Medical Imaging Workshop/Lab A
Introduction
Medical Imaging Workshop/Lab A
This lab aims to give the student an introduction to the use of scintillation detectors with radionuclides. The student will observe how the scintillation detector is able to produce spectra with peaks that correspond to the decay energies of the nuclides.
The student will also investigate the energy resolution of the detector – a key parameter when assessing the suitability of a particular system. The student will investigate the absolute sensitivities of scintillation and Geiger-Muller detectors. They will then discuss how these measurements affect the suitability of the detectors for clinical use.
The lab has been designed to run remotely based on the practical experiments of previous years. The lab runs using Jupyter Notebooks however requires no prior knowledge of python or programming, please ensure you can access the Jupyter Notebooks and all accompanying material prior to the lab session.
Complete all numbered Tasks highlighted in the grey boxes. The Additional Tasks in the blue highlighted boxes have been included to further your understanding and will not be assessed.
1. Spectral Measurements from a Scintillation Counter
Scintillation counters can be used to identify the energies of gamma radiation photons.
This exercise will use the “Gamma_Spectrometer” Jupyter notebook and the files within the “Spectra” folder. If you are unfamiliar with Jupyter notebooks/Python the demonstrators should be able to help you with any technical issues.
The key cell that you will need to alter is the following:
Task 1: Discuss with a demonstrator how a scintillation counter operates, how it relates to the lecture content, and how it measures photon energy.
Last updated 08/02/21 – Dr Ashley Lyons
It is here that you will be able to change the parameters needed for the experiment. The 1st variable dictates the radioactive sample that is to be loaded into the spectrometer. The available options are:
Task 2: Acquire the spectrum of the Cobalt 60 source using a range of acquisition times. What do you notice about the appearance of the final figure as the integration time is changed?
Additional Task: Take a look through the code. Can you identify how many photons are being simulated for each measurement? What do you notice about this number and why has it been included in this way?
As this is a simulation, we don’t need to worry about waiting this time for each measurement. The “frames_to_show” variable controls how often the figure updates, set this to a higher value (e.g. 100) to run the simulation more quickly.
Task 3: Use the two known peaks of energy 1.173 and 1.333 MeV to calibrate the scale. Ensure the data is of sufficient quality to allow you also to measure the full and half maximum of the peak with suitable consideration of uncertainty.
Task 4: Acquire a spectrum for Na22, again this should be suitable for analysis. Measure the energy of the peak. From this energy can you indicate the type of emissions from the sodium and how this peak is being produced?
Task 5: Acquire at least one further peak for the mixed Sr90; Am241; Cs137 source. Measure the energies of the photopeaks that you see and again ensure that these are suitable for subsequent analysis.
Task 6: Measure the full width at half maximum of the four peaks you have identified. Make sure you include an estimate of the uncertainty in the widths of these peaks.
Task 7: Plot a graph of the full width at half maximum of the peaks versus the energy of the emission. Explain the appearance of this graph.
2. AbsorptionofGammaRays
Gamma rays display exponential attenuation in matter given by the relation:
? = ? ?−?? 0
where ? is the linear absorption coefficient. We can define the half thickness, ?1/2, as the thickness at which the gamma ray intensity drops to half its original value.
Using the mixed Caesium 137 source, estimate the half thickness of lead.
The “thickness_of_lead” variable controls how much lead is placed between the radioactive source and the detector in mm.
Task 8: Measure the total number of events in the spectrum over a range of different thicknesses of lead e.g. 0 – 10 mm in steps of 2 mm. Make sure that the integration time is kept the same (or record the integration time) as well as all other variables. Make sure the spectrum is adequate enough to get reliable results.
Task 9: Plot a suitable graph and determine the half thickness value.
Additional Task: Take a look through the code. Can you identify where the attenuation is taking place? Does the value used for the attenuation match what you measured and why?
3. ComparisonofGammaRayDetectors
The objective of this exercise is to compare two of the common types of gamma detector, assess their sensitivity, and judge which will be more practical for measuring patient dosage
levels. We have already seen how a scintillation counter works, the other detector type will be a Geiger-Müller tube.
