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.

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Presentation transcript:

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 Fourier Transform (FT) Signal acquired by the NMR/MRI unit Line width  1/T 2

Linewidth is a f(T 2 )

Signal appearance in a Spin-echo Pulse Sequence

Graphical Representation of T1

T1 Measurement – Inversion Recovery

T2 Measurement by Spin Echoes

Exponential Recovery – T1

T1W Image contrast

T1W Brain Image

T2W Brain Image

T2W Human brain image

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 B o or z-axis and/or b) the dephasing of the NMR/MRI signal in the x-y plane

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