I have been in the medical field for over 5 years now and I still can’t get the hang of the following scans and what is each one used for. Like, I KNOW what they do but they’re all the same to me. What is an MRI, CT, Ultrasound, X-Ray, etc. etc. and what makes them different for their specific purposes? At work, I see sometimes radiological facilities want Mammograms with an US and the report says the same thing or sometimes insurance companies want an X-Ray to be done before covering an MRI but no sign of broken bone. I’m just curious lol
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Hey this is my job!
What are the different scans? Main differences are in how the images are acquired, and how they’re processed.
– XRay: Basic 2D imaging with radiation. Good for bones and basic soft tissue structures
– Fluoroscopy: XR but with video/live imaging. Used for functional tests (swallowing dysfunction, gastric reflux, diaphragm dysfunction, etc).
– DSA: Also live XR imaging but mainly used when diagnosing and treating vessel diseases (narrow blood vessels, blocked bile ducts, inserting stents, etc)
– Mammogram: XR but specifically for breast imaging.
– DEXA/DXA: XR but specifically for assessing bone density (risk of osteoporosis).
– CT: Higher dose radiation imaging. Images are acquired as a block of data over an area and can be cut up into different planes and axes. Good for most body structures. Can use IV injected medication to enhance images for more soft tissue detail or tracking blood vessels.
– MRI: Images acquired using magnetic fields. Gold standard in terms of image quality for almost all body structures (soft tissue, spinal cord, brain, etc). Like CT, can also use IV injected medication to enhance images for more soft tissue and blood vessel details.
– US: Images acquired with sound waves. Good for soft tissue structures. Also good because imaging is live while you’re scanning so you can assess functional things too (like blood vessel flow).
– PET and SPECT: This is outside my field, but imaging acquired by injecting a patient with a radioactive substance and scanning them to detect the uptake in the body. Used a lot for cancer detection.
Why one scan over the other? Main reasons I can think of:
– Pathology: certain things will appear on one type of scan but might not come up on another
– Cost: scans can be very expensive. Why spend more if you can diagnose the problem with a cheaper one?
– Accessibility: as an example, MRI scans tend to take longer compared to other modalities so there are limited slots of time you can scan somebody in a day. Things like CT and XR are quicker scans so it makes it easier to just fit someone in. Also most urgent pathologies can be detected on these other modalities anyway (like a brain bleed).
– Radiation: as useful as radiation is in terms of imaging we want to reduce exposure as much as possible. Does this young 20yo patient really need to have this CT Scan done? Can we do an US instead first and see if that answers your clinical question?
– Patient factors: Are they well enough to tolerate a whole scan/examination? Can they lie/sit still? Claustrophobic? Do they have metal implants (pacemakers? hip replacements?)? Past allergies to CT/MRI IV medication? Are they pregnant?
In your specific cases
1. Mammo and US usually done together since some tumours can appear on one modality but not the other, or may appear differently and mimic something else and you need the second one to correlate. Also, just with the way Mammo images are taken it’s difficult to look at breast tissue that’s far back and close to the chest wall. US can demonstrate this area a lot easier. Another thing is that some women (mainly younger women) tend to have denser breast tissue. Mammo is terrible at imaging this type of breast tissue so US is better in this regard. You can do breast MRI as well but that also has its own difficulties (cost, need specific equipment at the clinic, have to lie face down for whole scan).
2. XR before MRI – likely to do with billing policies of the insurance company. But also, sometimes very fine fractures don’t appear on XR but will appear on CT or MRI.
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