Material decomposition using a photon counting X-ray detector in low energy range

Sammanfattning: Material decomposition in X-ray imaging is used to separate different materials or elements in an image and to quantify their respective concentration. The material decomposition and quantification are made possible by the dependency of attenuation on the attenuating material and photon energy. The purpose of this thesis was to implement a material decomposition method, from the image acquisition using a photon counting detector in a laboratory setup to the actual material decomposition, for both radiography and computed tomography. The detector has a 450 µm silicon sensor with 1030 × 514 square pixels of 75 µm and two configurable energy thresholds. Due to the sensor material and thickness, the detectors quantum efficiency decreases rapidly after 10 keV limiting the photon detection to a low energy X-ray range. Energy thresholds were set in the range of 4 to 10.5 keV to acquire two images with specific photon energy windows on each side of an absorption edge of a selected element in the sample. The material decomposition was then performed in the image domain based on these images and three separate material images were created with pixel values representing the material fraction in the pixel. Two samples were used to verify the method, one with copper and silver grids and PMMA and one with a copper cable and aluminium foil for the radiography and computed tomography decompositions respectively. MultiHance, a gadolinium based contrast agent, was diluted with water to five different concentrations to study the concentration quantification for both radiography and computed tomography. Finally, a piece of an atherosclerotic carotid plaque, containing calcium and iron in soft tissue embedded in paraffin wax, was decomposed to test the method on a biological sample. The verification samples, where the impact of the copper K-edge on the attenuation was clear, decompose well with the exception of regions affected by ring artifacts and beam hardening. The ring artifact arises due to an insufficient amount of photons in the small energy windows and beam hardening is because of highly attenuating materials like copper and thick samples. These artifacts impacted the decomposition of all samples but can hardly be avoided in the current setup. The measured concentrations in the MultiHance samples were in reasonable agreement with the expected concentrations. The gadolinium L-edges had no measurable effect on the attenuation due to too high attenuation in the water in the solution and the PMMA holder that the solutions were filled in. For the plaque sample, iron, calcium and paraffin were decomposed with a reasonable distribution of the materials in the plaque. The materials suited for decomposition and the thickness of the samples are limited by the low energy range, but the method could be verified. The concentration quantification of the MultiHance samples were mainly achieved by the large difference in the attenuation value between the materials in the sample. But as the general method works for samples containing materials with visible absorption edges, it is reasonable to assume that the concentration quantification would yield accurate results for samples with materials of similar attenuation but with one visible absorption edge.