**2. Mechanism of localization**

The success of the molecular imaging technique using radioisotope labeled molecules commonly termed as radiopharmaceuticals relies on the mechanism of localization at disease cells. In the following sections we are explaining different mechanisms of radiopharmaceutical localization undertaken either for imaging process or therapy of diseases.

#### **2.1 Compartmental localization**

Generally, the phenomenon in which the desired species are disseminated in a bounded space is named as compartmentalization or may also be termed as compartmental localization and basically this bounded space is called as a compartment. Specifically, in radio pharmacy compartment-localization means to put a radiotracer in a bounded space and sustaining the tracer for time being enough to scan that bounded space. The bounded space contains fluids (either liquid or gas). The fluids of compartment move systematically in normal circumstances but the pathophysiological changes cause anomalies in the motion of compartmental fluids. These conditions if left unattended and untreated may become fatal. But the conventional diagnostic techniques fail to localize the exact location of abnormality, so, here radio pharmacy provides refuge and we can get exact pinpoint location along with treatment from molecular imaging.

**89**

*Localization Mechanisms of Radiopharmaceuticals DOI: http://dx.doi.org/10.5772/intechopen.94099*

a.Uniform distribution inside compartment

c.Outflow from compartment

d.Flow within the compartment

plasma in blood.

Examples:

tracer in affected kidney.

*2.1.1 Uniform distribution inside compartment*

the blood's cellular part uniformly.

*2.1.2 Non-uniform distribution within compartment*

b.Non-uniform distribution within compartment

The compartments in biological systems are: Vascular system (blood vessels), Airways of lungs, cerebrospinal fluid (CSF) space, Abdominal (peritoneal) cavity,

Vascular system is the most typical example of uniform dispersion inside compartment. By utilizing the tracer dilution method blood volume could be analyzed quantitatively. A radiopharmaceutical named I-125 RISA (Radio-Iodinated Serum Albumin) diffuses uniformly in blood plasma, is employed to determine volume of

• Cr-51 labeled RBCs is another radiotracer that is applied to evaluate the mass of red cells (volume of red cells in blood). This radiotracer distributes itself inside

• Another radiotracer technetium-99 m labeled RBCs homogenously diffuses in blood, is used to evaluate expulsion fraction of left ventricle and movement of

Radiopharmaceuticals are not distributed equally every time. In some conditions they exhibit non-uniform dispersion, therefore showing disordered physiological process (due to some disease or injury). The increased concentration of a radiopharmaceutical in any organ or tissue corresponds to the disturbance in normal physi-

• Hemangioma is a condition in which a bright-red bump having extra blood vessels appears on skin and is quite rubbery. Extra blood vessels mean extra blood in that region. So, technetium-99 m labeled RBCs shows amplified

• Hydronephrosis is the inflammation of a kidney triggered by the accumulation of urine in kidney. This condition prevails when urine could not be drained out from kidney to bladder owing to some sort of obstruction or blockade. MAG3 and DTPA radiolabeled with 99mTc are used for its imaging. But, MAG3 has preference over DTPA due to its good output. Mercaptoacetyltriglycine (MAG3) is a peptide radiolabeled with 99mTc and it is released in kidney tubules. So, this increased volume of urine results in escalated amount of 99mTc-mertiatide (MAG3) or 99mTc-pentetate (DTPA)

left ventricular wall by using gated blood-pool scanning.

ological function of that organ or tissue (pathologic changes).

localization in this region due to escalated volume of blood.

Alimentary (digestive/GI) tract, urinary system, lymphatic vessels. The compartmental localization could be in the form of:

*Localization Mechanisms of Radiopharmaceuticals DOI: http://dx.doi.org/10.5772/intechopen.94099*

*Medical Isotopes*

procedures to ensure purity [1, 3].

least teff must be moderately long.

**2. Mechanism of localization**

process or therapy of diseases.

**2.1 Compartmental localization**

along with treatment from molecular imaging.

examples are given.

multi-molecular cell localization, bio-energies, tissue metabolism, physiological functioning of the organ, intercellular and intracellular communicative pathways [2]. Different radiopharmaceuticals are used to image different organs based on the functioning of the organ. For example, the labeled iodine would be ideal for imaging thyroid malignancies, because inorganic iodine absorbed more in the thyroid. Similarly, radiolabeled phosphate widely used for the bone scan as it is observed that phosphate ions more accumulated in the bone. Hence one can use the same

A radiopharmaceuticals localization mechanism is specific to targeted organs depends on processes as varied as antigen–antibody reactions, physical particle trapping, receptor site binding, removal of deliberately damaged cells from circulation, and transportation of a chemical species across a cell membrane and into the cell via a normally operational metabolic cycle. Chemically, radiochemistry plays a crucial role in producing these compounds and in conducting quality assurance

Some other factors also important for the selection and action of radiopharmaceuticals like for diagnosis gamma emitters were preferably choose (beta emitter in case of therapeutic), energy threshold 100–250 Kev, high T/NT ratio last but not

Furthermore, insoluble radiopharmaceuticals such as 99mTc-MAA and 99mTc-SC are used to represent the lungs and liver/spleen, diagnostic tests, respectively. Since it is well known that these two organs extract particles from the bloodstream, selec-

The mechanisms explained are not specific to radiopharmaceuticals, but these may be appropriate for some instances to explain the localization mechanisms of nuclear medicines. Radiopharmaceuticals are not limited to a mechanism but requires a combination of more than one mechanism. Lastly, a comprehensive overview of radiopharmaceuticals characteristics, their mode of action and detailed

The success of the molecular imaging technique using radioisotope labeled molecules commonly termed as radiopharmaceuticals relies on the mechanism of localization at disease cells. In the following sections we are explaining different mechanisms of radiopharmaceutical localization undertaken either for imaging

Generally, the phenomenon in which the desired species are disseminated in a bounded space is named as compartmentalization or may also be termed as compartmental localization and basically this bounded space is called as a compartment. Specifically, in radio pharmacy compartment-localization means to put a radiotracer in a bounded space and sustaining the tracer for time being enough to scan that bounded space. The bounded space contains fluids (either liquid or gas). The fluids of compartment move systematically in normal circumstances but the pathophysiological changes cause anomalies in the motion of compartmental fluids. These conditions if left unattended and untreated may become fatal. But the conventional diagnostic techniques fail to localize the exact location of abnormality, so, here radio pharmacy provides refuge and we can get exact pinpoint location

labeled atoms for organ imaging, which are more accumulated there.

tion based entirely on particle size instead to chemical composition.

**88**

The compartments in biological systems are: Vascular system (blood vessels), Airways of lungs, cerebrospinal fluid (CSF) space, Abdominal (peritoneal) cavity, Alimentary (digestive/GI) tract, urinary system, lymphatic vessels.

The compartmental localization could be in the form of:

