7 ms·

We've had preclinical 12T and way above machines for a while now (Bruker will sell you a ~21T, 1 GHz Larmour frequency machine, for instance). They are great fo

We've had preclinical 12T and way above machines for a while now (Bruker will sell you a ~21T, 1 GHz Larmour frequency machine, for instance). They are great for some forms of spectroscopy, but the issue for imaging is that t2* becomes very short, t1 becomes long, and it becomes impossible to have good b1 or b0 homogeneity. This means that the images are not necessarily filled with as much contrast as you'd like, tend to be a bit artefact-filled, and will keep physicists like me in pay for a long time. Still very cool though, and well done to the CEA team.

xattt 5y ago
Is there a magnet strength sweet spot for diagnostic imaging?
- lostlogin 5y ago
  I’m an MR tech. 1.5T magnets are a joy to use. Ideally a small bore and with big gradients (please) - Siemens Avanto Fit or similar.
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 ago
  More powerful isn’t as practical as it sounds
- whatshisface 5y ago
  B0 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 ago
    That was less English than the first guy
  - l33tman 5y ago
    T2-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 ago
      Oh, 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 ago
  With 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.
  - hoseja 5y ago
    Sounds like the word impedance could be fitted in there somewhere.
  - phkahler 5y ago
    Why is relaxation important? Is it a precondition for starting a new measurement? If so why?
Goety 5y ago
What is it about a stronger magnetic field in practice that decreases b1, b0? Is it that we cant create a strong magnetic field with good homogeneity in a small space? Are sensors also a limiting factor?
- azalemeth 5y ago
  [2/2 -- see above about not knowing HN had a character limit!] Now, for B0. The simple explanation is this: as alluded to above, we're all made of very different stuff. Bone, air, fat and muscle all have very different electrodynamic properties. At their interfaces, there's a "rule" that comes from Maxwell's equations† that means that you _must_ have a discontinuity in both the normal component of the electric (D) field in media and the tangential component of the magnetic field (H). This isn't negotiable, under the assumption that your patient does not have a net overall charge or current flowing through them. What this means, practically, is you get tiny bound surface charges or currents appearing at the interface of different materials. These then change the magnetic field homogeneity -- how B_0 varies over space -- because they create magnetic fields that add to it (the principle of superposition). Remember, all of MRI is based on having extremely homogeneous fields so that you can make a mapping between frequency and phase of your signal to the space of your sample. Inhomogeneities destroy that -- they can e.g. make two places look the same if they should be different, for example. B0 homogeneity is a big problem: we need the field to be uniform across the sample. Practically, what that means is twofold. One, for big changes -- caused by air, or a dental filling typically -- you get large, far-field effects that distort the image macroscopically. The maxillary and ethymoid sinuses (in the front of your face) together with the hard palette make it harder to image certain regions of the brain -- the bits that are basically directly above the roof of your mouth. This is _entirely_ due to the difference in the dielectric properties of air and "meat". Lung imaging with MRI is very, very challenging (but possible!). What works well for the brain does _not_ work at all, really, on the long -- and especially not at ultrahigh field. Even something as "simple" as cardiac imaging becomes more difficult, because the heart contains oxygenated and deoxygenated blood in close proximity -- and they have different dielectric properties. Secondly, all tissue on a microscopic level is inhomogeneous. Cells have bilayers and a complex microstructure. Organs have microstructures made up of different types of cells. The liver has a a beautiful microstructure, and a lot of iron in it. This causes a