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catbrane t1_j2e0czv wrote

I've worked on this a little. It's usually a combination of gating and superresolution.

Superresolution -- MRI scanners get faster at lower resolutions. You can do a very low res or small volume scan in less than 100 milliseconds, which is fast enough to freeze most heart movement.

You can't see much in these very low res images, so instead you take 100s of them, with slight movement of the scanner each time, and then reconstruct a high-res image from all the low res images. The image reconstruction techniques in modern video games work a little like this.

Gating -- you attach a heart monitor and note the exact point in the heart cycle of each tiny scan you make. When you want to reconstruct the final movie, you put all the tiny scans into maybe 32 buckets, with one bucket for each 50ms period of the cycle, and then do the superresolution reconstruction on just the scans in that bucket. Put the 32 final images together and bingo, you have a movie you can loop.

People have experimented with extra techniques, like estimating motion vectors to remove movement and increase sharpness, but I don't know much about that. No doubt you could use ML to help as well.

tldr: the movies you see are composites of many, many heart cycles recorded over a long period of time.

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catbrane t1_j2e0un0 wrote

The MRI thing I did a little work on (I was on another part of the team, but I knew the person who did the MRI work) was fetal imaging, ie. scanning a child in the womb.

The little bastards won't stay still, so you have a similar problem of somehow detecting and freezing movement, then doing superresolution reconstruction. Fun stuff!

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Vespiri2d OP t1_j2e10c4 wrote

Fascinating. Is the heart monitor attached during the scan? If so wouldn't it be a health hazard with it possibly being magnetized?

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Fluffy-Jackfruit-930 t1_j2e5bsl wrote

Yes the monitor can be attached. It can be either ECG (electrical) or a finger pulse monitor. Both work fine.

ECG monitors do have problems with MRI, because the scanner generates an absolute ton of electrical interference which messes up the waveforms.

The waveform is also distorted because when you move something which conducts electricity (like a wire, but also blood or heart tissue) in the presence of a magnetic field, that movement generates electricity. The electricity generated by the moving blood and heart muscle can mess up the trace - it's still good enough for timing and syncing up the scan - but it doesn't look like the trace you get in textbooks.

Heart monitors are absolutely a hazard if used with MRI incorrectly. Obviously, you wouldn't use one with any magnetic parts. However, because of the enormous electromagnetic fields generated by the MRI scanner, there can be all sorts of weird effects with wires and metals. So, it's very important that the correct monitor and sticky electrodes be used. Electrodes with too much metal in them can get hot and burn the skin. Wires which are too long can pick up interference from the scanner, concentrate it and direct it into electrodes, also burning the skin. There are special monitors and electrodes designed specifically to withstand these effects and absorb the energy safely so as not to burn the skin. These monitors also contain special electronic filters to try to filter out most of the interference produced by the scanner so that the waveform isn't completely swamped by just static and interference.

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