Scans, Scans and More Scans: What’s the Difference Between PET, CT and MRI?

Scans. Scans. And more scans. If you’re going through bowel cancer treatment, or monitoring after treatment, you’ve probably had your fair share of medical imaging. But what’s the difference between all the types? Why do you need one scan instead of another? And what actually happens during each one?

Let’s break it down: PET, CT and MRI, what they do, how they feel, and what to expect.

What’s the purpose of each scan?

• CT (Computed Tomography): CT scans use X-rays to take detailed cross-section images of your body. Think of it like slicing a loaf of bread, except it’s your body, and the images help doctors see inside layer by layer. CTs are fast and often the first go-to for spotting tumours, checking organs, or monitoring treatment response.
• PET (Positron Emission Tomography): PET scans look at how your cells are functioning. You’re injected with a small amount of radioactive glucose, and areas with high activity (like cancer) light up. It’s often combined with a CT scan (PET/CT) for better detail.
• MRI (Magnetic Resonance Imaging): MRIs use magnets and radio waves, no radiation, to create high-resolution images, especially useful for soft tissues like the brain, spine or pelvis. They take longer than CTs and are often used when very fine detail is needed.

In short:

• CT: Fast and good for structure
• PET: Lights up active cancer
• MRI: Great for soft tissue detail

What the scan experience actually feels like

I’ve had all three, more times than I can count. Here’s what they’re like from a patient perspective.

CT Scan

CTs are quick. Usually done in 10 – 15 minutes. You lie on a narrow bed that slides through a donut-shaped scanner. Sometimes you drink a contrast liquid beforehand (tastes like chalky cordial), and sometimes you get a contrast dye through a cannula that gives you a warm flush or the feeling like you’ve wet yourself, it’s weird but normal.

PET Scan

PET scans are a whole different beast. I’ve had more than ten, and I dread the prep more than the scan itself. You have to drink about a litre of water, then sit in a room alone for 45 minutes while the radioactive tracer works its way through your body. As soon as the door shuts, I always feel like I need to pee, and you just sit there, alone, bored, uncomfortable, until you’re finally called in.

The scan itself can take 15 – 20 minutes. You lie still, sometimes with your arms overhead. It’s quiet, slow, and kind of meditative if you can zone out, but the waiting beforehand is honestly the worst part for me.

MRI

MRIs are loud. Really loud. You’re in a tunnel-like machine, and even if you’re not claustrophobic (I’m not), it can feel tight and confronting. They give you earplugs or headphones, but the knocking and buzzing still comes through. It can take 30–60 minutes, and you have to stay really still, which gets harder the longer it goes on.

If you need contrast dye, it’s usually done halfway through via cannula. And yeah, if you move, they sometimes have to restart parts of the scan. Not ideal if you’re already uncomfortable.

Why doctors choose one scan over another

Different scans give different kinds of information. Your oncologist isn’t just picking at random, they’re using the scan that’ll give them the clearest view of what they’re looking for. That might mean a CT to check organ spread, a PET to see if something is lighting up with activity, or an MRI to dig into a specific area in more detail.

Sometimes you’ll get a combination, like a PET/CT, to get both the function and structure in one go.

What about radiation exposure?

It’s a fair question, especially when you’re racking up scans over months or years. Here’s a rough guide:

• CT: Moderate radiation (more than a regular X-ray)
• PET: Higher radiation (due to the tracer + CT combo)
• MRI: No radiation (uses magnets instead)

Doctors will always weigh the risks vs benefits. If they order a scan, it’s because the information it gives could influence your care or outcome. The goal is always to use the minimum necessary while still keeping a close enough eye on what’s happening inside.

Is scanxiety a thing?

Absolutely. Waiting for results can be brutal, even if the scan itself was fine. That looming sense of “what if something’s changed” can stick in your gut for days. While this blog focuses on the differences between scan types, just know you’re not alone in feeling anxious about the outcome. It’s one of the most common emotional hurdles in cancer care and one we’ve talked about more in this blog on scanxiety.

Final thought

Scans are part of the cancer grind. They’re not fun, they’re not comfortable but they’re essential. Each type has a purpose, and while the prep and process can suck, the information they give your medical team is often critical. I’ve had more scans than I care to count, and I still hate parts of the process. But knowing what each one does, why it’s being used, and what to expect can make it a little easier to face the next one.