Glossary of CT terms

Beam Hardening Phenomenon referring to the effect of selective x-ray attenuation and scatter from polychromatic X-ray beams. An x-ray beam is composed of individual photons with a range of energies. As the beam passes through an object, it becomes "harder" (mean energy increases) because the lower-energy photons are selectively absorbed (or scattered) more rapidly, leaving behind only the high-energy photons. Both cupping and streak artefacts can occur as a result.

Cupping artifact

A ring of excessively bright voxels around the edge of the fossil reducing to excessively dark voxels at the centre – caused by beam hardening

CT numbers

Term coined by Godfrey Hounsfield to describe the voxel grey values

Electron volt (eV)

is a unit of energy equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt.

LUT plot

Image look up table, a bivariate plot of grey scale voxel values (CT numbers) versus the number of voxels

Noise

Unusually bright and dark voxels are superimposed over the CT data, which gives a CT scan a grainy or speckled appearance, leading to blurring of material boundaries.

Partial volume

Artefact resulting when numerous linear attenuation coefficients (i.e., material density) different within a single voxel being represented by an averaged grey value.

Segmentation

The process of defining regions of interest with a scan, which usually represent a fossil or part thereof. The regions of interest can be modelled and rendered independently. Segmentation invariably requires the user to threshold (i.e., define) material boundaries within an object e.g., fossil and matrix (note: the term segmentation refers to a different process in the SPIERS software suite).

Polychromatic

An X-ray beam composed of radiation of more than one photon energy (i.e., wavelength)

Steak artefacts

Dark bands or streaks between dense objects in ta CT slice image.

Threshold

The process of defining material boundaries within a scan. In its simplest form this entails selecting a grey value (threshold) above which any voxels are grouped in a region of interest and those, which fall below, are discarded or made transparent.

Reconstruction

is the process of transforming the set of 2D X-ray projections to a 3D volume.

Region of interest (ROI)

is a selected sub-region of a CT scan that can be identified e.g., a fossil tooth within a matrix or vascular voids with the tooth.

Rendering

the process of generating an image from a model

Surface Determination

defining the surface of an object to produce a hollow 3D model without internal features