Relaxation Exponential time constants T1 T2 T2*

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Presentation transcript:

Relaxation Exponential time constants T1 T2 T2* Longitudinal (spin-lattice) Inversion recovery experiment T2 Transverse (spin-spin) relaxation Spin-echo experiment T2* T2 with field inhomogeneities Gradient echo experiment TE = 15 ms TE = 20 ms TE = 30 ms TE = 60 ms Relaxation

Exponential decay  = 30 ms Relaxation

T2 TE = 15 ms TE = 20 ms TE = 30 ms TE = 60 ms  = T2= 30 ms Relaxation

Exponential recovery Inversion recovery Saturation recovery Relaxation

T1 Inversion recovery T1 = 2000 ms TI = 50 ms 100 ms 400 ms 800 ms Relaxation

Pulse sequence diagrams Magnetization preparation Excitation Read-out Nrep RF GSS GPE GRO DAC Relaxation

Spin Echo EPI pulse sequence (not to scale) Excitation pulse Refocusing pulse RF GSS … GPE … GRO … DAC Relaxation

Spin Echo measures T2, not T2* Excitation pulse Refocusing pulse Echo Read-out MT T2 T2* S Relaxation

Spin Echo EPI pulse sequence TE/2 TE/2 RF GSS GPE GRO DAC Relaxation

Inversion Recovery Spin Echo EPI TE/2 TE/2 RF GSS GPE GRO DAC Relaxation

Inversion Recovery FLASH NPE RF GSS GPE GRO DAC NRO Relaxation

Spin Echo: erasing magnetic field imperfections Imaging signal comes from protons on water molecules. Frequency map, zoomed in on lateral temporal cortex Hz On resonance Sensitive to macro- and microscopic variations in B0. 100 Hz off resonance 250 Hz off resonance Relaxation

Spin Echo: erasing magnetic field imperfections Summing all spins (e.g. axial slice) creates rapid signal decay Relaxation

Spin Echo: erasing magnetic field imperfections Applying a 180 pulse at TE/2 refocuses the inhomogeneity-induced dephasing at TE t = 0 ms t = TE/2 t = TE Relaxation

SE EPI: reduction of through-slice dephasing Gradient echo Spin echo Relaxation