**2.6 Ion exchange**

Ion exchange is a mechanism of localization in which ions exchange between a complex like hydroxyapatite and electrolyte solution. 18F radioisotope is typically used for imaging metastatic and primary tumors present in bone. In 18F-NaF the mechanism of localization followed by this radiopharmaceutical is Ion exchange mechanism and is used for studying metabolism of bones and also for bone imaging [19]. 18F obtained from cyclotron is diluted by using 5 mL of sterile water and then passed through a sealed unit containing cation exchanger and then anion exchanger. To obtain 18F-NaF, 10 ml saline (NaCl) is added in anion exchanger. And then eluted 18F-NaF from anion exchanger is ready to inject in patients. The localization mechanism of 18F-NaF in infected area involves the exchange of fluorine anion (F− ) from hydroxyl group (OH− ) in hydroxyapatite a bone crystal [Ca10 (PO4)6 (OH) 2]. After the exchange of F− with OH− , fluoroapatite [Ca10 (PO4)6 (F) 2] is formed as shown in **Figure 6** [1].

**97**

the whole body imaging [32].

to tissues of bone by this mechanism [19].

*Ion exchange mechanism of 18F-NaF for bone deposition [31].*

**2.7 Chemisorption**

**Figure 6.**

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

When 18F-NaF injected in the body of patient its distribution depends on the blood flow of body and the amount of 18F-NaF distributed in different bones at different ratio. In bone marrow the uptake of radioisotope is almost negligible. 18F-NaF can easily diffuse through membrane and almost 30% radiotracer present in erythrocytes. 18F-NaF has a fast plasma clearance rate. For bone deposition 18F-NaF must pass through extracellular fluid with the help of plasma. The incorporation of fluoroapatite in bone in a slow process and it depends on the area of infected bone. In case of malignant bone disorder, the incorporation time of fluoroapatite is high. Fluoroapatite has low binding with plasma protein and rapidly clear from nontargeted area. After 40–45 minutes of radiotracer injection, fluoroapatite permits

Chemisorption also known as physiochemical adsorption is localization mechanism refers to the binding of phosphate-type compounds like methylene diphosphate (MDP), pyrophosphate (PYP) and hydroxy diphosphate (HDP) onto the bone surface. So, with the increase in bone metabolism like tumor, fracture and infection, surface area increases and hence there is enhanced accumulation of radiopharmaceutical at that surface. 99mTc-MDP, 99mTc-PYP, and 99mTc-HDP all bind

The administration of radiopharmaceuticals with low-energy photons that are attached chemically to moiety having affinity with hydroxyapatite a bone mineral. This attachment permits selective radiation dose to the area of interest with no or minimum radiation dose to the non-infected tissues. Out of 100%, only 40–50% dose of injected radiopharmaceutical localizes in bone and the remaining 50–60% dose is excreted from body through kidneys. Since, the

**Figure 5.** *Schematic explanation of 99mTc-DRBC preparation [30].*

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

**Figure 6.**

*Medical Isotopes*

**2.5 Cell sequestration**

**2.6 Ion exchange**

OH−

macro-aggregated albumin) have been used. This delivery mechanism necessarily involves to the capillary beds via blood flow, localization of Tc-99 m MAA is a surrogate for relative blood flow in lungs. Therefore, this perfusion lung scan with Tc-99 m MAA aggregates also used to assess blood flow in pulmonary arteries. A similar procedure in which Tc-99 m MAA is injected in hepatic artery through a catheter, it is delivered via hepatic blood flow to the capillaries in the liver [19, 28].

Potentially saturable mechanism that is mostly associated with spleen and refers to the process where damaged and old RBC's removed from circulation [29]. It is unlikely for the relatively small numbers of cells used for imaging. The radiopharmaceutical preparation is carried out by in vitro labeling of red blood cells with technetium-99 m using modified Brookhaven labeling method and then damaging

Ion exchange is a mechanism of localization in which ions exchange between a complex like hydroxyapatite and electrolyte solution. 18F radioisotope is typically used for imaging metastatic and primary tumors present in bone. In 18F-NaF the mechanism of localization followed by this radiopharmaceutical is Ion exchange mechanism and is used for studying metabolism of bones and also for bone imaging [19]. 18F obtained from cyclotron is diluted by using 5 mL of sterile water and then passed through a sealed unit containing cation exchanger and then anion exchanger. To obtain 18F-NaF, 10 ml saline (NaCl) is added in anion exchanger. And then eluted 18F-NaF from anion exchanger is ready to inject in patients. The localization mechanism of 18F-NaF in

in hydroxyapatite a bone crystal [Ca10 (PO4)6 (OH) 2]. After the exchange of F−

, fluoroapatite [Ca10 (PO4)6 (F) 2] is formed as shown in **Figure 6** [1].

) from hydroxyl group (OH−

)

with

them by heating at 49°C for fifteen minutes (**Figure 5**).

infected area involves the exchange of fluorine anion (F−

**96**

**Figure 5.**

*Schematic explanation of 99mTc-DRBC preparation [30].*

*Ion exchange mechanism of 18F-NaF for bone deposition [31].*

When 18F-NaF injected in the body of patient its distribution depends on the blood flow of body and the amount of 18F-NaF distributed in different bones at different ratio. In bone marrow the uptake of radioisotope is almost negligible. 18F-NaF can easily diffuse through membrane and almost 30% radiotracer present in erythrocytes. 18F-NaF has a fast plasma clearance rate. For bone deposition 18F-NaF must pass through extracellular fluid with the help of plasma. The incorporation of fluoroapatite in bone in a slow process and it depends on the area of infected bone. In case of malignant bone disorder, the incorporation time of fluoroapatite is high. Fluoroapatite has low binding with plasma protein and rapidly clear from nontargeted area. After 40–45 minutes of radiotracer injection, fluoroapatite permits the whole body imaging [32].

#### **2.7 Chemisorption**

Chemisorption also known as physiochemical adsorption is localization mechanism refers to the binding of phosphate-type compounds like methylene diphosphate (MDP), pyrophosphate (PYP) and hydroxy diphosphate (HDP) onto the bone surface. So, with the increase in bone metabolism like tumor, fracture and infection, surface area increases and hence there is enhanced accumulation of radiopharmaceutical at that surface. 99mTc-MDP, 99mTc-PYP, and 99mTc-HDP all bind to tissues of bone by this mechanism [19].

The administration of radiopharmaceuticals with low-energy photons that are attached chemically to moiety having affinity with hydroxyapatite a bone mineral. This attachment permits selective radiation dose to the area of interest with no or minimum radiation dose to the non-infected tissues. Out of 100%, only 40–50% dose of injected radiopharmaceutical localizes in bone and the remaining 50–60% dose is excreted from body through kidneys. Since, the

radiopharmaceutical uptake in bone is low, so imaging starts after three hours of post injection [33].
