TE determines how much decay of the transverse magnetization
is allowed to occur before the signal is read.
The method in Section 2.1 is appropriate in applications that use the small-tip angle approximation (i.e., FA < 30[degrees] or so) as the longitudinal and transverse magnetization
components can be decoupled.
If the TR is shorter than the T2-relaxation times of the imaged tissues, which will typically be case for breathhold imaging, the transverse magnetization will not fully decay be fore the next radiofrequency (RF) pulse.
To remove this contrast, the TR could be increased so the transverse magnetization will relax fully after each RF pulse.
Whereas in the lower energy case [T.sub.2] is simply shortened, in the higher energy case the shape of the FID can be dramatically modified because the transverse magnetization can increase during the free precession.
For a initial magnetization M(0) and initial tip angle [theta](0), the transverse magnetization [M.sub.T](t) is given by
is the vector component of the magnetic moment in the x-y plane.
(7) This technique involves the application of a resonant 90 degree radiofrequency (RF) pulse at the time when the transverse magnetization
is refocused by the last 180 degree RF pulse.