A novel approach to image reconstruction and calibration for a multi-slit-slat SPECT system

In the context of the development of a simultaneous SPECT/MRI system, we have previously proposed a multi-minislit-slat (MSS) collimator, with multiple sections of short slits in order to improve the angular sampling. The data can be reconstructed using a 3D reconstruction algorithm that models the collimator geometry. One drawback, however, is that the projection data obtained with this collimator are difficult to interpret visually. Also, calibration can be problematic, as each mini-slit only covers part of the object FoV. We have therefore developed an algorithm for transforming the MSS projection data into the traditional sinogram format. These sinograms consist of multiple thin tilted lines with gaps in between due to the lack of detector rotation in this system. The data can be reconstructed using standard parallel-beam algorithms, taking into account the fact that there are data missing. We have shown with simulations and measurements that the algorithm can transform complex data, consisting of multiple rough broken line segments, into simple sine-curves. This algorithm can be useful for interpreting the acquired MSS data, reconstructing images, and calibrating the system

Shielding requirements of a SPECT insert for installation in a PET/MRI system

The objective of this work is to evaluate the shielding requirements of a SPECT insert for installation in the Siemens Biograph mMR in order to perform simultaneous SPECT/MR imaging of the human brain. We intend to use the radionuclides 99mTc, 123I and 111In. The main photopeaks of these radionuclides have the following energies: 140.5, 159.0, 171.3 and 245.4 keV. There is also about ∼3% of emission probability of high energy gamma photons for 123I in the range of 248-784 keV. The main constraints to the design of the gamma shielding are the presence of high energy photons, the weight, the MR compatibility and the PET LSO crystals intrinsic activity. We used GATE to simulate a SPECT acquisition, defining an MRI system with LSO crystals, a partial SPECT ring and a NEMA phantom. We also defined a lead (Pb) base plate (BP) to simulate the support system and three Pb shielding volumes with variable thickness: front and end (FE), back (B), and lateral (L) shield. These volumes reduce interference from out-of-field activity, LSO intrinsic activity and edge effects, respectively. We performed 4 sets of simulations, with variable FE, variable B, variable L and variable BP thickness, respectively, with a NEMA phantom filled with 185 MBq of 123I or 111In. For all simulations, we compared the different energy spectra and count-distribution plots. Results show that a Pb shielding configuration with a thickness of 6 mm-F, 2 mm-E, 3 mm-B, and 5 mm-L is appropriate for the insert. For 123I there is still a high contribution from high energy photons, as the amount of shielding is limited by weight, however this contribution is likely to be overestimated in the simulations as compared to practice. The effect of the LSO intrinsic activity is negligible at the energies of interest

Length functions on currents and applications to dynamics and counting

The aim of this (mostly expository) article is twofold. We first explore a variety of length functions on the space of currents, and we survey recent work regarding applications of length functions to counting problems. Secondly, we use length functions to provide a proof of a folklore theorem which states that pseudo-Anosov homeomorphisms of closed hyperbolic surfaces act on the space of projective geodesic currents with uniform north-south dynamics.Comment: 35pp, 2 figures, comments welcome! Second version: minor corrections. To appear as a chapter in the forthcoming book "In the tradition of Thurston" edited by V. Alberge, K. Ohshika and A. Papadopoulo

Collimator design for a clinical brain SPECT/MRI insert

This project's goal is to design a SPECT insert for a clinical MRI system for simultaneous brain SPECT/MR imaging. We assume the stationary SPECT insert will consist of two rings of ∼5x5-cm SiPM-based detectors insensitive to magnetic fields, with 0.8-mm intrinsic resolution. The maximum diameter is 44.5 cm, the minimum diameter is 33 cm to accommodate the patient and MRI receive/transmit coil, and the FOV has a 20 cm diameter. We have compared eight collimator designs: single-, 2x2-, 3x3- and 5+2½- pinhole, and single-, 2-, 3- and 1+2½-slit slit-slat, where ½-pinholes/slits are shared between two detectors. Analytical geometric efficiency was calculated for an activity distribution corresponding to a human brain and a target resolution of 10 mm FWHM at the centre of the FOV. Noise-free data were simulated with and without depth-of-interaction (DOI) information, and reconstructed for uniform, Defrise, Derenzo, and Zubal brain phantoms. For DOI it is assumed that the crystal's first and second half can be differentiated. Comparing the multi-pinhole and multi-slit slit-slat collimators, the former gives better reconstructed uniformity and trans-axial resolution, while the latter gives better axial resolution. Although the 2x2-pinhole and 2-slit designs give the highest sensitivities, they result in a sub-optimal utilization of the detector FOV. The best options are therefore the 5+2½-pinhole and the 1+2½-slit systems, with sensitivities of 4.9*10–4 and 4.0*10–4, respectively. The brain phantom reconstructions with multi-pinhole collimator are superior as compared to slit-slat, especially in terms of symmetry and realistic activity distribution. DOI information reduces artefacts and improves uniformity in geometric phantoms, although the difference is small for the brain phantom. These results favour a multi-pinhole configuration

