Magnetic Resonance Imaging (MRI) Practice Test

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

1. Equilibrium State

2. Steady State

3. Dynamic Equilibrium

4. Transient State

The correct answer is: Steady State

When the TR (repetition time) in a gradient echo sequence is less than T2 (or T2*), it leads to a condition known as the steady state. In MRI, the steady state is the condition achieved when the system maintains a balance between the longitudinal magnetization recovery after excitation and the steady state of signal acquisition. In this situation, because TR is shorter than T2 or T2*, significant amounts of transverse magnetization remain from previous excitations, allowing for higher signal intensities and less contrast decay. The repeated excitations maintain the signal while the spins are continually rephased, resulting in consistent image quality. This contrasts with conditions such as dynamic equilibrium or transient states, where either there's more fluctuation in signal due to longer TR values or other factors impacting the magnetization states. The concept of equilibrium state typically suggests that the magnetization has reached a point of stability after several cycles of repetition time, which is not the case with TR significantly less than T2 or T2*. Thus, the characterization of this condition as a steady state is both crucial for understanding the behavior of spins during faster imaging sequences and important for interpreting MRI results effectively.