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Comment by fqwefqwef902 | Hacker News Reader

Coming from the field of cognitive neuroscience, where MRI scans are commonplace (because you can get an estimate of brain activity by doing a continuous, "functional" MRI scan), CT feels like ancient technology to me. There are highly specific case where it has benefits, so it has a place. But if I went into the hospital and they sent me in for CT rather than MRI to get an anatomical scan, they better have a damn good reason for it.

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Well, you better be willing to wait a couple days for a non-emergent MRI.

Because MRIs take longer, they are a more scarce resource. If you are in the hospital and need a scan, presumably its because the doctors are trying to diagnose something somewhat urgently. If not, they should discharge you and let you get the scan scheduled as an outpatient. So, CT it is for almost all common conditions in the hospital as first line. (even stroke usually gets CT head first to rule out a bleed, where the sensitivity is still pretty good)

I had a mild stroke in 2001. CT did not find it, MRI did.

CT and MRI are simply different imaging modalities that provide different information, usually used to answer different questions. There are tradeoffs for each.

By the way, they were both invented around the same time (CT is only older by like ~5 years).

CT is simply more practical for full body scans, since an MRT would take forever. Imagine laying completely still for 2 hours... CT does it in 5 minutes.

They also differ in diagnostic value for different types of use cases. But I am lacking any expertise there.

Either way, CT plus constrast agent is the standard in cancer diagnostics/staging.

Excuse my ignorance, but I've always been told that CTs are significantly cheaper and faster to administer. According to the famous XKCD radiation graph, a full CT head scan is about 50% of the normal natural yearly background radiation you'll get. Not something to cheer over, but nothing to lose sleep over either. If a CT scan would suffice, why tie up the MRI machine?

The dosage is in a shorter amount of time (within seconds instead of months). I don’t think the radiation averaged over half a year is comparable to the head CT.

My understanding is that the dosage within those few seconds is the equivalent to absorbing half a year's worth of radiation. That's explicitly what Sieverts are designed to measure: https://en.wikipedia.org/wiki/Sievert

Correct me if I'm wrong, though?

It's disputed, if the linear, no threshold model is adequate. The problem is, 1:5 people die of cancer anyway and the people getting fullbody CTs tend to be either old and die rather soon, or are people who had cancer at some point to begin with. Really hard to find a signal there. The best data is still from the nuclear accidents/bombings.

Cells can repair (maybe faulty) a certain amount of damage, but may suicide when too much is broken (double-strand breaks). The amount and type of ROS generated by ionizing radiation also depends on your antioxidant state and how well tissue is saturated with oxygen (more ROS if you exercised before exposure). Generated ROS are a significant factor in cell damage, it's not just direct DNA hits. Some ROS can last for weeks and travel across cells to fuck things up.

I think we can confidently say, lowish radiation exposure is: not great, not terrible ;)

The problem is when you are screening lots of people, most of whom don't actually have cancer. The individual risk is low, but the population risk is high. And when we talk about cancer screening we have to think at the population level.

From what I know, the risk is not high enough to leave a strong signal. The problem is the overall high cancer rate for humans. As far as I know, we still don't have a model for predicting cancer rates after low exposure to ionizing radiation.

OK, but you'd be delaying scans by having to wait for the MRI, which takes significantly longer to setup and administer as well as being much more expensive (which would further reduce access).

Also (and this is according to the XKCD chart) you'd need 50 CT head scans before you'd hit the clear statistical cancer risk.

But it's not like the whole population gets a CT scan every year, or even during their entire life. If you need one anyways, there's probably a more pressing issue that 1/50th the clearly statistical risk level.

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