Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

ways. A 90-degree angle provides the largest possible transverse magnetization. Thetime required to flip the magnetic moment is linearly related to the displacementangle: For the same Bl field strength, a 90-degree angle takes half the time to producethat a 180-degree angle does. The time required to implement a rotation is onthe order of tens to hundreds of microseconds. With fast MR imaging techniques,30-degree and smaller angles are often used to reduce the time needed to displacethe longitudinal magnetization and generate the transverse magnetization. For flipangles smaller than 90 degrees, less signal in the Mxy direction is generated, but lesstime is needed to displace M" resulting in a greater amount of transverse magnetization(signal) per excitation time. For instance, a 45-degree flip takes half the time ofa 90-degree flip yet creates 70% of the signal, because the projection of the vectoronto the transverse plane is sin 45 degrees, or 0.707. In instances where short excitationtimes are necessary, small flip angles are employed.Free Induction Decay: 12 RelaxationThe 90-degree RF pulse produces phase coherence of the individual protons andgenerates the maximum possible transverse magnetization for a given sample volume.As Mxy rotates at the Larmor frequency, the receiver antenna coil (in the laboratoryframe) is induced (by magnetic induction) to produce a damped sinusoidal electronicsignal known as thefree induction decay (FID) signal, as shown in Fig. 14-11.EquilibriumMxy = zero90° RF pulseMxylargeMxyDephasingdecreasingDephasedMxy = zeroTimeAntennareceiver+ ··.~FID'.FIGURE 14-11. A: Conversion of longitudinal magnetization, Mz, into transverse magnetization,Mxy, results in an initial phase coherence of the individual spins of the sample.The magnetic moment vector precesses at the Larmor frequency (stationary in therotating frame) and dephases with time. B: In the laboratory frame, M xy precesses andinduces a signal in an antenna receiver that is sensitive to transverse magnetization. Afree induction decay (FID) signal is produced, oscillating at the Larmor frequency, anddecays with time due to the loss of phase coherence.