Chemistry Reference
In-Depth Information
5.2
Rare Earth Based Multimodal Molecular
Imaging Contrast Agents
Synergistic combination of two or more imaging modalities is made
possible by advanced hardware technology and the development of
novel multimodal imaging probes. Integrated imaging modalities can
provide more comprehensive and reliable biological data, which is
important for accurate diagnosis. Taking PET/MRI and MRI/optical
imaging as examples, the combination of high-sensitivity PET and
high-resolution MRI can be used in brain imaging, where tissue
remains relatively fixed in the skull [51]. Combination of optical
imaging and MRI allows the scalpel to be guided (via fluorescence
imaging) to remove lesions (via MRI) [52].
Rare earth based nanomaterials have been widely investigated as
contrast agents in integrated imaging modalities in recent years. Such
integrated imaging modalities include MRI/ fluorescence imaging,
PET/MRI and CT/MRI. The next section is broadly divided into two
sections to discuss the application of rare earth ions as contrast agents
in (1) MRI/fluorescence imaging and (2) PET/MRI and CT/MRI.
5.2.1
MRI/Optical Imaging Probes and Their
Bioapplications
The most investigated inorganic multimodal imaging probes
combine MRI and optical imaging capabilities. In MRI, the most
commonly used
contrast agents are low molecular weight
gadolinium ion complexes. Gadolinium ions have seven unpaired
electrons, which can efficiently alter the relaxation time of water
where they distribute. Superparamagnetic iron oxide nanoparticles
are commercially available as
T
1
contrast agents. Dysprosium (Dy)
is another lanthanide ion that has been investigated in MRI, and
in most cases, behaves as a
T
2
contrast agent. Based on the type
MRI contrast agent, rare earth based multimodal imaging probes
can be generally categorized into two groups: (1) Gd
T
2
-based
multifunctional nanomaterials, (2) superparamagnetic iron oxide
nanoparticles based multifunctional nanomaterial and (3) Dy
3+
3+
-
based multifunctional nanomaterials.
Gd
3+
-based multifunctional nanomaterials
5.2.1.1
Gd ions, as paramagnetic centers, have to be incorporated into
chelate ligands or doped into a matrix as MRI contrast agents due to
