On the magnetic field and the electrical potential generated by bioelectric sources in an anisotropic volume conductor

The electrical conductivity in biological tissue is often dependent on the direction of the fibres. In the paper the influence of this anisotropic nature on the electrical potential and magnetic field generated by a current dipole is studied analytically. Three different methods are discussed. The volume conductor is described by piecewise homogeneous compartments and the interfaces between compartments are either parallel or perpendicular to one of the principal axes. To illustrate the methods, the influence of the anisotropic nature is computed for a two-layered medium. It turns out that the influence on both the potential and the magnetic field cannot be ignored. However, for some commonly used models of the head and torso, a certain component of the magnetic field is not influenced by the anisotropy.     

TL-Ⅰ型磁场扫描式理疗机对人血清SOD活力及MDA含量的影响

目的:观察TL-Ⅰ型磁场扫描式理疗机对人血清超氧化物歧化酶(SOD)活力及丙二醛(MDA)含量的影响。方法:用此机给受试者每日磁疗一次,每次30分钟,两周为一疗程,于磁疗开始前及一疗程后取空腹静脉血测定血清SOD活力及MDA含量。结果:磁疗后血清SOD活力显著升高(P<0.01),MDA含量明显降低(P<0.01)。结论:此机可对人体起到保健和治疗作用。     

Dynamics of Sodium Ions and Water in Swollen Superabsorbent Hydrogels as Studied by 23Na‐ and 1H‐NMR

Dynamics of sodium ions and water in swollen superabsorbent polymer (SAP) hydrogels are studied by ²³Na‐ and ¹H‐NMR, respectively. The apparent diffusion coefficients of water in swollen SAPs, probed by ¹H pulsed field gradient NMR, decreases with increasing diffusion time. The degree of hindrance depends on structural and synthesis parameters. It is quantified within a tortuosity model. Based on the results, the swelling degree has the highest impact on the ion mobility, apart from synthesis parameters leading to different levels of physical and chemical crosslinks. ²³Na‐NMR relaxation and diffusion reveal the ²³Na⁺ mobility in swollen SAPs. A higher degree of neutralization leads to faster relaxation and to a smaller apparent diffusion coefficient. Surface crosslinking restricts water mobility, but has a smaller impact on the dynamics of sodium ions. The experimental results indicate an influence of SAP structure on the dynamics of ions and water molecules.     

Spatial and temporal variations in the magnetic fields produced by human gastrointestinal activity

Magnetoenterography (MENG) is a new, non-invasive technique that measures gastrointestinal magnetic signals near the body surface. This study was undertaken to evaluate the temporal and spatial characteristics of the magnetic signals generated by gastric and duodenal slow wave activity. The gastrointestinal magnetic fields of eight normal subjects were measured for 60 minutes in both the fasting and fed state using 36 magnetic sensors simultaneously. The results were displayed as a succession of maps over time showing the temporal evolution of the spatial distribution of the signal over the upper abdomen. In all subjects, slow wave activity of the stomach centred at 3.0 +/- 0.5 cycles min-1 in both the fasting and fed state was observed. The duodenal signal at 11.0 +/- 1.0 cycles min-1 was observed in four subjects. The spatial distribution of these two signals is distinctly different. The observed spatial and temporal variations are described in terms of a model used previously to explain the potentials observed in electrogastrography (EGG).     

Spatial and temporal variations in the magnetic fields produced by human gastrointestinal activity

Magnetoenterography (MENG) is a new, non-invasive technique that measures gastrointestinal magnetic signals near the body surface. This study was undertaken to evaluate the temporal and spatial characteristics of the magnetic signals generated by gastric and duodenal slow wave activity. The gastrointestinal magnetic fields of eight normal subjects were measured for 60 minutes in both the fasting and fed state using 36 magnetic sensors simultaneously. The results were displayed as a succession of maps over time showing the temporal evolution of the spatial distribution of the signal over the upper abdomen. In all subjects, slow wave activity of the stomach centred at 3.0±0.5 cycles min⁻¹ in both fasting and fed state was observed. The duodenal signal at 11.0±1.0 cycles min⁻¹ was observed in four subjects. The spatial distribution of these two signals is distinctly different. The observed spatial and temporal variations are described in terms of a model used previously to explain the potentials observed in electrogastrography (EGG).     

