Improvements of SPECT by a new collimator design and simultaneous transmission-emission tomography

Sammanfattning: SPECT (Single Photon Emission Computed Tomography) and planar scintigraphy are the most established and world wide-spread nuclear medicine imaging techniques for clinical use today. SPECT is a tomographic technique that allows 3-D visualization of biochemical processes or physiological flow in the human body by external detection of photons from an administered radiopharmaceutical. However, its ability to depict a "true" activity map depends largely on the imaging properties of the SPECT-system as well as on the methods used for converting the acquired data into values reflecting the activity distribution. Photon attenuation, as well as the contribution of "false" (scattered) events are two of the most disturbing factors for achieving high quantitative accuracy in SPECT. Since these factors are related to the density and composition of the body tissues, it is crucial to have access to individual attenuation maps when high quantitative accuracy is desired.Two methods for simultaneous acquisition of emission and transmission data have been developed and analyzed. These methods may either be applied on single-head cameras or on opposed dual-head camera systems. Advantages and disadvantages of various radio-nuclides for use in transmission tomography have been analyzed. Their activity distribution or their scanning speed was of great importance for minimizing the image noise. By careful collimation of the external photon sources for the transmission measurements and by introducing accurate routines of corrections for camera non-uniformity and interferences of photons during simultaneous emission and transmission tomography, accurate attenuation maps can be obtained.In the second part of this work, a new collimator was developed in order to reduce the gap between a conventional camera and the convex shape of most body regions. These new collimators are designed planar in one direction and concave in the other which improves the radial spatial resolution and reduces the non-isotropic blur in SPECT. An improved lateral spatial resolution in planar scintigraphy was achieved as well. The impact on imaging quality was investigated by Monte Carlo simulations. The non-isotropic image blurring was reduced by up to 60% for a PC-collimator as compared to that for a conventional collimator. The image noise distribution in SPECT was more uniform but higher than for a planar collimator due to the reduced lateral sensitivity. Simulations of a Hoffman brain phantom showed that the rCBF values achieved with an optimized PC-collimator, were up to 10% higher in the lateral cortex (Brodmann areas 18,19), than those obtained with a planar collimator. Finally, a combination of a planar-concave collimator and a transmission tomography system described may give the same advantages of low noise and reduced requirements on high dynamic range of the camera, as was obtained for photon sources with non-linear activity distributions.