Correction Of Spectral Artifacts And Determination Of Electron Density And Atomic Number From Computed Tomographic (CT) Scans

The usefulness of CT scanners has been demonstrated in imaging many areas of the body, particularly the brain. However, one serious problem common to all scanners still remains and is related to the x-ray source. Because the source of x-rays produces a spectrum of radiation rather than monoenergetic x-rays, the CT numbers will depend upon the size of the patient so that they are in fact functions of the source, the size and shape of the patient and the point of reconstruction in the patient. For this reason brain tissues near the bone are imaged with elevated CT values so that it appears that the bone extends into the brain and therefore lesions near the skull would be missed. This artefact is known as the "spectral artefact". Another problem due to the polychromatic nature of the x-ray beam is the fact that it is very difficult to obtain meaningful attenuation coefficients and hence the electron density (σ), and average atomic number (Ζ) for each pixel directly from the CT image. The p is required for realistic dose calculations for radiotherapy and may be useful in diagnosis of tumours in the rest of the body. Two methods for correcting the "spectral artefact" and obtaining ρ and Ζ from CT scans will be discussed: 1) Dual detector; 2) Split filter. Results obtained from an experimental CT scanner will be shown.