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ABSTRACT: Nonlinear spatial encoding fields for magnetic resonance imaging (MRI) hold great promise to improve on the linear gradient approaches. Unlike the linear techniques, the nonlinear encoding leads to a spatially varying signal-to-noise ratio (SNR). This paper demonstrates the possibility to tailor the encoding fields to focus the high SNR areas to a region of interest. To achieve this, a metric is derived to quantify the spatially dependent performance for arbitrary encoding schemes.
Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 08/2011; 2011:3740-3.
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ABSTRACT: An attractive technique to extract scorpion venom is the use of a physiologically stimulating electrical signal across the muscles of the venom gland, resulting in the expression of venom from the aculeus. A Grass™ stimulator is typically used for this purpose, but is difficult to use in the field. The present communication describes a circuit which is battery-powered and simply constructed. Also described is the technique for its construction and housing. The circuit was successfully tested on two species of scorpion. The method for calculating the required values of passive circuit components is given to allow the adaptation and refinement of this circuit for producing different signals, as may be required for use in other species.
Toxicon 02/2011; 57(2):244-7. · 2.51 Impact Factor
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ABSTRACT: The behaviour of spins at a classical level in the presence of an external magnetic field is completely described by the Bloch equation. This is the main equation governing the magnetic resonance imaging (MRI) system, and as such the Bloch equation is extensively used for design purposes. Here we have transferred the Bloch equation to the spherical coordinate system that, to the best of our knowledge, has not previously been applied in this field. The mathematical framework and the simulation results show that this fresh view of the Bloch equation provides better insight into the magnetic resonance (MR) phenomenon. In this mathematical framework, without using spinor space, the order of the Bloch equation is reduced in a much simpler way and can therefore provides a novel insight to the slice selection problem. Simulation results are presented for a variety of slice selective pulses, with and without post excitation rephasing gradients. In this paper nonlinear gradient is tried as well which shows an improvement in uniformity of the selected slice. It is feasible to find an analytic approximate solution to the Bloch equation in spherical coordinate system by adopting averaging techniques over spatial variables available available in nonlinear dynamical systems. We anticipate that our new description of the MR phenomenon will allow researchers to revisit the excitation pattern design question to achieve better slice selectivity or to find an optimal excitation pattern from a theoretical basis. This has the potential to result in practical improvements affecting all forms of MR imaging.
SPIE on Medical Imaging, Orlando-USA; 02/2009
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ABSTRACT: An excitation dependent rotating frame of reference to observe the magnetic resonance phenomenon is introduced in this paper that, to the best of our knowledge, has not been used previously in the nuclear magnetic resonance context. The mathematical framework for this new rotating frame of reference is presented based on time scaling the Bloch equation after transformation to the classical rotating frame of reference whose transverse plane is rotating at the Larmor frequency. To this end, the Bloch equation is rewritten in terms of a magnetisation vector observed from the excitation dependent rotating frame of reference. The resultant Bloch equation is referred to as the time scaled Bloch equation. In the excitation dependent rotating frame of reference whose coordinates are rotating at the instantaneous Rabi frequency the observed magnetisation vector is a much slower signal than the true magnetisation in the rotating frame of reference. As a result the ordinary differential equation solvers have the ability to solve the time scaled version of the Bloch equation with a larger step size resulting in a smaller number of samples for solving the equation to a desired level of accuracy. The simulation results for different types of excitation are presented in this paper. This method may be used in true Bloch simulators in order to reduce the simulation time or increase the accuracy of the numerical solution. Moreover, the time scaled Bloch equation may be employed to determine the optimal excitation pattern in magnetic resonance imaging as well as designing pulses with better slice selectivity which is an active area of research in this field.
Engineering in Medicine and Biology Society, 2008. EMBS 2008. 30th Annual International Conference of the IEEE; 09/2008
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ABSTRACT: This work details a method to compute a probability of outage of a set of concatenated optical links with respect to multiple simultaneous optical impairments.
Optics Express 08/2008; 16(14):10529-34. · 3.59 Impact Factor
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ABSTRACT: Optical fibre-based evanescent field sensors are proving to be viable and useful in the detection of chemicals, toxins, gases and other species. Such fibre sensors rely on weak interactions between the surrounding environment and the external “evanescent” field associated with bound modes of electromagnetic radiation propagating through an optical fibre. Of fundamental importance in the design of such sensors is the fraction of the total guided field power that propagates as the evanescent field. By increasing this relative evanescent field power, sensor gain can be improved. As the evanescent field is determined by the refractive index profile (RIP) of the sensor and the surrounding environment, design of the sensor RIP is critical. This paper presents a rigorous approach to this RIP design problem, formulated in terms of an optimization problem with the normalized evanescent field power as the payoff, RIP as the input, and a simple electric field model as the constraint. An RIP design procedure utilizing dynamic programming is then presented, along with an example that illustrates the improvement obtainable over existing designs.
