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In more complete analyses of the spin echo pulse sequence, one does need to account for the small amount of regrowth of longitudinal magnetization that occurs in the time interval between the 90° and 180° pulses. As we show in a later Q&A, the classic formula predicting the signal intensity (S) from a stationary tissue with spin density (ρ) and relaxation times (T1 and T2) subjected to a SE sequence with repetition time (TR) and echo time (TE) is given by
S = k ρ (1 − e−TR/T1) e−TE/T2
where k is a scaling factor. This equation is an approximate solution to the Bloch equations neglecting the inversion of the small amount of longitudinal magnetization that regrows in the interpulse interval. The complete equation, taking into account the longitudinal inverting property of the 180° pulse, is written
S = k ρ (1 − 2e−(TR−TE/2)/T1 + e−TR/T1) e−TE/T2
For TR>>TE/2, this equation reduces to the more simplified one above.
Elster AD, Burdette JH. Questions and Answers in MRI, 2nd ed. St. Louis: Mosby, 2001, pp 51-2.
How is a spin echo generated?