Bernard ten Haken

Dependence of the critical current in ex situ multi- and mono-filamentary MgB2/Fe wires on axial tension and compression

The transport critical current Ic of single- and four-core MgB2/Fe wires under tensile and compressive axial strain e was measured with a U-shaped spring set up. In all wires, Ic increases linearly and reversibly with applied tensile strain, up to a sample-dependent reversible strain limit eirr. The same reversible and linear Ic(e) relation is found when applying compressive strain. This shows how the Ic(e) increase with tension can be understood as a gradual release of the thermal pre-compression strain induced in the MgB2 filaments of the composite. The value of the reversible strain limit eirr depends mainly on the degree of pre-compression, but also on the shape of the composite wire. It is highest in a square wire and lowest in the flattest tape. Tensile strain levels above eirr cause an irreversible degradation of Ic. This Ic(e) degradation, due to filament cracking, is significantly steeper in heat-treated wires than in as-deformed ones. The different Ic(e) regimes are analysed and the mechanical behaviour of the MgB2/Fe composite is compared with that of typical Nb3Sn wires and Bi,Pb(2223) tapes.

Diffusing Substances during Spreading Depolarization: Analytical Expressions for Propagation Speed, Triggering, and Concentration Time Courses

Spreading depolarization (SD) is an important phenomenon in stroke and migraine. However, the processes underlying the propagation of SD are still poorly understood, and an elementary model that is both physiological and quantitative is lacking. We show that, during the onset and propagation of SD, the concentration time courses of excitatory substances such as potassium and glutamate can be described with a reaction–diffusion equation. This equation contains four physiological parameters: (1) a concentration threshold for excitation; (2) a release rate; (3) a removal rate; and (4) an effective diffusion constant. Solving this equation yields expressions for the propagation velocity, concentration time courses, and the minimum stimulus that can trigger SD. This framework allows for analyzing experimental results in terms of these four parameters. The derived time courses are validated with measurements of potassium in rat brain tissue.

Comparison of three magnetic nanoparticle tracers for sentinel lymph node biopsy in an in vivo porcine model

Introduction Breast cancer staging with sentinel lymph node biopsy relies on the use of radioisotopes, which limits the availability of the procedure worldwide. The use of a magnetic nanoparticle tracer and a handheld magnetometer provides a radiation-free alternative, which was recently evaluated in two clinical trials. The hydrodynamic particle size of the used magnetic tracer differs substantially from the radioisotope tracer and could therefore benefit from optimization. The aim of this study was to assess the performance of three different-sized magnetic nanoparticle tracers for sentinel lymph node biopsy within an in vivo porcine model. Materials and methods Sentinel lymph node biopsy was performed within a validated porcine model using three magnetic nanoparticle tracers, approved for use in humans (ferumoxytol, with hydrodynamic diameter dH =32 nm; Sienna+®, dH =59 nm; and ferumoxide, dH =111 nm), and a handheld magnetometer. Magnetometer counts (transcutaneous and ex vivo), iron quantification (vibrating sample magnetometry), and histopathological assessments were performed on all ex vivo nodes. Results Transcutaneous “hotspots” were present in 12/12 cases within 30 minutes of injection for the 59 nm tracer, compared to 7/12 for the 32 nm tracer and 8/12 for the 111 nm tracer, at the same time point. Ex vivo magnetometer counts were significantly greater for the 59 nm tracer than for the other tracers. Significantly more nodes per basin were excised for the 32 nm tracer compared to other tracers, indicating poor retention of the 32 nm tracer. Using the 59 nm tracer resulted in a significantly higher iron accumulation compared to the 32 nm tracer. Conclusion The 59 nm tracer demonstrated rapid lymphatic uptake, retention in the first nodes reached, and accumulation in high concentration, making it the most suitable tracer for intraoperative sentinel lymph node localization.

Intravoxel incoherent motion modeling in the kidneys: Comparison of mono‐, bi‐, and triexponential fit

To evaluate if a three‐component model correctly describes the diffusion signal in the kidney and whether it can provide complementary anatomical or physiological information about the underlying tissue.

A neural mass model based on single cell dynamics to model pathophysiology

Neural mass models are successful in modeling brain rhythms as observed in macroscopic measurements such as the electroencephalogram (EEG). While the synaptic current is explicitly modeled in current models, the single cell electrophysiology is not taken into account. To allow for investigations of the effects of channel pathologies, channel blockers and ion concentrations on macroscopic activity, we formulate neural mass equations explicitly incorporating the single cell dynamics by using a bottom-up approach. The mean and variance of the firing rate and synaptic input distributions are modeled. The firing rate curve (F(I)-curve) is used as link between the single cell and macroscopic dynamics. We show that this model accurately reproduces the behavior of two populations of synaptically connected Hodgkin-Huxley neurons, also in non-steady state.

The Twente high-current Conductor test Facility, First Results on Critical Current and Propagation in two Cables

A conventional measurement of the critical current and other relevant properties of superconducting cables within the field range of 0 to 8 tesla is complicated because of the large current supply required, the strong magnetic forces on the conductor, the minimum bending diameter of the cable and the high running costs of a conventional large scale facility. In our laboratory we have put into use a new small-size conductor test facility in which cables can be investigated concerning their critical current, a.c. losses, stability and propagation phenomena in the field range of 0 to 7.5 tesla. The test current is generated by a single-cycle superconducting rectifier which is designed to generate the cable current in the range to 25 kA nominal with an absolute maximum of 40 kA. In this low-cost facility we started with the investigation of several cables. In this paper we report on the first successful experiments during sample tests of two high-current conductors which are a monolithic conductor for the French TORE SUPRA fusion project and an 11-strand cable which is applied in the generator development program of Siemens. Critical current and propagation velocities were measured and analyzed over the mentioned field range.