Impairment of visual perception and visual short term memory scanning by transcranial magnetic stimulation of occipital cortex

Summary Transcranial magnetic stimulation (TMS) of occipital cortex was performed using a magneto-electric stimulator with a maximum output of 2 Tesla in 24 normal volunteers. The identification of trigrams, presented for 14 ms in horizontal or vertical arrays was significantly impaired when the visual stimulus preceded the occipital magnetic shock by 40 to 120 ms. The extent of impairment was related to TMS intensity. The latency of perceptual impairment was shorter for more intense TMS. No perceptual impairment was obtained by sham stimulation when TMS shocks were applied to the upper cervical region rather than the occipital region to rule out unspecific startle reactions affecting attention possibly responsible for the observed reduction in performance. Occipital TMS did not evoke systematic eye movements except for blink responses at latencies beyond 40 ms which were too late to interfere with visual input. Depending on the required serial order of readout of the trigram perceptual impairment was more marked for the second and third part of the trigram. This demonstrates that TMS interferes with the internal serial processing of visual input. To elucidate this further, TMS was used in a Sternberg short term visual memory scanning task. TMS caused a marked decrease in memory scanning rates whereas visual stimulus encoding and storage remained unaffected when tested at various TMS delays. TMS appears to be a useful method to study processes of visual perception and short term memory handling in the occipital cortex. Advantages over classical visual masking techniques especially regarding topical localisation are discussed.This work was partially supported by a grant from the Deutsche Forschungs Gemeinschaft (SFB 200/B9) to V.H.     

Spatial resolution of body surface potential maps and magnetic field maps: A simulation study applied to the identification of ventricular pre-excitation sites

The spatial resolution of body surface potential maps (BSPMs) and magnetic field maps (MFMs) is investigated by means of an anatomically accurate computer model of the human ventricular myocardium. BSPMs and MFMs are calculated for the simulated activation sequences initiated at 35 pre-excitation sites located along the atrioventricular (AV) ring of the epicardium. Changes in the BSPMs and MFMs corresponding to different pre-excitation sites are quantified in terms of the correlation coefficient r. The spatial resolution (selectivity) for a given pre-excitation site is defined as the half-distance between those neighbouring locations at which morphological features of maps, in terms of r, become distinct (r<0.95). It is found that, at 28 ms after the onset of pre-excitation and with no noise added, this distance ±SD, for all sites along the AV ring for the 117-lead BSPMs, is 0.83±0.32 cm, and for the 64-lead and 128-lead MFMs it is 1.54±0.84 cm and 1.15±0.43 cm, respectively. The findings suggest that, when features of non-invasively recorded electrocardiographic and magnetocardiographic map patterns are used for identifying accessory pathways in patients suffering from WPW syndrome, BSPMs are likely to provide more detailed information for guiding the ablative treatment than MFMs. For some sites MFMs provide more information. Both modalities may provide additional assistance to the cardiologist in locating the site of the accessory pathway.     

Theoretical model for investigating the magnetic and electric fields produced during pulsed magnetic field therapy for nonunion of the tibia

Pulsed magnetic field therapy for tibial nonunion has become an established orthopaedic procedure in many centres. The field is generated by passing pulses of current through coils positioned one each side of the limb but the magnitudes of the magnetic and induced electric fields produced are not usually known. The paper describes a method of calculating the fields that gives good agreement between theory and measurement. An improved model of a bone in a limb has been developed and this model predicts that the peak induced electric field close to the fracture site is between 0·03 and 0·6 Vm⁻¹, depending on which of the many clinically tested coil systems is used. The effect of changing geometry and the contribution of the outer surface of the limb are examined, and the implications for future experimental work are discussed.     

