spins and relaxation06

Spins and Relaxation Daniel Bulte FMRIB Graduate Program Lecture – MR Physics Objectives • • • • • • How does MRI work? What is resonance? Where does “the signal” come from? What are T1 and T2? Where does contrast come from? Please ask questions! Spin • Fundamental property of particles, like mass and charge • Spin comes in quanta of 1/2 (for Fermions) • Electrons, protons and neutrons all have spin 1/2 • Pairs of these subatomic particles tend to align with opposite spin and “cancel out” Nuclear Spin Some nuclei have Spin If a nucleus has an unpaired proton it will have spin and it will have a net magnetic moment or field ⇒ NMR phenomenon Common NMR Active Nuclei Isotope Spin I % abundance γ MHz/T H H 13 C 14 N 15 N 17 O 19 F 23 Na 31 P 1/2 1/2 1/2 5/2 1/2 3/2 1/2 99.985 0.015 1.108 99.63 0.37 0.037 100 100 100 42.575 6.53 10.71 3.078 4.32 5.77 40.08 11.27 17.25 γ = gyromagnetic ratio Alignment of Spins in a Magnetic Field M magnetic moment M=0 spin B0 field Energy in a Magnetic Field (Zeeman Splitting, Spin ½) mI = +½ E+1/2= −γ B0/2 mI = −½ P+1/2= 5000049 P(E) ∝ exp(−E/kT) E-1/2= +γ B0/2 P-1/2= 0.4999951 Larmor Frequency mI = −½ mI = +½ E+1/2= −γ B0/2 E-1/2= +γ B0/2 Allowed transitions ∆E = γ B0 = ω0 ω0 = γ B Larmor Frequency z B0 ω0 M0 Spins “precess” at Larmor frequency Net magnetisation M0 is static x ω = γ B0 Precession QuickTime™ and a YUV420 codec decompressor are needed to see this picture T2 Decay Curves Echo Amplitude Long T2 (CSF) Medium T2 (grey matter) Contrast Short T2 (white matter) TE T2 Weighted Image T2/ms CSF grey matter 500 80−90 white matter 70−80 1.5T SE, TR=4000ms, TE=100ms Relaxation in a Nutshell QuickTime™ and a YUV420 codec decompressor are needed to see this picture What is T2* • • Loss of coherence in transverse magnetisation also occurs as a result of bulk magnetic effects Spatial static B0 field variations within a voxel lead to identical effects on the signal as spin-spin interactions T2 vs T2* • • • • The T2 relaxation time depends primarily on spin-spin interactions - non-reversible T2* depends on both spin-spin interactions AND the homogeneity of the external magnetic field - reversible Homogeneity depends on how good your magnet is, and susceptibility-induced field distortions due to the presence of different tissues T2* can be considered as the observed or effective transverse relaxation time Optimal TR and TE for T1 Contrast TE 1 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 MR Signal MR Signal TR 0.5 0.4 0.3 0.2 T1 contrast 0.4 0.3 T2 contrast 0.2 0.1 0.5 0.1 0.2 0.4 0.6 0.8 1.2 T1 Recovery 1.4 1.6 1.8 sec 0 10 20 30 40 50 60 T2 Decay 70 80 90 100 ms Optimal TR and TE for T2* and T2 Contrast TE 1 0.9 0.9 0.8 0.8 0.7 0.7 MR Signal MR Signal TR 0.6 0.5 0.5 T1 Contrast 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 T2 Contrast 0.6 0.2 0.4 0.6 0.8 1.2 T1 Recovery 1.4 1.6 1.8 sec 0 10 20 30 40 50 60 T2 Decay 70 80 90 100 ms Important Point • • • • TR controls T1 weighting TE controls T2 weighting Short T2 tissues are dark on T2 images Short T1 tissues are bright on T1 images Reversing T2* Losses • • • A spin echo can refocus spins that are sitting in a time invariant B0 field A spin echo cannot refocus T2 dephasing A spin echo cannot refocus spins that have experienced a time varying field, for example diffusing molecules The Spin Echo t=0 The Spin Echo t=τ The Spin Echo t = 2τ Tutorial http://www.revisemri.com/questions/ basicphysics/ QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture ... and T2? Where does contrast come from? Please ask questions! Spin • Fundamental property of particles, like mass and charge • Spin comes in quanta of 1/2 (for Fermions) • Electrons, protons and. .. particles tend to align with opposite spin and “cancel out” Nuclear Spin Some nuclei have Spin If a nucleus has an unpaired proton it will have spin and it will have a net magnetic moment or field... B0 = ω0 ω0 = γ B Larmor Frequency z B0 ω0 M0 Spins “precess” at Larmor frequency Net magnetisation M0 is static x ω = γ B0 Precession QuickTime™ and a YUV420 codec decompressor are needed to