Feasibility of tracking involuntary head movement for MRI using a coil as a magnetic dipole in a time-varying gradient.

Accurate tracking involuntary head movements is fairly a challenging problem in MR imaging of the brain. Though there are few techniques available to monitor the head movement of the subject for a prospective motion correction, it is still an unsolved problem in MRI. In this theoretical study, we aim to describe an analytical investigation to track head movement inside an MR scanner by calculating the change in induced voltage in the head-mounted coils during the execution of time-varying gradients. We derive an expression to calculate the change in induced voltage in a coil placed in a time-varying gradient. We also derive a general equation to investigate the changes in the induced voltage in a set of coils mounted onto the head for the planar position and orientation of the coils. Each coil is considered as a magnetic dipole with location and sensitivity vectors. The changes of the vectors can track the head movement in the MR scanner by measuring the changes in the induced voltage in the coils. The dipole concept is valid for a wide range of coils. The changes in induced voltage in the coils are linear due to small changes in pose of the head. Movement parameters are estimated from the induced voltage changes. If the random noise voltage is less than 100 µV, it does not significantly affect movement parameters because the change in induced voltage in the coils dominates over the small noise voltage. This method and array of the coils may provide a real-life solution to the long-standing problem of head motion during MRI.

Magnetic field effects on the vestibular system: calculation of the pressure on the cupula due to ionic current-induced Lorentz force.

Large static magnetic fields may be employed in magnetic resonance imaging (MRI). At high magnetic field strengths (usually from about 3 T and above) it is possible for humans to perceive a number of effects. One such effect is mild vertigo. Recently, Roberts et al (2011 Current Biology 21 1635-40) proposed a Lorentz-force mechanism resulting from the ionic currents occurring naturally in the endolymph of the vestibular system. In the present work a more detailed calculation of the forces and resulting pressures in the vestibular system is carried out using a numerical model. Firstly, realistic 3D finite element conductivity and fluid maps of the utricle and a single semi-circular canal containing the current sources (dark cells) and sinks (hair cells) of the utricle and ampulla were constructed. Secondly, the electrical current densities in the fluid are calculated. Thirdly, the developed Lorentz force is used directly in the Navier-Stokes equation and the trans-cupular pressure is computed. Since the driving force field is relatively large in comparison with the advective acceleration, we demonstrate that it is possible to perform an approximation in the Navier-Stokes equations that reduces the problem to solving a simpler Poisson equation. This simplification allows rapid and easy calculation for many different directions of applied magnetic field. At 7 T a maximum cupula pressure difference of 1.6 mPa was calculated for the combined ampullar (0.7 µA) and utricular (3.31 µA) distributed current sources, assuming a hair-cell resting current of 100 pA per unit. These pressure values are up to an order of magnitude lower than those proposed by Roberts et al using a simplistic model and calculation, and are in good agreement with the estimated pressure values for nystagmus velocities in caloric experiments. This modeling work supports the hypothesis that the Lorentz force mechanism is a significant contributor to the perception of magnetic field induced vertigo.

Interaction of MRI field gradients with the human body.

In this review, the effects of low-frequency electromagnetic fields encountered specifically during magnetic resonance imaging (MRI) are examined. The primary biological effect at frequencies of between 100 and 5000 Hz (typical of MRI magnetic field gradient switching) is peripheral nerve stimulation, the result of which can be a mild tingling and muscle twitching to a sensation of pain. The models for nerve stimulation and how they are related to the rate of change of magnetic field are examined. The experimental measurements, and analytic and computational modelling work in this area are reviewed. The review concludes with a discussion of current regulation in this area and current practice as both are applied to MRI.

Measurement of electric fields induced in a human subject due to natural movements in static magnetic fields or exposure to alternating magnetic field gradients.

A dual dipole electric field probe has been used to measure surface electric fields in vivo on a human subject over a frequency range of 0.1-800 Hz. The low-frequency electric fields were induced by natural body movements such as walking and turning in the fringe magnetic fields of a 3 T magnetic resonance whole-body scanner. The rate-of-change of magnetic field (dB/dt) was also recorded simultaneously by using three orthogonal search coils positioned near to the location of the electric field probe. Rates-of-change of magnetic field for natural body rotations were found to exceed 1 T s(-1) near the end of the magnet bore. Typical electric fields measured on the upper abdomen, head and across the tongue for 1 T s(-1) rate of change of magnetic field were 0.15+/-0.02, 0.077+/-0.003 and 0.015+/-0.002 V m(-1) respectively. Electric fields on the abdomen and chest were measured during an echo-planar sequence with the subject positioned within the scanner. With the scanner rate-of-change of gradient set to 10 T m(-1) s(-1) the measured rate-of-change of magnetic field was 2.2+/-0.1 T s(-1) and the peak electric field was 0.30+/-0.01 V m(-1) on the chest. The values of induced electric field can be related to dB/dt by a 'geometry factor' for a given subject and sensor position. Typical values of this factor for the abdomen or chest (for measured surface electric fields) lie in the range of 0.10-0.18 m. The measured values of electric field are consistent with currently available numerical modelling results for movement in static magnetic fields and exposure to switched magnetic field gradients.