decrease in the effective static field inhomogeneity -- which manifests itself in terms of an NMR relaxation parameter called T2* -- meaning, in short words, that the MRI signal vanishes more quickly over time than it does at lower fields. That's why livers tend to look a bit darker than their surrounding organs on some types of MRI scans. If you play a single RF pulse and acquire signal on an NMR tube of pure water in a chemistry lab, you'll get a voltage you could measure with a hand-held AC voltmeter appearing for seconds after the initial excitation. Put a sphere of water in a 12T MRI scanner, and frankly you're doing well to have signal there after ~500 ms. In vivo, that's likely to be ~30-50 ms. That makes actually doing the experiments hard - your [gradient-echo] signal vanishes quickly. The whole B0 homogeneity thing is a coupled problem, best solved by considering Laplace's equation for the scalar magnetic potential (\nabla^2 \psi = 0). This famously has a solution space that can be expressed as an infinite sum of spherical harmonic functions. So, MRI scanners (yes, even clinical ones!) have a series of loops of wire that generate B-fields that approximate the first few spherical harmonics -- in Cartesian coordinates, they're weird things like x^2-y^2 and look like the pictures of the orbitals for electrons inside a hydrogen atom, if you've ever seen that. Patient goes in scanner, B0 field inhomogeneity is measured, magic regression‡ works out the right coefficients to dial a current into each coil, and the field is magically more homogeneous. Now, the trouble with this is that the spherical harmonics are a sum until infinity. And you can't build an infinite number of coils. The first few components get you most of the way there...but not all. And again, at higher field building effective shim coils, working out what your artefact is and where it comes from, and so on, all become more difficult. </end lecture> MRI is complicated -- it spans mathematics, physics, chemistry, engineering, biochemistry, and medicine. Frankly, that's why I like working in it as an academic area -- there's always something to learn. There are about 6000 others who do so technically, and we have a big, week-long worldwide conference once a year to both teach this stuff (where I have lectured) in quantitative detail, and show off our latest and greatest results (the usual big "evil" companies typically take academic work and put it in their products -- which I find rewarding, as it means your work can have a difference quickly and you don't have to deal with the FDA). I'm sure CEA / ISEULT will present there and I am genuinely excited that their mega-machine has finally been built. But there are a lot of challenges to overcome. Of course, there are also a huge number of opportunities -- it is (hopefully!) going to make it possible to image at higher spatial resolution, and also be much better for MR spectroscopy, and reveal information about low concentrations of biomolecules. But I guess the main point of my comment originally last night was trying to convey "the journey is really just beginning" and not "expect this in your local hospital in five years time". --- [1] https://sci-hub.st/10.1002/cmr.b.21319 https://sci-hub.st/10.1002/cmr.b.21319 † Specifically Gauss's law and Ampere's law at the boundaries for the D or H fields giving rise to the rule that there must be a discontinuity in D, with the difference normal components being equal to the bound surface charge density; and the difference in the tangential part of the H field being equal to the bound surface current. Putting these together with the constitutive relationships for D and H, and you get that the surface current is equal to B_{tangential, 1} / µ_1 - B_{tangential, 2} / µ_2 = J_s. In biology, µ is approximately fixed...so the surface current changes. ‡ Or a patient graduate student / radiographer...
Forge36 5y ago
Who are you working on this with?
- xcambar 5y ago
  FYI, the HN bio says it all > Medical physicist. Nerd. [...]
kergonath 5y ago
Yes, their claim is for the most powerful full-body scanner. AFAIK and for perspective, the highest man-made magnetic field is around 45 T, the LHC magnets are ~9 T, and those in ITER are (will be) 12 T as well.

