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Contrast in MRI

Contrast in MRI. Signal (measure of Mx, My). T 2 decay. TE. S= S o e -TE/T2. Perturbation of M o. When you perturb Mo, it will recover back to equilibrium with time constants T1 and T2. Signal (measure of Mz). TI. S = So (1-e -TI/T1 ). Dipole-dipole interaction.

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Contrast in MRI

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  1. Contrast in MRI

  2. Signal (measure of Mx, My) T2 decay TE S= So e -TE/T2 Perturbation of Mo • When you perturb Mo, it will recover back to equilibrium with time constants T1 and T2 Signal (measure of Mz) TI S = So (1-e-TI/T1)

  3. Dipole-dipole interaction The dipole field around the proton at A alters the field experienced by the proton at B

  4. Molecular relaxation mechanisms Protons Macromolecule Dipole coupling Spin exchange Proton exchange Molecular exchange Water

  5. Measuring T1 by inversion recovery

  6. T1 measurement TI = 100, 250, 500, 1000, 1500, 2000, 3000, 5000 Signal from amplitude of FID TI S = So (1-2e ) -TI/T1

  7. T2 (top) and T1 (bottom) maps

  8. T1 of normal appearing white matter in MS

  9. Vector coherence Schering

  10. Fast spins (Dw +ve) x’ Mxy=0 Mxy at t=0 Slow spins (Dw -ve) Complete dephasing Time Dephasing

  11. T2* artefacts Good(ish) shim Phantom with coin near it Bad shim

  12. BOLD effect due to T2* effects around blood vessels

  13. 180o 90o T2 envelope FID T2* envelope fast f s slow Spin echo

  14. Fate of multiphase meals Liver Meal liquid L R viscous Spleen Spinal cord 36 min 48 min 72 min Anatomical reference T = 90 mins Slow mixing of a porridge meal T= 14 mins Monitoring dilution

  15. T2 for measuring brain iron and myelin

  16. Weighted images

  17. Signal Signal T1 weighting T2 weighting TI High contrast Low contrast TE High contrast Low contrast

  18. SMA blood flow

  19. Elastography- internal palpation

  20. Pre and post Gd

  21. Contrast enhancement

  22. Gd-DTPA molecule Structure of Ln(III)-DTPA complexes. Oxygen atoms are red, nitrogen atoms blue, carbon atoms grey and lanthanides green.

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