Figure 4.

Measured reflection coefficient (S11, magnitude) as a function of frequency for the birdcage resonator. The resonance spectrum includes resonance modes at which an energy transmission to the RF resonator is highly efficient. The mode that provides a homogeneous RF field distribution and is therefore used for spin excitation is highlighted with a grey-shaded area. In contrast, the modes at lower frequencies do not provide a homogeneous field distribution and are therefore not of interest for MRI applications. Hence, a band-pass filter (also indicated by the grey-shaded area) is applied around the Larmor frequency at 3 T. To avoid noise coupling, the sensor die clock frequency is shifted in such a way that as little energy as possible is emitted in the frequency region of the band-pass filter.