Beam hardening
Beam hardening is a change in X-rays when it penetrates matter. With increasing penetration depth, the average energy (“hardness”) of the X-ray quanta ( photons ) increases because the harder photons are less strongly scattered. This is synonymous with a shift in the focus of the spectrum towards higher energies. In computed tomography (CT) the hardening occurs as a disturbing effect, which can lead to so-called hardening artifacts. An attempt is made to correct these artifacts as far as possible with the help of mathematical methods.
Computed Tomography
In computed tomography (CT), a three-dimensional image is calculated from the image information from X-ray fluoroscopy. In contrast to a simple fluoroscopy, this contains depth information and is typically displayed as a sequence of sections. The imaging is based on the fact that the X-ray photons are scattered ( Compton scattering ) or absorbed ( photo effect ) by the electrons . In matter with a higher electron density, more photons are scattered away from the beam direction. Bones, with a relatively high electron density, therefore appear bright in an X-ray exposure because the X-ray film is exposed to fewer photons.
The X-ray tubes used in computer tomography work with typical accelerator voltages between 80 and 140 kV . The x-rays emitted in this process have a broad energy spectrum. As a result of the beam hardening, this spectrum is now shifted towards higher energies after penetrating the object (patient). The hardening of the beam is stronger the longer the path through the object or the higher the electron density of the individual layers. The detectors used in CT today are not energy-resolving; that is, they do not distinguish between photons of different energies. Instead, the individual detector elements only measure the total energy deposited in them, which corresponds to averaging over the photon energies. As a result of this averaging, part of the information is lost, which is ultimately the cause of the hardening artifacts. Uncorrected CT images show a typical course in which the gray values become darker towards the center (rotation axis of the CT).
When viewing CT images of humans, it is difficult to decide what is real and what is an artifact created by beam hardening. One way to differentiate that is to use a phantom. These are human bones, but the soft tissues have been replaced with water. The phantom makes it possible to quantitatively estimate the falsifications of the CT values in the adjacent soft tissue caused by the bone. Ideally, they should be 0 Hounsfield units. Due to the hardening effect, the water bath simulating the soft tissues creates partly hypodense and partly hyperdense artifacts.
literature
- W. Schlegel, J.Bille (Ed.): Medical Physics 2 , Berlin Heidelberg New York (Springer) 2002, ISBN 3-540-65254-X
- WGH Schmitt, MO Mahmalat, HK Beyer: The measurement accuracy of computed tomographic densitometry in the vicinity of the pelvic skeletonFortschr Röntgenstr 1987; 147 (1): 34-38 Georg Thieme Verlag Stuttgart New York