42
D. Li et al.
modalities. However, ultrasonic contrast agents with size smaller than 700 nm could
pass through the vascular wall into the site of extra-vascular tumor tissues to generate
the imaging signal by enhancing the penetration and retention effects [56]. Targeted
nanobubble ultrasound contrast agents with aptamers as molecular probes still have
the clinical value to research.
2.5 Fluorescence Imaging
Recently, fluorescence imaging has gained significant attention owing to the emergence of appropriate reporter technologies that enable noninvasive detection of
molecular function in vivo. This type of imaging is similar to fluorescence microscopy
with significant advantages, including high sensitivity and contrast, real-time monitoring and feedback, and rapid acquisition, but only at the macroscopic level [57–59].
Unfortunately, unlike other molecular imaging techniques, fluorescence imaging is
more complicated in application. On the one hand, currently available small molecular fluorescent organic dyes are poorly soluble in water, highly susceptible to photobleaching and short lived. On the other hand, biological tissues absorb and disperse
photons, generate strong autofluorescence, obscuring the collection and quantification of signals. To address these limitations, mathematical models that described the
propagation of photons in tissues have made significant progress [60, 61]. Moreover,
various fluorescent nanoprobes with excellent water solubility, high quantum fluorescence yield, superior stability and long fluorescence lifetime have been broadly
investigated for sensitive and durable molecular imaging. In addition, the available illumination and detection instruments have improved. This type of imaging
is expected to apply in small rodent imaging and image-assisted human surgery [62],
and many fluorescent probes have been designed with aptamers thereof.
The aptamers can be easily modified with a variety of organic dyes, and
fluorophore-based detection method has been extensively demonstrated. Biological
tissues absorb less photons and generate less autofluorescence at the near infrared
wavelengths, fluorophores emitting in the near infrared band might be a better choice.
Epi-illumination (reflectance) imaging and fluorescence molecular tomography are
two major approaches have been used so far for fluorescence molecular imaging [63].
Epi-illumination (reflectance) imaging, known as fluorescence reflectance imaging
(FRI), provides qualitative or semiquantitative two-dimensional imaging and is
strongly affected by light scattering [64]. So, FRI is usually used for imaging surface
tissues such as subcutaneous tumor xenografts. Many aptamers against proteins
overexpressed at the surface of cancer cells are applied in FRI, which demonstrate
higher fluorescence signals at the tumor area after intravenous injection into bodies
from a few minutes up to several hours. Shi et al. presented a Cy5-labeled aptamer
TD05 (Cy5-TD05) as the fluorescence probe and tested the efficacy of aptamerbased molecular imaging in B-cell lymphoma xenograft nude mice [65, 66]. Gong
et al. generated a PEI/aptamer molecular complexes with the same aptamer, TD05,
D. Li et al.
modalities. However, ultrasonic contrast agents with size smaller than 700 nm could
pass through the vascular wall into the site of extra-vascular tumor tissues to generate
the imaging signal by enhancing the penetration and retention effects [56]. Targeted
nanobubble ultrasound contrast agents with aptamers as molecular probes still have
the clinical value to research.
2.5 Fluorescence Imaging
Recently, fluorescence imaging has gained significant attention owing to the emergence of appropriate reporter technologies that enable noninvasive detection of
molecular function in vivo. This type of imaging is similar to fluorescence microscopy
with significant advantages, including high sensitivity and contrast, real-time monitoring and feedback, and rapid acquisition, but only at the macroscopic level [57–59].
Unfortunately, unlike other molecular imaging techniques, fluorescence imaging is
more complicated in application. On the one hand, currently available small molecular fluorescent organic dyes are poorly soluble in water, highly susceptible to photobleaching and short lived. On the other hand, biological tissues absorb and disperse
photons, generate strong autofluorescence, obscuring the collection and quantification of signals. To address these limitations, mathematical models that described the
propagation of photons in tissues have made significant progress [60, 61]. Moreover,
various fluorescent nanoprobes with excellent water solubility, high quantum fluorescence yield, superior stability and long fluorescence lifetime have been broadly
investigated for sensitive and durable molecular imaging. In addition, the available illumination and detection instruments have improved. This type of imaging
is expected to apply in small rodent imaging and image-assisted human surgery [62],
and many fluorescent probes have been designed with aptamers thereof.
The aptamers can be easily modified with a variety of organic dyes, and
fluorophore-based detection method has been extensively demonstrated. Biological
tissues absorb less photons and generate less autofluorescence at the near infrared
wavelengths, fluorophores emitting in the near infrared band might be a better choice.
Epi-illumination (reflectance) imaging and fluorescence molecular tomography are
two major approaches have been used so far for fluorescence molecular imaging [63].
Epi-illumination (reflectance) imaging, known as fluorescence reflectance imaging
(FRI), provides qualitative or semiquantitative two-dimensional imaging and is
strongly affected by light scattering [64]. So, FRI is usually used for imaging surface
tissues such as subcutaneous tumor xenografts. Many aptamers against proteins
overexpressed at the surface of cancer cells are applied in FRI, which demonstrate
higher fluorescence signals at the tumor area after intravenous injection into bodies
from a few minutes up to several hours. Shi et al. presented a Cy5-labeled aptamer
TD05 (Cy5-TD05) as the fluorescence probe and tested the efficacy of aptamerbased molecular imaging in B-cell lymphoma xenograft nude mice [65, 66]. Gong
et al. generated a PEI/aptamer molecular complexes with the same aptamer, TD05,