What is the significance of using long TRs for T2 weighted images?

The use of long repetition times (TRs) in T2-weighted imaging is essential because it ensures that there is sufficient time for the longitudinal magnetization of the spins to recover before the next excitation pulse. In MRI, TR is the time interval between successive pulse sequences applied to the same slice. Longer TRs allow for full longitudinal relaxation, meaning that spins have adequate time to realign with the magnetic field, which is critical for T2-weighted imaging. This results in more accurate representation of T2 relaxation characteristics, producing clearer and more defined T2-weighted images.

When short TRs are used, there may not be enough time for spins to return to their equilibrium state, which can lead to incomplete relaxation and affect the quality and contrast of the images. Thus, long TRs are vital for achieving the desired T2 contrast and ensuring that the inherent T2 relaxation differences between tissues are appropriately captured, leading to improved diagnostic ability.