Production of Radionuclides Flashcards
Moly99
Can be made in 3 ways
Most common is in NUCLEAR REACTOR via Uranium fission
Yields higher specific activity product
Can also prepare by neutron capture (bombard Mo98)
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Moly-Tc generator
T: Tc, Transient equilibrium
1:10 ratio half lives dtr to parent (6 hrs:66 hrs)
vs S: Strontium (Sr), Secular equilibrium;
ie Sr-Rb generator for PET
Much bigger ration of parent:dtr half lives (Sr: 25 days, RB 72 sec)
Extract Tc by simple column chromatography
Saline passes thru Al column containing Mo and Tc;
Converts Tc to sodium pertechnetate (Na:TcO4-) which comes off the Al easily;
Moly stays fixed bc has 2 negative charges (binds tightly to + charges of Al atoms)
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Mo-Tc generator QC
After each elution, check:
Mo contamination
Al contamination
Hydrolyzed reduced Tc (TcO2)
Can be other contamination isotopes from the product of nuclear reactor fission process, but manufacturer checks for those
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Moly breakthrough
Small amt may come off column with saline
Limit: <0.15 uCi Moly / 1 mCi Tc
Detect it by using lead shield around eluate in dose calibrator;
Tx photons blocked but 740/780 keV photons from Moly get thru lead and are detected
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Aluminum contamination
If present, it interferes with prep of certain radiopharmaceuricals;
impacts RBC labeling efficiency;
Limit: <10ug/1 mL eluate solution
Test using colorimetric method–pink precipitant forms if too much Al present
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Kits to add Na pertechnetate to, to form mibi or tetrafosmin
Kit contains reducing agent (tin: stannous chloride), antioxidant
You need to test sample from your lot of the radiopharmaceutical for “radiochemical purity”–i.e. fraction of the total radioactivity from the stated radionuclide (Tc) that is present in the stated chemical form (sestamibi);
Must be 90% or higher.
contaminants are: free TcO4- (pertechnetate), hydrolyzed Tc (? TcO2)
These affect image quality bc don’t go to myocardium (light up thyroid and stomach), and affect radiation dose to pt
Test radiochemical purity using paper (thin-layer) chromatography
(Can use other chromatographic techniques or electrophoresis as well)
See migration of the various forms of Tc present to different places on the paper,
Other definitions:
“Radionuclidic purity”: how much of the total activity present comes from the intended (states) radionuclide;
eg the fraction of total activity that’s coming from Tc, vs Moly or other (fission reaction) contaminants;
These have big effects on radiation dose to pt (long half-lives)
“Chemical purity”: how much of other non-radioactive chemicals are present (eg Al), other than the chemical of interest (eg Tc)
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Thallium
Produced in cyclotron
Redistribution (bc myocardial clearance is about 3-4 hrs);
So inject at stress, then re-image 4 hrs later and those are your “rest” images
High first pass extraction (uptake is proportional to CBF at higher flow rates–ie roll-off occurs later)
Clears liver and gut rapidly–less wait time for imaging after injection
Behaves similar to K in body–Uptake is via N/K-ATPase (hence no uptake if cells not viable)
Cons:
Low energy photons (characteristic X-rays (from electron capture)–80 keV
Long half life: 74 hrs
Gated images can be done but worse quality
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Tc
Since myocardial clearance takes many hours, and don’t get redistribution, you can reimage if there was pt motion, excess liver counts, etc
Higher energy photons
Photon energy more tuned to camera performance (smaller FWHM–better energy resolution)
Cons:
Less linear relationship of uptake to CBF at higher flow rates
Need wait longer after injection to image (liver clearance an issue)
Can get artifacts related to liver/bowel activity
Tetrafosmin:
Faster liver clearance than mibi–can image sooner after inject
Lower extraction than mibi–roll-off occurs at lower CBF rate
Slightly worse imaging performance than mibi and Thall in head to head studies, but small differences in defect extent not clinically significant
Teboroxime:
Much higher extraction, better proportional uptake vs CBF than other Tc tracers
Fast myocardial washout: don’t need wait long time btw rest and stress studies
Cons:
Fast myocardial washout means you miss your imaging time window if any errors
Lingers in liver long time
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PET tracers
O-water Short half life 2 min Needs ON-SITE cyclotron Ideal flow tracer Research use (to measure blood flow)
N-Ammonia
Short half life 10 min
ON-SITE cyclotron
Very good extraction and relationship of uptake to CBF (BETTER THAN RUBIDIUM)–so best for borderline stenosis with mildly reduced CBF vs normal, or ? for microvascular dysfunction as well;
Half life long enough to do EXERCISE stress
Liver and bowel uptake
Uptake via N/K ATP pump
Rb Very short half life (72 sec) Sr generator--no cyclotron needed Good extraction/uptake at hyperemia No exercise (bc such short half life) Longer positron range than NH3--worse spatial resolution Myocardial uptake via K channels
(FDG can be offsite cyclotron–half life 110 min)
Less radiation to pt with PET bc much shorter half lives
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