Final answer:
In imaging techniques that involve RF pulses, such as MRI, the gradient magnetic field is turned on to differentiate specific body slices. The gradient fields cause protons to resonate at varying frequencies based on their location, which allows for precise imaging of thin cross-sectional slices.
Step-by-step explanation:
In any given pulse sequence, when there is a transmitted application of a 90 degree or 180 degree RF pulse, the gradient magnetic field must be turned on to allow a specific slice to be singled out. By manipulating the magnetic field, nuclei such as protons in the body will resonate at different frequencies depending on their spatial location. The process involves sweeping radio frequencies that resonate with the protons only in areas where the magnetic field strength is correct, which then emit these signals back to be received. The intensity and frequency of these reemitted radio waves provide detailed position information and enable the construction of an image that consists of slices through the body, revealing not only the density of the nuclear type but also chemical environment details of the tissues being imaged.