Radiation In Medicine

Types of Radiation:

Gamma Rays: movements of substances producing gamma rays are detected and positions are recorded.
X-Rays: High frequency electromagnetic waves that penetrate tissue but not bone, used in hospitals to locate injuries.
Light: Light reflected off objects to form an image.
Ultrasound: High frequency sound waves reflect off features inside the body to form an image.

X-Rays and Gamma Rays are ionising that's why they are used in killing of cancer cells in hospitals.

Intensity: Strength of a wave depending on the area and the power.

If the source is far away, the intensity will be low.
If the Area is bigger, the intensity will be low and vice versa. 

Intensity = Power of radiation/Area

W/m^2 = W/m^2