Final answer:
A fractional focal spot in mammography increases image resolution by providing a smaller point of focus, thereby producing sharper images that are crucial for detecting breast abnormalities.
Step-by-step explanation:
The use of a fractional focal spot in mammography is designed to enhance the image resolution of the mammogram. Considering the principles of optics, the resolution of an imaging system is influenced by factors such as wavelength and numerical aperture (NA). A smaller focal spot, often associated with a higher NA, allows for improved resolution because it can focus on small details more precisely, producing sharper images. In mammography, improved resolution is critical to discern fine structures within breast tissue which is vital for early detection of breast abnormalities and pathologies. Therefore, the correct answer is that a fractional focal spot used for mammography increases image resolution.