Emission computed tomography involves external measurements of gamma photons emitted from within the object under investigation in order to map the radioactive distribution into a two-dimensional array within a slice of interest. Both positron emission tomography (PET) and single photon emission computed tomography (SPECT) constitute the two types of emission computed tomography. PET and SPECT differ radically in almost every aspect of system design; radionuc1ide employed, radiation detectors and arrangement, collimation (electronic, mechanical), processing electronics as well as data acquisition, handling and correction. A prototype scanning-rig incorporating two collimated BOO scintillation detectors has been used to carry out PET experiments utilising 6SOe line sources (positron-emitter) and a perspex phantom of 50-mm in diameter to simulate a small animal i.e. a rat's head. Modifications for the experimental scanning-rig allowed the collection of the singles events in the PET studies in such a way that they could be reconstructed to provide SPECT images for the radioactive distribution under investigation. This property allowed a simultaneous collection of PET and SPECT data for the same object under exactly the same conditions. Two data sets are generated from each tomographic experiment; one is for PET and the other is for SPECT. Each data set is corrected separately for the required corrections i.e. scattering and attenuation before reconstruction, and then two images are produced for each study. The outcome from this work is the comparison between the two images of PET and positron SPECT obtained. The line spread function curves taken for various depths and the image profiles for studies in air and perspex show that PET provides better spatial resolution than positron SPECT. This property of PET is further confirmed by the MTF curves and the fidelity test. Using a collimation aperture of 3- mm wide, the spatial resolution values in air were found to be 3.2 +/- 0.45 mm and 7.4 +/- 0.45 mm FWHM for PET and SPECT respectively. The images of the two line sources with a 10-mm centre-to-centre separation are partially resolved in the SPECT images whereas a sufficient separation between the two sources is achieved in PET. Image combination has been applied in order to obtain a hybrid image which contains the advantages from both PET and SPECT. A straightforward averaging and multiplication of the two images of PET and SPECT were found useful to provide images with enhanced quality. The multiplication process provided images with significantly improved quality for the PE T images. When evaluating the image quality of the line source in air, the fidelity test values are 0.71 and -1.11 for PET and SPECT respectively. The image combination resulted in an image with fidelity values of 0.92 when the two images are multiplied and 0.12 when their averaging was obtained.

A non-covariant but approximately relativistic two-body wave equation describing the quantum mechanics of two fermions interacting with one another through a potential containing scalar, pseudoscalar and vector parts is presented. It is based on a generalisation of the equation introduced by Breit in 1929. After expressing the sixteen component twobody wavefunction in terms of a radial and an angular function by means of the multi pole expansion, the initial equation can be reduced into a set of sixteen radial equations which, in turn, can be classified in accordance to the parity and the state of the wavefunctions involved. The adequacy of the reduced equations in describing real problems is discussed, first, by applying the theory to a QED problem, the calculation of the lowest bound states, 1So and 351, of positronium to order 0'4. Second, the knowledge of the bottomium and charrnoniurn spectra serves as a laboratory to test both the efficiency of the potential which is supposed to represent the interaction between two quarks leading to the formation of mesons, and the reliability of the Breit equation. The final results are presented in such a form as to allow a direct comparison with both experimental data and existing theories. Results are, also, obtained for a stronger Coulomb-like vector potential as well as for a scalar square well potential. The former case is applied to a bound state of a monopoleantimonopole system.

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