Volume 14 (2008): Issue 1 (March 2008)

The Electron Paramagnetic Resonance (EPR) was used to investigate free radicals in the peripheral blood of patients subjected to angioplasty treatment. To detect these free radicals, a nitrosobenzene spin trap was used in this experiment. The EPR spectra of the blood with a spin trap conglomerate was measured at room temperature and at 170 K. To confirm the kind of free radicals in the conglomerate blood-spin trap, simulation and quantum-chemical calculations were made, and the conglomerate spin trap with ascorbic acid was measured. Two different types of radicals, one at room temperature and the other in a frozen sample of blood, were found.

Virtual human models, a.k.a. voxel models, are currently the state of the art in radiation protection for computing organ irradiation doses without difficult or morally unfeasible experiments. They are based on medical image data of human patients and offer a realistic, three dimensional representation of human anatomy.

We present our newest voxel model Katja, a virtual woman in the 24^th week of pregnancy. Katja integrates two previous voxel models, one obtained from the abdominal MRI scan of a pregnant patient and an already segmented model of a non-pregnant woman. The latter is the ICRP-AF, fitting the reference values for standard height, weight and organ masses given by the Internationals Committee of Radiological Protection (ICRP). The dataset was altered in order to fit the segmented foetus taken from the abdominal MRI scan. The resulting pregnant woman model, Katja, complies with the ICRP reference values for the adult female.

A pilot study of breast dose measurements on two mammography units in Bulgaria was conducted. The mean glandular doses (MGDs) to samples of approximately 60 women per unit were measured. MGD with a standard PMMA phantom was measured as well. The MGDs were calculated according to the European protocol on dosimetry in mammography as well as to the European protocol for the quality control of the physical and technical aspects of mammography screening. The measured women's MGDs were divided into three groups depending on the compressed breast thicknesses. The results for the group of thicknesses in the interval 40-60 mm were compared with the results from the measurements on the standard 45 mm PMMA phantom. Some differences were found which could be due to errors in breast thickness measurements, differences in breast and phantom densities and other factors. A standardized procedure was elaborated for patient dose measurement and calculation both from patient and phantom studies.

Many modern linear accelerators are equipped with asymmetric collimators or jaws that can be moved independently. Asymmetric jaws have got many clinical applications in radiation therapy. In the present study, the dosimetric characteristics of asymmetric collimators from our linear accelerator with 6 and 18 MV X-rays were carried out. The field size factors (FSF) and half value layer (HVL) were measured in a water phantom using 0.6 cc Farmer chamber for symmetric and asymmetric fields for both 6 and 18 MV X-rays. Measurements of beam penumbra, percentage depth dose (PDD), cross beam profiles and calculated isodose curves were measured by RFA 300 for both asymmetric and symmetric fields. The FSF were found to agree with in 3% for symmetric and asymmetric fields. The HVL in water was found to be 15.8 cm and 14.4 cm for 6 MV photons and 26 cm and 22.9 cm for 18 MV photons at the central axis and at 20 cm off the central axis. At 30 cm depth the percentage depth dose for symmetric and asymmetric fields were found to differ as high as 6% for 6 MV and 4% for 18 MV fields. No observable difference in penumbra was noticed for symmetric and asymmetric fields of same dimensions. The constrictions of isodose curves at the edge nearer to central axis were noticed for asymmetrically placed fields. The observed differences could be due to the passage of primary beam through differential thickness of the flattening filter which alters the beam quality.

The determination of three-dimensional (3D) temperature distribution within tissue during thermoablation is necessary to estimate procedure efficiency. The use of a thermo-camera combined with finite element modelling is discussed.

The temperature distributions in a metal phantom and an animal tissue sample were simulated. In the real experiment, temperatures were measured around the heating probe by a thermo-resistor set and the temperature distributions on samples' surface were acquired by a thermo-camera. The temperatures measured in the experiment were compared with the simulated ones. The differences between the measured and simulated temperatures were lower than 1.3°C and 3.0°C for a metal phantom and tissue sample, respectively.

Good agreement was achieved for homogenous material of well-defined parameters. Higher discrepancies for the tissue sample are due to in-homogeneity and to difficulties with describing tissue thermal properties.

The proposed method permits the precise prediction of a 3D temperature distribution in in-vitro studies. Potential application for in-vivo procedures requires further investigations.