[Scoundreller]

I may be MRI ignorant, but even if the Teslas are lower, if the radius is smaller (say, can only fit a head or arm or leg), does that make up for it?

I understand MRIs are largely a compromise. Organizations will buy only 1 or 2, and therefore buy one that will fit all but the most obese patients, to the imaging detriment of anyone/anything smaller. Or is that all wrong?

[SimplyUnknown]

No. More teslas are needed for more signal to improve the signal to noise ratio. And there is a lot of noise with portable non-shielded MRI. The only way to compensate for this is by having bigger voxels (3-5mm per voxel rather than the standard 1-2mm), or repeat scans and perform averaging. The more drastic way is to do funky image reconstruction with compressed sensing or deep learning.

The wider bore does indeed come at a cost. You have more geometric distortion and a less homogenous magnetic field at the edges of the bore. Also, you will need stronger magnetic gradients to form image, causing more energy disposition (i.e. Heating the patient) and higher probability of peripheral nerve stimulation (PNS, involuntary muscle twitches due electric fields induced in the nerve caused by MR gradients).

[lostlogin]

To add to this comment, use of strong gradients also slows down scanning When you hit SAR limits. The SAR has to be brought down with pauses between scans, reducing gradient amplitudes, reduced resolution or shorter echo trains. It’s really painful.

[ska]

The compromise for bore size is convenience in fitting things in it vs homogeneity of field, field strength, and cost. This is why standard bore 7T clinical imaging systems are pretty niche, but 9-12T mouse imaging systems aren't.

To get a good high resolution signal you need field homogeneity (and/or accurate mapping), high quality gradients, and good SNR. Field strength helps with the last part.

There are other trade offs too, for example imaging artifacts due to implants (or even being able to image them), susceptibility, etc. vary with main field strength.

[superkuh]

>need field homogeneity (and/or accurate mapping), high quality gradients

It always confused me that MRI did millimeter scale imaging with radio waves that are meters long. I learned, and other's might be interested to learn, the trick is that it's the magnetic gradient that does the imaging. Each nucleus in the volume is tagged to it's place along the magnetic gradient axis by the local magnetic field strength shifting the emission frequency proportionally. That frequency shift is then inverted back to position for that axis.

[ska]

Good point. It’s easy to forget this stuff isn’t obvious.