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RF Pulse – generates a B 1 field that realigns the precessing spins in the low energy state

RF Pulse – generates a B 1 field that realigns the precessing spins in the low energy state . In the case of a 90 o pulse the alignment is perpendicular to B o – the B 1 field is sufficiently strong to effect this realignment. The NMR/MRI Signal and FT. Signal acquired by

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RF Pulse – generates a B 1 field that realigns the precessing spins in the low energy state

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  1. RF Pulse – generates a B1 field that realigns the precessing spins in the low energy state In the case of a 90o pulse the alignment is perpendicular to Bo – the B1 field is sufficiently strong to effect this realignment

  2. The NMR/MRI Signal and FT Signal acquired by the NMR/MRI unit Fourier Transform (FT) Line width  1/T2

  3. Linewidth is a f(T2)

  4. Signal appearance in a Spin-echo Pulse Sequence

  5. Graphical Representation of T1

  6. T1 Measurement – Inversion Recovery

  7. T2 Measurement by Spin Echoes

  8. Exponential Recovery – T1

  9. T1W Image contrast

  10. T1W Brain Image

  11. T2W Brain Image

  12. T2W Human brain image

  13. Pulses, T1 and T2 • Pulses have nothing to do with either T1 or T2 • Relaxation mechanisms/processes pertain to the a) recovery of magnetization along the Bo or z-axis and/or b) the dephasing of the NMR/MRI signal in the x-y plane

  14. The Importance of Relaxation Phenomena for fMRI and Clinical MRI • Small differences in T1 and T2 can provide clear-cut diagnostic interpretations • Small changes in signal intensity from the decrease of deoxyhemoglobin and the increase of oxyhemoglobin due to capillary dilation with brain activation are the basis for BOLD fMRI • MRI ‘contrast agents’ all rely on changing T1 or T2 of nearby water protons to give better disease changes contrast

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