Safety testing and operational procedures for self-developed radiofrequency coils.

The development of novel radiofrequency (RF) coils for human ultrahigh-field (≥7 T), non-proton and body applications is an active field of research in many MR groups. Any RF coil must meet the strict requirements for safe application on humans with respect to mechanical and electrical safety, as well as the specific absorption rate (SAR) limits. For this purpose, regulations such as the International Electrotechnical Commission (IEC) standard for medical electrical equipment, vendor-suggested test specifications for third party coils and custom-developed test procedures exist. However, for higher frequencies and shorter wavelengths in ultrahigh-field MR, the RF fields may become extremely inhomogeneous in biological tissue and the risk of localized areas with elevated power deposition increases, which is usually not considered by existing safety testing and operational procedures. In addition, important aspects, such as risk analysis and comprehensive electrical performance and safety tests, are often neglected. In this article, we describe the guidelines used in our institution for electrical and mechanical safety tests, SAR simulation and verification, risk analysis and operational procedures, including coil documentation, user training and regular quality assurance testing, which help to recognize and eliminate safety issues during coil design and operation. Although the procedure is generally applicable to all field strengths, specific requirements with regard to SAR-related safety and electrical performance at ultrahigh-field are considered. The protocol describes an internal procedure and does not reflect consensus among a large number of research groups, but rather aims to stimulate further discussion related to minimum coil safety standards. Furthermore, it may help other research groups to establish their own procedures. Copyright © 2015 John Wiley & Sons, Ltd.

Extending the range for force calibration in magnetic tweezers.

Magnetic tweezers are a wide-spread tool used to study the mechanics and the function of a large variety of biomolecules and biomolecular machines. This tool uses a magnetic particle and a strong magnetic field gradient to apply defined forces to the molecule of interest. Forces are typically quantified by analyzing the lateral fluctuations of the biomolecule-tethered particle in the direction perpendicular to the applied force. Since the magnetic field pins the anisotropy axis of the particle, the lateral fluctuations follow the geometry of a pendulum with a short pendulum length along and a long pendulum length perpendicular to the field lines. Typically, the short pendulum geometry is used for force calibration by power-spectral-density (PSD) analysis, because the movement of the bead in this direction can be approximated by a simple translational motion. Here, we provide a detailed analysis of the fluctuations according to the long pendulum geometry and show that for this direction, both the translational and the rotational motions of the particle have to be considered. We provide analytical formulas for the PSD of this coupled system that agree well with PSDs obtained in experiments and simulations and that finally allow a faithful quantification of the magnetic force for the long pendulum geometry. We furthermore demonstrate that this methodology allows the calibration of much larger forces than the short pendulum geometry in a tether-length-dependent manner. In addition, the accuracy of determination of the absolute force is improved. Our force calibration based on the long pendulum geometry will facilitate high-resolution magnetic-tweezers experiments that rely on short molecules and large forces, as well as highly parallelized measurements that use low frame rates.

Low-field permanent magnets for industrial process and quality control.

In this review we focus on the technology associated with low-field NMR. We present the current state-of-the-art in low-field NMR hardware and experiments, considering general magnet designs, rf performance, data processing and interpretation. We provide guidance on obtaining the optimum results from these instruments, along with an introduction for those new to low-field NMR. The applications of lowfield NMR are now many and diverse. Furthermore, niche applications have spawned unique magnet designs to accommodate the extremes of operating environment or sample geometry. Trying to capture all the applications, methods, and hardware encompassed by low-field NMR would be a daunting task and likely of little interest to researchers or industrialists working in specific subject areas. Instead we discuss only a few applications to highlight uses of the hardware and experiments in an industrial environment. For details on more particular methods and applications, we provide citations to specialized review articles.

In vivo O-Space imaging with a dedicated 12 cm Z2 insert coil on a human 3T scanner using phase map calibration.

Recently, spatial encoding with nonlinear magnetic fields has drawn attention for its potential to achieve faster gradient switching within safety limits, tailored resolution in regions of interest, and improved parallel imaging using encoding fields that complement the sensitivity profiles of radio frequency receive arrays. Proposed methods can broadly be divided into those that use phase encoding (Cartesian-trajectory PatLoc and COGNAC) and those that acquire nonlinear projections (O-Space, Null space imaging, radial PatLoc, and 4D-RIO). Nonlinear projection data are most often reconstructed with iterative algorithms that backproject data using the full encoding matrix. Just like conventional radial sequences that use linear spatial encoding magnetic fields, nonlinear projection methods are more sensitive than phase encoding methods to imperfect calibration of the encoding fields. In this work, voxel-wise phase evolution is mapped at each acquired point in an O-Space trajectory using a variant of chemical shift imaging, capturing all spin dynamics caused by encoding fields, eddy currents, and pulse timing. Phase map calibration is then applied to data acquired from a high-power, 12 cm, Z2 insert coil with an eight-channel radio frequency transmit-receive array on a 3T human scanner. We show the first experimental proof-of-concept O-Space images on in vivo and phantom samples, paving the way for more in-depth exploration of O-Space and similar imaging methods.

