Introduction to CT physics and live concerns

Image generation

What is computed tomography (CT)?
Since the first CT scanner was developed in 1972 by Sir Godfrey Hounsfield,
the modality has become established as an essential radiological technique
applicable in a wide range of clinical situations.
CT uses X-rays to generate cross-sectional, two-dimensional images of the
body. Images are acquired by rapid rotation of the X-ray tube 360° around the
patient. The transmitted radiation is then measured by a ring of sensitive radiation
detectors located on the gantry around the patient (Fig. 1.1). The final image is
generated from these measurements utilizing the basic principle that the internal
structure of the body can be reconstructed from multiple X-ray projections.
Early CT scanners acquired images a single slice at a time (sequential
scanning). However, during the 1980s significant advancements in technology
heralded the development of slip ring technology, which enabled the X-ray tube
to rotate continuously in one direction around the patient. This has contributed
to the development of helical or spiral CT.
In spiral CT the X-ray tube rotates continuously in one direction whilst the
table on which the patient is lying is mechanically moved through the X-ray
beam. The transmitted radiation thus takes on the form of a helix or spiral. Instead
of acquiring data one slice at a time, information can be acquired as a continuous
volume of contiguous slices (Fig. 1.2a, b). This allows larger anatomical regions
of the body to be imaged during a single breath hold, thereby reducing the

possibility of artefacts caused by patient movement. Faster scanning also increases
patient throughput and increases the probability of a diagnostically useful scan
in patients who are unable to fully cooperate with the investigation.
Therefore to allow the observer to interpret the image, only a limited number of
HU are displayed. Aclinically useful grey scale is achieved by setting the WLand
WW on the computer console to a suitable range of Hounsfield units, depending
on the tissue being studied.
The term ‘window level’ represents the central Hounsfield unit of all the
numbers within the window width.
The window width covers the HU of all the tissues of interest and these are
displayed as various shades of grey. Tissues with CT numbers outside this range
are displayed as either black or white. Both the WL and WW can be set
independently on the computer console and their respective settings affect the
final displayed image.
For example, when performing a CT examination of the chest, a WW of 350
and WL of +40 are chosen to image the mediastinum (soft tissue) (Fig. 1.5a),
whilst an optimal WW of 1500 and WLof –600 are used to assess the lung fields
(mostly air) (Fig. 1.5b).
What is pitch?
Pitch is the distance in millimetres that the table moves during one complete
rotation of the X-ray tube, divided by the slice thickness (millimetres). Increasing
the pitch by increasing the table speed reduces dose and scanning time, but at the
cost of decreased image resolution (Fig. 1.6a, b).
Image reconstruction