Question

In MRI system, longitudinal relaxation time is slower than:

• A. Phase relaxation time
• B. Spin lattice relaxation time
• C. Transverse relaxation time
• D. De-phasing relaxation time

Hint:

MRI Relaxation Time Inequality: In magnetic resonance physics, $T_1$ is always significantly longer than $T_2$ ($T_1 > T_2$). - $T_1$ (Longitudinal Spin-Lattice): Involves physical energy transfer to the surroundings $\rightarrow$ Takes more time (Slower process). - $T_2$ (Transverse Spin-Spin): Involves simple loss of phase synchronization $\rightarrow$ Happens quickly (Faster process).

Answer 1

The Correct Option is C

Concept: Magnetic Resonance Imaging (MRI) tracks the behavior of hydrogen protons inside the body. When placed within a strong static magnetic field ($B_0$), these protons align their magnetic moments parallel to the field, creating a net longitudinal magnetization vector ($M_z$). Applying a Radiofrequency (RF) excitation pulse tips this magnetization vector into the transverse plane ($M_{xy}$), causing the protons to precess in phase with one another. Once the RF pulse turns off, the proton system returns to its original equilibrium state through two separate, simultaneous relaxation processes:

Step 1: Defining Longitudinal Relaxation ($T_1$).
Longitudinal relaxation (also called Spin-Lattice relaxation) is the process where protons transfer their absorbed RF energy back to the surrounding molecular lattice. This energy transfer allows the longitudinal magnetization vector ($M_z$) to recover back along the z-axis. The time constant governing this exponential recovery curve is called $T_1$.

Step 2: Defining Transverse Relaxation ($T_2$).
Transverse relaxation (also called Spin-Spin relaxation or phase relaxation) describes the loss of phase coherence among the precessing protons. This dephasing is caused by localized variations in micro-magnetic fields as protons interact with one another, which rapidly decays the transverse magnetization vector ($M_{xy}$). The time constant governing this exponential decay is called $T_2$.

Step 3: Comparing relaxation rates.
Because losing phase coherence (Spin-Spin dephasing) depends only on localized proton interactions, it happens much faster than transferring energy back to the wider molecular environment (Spin-Lattice recovery). Therefore, in all biological tissues, the longitudinal recovery process is significantly slower than the transverse decay process: \[ T_1 \text{ (Longitudinal Relaxation Time)} > T_2 \text{ (Transverse Relaxation Time)} \]
• For example, in typical soft tissues, $T_1$ values range from $500\text{ ms to } 2000\text{ ms}$, whereas $T_2$ values are much shorter, ranging from $40\text{ ms to } 150\text{ ms}$.
• Note that Option (B) is another name for $T_1$ itself, while Options (A) and (D) describe sub-components of the $T_2$ process. Thus, longitudinal relaxation time ($T_1$) is slower (larger time value) than transverse relaxation time ($T_2$), matching Option (C).