Nanomaterials for Medical Imaging …
339
3 Nanotechnology
Over the years, direct visualization of the diseased region using different imaging
modalities have shown conflicts between resolution and penetration depth. Thus,
an exogenous contrast agent that is biologically compatible with less toxicity is
required to improve current imaging systems. Conventionally used agents approved
by Food Drug Administration (FDA) are Iodine for X-ray imaging, fluor-deoxyglucose (FDG) for PET, and visible and NIR fluorophores for OI are used widely in
these systems [12]. Present contrast agents used in existing imaging systems have
many limitations; for instance, fast clearance & photo instability and are inappropriate
for multi-modal and hybrid imaging. Later, chemists worked with a clinical specialist
to produce some novel materials to handle critical issues to ease the diagnosis at a
faster rate. Imaging with nanomaterials compared to traditionally used agents can
provide enhanced sensitivity and specificity of the diseased region.
Recent improvements in nano-based contrast agents have triggered to modify and
enhance the current imaging techniques to produce hybrid and multi-modal imaging
systems. During the past two decades, nano-based medicines have revolutionized
medical imaging science. Early detection of cancer has become more rapid and
efficient. Although the concept and technology of imaging are old, the way we
look at the disease using nanomaterials is new. Imaging techniques, along with the
nano-based contrast agents were used as a tool to characterize the interaction of
drugs at both cellular and pre-clinical level [13]. The Fig. 3 summarizes different
nanoparticles used in preclinical imaging modalities.
Inherent properties of elements that can be converted into nanomaterial would
act as nanoprobe to either specific imaging systems. Nanoprobes show better performance compared to conventional contrast agents used for disease diagnosis. For
instance, gadolinium and iron oxide (IO) NPs have been used as a probe in an MRI
system as a T 1 /T 2 contrast. Compared to the gadolinium complex, the contrast of
IO NPs shows higher due to its superparamagnetic properties when converted into
nano form [14]. Likewise, other materials have also been explored and compared
with the conventional contrast used in specific imaging modalities. For example,
gold nanoparticles (GNPs) (mCT and photoacoustic tomography (PAT)), quantum
dots (mOI), and nanobubbles (mUS) show a better contrast compared to conventional agents [6]. Nano based contrast agents for pre-clinical imaging applications
are not entirely replaced with the conventional contrast agents. Figure 4 shows the
general requirements in designing nanomaterials for in vivo imaging as follows [15]:
(i) high surface area due to their size, (ii) ease of functionalization, (iii) modulation in
their physicochemical properties, (iv) high drug loading efficiency, (v) multi-modal
imaging, (vi) optimization in image quality.
The size of the nanoparticles is an essential factor to be considered due to change
in contrast for CT. For instance, Dong et al. showed that different sized GNPs ranging
from 4 to 150 nm were studied in both clinical and pre-clinical modalities. Particles
with size less than 15 nm have higher X-ray attenuation and more extended blood
339
3 Nanotechnology
Over the years, direct visualization of the diseased region using different imaging
modalities have shown conflicts between resolution and penetration depth. Thus,
an exogenous contrast agent that is biologically compatible with less toxicity is
required to improve current imaging systems. Conventionally used agents approved
by Food Drug Administration (FDA) are Iodine for X-ray imaging, fluor-deoxyglucose (FDG) for PET, and visible and NIR fluorophores for OI are used widely in
these systems [12]. Present contrast agents used in existing imaging systems have
many limitations; for instance, fast clearance & photo instability and are inappropriate
for multi-modal and hybrid imaging. Later, chemists worked with a clinical specialist
to produce some novel materials to handle critical issues to ease the diagnosis at a
faster rate. Imaging with nanomaterials compared to traditionally used agents can
provide enhanced sensitivity and specificity of the diseased region.
Recent improvements in nano-based contrast agents have triggered to modify and
enhance the current imaging techniques to produce hybrid and multi-modal imaging
systems. During the past two decades, nano-based medicines have revolutionized
medical imaging science. Early detection of cancer has become more rapid and
efficient. Although the concept and technology of imaging are old, the way we
look at the disease using nanomaterials is new. Imaging techniques, along with the
nano-based contrast agents were used as a tool to characterize the interaction of
drugs at both cellular and pre-clinical level [13]. The Fig. 3 summarizes different
nanoparticles used in preclinical imaging modalities.
Inherent properties of elements that can be converted into nanomaterial would
act as nanoprobe to either specific imaging systems. Nanoprobes show better performance compared to conventional contrast agents used for disease diagnosis. For
instance, gadolinium and iron oxide (IO) NPs have been used as a probe in an MRI
system as a T 1 /T 2 contrast. Compared to the gadolinium complex, the contrast of
IO NPs shows higher due to its superparamagnetic properties when converted into
nano form [14]. Likewise, other materials have also been explored and compared
with the conventional contrast used in specific imaging modalities. For example,
gold nanoparticles (GNPs) (mCT and photoacoustic tomography (PAT)), quantum
dots (mOI), and nanobubbles (mUS) show a better contrast compared to conventional agents [6]. Nano based contrast agents for pre-clinical imaging applications
are not entirely replaced with the conventional contrast agents. Figure 4 shows the
general requirements in designing nanomaterials for in vivo imaging as follows [15]:
(i) high surface area due to their size, (ii) ease of functionalization, (iii) modulation in
their physicochemical properties, (iv) high drug loading efficiency, (v) multi-modal
imaging, (vi) optimization in image quality.
The size of the nanoparticles is an essential factor to be considered due to change
in contrast for CT. For instance, Dong et al. showed that different sized GNPs ranging
from 4 to 150 nm were studied in both clinical and pre-clinical modalities. Particles
with size less than 15 nm have higher X-ray attenuation and more extended blood