xattt 5y ago

Is there a magnet strength sweet spot for diagnostic imaging?

lostlogin 5y ago
I’m an MR tech. 1.5T magnets are a joy to use. Ideally a small bore and with big gradients (please) - Siemens Avanto Fit or similar.

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 ago
More powerful isn’t as practical as it sounds
whatshisface 5y ago
B0 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 ago
  That was less English than the first guy
- l33tman 5y ago
  T2-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 ago
    Oh, 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 ago
With 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.
- hoseja 5y ago
  Sounds like the word impedance could be fitted in there somewhere.
- phkahler 5y ago
  Why is relaxation important? Is it a precondition for starting a new measurement? If so why?

Goety 5y ago

What is it about a stronger magnetic field in practice that decreases b1, b0? Is it that we cant create a strong magnetic field with good homogeneity in a small space? Are sensors also a limiting factor?

azalemeth 5y ago
[2/2 -- see above about not knowing HN had a character limit!] Now, for B0. The simple explanation is this: as alluded to above, we're all made of very different stuff. Bone, air, fat and muscle all have very different electrodynamic properties. At their interfaces, there's a "rule" that comes from Maxwell's equations† that means that you _must_ have a discontinuity in both the normal component of the electric (D) field in media and the tangential component of the magnetic field (H). This isn't negotiable, under the assumption that your patient does not have a net overall charge or current flowing through them. What this means, practically, is you get tiny bound surface charges or currents appearing at the interface of different materials. These then change the magnetic field homogeneity -- how B_0 varies over space -- because they create magnetic fields that add to it (the principle of superposition). Remember, all of MRI is based on having extremely homogeneous fields so that you can make a mapping between frequency and phase of your signal to the space of your sample. Inhomogeneities destroy that -- they can e.g. make two places look the same if they should be different, for example. B0 homogeneity is a big problem: we need the field to be uniform across the sample. Practically, what that means is twofold. One, for big changes -- caused by air, or a dental filling typically -- you get large, far-field effects that distort the image macroscopically. The maxillary and ethymoid sinuses (in the front of your face) together with the hard palette make it harder to image certain regions of the brain -- the bits that are basically directly above the roof of your mouth. This is _entirely_ due to the difference in the dielectric properties of air and "meat". Lung imaging with MRI is very, very challenging (but possible!). What works well for the brain does _not_ work at all, really, on the long -- and especially not at ultrahigh field. Even something as "simple" as cardiac imaging becomes more difficult, because the heart contains oxygenated and deoxygenated blood in close proximity -- and they have different dielectric properties. Secondly, all tissue on a microscopic level is inhomogeneous. Cells have bilayers and a complex microstructure. Organs have microstructures made up of different types of cells. The liver has a a beautiful microstructure, and a lot of iron in it. This causes a decrease in the effective static field inhomogeneity -- which manifests itself in terms of an NMR relaxation parameter called T2* -- meaning, in short words, that the MRI signal vanishes more quickly over time than it does at lower fields. That's why livers tend to look a bit darker than their surrounding organs on some types of MRI scans. If you play a single RF pulse and acquire signal on an NMR tube of pure water in a chemistry lab, you'll get a voltage you could measure with a hand-held AC voltmeter appearing for seconds after the initial excitation. Put a sphere of water in a 12T MRI scanner, and frankly you're doing well to have signal there after ~500 ms. In vivo, that's likely to be ~30-50 ms. That makes actually doing the experiments hard - your [gradient-echo] signal vanishes quickly. The whole B0 homogeneity thing is a coupled problem, best solved by considering Laplace's equation for the scalar magnetic potential (\nabla^2 \psi = 0). This famously has a solution space that can be expressed as an infinite sum of spherical harmonic functions. So, MRI scanners (yes, even clinical ones!) have a series of loops of wire that generate B-fields that approximate the first few spherical harmonics -- in Cartesian coordinates, they're weird things like x^2-y^2 and look like the pictures of the orbitals for electrons inside a hydrogen atom, if you've ever seen that. Patient goes in scanner, B0 field inhomogeneity is measured, magic regression‡ works out the right coefficients to dial a current into each coil, and the field is magically more homogeneous. Now, the trouble with this is that the spherical harmonics are a sum until infinity. And you can't build an infinite number of coils. The first few components get you most of the way there...but not all. And again, at higher field building effective shim coils, working out what your artefact is and where it comes from, and so on, all become more difficult. </end lecture> MRI is complicated -- it spans mathematics, physics, chemistry, engineering, biochemistry, and medicine. Frankly, that's why I like working in it as an academic area -- there's always something to learn. There are about 6000 others who do so technically, and we have a big, week-long worldwide conference once a year to both teach this stuff (where I have lectured) in quantitative detail, and show off our latest and greatest results (the usual big "evil" companies typically take academic work and put it in their products -- which I find rewarding, as it means your work can have a difference quickly and you don't have to deal with the FDA). I'm sure CEA / ISEULT will present there and I am genuinely excited that their mega-machine has finally been built. But there are a lot of challenges to overcome. Of course, there are also a huge number of opportunities -- it is (hopefully!) going to make it possible to image at higher spatial resolution, and also be much better for MR spectroscopy, and reveal information about low concentrations of biomolecules. But I guess the main point of my comment originally last night was trying to convey "the journey is really just beginning" and not "expect this in your local hospital in five years time". --- [1] https://sci-hub.st/10.1002/cmr.b.21319 https://sci-hub.st/10.1002/cmr.b.21319 † Specifically Gauss's law and Ampere's law at the boundaries for the D or H fields giving rise to the rule that there must be a discontinuity in D, with the difference normal components being equal to the bound surface charge density; and the difference in the tangential part of the H field being equal to the bound surface current. Putting these together with the constitutive relationships for D and H, and you get that the surface current is equal to B_{tangential, 1} / µ_1 - B_{tangential, 2} / µ_2 = J_s. In biology, µ is approximately fixed...so the surface current changes. ‡ Or a patient graduate student / radiographer...

Forge36 5y ago

Who are you working on this with?

xcambar 5y ago
FYI, the HN bio says it all > Medical physicist. Nerd. [...]

kergonath 5y ago

Yes, their claim is for the most powerful full-body scanner. AFAIK and for perspective, the highest man-made magnetic field is around 45 T, the LHC magnets are ~9 T, and those in ITER are (will be) 12 T as well.