Comparing Imaging Techniques (AQA A Level Physics)
Revision Note
Comparing Imaging Techniques
All imaging techniques are beneficial in their own context, but often the positive outcomes of the scan must be balanced against the potential risks
The scan must have a level of resolution high enough to image the target clearly, while minimising risk
Performing an X-ray scan on a foetus is too risky, when other types of scans provide a good enough resolution
However, when identifying a potential tumour, resolution must be high and the risks associated with the scan are much smaller than the risks associated with not catching cancer in its early stages
A table of advantage, disadvantage and resolution of each method
Imaging Method | Advantage | Disadvantage | Resolution (mm) |
|---|---|---|---|
CT Scan | Show different tissue types High resolution | Expensive Large radiation dose | 0.5 |
External X-rays | Cheap, fast and easy operation High resolution | Cannot distinguish different tissues Cannot image some organs | 0.5 |
MRI | Distinguish types of tissue No radiation risk | Expensive and time-consuming Uncomfortable for some patients | 1 |
PET | Shows the function of organs Good for imaging the brain | Expensive and time-consuming Does not work for every organ | 1 |
Ultrasound | No radiation danger Safe for use on a foetus | Cannot show certain organs (e.g. brain) Lower resolution, hard to operate | 2 |
Ingested gamma source | Shows function of organs Relatively simple to use | Lowest resolution Radiation risk associated | 6 |
Worked Example
Three patients visit a hospital. Patient 1 has a lower-body injury, patient 2 has a suspected tumour in their lung and patient 3 is suspected to have a hyperactive thyroid.
The hospital has the following facilities:
Ultrasound imaging
Gamma sources for ingestion
External X-ray imaging
CT scanner
For each patient, select an appropriate imaging method, explaining why this method is appropriate while the others are not.
Answer:
Patient 1 (lower body injury)
Step 1: Select an appropriate imaging method, explaining your answer
External X-ray imaging is most appropriate
Step 2: Compare this method to alternative imaging methods
This is quick and easy and shows potential bone damage in the injury
Whereas a CT scan has a higher radiation dose and an image that detailed is unnecessary
Real-time scanning is not needed, so an ultrasound would be inappropriate
The function of organs is not needed, so gamma imaging would be inappropriate
Patient 2 (lung tumour)
Step 1: Select an appropriate imaging method
A CT scan is best for this patient
Step 2: Compare this method to alternative imaging methods
This highlights cancer tissue against healthy tissue
Cross sections throughout the lungs can show all potential tumour locations
Ultrasound is absorbed by air in the lungs and the detail is not great enough
X-ray scanning may not differentiate tissues and the ribcage may obscure the view of a tumour
An ingested gamma source may not provide a detailed enough image
Patient 3 (hyperactive thyroid)
Step 1: Select an appropriate imaging method
An ingested gamma source
Step 2: Compare this method to alternative imaging methods
This shows the function of tissues and therefore if the thyroid is performing at a higher than normal rate
Ultrasound, X-ray and CT scans show structures and tissues but not the level of their function
Examiner Tips and Tricks
You will only be asked to compare image resolution, convenience and safety issues. Make sure you use clear comparative language in these answers.
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