Aptamers have already been included into a variety of molecular imaging modalities such as optical images, nuclear imaging, computed tomography (CT), magnetic
resonance imaging (MRI), and ultrasound. They differ in type, spatial resolution,
sensitivity, imaging time, and cost of currently available common systems. Details
are discussed below.
3.1 Optical Imaging
Optical imaging is one of the molecular imaging modalities that has been more
studied, including both in vitro and in vivo approaches. It is based on detecting the
transmission of light (photons) [59]. This includes photon emission in the range
between ultraviolet and infrared wavelengths.
Luminescence comprises the emission of photons produced by different phenomena, i.e., chemical or biological reactions, in the visible and near-infrared spectra
(400–1,700 nm). Fluorescence is more frequently applied and based on photons that
are emitted after light absorption from an external source. For fluorescence imaging
acquisition, the fluorophore is excited by a laser, and the light is detected by an
appropriate camera.
In the near-infrared (NIR) range wavelength (650–1,000 nm), background interference is minimum because of considerably lower tissue absorption coefficient in
the region [60].
Thus, near-infrared (NIR) optical imaging is widely used for in vivo applications
with adequate fluorophores [61], providing a micron-scale resolution at 0.2 mm
depth [62, 63].
These imaging modalities can principally be used for imaging of surface pathologies and surgically exposed organs or for intraoperative imaging but are limited in
their imaging depth (cm) and resolution [64]. Promising applications in the clinic
include fluorescence endoscopy, optical coherence tomography, and confocal
microendoscopy. These methods, collectively termed “optical biopsy,” are nondestructive in situ assays of mucosal histopathologic states using light that can provide
instantaneous tissue assessment, alternative to conventional biopsy [65, 66]. Recent
technological advances in fiber optics, light sources, and detectors have stimulated
the development of numerous optical methods that promise to significantly improve
our ability to visualize and evaluate the human epithelium in vivo.
To create these imaging probes, direct conjugation by covalent attachment of
fluorophores to aptamers is widely used. Either a standard chemical reaction at the 5
0
or 3
0 prime ends [67] or fluorescent-labeled bases [68] are commonly used (see
Fig. 2). Many of such produced aptamers are promising imaging probes for in vivo
studies [67–71].
A very useful strategy for molecular imaging consists of designing the probe
with a fluorophore and an acceptor included in the aptamer system. This fluorescence resonance energy transfer (FRET) can be detected as a consequence of
Aptamers in Diagnostic and Molecular Imaging Applications
151
resonance imaging (MRI), and ultrasound. They differ in type, spatial resolution,
sensitivity, imaging time, and cost of currently available common systems. Details
are discussed below.
3.1 Optical Imaging
Optical imaging is one of the molecular imaging modalities that has been more
studied, including both in vitro and in vivo approaches. It is based on detecting the
transmission of light (photons) [59]. This includes photon emission in the range
between ultraviolet and infrared wavelengths.
Luminescence comprises the emission of photons produced by different phenomena, i.e., chemical or biological reactions, in the visible and near-infrared spectra
(400–1,700 nm). Fluorescence is more frequently applied and based on photons that
are emitted after light absorption from an external source. For fluorescence imaging
acquisition, the fluorophore is excited by a laser, and the light is detected by an
appropriate camera.
In the near-infrared (NIR) range wavelength (650–1,000 nm), background interference is minimum because of considerably lower tissue absorption coefficient in
the region [60].
Thus, near-infrared (NIR) optical imaging is widely used for in vivo applications
with adequate fluorophores [61], providing a micron-scale resolution at 0.2 mm
depth [62, 63].
These imaging modalities can principally be used for imaging of surface pathologies and surgically exposed organs or for intraoperative imaging but are limited in
their imaging depth (cm) and resolution [64]. Promising applications in the clinic
include fluorescence endoscopy, optical coherence tomography, and confocal
microendoscopy. These methods, collectively termed “optical biopsy,” are nondestructive in situ assays of mucosal histopathologic states using light that can provide
instantaneous tissue assessment, alternative to conventional biopsy [65, 66]. Recent
technological advances in fiber optics, light sources, and detectors have stimulated
the development of numerous optical methods that promise to significantly improve
our ability to visualize and evaluate the human epithelium in vivo.
To create these imaging probes, direct conjugation by covalent attachment of
fluorophores to aptamers is widely used. Either a standard chemical reaction at the 5
0
or 3
0 prime ends [67] or fluorescent-labeled bases [68] are commonly used (see
Fig. 2). Many of such produced aptamers are promising imaging probes for in vivo
studies [67–71].
A very useful strategy for molecular imaging consists of designing the probe
with a fluorophore and an acceptor included in the aptamer system. This fluorescence resonance energy transfer (FRET) can be detected as a consequence of
Aptamers in Diagnostic and Molecular Imaging Applications
151