In a pulse sequence diagram, what does Gz refer to?

In a pulse sequence diagram, Gz specifically refers to the slice selection gradient. This gradient is crucial during the imaging process because it is responsible for determining which slice of the body will be excited and subsequently imaged.

When the Gz gradient is applied, it creates a variation in the magnetic field along the z-axis. This leads to different resonant frequencies for protons at various locations within that axis. Only protons in the plane of interest, corresponding to the location with the appropriate frequency, will be selectively excited by the radio frequency (RF) pulse applied during the imaging process. This slice excitation is fundamental to obtaining images from specific anatomical slices, allowing for the non-invasive examination of internal structures.

The other options pertain to different aspects of the imaging process. Phase encoding refers to the encoding of information about the spatial location of signals in the phase domain, frequency encoding is about how spatial information is encoded based on the varying frequencies of signals received, and gradient nulling is related to techniques that aim to cancel out certain signal contributions, but none of these pertain to the specific role of Gz in slice selection.