Volume 31, Issue 8, August 2004
Index of content:
- PH. D. THESES ABSTRACTS
Development and application of a tomographic model from CT images for calculating internal dose to a pregnant woman31(2004); http://dx.doi.org/10.1118/1.1772571View Description Hide Description
Assessment of radiation dose and possible risk to a pregnant woman and her fetus is an important task in radiation protection. Although stylized models for male and female patients of different ages have been developed, tomographic models for pregnant women have not been developed to date. This dissertation presents an effort to construct a partial-body model of a pregnant woman from a set of CTimages. The patient was 30-weeks pregnant, and the CT scan covered the portion of the body from above liver to below pubic symphysis in 70 slices, each 7 mm thick. The image resolution was in a field. The images were carefully segmented to identify 34 organs and tissues. It has been found that the masses are different from the Reference Woman. The characteristics of the resulting model are discussed and compared with one existing stylized mathematical model for pregnant women. Based on this tomographic model, a Monte Carlo code, EGS4-VLSI, was used to derive specific absorbed fractions. Monoenergetic and isotropic photon and electron emitters distributed in different source organs were assumed and the energies ranged from 10 keV to 4 MeV for photons and from 100 keV to 4 MeV for electrons. The results for high energy photons showed general agreement with previous studies, however, the results for lower energy photons showed differences of up to several hundred percent for some source and target organs. For electron results, several tens of percent differences were found. Those differences can be explained by mass differences and the relative geometry differences between source and target organs. In summary, the stylized models for pregnant women are satisfactory for a very large size patient for most of the photon energies (between 50 keV and 4 MeV). However, a tomographic model has to be used to obtain acceptable dose assessments for electrons. The newly calculated SAF data set can provide the nuclear medicinedosimetry field a new perspective involving internal electrons.
31(2004); http://dx.doi.org/10.1118/1.1774277View Description Hide Description
Positron Emission Mammography(PEM) is a relatively new nuclear medicine imaging technique for the detection and staging of breast cancer.PEM tries to overcome the limitations of existing whole body clinical PETscanners by means of a breast-dedicated design. We have designed a novel PEMscanner prototype, called YAP-PEM, based on a pair of opposing detector heads made up of a YAP:Ce scintillation matrix and position sensitive photo-detectors. This thesis work is dedicated to the detector development for the construction of the PEMscanner. The utilization of three different types of PSPMT have been evaluated: the R8520-00-C12, the well known R2486 and a very early version of the new H8500 “Flat Panel” PSPMT, all from Hamamatsu. These tubes have been fully characterized in terms of their imaging performance in the identification of small scintillation pixels. The H8500 confirmed the good expected performance and represents a noticeable advance in position sensitive photo-detection, in particular for its shape and size, that allows it to be tiled over large areas, as demanded by current and future application. The adopted final solution is based on an array of position sensitive PMTs, model R8520-00-C12, because of the size of the used YAP:Ce matrix that better fits with this solution. A complete signal processing and acquisition platform has been also developed for the utilization of an array of nine of these PS-PMTs. The first detector head composed of the YAP:Ce matrix and nine R8520 has been assembled, optimized and characterized. In the thesis the calibration procedures and evaluation results for the chosen detector solution for the PEM system are described. The work is completed by a full Monte Carlo simulation. Various parameters have been considered here; in particular the acquisition geometry and the tumor size. The Monte Carlo simulation on YAP-PEM indicates that our system could be able to detect very small tumors, down to 5 mm in diameter for a tumor-to-background uptake ratio of 10:1. An experimental study on a dual head system, based on the small animal YAP-(S)PET scanner, has been simulated, so as to validate the Monte Carlo code. The results obtained support that the YAP-PEM scanner under development could be a valuable tool for breast cancer investigation, showing excellent performance, similar or better than that of other existing (or in development) Positron Emission Mammography systems.