RTK/ImageQuality

This page summarizes the existing and the future solutions in RTK for improving image quality of cone-beam (CB) CT images.

X-ray source imperfections

• Geometric blurring can be corrected by the scatter glare correction detailed in the detector imperfections section.
• Exposure fluctuations from projection to projection are common. They can be corrected by rtk::I0EstimationProjectionFilter which automatically estimates a weighting constant per projection using an histogram analysis. This filter only works if there are pixels in each projections that measure x-rays that traversed air only (except maybe a few projections using a revursive least-square . The filter does not have any parameter except the bitshift template value for the reduction of the histogram size. It is implemented for integer pixel types only.
• Focal spot motion cannot be corrected currently. It would require geometric calibration for each acquisition.

Detector imperfections

• Lag: the [ rtk::] filter implements correction implements

, see [ Poludniowski et al, PMB, 2011] implemetend in [rtk::]

Beam hardening

• The acquired data may be linearized for a given material using rtk::LookupTableImageFilter. There are several solutions to compute the lookup table:
  • Compute it from the knowledge of the spectrum of the x-ray source and the detector response,
  • Measure the attenuation for several thicknesses of the material of interest,
  • Do a tomography of a homogenous object (e.g., a cylinder), fix the lookup table the rtk::WaterPrecorrectionImageFilter] which implements Kachelriess et al, Med Phys, 2006.
  • Estimate the p and a parameters of equation 1 in Ohnesorge et al, Eur Radiol, 1999
• The algorithm of Kyriakou et al, Med Phys, 1010 may easily be implemented from the existing code in RTK.

Truncated projection images

• The rtk::FFTRampImageFilter implements the heuristic solution of Ohnesorge et al, Med Phys, 2000. The parameter TruncationCorrection must be set.
• Exact reconstruction based differentiated backprojection and inverse Hilbert filtering (see, e.g., Noo et al, PMB, 2004 is investigated in Lyon.