Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

TABLE 14-3. GYROMAGNETIC RATIO FOR USEFULELEMENTS IN MAGNETIC RESONANCEy/2n(MHz/T)lHBe17 019F23Na31p42.5810.75.840.011.317.2direction. As the perturbed system goes back to its equilibrium state, the MR signalis produced.Typical magnetic field strengths for imaging range from 0.1 to 4.0 T (1,000 to40,000 G). For protons, the precessional frequency is 42.58 MHz in a I-T magneticfield (i.e., y/21t = 42.58 MHz/T for lH). The frequency increases or decreaseslinearly with increases or decreases in magnetic field strength, as shown in the example.Accuracy and precision are crucial for the selective excitation of a given nucleusin a magnetic field of known strength. Spin precession frequency must be knownto an extremely small fraction (l0- 12 ) of the precessional frequency for modernimaging systems.The differences in the precessional frequency allow the selective excitation of oneelemental species for a given magnetic field strength.By convention, the applied magnetic field Bo is directed parallel to the z-axis of thethree-dimensional Cartesian coordinate axis system. The x and y axes are perpendicularto the z direction. For convenience, two frames of reference are used: thelaboratory frame and the rotating frame. The laboratory frame (Fig. 14- 5A) is a stationaryreference frame from the observer's point of view. The proton's magneticmoment precesses about the z-axis in a circular geometry about the x-y plane. Therotating frame (see Fig. 14-5B) is a spinning axis system whereby the angular frequencyis equal to the precessional frequency of the protons.In this frame, the spins