Fast maximum intensity projections of large medical data sets by exploiting hierarchical memory architectures.

Maximum intensity projections (MIPs) are an important visualization technique for angiographic data sets. Efficient data inspection requires frame rates of at least five frames per second at preserved image quality. Despite the advances in computer technology, this task remains a challenge. On the one hand, the sizes of computed tomography and magnetic resonance images are increasing rapidly. On the other hand, rendering algorithms do not automatically benefit from the advances in processor technology, especially for large data sets. This is due to the faster evolving processing power and the slower evolving memory access speed, which is bridged by hierarchical cache memory architectures. In this paper, we investigate memory access optimization methods and use them for generating MIPs on general-purpose central processing units (CPUs) and graphics processing units (GPUs), respectively. These methods can work on any level of the memory hierarchy, and we show that properly combined methods can optimize memory access on multiple levels of the hierarchy at the same time. We present performance measurements to compare different algorithm variants and illustrate the influence of the respective techniques. On current hardware, the efficient handling of the memory hierarchy for CPUs improves the rendering performance by a factor of 3 to 4. On GPUs, we observed that the effect is even larger, especially for large data sets. The methods can easily be adjusted to different hardware specifics, although their impact can vary considerably. They can also be used for other rendering techniques than MIPs, and their use for more general image processing task could be investigated in the future.     

Theory of instability-generated divergence of intense ion beamsfrom applied-B ion diodes

Recent advances in the analytic theory and numerical simulation of applied-B ion diodes in general and, in particular, the diodes being tested experimentally on the Particle Beam Fusion Accelerator II at Sandia National Laboratories, Albuquerque, NM, are described. Numerical simulation of these diodes using the three-dimensional particle-in-cell code QUICKSILVER and analytic stability analysis have provided new insight into the physics governing instability-induced beam divergence. Experimental evidence of these instabilities is described and compared with the theory. When simulation parameters are chosen to model present experiments, QUICKSILVER calculates beam divergence near the measured value. A new understanding of the nature of the electromagnetic instabilities and their effects on beam divergence has led to suggestions for improved diode design. A simulation of one of these design concepts shows divergence levels in the vicinity of 10 mrad, a level considered adequate for ignition experiments     

Nichtlineare Trajektorienfolgeregelung für einen Laborhelikopter

This contribution is devoted to the nonlinear tracking control problem of the laboratory experiment helicopter 3DOF distributed by Quanser. The laboratory experiment belongs to the class of mechanical systems with three degrees-of-freedom and two control inputs. It is well known that the systematic design of nonlinear controllers for underactuated mechanical systems is a challenge compared to fully actuated systems. On certain simplifying assumptions, which very well apply to the operating range of practical interest, we can show that the mathematical model is configuration flat. Thereby, a mechanical system is said to be configuration flat if it is differential flat and the flat outputs solely depend on the generalized coordinates of the mechanical system. The controller design is based on a formulation of the mechanical system on a Riemannian manifold where the kinetic energy serves as a natural Riemannian metric. In a first step a nonlinear tracking controller including an integral part in the linear error system is designed by means of a quasi-static state feedback. In a second step the design of the tracking controller is based on the theory of exact linearization utilizing the so-called dynamic extension algorithm. The experimental results of both controllers are compared and discussed in detail. In particular, the quasi-static state feedback controller shows an excellent tracking behavior. The performance as being obtained by the nonlinear controlled cannot be achieved by conventional linear control strategies.     

Bioassay for determining 2,3,7,8-tetrachlorodibenzo-p-dioxin equivalents (TEs) in human hepatoma Hepg2 cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), and benz[a]anthracene (BA) highly induce cytochrome P4501A1, determined by aryl hydrocarbon hydroxylase (AHH) activity, in human hepatoma HepG2 cells within 24 h. AHH activity induced by TCDD and TCDF persists for at least 48 h. In contrast, AHH activity induced by BA rapidly declines, although the amounts applied are 4-5 orders of magnitude higher than those of TCDD or TCDF. AHH induction in HepG2 cells differs from that in rat hepatoma cells H4IIEC3/T in two aspects: (1) HepG2 cells are 20 times less sensitive to the test compounds than H4IIEC3/T cells. (2) TCDF-induced AHH activity does not persist in the rat cells. The results suggest that human HepG2 cells, because of their low sensitivity, are inferior to rat H4IIEC3/T cells for determining TCDD equivalents in environmental samples. They may be useful for investigating species dependent differences in the toxicokinetics of individual polyhalogenated aromatic hydrocarbon congeners.     

Attentional sensitization of unconscious cognition: Task sets modulate subsequent masked semantic priming.

