ELI5: How do they take an MRI of a heart when it's still pumping, and therefore moving?

Submitted by Vespiri2d t3_zzudio in explainlikeimfive

My understanding of an MRI is that it takes a cross-section image of your body and pieces it back together digitally. So it's not exactly taking an instant snapshot, but instead is compositing several images taken over a duration of time.

Now, if your heart is pumping during the scan (which I'm hoping it does), then wouldn't the size of the heart vary between each image?

Do they do something to account for this in the software, or do they physically do something during the scan?

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pottedflower t1_j2drhg3 wrote

I asked my supervisor this when I was at uni and she said one method is to record the heartbeat and synchronise the image capture so that it takes the scans at the same point of each pump :)

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comeonstupidurdumb t1_j2fb6jf wrote

This is called gating, and it's in another comment too. They use the same technique in other types of heart imaging as well as most are very motion sensitive.

I work in CT not MRI but in CT gating works by having the patient hooked up to an ECG machine that communicates with the CT computer, the computer assesses the patients heart rate while doing some breath hold exercises and gives a range of rates that it think the patients heart will be beating at during the duration of the scan. Then you can do two things, one is retrospective gating or prospective gating.

Retrospective gating scans the patient during their whole heart cycle from heart at rest to heart at rest again, and then takes all of that data to rebuild you a picture of the heart where the computer thinks it will be the most still. With this type of scanning you can go back and reconstruct better phases if the computer didnt end up making you the best possible images but it comes at the cost of radiating the patient when it is potentially unessessary and having a longer exposure time with increases overall dose to the patient.

With prospective gating you are only scanning when the computer thinks the patients heart is at rest, or in between beats, when the heart is the most still. With this type of scanning you have more limited reconstruction options because you have less scan time and less raw data but the patient receives less radiation dose.

Disclaimer that radiation does is not relevant to the MR imaging because magnetic resonance is not ionizing radiation.

Also, I am just learning cardiac scanning so feel free to point out any errors if there is someone in the comments with more experience!

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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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MattyHurricane t1_j2dx16f wrote

I have had both a regular and cardiac MRI. The regular one was no big deal, just kind of hang out and not focus on the confined space. The cardiac one was exhausting. Pretty much a 2 hour crunch/pilates/yoga/cardio session in a hamster tube.

My sense is that they took some backing images, but all of the important imaging was some variation of hold your breath and tense your muscles for 20 seconds, or exhale and tense your muscles for 20 seconds. Basically, intentionally ensure that you are removing any consciously controlled movement so that they can focus just on the heart and try to snap images throughout the beat.

Like taking screen shots of a moving video. 98% will be crap, but you will get some clear images. Now make 50 attempts over 2 hours, and each attempt requires you to hold a 20 second crunch while remaining perfectly still, in a tube. I literally had to rest in my car for 15 minutes before I drove home. Physically beat up.

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