Shortening of simple reaction time by peripheral electrical and submotor-threshold magnetic cortical stimulation

 Subthreshold transcranial magnetic stimulation (TMS) over the motor cortex can shorten the simple reaction time in contralateral arm muscles if the cortical shock is given at about the same time as the reaction stimulus. The present experiments were designed to investigate whether this phenomenon is due to a specific facilitatory effect on cortical circuitry. The simple visual reaction time was shortened by 20–50 ms when subthreshold TMS was given over the contralateral motor cortex. Reaction time was reduced to the same level whether the magnetic stimulus was given over the bilateral motor cortices or over other points on the scalp (Cz, Pz). Indeed, similar effects could be seen with conventional electrical stimulation over the neck, or even when the coil was discharged (giving a click sound) near the head. We conclude that much of the effect of TMS on simple reaction time is due to intersensory facilitation, although part of it may be ascribed to a specific effect on the excitability of motor cortex. Received: 15 July 1996 / Accepted: 25 February 1997     

Interaction of microwaves and a temporally incoherent magnetic field on spatial learning in the rat

The effect of a temporally incoherent magnetic field ('noise') on microwave-induced spatial learning deficit in the rat was investigated. Rats were trained in six sessions to locate a submerged platform in a circular water maze. Four treatment groups of rats were studied: microwave-exposure (2450-MHz continuous-wave microwaves, power density 2 mW/cm(2), average whole-body specific absorption rate 1.2 W/kg), 'noise' exposure (60 mG), 'microwave+noise' exposure, and sham exposure. Animals were exposed to these conditions for 1 h immediately before each training session. One hour after the last training session, animals were tested in a 2-min probe trial in the maze during which the platform was removed. The time spent during the 2 min in the quadrant of the maze in which the platform had been located was scored. Results show that microwave-exposed rats had significant deficit in learning to locate the submerged platform when compared with the performance of the sham-exposed animals. Exposure to 'noise' alone did not significantly affect the performance of the animals (i.e., it was similar to that of the sham-exposed rats). However, simultaneous exposure to 'noise' significantly attenuated the microwave-induced spatial learning deficit (i.e. 'microwave+noise'-exposed rats learned significantly better than the microwave-exposed rats). During the probe trial, microwave-exposed animals spent significantly less time in the quadrant where the platform was located. However, response of the 'microwave+noise'-exposed animals was similar to that of the sham-exposed animals during the probe trial. Thus, simultaneous exposure to a temporally incoherent magnetic field blocks microwave-induced spatial learning and memory deficits in the rat.     

Comparison of motor effects following subcortical electrical stimulation through electrodes in the globus pallidus internus and cortical transcranial magnetic stimulation

Current concepts of transcranial magnetic stimulation (TMS) over the primary motor cortex are still under debate as to whether inhibitory motor effects are exclusively of cortical origin. To further elucidate a potential subcortical influence on motor effects, we combined TMS and unilateral subcortical electrical stimulation (SES) of the corticospinal tract. SES was performed through implanted depth electrodes in eight patients treated with deep brain stimulation (DBS) for severe dystonia. Chronaxie, conduction velocity (CV) of the stimulated fibres and poststimulus time histograms of single motor unit recordings were calculated to provide evidence of an activation of large diameter myelinated fibres by SES. Excitatory and inhibitory motor effects recorded bilaterally from the first dorsal interosseus muscle were measured after SES and focal TMS of the motor cortex. This allowed us to compare motor effects of subcortical (direct) and cortical (mainly indirect) activation of corticospinal neurons. SES activated a fast conducting monosynaptic pathway to the alpha motoneuron. Motor responses elicited by SES had significantly shorter onset latency and shorter duration of the contralateral silent period compared to TMS induced motor effects. Spinal excitability as assessed by H-reflex was significantly reduced during the silent period after SES. No ipsilateral motor effects could be elicited by SES while TMS was followed by an ipsilateral inhibition. The results suggest that SES activated the corticospinal neurons at the level of the internal capsule. Comparison of SES and TMS induced motor effects reveals that the first part of the TMS induced contralateral silent period should be of spinal origin while its later part is due to cortical inhibitory mechanisms. Furthermore, the present results suggest that the ipsilateral inhibition is predominantly mediated via transcallosal pathways.This paper is dedicated to Bernd-Ulrich Meyer, who died in a plane accident     

