Low resolution. A ct has advantages and disadvantages: a ct is cheap, fast done and gives a great information about blood, bone and calfifications. However, it is done with x- ray, and has low resolution of tissue such as the brain. An MRI might be at time better, depending on the goal of the exam.
Head CT. CT of the brain is quick and provides an excellent screen for large pathology. Early strokes or subtle tumors are not as well seen. For better characterization of the tissue itself, MRI is far superior. In an emergency setting or screen for pathology, CT is usually the 1st choice. CT also uses radiation which also a consideration.