Introducing improved voxel navigation and fictitious interaction tracking in GATE for enhanced efficiency.

Geant4 Application for Emission Tomography (GATE) is a widely used, well-validated and very versatile application for Monte Carlo simulations in emission tomography. However, its computational performance is poor, especially for voxelized phantoms, partly due to the use of a very general particle tracking algorithm. In this work, two methods are proposed to reduce the time spent on particle tracking in the phantom: a newly introduced 'regular navigation algorithm' of Geant4 and fictitious interaction tracking (also known as Woodcock tracking) for photons. The speed-up introduced by the two methods was investigated by simulating a PET acquisition with the Allegro/GEMINI GXL PET/CT scanner. The simulation was based on a clinical head-and-neck [(18)F]FDG PET/CT scan. The total time spent for the simulation (including initialization, particle tracking and signal processing) was obtained for seven settings corresponding to different tracking options. All seven methods led to very close results with regard to the total number of detected coincidences (less than 0.5% differences), and trues, scatter and random fractions. Acceleration factors of approximately 2.7 (14 x 14 x 9 voxels) to 27.6 (378 x 378 x 243 voxels) were obtained in comparison with the fastest available tracking available in GATE 3.1.2.

Bleeding following deep hypothermia and circulatory arrest in children.

Deep hypothermic circulatory arrest (DHCA) is a technique of extracorporeal circulation commonly used in children with complex congenital heart defects undergoing surgical repairs. The use of profound cooling (20 degrees C) and complete cessation of circulation allow adequate exposure and correction of these complex lesions, with enhanced cerebral protection. However, the profound physiologic state of DHCA results in significant derangement of the coagulation system and a high incidence of postoperative bleeding. This review examines the impact of DHCA on bleeding and transfusion requirements in children and the pathophysiology of DHCA-induced platelet dysfunction. It also focuses on possible pharmacologic interventions to decrease bleeding following DHCA in children.

Massive intracardiac thrombosis during coronary artery bypass grafting surgery.

Thrombosis is a potential life-threatening complication in patients undergoing cardiac surgery. Various clinical and heritable conditions, like cancer, trauma, immobilization, the presence of factor V Leiden or prothrombin 20210A, deficiency of or resistance to the inhibitor proteins C, S, or antithrombin, elevated levels of coagulation proteins, antiphospholipid antibody syndrome, pregnancy, and the use of exogenous hormones, may contribute to catastrophic thrombosis. Massive thrombi with cerebrovascular and cardiovascular events develop in patients with polycythemia vera (PV). However, thrombus formation in the cardiac chambers is extremely rare. We report a case of massive intracardiac thrombosis in a patient undergoing coronary artery bypass grafting.

DagSolid: a new Geant4 solid class for fast simulation in polygon-mesh geometry.

Even though a computer-aided design (CAD)-based geometry can be directly implemented in Geant4 as polygon-mesh using the G4TessellatedSolid class, the computation speed becomes very slow, especially when the geometry is composed of a large number of facets. To address this problem, in the present study, a new Geant4 solid class, named DagSolid, was developed based on the direct accelerated geometry for the Monte Carlo (DAGMC) library which provides the ray-tracing acceleration algorithm functions. To develop the DagSolid class, the new solid class was derived from the G4VSolid class, and its ray-tracing functions were linked to the corresponding functions of the DAGMC library. The results of this study show that the use of the DagSolid class drastically improves the computation speed. The improvement was more significant when there were more facets, meaning that the DagSolid class can be used more effectively for complicated geometries with many facets than for simple geometries. The maximum difference of computation speed was 1562 and 680 times for Geantino and ChargedGeantino, respectively. For real particles (gammas, electrons, neutrons, and protons), the difference of computation speed was less significant, but still was within the range of 53-685 times depending on the type of beam particles simulated.