Christiaan Fivez

Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation

In X-ray projection radiography, especially if area detectors are used, scattered radiation can strongly degrade the image contrast. Although the amount of radiation scatter is partly reduced by the use of anti-scatter grids or air gaps, the contrast is mostly still substantially degraded in large areas of the images. Here, the radiation scatter in radiographs is modeled as the sum of a low-pass filtered version of the primary radiation distribution and a residual scatter component. The authors show how the Multiscale-Image-Contrast-Amplification (MUSICA) algorithm for contrast enhancement can automatically remove the degrading effect of scattered radiation on image contrast, by selectively suppressing low resolution components, without significantly affecting the primary radiation distribution at any spatial resolution.

Influence of Heel effect and of nonuniformity of emitted spectra on dual-energy subtraction in computed radiography

Because of beam hardening in the target, the energy distribution of the x-radiation emitted by a conventional x-ray tube differs from different directions of emission. This fact is generally neglected in dual-energy subtraction imaging. In this paper we study the influence of the non- uniformity of the energy spectrum of emitted radiation on dual-energy subtraction. Experiments with synthetic (aluminum) phantoms and with dual-energy images of humanoid chests (made with a computed radiography system) give more insight in the degree to which dual-energy subtraction is affected by the described effects.

A novel method for scattered radiation compensation in X-ray imaging systems, using partially transparent shields (PTS)

A new method for scattered radiation compensation in 2D projection radiographic imaging is presented. By comparing the detected signal under a small partially transparent shield (e.g. aluminum disk or strip), positioned between the X-ray source and the object being imaged, with the detected signal near the border of the shadow of the shield, the radiation scatter signal in the location of the shield is calculated. For polychromatic X-radiation, calibration with two known basis materials allows accurate calculation of the radiation scatter. Shields are positioned at several locations between object and source. By an interpolation technique, the radiation scatter in every region of interest can be calculated. The radiation scatter image is then subtracted from the original image of the object. The method needs only one exposure of the object. The primary radiation signals at the locations of the shields have undergone an extra drop, but the information about the object under the shields is not lost. One could restore the image by compensating for the effect of the shields on the primary signal. The theory of PTS is presented and results of experiments are given.<<ETX>>

Calibration phantom for dual-energy basis material absorption measurements

Dual-energy subtraction imaging uses the spectral differences in x-ray radiation attenuation caused by different tissues to obtain material-selective images. When the subtraction is based on decomposition of the object images into basis material images, accurate calibration measurements must be made. This calibration procedure consists of measuring the absorptions of the two x-ray spectra by a set of known thickness combinations of two chosen basis materials. In this paper a calibration phantom is presented which allows accurate measurements, which allows adjustment to the source-to-detector distance and which is easy to handle. In our laboratory and in the hospital we use the phantom for the implementation of the dual-energy technique on a computed radiography system.

Primitive-based contrast enhancement method

In this article we present a new solution for the problem of contrast enhancement. The method is based on the decomposition of the original image into its primitive components. In a first stage a multiresolution representation of the image is computed yielding transform coefficients proportional to the maximum of the local gradient at a specific scale and at a specific position in the image. As an image detail exists at several scales, the transform coefficients corresponding to the same image primitive are grouped. Contrast is enhanced by groupwise non-linear amplification of the transform coefficients. An inverse transform is then applied to the modified coefficients, resulting in a uniformly contrast-enhanced image without artifacts. Preliminary results demonstrate the effectiveness of this method. It could be used in a wide range of applications.

Linear model for the scattered radiation distribution in digital chest radiographs

We propose an empirical linear model for the scattered radiation distribution in chest radiographs in order to compensate digital x-ray images for the presence of radiation scatter. The total detected intensity distribution is convolved with two different convolution kernels and radiation scatter is modelled as the weighted sum of the two convolutions plus a constant factor. The weighting factors and the constant factor are optimized for a specific chest radiograph. This can be done by sampling the scattered radiation distribution in a limited area of the radiographic image. The scatter values are then used to determine the factors with a standard linear least squares technique. Another way to determine the factors is by utilizing lookup tables (LUTs), giving the relationship between the factors and the acquisition parameters. These LUTs are constructed with data from previous cases. We give some preliminary results and compare the model with some other convolution based models.

Compensation for scattered radiation in dual-energy imaging using partially transparent shields

In this paper we present a method for the compensation for scattered radiation in dual-energy imaging. The method uses shields (PTS), partially transparent for x-radiation, positioned between the x-ray source and the detector. By comparing the detected signals under and beside the shields, and using data from the dual-energy calibration procedure with basis materials, the radiation scatter can be calculated at the location of the shields. In a first implementation of the technique many shields are positioned at regular distances in the imaging field. The radiation scatter in the entire image can then be calculated by interpolation techniques. A second implementation of the technique makes use of a robust convolution with a low-pass filter. The parameters of the convolution kernel are determined by PTS-measurements at a limited amount of well-chosen locations of the object. After substraction of the radiation scatter, the images can be compensated for the effect of the shields. We tested the method with images of an artificial polystyrene phantom and with humanoid chest images, made with a computed radiography system.