A long train of radiowave pulses make up the imaging sequence. The duration and timing of the pulses determine which tissue properties will be reflected in the final image, e.g. the T1 or T2 relaxation rates (T1- or T2-weighted image). Images may also be weighted for the proton density of tissues, which corresponds to their free-water content. Therefore, unlike CT, tissue contrast in MRI is variable, and an understanding of the sequence used is required in order to interpret images.
TE is the time to echo, or the time between applying the RF pulse and listening for the signal. TR is the time to repeat or the time between RF pulses. TR and TE are measured in milliseconds (ms).
By looking at the TR and TE (which is normally printed on the sheet of film), it is possible to decide whether the image has been T1 or T2 weighted:
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