Nanoparticles as Contrast Agents for Optoacoustic Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 109

All nanoparticle-based OA contrast agents can be divided in two groups:
(1) nanoparticles based on endogenous chromophores and (2) nanoparticles based on
exogenous chromophores. OA imaging takes advantage of the high optical contrast
of hemoglobin over a broad optical spectrum in the NIR. There is no other biological
molecule that possesses strong NIR absorption close to that of hemoglobin, with the
exception of melanin. melanin is a protein that is naturally present only within the
very thin basal layer of the epidermis. Hemoglobin, on the other hand, is distributed
throughout the entire volume of the body. Both forms of hemoglobin, oxygenated
and deoxygenated, have much stronger optical absorption coefficients in the NIR
( μ a = 1-10 cm −1 ) compared with the NIR optical absorption of other tissues. Therefore,
OA imaging represents, possibly, the most powerful modality for imaging blood
distribution in organs, the circulation system, and micro blood vessels including
angiogenesis-related microvasculature of tumors [2, 3, 16, 17].
Administration of exogenous contrast agents in vivo opens another opportunity
for enhancement of the OA imaging contrast in organs and tissues of interest.
standard molecular contrast agents such as NIR organic dyes have been extensively
used in OA imaging of small animal models [18-21]. A variety of chromophores are
absorbed in the wavelength range of NIR. However, only a few types of strongly
absorbing nanoparticles can compete with endogenous contrast agents such as hemo-
globin and melanin. Nanoparticles are especially attractive as OA contrast agents
compared with molecules, since the signal is proportional to the total volume of the
optically absorbing chromophores. The volume of a nanoparticle is significantly
larger than that of a single chromophore molecule.
The two most effective types of exogenous contrast agents designed for optical
and OA imaging modalities are dye-containing nanoparticles (such as indocyanine
green (Icg)-doped nanoparticles) and nanoparticles that possess strong surface plas-
mon resonance (sPR) absorption (such as gold nanorods, hollow gold nanospheres,
and nanocages). NIR exogenous nanoparticles may be more effective as contrast
agents when being targeted and selectively delivered and bound with specific mole-
cules in cells and tissues with minimal NIR absorption. However, nanoparticles pas-
sively accumulating in organs with higher metabolic activity have the advantage of a
relatively simple and low-cost design. Both types, therefore, have merit and will be
discussed in the following text. Advances in nanotechnology have produced
significant contributions to the nanoparticle-based optical and OA contrast agents
[22-30]. In this review, we will describe the OA applications of nanoparticles based
on both endogenous and exogenous chromophores.
5.2 optoacoustic NaNoparticles Based
oN eNdogeNous chromophores
Two of the key advantages of using endogenous contrast agents for imaging applica-
tions are safety and the possibility of revealing the physiological environment,
because the physiological parameters of the tissue do not change during OA image
acquisition [9]. OA imaging technique is suitable for monitoring endogenous optical