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 de

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