*2.1.3.1 Mechanism of technetium-99 m labeled RBCs*

Technetium-99 m labeled RBCs are used to foresee the exact location of Gastrointestinal bleeding. Because the blood from hemorrhaged vessel leaks-out and piles in GI-tract. So, radio-images using technetium-99 m labeled RBCs tracer shows the exact pinpoint location of hemorrhage.

After administration, technetium-99 m labeled RBCs speedily disperse in the vascular spaces. Small amount to activity could be observed in the urinary tract, that is due to free activity. No considerable GI bleeding is visualized in the early angiographs. But with time lapse the angiographs shows the bleeding in case of hemorrhage (blood move out of the disrupted area and the tracers in blood give exact scan of image [5].

The other examples include:


### *2.1.4 Flow within the compartment*

The changes in extent, rate and direction of compartmental flow is the consequence of some pathophysiological changes, that needs to be assessed and treated.

**91**

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

**2.2 Passive diffusion/simple diffusion**

molecules could not cross the membrane system.

molecule could pass when in non-ionizes state (lipophilic).

There are certain characteristics of passive diffusion:

• It is fast at high conc. Gradient and slow at low conc. Gradient.

• It does not need any sort of input as it is a passive method.

move across the membranes.

than this size is thus restricted.

method.

bladder [6].

99mTc-sulfur colloids is preferably used for studying the rate at which gastric contents are emptied in the stomach by. The reason why 99mTc-sulfur colloid is suited for this study is that it is not absorbed by Gastrointestinal tract. For assessment of solid emptying rate 99mTc-sulfur colloid is bound in scrambled eggs, while for liquid emptying rate 99mTc-sulfur colloid is mixed in drinking water. Then experimental values are compared to normal values with a margin of ±2 standard deviation. The presence and rate of back reflux of contents (due to infections) of urinary bladder to kidney is also studied by the scans taken at specific time lapse using 99mTc-sulfur colloid (SC). This tracer is implanted by means of a catheter in

Passive diffusion refers to the random motion from higher to lower concentration of molecules to attain uniformity. Diffusion of tea into water from teabag is most common example of passive diffusion. But typically, in a biologic system this movement comprises motion of molecules across the membrane. Factors like pH, ionization, size of molecule and lipid solubility affect the mobility of molecules to

Lipid solubility is the primary factor as phospholipids, glycolipids, sphingolipids, and sterols are the common types of lipids that make up membranes, of which phospholipid is the chief constituent. So, only the molecules that are soluble in lipids (lipophilic) could move across the membranes while polar hydrophilic

pH and ionization impact also influence the mobility of molecules, as all molecules bear different charge (either neutral or charged according to pH). As pH varies the ionization state also varies. Like amines maybe neutral at high pH values and protonates at low pH values. So, according to the pH of surrounding, molecule could not move across membrane when ionized state (hydrophilic), but same

Size of molecules is another important parameter, allowing only molecules of certain size to pass through the pores on membranous surface. Generally, the particles that weigh less than 80 Daltons could pass only, the entry of molecules greater

• Concentration gradient is required for this type of movement. The membranes in human body segregates this concentration gradient so, in biologic systems it is movement across the membrane (from higher to lower concentration).

• It is a non-selective process because no carriers or receptors are included in this

Many radiotracers are localized in the targeted organs by mechanism of passive diffusion. And its flow is invariably from areas of high tracer concentration to lower. Initially, diffusion rate is in direct proportionality to the tracer concentration, until equilibrium is achieved. 133Xe,127Xe, and 81mKr are commonly used for ventilation and have non-reactive lipophilic nature. After administering the tracer via inhalation, diffusion process operates, and the ventilation gas is scattered in airways *Localization Mechanisms of Radiopharmaceuticals DOI: http://dx.doi.org/10.5772/intechopen.94099*

*Medical Isotopes*

hindrance.

*2.1.3 Outflow from the compartment*

*2.1.3.1 Mechanism of technetium-99 m labeled RBCs*

shows the exact pinpoint location of hemorrhage.

compartmental leakage.

exact scan of image [5].

fluid.