The sensitivity of the two detectors will be examined as a function of distance from the source. To simulate these measurements, the “Detector_Comparison” Jupyter notebook will be used. As before, the only variables you will need to change are found in the “Experiment Parameters” cell.
The variables here should be relatively straightforward.
“distance” controls the distance between the source and detector in cm.
“detector” determines the type of detector that will be used. The available options are:
- - Geiger-Müller tube (GM tube)
- - Scintillation counter (scintillator)
Note: again these much match exactly as in the parenthesis
Task 10: Discuss with a demonstrator how a G-M tube operates. This is also covered in the lecture material so only a “refresher” may be required.
Figure 1: Example Geiger counter. Image credit: https://www.admnucleartechnologies.com.au/geiger-counters
Last updated 08/02/21 – Dr Ashley Lyons
The image at the bottom mimics the display from the detector. When the code is running you should see the needle move left/right. This shows the rate of detected events in units of counts per second (cps).
Figure 2: Display that appears at the bottom of the notebook.
We will begin with the Geiger-Müller. The source is a vial of Technetium-99m (Tc99m), the same type of source that is used in Lab B.
Task 11: Set the detector to “GM tube” and measure the detection rate at 5 – 30 cm in steps of 5 cm.
Task 12: The Tc99m source was measured to have an activity of 13 MBq at 11am. The experiment began at 3pm the same day. Given that Tc99m has a half-life of almost exactly 6 hours, estimate the activity during the experiment.
Task 13: Calculate the sensitivity of the detector setup in units of cps/MBq. Calculate for all measured distances.
Task 14: Plot a figure to find the relationship between the sensitivity and the distance. Why does your figure have this shape?
Task 15: The absolute efficiency of the detector can be estimated if the area of the detector head is known. The diameter was measured to be 4.8 cm ± 0.5 cm. Calculate the absolute efficiency; you may need to discuss with a demonstrator how to do this.
4. Lab A Report – Outline of Requirements
You should write up a suitable lab report covering both parts of this workshop. Your report should cover the following points (see notes below), based on information in the lab script and your notes and results from the lab.
- A brief introduction outlining the aims of the experiment and any relevant theory
- A description of the equipment and methods used
- Results and analysis, including consideration of errors/uncertinaties
4. Conclusions
All reports will be marked anonymously and thus your name should not appear on your report – please use your University of Glasgow registration number instead. The suggested length is 6 pages however there is not strict page limit. The most important thing is to demonstrate that you have understood the principles involved and the techniques used and that you have an appreciation of how to assess errors and uncertainty when analysing your data. Ensure before you leave the lab that you have enough data to allow this report to be completed. The deadline for submission of the report is two weeks after you attend the workshop. The standard penalties for late submission of work will be applied.
Example report templates can be found on the P3 Moodle site under the Laboratory Last heading.
Experiment 1. Spectral Measurements from a Scintillation Counter
Your introduction and equipment / methods should cover measurements of spectra and the FWHM of spectra.
Ensure that you present your measured data in the report, and address each of the points in the grey task boxes. Remember to draw conclusions and to consider uncertainties.
Experiment 2. Absorption of Gamma Rays
Present your data and calculations, showing how you calculated the half-value thickness. Make sure you discuss the source of any experimental uncertainty and estimate their magnitude.
Experiment 3. Comparison of Gamma Ray Detectors
1. Absolute sensitivity of a Geiger-Muller detector
In the results section, ensure you include results/graphs for each part of the experiment. Where appropriate also include error analysis.
Make sure you draw some conclusions, which may also refer to other parts of this experiment.
2. Absolute sensitivity of a scintillation detector
In the results section, ensure you include results/graphs for each part of the experiment. Where appropriate also include error analysis.
Make sure you draw some conclusions, which may also refer to other parts of this experiment.
Discuss your findings and compare the two types of detector studied. Would either detector be suitable for calibration of activities for patient administrations of isotope? What would be used in practice and why?
