Beam hardening in X-ray reconstructive tomography

Abstract

As a polychromatic X-ray beam passes through matter, low energy photons are preferentially absorbed, and the (logarithmic) attenuation is no longer a linear function of absorber thickness. This leads to various artifacts in reconstructive tomography. If a water bag is used, the nonlinear attenuation in bone causes a distortion of the bone values and a spill-over inside the skull, or 'pseudo-cortex' artifact. If no water bag is used, there is an additional effect due to the varying thickness of soft tissue which causes a depression of interior values, or 'cupping'. Both artifacts can be remedied by additional prefiltering of the beam and by applying a linearization correction to the detector outputs.

Characterization and correction of cupping effect artefacts in cone beam CT

“…Removal of the low-energy photons from the X-ray spectrum reduces beam hardening and therefore also reduces the increase in transmitted intensity that results from its presence. The use of filtration to decrease beam hardening is supported by the findings of Brooks and Di Chiro, 16 who demonstrated a reduction of beam hardening effects from 9.2% in 20 cm of water using a 4.5 mm aluminium pre-filter to 1.5% using a 3.5 cm aluminium pre-filter and reported that using high atomic number (Z) materials such as copper, tin or Thoraeus filters could produce even better results. In fact, normal aluminium filters are approximately 10% less efficient than filters of other materials such as copper, brass or iron.…”

“…6 As a result, beam hardening and scatter produce a common artefact known as the cupping effect artefact. [6][7][8][9][10][11][12][13][14] McDavid et al 15 and Brooks and Di Chiro 16 demonstrated that the cupping effect is caused by beam hardening by reconstructing a uniform object with ideal projections and observing the absence of the cupping effect. The cupping effect caused by scatter occurs because of the scatter flux, resulting in an underestimation of the linear attenuation coefficient.…”

Synchrotron- and laboratory-based X-ray phase-contrast imaging for imaging mouse articular cartilage in the absence of radiopaque contrast agents

“…Ring artefacts, which may appear as a result of imperfections in the detector and cause concentric rings superimposed on the image, were minimized by randomly moving the object by a few micrometres throughout scanning [29] and by setting the ring artefact correction equal to 5 (as automatically set by the NRECON software) in the reconstruction process. To correct for beam hardening-which is a result of preferential absorption of low-energy X-rays on the surfaces of bone, leaving the inner portions apparently less dense owing to the fact that the remaining high-energy X-rays are absorbed less-the NRECON software applies mathematical corrections as documented in [30,31]. In this study, beam hardening was reduced both during the scans by using an aluminium filter and in the tomogram reconstruction by setting a constant beam hardening correction factor of 25% in the NRECON software.…”

Software architecture for multi-bed FDK-based reconstruction in X-ray CT scanners

“…The first one is a linearization correction, which assumes that all the materials in the sample have homogeneous X-ray attenuation (similar to water), and is applied to the projections prior to reconstruction [21,23,33]. It is a fast correction algorithm, currently implemented in most commercial scanners, based on a linearizing function that maps polychromatic projection data obtained from the real scanner into monochromatic projection data (Fig.…”