An alignment procedure for ambulatory measurements of lower limb kinematic using magneto-inertial sensors.

In this work, an alignment procedure of magneto-inertial units in the Special Orthogonal Space SO(3) is presented and discussed. The procedure, designed for ambulatory measurements of lower limb kinematic, is based on simple rotation movements around anatomical axes of hip joint and its accuracy is independent of the speed as well as the range of the movements. This is particularly important for movement analysis of subjects with motor impairments. Despite such procedure was designed for lower limb movement analysis, it can be applied to every anatomical compartment (e.g.: upper limb).

Feasibility of using inertial sensors to assess human movement.

The aim of this study was to determine the suitability of inertial sensors for motion analysis research. Inertial sensors (Xsens Technologies, Netherlands) consisting of 3D gyroscopes, accelerometers and a magnetometer were compared against an electromagnetic motion tracking system (Fastrak, Polhemus, USA) for measuring motions of an artificial hinge joint and random 3D motions. Subsequently, to assess the feasibility of using inertial sensors for human motion analysis, the movements of the hip joint during walking were recorded in 20 normal asymptomatic subjects. The comparative study demonstrated good agreement between the inertial and electromagnetic systems. Measurements obtained for hip joint movement during walking (flexion, extension and step length) were similar to those reported in previous studies (flexion 38.8 degrees , extension 6.6 degrees , step frequency 1.02Hz). We conclude that the inertial sensors studied have the potential to be used for motion analysis and clinical research.

Localization of epileptic foci in children with intractable epilepsy secondary to multiple cortical tubers by using synthetic aperture magnetometry kurtosis.

OBJECT: Magnetoencephalography (MEG) has been typically used to localize epileptic activity by modeling interictal activity as equivalent current dipoles (ECDs). Synthetic aperture magnetometry (SAM) is a recently developed adaptive spatial filtering algorithm for MEG that provides some advantages over the ECD approach. The SAM-kurtosis algorithm (also known as SAM[g2]) additionally provides automated temporal detection of spike sources by using excess kurtosis value (steepness of epileptic spike on virtual sensors). To evaluate the efficacy of the SAM(g2) method, the authors applied it to readings obtained in children with intractable epilepsy secondary to tuberous sclerosis complex (TSC), and compared them to localizations obtained with ECDs. METHODS: The authors studied 13 children with TSC (7 girls) whose ages ranged from 13 months to 16.3 years (mean 7.3 years). Video electroencephalography, MR imaging, and MEG studies were analyzed. A single ECD model was applied to localize ECD clusters. The SAM(g2) value was calculated at each SAM(g2) virtual voxel in the patient's MR imaging-defined brain volume. The authors defined the epileptic voxels of SAM(g2) (evSAM[g2]) as those with local peak kurtosis values higher than half of the maximum. A clustering of ECDs had to contain > or = 6 ECDs within 1 cm of each other, and a grouping of evSAM(g2)s had to contain > or = 3 evSAM(g2)s within 1 cm of each other. The authors then compared both ECD clusters and evSAM(g2) groups with the resection area and correlated these data with seizure outcome. RESULTS: Seizures started when patients were between 6 weeks and 8 years of age (median 6 months), and became intractable secondary to multiple tubers in all cases. Ictal onset on scalp video electroencephalography was lateralized in 8 patients (62%). The MEG studies showed multiple ECD clusters in 7 patients (54%). The SAM(g2) method showed multiple groups of epileptic voxels in 8 patients (62%). Colocalization of grouped evSAM(g2) with ECD clusters ranged from 20 to 100%, with a mean of 82%. Eight patients underwent resection of single (1 patient) and multiple (7 patients) lobes, with 6 patients achieving freedom from seizures. Of 8 patients who underwent surgery, in 7 the resection area covered ECD clusters and grouped evSAM(g2)s. In the remaining patient the resection area partially included the ECD cluster and grouped evSAM(g2)s. Six of the 7 patients became seizure free. CONCLUSIONS: The combination of SAM(g2) and ECD analyses succeeded in localizing the complex epileptic zones in children with TSC who had intractable epilepsy secondary to multiple cortical tubers. For the subset of children with TSC who present with early-onset and nonlateralized seizures, MEG studies in which SAM(g2) and ECD are used might identify suitable candidates for resection to control seizures.

Magnetomyographic recording and identification of uterine contractions using Hilbert-wavelet transforms.

