injected into a mouse model of vascular injury stimulated by a photochemical reaction of Rose
Bengal dye to green light. Mixed solutions of ICAM-1 antibody conjugated GNRs (715 nm) and E-selectin antibody
conjugated GNRs (800 nm) were injected to bind to their respective inflammatory markers on the luminal surface of the
inferior vena cava of a mouse. Photoacoustic intensity was measured by a commercial ultrasound probe synchronized to
a pulsed laser (10-18 mJ/cm2) at 715 nm or 800 nm clearly identified the upregulation of targeted biomarkers.
Histopathology on the harvested tissues confirmed inflammation. The feasibility of simultaneous photoacoustic
molecular imaging of inflammation responses in cardiovascular system using a commercial ultrasound system has been
demonstrated in vivo.
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