Beam hardening

Correction for beam hardening artefacts in computerised tomography... 

In actual practice, one uses modifications to Eq. (2), which include the so-caUed beam-hardening corrections to account for deviations from Eq. (1) caused by the broad-spectrum nature of the incident X-ray beam.)... 

Fig. 12 (Right) Comparison of thermal flows of 3.5 fim thick resist images at 250°C (a) no treatment, (b) electron beam hardening, (c) Deep UV-hardening.

Okoroanyanwu, J. Shields, and C.Y. Yang, Process for preventing deformation of patterned photoresist features, U.S. Patent No. 6,589,709 (2003) C. Gabriel, H.J. Levinson, and U. Okoroanyanwu, Selective photoresist hardening to facilitate lateral trimming, U.S. Patent No. 6,716,571 (2004) W.R. Livesay, A.L. Rubiales, M. Ross, S. Woods, and S. Campbell, Electron beam hardening of photoresists, Proc. SPIE 1925, 426 436 (1993). 

Accuracy of Reconstructions. Dense, circumferential calcifications can cause beam hardening and degrade the accuracy of vascular reconstructions. Although this limitation is addressed by dual- and spectral-source CT imaging, optimal methods to accurately measure the degree of vascular stenosis in patients with heavily-calcified plaque are under investigation. 

The patient should he positioned as comfortably as possible for any examination. For this purpose,special headrests, arm supports (particularly for supporting the arms in the above-head position), and cushionlike supports for the knees are available. The patient must be positioned on a soft surface, which should in addition be heat-insulated. Because most CT scanners are no longer water-cooled but dissipate their heat directly into the room air and because the scanner rooms must normally be fully air-conditioned for this reason, the patient should be covered with a protective sheet. None of the materials employed should contain silicate, glass or other absorbing substances, since the presence of these substances in the scan field can lead to artifacts or increased image noise as a result of beam hardening. 

As shown in Fig. 1, low energy electron beam irradiation hardened the treated surface layer to a indentation penetration depth of about 3 im. The electron-beam hardening decayed significantly with increasing penetration depth. Therefore, the hybrid surface reinforced specimen has a large gradient of hardness along the depth direction. However, the enhanced... 

Beam hardening artifacts (for polychromatic X-rays or neutrons) prevent exact quantification of the attenuation coefficients. 

At energies below 40 keV, X-ray absorption in air is a major limitation, and replacing air by low density helium has been recommended (77). This requires a special enclosure for X-ray source and imaging system, often limiting the size of the test object. X-ray beam hardening (78), ie, the preferential absorption of low energy X-rays in the test object, also occurs in polymers and PMC, but to a lesser extent than in denser materials. 

Sophisticated correction techniques are needed to obtain C-arm CT results with good low-contrast visibility. They are explained below. We start with overexposure correction, then turn to scatter correction, and next look into beam-hardening correction. After that, we briefly visit truncation correction and finish up with ring correction. 

The appearance of cupping and shadowing artifacts due to scatter can be very similar to those created by beam hardening (Ruehrnschopf and Kalender 1981). Contrary to beam hardening, however, a decrease in differential contrast and an increase in noise are typical consequences of scatter. 

After dealing with overexposure and scatter removal, 2D projections can be corrected for beam hardening. X-ray tubes emit photons with different energies. Low-energy photons are absorbed and attenuated more strongly by matter than higher energy photons. As a consequence. 

Because of beam hardening, the linear relationship between attenuation value and object thickness no longer applies. As a result, attenuation values towards the... 

Fig. 5.3. Diagram of the three-material decomposition showing the relation of CT densities of a voxel at 140 and 80 kVp for different body tissues. The blue lines indicate beam hardening by additional iodine content in a voxel...

With modern MDCT scanners, the indications for sequential CT scanning—as opposed to spiral scanning—are usually limited to dynamic protocols such as perfusion CT (PCT) and to sequential scans of the infratentorial brain or skull base. Sequential scanning can reduce beam hardening artefacts and overbeaming however, 3D-refomation options are limited. 

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