Physics of Medical Scans
Computer Tomography
Computer Tomography, otherwise known as CT, is a widely used scanning technique in hospitals that use X-rays to see internal organs and other tissues in the body without needing to perform any invasive surgeries.
When would I need a CT scan?
CT scans help medical professionals to monitor your internal body structure which may include checking your bones, your circulatory system via the blood vessels, or individual organs such as the heart or lungs. Because of the versatility of CT, it can be used throughout a patient’s journey to recovery.
During initial diagnosis, CT scans can be useful in identifying an illness/condition: it can diagnose bone diseases, discover blockages in blood flow (such as a pulmonary embolus), or find tumours. After diagnosis, it can also advise doctors to what the best way forward is during treatment. For example, CT scans are extremely helpful in determining the size, location and shape of tumour(s) in a cancer patient. This information helps oncologists to decide whether to suggest radiotherapy, chemotherapy, surgery or any other combination of treatments that would be most suitable for the patient. Furthermore, CT scans can be used for the monitoring process of a tumour to regularly check on its size/shrinkage during treatment [1].
What happens during a CT examination?
Before going into the scan medical staff may give you what’s called a contrast, a dye that improves the detail quality in the scan. This can be administered orally through a drink, injected, or passed through using an enema [2].
During a scan, you lie on a table that will then move into a ring-shaped part of the machine; this is the part that captures all the data for the CT computer to process. At this part of the scan, it’s important to stay still as movement (even uneven/large breathes) can blur the scan. The entire procedure takes up to 30 minutes [3].
How does it work?
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Does it hurt?
You’ll notice that during the scan, you won’t feel anything happening to your body. This is because you can’t feel X-rays as they are a form of light and your eyes aren’t able to see light beyond the visible spectrum (eg. red, yellow, blue).
X-Ray vision
X-rays can pass through certain solid objects because, compared to the visible light humans can see, X-rays have far more energy that allows them to reach a deeper penetration depth than with visible light [4].
What is Attenuation?
Attenuation is the weakening of the intensity of light as it travels through something. Just as sunlight is attenuated more strongly through tinted glass than clear glass, X-rays are attenuated more strongly in bone than other tissues [5].
CT machines use the penetrating properties of X-rays to detect how optically resistant (or attenuating) your internal body structures are; the more a specific body tissue attenuates light, the denser it is. For example, bones in our body attenuate X-rays at a much larger extent than a softer tissue like muscle does due to its greater density. This appears on a CT image as a lighter area (bone) and a darker area (muscle). The ring-shaped part of the machine, called the gantry, has both an X-ray emitter and a detector attached to it. The emitter produces X-rays that go through the part of the body being scanned and the detector on the other side is there to pick up the waves that are able to penetrate the body and come out the other side.
A diagram of the entire spectrum of light known in the universe. Visible light only constitutes a very small fraction of all light that is available to us.
The concentration of X-rays that the detector picks up indicates what sort of tissues may be in the path of the light; that is, the lower the intensity of X-rays at the detector means the more attenuating the tissue blocking it will be. As this is happening, the gantry rotates around and more attenuation data is collected at different angles around the body part that’s being imaged. This information is then processed in a computer which then converts it into an image.
CT - Simple
CT - Advanced
References:
[1] NHS, "CT Scan", https://www.nhs.uk/conditions/ct-scan/ [Accessed: 21/02/2019]
[2] Cancer Research UK, "CT Scan", https://www.cancerresearchuk.org/about-cancer/cancer-in-general/tests/ct-scan [Accessed: 21/02/2019]
[4] Radiology Info, "Computed Tomography (CT) - Body", https://www.radiologyinfo.org/en/info.cfm?pg=bodyct [Accessed: 22/02/19]
[5] Zagzebski J. A. (1996), "Essentials of Ultrasound Physics", Mosby Inc..