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Optimizing x-ray spectrum shape for different clinical computed tomography imaging tasks
CT is a high x-ray dose examination; in 2007 4% of all radiology examinations in Europe was done by CT but they contributed to 40% of delivered ionizing dose to the patient population. Many clinical CT examinations are not performed to yield optimal dose efficiency; instead image quality per se seems to have been the sole consideration. This has resulted in many examinations performed with higher than optimal x-ray energy (old and worked-in procedures and x-ray tube power limitations have also been a contributing factor). In a recent publication it was shown that for state of the art CT machines, the image quality can be increased by 40% for certain standard imaging tasks by reducing the x-ray tube voltages from the standard settings. Often the optimal reduction in voltage was up to 50%, resulting in kVp (peak kilo-volt) of 40-60 instead of 140. While physical limitations on the x-ray tubes prevent the entire gain to be materialized, this clearly indicates the magnitude of improvement achievable from carefully optimizing the spectrum.
We are developing a new CT system based on strip silicon detector. Silicon has the benefit to open up for photon counting CT with spectral imaging capabilities (making optimal use of each single detected photon quantum), which will introduce new clinical imaging tasks (such as better enhancement of cardiac vessels filled with contrast agent). Silicon as a detector material has the physical characteristic that detection efficiency and the ability to extract energy information from photon increase with decreasing kVp; the overall awareness of the benefit of shifting the clinical x-ray tube voltages to lower values will therefore benefit silicon detector based systems more than other.
How optimal x-ray spectra for a silicon detector system for CT looks like, both for “old” imaging tasks (soft tissue discrimination) and new imaging tasks, where the energy information is taken into consideration, is an open question. This thesis work would include simulation of CT images under different x-ray spectrum conditions, based on an existing model for the detector. Deposited dose in important internal organ (weighted by the biological effect) for the different settings will also be derived and used for normalization purposes. The output will be how the image quality normalized with the delivered dose depends on certain parameter settings. This method will allow determining the optimal settings and also allow fair comparisons with other systems.
The successful applicant should be proficient in mathematical modeling and complex simulations using Matlab and can expect to work in a small group where there is real interest in the results, with a corresponding interest to help the student get up and running quickly.
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