370
T. Vemulkar and R. P. Cowburn
In the imaging space, optimizing SPIONs for the MPI field is an extremely relevant challenge with significant expected impact in the next decade. The performance
of SPIONs as MPI tracers is generally evaluated by a combination of the Langevin
theory of paramagnetism and understanding the relaxation processes for a magnetized nanoparticle [159, 160]. Magnetic nanoparticles typically consist of a magnetic
centre and a coating of a capping polymer or organic molecules. Particles with a larger
core have a higher magnetic moment and generally produce higher signal increasing
imaging sensitivity. As particles get larger, however, their relaxation time (typically
dominated by Brownian processes in MPI) increases [161]. This affects spatial resolution since the particle response to the FFP should be as near to instantaneous as
possible to allow for the detection of the higher-order harmonics in the magnetization
response to the modulation field. Typically, for a given applied field configuration, the
goal is to maximize the size of the SPIONs until particle relaxation begins to impede
spatial resolution. The polydispersity of SPION formulations can also have a large
impact on MPI performance [160] since both the Langevin function for the magnetization and the Brownian function for relaxation are strongly dependent on particle
size [161]. Particles that are smaller than ideal do not have high enough signals,
and particles that are larger have long relaxation times, and so the monodispersity
of SPION tracers is crucial to maximize MPI signal per unit gram of tracer used.
Further, since SPIONs are typically fabricated with an encapsulation for stability and
Brownian relaxation processes are dependent on the effective hydrodynamic radius,
the optimum coating thickness to magnetic core ratio is also a factor of consideration. With MPI performance so closely tied to the quality of the SPION tracers,
there is significant potential for concerted SPION development that may be specific
to a given diagnostic application. The commercial interest in development of MPI
nanoparticle formulations is evident in that Lodespin Labs has been founded around
this goal and has already created the first commercial SPION-based tracer dedicated
for MPI [132].
From the thermal ablation perspective, there are two overarching goals moving
forward. The first is to develop nanoparticle formulations to maximize the SLP, with
a minimum figure of merit of 1000 W/g at 100 kHz and 20 mT [162]. This is already
significantly higher than the 200–600 W/g obtained from commercially available
SPION formulations [133]. The work in this area has moved towards modifying
the nanoparticle structure and material composition with a variety of core–shell
structures utilizing exchange coupling [28, 131, 163, 164] to massively increase
the SLP. Three materials properties that are of importance here are nanoparticles
diameter D, anisotropy K, the nanoparticle shape and saturation magnetization M S .
The particularly relevant parameter to tune is the anisotropy of the nanoparticles
used [75, 165–167], and it is here that the interfacial coupling between core–shell
nanoparticles becomes relevant. By using a hard magnetic core and soft magnetic
shell that are magnetically coupled, the anisotropy can be tuned to be close to the
theoretically ideal point for maximizing the SLP with values of 1000–4000 W/g
[28] demonstrated. With nanoparticle shape and size, optimized values in excess of
10,000 W/g have been observed [167]. Most of this work has focussed on Mn, Zn and
Co ferrites which may prove to be challenging with regard to regulatory approval
T. Vemulkar and R. P. Cowburn
In the imaging space, optimizing SPIONs for the MPI field is an extremely relevant challenge with significant expected impact in the next decade. The performance
of SPIONs as MPI tracers is generally evaluated by a combination of the Langevin
theory of paramagnetism and understanding the relaxation processes for a magnetized nanoparticle [159, 160]. Magnetic nanoparticles typically consist of a magnetic
centre and a coating of a capping polymer or organic molecules. Particles with a larger
core have a higher magnetic moment and generally produce higher signal increasing
imaging sensitivity. As particles get larger, however, their relaxation time (typically
dominated by Brownian processes in MPI) increases [161]. This affects spatial resolution since the particle response to the FFP should be as near to instantaneous as
possible to allow for the detection of the higher-order harmonics in the magnetization
response to the modulation field. Typically, for a given applied field configuration, the
goal is to maximize the size of the SPIONs until particle relaxation begins to impede
spatial resolution. The polydispersity of SPION formulations can also have a large
impact on MPI performance [160] since both the Langevin function for the magnetization and the Brownian function for relaxation are strongly dependent on particle
size [161]. Particles that are smaller than ideal do not have high enough signals,
and particles that are larger have long relaxation times, and so the monodispersity
of SPION tracers is crucial to maximize MPI signal per unit gram of tracer used.
Further, since SPIONs are typically fabricated with an encapsulation for stability and
Brownian relaxation processes are dependent on the effective hydrodynamic radius,
the optimum coating thickness to magnetic core ratio is also a factor of consideration. With MPI performance so closely tied to the quality of the SPION tracers,
there is significant potential for concerted SPION development that may be specific
to a given diagnostic application. The commercial interest in development of MPI
nanoparticle formulations is evident in that Lodespin Labs has been founded around
this goal and has already created the first commercial SPION-based tracer dedicated
for MPI [132].
From the thermal ablation perspective, there are two overarching goals moving
forward. The first is to develop nanoparticle formulations to maximize the SLP, with
a minimum figure of merit of 1000 W/g at 100 kHz and 20 mT [162]. This is already
significantly higher than the 200–600 W/g obtained from commercially available
SPION formulations [133]. The work in this area has moved towards modifying
the nanoparticle structure and material composition with a variety of core–shell
structures utilizing exchange coupling [28, 131, 163, 164] to massively increase
the SLP. Three materials properties that are of importance here are nanoparticles
diameter D, anisotropy K, the nanoparticle shape and saturation magnetization M S .
The particularly relevant parameter to tune is the anisotropy of the nanoparticles
used [75, 165–167], and it is here that the interfacial coupling between core–shell
nanoparticles becomes relevant. By using a hard magnetic core and soft magnetic
shell that are magnetically coupled, the anisotropy can be tuned to be close to the
theoretically ideal point for maximizing the SLP with values of 1000–4000 W/g
[28] demonstrated. With nanoparticle shape and size, optimized values in excess of
10,000 W/g have been observed [167]. Most of this work has focussed on Mn, Zn and
Co ferrites which may prove to be challenging with regard to regulatory approval