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Gradient Echo Imaging:
Partial flip angle pulses seem to be used only in GRE imaging. Why can't you use them with SE imaging also?
Actually, you can use partial flip angles in SE imaging, as long as certain precautions are observed.
In general, performing a SE sequence of the form α°-180°-echo creates a number of problems with the maintenance of longitudinal magnetization that you don't have to worry about in GRE imaging. The source of this problem in SE imaging stems from the use of 180° pulses. These 180° pulses play a vital role in routine SE imaging, flipping over the transverse magnetization within its plane and thereby compensating for the effect of static field inhomogeneities. In routine SE imaging, however, the 180° pulses occur soon after the 90° pulses have deflected all of the longitudinal magnetization into the transverse plane. This would not be the case, however, if the first pulse were α° (instead of 90°).
If a partial flip angle pulse were employed, the 180° pulse would do much more than merely compensate for field inhomogeneities. Not only would spins be flipped over in the transverse plane by this pulse, but so would the residual longitudinal magnetization (which could be considerable, since the α° pulse may have reduced it by only a small fraction). Thus the 180° pulse would invert the longitudinal magnetization, destroying the individual beneficial aspects of both partial flip angle and SE imaging!
Nevertheless, it is possible to work around this limitation. If one adds a 180° pulse at the end of the sequence, then the longitudinal magnetization can be returned to its steady-state alignment along the +z-axis with relatively little penalty incurred from the first 180° pulse. The simplest way to do this is merely to require the sequence to have an even number of echoes (i.e., be of the form α°-180°-echo-180°-echo). Alternatively, if only a single echo is desired, then one may use a large flip angle SE technique of the form α°-180°-echo, where α>90°. Both techniques have been used with limited success and at one time were available as commercial options on some scanners (e.g., Picker's THRIFT).
In general, performing a SE sequence of the form α°-180°-echo creates a number of problems with the maintenance of longitudinal magnetization that you don't have to worry about in GRE imaging. The source of this problem in SE imaging stems from the use of 180° pulses. These 180° pulses play a vital role in routine SE imaging, flipping over the transverse magnetization within its plane and thereby compensating for the effect of static field inhomogeneities. In routine SE imaging, however, the 180° pulses occur soon after the 90° pulses have deflected all of the longitudinal magnetization into the transverse plane. This would not be the case, however, if the first pulse were α° (instead of 90°).
If a partial flip angle pulse were employed, the 180° pulse would do much more than merely compensate for field inhomogeneities. Not only would spins be flipped over in the transverse plane by this pulse, but so would the residual longitudinal magnetization (which could be considerable, since the α° pulse may have reduced it by only a small fraction). Thus the 180° pulse would invert the longitudinal magnetization, destroying the individual beneficial aspects of both partial flip angle and SE imaging!
Nevertheless, it is possible to work around this limitation. If one adds a 180° pulse at the end of the sequence, then the longitudinal magnetization can be returned to its steady-state alignment along the +z-axis with relatively little penalty incurred from the first 180° pulse. The simplest way to do this is merely to require the sequence to have an even number of echoes (i.e., be of the form α°-180°-echo-180°-echo). Alternatively, if only a single echo is desired, then one may use a large flip angle SE technique of the form α°-180°-echo, where α>90°. Both techniques have been used with limited success and at one time were available as commercial options on some scanners (e.g., Picker's THRIFT).
References
Elster AD, Provost TJ. Large-tip-angle spin echo imaging: theory and applications. Invest Radiol 1993; 28:944-953
Elster AD, Provost TJ. Large-tip-angle spin echo imaging: theory and applications. Invest Radiol 1993; 28:944-953
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