Justification Optimisation Limiting dose in Imaging (JOLI) 2022
Guidance for students
As part of the JOLI module students are expected to be able to make measurements of radiation dose, using Thermo |Luminescent Dosimeters (TLDs) and demonstrate knowledge of experimental design with respect to radiation dose. These are explained in the JOLI handbook learning outcomes 3 and 4.
This document acts as a guide to undertaking these dose experiments. These experiments require a number of stages which must be completed in the correct order to ensure repeatability and accurate results every time. The stages are described below and will be explained individually.
1) TLD Preparation
2) TLD Calibration
3) Experiment set up
4) TLD reading
5) Recording of the results.
It is important that you understand the principle of each stage, the equipment used, and how the experimental process is used to measure dose.
The TLDs cost around £10 each. They are 1.5mm round or square white ceramic discs which will absorb radiation dose and then, when read, will emit light directly proportional to the amount of radiation received. Each TLD will react slightly differently to the same Radiation dose, so it is important that they are prepared and calibrated before initial use.
Fig 1 TLDs in the platen
In order to remove any residual readings from the TLDs they will go through a process known as “annealing”. This will “wipe the TLDs clean” and prepare them for use.
Annealing can be performed in 1 of 2 ways; through ‘cooking’ heating them in an annealing oven or as part of the final stage of the TLD reading process. The TLDs should be annealed in the annealing oven at the beginning of any session. As this process can take up to an hour this is often done early in the morning before experiments begin.
Fig 2 ANNEALING OVEN IMAGE
We need to predict how many TLDs we will need for our experiments. It is suggested that you have 3 TLDs for each area you wish to measure as well as 3 spares, (in case of loss or breakage). If we are going to measure dose at 5 positions in the body, we would therefore need 18 TLDs (3 TLDs X 5 areas + 3 spare).
The TLDs are placed in a square metal tray with a lid and placed within the annealing oven. The oven will slowly heat the TLDs to 2400 C degrees and the slowly cool them. NO NOT remove the TLDs from the oven until cooled to at least 400 C degrees.
FIG 3 IMAGE OF TRAY WITH TLDs
On the front of the annealing oven there is a control pad.
Press and hold ‘Run hold’ to select programme 1
Press ‘Run hold’ again to start the sequence
1. Gentle warming over 20 minutes to 240 degrees.
2. 10 minutes at 240 degrees
3. Forced cooling down to room temperature (using air pulled in from the room).
Immediately after annealing process, the TLDs will need to be “read” within the TLD reader (see below) in order to ensure consistent results throughout your experiment. The results from this read will be discarded (as the TLDs will have had no radiation exposure.
FIG 4 THE TLD READER
Before the TLDs can be used to measure incident radiation dose, they will need to be calibrated. This process is vital as it will allow us to determine how many “light counts” read by the TLD reader equate to 1 µGy.
We will be exposing each TLD to a known radiation dose and then reading the light count from each TLD. This will tell us “How many light counts equate to 1 µGy” – I.e., the calibration factor.
The process for calibrating the TLDs is shown below:
1. On the x-ray table put a slab of solid water (human tissue equivalent substitute).
2. Place the ionisation chamber on top (TNT ionising chamber x-ray test tool kit).
3. Link up the equipment and take the display over to the lead glass, place it on the trolley so that you can see the dose through the glass.
4. Open out the collimation to cover all of the ionisation chamber. The slab will mimic normal scatter radiation experienced on a patient.
5. Expose twice using your ‘normal’ exposure for the body part under investigation (elbow – 57 kV / 4 mAs). Write down the reading from the ionisation chamber as well as the DAP reading to ensure that the exposure of the equipment is the same each time you expose.
