Which of the following best defines echo time (TE)?

• A. Time between the 90º RF pulse and the peak of the signal in the receiver coil
• B. Time between two 90º RF pulses
• C. Time between two 180º RF pulses
• D. Time between the RF pulse and the start of data acquisition

Answer: (A)

Echo time is the interval from the RF excitation pulse to the peak of the echo signal that is detected by the receiver. This timing marks when the signal used to form the image reaches its maximum, whether the echo comes from a spin-echo (after the 180° refocusing pulse) or a gradient-echo (generated by gradient rephasing).

This concept differs from the repetition time, which is the gap between successive excitation pulses, and from the echo spacing in sequences that have more than one refocusing pulse. It also isn’t simply the time to start data acquisition; TE specifically targets the moment the echo peak is measured for image encoding.

Knowing TE helps explain why longer TE reduces signal from tissues with short T2 and increases T2 weighting in the image.