Patent Application: US-201615196098-A

Abstract:
a passive magnetic flux focusing element having electrically conductive wires or faces containing an outer area defined by an outer , an inner and connecting edges forming a closed current loop enclosing a surface area penetrated by a time varying magnetic field flux and through induction sets up a time varying electrical current in the conducting loop thereby achieving a counter magnetic field to the penetrating field completely canceling the penetrating field in the interior of the loop , is characterized in that the element is part of an rf volume - or surface - coil arrangement adapted for receiving and / or transmitting rf signals . such elements increase the sensitivity and the snr in mri and mr spectroscopy experiments due to an increased magnetic flux density by means of lenz lenses , in combination with a conventional probe .

Description:
in this disclosure , a device is proposed to increase the flux density of an nmr detector , thereby achieving a higher localized intensity of rf excitation ( the b 1 - field ), and by the principle of reciprocity , a higher detection sensitivity or snr . recently , a method was published which demonstrated the ability to focus the field strength within a helmholtz coil in a localized region ( see reference [ 1 ]). whereas the explanation in the paper is broadly correct , an assumption presented in the paper is incorrect , leading to an incorrect prediction of the flux concentration . furthermore , the paper exclusively describes the effect , while omitting an actual application . the paper especially fails to recognize the potential of the concept in nuclear magnetic resonance nmr , only considering applications in the khz frequency domain , and only considering effects in the direction of production of a magnetic field , whereas nuclear magnetic resonance ranges over the khz to ghz domain depending on the nuclear polarization field , and coils are used as detectors . that is , the authors failed to recognize the inverse action of a lenz lens under the reciprocity theorem of magnetic resonance , i . e ., the induction of current into a detector coil due to a precessing magnetization . in this disclosure , we detail how the concept can also be used to improve the detection of nuclear magnetic resonance signals , especially from small regions . the lenz lens effectively acts as a magnetic field “ lever ”. this highly localized gi field profile of the combined coil plus lenz lens defines the most sensitive volume for nmr detection . by the theory of reciprocity ( see reference [ 3 ]), a precessing magnetization of the same spatial distribution as the b 1 field profile of the coil ( in the coil &# 39 ; s sensitive volume ) will induce an electromotive force in the accompanying coil , and its strength v ( t ) is by reciprocity the same as if an alternating voltage was applied to the coil in order to produce an equivalent magnetic field . in summary then , the use of a lenz lens , together with an nmr coil detector , possesses the following advantages : it localizes the sensitive volume of the combined assembly . it increases the local field efficiency b 1 / ī of the original coil detector by approximately the root radius ratio f =√{ square root over ( r big / r small )} of the lenz lens . it is a broad band technique , working for samples independent of the resonance frequency of the excitation field , thus facilitating the study of samples through a range of field strengths . it provides a means for remote amplification , e . g ., it may be encapsulated wholly within the sample , owing to the discrete nature of the lens itself . the following lists some of the potential applications that could profit from the advantages of the lenz lens , some of which have already been demonstrated and documented in experiments . numerous further applications are expected : nmr microscopy on large samples in conjunction with a large sample - conformal nmr detector . we show that the lenz lens , placed on a flexible substrate , could be used to separately move the focus of attention to a specific region ( or regions ) of interest , without moving the entire detector assembly . nmr microscopy of wounds . by placing a lenz lens on a bandage or on a wound plaster , especially a lenz lens with a transparent inner loop , the lenz lens focus would always be properly aligned with the wound region of interest , and hence the assembly would be less susceptible to the accuracy of placement of the nmr detector . implantable nmr microscopy . by implanting a lenz lens , for example into the brain of a patient receiving invasive therapy but not limited to this application , the subsequent nmr inspection would benefit from the correct placement , high resolution , and wireless nature of the lenz lens implant , making possible