The effect of head and coil modeling for the calculation of induced electric field during transcranial magnetic stimulation

In the present work we studied some of the features related to transcranial magnetic stimulation (TMS) computational modeling. Particularly we investigated the impact of head model resolution on the estimated distribution of the induced electric field, as well as the role of the stimulating magnetic coil model in TMS. Using the impedance method we calculated the induced electric field inside a realistic numerical phantom of the human head from a commercially available eight-shaped coil, which was modeled in two ways. The results showed that finer resolution of the model has better performance at tissue interfaces eliminating numerical artifacts of local peaks. Furthermore, the geometrical details of a TMS coil must be taken into account since the predicted amount of volume of brain tissue involved can have great variation. Finally, the secondary magnetic field that is generated by the induced eddy currents in the tissues can be neglected.     

Comparison of the pedalling performance induced by magnetic and electrical stimulation cycle ergometry in able-bodied subjects

The purpose of the study was to compare the mechanical power and work generated by able-bodied subjects during functional magnetic stimulation (FMS) vs. functional electrical stimulation (FES) induced ergometer training conditions. Both stimulation methods were applied at a 30 Hz frequency to the quadriceps muscles of 22 healthy able-bodied subjects to induce cycling for 4× four minutes or until exhaustion. FMS was performed via large surface, cooled coils, while FES was applied with a typical stimulation setup used for cycling. Significantly more (p < 10⁻³) muscular power was generated by FMS (23.8 ± 9.1 W [mean ± SD]) than by FES (11.3 ± 11.3 W). Additionally, significantly more (p < 10⁻⁶) work was produced by FMS than by FES (4.413 ± 2.209 kJ vs. 0.974 ± 1.269 kJ). The increase in the work was paralleled by a significant prolongation of time to cycling failure (181.8 ± 33.4 s vs. 87.0 ± 54.0 s, respectively, p < 10⁻⁵). Compared to FES, FMS can produce more intense and longer cycling exercise in able-bodied subjects. The differing dynamic behaviour of FMS and FES in the presented measurement setup might be related to stimulation induced pain and fatigue mechanisms of the neuromuscular system.     

Investigation of lithium distribution in the rat brain ex vivo using lithium‐7 magnetic resonance spectroscopy and imaging at 17.2 T

Lithium is the first‐line mood stabilizer for the treatment of patients with bipolar disorder. However, its mechanisms of action and transport across the blood–brain barrier remain poorly understood. The contribution of lithium‐7 magnetic resonance imaging (⁷Li MRI) to investigate brain lithium distribution remains limited because of the modest sensitivity of the lithium nucleus and the expected low brain concentrations in humans and animal models. Therefore, we decided to image lithium distribution in the rat brain ex vivo using a turbo‐spin‐echo imaging sequence at 17.2 T. The estimation of lithium concentrations was performed using a phantom replacement approach accounting for B₁ inhomogeneities and differential T₁ and T₂ weighting. Our MRI‐derived lithium concentrations were validated by comparison with inductively coupled plasma‐mass spectrometry (ICP‐MS) measurements ([Li]_MRI = 1.18[Li]_MS, R = 0.95). Overall, a sensitivity of 0.03 mmol/L was achieved for a spatial resolution of 16 μL. Lithium distribution was uneven throughout the brain (normalized lithium content ranged from 0.4 to 1.4) and was mostly symmetrical, with consistently lower concentrations in the metencephalon (cerebellum and brainstem) and higher concentrations in the cortex. Interestingly, low lithium concentrations were also observed close to the lateral ventricles. The average brain‐to‐plasma lithium ratio was 0.34 ± 0.04, ranging from 0.29 to 0.39. Brain lithium concentrations were reasonably correlated with plasma lithium concentrations, with Pearson correlation factors ranging from 0.63 to 0.90.     

