SAR reduced pulse sequences

Three techniques were considered for reducing the RF (radiofrequency) power deposition in the body while maintaining scan time efficiency: reducing the RF peak amplitude while increasing the pulse width, substituting gradient echoes for spin echoes, and reducing the flip angle of the phase reversal pulse. The use of gradient echoes was found to be the most efficient means to reduce the power delivered to the patient and to obtain rapid data acquisition. The effect upon SAR (specific absorption rate) and SNR (signal-to-nois ratio) was demonstrated on a phantom when the phase reversal pulse was reduced from the standard 180° to 90°. Data in the body indicated a fairly constant SNR down to a refocusing flip angle between 110° and 135°. An initial clinical evaluation was performed at three institutions using the method of reducing the flip angle of the phase reversal pulse. The scan with θ = 120° was rated by readers in a blinded study as having acceptable diagnostic image quality while the 135° scan had comparable image quality to a conventional 90°-180° pulse sequence. The use of reduced phase reversal pulses was seen as an efficient protocol to obtain T₁-weighted images at rapid data rates while reducing the power delivered to the body by about 40%.