Chemistry Reference
In-Depth Information
Molecular imaging modalities such as positron emission
tomography (PET), single photon emission computed tomography
(SPECT), magnetic resonance imaging (MRI), computed tomography
(CT), ultrasound and optical imaging are widely used clinically to
understand and diagnose various diseases. The characteristics of
every modality are summarized in Table 5.1. Every imaging modality
has its own advantages and disadvantages and no single technique
possesses full capabilities to obtain comprehensive and accurate
biological information. For example, MRI and CT have the advantages
of high spatial resolution, but they are limited by low sensitivity. On
the contrary, optical imaging has very high sensitivity but suffers from
low tissue penetration depths. Therefore, by integrating different
imaging modalities into a single system, multimodal imaging method
can offset the limitations of single imaging modalities. PET/CT is one
of the multimodal imaging instruments. The first PET/CT scanner,
developed in 1998 by Townsend and his colleagues in collaboration
with Siemens Medical, was commercially available in 2001 [6, 7].
By 2003, PET/CT instruments were available from all major clinical
instrument manufactures such as GE, Philips, CTI, and Siemens.
Today, more than 95% of new PET scanners installed are integrated
PET/CT scanners. SPECT/CT was introduced commercially in 2004
[8]. PET/MRI instruments, which have brought much hope for
improved patient safety and imaging capacity over PET/CT, are also
on the horizon [9].
Imaging probes are used to monitor and trace molecular
processes. With the advent of multimodal technology, design and
development of new functional molecular imaging probes are of great
importance. There are two prerequisites for functional molecular
imaging probes: first, they have to provide high spatial resolution as
well as high sensitivity in imaging; second, target-specific molecular
probes have to be available [10]. Many possible combinations of
imaging modalities result in different types of imaging contrast
agents. Increasing number of works have reported the fabrication
of different inorganic multimodal contrast agents. Some examples
include iron oxide nanoparticles conjugated with dye molecules
[11, 12], iron oxide/quantum dots (QDs) heterodimers [13-16], and
paramagnetic ion (Mn
2+
)-doped QDs [17, 18].
Rare earth ions possess unique optical and magnetic properties
due to their 4f electronic configuration. The 4f
n
electronic
