What are the two primary relaxation time constants used in MRI?

• A. T1 and T2
• B. TE and TR
• C. T2 and T3
• D. T1 and T3

Answer: (A)

In MRI, image contrast is governed by how spins relax after the radiofrequency pulse. The two main relaxation time constants are T1 and T2. T1, the longitudinal relaxation time, is the time it takes for magnetization to recover along the main magnetic field toward equilibrium after excitation. Tissues with shorter T1 recover faster and tend to appear brighter on T1-weighted images, while those with longer T1 recover more slowly. T2, the transverse relaxation time, is the time for transverse magnetization to decay due to spin-spin interactions, causing the signal to fade. T2-weighted images highlight differences in T2 values, so tissues with longer T2 remain bright relative to those with short T2. The other options aren’t the intrinsic relaxation constants: TE (echo time) and TR (repetition time) are sequence timing parameters that shape contrast but do not define tissue properties themselves, and T3 is not a standard relaxation constant in MRI.