FIGURE 7.

Simplified model of metal-induced changes in the precession frequency (Δf_m) as a function of distance from the metal surface (z) for 1.5 T (red) and 0.55 T (blue) field strengths. For similar RF pulse bandwidths (Δf(RF)), those spins outside the RF pulse bandwidth (z < z₁ for 0.55 T and z < z₂ for 1. 5 T) will not be excited, which explains the larger areas of signal loss due to off-resonance at 1.5 T. Through plane signal displacement for each excited point with a frequency offset of Δf_m equals $\mathrm{\Delta }\mathit{z}\mathrm{=}\frac{\mathrm{\Delta }{f}_m}{\gamma G_{ss}}\mathrm{,}$ with γ representing the gyromagnetic ratio and G_ss representing the slice-selection gradient. Whereas the signal displacement for points A₁ at 0.55 T $\mathrm{(}{\mathit{A}}_{\mathrm{1}}\mathrm{\to }{\mathit{A}}_1^{\prime }\mathrm{)}$ and B₁ at 1.5 T $\mathrm{(}{\mathit{B}}_{\mathrm{1}}\mathrm{\to }{\mathit{B}}_1^{\prime }\mathrm{)}$ are equal, the signal from points z₁ < z < z₂ at 0.55 T (not excited at 1.5 T) will be distorted (eg, compressed) into the adjacent slices and appear as signal pileups.