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This may not be the appropriate forum for this but seemed to be the closest.

I am trying to understand some concepts around MRI physics and it is common to use external magnetic fields created using gradient coils to manipulate the main magnetic field strength at different locations.

Now, the books talk in terms of gradient amplitude and the units they typically use is mT/m (microtesla/metre). I am not sure why there is this per meter as it is just the gradient amplitude should it not just be microtesla or teslas? Why is it defined per unit distance?

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A 'gradient' measures how quickly something changes with respect to something else. In this case, it's how much the magnetic field strength changes per unit length.

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  • $\begingroup$ Ah thanks. I missed the point that these gradient coils actually generate a magnetic field gradient! $\endgroup$ – Luca Apr 27 '16 at 19:00
  • $\begingroup$ In addition: For data sheets of MRI machines, you often find the term "slew rate" measured in T/m/s, which gives the peak or maximum gradient over the time needed to ramp up that gradient. $\endgroup$ – Dschoni Apr 5 '17 at 12:12
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The term "gradient" implies a change in some quantity versus a change in second quantity, usually over a distance. It's very much like a slope. For example, the gradient of a roof line on a house is given as rise/run like 15 cm/m or 5 inches/foot. The gradient of potential is the electric field magnitude, with SI units of volts/meter.

A magnetic field is measured in tesla or sub-units thereof, so the gradient of the field would be tesla per meter. It tells you how much the field changes when you change position in the field.

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