Modeling of strain in multifilamentary wires deformed by thermal contraction and transverse forces

A previously published analytical model that describes a simplified wire geometry with three stacked cylinders is compared with finite element model calculations. The thermal strain from the matrix on the superconducting filaments is considered first. It appears that the analytical model is able to describe the strain that occurs in the filaments relatively accurate. Especially the radial dependence of the strain if a central core of normal material is present, is described quit well by the analytical model. The strain inside a wire surrounded by epoxy and subjected to a transverse load is almost uniform and can be approximated with an analytical model too. When yielding is involved to simulate a more localised transverse load inside a multifilamentary wire it is necessary to consider a numerical model.

Photoacoustic staging of nodal metastases using SPIOs: Comparison between in vivo, in toto and ex vivo imaging in a rat model

Background and objectives:nTo determine prognosis and treatment, accurate nodal staging is essential in many tumor types. After injection of clinical grade superparamagnetic iron oxide (SPIO) nanoparticles, it has been shown that metastatic lymph nodes can be distinguished from benign specimens using MR imaging. However, MR does not benefit per-operative nodal staging which requires a non-ionizing, small volume, high resolution, fast imaging technique. In vivo non-invasive photoacoustic (PA) imaging of lymph nodes might facilitate nodal staging during surgery, thereby benefiting both surgeon and patient.nnMaterials and methods:nIn order to investigate the feasibility of an in vivo nodal staging approach using photoacoustics, six Mat-lylu inoculated Copenhagen rats were photo-acoustically imaged after injection of a new Class IIa medical device SPIO magnetic tracer (Sienna+). Lymph nodes were imaged in vivo, in toto (after euthanization) and ex vivo using multiple wavelength illumination. Results were compared with MRI, immunohistochemistry and photographs of the sectioned nodes.nnResults:nThese experiments demonstrate that in an ex vivo setting, the PA contrast of Sienna+ is able to facilitate a distinction between metastatic and benign nodes. A non-invasive distinction between both groups is partially impeded by the low amount of PA contrast generated by the SPIO particles compared to that of endogenous absorbers such as hemoglobins.nnConclusions:nThis comparison between in vivo, in toto and ex vivo PA imaging of lymph nodes after SPIO injection demonstrates that the clinical potential of combined PA/SPIO staging should initially be exploited in an ex vivo setting. Improved distinction between chromophores by for example multi-spectral unmixing might in the near future enable non-invasive assessment of nodal involvement.

Phantom study quantifying the depth performance of a handheld magnetometer for sentinel lymph node biopsy

PURPOSEnThe use of a magnetic nanoparticle tracer and handheld magnetometer for sentinel lymph node biopsy (SLNB) was recently introduced to overcome drawbacks associated with the use of radioisotope tracers. Unlike the gamma probe, the used magnetometers are not only sensitive to the tracer, but also the diamagnetic human body. This potentially limits the performance of the magnetometer when used clinically.nnnMETHODSnA phantom, mimicking the magnetic and mechanical properties of the human axilla, was constructed. The depth performance of two current generation magnetometers was evaluated in this phantom. LN-phantoms with tracer uptake ranging from 5 to 500μg iron were placed at clinically relevant depths of 2.5, 4 and 5.5cm. Distance-response curves were obtained to quantify the depth performance of the probes.nnnRESULTSnThe depth performance of both probes was limited. In the absence of diamagnetic material and forces on the probe (ideal conditions) a LN-phantom with high uptake (500μg iron) could first be detected at 3.75cm distance. In the phantom, only superficially placed LNs (2.5cm) with high uptake (500μg iron) could be detected from the surface. The penetration depth was insufficient to detect LNs with lower uptake, or which were located deeper.nnnCONCLUSIONnThe detection distance of the current generation magnetometers is limited, and does not meet the demands formulated by the European Association for Nuclear Medicine for successful transcutaneous SLN localization. Future clinical trials should evaluate whether the limited depth sensitivity is of influence to the clinical outcome of the SLNB procedure.

Analyzing magnetic nanoparticle content in biological samples: Acsusceptometry using offset fields

The clinical application of magnetic nanoparticles is a developing field with promising perspectives in treatment and diagnosis [1]. After the first applications as a contrast agent in MRI, other magnetic methods have been developed for excitation and detection of magnetic nanoparticles. For magnetic detection, the nonlinear behavior of superparamagnetic iron oxides provide excellent contrast in the linear magnetic human body. To exploit these properties, the design of magnetic nanoparticles as well as detection systems has to be optimized for clinical practice. The particles have to provide optimal sensitivity in contrast to tissue, whereas the signal-to-noise ratio and applicability of a measurement system are important for successful clinical implementation. In this contribution a setup is presented that is able to assess these both elements for sentinel lymph node mapping. Small intact biological samples, such as lymph nodes, can be measured at room temperature to characterize the magnetic nanoparticle content by differential magnetometry. Furthermore, the system can be used as a tool to analyze the magnetic properties of nanoparticles, providing insight in the quality for nonlinear particle detection.