Effects of the Collimator Magnification Factor in the Geometrical Calibration of SPECT Systems

In compact systems, precise measurement in the projection space may be compromised due to minification. The objective of this work is to investigate the impact of the magnification factor in a model-based calibration procedure. This has direct relevance to the geometrical calibration of the clinical INSERT camera. Projection data from three point sources were simulated for a single pinhole collimator with magnification and single pinhole and slit-slat collimators with minification, for 100 noise realizations and 3 count levels. Model-based calibration was performed to estimate geometric parameters and data corresponding to a Derenzo phantom were simulated and reconstructed with true and worst-case estimates for each collimator. Experimental projection data were acquired with an INSERT prototype camera and four 99mTc line sources in different locations within the FoV. The collimator-CoR distance was varied in order to obtain different minifications and model-based calibration was performed. The results from the simulations suggest that calibration is less robust when minification is present, with higher biased calibration parameters, which result in activity underestimation. For experimental data, estimated parameters improved with a higher magnification factor, in line with the simulation results. However, some inconsistencies in the results suggest that there is still room for improvements. To conclude, geometric calibration of SPECT systems is more sensitive to minification than magnification, which will impact image quality

Moral preferences in helping dilemmas expressed by matching and forced choice

This paper asks whether moral preferences in eight medical dilemmas change as a function of how preferences are expressed, and how people choose when they are faced with two equally attractive help projects. In two large-scale studies, participants first read dilemmas where they “matched” two suggested helping projects (which varied on a single attribute) so that they became equally attractive. They did this by filling in a missing number (e.g., how many male patients must Project M save in order to be equally attractive as Project F which can save 100 female patients). Later, the same participants were asked to choose between the two equally attractive projects. We found robust evidence that people do not choose randomly, but instead tend to choose projects that help female (vs. male), children (vs. adult), innocent (vs. non-innocent), ingroup (vs. outgroup) and existing (vs. future) patients, and imply no (vs. some) risk of a harmful side-effect, even when these projects have been matched as equally attractive as, and save fewer patients than the contrasting project. We also found that some moral preferences are hidden when expressed with matching but apparent when expressed with forced choice. For example, 88–95% of the participants expressed that female and male patients are equally valuable when doing the matching task, but over 80% of them helped female patients in the choice task

Estimation of an image derived input function with MR-defined carotid arteries in FDG-PET human studies using a novel partial volume correction method

Kinetic analysis of (18)F-fluorodeoxyglucose positron emission tomography data requires an accurate knowledge the arterial input function. The gold standard method to measure the arterial input function requires collection of arterial blood samples and is an invasive method. Measuring an image derived input function is a non-invasive alternative but is challenging due to partial volume effects caused by the limited spatial resolution of the positron emission tomography scanners. In this work, a practical image derived input function extraction method is presented, which only requires segmentation of the carotid arteries from MR images. The simulation study results showed that at least 92% of the true intensity could be recovered after the partial volume correction. Results from 19 subjects showed that the mean cerebral metabolic rate of glucose calculated using arterial samples and partial volume corrected image derived input function were 26.9 and 25.4 mg/min/100 g, respectively, for the grey matter and 7.2 and 6.7 mg/min/100 g for the white matter. No significant difference in the estimated cerebral metabolic rate of glucose values was observed between arterial samples and corrected image derived input function (p > 0.12 for grey matter and white matter). Hence, the presented image derived input function extraction method can be a practical alternative to noninvasively analyze dynamic (18)F-fluorodeoxyglucose data without the need for blood sampling

Adsorption of Fibrinogen on Thin Oriented Poly(Tetrafluoroethylene) (PTFE) Fibres Studied by Scanning Force Microscopy

We have investigated fibrinogen adsorption on ordered poly(tetrafluoroethylene), PTFE, fibres deposited on hydrophilic and hydrophobic silicon substrates. Fibrinogen molecules appear to adsorb with their long axis perpendicular to the fibre direction for PTFE fibres having widths of less than 100 nm. On these thin fibres, fibrinogen apparently forms close packed bands or clusters, consisting of small integer numbers of molecules arranged parallel to each other. On broader (\u3e 100 nm) PTFE fibres, the fibrinogen forms two dimensional networks. The orientation of the molecules in these networks is random in the central flat part of the fibres but perpendicular to the fibre direction at the fibre edges. As a tentative explanation, we propose that the observed orientation may be linked to the radius of curvature of the fibre surface