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I always thought an MRI worked a bit like a microwave oven that it would jiggle water or the hydrogen molecules in a person. The jiggling made the molecules emi

I always thought an MRI worked a bit like a microwave oven that it would jiggle water or the hydrogen molecules in a person. The jiggling made the molecules emit energy, radio emissions. But it's much more precise than that. nibib.nih.gov has a nice explanation. >MRIs employ powerful magnets which produce a strong magnetic field that forces protons in the body to align with that field. When a radiofrequency current is then pulsed through the patient, the protons are stimulated, and spin out of equilibrium, straining against the pull of the magnetic field. When the radiofrequency field is turned off, the MRI sensors are able to detect the energy released as the protons realign with the magnetic field. The time it takes for the protons to realign with the magnetic field, as well as the amount of energy released, changes depending on the environment and the chemical nature of the molecules. Physicians are able to tell the difference between various types of tissues based on these magnetic What's strange is for clarity of images an old-fashioned x-ray/CAT scan is better it's not as fuzzy as 1T or 3T MRI. From what I can gather is the more powerful an MRI is the faster it can capture images and with more detail. The speed is why they are clearer you the patient has less time to squirm around. It's also mean less time for the patient to be in the machine and the machine is less constrictive (bigger hole).

RL_Quine 5y ago
Pretty often for cerebral scans you don't have a whole lot of room to move due to being literally restrained by the focusing gear. Most people who have done MRIs regularly get pretty good at not blurring the images; the more still you remain the less chance of them having to do another round.
- johnisgood 5y ago
  I stayed still for my first, too. It was really noisy. I dislike the part where it suddenly changes the "rhythm" but I prepared myself for that now. Going to get an MRI soon again.
  - RL_Quine 5y ago
    You do get used to it, believe it or not. One thing that helped me understand what was going on was looking at the output files, each pulse of the magnet is a slice within the images, and each sound corresponds to different types of measurement. I tend to just count them to work out what type of imaging is currently going on, it's ultimately pointless but it's something to focus on given the lack of other stimulus. You'll know you're there when the MRI tech comments that it's obvious you've done a lot of them because every image was perfect.
    - johnisgood 5y ago
      Thank you. It is not exactly an issue to me, I just know it is necessary, so I lay through it without movement. You gotta do what you gotta do. :)
version_five 5y ago
X-ray / CT use ionizing radiation and basically trade off image quality with how much damage they are doing to you (I don't mean they are dangerous, but they deal with high power radiation). MRI uses harmless magnetic fields, the tradeoff being that the Zeeman interaction is orders of magnitude weaker than whatever happens in an xray and so the signal to noise ratio is much lower. In an MRI, stronger field means more magnetism to work with an a higher db/dt so higher voltage induced in detection coils so you end up with a notional square law (actually 7/4 due to skin depth) scaling of sensitivity with field strength. But higher field introduces other problems - more RF power deposited into the patient, field variations over the spatial scale of the patient, static field homogeneity, others, so it's not automatically better. Also, making the machine bigger means more challenges creating a homogeneous field over the large region (for example, its easier to make a narrow bore nmr magnet than a big clinical MRI (but the nmr would want better homogeneity). Speed really again trades off against quality. You end up with essentially a fixed amount of signal per time, which you can use for more resolution or more SNR
- ahartmetz 5y ago
  > I don't mean they are dangerous But they are. CT uses pretty high doses, and each CT scan increases lifetime risk of cancer by order of magnitude 0.1%. CT scans absolutely should not be done willy-nilly.
  - posterboy 5y ago
    ...increase by 0.1% on average, over the average a priori risk. If I didn't know I was a risk to begin with, I certainly won't care about a rounding error magnitude increase.
jiggunjer 5y ago
"better" depends on the problem domain. MRI is useful because it offers better contrast between different tissue types that look the same on CT. The modalities can also be combined.
thehappypm 5y ago
Massachusetts has almost universal health care coverage due to it being a priority of the Commonwealth. We have the lowest uninsured population in the USA at ~97% coverage. (Lowest coverage state? Texas, ~82%.) I used to go to a local medical center in Boston that was apparently mostly serving Medicaid recipients. It had an MRI area that was like an assembly line -- huge waiting room, dozens of people constantly shuffling in and out of MRI rooms to get scans for whatever reason. So, a faster MRI machine means more people can get more scans. Which might be the difference between life and death, especially when you're on Medicaid. (Source: https://www.kff.org/other/state-indicator/total-population/?currentTimeframe=0&sortModel=%7B%22colId%22:%22Uninsured%22,%22sort%22:%22asc%22%7D https://www.kff.org/other/state-indicator/total-population/?..., 2019 data)

