Relaxation Parameters in NMR Imaging

Contrast in NMRI depends on both material-specific and operator-selected parameters. The material-specific parameters include the spin density and the relaxation times Tj and T2. The operator-selected parameters include the pulse sequence (inversion recovery, spin-echo, etc.) and the pulse delay and repetition times (timing parameters). For a given imaging system and pulse sequence, it is the delay and repetition times in conjunction with the intrinsic material parameters which dictate the appearance of the final image. If the correct pulse sequence is employed and the relaxation times of the two materials are known, it is possible to calculate the delay and/or repetition times that will produce the maximum diflerence in signal intensity between those materials. 

F%are 6.2 The timing diagram for a spin-echo imaging pulse sequence using a selective 90° pulse 

Spin-echo techniques have a unique position in NMR applications. The main problem with NMR imaging is the long data collection time, due mainly to the spin-lattice relaxation time Each measurement necessitates a time period of the order of T, (which is 0.5 s for aqueous systems) for the system to return to 

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