30. Isotope diagnostics.

Question 1

Explain the importance of isotope diagnostics

Answer 1

Importance of isotope diagnostics:

due to emitted radiation of isotopes organism (metabolism, uptake, excretion, etc.) studied with high sensitivity and reliability

• monitor physiological pathways
• identify, localize pathological changes

Question 2

What are the selection principles of isotopes used for in-vivo diagnostics? What would be a good isotope for that reason?

Answer 2

Selection principles in in-vivo diagnostics:

1. **γ-radiation: **
   
   • least dangerous nuclear radiation (smalles ionization)
   • deepest penetration distance (effective range)
2. short half-life:
   
   • should match biological half life and time of measurement
   • certain level of activity needed to get a good signal
3. E_photon ~ 0.1 MeV:
   
   • too long γ-rays → high risk for the body (high absorption), but good for detection
   • too short γ-rays → low risk for the body (low absorption), but bad for detection
4. part of a biological molecule
5. cheap and easily obtained

**→ **^99mTc: emits γ-radiation, has short half life (6 hours), E = 0.14 mEv, it can be attached to biological molecules.

Question 3

How do selection principles of isotopes used for in-vitro diagnostics differ from in-vivo diagnostics? What other differences are there?

Answer 3

Selection principles in in-vitro diagnostics:

1. β- and γ-radiation
2. half-life may be longer

Aims and other differences:

• usually concentration in body fluid sample determined
• plexi plates are used for radiation protection

Question 4

Explain what static examinations are

Answer 4

Static examination:

one detection in order to visualize spatial distribution of the radioactive isotope at a certain point of time

Question 5

Explain what dynamic examinations are

Answer 5

Dynamic examination:

2+ detections in order visualize how spatial distribution of the radioactive isotope changes in time

→ isotope accumulation curve can be obtained

Question 6

What are the phases in the isotope accumulation curve? Refer to the diagram

Answer 6

Isotope accumulation curve:

after isotope injection:

1. lag phase: time until the appearance of the activity (T₀) → characterizes the transport capacity to the organ
2. clearance: slope of the curve between T and T_max characterizes the organ → uptake rate of the radioactive isotope
3. elimination: decreasing part of the curve → T_eff which characterizes the decay of the labeled substance and its elimination from the target organ

​

→ area under curve gives mean isotope content of the organ during that period

Question 7

How can half-life be classified? Define the terms and give a formula

Answer 7

Half-lifes:

• phyiscal half-life T_phys: time in which the initial no. of radioactive isotopes decreases by half, because of physical decay
• biological half-life T_bio: time in which the initial no. of radioactive isotopes decreases by half in the body, because of biological process, can be determined if a phantom is applied
• effective half-life T_eff: ​time in which the initial no. of radioactive isotopes decreases by half in the body, because of physical decay and biological processes

→ 1/T_eff = 1/T_phys + 1/T_bio

Question 8

When is a phantom used? What is it?

Answer 8

Determination of T_bio:

Comparison of physical decay of a radioactive isotope outside (= phantom.

additional decrease of the amount of isotope inside the body shoes Tbio