MR‐based measurements and simulations of the magnetic field created by a realistic transcranial magnetic stimulation (TMS) coil and stimulator

Transcranial magnetic stimulation (TMS) is an emerging technique that allows non‐invasive neurostimulation. However, the correct validation of electromagnetic models of typical TMS coils and the correct assessment of the incident TMS field (B_TMS) produced by standard TMS stimulators are still lacking. Such a validation can be performed by mapping B_TMS produced by a realistic TMS setup. In this study, we show that MRI can provide precise quantification of the magnetic field produced by a realistic TMS coil and a clinically used TMS stimulator in the region in which neurostimulation occurs. Measurements of the phase accumulation created by TMS pulses applied during a tailored MR sequence were performed in a phantom. Dedicated hardware was developed to synchronize a typical, clinically used, TMS setup with a 3‐T MR scanner. For comparison purposes, electromagnetic simulations of B_TMS were performed. MR‐based measurements allow the mapping and quantification of B_TMS starting 2.5 cm from the TMS coil. For closer regions, the intra‐voxel dephasing induced by B_TMS prohibits TMS field measurements. For 1% TMS output, the maximum measured value was ~0.1 mT. Simulations reflect quantitatively the experimental data. These measurements can be used to validate electromagnetic models of TMS coils, to guide TMS coil positioning, and for dosimetry and quality assessment of concurrent TMS‐MRI studies without the need for crude methods, such as motor threshold, for stimulation dose determination.     

基于灰度差分法的7.0 T及3.0 T MRI测量正常大鼠海马容积的对比研究

目的 评价7.0 T MRI对正常大鼠海马结构及亚区容积的检测能力。方法 40只正常Wistar大鼠分别行7.0 T 和3.0 T MRI T₂WI扫描,图像导入IMAGE J软件,利用灰度差分法辨识海马及亚区的解剖点,测得两种场强MRI海马及亚区层面积、容积并进行比较。结果 7.0 T MRI中,侧脑室、环池辨认率达100%(40/40),腹侧海马裂与下托辨认率达95%(38/40),丘脑外侧核、外侧膝状体背侧核辨认率达90%(36/40)。3.0 T MRI上只能清晰辨认双侧侧脑室及环池,不能分辨大鼠海马亚区结构。3.0 T MRI所测海马层面积左侧 (2.81±0.86) mm²、右侧 (2.77±0.80)mm²,容积左侧(56.36±5.98) mm³、右侧(55.61±6.03 )mm³;7.0 T MRI所测海马层面积左侧(3.25±0.92) mm²、右侧 (3.14±0.81)mm²,容积左侧(64.29±7.13)mm³、右侧(65.34±7.74)mm³;两者比较差异均有统计学意义(P值均<0.05)。7.0 T MRI所测海马CA1区层面积左侧为 (2.81±0.98) mm²、右侧 (2.88±0.92) mm²,容积左侧为(27.02±4.62)mm³、右侧(27.64±4.13) mm³;CA3和DG区合并计为CA3-DG区,其层面积左侧为 (4.21±1.21) mm²、右侧 (4.19±1.40)mm²,容积左侧为(38.73±4.17) mm³、右侧(38.11±5.09) mm³。结论 依据灰度差分法,大鼠海马7.0 T MRI能够准确辨认海马结构、亚区边界标志点,获得其较为可靠的容积大小,其相关数据可为此类研究提供参照和依据。