Impact of random and scattered coincidences from outside of field of view on positron emission tomography/computed tomography imaging with different reconstruction protocols

Image quality in positron emission tomography(PET)is affected by random and scattered coincidences and reconstruction protocols.In this study,we investigated the effects of scattered and random coincidences from outside the field of view(FOV)on PET image quality for different reconstruction protocols.Imaging was performed on the Discovery 690 PET/CT scanner,using experimental configurations including the NEMA phantom(a body phantom,with six spheres of different sizes)with a signal background ratio of 4:1.The NEMA phantom(phantom I)was scanned separately in a one-bed position.To simulate the effect of random and scatter coincidences from outside the FOV,six cylindrical phantoms with various diameters were added to the NEMA phantom(phantom II).The 18 emission datasets with mean intervals of 15 min were acquired(3 min/scan).The emission data were reconstructed using different techniques.The image quality parameters were evaluated by both phantoms.Variations in the signal-to-noise ratio(SNR)in a 28-mm(10-mm)sphere of phantom II were 37.9%(86.5%)for ordered-subset expectation maximization(OSEM-only),36.8%(81.5%)for point spread function(PSF),32.7%(80.7%)for time of flight(TOF),and 31.5%(77.8%)for OSEM+PSF+TOF,respectively,indicating that OSEM+PSF+TOF reconstruction had the lowest noise levels and lowest coefficient of variation(COV)values.Random and scatter coincidences from outside the FOV induced lower SNR,lower contrast,and higher COV values,indicating image deterioration and significantly impacting smaller sphere sizes.Amongst reconstruction protocols,OSEM+PSF+TOF and OSEM+PSF showed higher contrast values for sphere sizes of 22,28,and 37 mm and higher contrast recovery coefficient values for smaller sphere sizes of 10 and 13 mm.

Timed RR-interval Scatter Plots and Reverse Technology

Lorenz-RR scatter plot has an obvious shortcoming in that it does not indicate the time when the scatter point happens.On the Lorenz RR scatter plot,one cannot know the time during which the cardiac rhythms take place.Since occurrence of cardiac rhythms is time-related,time should be introduced to such plots.In this study,time was used as abscissa and RR interval(the time interval between the previous RR wave and the R wave)as the ordinate and time was compressed into a visually observable length,and thereby a timed RR-interval scatter plot,or t-RR scatter plot,for short,was developed.On t-RR scatter plot,the patterns were band-shaped or were of linear type.On the t-RR plot,the sinus rhythm presented bands of various widths,with the spiculate or burred upper and lower boundaries,having diurnal variation.Premature beats showed separate layers(“stratification”),the layer number corresponding the number of RR-intervals.With simple premature beats,the layers were clearly separated.With parasystole rhythm,the upper and lower bands or layers might become thicker.With arial premature beats,the space or distance between layers varied.Ventricular premature beats presented equal space or distance between layers.With tachycardia,the lower layer became a“solid”layer.With atrial fibration,the“stratification”disappeared,presenting thicker or widened layers or bands,with neat lower boundary.With atrial flutter,the layers went parallel,with the layers being evenly separated or some distances being exact multiples of others.The second degree atrioventricular block displayed two layers,the lower and upper bands being equally away from the X-axis,presenting a straight line(pacing at a fixed rate)or a thicker or wider bands,with a neat upper boundary(the lowest pacing rate).When the scatter plot presented uncharacteristic patterns or had some scattered points,which rendered diagnosis difficult,a reverse technology could be used.Briefly,upon selection of scattered points,they were subjected to computerization,by regression,to reveal the piece of electrocardiogram(ECG)containing an R wave(QRS complex).Then ECG was analyzed to diagnose the cardiac rhythms.In conclusion,t-RR is a novel methodology which helps us understand heart rhythms from a new perspective.

Design of the sample cell in near-field surface-enhanced Raman scattering by finite difference time domain method

The rational design of the sample cell may improve the sensitivity of surface-enhanced Raman scattering （SERS） detection in a high degree. Finite difference time domain （FDTD） simulations of the configuration of Ag film-Ag particles illuminated by plane wave and evanescent wave are performed to provide physical insight for design of the sample cell. Numerical solutions indicate that the sample cell can provide more ＂hot spots＂ and the massive field intensity enhancement occurs in these ＂hot spots＂. More information on the nanometer character of the sample can be got because of gradient-field Raman （GFR） of evanescent wave. OCIS codes： 290.5860, 240.0310, 240.6680, 999.9999 （surface-enhanced Raman scattering）.

A Hybrid Finite Element-Laplace Transform Method for the Analysis of Transient Electromagnetic Scattering by an Over-Filled Cavity in the Ground Plane

A hybrid finite element-Laplace transform method is implemented to analyze the time domain electromagnetic scattering induced by a 2-D overfilled cavity embedded in the infinite ground plane.The algorithm divides the whole scattering domain into two,interior and exterior,sub-domains.In the interior sub-domain which covers the cavity,the problem is solved via the finite element method.The problem is solved analytically in the exterior sub-domain which slightly overlaps the interior subdomain and extends to the rest of the upper half plane.The use of the Laplace transform leads to an analytical link condition between the overlapping sub-domains.The analytical link guides the selection of the overlapping zone and eliminates the need to use the conventional Schwartz iteration.This dramatically improves the efficiency for solving transient scattering problems.Numerical solutions are tested favorably against analytical ones for a canonical geometry.The perfect link over the artificial boundary between the finite element approximation in the interior and analytical solution in the exterior further indicates the reliability of the method.An error analysis is also performed.

High Performance Algorithms Based on a New Wavelet Expansion for Time Dependent Acoustic Obstacle Scattering

This paper presents a highly parallelizable numerical method to solve time dependent acoustic obstacle scattering problems.The method proposed is a generalization of the“operator expansion method”developed by Recchioni and Zirilli[SIAM J.Sci.Comput.,25(2003),1158-1186].The numerical method proposed reduces,via a perturbative approach,the solution of the scattering problem to the solution of a sequence of systems of first kind integral equations.The numerical solution of these systems of integral equations is challenging when scattering problems involving realistic obstacles and small wavelengths are solved.A computational method has been developed to solve these challenging problems with affordable computing resources.To this aim a new way of using the wavelet transform and new bases of wavelets are introduced,and a version of the operator expansion method is developed that constructs directly element by element in a fully parallelizable way.Several numerical experiments involving realistic obstacles and“small”wavelengths are proposed and high dimensional vector spaces are used in the numerical experiments.To evaluate the performance of the proposed algorithm on parallel computing facilities,appropriate speed up factors are introduced and evaluated.