RL_Quine 5y ago

Pretty often for cerebral scans you don't have a whole lot of room to move due to being literally restrained by the focusing gear. Most people who have done MRIs regularly get pretty good at not blurring the images; the more still you remain the less chance of them having to do another round.

johnisgood 5y ago
I stayed still for my first, too. It was really noisy. I dislike the part where it suddenly changes the "rhythm" but I prepared myself for that now. Going to get an MRI soon again.
- RL_Quine 5y ago
  You do get used to it, believe it or not. One thing that helped me understand what was going on was looking at the output files, each pulse of the magnet is a slice within the images, and each sound corresponds to different types of measurement. I tend to just count them to work out what type of imaging is currently going on, it's ultimately pointless but it's something to focus on given the lack of other stimulus. You'll know you're there when the MRI tech comments that it's obvious you've done a lot of them because every image was perfect.
  - johnisgood 5y ago
    Thank you. It is not exactly an issue to me, I just know it is necessary, so I lay through it without movement. You gotta do what you gotta do. :)

version_five 5y ago

X-ray / CT use ionizing radiation and basically trade off image quality with how much damage they are doing to you (I don't mean they are dangerous, but they deal with high power radiation). MRI uses harmless magnetic fields, the tradeoff being that the Zeeman interaction is orders of magnitude weaker than whatever happens in an xray and so the signal to noise ratio is much lower. In an MRI, stronger field means more magnetism to work with an a higher db/dt so higher voltage induced in detection coils so you end up with a notional square law (actually 7/4 due to skin depth) scaling of sensitivity with field strength. But higher field introduces other problems - more RF power deposited into the patient, field variations over the spatial scale of the patient, static field homogeneity, others, so it's not automatically better. Also, making the machine bigger means more challenges creating a homogeneous field over the large region (for example, its easier to make a narrow bore nmr magnet than a big clinical MRI (but the nmr would want better homogeneity). Speed really again trades off against quality. You end up with essentially a fixed amount of signal per time, which you can use for more resolution or more SNR

ahartmetz 5y ago
> I don't mean they are dangerous But they are. CT uses pretty high doses, and each CT scan increases lifetime risk of cancer by order of magnitude 0.1%. CT scans absolutely should not be done willy-nilly.
- posterboy 5y ago
  ...increase by 0.1% on average, over the average a priori risk. If I didn't know I was a risk to begin with, I certainly won't care about a rounding error magnitude increase.

jiggunjer 5y ago

"better" depends on the problem domain. MRI is useful because it offers better contrast between different tissue types that look the same on CT. The modalities can also be combined.

thehappypm 5y ago

Massachusetts has almost universal health care coverage due to it being a priority of the Commonwealth. We have the lowest uninsured population in the USA at ~97% coverage. (Lowest coverage state? Texas, ~82%.) I used to go to a local medical center in Boston that was apparently mostly serving Medicaid recipients. It had an MRI area that was like an assembly line -- huge waiting room, dozens of people constantly shuffling in and out of MRI rooms to get scans for whatever reason. So, a faster MRI machine means more people can get more scans. Which might be the difference between life and death, especially when you're on Medicaid. (Source: https://www.kff.org/other/state-indicator/total-population/?currentTimeframe=0&sortModel=%7B%22colId%22:%22Uninsured%22,%22sort%22:%22asc%22%7D https://www.kff.org/other/state-indicator/total-population/?..., 2019 data)