3.2 Aptamer-Based CT and MR Imaging
Computed tomography (CT) and magnetic resonance imaging (MRI) are conventional imaging techniques commonly used in clinics, known as anatomical imaging
modalities. Both usually use contrasting agents, which alter image contrast to help
distinguish between normal and abnormal conditions. However, using a proper
probe, these modalities can provide more information. Using aptamers as targeting
components, these modalities can be part of the molecular imaging field.
In CT, the patient is exposed to an external source of X-rays, and the image is
based on X-ray attenuation by tissue. Analogous to the 2D X-rays, CT includes a
rotating detector producing a 3D image. Results include anatomical information with
high resolution, and there is no limitation on tissue depth. Soft tissues have low
X-ray attenuation, and CT results are poor in contrast. Elements with high atomic
weight, such as iodine, are administrated to increase the attenuation but are generally
not specific and require considerable quantities. To overcome these hurdles, molecular imaging approaches have been developed using contrast agents attached to
aptamers for CT molecular imaging [80, 81].
Magnetic resonance imaging (MRI) offers a primarily diagnostic method, given
its high soft tissue contrast, spatial resolution, low risk, availability, and relatively
low cost [82]. For anatomical and physiological information, MRI is an excellent
technique.
Images are obtained by exposing endogenous nuclei (most commonly
1 H in H 2 O)
to a static magnetic field, perturbing a steady-state equilibrium with time and space
varying magnetic fields. After perturbation, all nuclei relax by two unique and
co-dependent relaxation mechanisms: T1 (spin-lattice relaxation) and T2 (spin–
spin relaxation) [64]. Images depend on different levels of magnetic nuclei relaxation in tissues.
MRI also provides micrometer resolution, and contrast agents are frequently
used. Commonly, these agents include the injection of small complexes based on
gadolinium (Gd
3+ ) or include biocompatible superparamagnetic iron oxide
nanoparticles (SPIONs) [83, 84], which increase the relaxation times.
The development of MRI has advanced to create contrast agents based on specific
targeting strategies. Aptamers have been complexed with contrast agents to develop
MRI-based molecular imaging [85]. Some authors indicate SPION disrupted conformational changes due to aptamer-target interactions displaying larger relaxation
values [86, 87]. Recently, preclinical studies showed a target aptamer conjugated to
the gadolinium on nanocarriers with promising results [88]. A multimodal
nanoprobe was designed against a type I membrane glycoprotein, endoglin
(CD105), which is expressed abundantly in the tumor periphery of hepatocellular
carcinoma. The nanoprobe included a single-stranded DNA aptamer conjugated
with gadolinium in a G5 dendrimer (aptamer/DTPA/Gd/PEG/dendrimer). The invasive tumor margins were clearly delineated in a model mouse tumor [89].
Aptamers in Diagnostic and Molecular Imaging Applications
153
Computed tomography (CT) and magnetic resonance imaging (MRI) are conventional imaging techniques commonly used in clinics, known as anatomical imaging
modalities. Both usually use contrasting agents, which alter image contrast to help
distinguish between normal and abnormal conditions. However, using a proper
probe, these modalities can provide more information. Using aptamers as targeting
components, these modalities can be part of the molecular imaging field.
In CT, the patient is exposed to an external source of X-rays, and the image is
based on X-ray attenuation by tissue. Analogous to the 2D X-rays, CT includes a
rotating detector producing a 3D image. Results include anatomical information with
high resolution, and there is no limitation on tissue depth. Soft tissues have low
X-ray attenuation, and CT results are poor in contrast. Elements with high atomic
weight, such as iodine, are administrated to increase the attenuation but are generally
not specific and require considerable quantities. To overcome these hurdles, molecular imaging approaches have been developed using contrast agents attached to
aptamers for CT molecular imaging [80, 81].
Magnetic resonance imaging (MRI) offers a primarily diagnostic method, given
its high soft tissue contrast, spatial resolution, low risk, availability, and relatively
low cost [82]. For anatomical and physiological information, MRI is an excellent
technique.
Images are obtained by exposing endogenous nuclei (most commonly
1 H in H 2 O)
to a static magnetic field, perturbing a steady-state equilibrium with time and space
varying magnetic fields. After perturbation, all nuclei relax by two unique and
co-dependent relaxation mechanisms: T1 (spin-lattice relaxation) and T2 (spin–
spin relaxation) [64]. Images depend on different levels of magnetic nuclei relaxation in tissues.
MRI also provides micrometer resolution, and contrast agents are frequently
used. Commonly, these agents include the injection of small complexes based on
gadolinium (Gd
3+ ) or include biocompatible superparamagnetic iron oxide
nanoparticles (SPIONs) [83, 84], which increase the relaxation times.
The development of MRI has advanced to create contrast agents based on specific
targeting strategies. Aptamers have been complexed with contrast agents to develop
MRI-based molecular imaging [85]. Some authors indicate SPION disrupted conformational changes due to aptamer-target interactions displaying larger relaxation
values [86, 87]. Recently, preclinical studies showed a target aptamer conjugated to
the gadolinium on nanocarriers with promising results [88]. A multimodal
nanoprobe was designed against a type I membrane glycoprotein, endoglin
(CD105), which is expressed abundantly in the tumor periphery of hepatocellular
carcinoma. The nanoprobe included a single-stranded DNA aptamer conjugated
with gadolinium in a G5 dendrimer (aptamer/DTPA/Gd/PEG/dendrimer). The invasive tumor margins were clearly delineated in a model mouse tumor [89].
Aptamers in Diagnostic and Molecular Imaging Applications
153