Investigation of an amorphous silicon flat-panel detector for ion radiography

Using heavy ions in radiotherapy offers a good potential for targeted radiation of tumors and the ability to spare healthy tissue. Their characteristic interaction with matter holds the potential to employ ions for high-contrast radiographic imaging at a decreased dose in comparison to conventional X-ray imaging; however, it lacks simple detectors suitable for this purpose. In this study the performance of an amorphous silicon flat-panel detector, originally designed for photon imaging, was investigated for radiographic and tomographic imaging with ions. After the application of a newly-developed image correction routine, the flat-panel detector is capable of ion-radiographic imaging with high soft-tissue contrast. Further, the flat-panel detector was exploited to measure the water equivalent thickness (WET) of phantoms. To do so, the ambiguous correlation of detector signal to particle energy was overcome by active carbon ion beam energy variation and measurement of the signal-to-energy correlation. Using this method has enabled the study to determine the WET of the imaged object with an accuracy better than 1.5 mm WET. It is an improvement by a factor of about 2 with respect to the standard clinical method. Finally, the study has shown the feasibility of this imaging technique for high-resolution carbon ion computed tomography and determination of water equivalent path length (WEPL) accordingly. The developed imaging techniques present a method to measure the two-dimensional maps of WET and WEPL of complex phantoms with a simple and commercially available detector. The required doses in this configuration are too high (e.g. WET: 0.05 Gray, WEPL: 8 Gray) to be used for patient imaging, but this method presents a powerful tool to evaluate the performance of the treatment planning algorithm by studying range uncertainties

Dose ratio proton radiography using the proximal side of the Bragg peak

Purpose: In recent years there has been a movement towards single-detector proton radiography, due to its potential ease of implementation within the clinical environment. One such single-detector technique is the dose ratio method, in which the dose maps from two pristine Bragg peaks are recorded beyond the patient. To date, this has only been investigated on the distal side of the lower energy Bragg peak, due to the sharp fall-off. We investigate the limits and applicability of the dose ratio method on the proximal side of the lower energy Bragg peak, which has the potential to allow a much wider range of water-equivalent thicknesses (WET) to be imaged. Comparisons are made with the use of the distal side of the Bragg peak. Methods: Using the analytical approximation for the Bragg peak we generated theoretical dose ratio curves for a range of energy pairs, and then determined how an uncertainty in the dose ratio would translate to a spread in the WET estimate. By defining this spread as the accuracy one could achieve in the WET estimate, we were able to generate look-up graphs of the range on the proximal side of the Bragg peak that one could reliably use. These were dependent on the energy pair, noise level in the dose ratio image and the required accuracy in the WET. Using these look-up graphs we investigated the applicability of the technique for a range of patient treatment sites. We validated the theoretical approach with experimental measurements using a complementary metal oxide semiconductor active pixel sensor (CMOS APS), by imaging a small sapphire sphere in a high energy proton beam. Results: Provided the noise level in the dose ratio image was 1% or less, a larger spread of WETs could be imaged using the proximal side of the Bragg peak (max 5.31 cm) compared to the distal side (max 2.42 cm). In simulation it was found that, for a pediatric brain, it is possible to use the technique to image a region with a square field equivalent size of 7.6 cm2, for a required accuracy in the WET of 3 mm and a 1% noise level in the dose ratio image. The technique showed limited applicability for other patient sites. The CMOS APS demonstrated a good accuracy, with a root-mean-square-error of 1.6 mm WET. The noise in the measured images was found to be σ =1.2% (standard deviation) and theoretical predictions with a 1.96σ noise level showed good agreement with the measured errors. Conclusions: After validating the theoretical approach with measurements, we have shown that the use of the proximal side of the Bragg peak when performing dose ratio imaging is feasible, and allows for a wider dynamic range than when using the distal side. The dynamic range available increases as the demand on the accuracy of the WET decreases. The technique can only be applied to clinical sites with small maximum WETs such as for pediatric brains

Charakterisierung der Dynamik in Gelen und Lösungen von hydrophob modifizierten Polyacrylamiden

