On the use of flat-fields for tomographic reconstruction

Abstract

Seeking for quantitative tomographic images, it is of utmost importance to limit reconstruction artifacts. Detector imperfections, inhomogeneity of the incident beam, as classically observed in synchrotron beamlines, and their variations in time are a major cause of reconstruction bias such as `ring artifacts'. The present study aims at proposing a faithful estimate of the incident beam local intensity for each acquired projection during a scan, without revisiting the process of data acquisition itself. Actual flat-fields (acquired without specimen in the beam) and sinogram borders (when the specimen is present), which are not masked during the scan, are exploited to construct a suited instantaneous detector-wide flat-field. The proposed treatment is fast and simple. Its performance is assessed on a real scan acquired at ESRF ID19 beamline. Different criteria are used including residuals, i.e. difference between projections of reconstruction and actual projections. All confirm the benefit of the proposed procedure.

Use of fast realistic simulations on GPU to extract CAD models from microtomographic data in the presence of strong CT artefacts

“…with 𝑰(𝑥, 𝑦) the value of the raw X-ray projection at pixel location (𝑥, 𝑦), and with the sample and with the X-ray beam turned on; 𝑖 the 𝑖-th energy channel in the beam spectrum; 𝐸 𝑖 the energy in eV; 𝑹 𝑖 and 𝑵 𝑖 the detector response and the number of photons at that energy respectively; 𝑗 the 𝑗-th material being scanned, 𝜇 𝑗 (𝐸 𝑖 ) its linear attenuation coefficient at energy 𝐸 𝑖 , and 𝒅 𝑗 (𝑥, 𝑦) path length in cm -1 of the ray crossing the 𝑗-th material from the X-ray source to pixel (𝑥, 𝑦). Projections are then corrected to account for variations in beam homogeneity and in the pixel-to-pixel sensitivity of the detector [15]. This is the projection with flat-field correction (𝐏𝐫𝐨𝐣):…”

Separation of superimposed images with subpixel shift

“…The flat-field correction procedure with stationary intensity correction has been presented in detail by Jailin et al (2017). The principle can be summarized as follows: taking into account the multiplicative nature of the corrections, the logarithm of the radiographs Gðr; tÞ = log½Iðr; tÞ, i.e.…”

“…The result of the flat-field correction around the fourth spot is shown in Fig. 8 for the standard correction [proposed by Jailin et al (2017)] (Fig. 8a) (i.e.…”

Full-field spectroscopic measurement of the X-ray beam from a multilayer monochromator using a hyperspectral X-ray camera

“…Using adequate design parameters for the monochromators, the modified Bragg law followed by MLMs ensures a strong suppression of the second-order harmonic and usually also a mismatch between higher-order MLM harmonics and undulator harmonics, effectively resulting in very limited contamination with higher energies. However, the resulting beam often exhibits intensity fluctuations in space and time due to thermal instability of the optics (Titarenko et al, 2010) and roughness of the surfaces (Rack et al, 2010), which have a negative impact on the flat-field correction in tomographic reconstruction (Jailin et al, 2017). The effects of these fluctuations are in many cases left unprocessed, taking advantage of the temporal averaging of X-ray microCT over the complete trajectory.…”