Isotope diagnostics Flashcards
What are “in vivo” isotope diagnostics?
How do you call the administered substances?
What type of measurements can you do?
host molecule labeled with the radioactive isotope (= radiotracer) that is administered accumulates in the target organ
⇒ either:
- emitted γ-radiation detected outside the body and the spatial distribution is reconstructed
- time sequence of the spatial distribution (time change of the activity), the isotope accumulation curve can be calculated by the computer
Describe the aspects of the selection of the isotope that must be considered for in vivo isotope diagnostics?
- only γ emitting isotopes
- physical half-life should match biological half-life and the time of the measurement, but all as short as possible
- photon E of the emitted γ-radiation has to be high enough to pass through the parts of the body
- activity should be high enough to reach the required signal-to-noise ratio, but it should be the lowest possible
What is the biggest advantage of radiotracer technique compared to chemical microanalytical procedures?
only concentrations 106 - 108 times lower needed
What is the most common type of isotope generator?
Explain how its isotope is produced.
Tc generator (also: Tc cow)
- 235U decays to 98Mo
- 98Mo bombarded with neutrons → 99Mo produced (= “parent” isotope)
- 99Mo undergoes β-decay with 66h half-life → excited, metastable 99mTc produced (= “daughter” isotope)
- nuclear isomerism: 99mTc loses excess energy (γ-radiation) with 6h half-life → 99Tc
⇒ 99mTc must be seperated chemically to get an isotope which emits soft, monochromatic, purely γ-radiation
Explain how 99mTc is chemically seperated from Mo.
Draw the structure of a Tc generator.
- Mo applied in form of a water insoluble ammonium molibdenate (NH4MoO4) carrier compound
- adsorbed to aluminium oxide (Al2O3) granules and placed into generator column
- NH4MoO4 decays to ammonium pertechnetate (NH4mTcO4)
- elution: NaCl-containing solution drained up through the column by upper vacuum vial
- NH4+ ions exchange with Na+ ⇒ water soluble NamTcO4 produced
⇒ γ-radiating 99mTc washed off → gets into vacuum vial (removable and used for diagnostics)
What is the initial activity of Mo?
Explain the time course of the formation and elution of the mTc in the Tc generator using a graph.
initial activity of Mo = 5 GBq
⇒ mTc can be “milked” every 24h for about one week with decreasing activites
How do you call the organic carrier compound where the isotope is attached in order to administer it?
radiopharmaceutical
Explain the different types of half-life.
Why are they an important aspect of the selection of isotopes?
Relate them in one graph.
- physical half-life (Tphys): in vitro half-life
- biological half-life (Tbiol): half-life in organism if there was no physical decay
- effective half-life (Teff): Tphys + Tbiol
diagnostic relevance:
⇒ Tphys not too short, would end before end of examination
⇒ Tphys not too long, would cause unwanted exposure
Which photon energies are used in practice for isotope diagnostics?
Why?
Eγ > 100 keV
⇒ if E too low, radiation absorbed by the tissue
Which activities are used in practice for isotope diagnostics?
Why?
What does it depend on?
1 - 100 MBq
⇒ minimal activity depends on signal-to-noise ratio of the measuring device (if low ratio, lower activity needed)
What is the difference btw dynamic and static examinations?
- static examinations: spatial distribution of isotope measured
- dynamic examinations: time dependence of activity measured (2+ measurements)
What is an isotope accumulation curve?
Draw the graph and explain its different regions.
What are the most important parameters that can be taken from it?
represents time dependence of the activity (in fractions) in the organ
-
lag time: time until appearance of the activity T0
⇒ transport capacity to the organ -
ascending slope = clearance
⇒ uptake rate of the organ -
Tmax: time until reaching maximum activity
⇒ uptake + elimination activity, esp. important when paired organs are compaired -
descending slope = decay + elimination
⇒ Teff, Tbiol can be calculated - integral: mean isotope content
Why is the size of the collimator hole relevant for the measurement?
smaller hole → higher resolution
more precise location of isotopes since only parallel γ-photons measured
bigger hole → higher sensitivity
lower activity needed since more γ-photons are measured
What is a scintigraph?
- detector equipped with a single collimator hole scans the examined area
- activity values measured at different locations are recorded by a plotter device moving parallel with the detector
- spots of different sizes (brightness or color) resemble activity at the corresponding locations
What is a gamma camera?
What is a SPECT?
Draw a gamma camera.
Gamma camera:
- collimator plate (numerous holes), a large size scintillation crystal and several PMTs
- computer reconstructs 2D image
SPECT:
- Single Photon Emission Computed Tomography
- 1+ gamma camera measuring heads
- pictures from different directions ⇒ 3D image
What is a PET?
What is its main advantage over a gamma camera or SPECT?
Positron Emission Tomography
- β-radiating isotope administered
- annihilation occurs when e+ and e- collide → 2 γ-photons of 511 keV produced that travel into opposite directions
- detected by detectors in a ring around the patient
- coincidence of the detected photons analyzed andlocation of the decays located → 2D or 3D image (if several rings) constructed
⇒ very sensitive since no collimators used, but image is constructed according to differences in the timing of γ-photons striking the detectors
Draw the graph of the experimental setup used for the measurement.
