If the TR in a gradient echo sequence is less than the T2 (or T2*), what is the resulting condition called?

In MRI, the condition where the repetition time (TR) in a gradient echo sequence is shorter than the T2 or T2* relaxation time leads to the situation known as a steady state. This occurs because the TR is set to a value that allows the system’s longitudinal magnetization to recover partially, but not completely, before the next excitation pulse occurs. As a result, the magnetic moments of the spins are consistently partially aligned, leading to a stable signal that can produce a high signal-to-noise ratio.

In this steady state, the magnetization remains in a balance where the residual longitudinal magnetization is consistent with a repetitive sequence of excitations. This characteristic is essential in certain MRI techniques, especially in time-of-flight imaging and gradient echo sequences, where rapid imaging is critical.

Understanding the concept of steady state is vital for optimizing sequences in MRI for specific applications. It highlights the iterative and cumulative nature of magnetization in sequences with repeated excitations, which is a key factor in achieving desirable imaging outcomes.