With multi-pinhole collimation systems, high resolutions can be reached with both SPECT and PET. Using tracers with higher-energy gamma-photons increases the effects of pinhole edge penetration, decreasing the resolution of the system. This research aims to design a collimator using 3x3 twisted pinhole clusters to be used in the VECTor system. This new cluster design should allow high-energy tracers such as 89-Zr to be imaged. The sensitivity of this new design was compared at 511 keV and 909 keV with the sensitivity of the VECTor collimator by simulating the sensitivity using GATE. A collimator design is characterised by its degree of multiplexing and detector coverage. These values are aimed to be the same as for the VECTor collimator to ensure a fair comparison at the simulation stage. Several design approaches have been tested. The final design had the clusters placed in 5 rows on the collimator. The three inner rows had 21 clusters, the two outer rows had 15 clusters. The inner radius of the collimator was increased to prevent pinholes from intersecting. The pinhole diameters of the final design were modified such that the resolution of the system was the same as the VECTor resolution for both 511 keV and 909 keV photons. To prevent the pinholes from intersecting, either the inner radius of the collimator had to be further increased, or the pinholes had to be smaller. It was chosen to test both options, resulting in a total of three collimator designs to be simulated. A scan lasting 1 hour was simulated with a source the size of the VECTor CFOV. This was done with 511~keV photons using 2 MBq/mL 18-F, and with 909 keV photons using 2 MBq/mL 89-Zr with their respective designs. These simulations resulted in a 1.77 times higher sensitivity to direct photons for the 511 keV collimator, and either a 2.04 or a 2.69 times higher sensitivity to direct photons for the 909 keV photons. Additionally, the total sensitivity of the 511 keV collimator did not significantly change, and the total sensitivity of the 909 keV collimator designs increased with either 8% or 40%. It is concluded that the implementation of 3x3 twisted pinhole clusters in a collimator to be used in the VECTor system has significant benefits over the current collimator when used with high-energy gamma-photons.

Magnetic resonance fingerprinting is an MRI-based technique that allows for fast, simultaneous quantitative mapping of multiple tissue parameters. Multi-component MRF (MC-MRF) additionally allows for the mapping of multiple tissue components per voxel. Currently, most MC-MRF implementations ignore any nonlinear effects on signals resulting from multi-component systems, such as the effects introduced by magnetisation transfer (MT). Here, we investigate the effects of free pool to free pool magnetisation transfer on the accuracy of MC-MRF, with a focus on the application of this technique for myelin water fraction (MWF) imaging. Assessment of the different MC-MRF techniques is done through application of these algorithms to two-component numerical phantoms, where the signals from two interacting components were simulated using the EPG-X framework. Results show that MT has a negative effect on the accuracy of the acquired parameter estimates for all estimated parameters, resulting in biases and incorrect parameter estimates. Several adjusted methods for MC-MRF including magnetisation transfer were proposed and tested. Although theoretically improvements were expected, the used sequences showed to be inadequate for accurate MT estimates. More research into these techniques is still required to improve their performance and accuracy. A technique left mainly unexplored here is sequence optimisation to minimise the effects of magnetisation transfer on the resulting signals. A quick exploration, however, showed that this might be a viable approach for future research as well.