Relaxation Effects

T1 and T2 relaxation rates effect the SNR in an image. Improvement in the SNR is seen when the TR is increased up to about 3-5 T1 times. Changing the TR time will also effect the T1 weighting of the image and the acquisition time. T1 weighting occurs in a short TR spin echo sequence because of incomplete recovery of longitudinal magnetization. The effect of TR on SNR can be shown graphically by a T1 relaxation curve as illustrated in the exponential growth curve below:

As the TR is increased to several times the T1, the longitudinal magnetization has time to recover with resulting improvement in SNR.
Decrease in SNR is seen with an increase the TE secondary to T2 effects in spin echo sequences, and T2* effects in gradient echo sequences. The exponential decay curve below illustrates the lose of signal with time.

In spite of the decrease in SNR, we deliberately increase the TE to improve the T2 weighting (contrast) of an image. Ideally, we would want a long TR of several T1 times to minimize the T1 effects in a T2 weighted sequence.