Question

In fast spin echo, the readout gradient is turned on

A) Before the 90-degree RF pulse
B) After the 90-degree RF pulse
C) Simultaneously with the 90-degree RF pulse
D) Only during the 180-degree RF pulse

Answer 1

The readout gradient in a fast spin echo MRI sequence is typically turned on after the 90-degree RF pulse, and also during the application of 180-degree RF refocusing pulses.

Step-by-step explanation:

In fast spin echo (FSE) MRI sequences, the readout gradient is not turned on before the 90-degree Radio Frequency (RF) pulse, nor is it turned on simultaneously with it. Instead, the readout gradient is typically applied after the 90-degree RF pulse. This readout gradient, sometimes referred to as the frequency encoding gradient, is also utilized during the application of the 180-degree RF refocusing pulses that are characteristic of the FSE sequence to form the multiple spin echoes that are rapidly acquired. This acquisition technique is crucial for image formation and relies on the principle that the magnetic field in the coil changes rapidly due to the spinning of the magnet, which creates an electromotive force (emf) in the coil that is proportional to the rate of change of the magnetic flux.