Which of the following are true about gradient-recalled echo (GRE) in

1.	Which of the following are true about gradient-recalled echo (GRE) in comparison with spin–echo sequences: a. Excitation flip angles less than 90° are generally used b. Imaging can be performed faster due to shorter TR times c. Rephasing of the spins and hence echo formation is achieved by using a phase-encoding gradient instead of a 180° pulse d. The excitation flip angle that maximizes the contrast between two tissues is known as the Ernst angle e. GRE is more prone to magnetic susceptibility artefacts than spin–echo sequences
Answer» a. True. This allows shorter repetition times (TRs) to be used as not all the longitudinal magnetization is tipped into the transverse plane with each excitation. The image contrast is therefore dependent not only on echo time (TE) and TR but also on the flip angle. b. True. c. False. The echo is formed by a reversal of the frequency-encoding gradient. A prephasing gradient is used initially to dephase the spins, followed by the standard frequency-encoding gradient of the opposite polarity, which rephases the spins. Rephasing (and therefore signal amplitude) is maximal in the middle of the frequency-encoding gradient. d. False. The Ernst angle is the excitation flip angle that maximizes the magnetic resonance signal for a given tissue T₁and for a given TR. The Ernst angle (x_E) is given by cos(x_E) = exp(-TR/T₁). This angle may not necessarily provide the optimum contrast between any two tissues. e. True. This is because local field inhomogeneities are not eliminated due to the absence of a 180° refocusing pulse

Which of the following are true about gradient-recalled echo (GRE) in comparison with spin–echo sequences: a. Excitation flip angles less than 90° are generally used b. Imaging can be performed faster due to shorter TR times c. Rephasing of the spins and hence echo formation is achieved by using a phase-encoding gradient instead of a 180° pulse d. The excitation flip angle that maximizes the contrast between two tissues is known as the Ernst angle e. GRE is more prone to magnetic susceptibility artefacts than spin–echo sequences

Answer»

a. True. This allows shorter repetition times (TRs) to be used as not all the longitudinal magnetization is tipped into the transverse plane with each excitation. The image contrast is therefore dependent not only on echo time (TE) and TR but also on the flip angle.

b. True.

c. False. The echo is formed by a reversal of the frequency-encoding gradient. A prephasing gradient is used initially to dephase the spins, followed by the standard frequency-encoding gradient of the opposite polarity, which rephases the spins. Rephasing (and therefore signal amplitude) is maximal in the middle of the frequency-encoding gradient.

d. False. The Ernst angle is the excitation flip angle that maximizes the magnetic resonance signal for a given tissue T₁and for a given TR. The Ernst angle (x_E) is given by cos(x_E) = exp(-TR/T₁). This angle may not necessarily provide the optimum contrast between any two tissues.

e. True. This is because local field inhomogeneities are not eliminated due to the absence of a 180° refocusing pulse

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