Week 08 Isotope Diagnostics Lab

Question 1

What are the two main categories of isotope diagnostic examinations?

Answer 1

**static **- only the momentary spatial distribution of the isotope is measured

dynamic - the distribution of the isotope is measured in relation to time

Question 2

What is the parent isotope used in a Tc-generator?

Answer 2

⁹⁹Mo (Molybdenum)

Question 3

Describe the process of Technitium generation from its parent isotope via decay processes.

Answer 3

1. ⁹⁹Mo undergoes β^- decay (66-hr T_1/2)
2. decayed ⁹⁹Mo forms metastable ^99mTc
3. **^99mTc undergoes γ decay **(6-hr T_1/2)
4. **⁹⁹Tc **forms

Question 4

How is Mo applied to the Tc generator?

Answer 4

as a water insoluble ammonium salt (ammonium molybdenate) adsorbed to aluminum oxide

NH₄MoO₄ > Al₂O₃

Question 5

How is metastable Tc separated from Mo and removed from the generator?

Answer 5

• physiological NaCl solution is run over the Mo, some of which has decayed into ^mTc
• Pertechnate ions exchanged with Cl ions, while Mo ions do not
• Na^mTcO₄washes out of the column into a vacuum vial, while Mo stays behind

Question 6

How are γ-radiating isotopes usually administered into the body?

Answer 6

• attached to a carrier organic compound as a radiopharmaceutical
• designed to bind selectively to the target organ

Question 7

What are the 3 different types of half lives of an isotope used in in-vivo diagnostics?

Answer 7

1. physical - decay observed “on the table” without biological interference from metabolism, etc.
2. biological - decay as a result of metabolism, detoxification processes of the body, etc.
3. effective - combination of both physical and biological

Question 8

How is effective half life calculated?

Answer 8

the reciprocal of effective half life is equal to the sum of the reciprocals of both physical and biological half lives

Question 9

What must be considered about half life when selecting an isotope?

Answer 9

• physical half life should be comparable to biological
• effective half life should be comparable to examination time
  
  • too short - decay before measured
  • too long - unwanted exposure

Question 10

What should be considered about photon energy when selecting an isotope?

Answer 10

• must be sufficiently high that it is not too heavily absorbed

(low E photons absorb more)

Question 11

How is the minimal needed activity of the isotope determined?

What is the typical activity range of isotopes used?

Answer 11

**signal-to-noise ratio **of the measuring device determines activity because a sufficient number of photons greater than background noise levels must escape the body for a good measurement

typically 1 - 100 MBq

Question 12

What kind of graph can be made to follow the metabolic processes of an organ undergoing an isotope-diagnostic procedure?

Draw it.

Answer 12

isotope accumulation curve

• a graph of % administered activity vs. time

Question 13

What does the time before activity is seen on an isotope accumulation curve represent?

What is it called?

Answer 13

lag phase or minimal transit time

transport of the isotope to the organ

via blood vessels, GI tract, etc.

Question 14

What does the slope of the ascending part of the iso acc curve represent?

Answer 14

uptake rate

• rate at which the isotope enters the organ
• also called clearance… but that sounds paradoxical and stupid

Question 15

What does the peak of the isotope accumulation curve represent?

Answer 15

both the time of maximum uptake

and the

uptake capacity (meaning how much of the total activity the organ was able to absorb… important for paired organs such as kidneys)

Question 16

What is the most important part of the descending part of the isotope accumulation curve?

What can be calculated from this?

Answer 16

effective half life (T_eff)

• determined by the time between Λ₀ and Λ₀/2
• can be used to calculate biological half-life (using also the known physical half-life)

Question 17

What does the area under an isotope accumulation curve represent?

Answer 17

mean isotope content of the organ during the examination period

Question 18

What is the index of refraction for γ-radiation of different materials?

How does this effect mapping techniques for γ-radiation detection?

Answer 18

n = 1 for all materials

• because of this, refraction-based imagine systems are not used
• collimator systems are used instead

Question 19

What are collimators?

How do they help in radio-isotope diagnostic imaging?

Answer 19

• thick plates of lead with 1 or more holes through which only parallely-oriented γ-photons can travel
• allow for precise determination of high-activity areas

Question 20

What determines spatial resolution of a radio-isotope imagine detector?

What other aspect of imaging is affected by the same resolution-determining factor?

Answer 20

width of the collimator hole

• smaller holes = higher res

sensitivity is also affected

• smaller holes = lower sensitivity

… so there is a trade-off

Question 21

What is the simplest imaging device for radio-isotope diagnostics?

Describe its use and draw it.

Answer 21

Scintigraph

• detector with single collimator hole
• writing device moving parallel with collimator/detector
• detector scans back and forth over examined body part
• writing device moves in same pattern over paper, drawing dots of size corresponding to detected γ-radiation intensity
• creates 2D greyscale image image of isotope distribution

Question 22

What imaging methods is an improved version of the scinitigraph?

How does it work?

Draw it.

Answer 22

Gamma Camera

• large collimator w/ many holes
• large scintillation crystal above collimator
• many PMTs
• computer-calculated 2D image

Question 23

What imaging method is a further improvement upon gamma cameras?

How does it work?

Answer 23

SPECT

Single Positron Emission Computed Tomography

• 1 or more gamma cameras record from several directions
• computer reconstructs 3D image

Question 24

What imaging method uses a β-radiating isotope?

How does it work?

Draw it.

Answer 24

Positron Emission Tomography

• short half-life β-emitting isotope administered
• emitted positron is annihilated
• two γ-photons (511 keV) are released in opposite directions
• a ring of ~200 scintillation detectors detects photons
• location of radiation source is identified by intersections of the multiple pairs of detected photons
• high sensitivity due to lack of collimators
• constructs 3D images if multiple detector rings are used