99m-Tc QC part II Flashcards
99m-Tc complex
- purity
- desired RP
- bound
Free 99m-Tc
- Impurity
- sodium pertechnetate
- not reduced by stannous
- reduced and re-oxidized
- free radicals produced from radiolysis
Hydrolyzed-reduced 99m-Tc
- Impurity
- reacts with water to form hydrolyzed species (99m-Tc O2)
- 99m-Tc-Sn2+ colloids formed by the binding of reduced 99m-Tc to hydrolyzed stannous ion
Radiochemical impurities result in:
○ Poor quality images
■ Decreased T:NT
■ Altered biodistribution
● Ie. MDP & SC
■ Changing the diagnosis and/or quantitative data.
○ Increased radiation dose to patient
● May not be excreted in the same way (Biological T1/2 )
● May need to re-inject
● Burden different for different organs
■ Same effects as radionuclide impurities
Impurities arise from:
○ Solvent
○ Changes in:
■ Temp
■ pH
■ Light
○ Oxidizing agents
○ Reducing agents (too much/not enough)
○ Radiolysis
● Specific activity, energy, and half life
Free 99m-Tc O4 is caused by:
- Oxygen
- free radicals produced from radiolysis
- hydroxyl (OH.), alkoxy (RO.), peroxy (RO2.)
How to reduce free 99m-Tc O4
hydroxyl (OH.), alkoxy (RO.), peroxy (RO2.)
Hydrolyzed species depends on:
▪ pH
▪ Duration of hydrolysis
▪ Presence of other agents
● Sn2+ also may undergo hydrolysis in aqueous solutions at pH 6 to 7 and form
insoluble colloids
● Reduced 99mTc then complexes with the tin colloid
What is the result of hydrolyzed reduced technetium
Decreased yield of the desired 99mTc complex (RP)
To minimize the amount of HR impurity in RPs:
▪ Maintain the correct pH
▪ Ensure sufficient chelating agent (complexing agent)
What methods are used to determine radiochemical purity
● There are various methods used to determine radiochemical purity:
▪ Precipitation
▪ Paper and instant thin-layer chromatography
▪ Paper and gel electrophoresis
▪ Ion exchange
▪ High performance liquid chromatography
▪ Solid phase extraction
▪ Sep-Pak ®
▪ Distillation
Paper chromatograpy
● The chromatography strip (stationary phase) is paper such as:
▪ Whatman No. 1 – fragile in aqueous solution
▪ Whatman 3 mm – thicker, less spreading of sample drop
▪ Whatman 31 ET – faster to run
Instant thin-layer chromatography
● Stationary phase is a modified thin-layer support made of:
○ Glass microfiber mesh impregnated with silica gel or silicic acid
▪ ITLC – SG (gel)
▪ ITLC – SA (acid)
▪ appear very similar to the paper strips
▪ Important to correctly label containers and select the correct strip when performing chromatography
Chromatography strip considerations
● Must be handled gently
▪ They can crack or chip easily
▪ Paper can absorb moisture and tear
● Should be stored in a tightly sealed container with a desiccant
● If preparing strips, they should be marked lightly
▪ Use felt tip to mark the solvent front
▪ Draw origin and cutting line lightly with a soft lead pencil
list solvents
● Some commonly used solvents are:
▪ Acetone
▪ 0.9 % NaCl (Normal Saline)
▪ 20 or 30 % NaCl
▪ MEK (Methyl Ethyl Ketone)
▪ 85% Methanol
▪ 95% Ethanol
Solvents considerations
● The purity of the solvents used for chromatography is extremely important
○ Ensures the reliability of the results
● Contaminants can alter the system and give inaccurate results
○ H2O
○ Other chemicals
○ Evaporation
○ Absorb moisture from the environment
● Must be stored appropriately following MSDS (Material Safety Data Sheets)
○ Flammable solvents should be stored in a fire safe cabinet
○ Corrosive solvents should be stored separately
● All glassware and equipment should be clean and dry
Paper and ITLC procedure
1) A small sample (drop from a tuberculin syringe) is spotted on the appropriate strip (stationary phase) approx. 1 cm from the bottom of the strip
2) The strip is then placed in the appropriate solvent, in a chamber (mobile phase)
3) The solvent carries the RP over the chromatography strip by capillary action andadsorption
4) Electrostatic attractive forces (polarity) of the chromatography strips slows the
movement of the various radiochemical species while their solubility in the solvent
moves them forward
5) The different species in the RP move at different speeds and appear at different
distances along the strip
6) After the strip has been ‘run’, it is removed, allowed to dry and measure in DC
7) The tagging/labeling efficiency is calculated
Solvent front analysis
○ Distance the solvent travels from the origin (mobile phase)
○ As Sf moves different radiochemical species are distributed along the strip
Relative front analysis
○ The Rf values for the various species are calculated and compared to the Rf
values for a known substance/solvent
○ Rf value is the ratio of the distance the species travels to the distance the
solvent travels from the origin
○ Rf ranges from 0 to 1 in value
How to calculate Rf values
● There are several ways to calculate the Rf values:
1. The strip is scanned with a chromatogram scanner. This gives a spectrum of the radioactivity. The position of the peak indicates the presences of species and the Rf value. (ie. 18F-FDG QC in the cyclotron)
2. The chromatography strips are cut into cm segments; each are counted and
the amount of radioactivity of each species is calculated.
