# Time Constant Tool: Introduction

*Answer:* The time constant of an exponential is simply the denominator of the fraction to which *e* is powered.

A simple exponential decay may be defined in general form as

*f*(*t*) = *A*·*e*^{-t/τ }

where *f* is a function of time (*t*), *A* is the peak amplitude (a scalar) and *τ* is the time constant of the exponential. The time constant is the denominator of the fraction to which *e* is powered.

In MRI, T1, T2 and T2* are time constants.

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

Further reading on this topic:

Books: Q&A in MRI p29-30

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