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T. Vemulkar and R. P. Cowburn
neighbouring tissue resulting in reduced resolution for diagnosis [55]. Reference
[55] provides a thorough overview of nanoparticle design for MRI contrast and will
be of relevance to the reader.
15.1.1.2 Magnetic Particle Imaging
While MRI had its origins in the 1970s [60], the concept of magnetic particle imaging
(MPI) is relatively recent. It was demonstrated in 2005 by Gleich and Weizenecker
[23]. Core to this imaging technique is the use of SPIONs as tracers (not contrast
agents) due to their zero magnetization remanent state, and their nonlinear, saturating
magnetization response in an applied field [61–63].
In the presence of a driving alternating applied field at a specific frequency termed
the modulation field, the magnetization response of SPIONs is time-dependent and
contains both the frequency of the driving field, but also contains higher harmonics.
If a strong saturating static field is also applied to the SPIONs, then they remain
saturated despite the presence of the alternating field and the generation of the higher
harmonics in their magnetization response with time is suppressed. If the spatial
configuration of the static field is such that it has a zero (or low) field at its centre
and higher fields at the edges, then any SPIONs in the centre of this field retain their
higher harmonics in conjunction with the driving field, whereas the SPIONs in the
high static field region lose their higher harmonic responses. This is known as the
application of a “selection field”. MPI thus works by scanning the position of the
field-free point (FFP) in the selection field throughout a sample volume containing
SPIONs. By mapping the suppression of the higher harmonics of the SPIONs, a
tomographic image of the sample volume is generated (Fig. 15.2).
This imaging modality is of extreme interest in the biomedical space. The magnetization response of the SPIONs is the sole contribution to the signal in this technique (hence “tracers”), and it is not confounded with artefacts related to proton
relaxation. Further, the higher magnetization and much shorter relaxation times of
SPIONs in MPI compared to protons in a 1.5 T applied field means MPI has comparatively extremely high temporal resolution [64] and higher signal to noise than MRI
[65]. Further, MPI systems do not need to necessarily be as large as MRI systems
and offer flexibility in their design for use [66]. MPI is thus a technique that is of
extremely high interest, particularly for cardiovascular diagnostics, tissue perfusion
and vascular anomalies, as well as situations where the use of standard MRI contrast
agents is not possible from a toxicology perspective such as in patients with compromised renal function [52]. MPI has advanced rapidly since its conception because of
its huge potential in the imaging space, and [67–69] will provide the reader with a
more detailed overview on MPI in its current state.
T. Vemulkar and R. P. Cowburn
neighbouring tissue resulting in reduced resolution for diagnosis [55]. Reference
[55] provides a thorough overview of nanoparticle design for MRI contrast and will
be of relevance to the reader.
15.1.1.2 Magnetic Particle Imaging
While MRI had its origins in the 1970s [60], the concept of magnetic particle imaging
(MPI) is relatively recent. It was demonstrated in 2005 by Gleich and Weizenecker
[23]. Core to this imaging technique is the use of SPIONs as tracers (not contrast
agents) due to their zero magnetization remanent state, and their nonlinear, saturating
magnetization response in an applied field [61–63].
In the presence of a driving alternating applied field at a specific frequency termed
the modulation field, the magnetization response of SPIONs is time-dependent and
contains both the frequency of the driving field, but also contains higher harmonics.
If a strong saturating static field is also applied to the SPIONs, then they remain
saturated despite the presence of the alternating field and the generation of the higher
harmonics in their magnetization response with time is suppressed. If the spatial
configuration of the static field is such that it has a zero (or low) field at its centre
and higher fields at the edges, then any SPIONs in the centre of this field retain their
higher harmonics in conjunction with the driving field, whereas the SPIONs in the
high static field region lose their higher harmonic responses. This is known as the
application of a “selection field”. MPI thus works by scanning the position of the
field-free point (FFP) in the selection field throughout a sample volume containing
SPIONs. By mapping the suppression of the higher harmonics of the SPIONs, a
tomographic image of the sample volume is generated (Fig. 15.2).
This imaging modality is of extreme interest in the biomedical space. The magnetization response of the SPIONs is the sole contribution to the signal in this technique (hence “tracers”), and it is not confounded with artefacts related to proton
relaxation. Further, the higher magnetization and much shorter relaxation times of
SPIONs in MPI compared to protons in a 1.5 T applied field means MPI has comparatively extremely high temporal resolution [64] and higher signal to noise than MRI
[65]. Further, MPI systems do not need to necessarily be as large as MRI systems
and offer flexibility in their design for use [66]. MPI is thus a technique that is of
extremely high interest, particularly for cardiovascular diagnostics, tissue perfusion
and vascular anomalies, as well as situations where the use of standard MRI contrast
agents is not possible from a toxicology perspective such as in patients with compromised renal function [52]. MPI has advanced rapidly since its conception because of
its huge potential in the imaging space, and [67–69] will provide the reader with a
more detailed overview on MPI in its current state.