the long term monitoring of healing processes , or of disease advancement , or of the monitoring of tissue - device interactions for example for neural microprobes . complicated sample geometries . sometimes the geometry of a technical or biological system does not facilitate the insertion of a wired device . for example , for technical systems such as microfluidics , or inside human arteries , or other confined spaces , it is desirable to obtain high resolution nmr signals . by placing a lenz lens on a catheter , for example , or other interventional mr device , or inside a microfluidic assembly , and by placing externally a larger nmr sensor that completely encloses the assembly , such as the head of a patient , or a complete technical system , it remains possible to localize the snr of the detector chain and thereby to achieve a very high fidelity nmr signal . we demonstrate this capability . nmr microscopy for biopsies and cell cultures . by including a lenz lens on a microscope glass slide , or on a culture dish , or on a well plate base ( possibly including multiple regions of interest ), we show that it is possible to achieve both transparency for optical microscopy , as well as enhanced nmr detection for nmr microscopy . inhomogeneities . by using micro - or nano - engineering to produce the lenz lens , it is possible to achieve an exceptionally uniform effective b 1 field in the focus of the lenz lens , despite a perhaps less uniform field in the nmr detector . this is because the lenz lens refocuses the flux through the use of a current , and the path of the current can be controlled very precisely using micro - manufacturing and nano - manufacturing . we provide a number of exemplary demonstrations and nmr measurements . design for improved lenz lens performance . we show that , by using numerical optimization , such as topology optimization , it is possible to take account of the coupling interaction between a lenz lens and the outer nmr coil detector , and thereby achieve an optimal placement of the conductors of the lenz lens to achieve even further fine tuning of the field homogeneity and flux leverage factor . design for novel lenz lens arrangements . we show that , by using numerical optimization , such as topology optimization , it is possible to rearrange the lenz lens topology so as to be able to be sensitive to multiple regions of interest , for example by using multiple lenz lens loops , or to further improve b 1 homogeneity , for example by using helmholtz arrangements of lenz lenses . further generalizations . since many nmr detectors also exhibit fields outside of their primary region of interest , it is possible to conceive of an arrangement for the lenz lens that also collects this flux . in order to do so , the lenz lens can no longer be purely planar , but requires a cross - over loop so as to collect the outer flux with the correct sense , since the detector &# 39 ; s outer b 1 field will point in the opposite direction as it closes the magnetic field line loops . especially for helmholtz type detectors where the lenz lens outer loop is ideally of a similar size to the helmholtz loop , such an arrangement is very advantageous , because up to 50 % more flux can thus be collected . waveguide nmr detectors . a lenz lens can also be operated in conjunction with non - coil inductive nmr sensors , such as striplines ( reference [ 6 ]), or microslots ( reference [ 7 ]) etc . in such an arrangement , the lenz lens can be used to locally modify the field density per unit length of the sensor in its most sensitive region , or to rotate the field locally to achieve more practical arrangements . magnetoinductive waveguides . a lenz lens can be used in conjunction with a magnetoinductive waveguide , for example , to achieve a more efficient waveguiding principle , by better gathering flux around the waveguide elements , or by forming a more efficient termination of the magnetoinductive waveguide . metamaterials . it is possible to form materials into a metamaterial so as to achieve effective permeability properties that are not available with natural materials . in this context , a lenz lens can for example be employed to achieve a very high local inductance value . furthermore , lenz lenses can be arranged in a metamaterial sense , so as to achieve regions with very strong shielding of magnetic field , and channeling of induction into areas where low losses are expected , thereby achieving efficient material arrangements . biochemistry sensor . by placing the lens on a substrate with material properties that are sensitive to the local chemical environment , it is possible to conceive of a device capable of remote detection of changes to this environment . such a device may act as a switch with an ‘ off ’ state invisible to nmr and an ‘ on ’ state that can be detected