According to classical theories, automatic processes are autonomous and independent of higher level cognitive influence. In contrast, the authors propose that automatic processing depends on attentional sensitization of task-congruent processing pathways. In 3 experiments, the authors tested this hypothesis with a modified masked semantic priming paradigm during a lexical decision task by measuring event-related potentials (ERPs): Before masked prime presentation, participants attended an induction task either to semantic or perceptual stimulus features designed to activate a semantic or perceptual task set, respectively. Semantic priming effects on the N400 ERP component, an electrophysiological index of semantic processing, were obtained when a semantic task set was induced immediately before subliminal prime presentation, whereas a previously induced perceptual task set attenuated N400 priming. Across experiments, comparable results were obtained regardless of the difficulty level and the verbal or nonverbal nature of the induction tasks. In line with the proposed attentional sensitization model, unconscious semantic processing is enhanced by a semantic and attenuated by a perceptual task set. Hence, automatic processing of unconscious stimuli is susceptible to top-down control for optimizing goal-related information processing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

MR imaging of focal liver lesions: comparison between turbo spin-echo and conventional spin-echo pulse sequences

The aim of this study was to compare conventional spin-echo (CSE)T ₂-weighted (T2W) images with turbo spin-echo (TSE) T2W pulse sequences in their ability to detect focal liver lesions. Seventy-eight consecutive patients with focal liver lesions were entered into this study. All patients were imaged using the gradient-echo (GE) sequence with the breath-hold technique forT ₁-weighted (T1W) images, and CSE and TSE sequences for T2W images. Qualitative evaluation included lesion detection (number of lesions detected) and conspicuity (extent of visualization of lesional borders); quantitative evaluation included the signal-to-noise (S/N) ratio and the contrast-to-noise (C/N) ratio. TSE showed the best performance in terms of lesion detection; however, the difference between TSE and CSE was significant only in the case of benign cysts (p<0.01). Conspicuity was higher with TSE and CSE, and lower with GE. The S/N and C/N ratios of the two T2W sequences were also comparable, and better than those of GE. However, the combined use of GE and TSE resulted in improved lesion detection. The results show that, because the acquisition time is greatly reduced with TSE sequences, these should be considered as first-line approach to magnetic resonance imaging of the liver for the study of focal lesions.     

Augmentation techniques: are they out?

Basics of ligament augmentation are reviewed from the literature. As biomechanical in-vitro studies, animal experiments, and clinical studies show different designs and data, the results may be interpreted variably. Theoretically there is a positive effect of augmentation, but no significant improvement of clinical results is evident. The goal of our own prospective study was to check the healing capacity of a repaired and augmented ACL with respect to stability compared to a primary or secondary BTB ligament replacement with and without augmentation. 200 patients were allocated to 4 groups. In group I (G1) acute proximal ruptures were repaired arthroscopically and augmented with PDS II-cord. Patients in G2 received an acute and in G3 a late BTB ligament replacement. Additionally in G4 a late ligament replacement was augmented with a PDS II-cord. The clinical results reveal a comparative stability between G1 and G2, and gradually less stability in G3 and G4. Subjectively, knee function was felt best in G1 and G2 while being worst in G3 and G4. There is no significant difference in the mobility of knees between primary and secondary ligament replacement. An additional augmentation doesn't improve the knee stability. CLINICAL RELEVANCE: There is no positive effect of an augmentation technique of a biologic ligament replacement concerning stability, morbidity, complication rate, and costs. Very marginal freshly ruptured cruciate ligaments may be reinserted successfully by a biodegradable augmentation. Subjective feeling and functional stability may be at least equal to a primary ligament replacement. There are no advantages in knee mobility, when a secondary ligament replacement is performed.     

A strongly polynomial algorithm for criticality of branching processes and consistency of stochastic context-free grammars

We provide a strongly polynomial algorithm for determining whether a given multi-type branching process is subcritical, critical, or supercritical. The same algorithm also decides consistency of stochastic context-free grammars.     

Diffusion-weighted echo planar MR imaging of the neck at 3 T using integrated shimming: comparison of MR sequence techniques for reducing artifacts caused by magnetic-field inhomogeneities

Objective

Our objective was to compare available techniques reducing artifacts in echo planar imaging (EPI)-based diffusion-weighed magnetic resonance imaging MRI (DWI) of the neck at 3 Tesla caused by B0-field inhomogeneities.

Materials and methods

A cylindrical fat–water phantom was equipped with a Maxwell coil allowing for additional linear B0-field variations in z-direction. The effect of increasing strength of this superimposed gradient on image quality was observed using a standard single-shot EPI-based DWI sequence (sEPI), a zoomed single-shot EPI sequence (zEPI), a readout-segmented EPI sequence (rsEPI), and an sEPI sequence with integrated dynamic shimming (intEPI) on a 3-Tesla system. Additionally, ten volunteers were examined over the neck region using these techniques. Image quality was assessed by two radiologists. Scan durations were recorded.

Results

With increasing strength of the external gradient, marked distortions, signal loss, and failure of fat suppression were observed using sEPI, zEPI, and rsEPI. These artifacts were markedly reduced using intEPI. Significantly better in vivo image quality was also observed using intEPI compared with the other techniques. Scan time of intEPI was similar to sEPI and zEPI and shorter than rsEPI.

Conclusion

The use of integrated 2D shim and frequency adjustment for EPI-based DWI results in a significant improvement in image quality of the head/neck region at 3 Tesla. Combining integrated shimming with rsEPI or zEPI can be expected to provide additional improvements.