Magnetic particle imaging (MPI)

Magnetic particle imaging is a developing scanning technique which can produce real-time internal images of the body. This is done using superparamagnetic iron oxide nanoparticle tracers known as SPIOs, which are inserted into the patient's body, and an external magnetic field. It is a very high-resolution, high-speed process. [1][2]

What are superparamagnetic iron oxide nanoparticles?

Also known as SPIOs, superparamagnetic iron oxide nanoparticles are iron oxide particles with diameter ranging between 1 and 100 nm. They exist in two main forms: magnetite (Fe3O4) and the oxidised form of magnetite, maghemite (g-Fe2O3). [3]

Superparamagnetism occurs in nanoparticles which are single (magnetic) domain. As such, the nanoparticles act like nanoscale bar magnets. Since each nanoparticle is discrete, there are no interactions or ordering of domains within a sample of nanoparticles. [5]

When placed within a magnetic field, the nanoparticles align to the applied magnetic field. When the nanoparticles are aligned, the magnetisation is said to be ‘saturated’, and the magnetisation, M is at a maximum. The magnetisation is ‘unsaturated’ when the nanoparticles are randomly organised in the absence of a magnetic field. [4]

To find out more about domains and magnetism, click here:

Why iron and not cobalt or nickel?

Although cobalt and nickel are the two other highly magnetic elements, they are toxic and easily oxidised, meaning they wouldn’t be appropriate for ingestion.

Figure 1: The alignment of superparamagnetic nanoparticles when placed in magnetic fields of varying strength and the resultant magnetisation. Source: magneticinsight.com

How SPIOs are used to produce a signal

When carrying out MPI, two strong magnets pointed towards each other are used to produce a magnetic field gradient with what is known as the field free region (FFR) at the centre. This is due to the superposition of magnetic fields.

The FFR is rapidly moved across the sample. Doing this causes the nanoparticles passing through the FFR to flip from a saturated negative value to a saturated positive value. This change in magnetisation induces as signal in a receiver coil. The signal is induced in the receiver coil by electromagnetic induction. The strength of this signal is dependent upon the density of SPIOs in the FFR. [4]

Since the locus of the FFR is known, the received signal can be assigned to the locus of the FFR. Thus, a mapping of the strength of the signal across the sample can be produced in order to form an image.

Figure 2: Superposition of magnetic fields surrounding two magnets directed towards each other. The field free region where net magnetic flux is zero is indicated. Source: magneticinsight.com

Further important features of SPIOs

Since SPIOs are superparamagnetic, they do not show any remnant magnetisation once the externally applied field is removed, as a consequence of Néel relaxation. [1]

Néel relaxation is the process whereby the magnetisation of a superparamagnetic nanoparticle flips and reverses its direction. The nanoparticles only have two stable orientations which are antiparallel to each other and separated by an energy barrier. This occurs as a consequence of the magnetic anisotropy of the nanoparticles. The mean time between two flips is called the Néel relaxation time, τ_N given by the following equation:

where K is the magnetic anisotropy energy density, V is the volume, k_B is the Boltzmann constant, T is the temperature and τ_0 the 'attempt period', characteristic to the material. [6]

In addition, SPIOs have a nonlinear magnetisation curve. This means that they can be magnetically saturated, and as such the SPIOs’ signal can be differentiated from the externally applied field. This enables the allocation of the SPIO signal to a precise location: a process also known as spatial encoding. [1]

The steeper the slope of the SPIOs’ magnetisation curve, the smaller and thus more precise the area within which the SPIOs’ signal can be confined. This leads to a greater resolution.

What is a magnetic anisotropy?

Magnetic anisotropy refers to the directional dependence of a material's magnetic properties: the magnetic moment of anisotropic materials tend to align along what is known as the easy axis. Meanwhile, the magnetic moment of magnetically isotropic materials have no preferential direction unless placed within an externally applied field.

Figure 3: SPIO magnetisation curve

Source: researchgate.net

References:

[1] Panagiotopoulos, N. et al., 'Magnetic particle imaging: current developments and future directions' Int J. Nanomedicine, 2015; 10: 3097–3114

[2] Borgert J. et al. 'Fundamentals and applications of magnetic particle imaging.' J Cardiovascular Computed Tomography, 2012;6(3):149–153.

[3] Aharoni, A., 'Introduction to the Theory of Ferromagnetism', Clarendon Press, 1996

[4] www.magneticinsight.com/about-mpi/ [Accessed: 21/02/19]

[5] Bromley, S., Woodley, S., 'Computational Modelling of Nanoparticles Vol. 12', Elsevier, 2018

[6] https://en.wikipedia.org/wiki/Néel_relaxation_theory [Accessed: 24/02/19]