by nmr ( or vice versa ) and located spatially within the sample , as well as temporally in the case of real - time imaging . tissue growth sensor . by placing the lens on a flexible substrate and affixing to damaged or diseased tissue , it is possible to conceive of a device capable of measuring morphological properties of the regrowth of healthy tissue or the proliferation of diseased tissue such as cancerous tumors ( in addition to measuring chemical properties ), by virtue of the changing morphology of the lens itself . field rotation for complex systems . by placing the inner element of the lens on a plane that lies at a non - zero angle θ to the plane of the outer element of the lens , it is possible to rotate the orientation of the field within the sensitive volume of the lens . such an arrangement may allow the lens to access sample regions that are otherwise hindered by geometry constraints or magnetic shielding . for example , such an arrangement may be useful nearby metal implants or air cavities , or when the positioning of the external nmr detector is restricted and cannot be placed in such a way that the induced field is orthogonal to the primary static field of the nmr magnet . circular polarization . it is possible to take two lenses that have a single slit from the outside to the inside and to slide them into one another with their faces at right - angles . such an arrangement would preserve circularly polarized waves and would therefore provide the lenz lens benefit to quadrature excitation , such as when using birdcage coils . magnetic nanomanipulation . it is possible to move and steer 3d chiral magnetic particles ( such as possessing of a screw or solenoidal arrangement ) by magnetic field using a three - pair helmholtz field arrangement ( one pair of coils along each of the three orthogonal coordinate axes ). by suitable arrangement , three orthogonally crossed lenz lens arrangements placed within such a helmholtz arrangement will focus the vector field to the inner region of the lenz lens , essentially decomposing it into three cartesian coordinates and then reinstating it within the focus region . this in turn will enable , due to the small size of the inner region , the steering of very small chiral particles in a very small volume . furthermore , the field required in the helmholtz arrangement need not be very high , due to the focusing and intensifying effect of the lenz lens . this can for example be very interesting for interventional operation techniques , for example when operating in the eye or in inner organs . cross polarization , such as dnp . when applying cross polarization techniques , the same region in space must be subjected to two different radio - frequency signals . for some cases it is possible to double tune a resonator . when the frequencies are vastly different , such as is the case for dynamic nuclear polarization , or dnp , the effect is that the resonator for the lower frequency effectively screens the radiation at the higher frequency . a lenz lens can be used to provide a path into the low frequency resonator , by having part of its structure extending outside of the low frequency resonator , and perpendicular to its excitation field for minimal interference . compact pulsed nmr device . it is possible to create very high magnetic fields ( currently up to 100 t ) for very short times by pulsing energy from a capacitor store through a resonant coil , but this requires a very expensive infrastructure . by creating a lenz lens with a focusing factor f , it is possible to concentrate the flux of a coil , which generates a much lower ( 1 / f ) field strength into a region of interest . because the high field is only within the lenz lens , it could even be sacrificed as a consumable item during each pulsed experiment . furthermore , by using a high field pulsed nmr experiment , an even higher field can be achieved by using a lenz lens to focus the 100 t pulse into , for example , a 200 t field . any of the following methods could be used to produce lenz lenses on a variety of substrates , for example glass , paper , or polymer , as well as on technical substrates made of composite materials : direct write inkjet printing . inkjet printing ( reference [ 8 ]) is very suitable to produce temporary or permanent lenz lens structures on flat or non - flat surfaces . the ink can either be a nanoparticulate ink ( for example silver , or copper , or gold nanoparticles with a surfactant to avoid clumping of the particles ), or it can be a chemical precursor of the metal . subsequent thermal treatment will , for the case of nanoparticles , sinter them together into a continuous film , and for the case of a precursor ink , will promote the reaction to form a nanoparticular ink . liftoff . by forming the inverse shape of a lenz lens using micro - or