Detection and Description of Tissue Disease: Advances in the Use of Nanomedicine for Medical Imaging - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 245

to allow increase the tissue density versus that of adjacent tissue.
Improved carrying capacity can be achieved through using larger
hydrophobic molecules, but with the downside that this increases
the CMC, which in turn means that a higher micelle concentration is
required to achieve adequate in vivo stability. Therefore, the use of
micelles in CT imaging is inherently limited.
In early studies of the use of micelles as CT contrast agents,
micelles were used as carriers for iodinated molecules. However, a
significant weakness of this approach was the loss of the iodinated
molecules from the core of the micelle. In a more recent study of
micelles as CT contrast agents, Torchilin et al . [2] used amphipathic
block-polymers containing iodine. These compounds form micelles
with an average diameter of 80 nm and an iodine content of 33.8%.
Detectable organ opacification was observed from 5 min to 3 h
post-injection in rats; attenuation increased from 85 HU to 253
HU in the aorta and from 92 HU to 156 HU in the liver post-micelle
injection over this 3 h window. The authors note that while this
was a significant achievement in the use of micelles as CT contrast
agents, much more work must be carried out before these materials
are ready for clinical use. For recent reviews, see Ref. [3].
8.2.1.2 Liposomes
As candidate nanoparticles, liposomes have several advantages
over micelles: Their manufacture is more controllable, they are
mechanically stronger, they are less susceptible to collapse on
dilution, they can be used as carriers for hydrophilic and hydrophobic
molecules, and they can be derivatized with targeting moieties more
easily.
Liposomes, first described by Bangham in 1964 [4], consist of
amphipathic phospholipids that form bilayered vesicles in aqueous
solutions. In contrast to micelles, the core of liposomes is hydrophilic.
They can, therefore, act as vehicles for either hydrophilic compounds
(in the core) or hydrophobic compounds, for example iodinated
polymers, in the lipid bilayer. Early liposomes comprised phosphatidyl
choline (PC) from egg and cholesterol. These “conventional,” or “C,”
liposomes have been superseded by sterically stabilized, or “S,”
liposomes (see Ref. [5] for further discussion). Improvements have
also been made in the combinations of fatty acids and lipids used
in the manufacture of liposomes and in surface modification of
the particles so as to evade in vivo clearance mechanisms, thereby