PHYS1008 Study Guide - Final Guide: Spin Echo, Physics Of Magnetic Resonance Imaging
2 Aug 2018
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Tr (repetition time) = time between 90 degree rf pulses. Te (echo time) = time between 90 rf pulse and central point of signal detection. Tr does not change the rate at which t1 recovers, this is determined by the tissue properties. One gets to 100% of mo, the other only gets back to 80%, so when you flip the spins back in the xy plane there is only 80% that are coherent. Doesn"t start from the same point, as it never made it back to the 100% signal. There is less protons aligned from before we moved it (less coherence to start with). By changing the tr you change the t2, because of the t1 not regaining fully. Top is t1, do not change the t1, tr does not change the rate that t1 recovers, this is determined by the tissue. When you change the tr you do change the initial value of magnetisation that the system gets to.
