Radionuclide Calibrator

Question 1

What type of detector is commonly used in a radionuclide calibrator?

Answer 1

A gas-filled ionization chamber is commonly used to detect radiation by measuring ionisation produced in the gas.

Question 2

How does the ionisation chamber detect radiation?

Answer 2

Radiation interacts with the gas in the chamber, producing ion pairs (positive ions and free electrons). An applied voltage collects these charges, generating an electrical current proportional to the activity of the radionuclide.

Question 3

What role does the applied voltage play in the ionisation chamber?

Answer 3

The voltage creates an electric field that collects ion pairs, preventing recombination and ensuring an accurate current measurement.

Question 4

What is the current proportional to?

Write the equation

Answer 4

• Number of incoming particles (i.e. source activity)
• Radiation energy

k x A = I

Question 5

How does the calibrator know which factor to use?

Answer 5

It doesn’t!
Current is proportional to both activity and radiation energy
Must be user selected

Question 6

What factors go into a calibration factor?

Answer 6

• Radionuclide characteristics
• Emission energy
• Branching ratio
• Energy response of chamber (chamber design)

Question 7

How does photon energy affect the photon interactions in a radionuclide calibrator?

< 13 keV, 13-50 keV and > 200 keV

Answer 7

• <13 keV – Photons are attenuated by vial/syringe, dipper, or chamber wall.
• 13 – 50 keV – Photons interact with the chamber wall, but some energized electrons enter the chamber.
• > 200 keV – Compton scattering dominates and is largely linear; more photon energy leads to more electrons and thus more current.

Question 8

What would the effect on the response be if the chamber wall thickness was increased?

Answer 8

Attenuate photons before they can enter chamber
Lower energy photons affected more than high energy photons

Question 9

How are Capintec calibrator factors assigned?

Answer 9

Na = 1076(Ra - 0.080)
Where Na is the numerical calibration factor and Ra is the calibrator response relative to Co60

Question 10

What is the traceability chain?

Answer 10

• National Physical Laboratory
• Primary reference standard (Coincidence counting determine current from first princple)
• Secondary reference (Ionisation chamber to determine current of know sample)
• Regional reference calibrated from the secondary reference (annual)
• Field instruments are then calibrated by the regional reference (annual)

Question 11

How does the annual tracability take place?

Answer 11

1. Radioactivity in P6 vial, with a clinical representative amount of activity
2. Measure in local instrument
3. Send to NPL
4. NPL measures in the secondary reference
5. Send certificate back
6. Adjust calibration number of necessary

Question 12

Why are calibration factors dependent on the container mass?

Answer 12

• Self-attenuation (geometry of source)
• Attenuation in container (material and thickness)

Question 13

What is the volume correction equation?

Answer 13

𝐼_0/𝐼_𝑚 =𝑎_2 (𝑚−𝑚_0 )^2+𝑎_1(𝑚−𝑚_0)+1

𝑎2 and 𝑎1are volume correctionn factors
m is the measured mass
m0 mass relating to the pA/MBq correction factor

Question 14

How are beta particles measured in a calibrator?

Answer 14

Most beta emitters also emit gamma rays. Beta particles attenuated by the chamber wall will emit Bremsstrallung x-rays that will be detected. Specialist units exist for pure beta emitters using a Sodium Iodine crystal rather than a gas ion chamber.

Question 15

I123 has a whole bunch of low energy emissions energetic enough to contribute to signal

• Why does this make accurate measurement difficult?
• What can we do about it?

Answer 15

• Low energy emissions – highly affected by geometry and positioning.
• Remove with filtration.

Question 16

Why is the chamber less sensitive at the top?

Answer 16

A lot of the emissions wil escape and not be detected
In the radial direction, the difference might be a maximum of 2% for Tc99m
Calibrators have source holders to minimise effect.

Question 17

Why might increasing external shielding increase response and when might external shielding be used?

Answer 17

Shielding and other objects increase back scatter of photons back into the ion chamber, and Pb (lead) k-edge photons.

Minimise dose to operator, minimise background signal in calibrator

Question 18

What are calibration factors specific to?

Answer 18

• Radionuclide
• Volume
• Container
• Source position
• Filtration
• Calibrator position / external shielding

Question 19

What is absorption phenomenom?

Answer 19

When the radioactvity within the vial/syringe interacts and binds with the material taht the vial/syringe is made of

Question 20

What are the different quality control tests for radionuclide calibrators?

Daily and Annual

Answer 20

Daily (usually technologist task)
* Background
* Start up checks
* Constancy on each calibration factor to be used

Annual (usually scientist task)
* Accuracy / traceability
* Linearity
* Reproducibility / repeatability

Question 21

Why is different circuitry required at different levels?

Answer 21

• Calibrators measure from > 100 GBq to 10 MBq, 4 orders of magnitude! and the calibrator needs to be linear across the entire range
• Produces a ‘saw tooth pattern’ as electrometer switches between circuits
• Less pronounced on modern unit and errors are smaller as well

Question 22

(a) How can we measure linearity?
(b) Do we need to measure linearity for every radionuclide?

Answer 22

(a) Using a decaying source method
(b) No need to measure for every radionuclide. In the end its just current. Just need to measure with a source (or sources) that cover whole range of currents from maximum to minimum current generated clinically.

Question 23

What are the advantages and disadvantages of the decaying dource method for linearity?

Answer 23

Advantages:
* Covers entire range
* Sensitive

Disadvantages:
* Takes a long time
* Long processing time
* Prone to interuption
* Radioactive source left out
* Need data logger

Question 24

How to perform the decaying source method practically?

Answer 24

1. Measure background
2. Make sure the calibrator isn’t automatically subtracting background from the display
3. Get as much Tc99m as you can lay your hands on
4. At least roughly the same as the largest amount of activity that may be measured on that calibrator or 100000x background (~2000Mbq), whichever larger
5. Put in calibrator, set data-logger to record at least every 2 hours
6. I tend to use 30 mins
7. Some calibrators have software for data-logging. Sometimes just easier to use laptop and webcam
8. Record until source is 10x background (~2MBq)

Question 25

How to perform the analysis for the decaying source method?

Answer 25

1. Use the data to create a measured decay constant for Tc99m
2. NPL guide 93 suggests that this should be done from 100,000 times the background to 100 times the background
3. Find the max deviation across the relevant range by using the data point closest to the amount of activity that was used for traceability measurements as “truth” and extrapolate from there

Question 26

What are the other method for determining linearity asside from the decaying source method and the problems associated with each method?

Answer 26

• Graduated shielding (awkward and heavy)
• Graduated sourced (need to be careful with gravimetric dispensing
• Neither method gives the decay constant

Question 27

What is the difference between repeatability and reproducibility?

Answer 27

• Repeatability – measures the stability of the unit. Leave source in place
• Reproducibility – measures the stability of the process. Move the source in and out as you would clinically.