MRI Image Accquisition

Main coil: running a current through a wire generates a magnetic field around, running it in a coil superimposes the magnetic fields, increasing its size (Bo). Dependent on:

  • number of coils of wire

  • amount of current

Increase current = increase resistance

Superconductor: generates enough current to generate a superimposed magnetic field. Must be cooled using liquid helium to reduce vibration of the coil due to heat (which is due to current) which disrupts the path of electrons, causing resistance.

Shims alter the main magnetic field to reduce inhomogeneities in the magnetic field. Passive shim is a magnetic sheet in the mri bore to manipulate the magnetic field passing through. Active shim have a separate electric current which can be changed to manipulate the main magnetic field, can be in the bore or in the cooling chamber.

Gradient coils: coils in three different planes, changing the strength of the magnetic field along the coil or axis

Isocenter: part of the magnetic field along the gradient coil with the same strength at the original magnetic field

Radiofrequency coil

  • the longer the radiofrequency pulse is generated, the greater the flip angle in precession

Spin echo

  1. 90 degree excitation pulse

  2. NMV precesses in transverse plane inducing voltage

  3. Relaxation = free induction decay

  4. T2* dephasing due to magentic field inhomogeneities

  5. Loss of transverse magnetisation

  6. 180 degree regeneration pulse

  7. Faster spins catch up to slower spins = in phase synchronisation

  8. Maximum signal induced in receiver coil

Gradient echo

Spatial localisation: determining coordinates where signal is coming from in all three axes

Varying flip angles used and slice selection to target only hydrogens in a specific slice to flip into the transverse plane.

  1. Slice selecting gradient: z axis coordinates

    • precession in hydrogens must be differentiated using a gradient field in order for slice selection to take place

    • radiofrequency bandwidth (range of radiofrequency pulse) is applied so only resonating hydrogens will be flipped to the transverse magnetisation, producing signal

    • Changing slice selection by changing radiofrequency pulse, matching hydrogens with different precessional frequency

    • Changing slice selection by changing gradient magnetic field strength, matching hydrogens with different precessional frequency

    • Changing slice selection by physically moving the patient to the slice selecting region

    • Change slice thickness by changing radiofrequency bandwidth, changing steepness of gradient magnetic field

    • Slice phase is inhomogeneity in gradient magnetic field within a slice due to gradient steepness, therefore have a different precessional frequency, requires a rephasing gradient

  2. Frequency encoding gradient: x axis coordinates

  3. Phase encoding gradients: y axis coordinates