Measurement of electric fields due to time-varying magnetic field gradients using dipole probes.

The operation of dipole probes in measuring electric fields in conductive media exposed to temporally varying magnetic fields is discussed. The potential measured by the probe can be thought of as originating from two contributions to the electric field, namely the gradient of the scalar electric potential and the temporal derivative of the magnetic vector potential. Using this analysis, it is shown that the exact form of the wire paths employed when using electric field probes to measure the effects of temporally varying magnetic fields is very important and this prediction is verified via simple experiments carried out using different probe geometries in a cylindrical sample exposed to a temporally varying, uniform magnetic field. Extending this work, a dipole probe has been used to measure the electric field induced in a cylindrical sample by gradient coils as used in magnetic resonance imaging (MRI). Analytic solutions for the electric field in an infinite cylinder are verified by comparison with experimental measurements. Deviations from the analytic solutions of the electric field for the x-gradient coil due to the finite length of the sample cylinder are also demonstrated.

Magnetic-field-induced vertigo: a theoretical and experimental investigation.

Vertigo-like sensations or apparent perception of movement are reported by some subjects and operators in and around high field whole body magnetic resonance body scanners. Induced currents (which modulate the firing rate of the vestibular hair cell), magneto-hydrodynamics (MDH), and tissue magnetic susceptibility differences have all been proposed as possible mechanisms for this effect. In this article, we examine the theory underlying each of these mechanisms and explore resulting predictions. Experimental evidence is summarised in the following findings: 30% of subjects display a postural sway response at a field-gradient product of 1 T(2)m(-1); a determining factor for experience of vertigo is the total unipolar integrated field change over a period greater than 1 s; the perception of dizziness is not necessarily related to a high value of the rate of change of magnetic field; eight of ten subjects reported sensations ranging from mild to severe when exposed to a magnetic field change of the order of 4.7 T in 1.9 s; no subjects reported any response when exposed to 50 ms pulses of dB/dt of 2 Ts(-1) amplitude. The experimental evidence supports the hypothesis that magnetic-field related vertigo results from both magnetic susceptibility differences between vestibular organs and surrounding fluid, and induced currents acting on the vestibular hair cells. Both mechanisms are consistent with theoretical predictions.

Quantitative density profiling with pure phase encoding and a dedicated 1D gradient.

A new centric scan imaging methodology for density profiling of materials with short transverse relaxation times is presented. This method is shown to be more robust than our previously reported centric scan pure phase encode methodologies. The method is particularly well suited to density imaging of low gyro-magnetic ratio non-proton nuclei through the use of a novel dedicated one-dimensional magnetic field gradient coil. The design and construction of this multi-layer, water cooled, gradient coil is presented. Although of large diameter (7.62 cm) to maximize sample cross section, the gradient coil has an efficiency of several times that offered by conventional designs (6 mT/m/A). The application of these ideas is illustrated with high resolution density-weighted proton (1H) images of hazelnut oil penetration into chocolate, and lithium ion (7Li) penetration into cement paste. The methods described in this paper provide a straightforward and reliable means for imaging a class of samples that, until now, have been very difficult to image.

A novel high temperature 1H NMR imaging probe for combustion studies: the behaviour of both a lit and unlit methane gas jet.

The design of a NMR probe suitable for very high temperature samples is described. The loop gap resonator is water cooled and tuned to 100 MHz for use in a 2.4 T horizontal bore magnet. The probe has been specifically designed for imaging of the combustion process. An experiment is described in this paper which shows the behaviour of a methane gas jet when both lit and unlit. The jet of gas may be observed in its unlit state flowing at up to 2 ms(-1) from a 1 mm diameter orifice using a Single Point Imaging technique. Images of the lit gas show loss of nuclear polarisation within 3 mm of the orifice. A residual amount of un-decomposed gas is visible in the first few millimetres of the flame neck. A computational fluid dynamics model is used to verify the distribution of molecular methane, as well as the temperature of the flame.

Effect of a range of microbial polysaccharides on the diffusion of manganese ions using spatially resolved NMR relaxometry.

