3.3 Ultrasound Imaging with Aptamers
Ultrasound is one of the most widely used conventional imaging modalities in the
world because it is the most accessible (transportable) and safe and incurs a lower
cost. Here, high frequency sound waves are transmitted to the tissue and scattered by
differences in density and compressibility. Micrometer resolution and real-time
images demonstrate the utility of this anatomical imaging modality.
However, the visuality and veracity of ultrasound imaging for the diagnostic and
prognostic prediction of some pathologies are controversial [90]. Limitations include
depth and inability to discern abnormalities from surrounding tissue, owing to the
similar physical properties of soft tissues within the body [91].
New ultrasound probes have been designed to include this modality into molecular imaging using aptamers [92]. This includes modified contrast agents as probes
for target recognition. Multi-walled carbon nanotubes have been reported as
promising ultrasound contrast material due to their particular structure and properties. Recently, these nanostructures with pegylated aptamers were studied as ultrasound contrast agent [93]. In this case, an aptamer anti-PSMA was attached to the
surface of multi-walled carbon nanotubes to further enhance their targeting ability
and biocompatibility. The nanotubes were modified with a bifunctional polyethylene
glycol (by amino and carboxyl group), and the aptamer was attached. The intravenous injection of the probe was performed in BALB/c xenograft nude mice models
of a prostate cancer. The results showed a better visuality and veracity of the images
as compared with the traditional contrast agent [93]. This exemplary work could
have a significant impact on the improvement and adaptation of ultrasound contrast
agents, available for new developments, but it still remains in the preclinical phase.
3.4 PET and SPECT Imaging
Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are both medical imaging modalities. These diagnostic techniques
include the use of radioactivity for high tissue penetration and are well-known
nuclear imaging or emission tomography imaging techniques.
The basis of these modalities consists of detecting emitted high energy photons
prevenient of a radioactive decay. They include the administration of a radioactive
probe to the patient, known as radiopharmaceutical formulation, generally by intravenous injection. After distribution, the probe reaches the pathology area, and its
retention there is a result of transport, binding to receptors and antigens, enzymatic
reactions, biochemical or mechanistic trapping, phagocytosis, or combinations of
the aforementioned [56]. Finally, the probe emission is measured by an external
detector.
Thus, essentially PET and SPECT are molecular imaging techniques, with poor
morphologic information. Despite millimeter spatial resolution, both modalities
154
V. Calzada
Ultrasound is one of the most widely used conventional imaging modalities in the
world because it is the most accessible (transportable) and safe and incurs a lower
cost. Here, high frequency sound waves are transmitted to the tissue and scattered by
differences in density and compressibility. Micrometer resolution and real-time
images demonstrate the utility of this anatomical imaging modality.
However, the visuality and veracity of ultrasound imaging for the diagnostic and
prognostic prediction of some pathologies are controversial [90]. Limitations include
depth and inability to discern abnormalities from surrounding tissue, owing to the
similar physical properties of soft tissues within the body [91].
New ultrasound probes have been designed to include this modality into molecular imaging using aptamers [92]. This includes modified contrast agents as probes
for target recognition. Multi-walled carbon nanotubes have been reported as
promising ultrasound contrast material due to their particular structure and properties. Recently, these nanostructures with pegylated aptamers were studied as ultrasound contrast agent [93]. In this case, an aptamer anti-PSMA was attached to the
surface of multi-walled carbon nanotubes to further enhance their targeting ability
and biocompatibility. The nanotubes were modified with a bifunctional polyethylene
glycol (by amino and carboxyl group), and the aptamer was attached. The intravenous injection of the probe was performed in BALB/c xenograft nude mice models
of a prostate cancer. The results showed a better visuality and veracity of the images
as compared with the traditional contrast agent [93]. This exemplary work could
have a significant impact on the improvement and adaptation of ultrasound contrast
agents, available for new developments, but it still remains in the preclinical phase.
3.4 PET and SPECT Imaging
Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are both medical imaging modalities. These diagnostic techniques
include the use of radioactivity for high tissue penetration and are well-known
nuclear imaging or emission tomography imaging techniques.
The basis of these modalities consists of detecting emitted high energy photons
prevenient of a radioactive decay. They include the administration of a radioactive
probe to the patient, known as radiopharmaceutical formulation, generally by intravenous injection. After distribution, the probe reaches the pathology area, and its
retention there is a result of transport, binding to receptors and antigens, enzymatic
reactions, biochemical or mechanistic trapping, phagocytosis, or combinations of
the aforementioned [56]. Finally, the probe emission is measured by an external
detector.
Thus, essentially PET and SPECT are molecular imaging techniques, with poor
morphologic information. Despite millimeter spatial resolution, both modalities
154
V. Calzada