| 摘要 |
In a magnetic resonance imaging or spectroscopy apparatus, radio frequency pulses of a selected size are applied to cause a corresponding tip angle. However, the actual tip angle which results varies with, for example, coil loading and patient geometry. For accuracy of the resultant image or data, the RF pulse size is adjusted or calibrated to produce a selected tip angle precisely - most commonly 90 DEG and 180 DEG tip angles. To calibrate the 90 DEG tip angle, a sequence of three like pulses ( alpha - alpha - alpha ) is applied. In each repetition, the pulses are varied in magnitude to produce tip angles around 90 DEG and the actual resulting tip angles are determined. The RF pulse size that produces or is projected by a least-squares fit to produce the exact 90 DEG tip angle is determined. To calibrate the 180 DEG tip angle, the pulse sequence includes the RF pulse ( alpha ) determined to produce the 90 DEG tip followed by two additional RF pulses ( beta ). The sequence ( alpha - beta - beta ) is repeated a plurality of times with the second and third RF pulses varied to produce tip angles around 180 DEG . From the zero frequency terms of Fourier transforms of the induced primary and stimulated echoes, the actual tip angle is determined. The RF pulse for producing a 180 DEG tip angle is calibrated to the size which produces or is projected to produce the 180 DEG tip angle.
|
