5 ms·

MRI machines work using superconducting magnets as does the LHC. So you better believe it buddy.

by ramraj07 3y ago

Waterluvian 3y ago
Why do MRIs need zero resistance vs incredibly low resistance?
- wbl 3y ago
  The more you crank up the field and gradients the better resolution you can get. Being able to take MRI of a beating heart depends on that sort of thing.
- jacquesm 3y ago
  Because the resistance is the limiting factor in how strong a magnetic field can be made in a given volume using the conducting material. Copper across an area of 1 cm ^2 does about 300A, compare with https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8047038/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8047038/ for what some superconductors can do.
  - robotresearcher 3y ago
    Link says 90 MA per cm^2, or 30,000X more than copper. Gulp!
    - patrec 3y ago
      I think you are missing a zero.
      - robotresearcher 3y ago
        You are right! 300,000X more than copper. Typo converting from brain to screen. I should have used exponential notation and the mistake would have been more obvious.
  - yreg 3y ago
    That's not an answer to that question.
- thelazyone 3y ago
  I think it's a fair question. I guess the answer is "because we have superconductors, and no material with 'very small resistance' instead". Also note that while there is zero resistance you still have parasitic currents and general interaction with the rest of the environment. So no perpetual motion for us today!
  - jacquesm 3y ago
    Even a 'very small resistance' would likely be prohibitive. Pure silver for instance is an excellent conductor, but when you're talking about the last little bits it's orders of magnitude difference between that and a superconductor, and those orders of magnitude difference in resistance translate into orders of magnitude more current. So even a small resistance would cause your MRI machine to have a resolution so low as to be unusable.
    - jychang 3y ago
      Actually, there are infinite orders of magnitude between the resistance of silver and that of a superconductor.
      - deleted 3y ago
        [deleted]
    - obrix 3y ago
      MR imaging is possible with permanent magnets and resistive (copper wire wound) magnets at low field strengths, below 0.35T or so. Above that the heating of the magnet windings becomes excessive and it would be very difficult to maintain a stable enough field strength. Superconducting magnets are very nice as long as there is no quenching. The material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular...
      - stcredzero 3y ago
        The material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular I was going to ask: Why don't we use the current high temperature superconductors, and start building grid interconnects? I guess part of the answer lies in the cost that would be incurred because of the mechanical properties of the existing superconductors.
- userbinator 3y ago
  To create very, very strong magnetic fields.
- scythe 3y ago
  The current is ridiculously large and would lead to an unacceptably high power consumption and thermal expansion with the resistivity of even a good metal (copper).
- crote 3y ago
  They don't! MRI scanners work just fine with regular electromagnets, or even plain old normal magnets. However, work better with stronger magnetic fields. In practice normal magnets end up being extremely large and heavy, and electromagnets end up using massive amounts of power. Both options are also limited in their field strength because getting enough stuff close together is tricky. A superconducting magnet is an electromagnet which is way smaller and uses orders of magnitude less power. For extremely high field strength MRIs they are the only viable option, and for regular MRIs they are often the best option.

MRI machines work using superconducting magnets as does the LHC. So you better believe it buddy.

Waterluvian 3y ago

Why do MRIs need zero resistance vs incredibly low resistance?

wbl 3y ago
The more you crank up the field and gradients the better resolution you can get. Being able to take MRI of a beating heart depends on that sort of thing.
jacquesm 3y ago
Because the resistance is the limiting factor in how strong a magnetic field can be made in a given volume using the conducting material. Copper across an area of 1 cm ^2 does about 300A, compare with https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8047038/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8047038/ for what some superconductors can do.
- robotresearcher 3y ago
  Link says 90 MA per cm^2, or 30,000X more than copper. Gulp!
  - patrec 3y ago
    I think you are missing a zero.
    - robotresearcher 3y ago
      You are right! 300,000X more than copper. Typo converting from brain to screen. I should have used exponential notation and the mistake would have been more obvious.
- yreg 3y ago
  That's not an answer to that question.
thelazyone 3y ago
I think it's a fair question. I guess the answer is "because we have superconductors, and no material with 'very small resistance' instead". Also note that while there is zero resistance you still have parasitic currents and general interaction with the rest of the environment. So no perpetual motion for us today!
- jacquesm 3y ago
  Even a 'very small resistance' would likely be prohibitive. Pure silver for instance is an excellent conductor, but when you're talking about the last little bits it's orders of magnitude difference between that and a superconductor, and those orders of magnitude difference in resistance translate into orders of magnitude more current. So even a small resistance would cause your MRI machine to have a resolution so low as to be unusable.
  - jychang 3y ago
    Actually, there are infinite orders of magnitude between the resistance of silver and that of a superconductor.
    - deleted 3y ago
      [deleted]
  - obrix 3y ago
    MR imaging is possible with permanent magnets and resistive (copper wire wound) magnets at low field strengths, below 0.35T or so. Above that the heating of the magnet windings becomes excessive and it would be very difficult to maintain a stable enough field strength. Superconducting magnets are very nice as long as there is no quenching. The material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular...
    - stcredzero 3y ago
      The material used for conductors must be mechanically stable and perform consistently from one production batch to another. One reason why current high tc superconductors are not popular I was going to ask: Why don't we use the current high temperature superconductors, and start building grid interconnects? I guess part of the answer lies in the cost that would be incurred because of the mechanical properties of the existing superconductors.
userbinator 3y ago
To create very, very strong magnetic fields.
scythe 3y ago
The current is ridiculously large and would lead to an unacceptably high power consumption and thermal expansion with the resistivity of even a good metal (copper).
crote 3y ago
They don't! MRI scanners work just fine with regular electromagnets, or even plain old normal magnets. However, work better with stronger magnetic fields. In practice normal magnets end up being extremely large and heavy, and electromagnets end up using massive amounts of power. Both options are also limited in their field strength because getting enough stuff close together is tricky. A superconducting magnet is an electromagnet which is way smaller and uses orders of magnitude less power. For extremely high field strength MRIs they are the only viable option, and for regular MRIs they are often the best option.