| Absorbed dose |
Energy absorbed from an ionising radiation per unit mass. Unit = Gray (Gy) |
| Collimation |
This refers to limiting the x rays to a certain area in space. For instance, in CT this often refers to the slice thickness. |
| CNR |
Contrast‐to‐noise ratio. The higher the CNR, the better the image quality and vice versa. Unfortunately higher CNR often involves imparting a higher dose to the patient. |
| CTDI |
CT dose index. Determined on phantoms, usually 16 cm and 32 cm acrylic phantoms. Does not represent the dose to the individual patient, but is an indicator of the dose per CT slice. |
| Deterministic effects |
These depend on cell killing and have a threshold dose for their induction. |
| Effective dose |
Equivalent dose received by each individual organ or tissue multiplied by the appropriate tissue weighting factor and summing for all tissues involved. Unit = Sievert (Sv) |
| Equivalent dose |
A measure of absorbed dose that accounts for the increased biological effect of high linear energy radiation. The absorbed dose is multiplied by the radiation weighting factor to produce the equivalent dose. This weighting is 1 for x rays, gamma rays and electrons, 5–20 for neutrons (depending on energy), 5 for protons and 20 for alpha particles, fission fragments and heavy nuclei. Unit = Sievert (Sv) |
| Gantry rotation time |
The time taken for the CT x ray source to make a 360° revolution around the patient. |
| Grid |
A grid is used for taking plain radiographs or fluoroscopy images in larger patients. A grid usually contains strips of lead and is placed between the patient and the film. The lead absorbs most of the scattered x rays which hit the grid obliquely while allowing the primary beam through. This increases resolution but unfortunately also increases dose. |
| Scan pitch |
The ratio of the table movement to the x ray beam width for one tube rotation. |
| Stochastic effects |
These depend on cell transformation, are random and have no threshold dose. Severity is unrelated to dose and may be heritable. |
