Shaping and Focusing Magnetic Field in the Human Body: State-of-the Art and Promising Technologies

Abstract

In recent years, the usage of radio frequency magnetic fields for biomedical applications has increased exponentially. Several diagnostic and therapeutic methodologies exploit this physical entity such as, for instance, magnetic resonance imaging, hyperthermia with magnetic nanoparticles and transcranial magnetic stimulation. Within this framework, the magnetic field focusing and shaping, at different depths inside the tissue, emerges as one of the most important challenges from a technological point of view, since it is highly desirable for improving the effectiveness of clinical methodologies. In this review paper, we will first report some of the biomedical practices employing radio frequency magnetic fields, that appear most promising in clinical settings, explaining the underneath physical principles and operative procedures. Specifically, we direct the interest toward hyperthermia with magnetic nanoparticles and transcranial magnetic stimulation, together with a brief mention of magnetic resonance imaging. Additionally, we deeply review the technological solutions that have appeared so far in the literature to shape and control the radio frequency magnetic field distribution within biological tissues, highlighting human applications. In particular, volume and surface coils, together with the recent raise of metamaterials and metasurfaces will be reported. The present review manuscript can be useful to fill the actual gap in the literature and to serve as a guide for the physicians and engineers working in these fields.

Keywords: biomedical applications; focusing; magnetic field; magnetic hyperthermia; metamaterials; metasurfaces; transcranial magnetic stimulation.

Figure 1

Scheme of the radiating solutions for magnetic field shaping classified by the adopted approach.

Figure 2

Sketch of hyperthermia and thermoablation procedures. The heating medium (i.e., magnetic fluid) is injected in the targeted region; by exciting such fluid with an external RF magnetic field, it is feasible to significantly increase the temperature in a local manner.

Figure 3

Sketch of the typical coil configurations employed in hyperthermia experiments: a solenoid (a), Helmholtz configuration (b), birdcage (c) and pancake (d) coils.

Figure 4

Different steps of inductors development for hyperthermia applications. Numerical design is followed by fabrication and workbenching tests.

Figure 5

Biconical stimulator coils system: pictorial representation.

Figure 6

Perspective view of TMS figure-of-eight enhanced coils configurations (a), and the “four leaf solution” (b) [72].

Figure 7

Different coil arrays configurations proposed in [75]: (a) hemispherical; (b) plane and (c) torus array.

Figure 8

Bending-angle of the figure-of-eight coil evaluated in [80].

Figure 9

An ideal metamaterial slab placed in proximity of an active RF coil (a) can be practically realized through an opportunely designed array of resonating spiral resonators (b). For the same circulating current amplitude in the active RF coil, the metasurface presence is able to significantly enhance the produced magnetic field, facilitating the penetration inside biological tissues (c,d).