Increasing slice thickness results in a decrease in which of the following?

Increasing slice thickness leads to a decrease in spatial resolution. Spatial resolution refers to the ability of an imaging system to distinguish small details and separate adjacent structures. When slice thickness is increased, the volume of tissue that is sampled also increases, which can blur fine details and reduce the system's ability to differentiate closely spaced structures. Consequently, thicker slices merge information from a wider area, resulting in a loss of clarity and sharpness in the images.

In the context of the other options, increasing slice thickness does generally improve signal-to-noise ratio (SNR) because more signal is collected from a larger volume of tissue, but it sacrifices spatial resolution. The scan time may not be significantly affected by changes in slice thickness in some imaging sequences, and the phase matrix primarily relates to the number of phase encoding steps used in an image acquisition rather than directly linking to slice thickness. Therefore, while SNR may increase or remain stable and scan time and phase matrix may be less directly impacted by slice thickness changes, the clear and well-defined impact is seen in the reduction of spatial resolution.