CT Image Formation and Presentation

CT image formation and presentation
Session overview
Description
This session provides an introduction to the reconstruction and display of x-ray computed tomography (CT) images.

Learning objectives
By the end of this session you will be able to:

• State the physical property of tissue represented by the pixel values in a CT image
• Explain the advantage of using suitably-adjusted windowing when viewing a CT image
• Recall that the method of image formation is filtered back-projection
• Explain why it is necessary to filter the projections before back-projecting
• Name the projection interpolation methods used to reconstruct helical scans in (i) single-slice, and (ii) multi-slice scanners

Prerequisites
Before commencing this session you should have completed the following sessions:

• Module 8a Physics/Components of the CT Imaging System (300-0723)
• Module 8a Physics/Interaction of Charged Particles with Matter (0670)
• Module 8a Physics/Interaction of X and Gamma Rays with Matter (300-0671)
• Module 8a Physics/The X-ray Tube (300-060)

X-ray CT, as its name suggests, is a purely digital modality. Indeed, advances in computer technology played an important role in allowing the first CT images to be produced.

A CT scan generates a digital image (Fig 1) made up of an array of individual pixels, stored in computer memory.

The CT image is a transverse (sometimes called transaxial) slice through the body. In practice, a CT investigation produces a large number of such transverse slices, which are often contiguous, i.e. there are no gaps between them. Slices in other orientations, such as sagittal or coronal, may be produced by interpolation from a stack of transverse slices.