Biomedical Engineering Reference
In-Depth Information
Ethylene glycol
Glycerol
Propylene glycol
Sorbitol
H
H
H
H
OH
OH
OH
H
H
H
H
H
H
OH
OH
OH
OH
OH
OH
H
H
H
H
H
H
H
H
H
OH
OH
OH
OH
H
H
H
H
Glucose
Fructose
CH
2
OH
CH
2
OH
O
OH
O
H
H
H
H
H
OH
OH
OH
H
CH
2
OH
HO
HO
H
H
OH
Sucrose
Dimethylsulphoxide
(DMSO)
CH
2
OH
CH
2
OH
O
OH
CH
3
O
H
H
H
H
OH
OH
H
S
O
O
CH
2
OH
HO
CH
3
H
OH
HO
H
FIgurE 8.1
Chemical structure representations of the most common optical clearing agents.
scattering. Optical clearing for tissues was first proposed by Tuchin et al. [7], and quite encouraging
results have been reported by several research groups since then. For example, immersion of a sample
of murine tail tendon in a test tube with 50% glycerol solution gives a rather dramatic demonstration
of optical clearing. In fact, the sample turns literally transparent, and locating it in glycerol becomes
somewhat challenging. Yet, upon transfer to PBS (phosphate-buffered saline), the sample regains its
native white appearance [8].
Successful optical clearing
in vitro
has been reported for diverse tissues such as skin (epidermis and der-
mis) [9-11], cerebral membrane (
dura mater
) [12], blood [13,14], tendon [6], gastrointestinal tissue [15,16],
and muscle [17], where imaging was carried out with bright-field, OCT, MPM, and SHG. More recently,
a clearing effect was observed
in vivo
in human [10,18,19] and rat skin [20], human and rabbit sclera [21],
and human
dura mater
[22] in some cases within as few as 10 min after injection or topical application
of optical clearing agents (OCAs). The clearing agents investigated so far include glycerol, glycerol-water
solutions, sugars and sugar alcohols, propylene glycol, polyethylene glycol, dimethylsulfoxide (the most
typical clearing agents are shown in Figure 8.1), and even sunscreen creams and other pharmaceutical
products. This chapter offers an overview of the current understanding of optical clearing mechanism, its
capabilities, applicability, and toxicity considerations, with special attention given to SHG imaging.
8.2 Physical Background
Consider a typical soft tissue on a microscopic scale. It is formed by a hydrated network of fibers that
hosts groups of cells. The individual tissue components have quite different refractive indices. In other
words, tissues consist of scatterers with high refractive index such as fibers and cell organelles, distrib-
uted in a medium of lower background index consisting of interstitial fluid and cytoplasm. A photon
of light traveling through the tissue encounters a continuous structure with local spatial variations of
