What is the main effect of the gradient magnetic field on spins?

The primary effect of the gradient magnetic field on spins is that it changes their precessional frequency. In magnetic resonance imaging (MRI), the gradient magnetic fields are used to create variations in the magnetic field strength across different areas of the imaging volume. This results in spins experiencing different magnetic environments based on their position within the magnetic field.

As a consequence, spins that are located in regions of different magnetic field strengths will precess at different frequencies. This frequency difference is the basis for spatial encoding in MRI, allowing the construction of images from the data collected. By manipulating the gradient fields, it is possible to selectively excite spins within specific regions, contributing to the spatial resolution of the images produced.

Utilizing the gradient magnetic field to alter the precessional frequency is crucial for decoding the spatial information needed in MRI, making it a vital aspect of producing diagnostic images.