Biomedical Engineering Reference
In-Depth Information
and more efficient ways of labeling biological molecules with radioactive isotopes.
These biological molecules are usually composed of elements of low atomic number
(Z), which limit the choice of possible radioisotopes. This limitation arises from the
fact that conventional nuclear medicine requires gamma emitters, usually of much
larger Z and with suitable periods for diagnosis purposes. Consequently, there was
(and still exists) the need to develop ways of labeling biological molecules with
gamma emitters of high Z that do not induce substantial changes in the physiological
behavior of these molecules. Another issue is the process of image reconstruction
where the research on faster and more accurate algorithms has been a constant.
Hence, there are several aspects that may worsen the final image and, in practice,
there is almost a corresponding discipline dedicated the improvement of each of
these factors (e.g. overall efficiency of the detector, radiopharmacy,
:::
). One of
the factors that impair the quality of the image, and consequently the clinical
information, is the movement of the subject during the image acquisition.
2.1
The Signal Source
Gamma photon or positron emission is considered the signal used for nuclear
imaging, which when combined with the capacity of labeling numerous biolog-
ical molecules significantly enhances the ability to study several physiological
processes. Hence, whenever a relevant process has to be inspected, a specific
molecule is chosen and labeled with a proper radionuclide, seeking to map the entire
process and attempting to achieve all the necessary information. One of the most
important eligibility conditions for the target molecule is to be present with high
concentration, in order to enhance the signal to noise ratio even when it exists only
at infinitesimal quantities. Nonetheless, the possibility to visualize the structures
considering the spatial resolution of the scanner should also be taken into account
[
1
]. It is also critical for the success of the technique that the tracer molecule reaches
the compartment of interest which depends both on the abundance in blood plasma
and on the permeability of the biological interfaces. Actually, in order to the free
molecule be in sufficient quantity and at the right time in the compartment, not
only the molecule availability in plasma is of extreme importance but also the
nature and the kinetic of the chemical bond while the molecule is transported by
the blood. On the other hand, the transport across biological membranes depends
either on a specific transport mechanism or on an adequate nature of the substance
that allows to be passively transported. Once at the compartment, the radionuclide
labeled molecule connects not only with the target of interest (specific binding) but
also with other structures (non-specific binding). Consequently, the SNR depends
on the ratio between the specific and non-specific binding, and generally this is
the parameter that is quantified in molecular imaging. Since the specific and non-
specific binding naturally depend on the kinetics and on the extent of reaction, the
signals due to each of the bindings may be temporally separable. That's why in some
cases there is a time delay between the administration of the radiopharmaceutical
and the image acquisition.