We propose a multi-stage approach using Wavelet and Hilbert transforms to identify uterine contraction bursts in magnetomyogram (MMG) signals measured using a 151 magnetic sensor array. In the first stage, we decompose the MMG signals by wavelet analysis into multilevel approximate and detail coefficients. In each level, the signals are reconstructed using the detail coefficients followed by the computation of the Hilbert transform. The Hilbert amplitude of the reconstructed signals from different frequency bands (0.1-1 Hz) is summed up over all the sensors to increase the signal-to-noise ratio. Using a novel clustering technique, affinity propagation, the contractile bursts are distinguished from the noise level. The method is applied on simulated MMG data, using a simple stochastic model to determine its robustness and to seven MMG datasets.

Reliability and validity of pendulum test measures of spasticity obtained with the Polhemus tracking system from patients with chronic stroke.

BACKGROUND: Spasticity is a common impairment accompanying stroke. Spasticity of the quadriceps femoris muscle can be quantified using the pendulum test. The measurement properties of pendular kinematics captured using a magnetic tracking system has not been studied among patients who have experienced a stroke. Therefore, this study describes the test-retest reliability and known groups and convergent validity of the pendulum test measures obtained with the Polhemus tracking system. METHODS: Eight patients with chronic stroke underwent pendulum tests with their affected and unaffected lower limbs, with and without the addition of a 2.2 kg cuff weight at the ankle, using the Polhemus magnetic tracking system. Also measured bilaterally were knee resting angles, Ashworth scores (grades 0-4) of quadriceps femoris muscles, patellar tendon (knee jerk) reflexes (grades 0-4), and isometric knee extension force. RESULTS: Three measures obtained from pendular traces of the affected side were reliable (intraclass correlation coefficient > or = .844). Known groups validity was confirmed by demonstration of a significant difference in the measurements between sides. Convergent validity was supported by correlations > or = .57 between pendulum test measures and other measures reflective of spasticity. CONCLUSION: Pendulum test measures obtained with the Polhemus tracking system from the affected side of patients with stroke have good test-retest reliability and both known groups and convergent validity.

Calibration of RF transmitter voltages for hyperpolarized gas MRI.

MRI with hyperpolarized gases, (3)He, (129)Xe, (13)C, and others, has the potential to become an important diagnostic technique for clinical imaging. Due to the nonreversible loss of magnetization in hyperpolarized gas imaging, the choice of the flip angle is a major factor that influences the signal intensity, and hence, the signal-to-noise ratio. Conventional automated radiofrequency (RF) calibration procedures for (1)H imaging are not suitable for hyperpolarized gas imaging. Herein, we have demonstrated a simple procedure for RF calibration for magnetic resonance imaging (MRI) with hyperpolarized gases that is easily adaptable to clinical settings. We have demonstrated that there exists a linear relationship between the RF transmitter voltages required to obtain the same nutation angle for protons (V(1H)) and hyperpolarized gas nuclei (V(3He)). For our (1)H and (3)He coils we found that V(3He) = 1.937 . V(1H) with correlation coefficient r(2) = 0.97. This calibration can be done as a one-time procedure during the routine quality assurance (QA) protocol. The proposed procedure was found to be extremely robust in routine scanning and provided an efficient method to achieve a desired flip angle, thus allowing optimum image quality.

Safety aspects of transcranial brain stimulation in man tested by single photon emission-computed tomography.

Single photon emission-computed tomography (SPECT) using 99mTc-labelled hexamethylpropyleneamine oxime (99mTc-HMPAO), a new method to visualize regional cerebral blood flow (rCBF) and epileptogenic foci, was used to study acute and long-term effects of transcranial brain stimulation. Magnetic and electric brain stimulation increase rCBF not more than voluntary muscle activation mimicking the motor effects of transcranial brain stimulation. Focal rCBF increase, typical for epileptogenic foci, or other pathological findings could not be detected even when the subject had received several thousand stimulations in the past. Transcranial brain stimulation does not produce rCBF patterns indicating acute or chronic adverse effects.

[Magnetic immunoenzyme analysis of Yersinia pestis antigens]. / Magnitnyi immunofermentnyi analiz antigenov Yersinia pestis.

The detection of Y. pestis cells in magnetic enzyme immunoassay is carried out with the use of magnetic polyacrylamide microgranules. In the assay system for the determination of the antigen commercial Y. pestis antigens, peroxidase-labeled antibodies, the substrate mixture consisting of sodium salt of 2,2-azino-bis-3-ethylbenzthiazoline-6-sulfonic acid and H2O2 in citrate-phosphate buffer solution, pH 4.5, are used. The sensitivity of the method is 5 X 10(4) microbial bodies per ml.