Clinical Assignment 7 [Physics Question 1]

Started by: Leinna
On: 28 Jul 2010, 14:22 +1000


Inhomogeneities are important factors that can influence imaging and dose distribution.

Mr. Brown is a hypothetical patient with localised prostate cancer, who is to be treated with curative intent using a combination of external beam radiation therapy and high dose-rate brachytherapy. Mr. Brown has bilateral hip prostheses.

How do you expect the planning CT scan images to appear? What are the underlying physical principles? How would MV images appear?

What problems and challenges do the bilateral hip prostheses present for treating this patient with external beam radiation therapy? Explain how you may overcome (or avoid) these specific challenges in radiation therapy planning and delivery.


Every time I perform a search on the topic of 'CT Artifact' or 'CT Artifact Metal Inhomogeneity' I am delivered many papers describing methods of avoiding these problems. They generally start with "The presence of metal in the CT image may lead to streaking/starburst artifact. This is due to beam hardening / scatter / incorrect function of the correction software. It can be fixed by xyz".

I've managed to glean from the various papers that:

  • Streaking or Starburst artifact is the most common name for these phenomenon, giving rise to bright lines radiating out from the inhomogeneity and dark spaces between the inhomogeneities (if there are two present)
  • It occurs because

a) the hardening in the metal confuses the reconstruction software so it assumes that that inhomogeneity is much larger than it really is (bright lines) and
b) the rapid attenuation / hardening of the beam reduces the information on attenuation normally gathered from normal human tissues, leading to dark spaces.

I get the feeling there is a lot more to it than this.

Discussion

I think you should start reading here! The predominant issue is that the CT calculations algorithms assume that a certain range of densities and attenuation coefficients exist in the patient. All these assumptions are made in the 80-100kV range where photoelectric effects dominate. The presence of a large very dense mass [remember that bone density is not much more than 1.2-1.5 (if my memory serves me well!)], where as the metal has densities of 5-8gm/mL. As a result the calculations are all wrong. However in the MV range, this does not occur (because our CBCT algorithms have been deliberately modified to include the density range?), but whatever, the star artifact disappears (as shown in the link)

This blog has been kind enough to also provide a reference list.

If the link above no longer works, try this one.

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