Which formula is used to define time-varying gradient magnetic fields?

The correct formula used to define time-varying gradient magnetic fields is expressed as dB/dt. This notation indicates the rate of change of the magnetic field strength (B) with respect to time (t). In the context of magnetic resonance imaging (MRI), time-varying gradient magnetic fields are essential for spatial encoding of the MR signals, allowing for the acquisition of high-resolution images.

Gradient magnetic fields are applied rapidly and vary with time to delineate spatial locations within the scanned volume. Understanding the rate of change of these fields is crucial for ensuring patient safety because rapid fluctuations can lead to induced currents in nearby conductive materials, potentially resulting in biological effects or equipment malfunctions.

Other options represent different physical concepts but do not directly relate to time-varying gradient magnetic fields. Tesla is a unit of magnetic flux density, whereas dB/dx refers to the spatial gradient of the magnetic field, which is not necessarily related to the time component. W/kg is a measure of specific absorption rate, relevant in assessing RF exposure but not specifically in defining the gradient magnetic field's time variation.