A single element coil using AWG 24 magnet wire was wound on the tip of a plastic tube with an internal signal source and tuned to 63.58 MHz using surface mount capacitors. A PIN photodiode was placed in series with a capacitor and then placed in parallel with the circuit. The prototype coil is shown in Figure 1. Imaging was performed on a Siemens Magnetom Sonata 1.5T imager. Circuit detuning was accomplished using a laser diode driver coupled with a 10 mW laser diode. A TTL output programmable through the pulse sequence software was connected to the analog input of the driver circuitry. Temperature imaging was conducted with RF intensive sequences to examine whether any RF induced heating occurred.
Device imaging shows the presence of a bright spot corresponding to the location of the resonant microcoil (Figure 2, Panel A), which disappears when the light is switched on and the circuit is detuned (Figure 2, Panel B). Temperature imaging experiments show no appreciable heating in the region surrounding the coil.
By combining this novel optical detuning technique with inductive coupling, a system that can be detuned during RF transmission and which uses no conductive leads has been developed. The result of which is the elimination of RF induced heating, a patient-safety issue of great concern on clinical 1.5T systems.
