Simulation Design And Optimization Of An Energy-independent SPECT Collimator

In single photon emission computed tomography,the qualities of images are largely dependent on the properties of collimators,while the specifications of collimators vary as the energy levels of radionuclides differ.In clinic,collimators have to be changed frequently due to the broad spectrum of radiotracers.Such inevitable exchange not only hinders throughput,but also subjects the camera to operational errors and damage.Hence,the objective of this work is to design a collimator,which can perform reasonably well for radiotracers with low-to medium energy levels of gamma emissions,with the hope to reduce the frequency of collimator exchanges and improve the throughput of hospitals.To design the optimum scheme of the energy-independent collimator successfully,some important techniques were proposed in this report.A large-area detector with a dimension of 20.48 cm×20.48 cm was made of the noval semi-conduct material Cadmium Zinc Telluride（CZT）.Combined with the pixel-geometry-matching model,the detection efficiency was maximized as the collimator was designed such that the center of each hole matched the center of each pixel in the CZT detector with the same number of collimator holes and detector pixels.The iterative reconstruction algorithm was improved by implementing the corrections for point spread function and attenuation.CZT material provides superior radiation detection and high stopping power to the rays by directly converting photons absorbed to charges.Compared to the conventional and continuous scintillation detectors,pixelated CZT-based detectors yield a better spatial resolution and a higher sensitivity.In order to maximize the detection possibility of the gamma rays coming through the holes of the collimators,i.e.,to make gamma photons be detected by the effective detector element instead of by the gaps between adjacent pixels,the pitch of the collimator holes was chosen equal to the pitch of the detector,and,the center of each hole was set as a one-to-one correspondence to that of each element of the detector.The collimator will be fabricated from high-density materials instead of conventional-used lead,decreasing the rate of penetration for medium energy.The quality of reconstructed images is mainly decided by reconstruction algorithms.Both the effect of the detector thickness on the point spread function and the attenuation to gamma photons were taken into consideration and corrected.In the attenuation correction,the effective energy was utilized to calculate the unique and equivalent linear attenuation coefficient for the radioactive nuclides with two photo-peaks,which reduced the complexity of the reconstruction.The energy-independent collimation presented in this work can find applications in imaging small-volume organs such as human brain,breast and prostate.According to the potential applications,four radionuclides including ⁵⁷Co,^99mTc,¹²³I and ¹¹¹In representing different energy levels,were selected to evaluate the imaging performance of the collimator under250 keV.The variables of the collimator geometries such as material,hole shape,hole length,hole radius and hole septa were studied in term of optimizing the collimator.Based on the idea of Monte Carlo,Geant4 toolkit was used to simulate the collimators with different combinations of parameters.Figures of merit such as spatial resolution,detection efficiency,and scattering and penetration ratios were calculated.Meanwhile,the total relative sensitivity（TRS）was defined to assist in deciding an optimum collimator.The optimum is the one with the maximum TRS while maintaining other performance parameters（including spatial resolution,penetration and so on）in permissible ranges.Based on lots of simulations and calculations of the indexes,we tentatively decide the design characteristics of the optimum collimator:the material of the collimator is tungsten alloy,the hole shape is rectangular,the thickness of the hole is 23 mm,the diameter is 1.28 mm and hole septa is0.32 mm.To validate the imaging performance of the specific collimator for low-to medium-energy,digital hot-rod phantom and Hoffman brain phantom were implemented to be flooded with different kinds and activities of the radioactive nuclides.The projection data were simulated and iteratively reconstructed.The results were assessed by calculating contrast and recovery ratio,confirming the possibility and reasonability of such a collimator.Finally,we found that the proposed energy-independent collimator has comparable or even better imaging performance versus commercial or studied collimators such as low energy high resolution and medium energy general purpose collimators.