In accordance with the theory of contact exchange, it is hypothesized that the presence of negative charge in microbial exopolysaccharides increases the rate of cation transport. These typically acidic materials may provide a fast-track for the diffusion of nutrient cations through the polymer layer for uptake at the organism cell surface. We have measured the diffusion coefficient of a model cation, Mn(2+,) through xanthan, de-acetylated xanthan, scleroglucan and chitosan using spatially resolved NMR relaxometry. The concentration of Mn(2+) in solution was measured by recording the change in the spin-spin (T(2)) relaxation time of water (1)H over time in compartments either side of a polymer layer. This approach provides a sensitive, in situ, non-invasive method of measuring the rate of diffusion of paramagnetic cations through hydrophilic polysaccharides. The negatively-charged polysaccharides, xanthan and de-acetylated xanthan, permitted a significantly faster rate (2-2.5x) of cation transport compared to the uncharged polymer, scleroglucan. The positively-charged polysaccharide chitosan reduced the rate of Mn(2+) diffusion to around half the value obtained for scleroglucan. These results suggest that the presence and nature of fixed charges on the polysaccharide molecule affects the rate of cation transport in accordance with the theory of contact exchange. The presence of negative charge on microbial exopolysaccharides may thus improve the availability of nutrient cations at the organism cell surface.

An optical method for three-dimensional dosimetry.

Accurate determination of the spatial distribution of the absorbed dose of ionising radiation plays an important role in radiotherapy, industrial radiation processing and many other applications. Computer calculations have frequently been used to estimate three-dimensional (3D) dose distributions in complex geometries and it becomes important to validate these by accurate 3D measurements. For this purpose we have been investigating the use of gelatin gels loaded with a modified Fricke solution which are pale orange in appearance and which, upon irradiation, become increasingly purple when viewed in normal light. This ferrous sulphate xylenol orange in gelatin gel (FXG) system displays very good properties, such as sensitivity, linearity and dynamic range, that make it suitable for 3D dosimetry applications. A high-speed optical tomography readout technique has been developed enabling two-dimensional projections of optical absorption data to be recorded rapidly. From these data the 3D absorbed dose distribution can quickly be derived with minimal degradation due to ion diffusion.

A novel high-gradient permanent magnet for the profiling of planar films and coatings.

The design and construction of a low-cost, permanent magnet is described. The magnet is intended for applications which require a large static gradient, such as those for which stray field imaging or fringe field diffusometry are conventionally employed. The magnet has been designed using the scalar potential method. Particular features of the magnet include a field profile such that ||B || is constant in the horizontal plane and such that B is horizontal at the midpoint between the poles. There is a vertical, and therefore orthogonal, strong gradient, G, in ||B ||. The ratio G/ ||B || is constant within a large volume and so allows measurements at a range of gradient strengths. It is this ratio which governs the shape of the pole-pieces. The constructed magnet has a typical operating field of 0.8 T, gives a gradient of 20 Tm-1, and has a useable interpole access of 20 mm. Field plot data show values consistent with the theory. In particular ||B || has a curvature of less than +/-5 microm over a 5 x 5 mm area at the target field. The magnet is most suitable for the one-dimensional profiling of thin planar samples. As an example of the magnet's use, a profile of a sandwich structure made of several polymer layers is shown. In addition, a set of one-dimensional profiles of an alkyd coating, recorded during solvent loss and cross-linking, is presented. This example demonstrates quantitative T2 measurements at a resolution of 6.5 microm across a 70-microm-thick film.

Sound generation in gradient coil structures for MRI.

When supporting plates of plastic material are subjected to alternating transverse Lorentz forces while in a strong magnetic field normal to the plate surface, compressional waves within the solid produce a modulation of the plate surface that launches an acoustic wave in air along the magnetic field axis. We have extended our previous theory describing this process to include a detailed description of the formation of an acoustic interference pattern in air described by Fraunhofer diffraction at a distance from the plate surface. The extended theory predicts that the observed acoustic signal midpoint and normal to the plate surface gives a variation with frequency in approximate agreement with our previous measurements. The acoustic output off axis shows acoustic blazing that produces two main diffraction peaks with a splitting inversely proportional to the velocity of sound in the plate material. The new results could have important ramifications for the minimization of sound output in gradient coil design for MRI. A new arrangement of coils is proposed to ameliorate the acoustic output problem centrally and normal to the plate by extending the frequency response of the supporting plates to much higher frequencies. Also presented are estimates of the compressional wave velocities deduced from frequency response data recorded at the center-point of a number of different plates.

A robust single-shot partial sampling scheme.

A method of reducing the amount of data required to reconstruct an image is described. In this scheme, fully sampled low spatial frequency data are acquired up to a given cutoff frequency and above this point, only alternate lines are sampled. Two images are produced, one of low definition and one of high definition but aliased. The proposed algorithm unwraps the aliased data, which are then used to enhance the low pass image, yielding a best estimate of the true image. The reduced sampling technique is shown to afford biological images that are almost indistinguishable from those obtained from a complete data set.

A microscope slide probe for high resolution imaging at 11.7 Tesla.

We present details of a novel RF coil for use in NMR microscopy. This coil is an inductively coupled surface coil which is built into a standard microscope slide. The coil is highly sensitive and is also designed so that the sample to be imaged can be viewed under an optical microscope. This facility allows comparison of optical and NMR micrographs, as well as accurate sample positioning. Using the slide coil, images of onion epidermal cells with an in-plane resolution of 4.5 microns have been produced.