Comment by Shinkei | Hacker News Reader

Comment by Shinkei | Hacker News Reader

A topic I can speak intelligently about! (I am a Radiologist.)

This article is a great discussion of modern imaging technology and I 100% agree that MRI is incredible. But CT has become much more impressive than the single-slice scanners of yore.

Watch this video of a modern scanner rotating at full speed:

http://www.youtube.com/watch?v=CWnjqeB7Mk8

Currently, the race to improvement is about speed and lower radiation dose. The newest scanners can take hundreds of 2D slices in a single rotation of less than a second and reconstruct the image with a huge variety of algorithms accounting for body part, patient size, etc. that all allow for lower dose and noise resulting in better diagnostic quality.

MRI is truly the cutting edge of human achievement. Watch this video to see the forces involved:

http://www.youtube.com/watch?v=6BBx8BwLhqg

And this for a detailed explanation of MRI principles:

http://www.youtube.com/watch?v=pGcZvSG805Y

Medicine uses 1.5T and 3T routinely for imaging, but higher field strengths are in use for research purposes. Innovation lately has been about developing new sequences (ways to differentiate types of tissue), and increase speed of aquisiton and comfort.

So, if we're late 20s, early 30s, what's the risk in taking a head CT?

Off topic but what would you recommend for brain imaging, a CT scan or MRI?

CT is a great screening exam because it is cheap, quick and more readily available. MRI is much more specific in diagnosing problems in the brain and has a lot more 'features' that can be added to aid in diagnosis. So generally, everyone gets a CT and then moves on to an MRI if there is an abnormality... or if they fail to find an abnormality when the clinical exam remains abnormal/severe. There are a lot of exceptions... like in little kids, you should always avoid radiation when possible and people will go straight to MRI.

Why is there such a big difference in patient price between a CT and MRI scan?

My doctor told me to have a CT scan done but I am worried about the radiation. I feel that if an MRI was cheaper, then I would have been prescribed an MRI.

Ask them about the radiation dose in banana-equivalent, then be less worried.

You're confusing CT scans with conventional x-ray images. CT scans deliver the radiation equivalent of hundreds of conventional x-ray images. The radiation dose is high enough to epidemiologically significant (meaning higher cancer rates in people who have been scanned, especially children).

Believe me, I'm not confusing them - I just think it's significantly less risky than the parent does. And yes, there can be higher cancer rates in people who have a _lot_ of CT scans done e.g. because of a serious illness.

I'd rather get that back to me as a percentage compared to a Phone Booth filled with nuclear waste (but only 75% of the way up) while standing 13.43 meters away with the humidity levels at 95%.

The MRI scanner requires more precautions and safeguards (being built around a 1+ tesla magnet), and acquiring images takes more time, thus more human resources.

Edit: to illustrate ris' point (the sibling comment), check this XKCD chart, linked in the article: http://xkcd.com/radiation/

The equipment and maintenance is much more expensive for MRI. The liquid helium bill alone for an MRI machine is not something you want to pay for.

To expand a bit on what Shinkei has said, and mostly this applies outside the brain as well:

CT is basically xrays in the round. You'll nominally use a radon transform (the actual approach is usually a back projection algorithm) to reproduce 3D position from a number of individual images. There are many good properties of this, it is relatively fast and cheap, it has very good spatial accuracy (e.g. a distance you measure on the image corresponds to a distance in the subject), but you have very little flexibility with tissue contrast. If an x-ray won't differentiate the features you are looking for, mostly you can only hope to add a contrast agent (typically expensive, fiddly, and more dose) where that is applicable.

By comparison, an MRI is more of a programmable imaging machine. You can get a lot of different contrasts by use of different pulse sequences, so you may be able to see features that are completely invisible in CT. On top of that, you can do interesting things with the time dimension and how things are changing while you image. This leads to approaches like functional MRI mapping which most people have heard of, but also diffusion and perfusion imaging. This can give you an idea of what is happening functionally, particularly in the brain. Downsides: time consuming (hence expensive), poor spatial accuracy, artifacts.

So for diagnostics or surgical planning, you get a lot more information from an MRI, but this also can take a lot more effort to interpret and understand.

Wow... that is some scary stuff. The spinning sensors make me really weary of putting my head / body in there.

The machine is entirely closed up when patients are in the mix--you don't see any spinning.

Well, the momentum of the pieces means that if anything goes flying off, it will travel in a straight line i.e. outward, away from your head :)

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