3 ms·
Could you repeat this in english? It's good for imaging big features but not small features, or high frequency density changes, or takes too long, or what?
by Sophistifunk 5y ago
Could you repeat this in english? It's good for imaging big features but not small features, or high frequency density changes, or takes too long, or what?
- m3kw9 5y agoMore powerful isn’t as practical as it sounds
- whatshisface 5y agoB0 is the magnetic field that separates the nuclear spin energy levels. It's the one that's really big in this machine. B1 is the radiofrequency pulse that excites the nuclear spins. Poor B0 and B1 homogeneity means you are taking the MRI with a slightly different machine at every point. T1 is the time it takes for the energy that B1 adds to the nuclear spins to leak back out of the nuclei. It generally happens faster when the energy per nucleus, which is proportional to the degree to which B0 has split the energy levels, is low. If I had to guess, it's probably because there are more weak couplings by which a nucleus can get rid of a little energy than there are strong couplings that can support a big jump. One of the major things that limits the speed at which NMR can be done is the fact that you have to wait for this energy to leak out before you can put more in for another round of imaging. (Maybe there is a way around this but if so they never taught me about it.) T2 is the time it takes for the excited spins to fall out of phase with each other. It happens faster when there is a greater diversity of electronic influences on each nuclear environment - a bunch of identical atoms in a perfect lattice would never go out of phase. If you "zoom in" on T2 and study it with incredible precision you can pick apart each distinct environment around each population of similar nuclear neighborhoods. A population of identical neighborhoods, in practice, means a chemical. Chemists use that a lot but it's not done in imaging.
- simooooo 5y agoThat was less English than the first guy
- l33tman 5y agoT2-weighted imaging is definitely used in medical settings. Generally MR is good for soft tissue and nervous system diagnosis and T2 is good because it gives contrast between otherwise very similar intensity regions in the brain for example.
- whatshisface 5y agoOh, I know about T2 imaging, I was talking about extracting the detail of T2 to compute an NMR spectrum, like they use in chemistry labs.
- fabian2k 5y agoWith stronger magnets the part of the signal that contains the actual information decays faster in this case. At the same time the total magnetization, most of which is useless to you, decays slower. And you need to wait until it decays to measure again. This explanation is extremely simplified, and the details here depend on the situation. Relaxation is a critical part of MRI and NMR, and there are different mechanisms that contribute to it. The sensitivity of magnetic resonance methods increases with the magnetic field strength to the power of 3/2. But other effects like the mentioned relaxation cause certain practical issues at higher fields. So you don't always get the full benefit of the higher magnetic field, depending on what exactly you are doing.