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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Ultrasounds use sound. But sound you cannot hear. They put an emitter up against the person’s body, which sends out “pings” of sound, which bounce off the stuff in the person’s body. The emitter then also has a receiver which hears the sounds, and uses that to figure out what is there. Some tech turns that into an image. It is basically echolocation.
X-Rays are a type of light. But they pass through more stuff than visible light, including skin, muscle and so on, but they don’t pass through bone. An X-ray machine shines a load of x-rays at a target body part, and then there is a screen on the other side. The screen changes colour where X-rays hit it, but not where they don’t – leaving a shadow in the shape of any bones (or other things that block X-rays) in the target area.
CT scanners are fancy X-ray machines. It stands for “computed tomography” – they take a whole load of X-rays from a load of different angles, and then the computer does some fancy stuff to turn it into a nice 3D model of what they are looking at.
MRIs (nuclear magnetic resonance imaging) use powerful magnetic fields and radio waves, and some fancy physics, to take pictures of water (or a contrast material put into the patient), and from that build up a picture of the insides of people.
In terms of why they would use one over the other, ultrasounds are quick, easy, and produce live images, so are great at looking for things right now. X-rays are good for looking at bones, but limited for looking at other things, and X-rays are harmful in large quantities (too many x-rays can cause cancer). CT scans have the same downsides as x-rays, but produce far better pictures. However they take longer and are more expensive (as they are a load of x-rays combined together). MRIs are great at showing the inside of people and are completely safe (unlike X-rays) unless there is anything potentially magnetic inside them – the strong magnetic fields can be very dangerous. They’re also loud, and can be a bit cramped for the patient, and the machines themselves are very complicated and expensive.
I can’t really help with the medical advantages (or why insurance companies would care) – but that is a rough idea of the physics. However, I can see someone wanting an X-ray before an MRI as the X-ray would show up any metal in the patient, which might cause serious problems (including killing them) in the MRI – particularly if they don’t have a full medical history of the patient, or if their may be splinters or fragments of something (like bullets) in them. Plus for the insurance companies I imagine they want the cheaper (if more harmful) option first.
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*Edit: I forgot about PET scans*
PET scanners are kind of complicated; for a PET scan the patient is injected with a radioactive drug. This travels through the body naturally, and different drugs will be used depending on where they want it to go. The radioactive nuclei decay (randomly, as normal radioactive decay), usually by a thing called “beta plus decay”, which emits a positron (hence *positron* *emission* tomography). Positrons are anti-matter; anti-versions of electrons. When emitted in the middle of a person they quickly hit one of the many regular electrons nearby, annihilate, and give off two gamma rays, which are detected by scanners around the patient. These can provide a nice 3d image of stuff inside the person – wherever the drug has gathered (often used for cancers, by finding drugs that will cluster around cancerous cells).
PET scans can be even more expensive than MRIs (as you also need the fancy radioactive drugs, as well as the fancy scanner and computers to put all the images together). They are also a bit more dangerous (other than for things with metal) as they use radioactive materials that do cause damage to the patient.
They use different forms of energy to look at the body, different parts of the body and at different definition, and some with more or less dangerous forms of energy.
Like, xray, can only see hard o texts, and only a 2d image. It is also low danger, and extremely cheep.
Mri, looks at soft body in great detail, and takes many many pics of slices to construct and image. It is no danger, takes a while and is very expensive.
CT, is just lots of xrays for form a better picture ? Maybe from all angles. Moderate danger (lots of xrays) moderate expensive.
Ultrasound, no danger, uses sound waves, looks at soft bodies in motion real time, low definition. Cheep.
Pet scans do something with very dangerous radiation to image stuff. Very Expensive
MRI stands for Magnetic Resonance Imaging. Uses the principle of nuclear magnetic resonance. You apply a very strong magnetic field to the body and different molecules react differently, allowing a computer to form pictures of what’s where.
X-Rays use x-ray radiation, which is a form of ionizing radiation (meaning it will damage your tissues if exposed too long), and if you send it through the body, harder objects like bones let it pass through differently than soft tissues. Used to form single picture of an area.
CT scan stands for Computed Tomography, which uses lots of X-rays (which is why you don’t do them often) to form “slices” of the body in images. CAT scan is an old way of talking about CT.
Ultrasound uses ultra-high-frequency sounds (much too high-pitched to hear) to create images of an area.
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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