Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 14-6. Longitudinal magnetization, Mz, is the vector component of the magneticmoment in the z direction. Transversemagnetization, Mxy, is the vector componentof the magnetic moment in the x-y plane. Equilibrium magnetization, Mo, is themaximal longitudinal magnetization of the sample; in this illustration it is shown displacedfrom the z-axis.plane and cancel each other. When the system absorbs energy, Mz is "tipped" intothe transverse plane; why this is important is explained in the next section. Figure14-6 illustrates these concepts.The famous Bloch equations are a set of coupled differential equations thatdescribe the behavior of the magnetization vectors under any conditions. Theseequations, when integrated, yield the x', y', and z components of magnetization asa function of time.14.2 GENERATION AND DETECTION OF THE MAGNETICRESONANCE SIGNALApplication of radiofrequency (RF) energy synchronized to the precessional frequencyof the protons causes displacement of the tissue magnetic moment fromequilibrium conditions (i.e., more protons are in the antiparallel orientation).Return to equilibrium results in emission of MR signals proportional to the numberof excited protons in the sample, with a rate that depends on the characteristicsof the tissues. Excitation, detection, and acquisition of the signals constitute thebasic information necessary for MR spectroscopy and imaging.The displacement of the equilibrium magnetization occurs when the magneticcomponent of the RF pulse, also known as the B 1 field, is precisely matched to theprecessional frequency of the protons to produce a condition of resonance. This RFfrequency (proportional to energy) corresponds to the energy separation betweenthe protons in the parallel and anti parallel directions, as described by either a quantummechanics or a classical physics approach. Each has its advantages in the understandingof MR physics.The quantum mechanics approach considers the RF energy as photons(quanta) instead of waves. Spins oriented parallel and antiparallel to the external