Systems & Control Letters. 01/2007;
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ABSTRACT: Nondestructive images of refractive-index variation within a type I fiber Bragg grating have been recorded by the differential interference contrast imaging technique. The images reveal detailed structure within the fiber core that is consistent with the formation of Talbot planes in the diffraction pattern behind the phase mask that had been used to fabricate the grating.
Optics Letters 06/2003; 28(10):789-91. · 3.40 Impact Factor
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ABSTRACT: Here we demonstrate the use of differential interference contrast microscopy with the inverse Abel transform to accurately obtain the refractive-index profile of a single-mode optical fiber. The application of this nondestructive imaging analysis technique provides high precision refractive-index information about the fiber. This technique is robust, rapid, and has diffraction limited spatial resolution
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ABSTRACT: Quantitative phase microscopy with polarized light has been used to determine the change in refractive index introduced into an optical fiber when the fiber is bent through a constant radius of curvature. By obtaining phase images for two orthogonal directions of the polarization of the incident light, one can infer the induced axial strain profile within the fiber. Radii of curvature from 1 to 8 cm were considered, and in each case excellent agreement, within λ/100, was obtained between experimental results and theory.
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ABSTRACT: Rare-earth-doped optical fibres are of much interest as fibre lasers and amplifiers. They have also been used successfully as temperature sensors, by exploiting the thermodynamic properties of the energy levels of these rare-earth ions in which the relative population of a group of closely spaced levels follows the Boltzmann ratio. This paper explains the modelling of the concentration dependence of fluorescence from Neodymium-doped Optical Fibres. It also states the application to the sensing.
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ABSTRACT: A quadrature rule for the finite Hilbert transform via midpoint type inequalities is obtained. Some numerical experiments for different divisions of the interval of (a,b) are also presented.
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ABSTRACT: A quadrature rule for the finite Hilbert Transform via trapezoid type inequalities is obtained. Some numerical experiments for different divisions of the interval [a, b] are also presented
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ABSTRACT: Strain sensitivities of various optical fibre temperature sensors based on the fluorescence from rare-earth-doped fibre are presented. Their properties depend upon the dopant ion used and its concentration. These sensors may be co-located with other sensors such as Bragg gratings to provide a simultaneous measurement of strain and temperature, with the possibility of reduced uncertainties compared with other simultaneous measurement arrangements involving two sensors.
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ABSTRACT: At the present time there is much interest in sensors for the simultaneous measurement of temperature and strain. To engineer strain or temperature sensors, based on the fluorescence lifetime or fluorescence intensity ratio techniques, with desirable characteristics requires detailed understanding of the physical origin of the strain dependency of these fluorescence effects. It has been suggested that the cause is slight shifts in the energy levels, since some of the important levels in rare-earth-doped crystals shift when subjected to considerable pressure. However, recent theoretical work analysing the two techniques, and outlined in the next section, does not support this idea. Instead, in that work, it was proposed that this sensitivity is due to a volumetric distortion of the energy transfer rates between the dopant ions. In this model an applied strain results in a slight decrease in concentration. To explore this latter idea, strain and temperature dependencies of Nd3+-doped optical fibres of various concentrations have been analysed using this model. Existing data have been supplemented by new measurements, giving the sensitivities for the two techniques at a number of dopant concentrations.
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ABSTRACT: Overcoming the temperature cross sensitivity inherent in optical fiber strain sensors is currently of much interest. Many groups are investigating this cross-sensitivity in Bragg grating (FBG) sensors by seeking ways to have both parameters determined simultaneously from two separate sensor elements having different temperature and strain responsivities. It is therefore of interest to take advantage of the strain insensitivity of the fluorescence intensity ratio (FIR) technique by splicing a length of rare-earth-doped fiber to a FBG sensor. This paper reports initial measurements of temperature and strain using the proposed sensing scheme, in erbium-doped fibre.
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ABSTRACT: The effects of temperature and concentration on the fluorescence lifetime of the 3P0 excited state in praseodymium-doped ZBLAN glass were studied. Above a critical dopant concentration, at which the lifetime exhibits no temperature dependence, the fluorescence lifetime increases with increasing temperature while below this concentration, the fluorescence lifetime decreases with increasing temperature. This critical concentration is determined to be 6900±220 ppm from measurements of the lifetime for varying temperatures and concentrations.
