A Simple Contrast Matching Rule for OSEM Reconstructed PET Images with Different Time of Flight Resolution

Background: Time-of-Flight (TOF) is a leading technological development of Positron Emission Tomography (PET) scanners. It reduces noise at the Maximum-Likelihood solution, depending on the coincidence–timing–resolution (CTR). However, in clinical applications, it is still not clear how to best exploit TOF information, as early stopped reconstructions are generally used. Methods: A contrast-recovery (CR) matching rule for systems with different CTRs and non-TOF systems is theoretically derived and validated using (1) digital simulations of objects with different contrasts and background diameters, (2) realistic phantoms of different sizes acquired on two scanners with different CTRs. Results: With TOF, the CR matching rule prescribes modifying the iterations number by the CTRs ratio. Without TOF, the number of iterations depends on the background dimension. CR matching was confirmed by simulated and experimental data. With TOF, image noise followed the square root of the CTR when the rule was applied on simulated data, while a significant reduction was obtained on phantom data. Without TOF, preserving the CR on larger objects significantly increased the noise. Conclusions: TOF makes PET reconstructions less dependent on background dimensions, thus, improving the quantification robustness. Better CTRs allows performing fewer updates, thus, maintaining accuracy while minimizing noise.

Optical Simulation and Experimental Assessment with Time–Walk Correction of TOF–PET Detectors with Multi-Ended Readouts

As a commonly used solution, the multi-ended readout can measure the depth-of-interaction (DOI) for positron emission tomography (PET) detectors. In the present study, the effects of the multi-ended readout design were investigated using the leading-edge discriminator (LED) triggers on the timing performance of time-of-flight (TOF) PET detectors. At the very first, the photon transmission model of the four detectors, namely, single-ended readout, dual-ended readout, side dual-ended readout, and triple-ended readout, was established in Tracepro. The optical simulation revealed that the light output of the multi-ended readout was higher. Meanwhile, the readout circuit could be triggered earlier. Especially, in the triple-ended readout, the light output at 0.5 ns was observed to be nearly twice that of the single-ended readout after the first scintillating photon was generated. Subsequently, a reference detector was applied to test the multi-ended readout detectors that were constructed from a 6 × 6 × 25 mm3 LYSO crystal. Each module is composed of a crystal coupled with multiple SiPMs. Accordingly, its timing performance was improved by approximately 10% after the compensation of fourth-order polynomial fitting. Finally, the compensated full-width-at-half-maximum (FWHM) coincidence timing resolutions (CTR) of the dual-ended readout, side dual-ended readout, and triple-ended readout were 216.9 ps, 231.0 ps, and 203.6 ps, respectively.

Fast cyclic stimulus flashing modulates perception of bi-stable figure

Many experiments have demonstrated that the rhythms in the brain influence an initial information processing. We investigated whether the alternation rate of the perception of a Necker cube depended on the degree of synchronization between two streams of spikes, one stemming from an external flashing image and the other from the action of an internal impulse stream. Knowing how a flickering stimulus with a given frequency and duration affects the alternation rate of bi-stable perception we could estimate properties of the internal signal. As the internal spike frequency is difficult to control, we varied the frequency of the flicker stimulus. Our results show that the duration of the dominant stimulus perception depends on the frequency or duration of the flashing stimuli. The values of the stimuli, at which the changes of the duration of the perceived image was maximal, we have called ‘extremal’. While changing the flash duration, the extremal parameters repeated periodically at 4ms intervals. Increasing the duration of the extremal stimuli by less than 4 ms shortens the duration of the dominant stimulus perception. Hence we may conclude that it is not the stimulus duration but the accurate coincidence (timing) of the moments of switching on of external stimuli to match the internal stimuli which explains our experimental results.

Fast cyclic stimulus flashing modulates perception of bi-stable figure

Many experiments have demonstrated that the rhythms in the brain influence an initial information processing. We investigated whether the alternation rate of the perception of a Necker cube depended on the degree of synchronization between two streams of spikes, one stemming from an external flashing image and the other from the action of an internal impulse stream. Knowing how a flickering stimulus with a given frequency and duration affects the alternation rate of bi-stable perception we could estimate properties of the internal signal. As the internal spike frequency is difficult to control, we varied the frequency of the flicker stimulus. Our results show that the duration of the dominant stimulus perception depends on the frequency or duration of the flashing stimuli. The values of the stimuli, at which the changes of the duration of the perceived image was maximal, we have called ‘extremal’. While changing the flash duration, the extremal parameters repeated periodically at 4ms intervals. Increasing the duration of the extremal stimuli by less than 4 ms shortens the duration of the dominant stimulus perception. Hence we may conclude that it is not the stimulus duration but the accurate coincidence (timing) of the moments of switching on of external stimuli to match the internal stimuli which explains our experimental results.

Effects of the task complexity on the performance of a Coincidence Timing Task of people with Down syndrome

With the present study, we aimed to investigate the task complexity effects on the performance of a coincidence timing task in individuals with Down syndrome. The experimental group was consisted by 26 subjects with Down syndrome, 20 years old (± 5), and classified as mild or moderate in International Classification of Functioning and Health. Each participant in the experimental group was matched by in function of gender and age with a participant in a control group without Down syndrome. Both groups performed a coincident timing task, in which the participant had different levels of structural and functional complexity. Performance measure was derived from the consistency (absolute error), accuracy (variable error) and direction of response (constant error). Individuals with Down syndrome presented spatiotemporal organization impairments related to the difficulty in dealing with motor demand in order to interact with the perceptive demand. They also experienced difficulty in adapt to task complexity, causing performance error in the task. Individuals with Down syndrome have deficits in perceptive and space-time organization, and this phenomenon may be explained by the difficulty in integrating perceptive stimulus to motor actions with a high number of elements involved.