Volume 30, Issue 12, December 2003
Index of content:
- PH. D. THESES ABSTRACTS
A comparison of TLD dosimeters: LiF:Mg,Ti and LiF:Mg,Cu,P for measurement of radiation therapy doses30(2003); http://dx.doi.org/10.1118/1.1624754View Description Hide Description
TLDs (thermoLuminescent dosimeters) are used in radiation therapy to verify the radiationdosecancer patients receive. University of Virginia RadiationOncology clinic TLD results must be fully trustworthy so physicians can confidently monitor and determine a patient’s treatment. A systematic investigation of major factors affecting TLD response and accuracy was carried out. The factors investigated were dose response, energy response, and fading response for two types of TLDs. The supralinear dose response of LiF:Mg,Ti and slightly sublinear dose response of LiF:Mg,Cu,P is documented. In energy response, LiF:Mg,Ti over-responds 46% at 33 keV while LiF:Mg,Cu,P under-responds by 10%–20% in the kilovoltage region. Fading of LiF:Mg,Ti was measured at 9% in 63 days while the output of LiF:Mg,Cu,P enhanced by 8% 23 days after irradiation. Overall accuracy at the 95% CL is 2.18% for LiF:Mg,Ti and 4.09% for LiF:Mg,Cu,P. Confidence in UVa RadiationOncologyTLD measurements was enhanced with the use of improved procedures. The improvements include reading LiF:Mg,Cu,P with a neutral density filter in the TLD reader, labeling of TLDs with identifying numbers, and use of a circular annealing tray for cooling. LiF:Mg,Cu,P releases about 34 times more light than LiF:Mg,Ti, and a neutral density filter prevents saturation of the TLD reader. Labeled TLDs are easier to work with, resolve mix-ups, and increase accuracy by allowing TLDs to be read always in the same orientation (although labeling slightly reduces the light output). TLDs all cool at the same rate on the circular tray, which maintains consistent TLD sensitivity between TLDs and multiple uses. The improved University of Virginia TLD procedure is documented.
30(2003); http://dx.doi.org/10.1118/1.1625441View Description Hide Description
This thesis is a theoretical evaluation of the (single) first photondetection(FPD) technique as a limiting case of time-resolvedtransilluminationimaging (TI) for diagnostic purposes. It combines analytic and Monte Carlo(MC) simulation methods to derive the single photon statistics and to solve the radiative transfer equation (RTE) for a given source-medium-detector geometry. In order to efficiently simulate very early arriving photons, an Indeterministic Monte Carlo (IMC) technique based on path integrals is devised and validated. The IMC extends controlled MC techniques to accelerate and enhance the probability of detecting shorter trajectories thereby improving the statistics. The IMC technique provides a tool for the construction of a temporal point spread function (TPSF) of the emerging photons for the entire time scale. It is then used to predict the spatial resolution of these systems for shorter (sub-100 picosecond) time scales. The calculation of the TPSF at short time scales for a pulse made incident onto the medium enables the mathematical derivation of the temporal probability density functions (p.d.f.) for the first arriving photon, This facilitates the investigation of a firstphotondetection(FPD)system as applied to a diagnostic TI configuration. A FPDsystem produces a signal representing from which the mean transit time of the first arriving photon, may then be estimated for a sequence of incident pulses at each scan position. By rectilinear scanning across the medium, a two-dimensional (2-D) map of can be created and displayed as a gray scale image. The application of FPD to TI is evaluated assuming an ideal detector capable of detecting the first arriving photon with 100% efficiency (infinite extinction coefficient). However, a model for a FPDsystem corresponding to a nonideal (single first photon)detector is also considered through the evaluation of the p.d.f. for the later (first, second,…) arriving photons. This enables a detection time limit to be specified to eliminate the later arriving photons and thereby overcome distortions in the first photon p.d.f., which may be caused by any inefficiency in the response of the detector. The FPD technique is then applied to obtain for various laser pulse intensities. The FPDsystem is also examined for the case of spherical inhomogeneities (representing tumors) embedded in the center of an otherwise homogeneous medium. The effect of the variations in the embedded inhomogeneity (size and optical density) and optical properties of the medium are also studied. A heterogeneous medium that resembles tissue more realistically is considered. For a FPDsystem where the received signal does not change (one photon) per incident pulse, the signal contrast is redefined and is examined as a function of the incident laser power and medium absorption and scattering properties. The signal–noise-ratio is also evaluated for the FPDsystem as the error in the estimation of Based on the analysis of the SNR, the number of incident pulses (per scan position), needed to achieve a required SNR, is also derived. It is shown that the p.d.f. of the first arriving photon for a totally absorbing inhomogeneity located at the midplane of a 50 mm thick tissue-like medium may be distinguished (95% confidence level) from the p.d.f. of a medium without the inhomogeneity. This theoretical study provides an introduction aimed at assisting further experimental research into the limits of transilluminationimaging employing a first photondetection(FPD)system.
Analysis on imaging features of mammography in computer radiography and investigation on gray scale transform and energy subtraction30(2003); http://dx.doi.org/10.1118/1.1628411View Description Hide Description
In this dissertation, a novel transform method based on human visual response features for gray scale mammographicimaging in computerradiography(CR) is presented. The parameters for imaging quality on CRimaging for mammography were investigated experimentally. In addition, methods for image energy subtraction and a novel method of image registration for mammography of CRimaging are presented. Because the images are viewed and investigated by humans, the method of displaying differences in gray scale images is more convenient if the gray scale differences are displayed in a manner commensurate with human visual response principles. Through transformation of image gray scale with this method, the contrast of the image will be enhanced and the capability for humans to extract the useful information from the image will be increased. Tumors and microcalcifications are displayed in a form for humans to view more simply after transforming the image. The method is theoretically and experimentally investigated. Through measurement of the parameters of a geometrically blurred image,MTF, DQE, and ROC on CRimaging, and also comparison with the imaging quality of screen–film systems, the results indicate that CRimaging qualities in DQE and ROC are better than those of screen–film systems. In geometric blur of the image and MTF, the differences in image quality between CR and the screen–film system are very small. The results suggest that the CR system can replace the screen–film system for mammographyimaging. In addition, the results show that the optimal imaging energy for CRmammography is about 24 kV. This condition indicates that the imaging energy of the CR system is lower than that of the screen–film system and, therefore, the x-ray dose to the patient for mammography with the CR system is lower than that with the screen–film system. Based on the difference of penetrability of x ray with different wavelength, and the fact that the part of the x-ray beam will pass through the image plate in the procedure of CRimaging, the method of subtraction of the two images which were taken in the same time with one exposure can increase the diagnostic information. Image registration for mammography with CRimaging is usually ignored because the two images are taken in one exposure time. This dissertation investigated the necessity of image registration for image energy subtraction in CRmammographyimaging. A novel method for image registration that can reduce the computing time is established, based on the features of CRimaging for mammography.