Week 08 Isotope Diagnostics Lab Flashcards
What are the two main categories of isotope diagnostic examinations?
**static **- only the momentary spatial distribution of the isotope is measured
dynamic - the distribution of the isotope is measured in relation to time
What is the parent isotope used in a Tc-generator?
99Mo (Molybdenum)
Describe the process of Technitium generation from its parent isotope via decay processes.
- 99Mo undergoes β- decay (66-hr T1/2)
- decayed 99Mo forms metastable 99mTc
- **99mTc undergoes γ decay **(6-hr T1/2)
- **99Tc **forms
How is Mo applied to the Tc generator?
as a water insoluble ammonium salt (ammonium molybdenate) adsorbed to aluminum oxide
NH4MoO4 > Al2O3
How is metastable Tc separated from Mo and removed from the generator?
- 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
- NamTcO4washes out of the column into a vacuum vial, while Mo stays behind
How are γ-radiating isotopes usually administered into the body?
- attached to a carrier organic compound as a radiopharmaceutical
- designed to bind selectively to the target organ
What are the 3 different types of half lives of an isotope used in in-vivo diagnostics?
- physical - decay observed “on the table” without biological interference from metabolism, etc.
- biological - decay as a result of metabolism, detoxification processes of the body, etc.
- effective - combination of both physical and biological
How is effective half life calculated?
the reciprocal of effective half life is equal to the sum of the reciprocals of both physical and biological half lives
What must be considered about half life when selecting an isotope?
- 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
What should be considered about photon energy when selecting an isotope?
- must be sufficiently high that it is not too heavily absorbed
(low E photons absorb more)
How is the minimal needed activity of the isotope determined?
What is the typical activity range of isotopes used?
**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
What kind of graph can be made to follow the metabolic processes of an organ undergoing an isotope-diagnostic procedure?
Draw it.
isotope accumulation curve
- a graph of % administered activity vs. time
What does the time before activity is seen on an isotope accumulation curve represent?
What is it called?
lag phase or minimal transit time
transport of the isotope to the organ
via blood vessels, GI tract, etc.
What does the slope of the ascending part of the iso acc curve represent?
uptake rate
- rate at which the isotope enters the organ
- also called clearance… but that sounds paradoxical and stupid
What does the peak of the isotope accumulation curve represent?
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)
What is the most important part of the descending part of the isotope accumulation curve?
What can be calculated from this?
effective half life (Teff)
- determined by the time between Λ0 and Λ0/2
- can be used to calculate biological half-life (using also the known physical half-life)
What does the area under an isotope accumulation curve represent?
mean isotope content of the organ during the examination period
What is the index of refraction for γ-radiation of different materials?
How does this effect mapping techniques for γ-radiation detection?
n = 1 for all materials
- because of this, refraction-based imagine systems are not used
- collimator systems are used instead
What are collimators?
How do they help in radio-isotope diagnostic imaging?
- 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
What determines spatial resolution of a radio-isotope imagine detector?
What other aspect of imaging is affected by the same resolution-determining factor?
width of the collimator hole
- smaller holes = higher res
sensitivity is also affected
- smaller holes = lower sensitivity
… so there is a trade-off
What is the simplest imaging device for radio-isotope diagnostics?
Describe its use and draw it.
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
What imaging methods is an improved version of the scinitigraph?
How does it work?
Draw it.
Gamma Camera
- large collimator w/ many holes
- large scintillation crystal above collimator
- many PMTs
- computer-calculated 2D image
What imaging method is a further improvement upon gamma cameras?
How does it work?
SPECT
Single Positron Emission Computed Tomography
- 1 or more gamma cameras record from several directions
- computer reconstructs 3D image
What imaging method uses a β-radiating isotope?
How does it work?
Draw it.
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
