What is the purpose of time-varying gradient coils during data acquisition?

The purpose of time-varying gradient coils during data acquisition is to achieve spatial encoding. These coils generate varying magnetic fields that are superimposed on the static magnetic field within the MRI scanner. By altering the magnetic field strength across the imaging volume, they enable the precise localization of the signals emitted from different tissues within the body.

As the gradient fields change over time, they create different resonance frequencies for protons located at different spatial positions. This change allows the MRI system to distinguish between signals coming from different areas in the imaging slice, creating a two-dimensional or three-dimensional spatial map of the tissue being imaged. Thus, the data collected can be correctly reconstructed into a coherent image that reflects the internal structures of the subject being scanned.

The other options, while related to magnetic resonance imaging, do not directly encapsulate the primary function of time-varying gradient coils. For example, aligning nuclei involves static magnetic fields rather than varying gradients, and tilting nuclei or resonance production pertains to other aspects of the imaging process rather than the spatial positioning integral to the role of the gradient coils.