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### TOOLS» Time Constants

An understanding of time constants allows easier understanding of MRI signal equations. This tool allows you to build up the equations describing the change of magnetisation in basic pulse sequences.

# Time Constant Tool: y = exp(x/400)

We'll start with this simple equation: y = exp(x/400); an increasing exponential curve.

Do this...

## Sketch the following curves: f(x) = ex, f(x) = e-x, f(x) = e0.

We can see the general shape of exponential growth and decay curves:

We can take the decay curve (that is, the one with the minus sign: e-x) and amend the mathematical formula in order to create equations which accurately describe the decay of transverse magnetisation (Mxy), and the recovery of longitudinal magnetisation (Mz).

Navigate below to the next equation in the list: exp (-x/400).

Navigate through the tool...

Other sections of this tool:

Remember, T1, T2 and T2* are time constants.