nano - lithography in for example a polymer resist layer , and anisotropically depositing a conducting metal layer onto the structure , so that the direct shape of the lenz lens is formed permanently on a substrate , and subsequently removing the structure and thereby removing the inverse shape of the lenz lens , one obtains a direct lenz lens structure . if the deposition step produces thin metal layers , then the layer thickness can be increased for example using electroless or conventional galvanic electro - deposition . sputter and lithography . by sputtering a metal film onto a substrate , and then forming a lithographic mask using a photoresist material on top which contains openings in the inverse shape of the lenz lens , and then etching the metal in those areas where the photoresist does not cover the metal , it is possible to obtain one or more micro - or nano - structured lenz lenses . milling . a metal film of sufficient thickness can be permanently fixed to a flexible or rigid substrate . subsequently , the metal film can be milled so as to remove the inverse shape of the lenz lens , but avoiding to mill away the substrate material . printed circuit board ( pcb ) manufacturing . printed circuit board manufacturing of sufficient precision is very well suited for manufacturing lenz lenses , especially if the pcb substrate material is a low - loss material as is available for the manufacture of radiofrequency components and circuits . laser machining . a metal film of sufficient thickness can be permanently fixed to a flexible or rigid substrate . by using a laser plotter with precise control over the position , size , strength , pulse frequency , and wavelength of the laser beam , it is possible to remove part of the metal film without affecting the underlying substrate . if the cut away shape is the inverse shape of a lenz lens , a metallic lenz lens is thereby formed . additionally , a lenz lens may be cut out of flexible metal foil where , prior to cutting , an adhesive layer or tape has been applied on one side of the metal foil . thereby , a lenz lens may be easily attached to arbitrary samples or objects . it is possible to model analytically the induced current within the lenz lens by calculating the electromotive force that is induced by an external field and matching this to the electric potential across the equivalent rl - circuit of the lens . for example , for a single lens placed half - way between and coaxial with two helmholtz coil loops , the induced current in the lens , i l , can be calculated as follows : where ω is the angular frequency , i h is the current in the helmholtz loops , r l is the resistance of the lens , l s and l b are the self - inductances of the inner ( s — small ) and outer ( b — big ) loops of the lens , and m bh , m sh and m sb are the mutual inductances between different combinations of the various loops ( as denoted by the subscripts ). note that for high - frequency applications , the skin effect should be considered in the calculation of the resistance and self - inductance of the lens elements . it is straightforward to extend the model to consider multiple lenses by introducing additional mutual inductance terms and considering each loop of the helmholtz pair separately ( h 1 and h 2 ). note that the choice of using helmholtz loops as the source of the external field has been made for demonstrative purposes only and the model described herein is also applicable for any external field , regardless of its source . the induced currents for all the lenses in the system ( contained within vector i l ) can be obtained by solving a matrix equation of the form i l = h , where the elements of matrix and vector h are given by : kk = r lk − i ω ( l sk + l bk − 2 m skbk ) ( 3 . 2 ) kj =− i ω ( m bkbj − m skbj − m bksj + m sksj ) ( 3 . 3 ) h k = i ω ( m bkh1 + m bkh2 − m skh1 − m skh2 ) i h , ( 3 . 3 ) and k = 1 : k , j = 1 : k and j ≠ k , where k is the number of lenses . note , for example , that m skbj is the mutual inductance between the inner loop of the k th lens and the outer loop of the j th lens . this model can be used to predict the behaviour of the lens ( es ) and also to permit optimization of the geometry and placement of the lens ( es ), with respect to maximizing the gain in field sensitivity and / or other properties such as field homogeneity . alternatively , it is also possible to model the system using finite element techniques . schoenmaker , j ., k . r . pirota , and j . c . teixeira : magnetic flux amplification by lenz lenses . review of scientific instruments , 84 ( 8 ), 2013 . http :// dx . doi . org / 10 . 1063 / 1 . 4819234 . webb , a . g . : radiofrequency microcoils in magnetic resonance , progress in nuclear magnetic resonance spectroscopy , 31 ( 1 ): 1 - 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