Radioactive Contamination :

It is radioactive material distributed over some area, equipment or person. It tends to be unwanted in the location where it is, and has to be cleaned up or decontaminated.

   Radioactive Material :

It is any material that contains radioactive atoms.

   Radioactivity :

It is the spontaneous transformation of an unstable atom and often results in the emission of radiation. This process is referred to as a transformation, a decay or a disintegrations of an atom.

   Reconstruction :

The production of an image from the “raw data”.

   Reformation (reformatted image) :

The generation of images in an alternative plane from the original CT images. Reformats can now be in any plane or even curved, as computer speeds have improved. Multiple overlapping sections such as those obtained from spiral acquisitions produce markedly improved reformat image quality.

   Region of interest (ROI) :

Any part of a CT image on which measurements are being made, particularly measurements of the mean CT number.

   Rem (roentgen equivalent man) :

The rem is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of thousandths of a rem, or mrem. To determine equivalent dose (rem), you multiply absorbed dose (rad) by a quality factor (Q) that is unique to the type of incident radiation.

   Repetition time (TR) :

The time between successful excitations of a slice (i.e. the time from the beginning of one RF pulse sequence to the start of the next). In cardiac gated studies this can vary depending on the heart rate of the patient and as such an effective repetition time is obtained.

   Saturation :

A non-equilibrium state in which equal numbers of spins are aligned with and against the magnetic field. This occurs immediately following a 90ş pulse with the longitudinal magnetization aligned in the x-y (transverse) plane and thus the system is said to be saturated. A few moments later, with some T1 recovery, the system is partially saturated. Saturation effects are used in the slice direction to minimize signal from following blood.

   Scout view :

CT scanner digital radiograph acquired by keeping the X-ray tube and detectors stationary while moving the table through the gantry. The resultant image can he used to identify the start and stop points for the subsequent CT Sections.

   Segmentation :

Removal of part of each image in a stack in order to allow subsequent manipulations to be applied in the area of interest, e.g. removing the spine from an angiographic CT to allow MIP projections without the bone densities obscuring the vascular contrast densities.

   SI Prefixes :

Many units are broken down into smaller units or expressed as multiples, using standard metric prefixes. As examples, a kilobecquerel (kBq) in 1000 becquerels, a millirad (mrad) is 10^-3 rad, a microrem (µrem) is 10^-6 rem, a nanogram is 10^-9 grams, and a picocurie is a 10^-12 curies.

   Sievert (Sv) :

The sievert is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of millionths of a sievert, or micro-sievert. To determine equivalent dose (Sv), you multiply absorbed dose (Gy) by a quality factor (Q) that is unique to the type of incident radiation. One sievert is equivalent to 100 rem.

   Signal-to-noise ratio (SNR) :

The ratio of signal amplitude to the noise. Methods of improving the SNR are: increasing the number of signal excitations (NEX) with subsequent time penalty; increasing the field of view (FOV) with corresponding reduction in spatial resolution of the resultant image; and increasing the strength of the main magnetic field used.

   Slice thickness :

The nominal thickness of the slice which the image represents. As the X-ray beam is divergent, the sampled volume in the patient is thicker around the edges than centrally. The alignment of pre- and post- patient collimation may also affect the true slice thickness. Image noise is reduced with thicker slices but partial volume averaging effects become apparent in larger structures.