Myths and maths in MR safety 3

Why the value of magnetic susceptibility doesn’t really matter

The British Institute of Radiology recently published three information sheets on MRI safety. Myths and maths was one of them. In this series I’d like to expand on themes of that sheet. More details can be sourced from my book Essentials of MRI Safety.

Myth: the translational force on a ferromagnetic object depends upon its magnetic susceptibility.

Maths: for all strongly ferromagnetic objects (iron, steel, nickel, etc.) the value of magnetic susceptibility does not matter. They are all subject to the same amount of force and are equally dangerous in the MR environment.

BIR Information sheet

The complex answer

Paramagnetic and diamagnetic materials have very small magnetic susceptibilities and, whilst the translational force is proportional to magnetic susceptibility, the small values mean that the object’s magnetisation and ensuing forces (and torques) are negligible (shown as blue line in chart)- much less than their weight.

Ferromagnetic objects have very high susceptibilities but the forces are usually limited by the saturation value of the material’s magnetisation (red dotted line in the chart). Even an object is not magnetically saturated, the force reaches a limit for susceptibilities in the tens to low hundreds (orange dashed line). This is well below the susceptibility of most ferromagnetic materials, e.g. iron, nickel and steel. Rather than the value of susceptibility, what matters is the shape of the object and its saturation magnetisation. For a deeper understanding check out Chapters 2 and 9 of Essentials of MRI Safety or have a play with the magnetic force calculator.

Predicted translational force on cylindrical 0.1 kg object (length/diamter=5), aligned with B=1.5 T, dB/dz=2 T/m. The objects have density of 8000 kg m-3 with “Bsat“=1.6 T. The force from gravity is 1 N.

The simpler answer

A metallic object will almost always either be entirely harmless with regard to magnetic magnetisation-related forces (but not necessarily from induction effects: SAR and heating) – or it has the potential to be a dangerous projectile with the possibility of extremely strong and dangerous twisting forces. There is no middle ground. Even a weakly ferromagnetic material like 17-7PH stainless steel (formerly used in some implants) with a susceptibility of around 1 presents a serious hazard in the MR environment.

Traffic lights

In MRI our amber is the MRI Conditions

As with traffic management there are really only two actions: proceed or stop. Green or red. As a precaution traffic signals have a third state: amber. In MRI our amber is the MRI Conditions. Always check and observe the conditions when scanning patients with implants. See current FDA guidance here. Chapter 11 of Essentials of MRI Safety contains many practical examples on how to comply with the MR conditions.


Essentials of MRI Safety

Myths and maths in MR safety 2

Magnetic force calculator

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