How does TR (repetition time) affect SNR?

Repetition time (TR) plays a significant role in determining the signal-to-noise ratio (SNR) in MRI. As TR increases, it allows for more time between successive excitations of the same slice. This additional time provides the longitudinal magnetization of tissues the opportunity to recover before the next excitation occurs, leading to a stronger signal as more protons are available for detection.

Initially, as TR increases, the SNR tends to improve because of this increased opportunity for longitudinal recovery. However, beyond a certain point, the SNR gain reaches a plateau where further increases in TR no longer significantly enhance the SNR. This phenomenon occurs due to the inherent limits of tissue relaxation times and the signal generated reflecting a balance between the amount of available longitudinal magnetization and the noise present in the system.

As a result, the correct choice acknowledges that SNR increases with TR, but this increase is not indefinite and will eventually level off, which is an important aspect of MRI physics to understand in the context of pulse sequence design and image quality optimization.