Final answer:
b. reconstruction interval
The reconstruction interval is the parameter that causes helical slices in CT scanning to overlap, improving image quality with smoother transitions.
Step-by-step explanation:
The parameter that causes helical slices in computed tomography (CT) to be reconstructed so they overlap is the reconstruction interval. The reconstruction interval is the thickness of the slices as they are reconstructed from the gathered data. When the reconstruction interval is set to a smaller value than the table increment, the result is overlapping slices, which can improve image quality by providing smoother transitions between slices. In contrast, pitch, which is the relationship between the table movement speed and the x-ray beam width, and collimation, which refers to the thickness of the x-ray beam, do not directly influence the overlapping of slices but rather impact scan speed and resolution.