Gamma Rays :

Gamma rays are electromagnetic waves or photons emitted from the nucleus (center) of an atom.

   Gantry :

The scanner gantry contains the X-ray tube and detectors mounted around a central tunnel through which the patient and table pass. The gantry can be angled away from the vertical over a limited range. Some spiral scanners also have the generator mounted on the gantry.

   Gauss (G) :

An old unit used for measuring magnetic field strength. The internationally accepted unit is the Tesla (1 Tesla = 10000 Gauss). The earth’s magnetic field is approximately 0.5 Gauss.

   Gradient coils :

These are magnetic coils designed to alter he main magnetic field (in order that the magnetic field is stronger in some areas compared with other parts) by a few percent. The magnetic field gradient generated by a coil is controlled by the electrical current passed through the coil. They are used to localize a slice and spatially encode slice information.

   Gradient echo (CE/GRE) :

A basic pulse sequence which only uses magnetic field gradient reversal to rephase the transverse magnetization and produces echoes of the MR signal. This allows shorter repetition times thus scanning, and flip angles less than 90º.

   Gray (Gy) :

The gray is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One gray is equal to one joule of energy deposited in one kg of a material. The unit gray can be used for any type of radiation, but it does not't describe the biological effects of the different radiations. Absorbed dose is often expressed in terms of hundredths of a gray, or centi-grays. One gray is equivalent to 100 rads.

   Health Physics :

Health Physics is an interdisciplinary science and its application, for the radiation protection of humans and the environment. Health Physics combines the elements of physics, biology, chemistry, statistics and electronic instrumentation to provide information that can be used to protect individuals from the effects of radiation.

   Hounsfield unit (HU) :

This is the corrected or accurate value of the CT number, measured from the scanner image, although CT number and Hounsfield unit are often used interchangeably. The Hounsfield unit is defined by the relationship between the linear attenuation value of the material being scanned and that of water. A vacuum has a Hounsfield unit -1000, while water has a value of zero. The upper end of the scale is open ended but image data storage limitations usually result in a scale from -1024 to +3096 (12 bit).