Physics of Medical Scans
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 magnetic tracers which are inserted into the patient's body, and an external magnetic field. It is a very high-resolution, high-speed process. [1] [2]
How it works
In MPI, images of the cross-section of the body are created using magnetic fields. A magnetic field is a region in which magnetic forces act upon objects within that space. When a magnetic force acts on a magnetic object, the object will move somehow, either by rotating or changing its position.
The magnetic fields generate a signal from the nanoparticles within the tracer. Since there is nothing in normal human tissue that would lead to a signal being produced from the magnetic field, a signal is only seen where there is the tracer. These signals can be created at any depth within the body. This provides images without any background showing where the tracer is in the body. [1] [2]
​
The imaging is performed in a range of milliseconds to seconds. This very small delay means that the images produced are effectively real-time images. Additionally, many images can be taken in quick succession to create a real-time video. [3]
The tracer used in MPI is naturally removed from the body by the immune system.
Where and how will it be used?
MPI uses magnetic fields which are harmless to the body, making it a favourable imaging method. So far, it has been developed to be used for the following:
​
1. Blood pool imaging
​
The tracer is carried in the blood, meaning that we can see the blood flow in the body. This is particularly useful when we need to see how the blood is flowing through the patient's heart.
​
2. Detecting tumour sites
​
The tracer accumulates within solid tumours. Therefore, during MPI scanning, sites where there is a large signal due to an accumulation of the tracer are likely to be tumour sites.
​
3. Cell tracking
​
Cell tracking is carried out alongside cell therapy, a treatment method for diseases such as cancer. Cell therapy involves the injection of white blood cells into the patient's body in order to combat the disease. By attaching the tracer to the injected cells, it is possible to track the cells as they move through the body by MPI. [4]
What is a nanoparticle?
A nanoparticle is a particle that is between 1 and 100 nanometers in size. A nanometre is a billion times smaller than a metre!
What does the immune system do to the tracer?
The iron from the tracer is removed and used to form haemoglobin, which is the molecule in our red blood cells that allows us to transport oxygen around the body!
MPI is favourable to other scanning techniques because it is:
​
Low noise
Radiation-free
​
Iodine-free
​
High resolution
High speed
What is a white blood cell?
White blood cells are cells which move through the body via blood and tissue fluid to counteract disease and foreign substances as part of the immune response.
Currently, there are three main geometries for MPI scanners: closed-bore scanners, open-bore scanners and single-sided coil arrangements for integration into beds:
Figure 1: Different geometries for MPI scanners, A is a closed-bore scanner, B is an open-bore scanner, C is a single-sided scanner. Source: ncbi.nlm.nih.gov
If you'd like to learn more about the physics behind how magnetic particle imaging works, try the advanced-level description by clicking this button:
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] https://en.wikipedia.org/wiki/Photoacoustic_imaging [Accessed: 20/02/19]
[4] https://www.cancer.org/treatment/treatments-and-side-effects/complementary-and-alternative-medicine.html [Accessed: 20/02/19]