3. The chromatography strip is cut into TWO pieces and counted. The labeling
efficiency is calculated. ( MOST COMMON METHOD)
what determines the location of HR and fre 99m-Tc
Chemical species @ Origin depends on:
○ RP
○ Impurities
○ Stationary phase
○ Mobile phase
Where is Hydrolyzed reduced located?
- remains at the origin
- rf=0
- insoluble particles
Location of free technetium
- travels to the solvent front
- Rf=1
Single strip chromatography
- a simple chromatography system for RCP determination
- allows for the 99m-Tc labeled complex to migrate to the Sf with the free and hydrolyzed reduces remaining at the origin
Method of RCP analysis
- Hydrolyzed reduced remains at the origin Rf=0
- insoluble particle
- Free 99m-Tc travels to the solvent front Rf=1
- In most solvents
- Labeled complex is always associated with one of the 2 impurities (HR or free) and cannot be isolated by itself
- Some RPs require 2 strips, while others only need to run one strip
- depends on solubilitiy of RP in solvent
List insoluble radiopharmaceuticals
- MAA
- DMSA
- SC
How do insoluble radiopharmaceutical chromatography results present
- the RP remains at the origin with the hydrolyzed reduced
- the free 99m-Tc O4 moves with the solvent
- ONLY the free 99m-Tc O4 impurity can be assessed for these RPs
- use ONE strip to assess % RCP
- ie. stationary phase- whatman 31ET
- Mobile phase- Acetone
List soluble radiopharmaceuticals
- MDP
- DTPA
- choletec
- DISIDA
- PYP
How do soluble chromatography results present
- BOTH HR and free 99m-TcO4 impurities can be assessed
- 2 STRIPS and two different solvents are needed
1) Isolate HR and bound at origin from Free 99m-TcO4 (solvent front)
2) Isolate Free and bound at solvent front from Hydrolyzed reduced (origin)
Dose calibrator strip measurement
▪ Ensure background counts are insignificant
▪ Strips should contain at least 100 uCi of activity to reduce counting error < 1%
▪ Dead time is not an issue; strips can be any length
(Most commonly used)
Well counter for strip measurements
▪ Must be counted far enough away from the bottom of the well to reduce dead
time
▪ Use count rate between 10 000 - 200 000 cpm
Chromatogram scanner strip measurement
▪ Dead time is not an issue
▪ Strips must be at least 10 cm long
ITLC considerations
- should not be too large
- should not be immersed below the level of the solvent
- -> prevents movement of the various species
- spot should be left to dry when using acetone or other organic solvents, complex could streak up the strip affecting results
- prolonged drying can cause oxidation of 99m-Tc complex
- dont splash RP onto strip
- make sure tweezers, scissors, tongs, tubes, etc are not contaminated
- fresh solvents and strips used
If the RCP test fails:
- Review technique
- repeat procedure
- use all new equiment to r/o contamination issues
- If RCP is still below limit: kit is disposed
- manufacturer should be called if kits from the same LOT number fail
