A gamma camera, also called a scintillation camera or Anger camera, is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy. The applications of scintigraphy include early drug development and nuclear medical imaging to view and analyse images of the human body or the distribution of medically injected, inhaled, or ingested radionuclides emitting gamma rays.
Basically, picture the device similar as a digital camera except this camera detects radioactive particles (for example, i131 radio-iodine) from a source--the patient is the source!
Hal Anger developed the first gamma camera in 1957 and did not rely on continuous motion. This became the predecessor of the present day gamma cameras where the input is stored on the hard disk of a computer and the output can be manipulated and recorded on a variety of media.. His original design, frequently called the Anger camera, is still widely used today. The Anger camera uses sets of vacuum tube photomultipliers. 
- ↑  Dept of Nuclear Medicine, "General Overview: History of Nuclear Medicine", Austin Hospital, Melbourne, Australia, 2007