Phased arrays technologies are used in widely different fields like radar, radiotelescope, sonar, MR receive coils...

Principle

Phased array operation is a straightforward application of wave theory so it is used both in emission and reception. It is applicable to electromagnetic waves like in radars, radio-telescope and MR receive coils, or acoustic waves like in sonar, echosonograhy and FUS transducers. The principle is both simple and elegant: rather than having one large wave emitter which has to be oriented mechanically, the phased array system uses a large number of small emitters whose signals are independently adjusted create the desired wavefront. The major advantage of such a system is that it does not require mechanical motion of the emitter to change the orientation of the beam. In sonar this phased array operation is called beam forming.

phased array FUS system.

Phased array system in focused ultrasounds

With a focused ultrasound device operating as a phased array, the tumor can be swept by the focal spot without any transducer displacement. This mode of operation is called electronic steering of the focal spot. The advantages of electronic steering in the case MR guided thermal ablation are:

• no transducer motion during the treatment and therefore no susceptibility changes while heating . These susceptibility changes induce large artefactual temperature variations (>20°C) in MR images making actual temperature measurements useless. So reducing transducer motion during treatment is the best way to ensure reliable temperature maps.
• the tumor can be repeatedly swept by the focal spot in a fast trajectory in order to increase the temperature of the whole tumor volume. This fast trajectory of the focal spot combined with continuous sonication is called volumetric ablation.

The main disadvantage of phase array systems is the technical complexity. Indeed, a single transducer coupled with a single signal power generator is replaced by a composite transducer with a large (256 in our case) number of emitting elements all fed a different signal generated by a massively multi channel generator. Phase coherence across all channels is required for proper operation. And of course all those channels must not interfere with MR imaging.

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