The other examples include:

*2.1.4 Flow within the compartment*

The decreased concentration of the radiotracer in a compartmental cavity is usu-

• Xenon-133 ventilation imaging of lungs is used to confirm the obstruction in airways of lungs. This radiotracer will not be present past the block in the case of complete obstruction. While in partial hindrance, Xenon-133 would not be present in affected region after preliminary breathing but with time thru equilibrium rebreathing the radiotracer travel through the areas of partial

• The obstruction in cerebrospinal fluid space could be monitored by intrathecal injecting 111Indium-pentetate (DTPA). After injection 111Indium-DTPA normally drifts up the spine and all over the brain. But in case of obstructing

An uncharacteristic escape of content from compartmental space occurs owing to some pathologic changes (disturbances in normal physiological function). Radio pharmacy has a good lot of tracers that can precisely sense and find the location of

hydrocephalus, hindrance impedes the movement of 111In-DTPA [4].

Technetium-99 m labeled RBCs are used to foresee the exact location of Gastrointestinal bleeding. Because the blood from hemorrhaged vessel leaks-out and piles in GI-tract. So, radio-images using technetium-99 m labeled RBCs tracer

After administration, technetium-99 m labeled RBCs speedily disperse in the vascular spaces. Small amount to activity could be observed in the urinary tract, that is due to free activity. No considerable GI bleeding is visualized in the early angiographs. But with time lapse the angiographs shows the bleeding in case of hemorrhage (blood move out of the disrupted area and the tracers in blood give

• 111In-DTPA imaging is employed to demonstrate the leakage of cerebrospinal

• Post cholecystectomy, 99mTc-disofenin or 99mTc-mebrofenin could be used to

• The 99mTc-MAG3, a peptide, binds 99mTc and could be employed to assess reno-vascular hypertension, kidney-transplant, hydronephrosis and urological anomalies. 99mTc-MAG3 (mercaptoacetyltriglycine) is also used to track leakage of urine into the abdominal cavity. Usually after kidney or urinary tract surgeries this complication occurs and urine seeps into the abdominal cavity.

The changes in extent, rate and direction of compartmental flow is the consequence of some pathophysiological changes, that needs to be assessed and treated.

monitor whether the bile is leaking out in abdomen or not.

ally the outcome of block in the cavity as mentioned in following examples:

**90**

99mTc-sulfur colloids is preferably used for studying the rate at which gastric contents are emptied in the stomach by. The reason why 99mTc-sulfur colloid is suited for this study is that it is not absorbed by Gastrointestinal tract. For assessment of solid emptying rate 99mTc-sulfur colloid is bound in scrambled eggs, while for liquid emptying rate 99mTc-sulfur colloid is mixed in drinking water. Then experimental values are compared to normal values with a margin of ±2 standard deviation. The presence and rate of back reflux of contents (due to infections) of urinary bladder to kidney is also studied by the scans taken at specific time lapse using 99mTc-sulfur colloid (SC). This tracer is implanted by means of a catheter in bladder [6].

## **2.2 Passive diffusion/simple diffusion**

Passive diffusion refers to the random motion from higher to lower concentration of molecules to attain uniformity. Diffusion of tea into water from teabag is most common example of passive diffusion. But typically, in a biologic system this movement comprises motion of molecules across the membrane. Factors like pH, ionization, size of molecule and lipid solubility affect the mobility of molecules to move across the membranes.

Lipid solubility is the primary factor as phospholipids, glycolipids, sphingolipids, and sterols are the common types of lipids that make up membranes, of which phospholipid is the chief constituent. So, only the molecules that are soluble in lipids (lipophilic) could move across the membranes while polar hydrophilic molecules could not cross the membrane system.

pH and ionization impact also influence the mobility of molecules, as all molecules bear different charge (either neutral or charged according to pH). As pH varies the ionization state also varies. Like amines maybe neutral at high pH values and protonates at low pH values. So, according to the pH of surrounding, molecule could not move across membrane when ionized state (hydrophilic), but same molecule could pass when in non-ionizes state (lipophilic).

Size of molecules is another important parameter, allowing only molecules of certain size to pass through the pores on membranous surface. Generally, the particles that weigh less than 80 Daltons could pass only, the entry of molecules greater than this size is thus restricted.

There are certain characteristics of passive diffusion:


Many radiotracers are localized in the targeted organs by mechanism of passive diffusion. And its flow is invariably from areas of high tracer concentration to lower. Initially, diffusion rate is in direct proportionality to the tracer concentration, until equilibrium is achieved. 133Xe,127Xe, and 81mKr are commonly used for ventilation and have non-reactive lipophilic nature. After administering the tracer via inhalation, diffusion process operates, and the ventilation gas is scattered in airways of lungs. The flow is smooth until and unless discontinued due to the presence of hinderance in airways. After entering the pulmonary circulation, gases leave lungs by alveolar-capillary diffusion method.
