T1 and T2

Excitation causes parallel nuclei spinning in direction of the magnetic field axis of rotation to change in longitudinal magnetisation, forcing them into another plane. High energy nuclei are aligned anti parallel to the magnetic field ordinarily however.

Excitation also causes nuclei to synchronise in precession, called spinning in phase with each other. Precession is the spinning of the nuclei along the transverse plane so is the transverse magnetisation of the protons.

Protons recover and return to original state of orientation with the magnetic field and asynchronous precession.

Spin Echo sequence

Flip angle is the change in orientation of the net magnetisation vector in the longitudinal magnetisation and is dependent on the amount of energy applied by the radiofrequency pulse.

  1. 90 degree pulse applied reaching a 90 degree perpendicular net magnetisation vector. This eliminate longitudinal magnetisation and establishes a transverse magnetisation vector by synchronising proton precession.

  2. Recovery causes dephasing of protons, decreasing the transverse magnetisation and increasing the longitudinal magnetisation. Recovery of net magnetisation vector occurs in a spiral pathway inducing an electrical signal, this is called free induction decay. (Inducing a current by rotating a magnetic field).

  • positively charged particles spin and produce small magnetic field, which cross the receiver coil and induce a voltage when they relax

  • greater voltage = brighter tissue

T1: time at which 63% of the longitudinal magnetisation has been recovered (recovery caused by interactions with the environment)

  • T1* is not present because dephasing does not affect longitudinal magnetisation and t1 is the average time constant of the magnetic field as homogeneities in the magnetic field still affect t1

  1. 90 degree pulse applied

  2. Signal sampled at echo time

  3. Recovery occurs in longitudinal plane

  4. 90 degree pulse reapplied

  • longitudinal magnetisation vectors are equal to the x axis value of the longitudinal magnetisation due to loss of transverse magnetisation (precession)

T2: time at which 63% of the tranverse magnetisation has been lost (dephasing caused by spin spin interactions)

T2* constant: speed of protons dephasing

T2* decay is T2 decay (precession) affected not only by spin spin interactions but in practically also affected by magnetic field inhomogeneities, so that each proton experiences the magnetic field at a slightly different strength, meaning there is never true uniformity in precession.

  • MRI scanner cannot make a perfect strength magnetic field that’s equal all the way through the magnetic field

  • Substance in patient causing disruption in local magnetic fields

  • Spins start to dephase, magnetic vectors are out of phase with one another and disrupt the local magnetic field

Differences in precession compile, inhomogeneity in magnetic field make dephasing worse and therefore signal dropout worse. T2* effects.

COMPENSATING FOR T2* DECAY

or

ECHO

Differences in precession speed are fixed and predictable in protons so can be made up for using a 180 degree refocusing pulse, instructing protons to precess in the opposite direction. More energy is released back into the system once the protons return to spinning in the opposite direction. Eventually dephasing carries out completely as all energy applied to protons is exhausted and the sequence must be restarted.

PROTON DENSITY

Number of protons in each type of tissue is different therefore in reality different signals are recorded when rf pulse is applied despite us assuming all tissues have the same starting point in the longitudinal magnetisation.

Long repetition time and short echo time to negate the values of t1 and t2 weighted images and focus on the difference in magnetisations caused only by magnetic fields of hydrogens, therefore representative of hydrogen concentration.