7.2 - Principles Of CT Scanning

Physics of CT Scanning

CT uses multiple pencil or fan beams of kilovoltage photons that pass through a desired volume from multiple angles (usually over 180 degrees). On the opposite side of the volume is a dosimeter which measures the amount of ionising radiation reaching it. This allows determination of the attenuation of individual beams as they pass through the volume.
Each part of the volume may be considered a 'voxel' (a three dimensional pixel) with width, height and depth. Each beam will pass through a number of voxels as it traverses the volume. The attenuation of the beam as it passes through the volume may be considered to be the sum of attenuations in each voxel it has passed through. This may be up to 512 voxels for modern scanners.
A computer is then used to solve a simultaneous equation with up to 512 variables, using the attenuation information from each beam. This is a process which computers are able to perform quickly and precisely, so long as they have been given good information from the photon attenuation.l
Once the attenuation for each voxel is determined, the computer system assigns a Hounsfeld Unit to each part of the volume. Hounsfeld Units range from -1000 (air), to 0 (water), to +1000 (cortical bone).
Because kilovoltage photons are used, there is good discrimination between tissues of different atomic numbers. Because so many beams are used, CT is able to discriminate between different soft tissues (such as adipose and muscular tissue) even though they have similar atomic numbers.
Another important feature is the kilovoltage photons are only affected by the intervening tissue, and spatial resolution (the accuracy of determining the physical position of each voxel) is very high and allows accurate planning.

Use of CT Scanning

CT scanning is now the preferred method of planning in modern radiotherapy departments. It allows acquisition of patient contour and inhomogeneities in a single procedure. It also gathers 3D attenuation data which is usable in treatment planning systems.


  • Relatively inexpensive compared with MRI and PET scanning
  • Accurate, 3-dimensional data including attenuation information
  • Rapid acquisition of data and no need for patients to remain for planning process
  • 4-dimensional data acquisition is possible using gating technology


  • Relatively high amount of ionising radiation per scan (increased for 4-D imaging)
  • Subject to artefacts due to patient movement
  • Contrast required for certain structures, particularly vessels. Some people may have an anaphylactic response to intravenous contrast, which may occasionally be fatal.