In der vorliegenden Arbeit wurde der Zusammenhang zwischen mikroskopischen und makroskopischen Eigenschaften hydrophob modifizierter Polyacrylamidketten betrachtet. Bei den hydrophob modifizierten Polyacrylamidketten handelt es sich um literaturbekannte Blockcopolymere, die durch mizellare Copolymerisation aus Acrylamid und Stearylmethacrylat hergestellt wurden. Für diese Arbeit wurde zusätzlich der polymerisierbare Fluoreszenzfarbstoff Acryloylfluorescein eingebracht, so dass ein Terpolymer mit hydrophoben Blöcken entstand. Für die mizellare Copolymerisation sind die Anwesenheit von Salz (Natriumchlorid, NaCl) und Tensid (Natriumdodecylsulfat, SDS) ausschlaggebend. Die Mizellen bringen vor der Polymerisation die hydrophoben Monomere und anschließend die hydrophoben Blöcke entlang der Polymerkette in Lösung. Die Eigenschaften der Lösungen wurden bei variierendem Salz-, Tensid- und Polymergehalt untersucht. Die mikroskopischen Eigenschaften der Polymerketten wurden an Hand ihres Diffusionskoeffizienten betrachtet. Dieser wurde durch die FRAP-Methode (Fluorescence Recovery After Photobleaching) bestimmt. Für die Betrachtung der makroskopischen Eigenschaften der Gele und Lösungen wurde die Rheologie herangezogen. Die Polymerkonzentration wurde im Bereich zwischen 0.25 und 5 Gew % Polymer in Lösung variiert. Die Ergebnisse zeigen, dass die hydrophoben Wechselwirkungen zu einem Diffusionsverhalten führen, dass stark von den gängigen Theorien für Polymere in Lösung abweicht. Die Variation des Tensidgehaltes von 0.35 bis 7 Gew % zeigte eine Beschleunigung der Diffusion bei Verringerung der Tensidkonzentration. Diese Beschleunigung folgt, wenn weniger Mizellen zur Verfügung stehen, die als Verknüpfungspunkte dienen können. Die Verringerung der Salzkonzentration von 0.8 auf bis zu 0.04 M beschleunigt die Diffusion der Polymerketten ebenfalls. Der zu beobachtende Effekt ist für die Variation des Salzgehaltes wesentlich stärker als für die Variation des Tensidgehaltes. In diesem Fall ist die Beschleunigung auf die Verringerung der Aggregationszahl und die damit einhergehende Verkleinerung der Mizellen zurückzuführen. Kleine Mizellen können nur einzelne hydrophobe Blöcke lösen, während große Mizellen hydrophobe Blöcke mehrerer Ketten gleichzeitig lösen und diese temporär miteinander verknüpfen. Diese Ergebnisse konnten sowohl für die mikroskopischen als auch für die makroskopischen Eigenschaften beobachtet werden. Die Auftragung der Ergebnisse beider Messmethoden gegeneinander zeigte eine klare Proportionalität. Dies bestätigt, dass das makroskopisch bereits literaturbekannte Verhalten direkt vom mikroskopischen Verhalten der Polymerketten abhängig ist.In this work the correlation between macroscopic and microscopic properties of hydrophobically modified polyacrylamide chains was considered. Hydrophobically modified polyacrylamides are block copolymers that are known from literature. They are being synthesized by micellar copolymerization of acrylamide and stearylmethacrylate. In this work a polymerizable fluorescent dye, acryloylfluorescein, was additionally incorporated into the chain to achieve a terpolymer with a blocky structure. In order to perform a micellar copolymerization the presence of salt (Sodiumchloride, NaCl) and surfactant (Sodiumdodecylsulfate, SDS) is crucial. The micelles dissolve the hydrophobic monomers before polymerization and keep the hydrophobic blocks that evolve during the polymerization in solution. The properties of the solutions were characterized for a varying concentrations of salt, surfactant and polymer. As microscopic properties of the polymer chains the diffusion coefficient was observed. This was obtained by the FRAP-method (Fluorescence Recovery After Photobleaching). In order to survey the macroscopic properties of the gels and solutions rheology measurements were performed. The polymer concentration was varied in the range between 0.25 and 5 wt % polymer in solution. The results show that the hydrophobic interactions lead to a diffusive behavior that differs strongly from the well-established theories of polymers in solution. The variation of the surfactant content from 0.35 to 7 wt % show an increase of the rate if diffusion of diffusion with decreasing surfactant concentration. This acceleration takes place, if fewer micelles are present, that can act as crosslinks. The decrease in salt concentration from 0.8 to 0.04 M also increases the diffusion of the polymer chains. This effect is much stronger for the variation of the salt concentration than for the variation of the surfactant content. In this case the acceleration is attributed to the decrease of the aggregation number and the reduction in size that comes along with it. Smaller Micelles only dissolve single hydrophobic blocks, bigger micelles are able to dissolve several hydrophobic blocks at the same time and thereby crosslinking them temporarily. These results could be seen for both, the microscopic an macroscopic properties. The comparison of the results of both methods clearly shows a proportionality between them. This confirms, that the macroscopic behavior which is known from literature depends directly on the microscopic behavior of the polymer chains