Purpose: X-Rays (of which a CT scan is comprised of) are used to see inside of the human body to look for fractures and other bony or soft tissue abnormalities.
Issue: X-rays are ionizing radiation. They harm cells and DNA. Exposure is cumulative and life long.
Frequency: X-rays have been used for almost 120 years, but the introduction of computed tomography, or CT scans, in the 1970s, was revolutionary. The new tests, which use multiple X-ray images, allowed doctors to see with unprecedented precision the inner workings of the human body, and earned the inventors of the device the 1979 Nobel Prize in medicine.
Better choice: Ultrasound is a better option since it is not ionizing radiation. It may not always be an available option though depending on the medical condition.
Source:
http://www.consumerreports.org/cro/magazine/2015/01/the-surprising-dangers-of-ct-sans-and-x-rays/index.htm