contrast t1 weighted – (mprage-anatomical) t2 weighted – (fmri)
DESCRIPTION
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 – T 1 decay - PowerPoint PPT PresentationTRANSCRIPT
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!