Contrast T1 weighted – (MPRAGE-anatomical) T2 weighted – (fmri)

Contrast

T1 weighted – (MPRAGE-anatomical)T2 weighted – (fmri)

Spin Relaxation• Spins do not continue to precess forever• Longitudinal magnetization returns to equilibrium

due to spin-lattice interactions – T1 decay

• Transverse magnetization is reduced due to both spin-lattice energy loss and local, random, spin dephasing – T2 decay

• Additional dephasing is introduced by magnetic field inhomogeneities within a voxel – T2' decay. This can be reversible, unlike T2 decay

T1 decay – “spins back down”

Collective MagneticMoment of Protons

end

start

B0

signalwe “hear”

V

Time

T1 Recovery

MRSignal

Time

Typical T1 Graph

1 s1 s

T2 decay – separation (dephasing) of “collective magnetic moment”

sometime after RF excitationImmediately after RF excitation

=

collective magnecticmoment

individual spins

separation (dephasing)

a little time later

T2 Decay

MRSignal

Typical T2 Graph

Time50 ms50 ms

Spin Echo

Y

X

T2 Decay

MRSignal

T1 Recovery

MRSignal

50 ms50 ms 1 s1 s

Proton Density Contrast

TE – echo time TR – repeat time

Proton Density Weighted ImageProton Density Weighted Image

T2 Decay

MRSignal

T1 Recovery

MRSignal

50 ms50 ms 1 s1 s

T1 Contrast

time time

TE – echo time TR – repeat time

T1 Weighted ImageT1 Weighted Image

T2 Decay

MRSignal

T1 Recovery

MRSignal

50 ms50 ms 1 s1 s

T2* and T2 Contrast

TE – echo time TR – repeat time

T2 Weighted IMageT2 Weighted IMage

ProtonProtonDensityDensityWeightedWeightedImageImage

T1 T1 Weighted Weighted ImageImage

T2 T2 Weighted Weighted ImageImage

Properties of Body TissuesTissue T1 (ms) T2 (ms)

Grey Matter (GM) 950 100

White Matter (WM) 600 80

Muscle 900 50

Cerebrospinal Fluid (CSF) 4500 2200

Fat 250 60

Blood 1200 100-200

MRI has high contrast for different tissue types!