6. Place all of the TLD’s on the slab (remove ionisation chamber) and expose once without changing the exposure factors or collimation. The TLDs should be separated and should not overlap each other. Check the DAP reading is same as previous exposures (ensuring similar exposures)
7. Remove the TLD’s, replace the ionisation chamber and expose two more times (recording the dose for each exposure).
8. The exposure reading should be the same for all 4 ionisation chamber exposures.
9. Place the TLD’s in the platen (in any order, as it doesn’t matter at this stage) and put platen in the TLD reader. Be careful not to touch them and use he air pippetor to carry out the transfer.
10. Record all of the light counts of the TLD’s on the provided excel spreadsheet. The spreadsheet will automatically calculate the calibration factor for each TLD.
11. Using the spreadsheet, order the TLD’s by sensitivity, highest to lowest. (Highest doses will be most sensitive, lowest will be the lowest sensitivity). Do not lose the order of the TLD’s on the platen, we need to know which is which.
12. Least sensitive TLD’s need to go to the area of expected highest dose. (e.g., point of entry measurement). Most sensitive TLD will go in the area of lowest expected dose. (In this experiment this was the bladder).
13. Middle-ground sensitivity should be the ones that are used for spare TLD’s so that they are not too under/over sensitive.
14. The order of each TLD on the numbered platen should not be changed. Select each TLD and place in a single annotated bag. The bag should be annotated with the area measured as well as the platen ID number of the TLD.
15. You should now have 3 TLDs in separate bags for each area measured as well as 3 spare TLDs.
Experiment set up.
Male and female phantoms are available. Set up the phantom as close to the normal patient positioning, any inaccurate positioning should be noted as a limitation of the study. Take a test exposure to test for:
• Accuracy of phantom placement/positioning
• Adequacy of differing collimation (tight, normal, wide)
• Exposure index (optimum is 400, 10% leeway is ok)
• Change the mAs according to exposure index and use this exposure throughout the experiment (even though the exposure index will be different for different collimations)
There are slots within the phantom for the TLD’s to be placed on the inside, otherwise TLD’s can be taped on the outside. TLD’s are grouped together in their bags and taped in position on/in the phantom. Care must be taken to not crush the TLD’s. Photographic evidence should be used to document where they are placed so that students can reproduced each time one of the variables are changed (lead use, collimation change). Use tape to hold phantom together, easier to handle.
When reading the TLDs we put them into the platen in the same ID number position as written on the bag. This will ensure that we know each individual TLDs calibration factor and can therefore calculate the dose.
When all TLDs are placed in the platen, the platen is inserted carefully into the TLD reader(A little bit like a DVD)
The platens have multiple holes (numbered) that go into the machine just like a CD/DVD drive.
The platen is placed on the tray with the numbers reading 1 at 6 O’Clock (or thereabouts). It is very easy to drop the TLDs out of the platen. The TLD’s must sit squarely in the platen (there is a very small square hole in each receptacle). The TLD machine uses nitrogen to heat the TLD’s. The machine above the reader sucks and filters air in order to create nitrogen. The cylinder is underneath the counter.
There is a PC which controls the reader and provides the readings. No login details required. The software you need to use is WinREM.
1. File -> TLD experiments
2. TTP (time temperature profile) [There will be an error message about username, clear it] 3. Read
5. Start (do not need to enter any details about the measurement name, etc)
Finding results if you don’t write it down straight away.
2. Response database
3. Enter date (or risk displaying all results ever taken)
When reading TLD’s the machine goes through a standard check before actually reading. Such as testing the light and temperature. The graph that is displayed will have two peaks for different electron traps. The light counts will be shown top left in nano coulombs. The TLD’s will be read in order of the platen numbers.
Recording the results.
The reader will read each TLD and provide a light count reading for each one. This light count reading IS NOT the dose. If the light count is entered into the spreadsheet the dose will be calculated automatically using the formula “dose = Light counts / calibration factor”.
After reading, the TLDs are automatically annealed and will be ready for the next experimental condition.
Remember to place the TLDs in the correctly numbered bag and do not get them mixed up – this will invalidate your results significantly.
The spreadsheet explains the process of recording each of the light counts from the TLDs, converts them into dose readings and provides a results summary sheet which can be used to inform your results.