MRI Physics

Basics

• Protons have a positive charge and normally spin at baseline

  • When there is a moving electrical charge this creates a magnetic field (physics property you just have to know and accept)

• When the proton is put into an external magnetic field the protons will do one of 2-3 things

  • Align themselves parallel to the magnetic field

  • Align themselves anti-parallel to the magnetic field

  • Note that being in parallel and anti-parallel require different amounts of energy/energy levels to be able to do this

    • The protons will preferentially align with the position that requires the lower energy

      • However a stronger magnetic field can allow the protons to go to the higher energy alignment

  • The following example is great:

    • People can walk around their house on their feet like normal, or by standing on their hands and walking upside down

      • Obviously walking on our feet would be easier and require less energy so this will be the preferred alignment

        • However if I give you an energy drink you may feel energized to walk on your hands for a period of time

          • The hand vs foot walking is the parallel vs anti parallel orientation

          • The energy drink is the strength of the magnetic field

• It should also be noted that the alignment and pull of the protons in their respective directions is additive

  • If there are 10 protons processing upward (parallel) and 5 processing downward (anti-parallel) the net magnetic field will be upward

  • This concept applies for the X-Y-Z axes

• SO…

  • When we put the protons in your body into an external magnetic field (MRI machine) the protons will align in the direction of the magnet (MRI machine)

    • The issue is that we cannot measure the strength of the magnetic force of just the protons in the body because they are in the same direction as the larger magnetic field made by the MRI machine which basically covers the small one formed by the body

      • So what can we do about this?

        • We can change the direction of the magnetic field so that it is transverse to the body and then record the difference

    • Big Takeaway is that we cannot measure the magnetization that occurs longitudinal to the magnetic field

  • Another great example:

    • A man is on a boat with a watering hose

      • If he points the hose into the ocean there is no telling how much water has been added to the ocean

      • However if we change the direction of the hose and point it to land then we can measure how much water is coming out of the hose

Procession

• The protons will not just sit still within the magnetic field, they will move in a somewhat cylindrical/circular pattern which is called procession

• Procession frequency is related to the strength of the magnetic field

  • Stronger magnetic field = faster procession = higher procession frequency

What happens when the patient is placed in the scanner

• We send a radiofrequency (RF) pulse at the patient (radio wave)

  • This disrupts the peace of the protons

  • Note that not all RF pulses can be used, we need one that has the same frequency as the protons as this allows them to exchanged energy which then results in the disruption of the protons that we want

    • When the RF pulse and protons have the same frequency energy can be transferred between the two = termed resonance

      • Hence resonance in MRI

• When the RF pulses gives energy to the protons it causes them to leave their low energy state (parallel/pointing upward) and become anti-parallel (point downward)

  • This results in a decrease in the magnetic field because now there are more protons pointing downward (anti-parallel) than before and this will causes a cancellation of the protons pointing upward (parallel)

Shimming/Shim Coils

• Shimming makes the magnetic field (B0) as homogenous as possible

• Reducing field inhomogeneities reduces artifacts with gradient echoes and chemical fat-saturation

  • Does not affect chemical shift artifact

• In spin-echo it is the refocusing pulse that reduces field inhomogeneities not all this shit

• How does it reduce field inhomogeneities

  • Passive shimming at time of installation (small strips of metal around the bore of the scanner)

  • Gradient offset shimming

  • Dedicated shim coils.

Diffusion Weighted Imaging

• B value

  • Higher b values/ADC values means

    • More signal loss = decreased signal to noise ration

    • Increases sensitivity to diffusion

    • Increased sensitivity to patient motion

Spin Echo MRI

• Has a 180 degree rephasing pulse

• Gradient refocusing refers to the bilobed frequency encoding gradient where a preparation part of the gradient dephases the spins, while the main part of the frequency encoding gradient rephases (refocuses) the dephased spins such that they are in phase in the middle of the readout. This refocusing is always done with the frequency encoding gradient

• Less affected by field inhomogenities