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1.  Fast Simulation of X-ray Projections of Spline-based Surfaces using an Append Buffer 
Physics in medicine and biology  2012;57(19):6193-6210.
Many scientists in the field of x-ray imaging rely on the simulation of x-ray images. As the phantom models become more and more realistic, their projection requires high computational effort. Since x-ray images are based on transmission, many standard graphics acceleration algorithms cannot be applied to this task. However, if adapted properly, simulation speed can be increased dramatically using state-of-the-art graphics hardware.
A custom graphics pipeline that simulates transmission projections for tomographic reconstruction was implemented based on moving spline surface models. All steps from tessellation of the splines, projection onto the detector, and drawing are implemented in OpenCL. We introduced a special append buffer for increased performance in order to store the intersections with the scene for every ray. Intersections are then sorted and resolved to materials. Lastly, an absorption model is evaluated to yield an absorption value for each projection pixel.
Projection of a moving spline structure is fast and accurate. Projections of size 640×480 can be generated within 254 ms. Reconstructions using the projections show errors below 1 HU with a sharp reconstruction kernel. Traditional GPU-based acceleration schemes are not suitable for our reconstruction task. Even in the absence of noise, they result in errors up to 9 HU on average, although projection images appear to be correct under visual examination.
Projections generated with our new method are suitable for the validation of novel CT reconstruction algorithms. For complex simulations, such as the evaluation of motion-compensated reconstruction algorithms, this kind of x-ray simulation will reduce the computation time dramatically. Source code is available at
PMCID: PMC3480228  PMID: 22975431
x-ray imaging; simulation; hardware acceleration; GPU; digitally reconstructed radiographs
2.  Influence of chronic hypoxia and radiation quality on cell survival 
Journal of Radiation Research  2013;54(Suppl 1):i13-i22.
To investigate the influence of chronic hypoxia and anoxia on cell survival after low- and high-LET radiation, CHO-K1 cells were kept for 24 h under chronic hypoxia (94.5% N2; 5% CO2; 0.5% O2) or chronic anoxia (95% N2; 5% CO2). Irradiation was performed using 250 kVp X-rays or carbon ions with a dose average LET of 100 keV/μm either directly under the chronic oxygenation states, or at different time points after reoxygenation. Moreover, the cell cycle distribution for cells irradiated under different chronic oxic states was measured over 24 h during reoxygenation. The measurements showed a fairly uniform cell cycle distribution under chronic hypoxia, similar to normoxic conditions. Chronic anoxia induced a block in G1 and a strong reduction of S-phase cells. A distribution similar to normoxic conditions was reached after 12 h of reoxygenation. CHO cells had a similar survival under both acute and chronic hypoxia. In contrast, survival after irradiation under chronic anoxia was slightly reduced compared to that under acute anoxia. We conclude that, in hamster cells, chronic anoxia is less effective than acute anoxia in inducing radioresistance for both X-rays and carbon ions, whereas in hypoxia, acute and chronic exposures have a similar impact on cell killing.
PMCID: PMC3700502  PMID: 23824117
radiosensitivity; hypoxia; anoxia; carbon ions; cell cycle distribution
3.  Influence of acute hypoxia and radiation quality on cell survival 
Journal of Radiation Research  2013;54(Suppl 1):i23-i30.
To measure the effect of acute oxygen depletion on cell survival for different types of radiation, experiments have been performed using Chinese hamster ovary (CHO) cells and RAT-1 rat prostate cancer cells. A special chamber has been developed to perform irradiations under different levels of oxygenation. The oxygen concentrations used were normoxia (air), hypoxia (94.5% N2, 5% CO2, 0.5% O2) and anoxia (95% N2, 5% CO2). Cells were exposed to X-rays and to C-, N- or O-ions with linear energy transfer (LET) values ranging from 100–160 keV/µm. The oxygen enhancement ratio (OER) and relative biological effectiveness (RBE) values have been calculated from the measured clonogenic survival curves. For both cell lines, the X-ray OER depended on the survival level. For particle irradiation, OER was not dependent on the survival level but decreased with increasing LET. The RBE of CHO cells under oxic conditions reached a plateau for LET values above 100 keV/µm, while it was still increasing under anoxia. In conclusion, the results demonstrated that our chamber could be used to measure radiosensitivity under intermediate hypoxia. Measurements suggest that ions heavier than carbon could be of additional advantage in the irradiation, especially of radioresistant hypoxic tumor regions.
PMCID: PMC3700520  PMID: 23824123
hypoxia; LET; OER; radiosensitivity
4.  Respiratory Motion Compensation Using Diaphragm Tracking for Cone-Beam C-Arm CT: A Simulation and a Phantom Study 
Long acquisition times lead to image artifacts in thoracic C-arm CT. Motion blur caused by respiratory motion leads to decreased image quality in many clinical applications. We introduce an image-based method to estimate and compensate respiratory motion in C-arm CT based on diaphragm motion. In order to estimate respiratory motion, we track the contour of the diaphragm in the projection image sequence. Using a motion corrected triangulation approach on the diaphragm vertex, we are able to estimate a motion signal. The estimated motion signal is used to compensate for respiratory motion in the target region, for example, heart or lungs. First, we evaluated our approach in a simulation study using XCAT. As ground truth data was available, a quantitative evaluation was performed. We observed an improvement of about 14% using the structural similarity index. In a real phantom study, using the artiCHEST phantom, we investigated the visibility of bronchial tubes in a porcine lung. Compared to an uncompensated scan, the visibility of bronchial structures is improved drastically. Preliminary results indicate that this kind of motion compensation can deliver a first step in reconstruction image quality improvement. Compared to ground truth data, image quality is still considerably reduced.
PMCID: PMC3690260  PMID: 23840198
5.  Prospective, Paired Crossover Comparison of the in vitro Quality of Red Blood Cells Collected by the Automate for Blood Collection Device or by a Conventional Method 
The prevention of the citrate shock should improve the quality of red blood cells (RBCs). We compared a conventional whole blood donation method (CONV) with a ‘Automate for Blood Collection’ (ABC), enabling a metered addition of anticoagulant and hence a correct and constant RBC-to-anticoagulant ratio throughout donation. We evaluated the performance of the ABC device and the storage quality of RBC units.
Material and Methods
The study was designed as prospective, paired crossover study with two groups of 20 donors donating first with the ABC or CONV and switching to the alternative method after 12 weeks. We measured the processing data of donations and the storage quality of RBCs on days 1, 28, and 42.
ABC whole blood donations showed a slightly higher volume before and after filtration. ABC-derived RBC units revealed higher values for haematocrit, mean cellular volume, potassium and lower values for mean corpuscular haemoglobin concentration and sodium until day 42. They further showed faster glucose consumption and lactate production until day 28.
The ABC device is suitable for whole blood collection. The quality of the obtained RBCs is comparable to that of CONV. Avoiding the citrate shock by the described method did not improve the investigated RBC storage quality parameters.
PMCID: PMC2941835  PMID: 20877668
Whole blood donation; Citrate shock; Red blood cell storage quality

Results 1-5 (5)