A CT (computed tomography) scanner is a medical imaging device that uses X-rays to create detailed pictures of the inside of the body. CT scans can be used to diagnose a variety of conditions, including:
Bone fractures and tumors
Lung conditions like emphysema, pneumonia, and blood clots
Heart disease
Blood vessel aneurysms
Kidney and bladder stones
Brain bleeds
Alzheimer's disease
Inflammatory diseases like ulcerative colitis and sinusitis 
 
CT scans work by rotating an X-ray tube around the patient's body while taking multiple X-ray images from different angles. A computer then processes the images to create cross-sectional or 3D images. 
CT scans are generally considered safe, but they do expose patients to ionizing radiation, which can damage DNA and increase the risk of cancer. The risk is very small, but it increases with each CT scan a patient receives. For children, healthcare providers adjust the dose to reduce radiation exposure. 
CT scans are also known as CAT scans, which is short for computed axial tomography.