Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Correction and Preprocessing Methods Veselin Dikov Moscow-Bavarian Joint Advanced Student School March 2006 Moscow, Russia

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Outline ●Beam hardening ●Scattered radiation ●Ring artefacts ●References

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Beam hardening ●Monochromatic vs. Polychromatic X-ray source ●Absorption of low energy photons ●Additive detector ●Mean energy higher - beam hardening

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Thickness measurement ●Monochromatic ●Polychromatic for homogenous medium

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Thickness measurement ●“Non-linear” deviation of the polychromatic signal

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Beam hardening artefacts ●Cupping ●Simulated skull

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Beam hardening artefacts ●Streaks ●Hard (e.g. bone) tissues

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Correction Strategies ●Preprocessing  On projection data ●Postprocessing  On reconstructed data ●Dual-energy imaging  Considered most elegant

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Preprocessing ●Select “appropriate” absorber ●Measured value A is corrected to A’ ●Assumes “homogeneous” medium ●Fails if bone present

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Postprocessing ●Iterative approach:  first: preprocessing step  second: threshold to identify hard structures  third: forward-project the contribution of the hard structures into projection data ●Good results

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Dual-energy image ●Modeling of the attenuation coefficient ●Substitute in ●Gives

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia ●Two energies - two integral equations ●Reconstruct and  Now can be reconstructed for any E ●Two scans? Dual-energy image

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Outline ●Beam hardening ●Scattered radiation ●Ring artefacts ●References

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Scattered radiation ●Scattered radiation ●Components of scatter  Off-focus radiation (OFR)  Scattered radiation in the patient  Veiling glare

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Scatter measure ●SPR – Scatter-to-Primary-Ratio ●Broad beam model estimation for SPR

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Scatter artefacts – loss of contrast ●Loss of contrast  Contrast without scatter  Contrast with scatter

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Scatter artefacts – loss of sharpness ●Loss of sharpness  Transmitted radiation  - blurring kernel  - 2D convolution

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Scatter artefacts – loss of sharpness ●Loss of sharpness  Veiling glare contribution

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Scatter reduction methods ●Anti-scatter grids  4G vs. 3G  Higher voltage ●Air gaps  Magnifying effects  Increased blurring ●Beam collimation  Complicated systems reminder:

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Measurement of scatter ●Opaque disc techniques  Scatter in the “shadow”  Arrays of discs with decreasing size ●Aperture techniques  Narrow beam “clean” of scatter  Arrays of apertures ●Hybrid techniques ●One vs. Two scan techniques

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Correction schemes - Convolution Filtering ●Detected image ●Use as an estimation for ●Spatial vs. frequency domain filtering ●Analytical vs. empirical approaches for filtering schemes

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Correction schemes – Scatter sampling ●Sampling of the scatter measurement  Opaque discs arrays  Aperture grid ●Interpolation of the smooth surface  -6 dB

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Correction schemes – Scatter sampling ●Interpolation schemes  2D least squares fitting  Filtration with sinc and jinc  2D polynomial fitting  2D bicubic splines ●Sampling rate depends on surface frequencies

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Results

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Outline ●Beam hardening ●Scattered radiation ●Ring artefacts ●References

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Ring artefacts

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Ring Artefacts ●Reason ●4G vs. 3G ●Postprocessing correction only

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Correction scheme 1.ROI 2.I P 3.Artefact pattern 4.Artefact subtraction 5.P I

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia ROI ●Select ROI with morphological operations

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Find artefact pattern ●Scan for irregularities  Use variance of signal  or Use median of signal  etc. ●Extract ring pattern

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia Results

Moscow-Bavarian Joint Advanced Student School March 2006, Moscow, Russia References ●T.Buzug, Einfürung in die Computertomographie, Springer-Verlag (2004) ●A.Kak and M.Slaney, Principles of Computerized Tomographic Imaging, IEEE Press (1988) ●K.P.Maher and J.F. Malone, Computerized scatter correction in diagnostic radiology, Contemporary Physics 38, (1997). ●J.Sijbers and A.Postnov, Reduction of ring artefacts in high resolution micro-CT reconstructions, Physics in Medicine and Biology Vol. 49 (07/2004) ●M.Zellerhoff et al, Low contrast 3D-reconstruction from C-arm data, Medical Imaging SPIE, (04/2004) Thank you for your attention!