Magnetic Metamaterials For MRI Applications

The main distinguishing feature of metamaterials was purposely engineered material to achieve electromagnetic properties that were not available in natural materials.There were roughly two broad categories of metamaterials namely resonant type and non resonant type.Resonant type metamaterials contained left handed(LH)metamaterials which could be attributed as refractive index less than one metamaterials.In resonant metamaterials,the periodicity of evanescent wave was too small to scatter between two adjacent metallic conductor elements.So it's not an evanescent wave scattering phenomena as evanescent wave itself was not resonant but there were metallic elements in the propagating path of evanescent waves,in which evanescent wave induced oscillating currents and finally metallic elements were self resonant which re-radiated little tiny secondary wave.Tiny secondary wave produced by conductor elements encountered out of phase with applied evanescent wave and we got some kind of interesting response.This yielded in refurbishment of applied evanescent wave.The periodicity of revamped evanescent wave was 10th of wavelength of original applied evanescent wave which resulted in emulation of atomic scale resonance.Non resonant type metamaterials contained anisotropic,i.e hyperbolic metamaterials.In non resonant metamaterials,the period infect was much less then wavelength,i.e;a millionth of a wavelength {photonic crystals} and behaved exactly as a period of 10th of wavelength.For non resonant metamaterials,cut off frequency was that frequency at which,the material became resonant.No currents oscillated neither secondary wave were produced in non resonant metamaterials as compare to resonant metamaterials.Non resonant metamaterials derived their properties simply due to applied power supply techniques.The theme to design metamaterials was to obtain the properties which were indirectly related to materials from which the metamaterials were made from,and the method that how they were structured {i.e;metal rings,bars,photonic crystals,etc},which provided different values of material parameters depending upon the structure of design.The significant characteristic of non resonant metamaterial was its potential application for broadband technology as its period was millionth or billionth of wavelength.In result we could fabricated structure to refurbish the applied evanescent wave for the applications operating at very high frequency.Moreover non-resonant metamaterials were robust with great structural tolerance.however,the drawback for non resonant metamaterials were their deficiency in exhibiting magical properties like left handed,negative refractive index,zero epsilon/Mu metamaterials which were the main important ingredients of resonant metamaterials.To design metamaterial for some assigned application,we needed to find and fabricate a material by knowing their dielectric and magnetic responses.In every material,an electron cloud bounded to a nucleus with some bound charge.Without applying an electric field,the bounded charge of nucleus remained neutral.As soon as we applied an electric field to a material then its electron cloud stretched and displaced which in resulted in polarization of atoms in a material and finally material started storing electric energy.If we released electric energy then electron cloud reverted to their neutral position but they bounced back&forth like a mass on a spring.In consequence,the oscillating electron charge radiated a tiny secondary wave which then combined out of phase with the applied wave and produced an overall slowing response due to the interference because of collisions of millions or billions of atoms.This effects was what we called the dielectric response of the material.Similarly we could have a magnetic response of the material.Here the electron motion is restricted in a way that it tended to circulate like a planet orbiting a Sun.We know that the circulating charges induced a magnetic field at the center of that circulation,where we could got a little magnetic dipole.Well,if we applied a magnetic field to that magnetic dipole then in result,those magnetic dipoles aligned themselves in the direction of applied magnetic field.That resulted in the storing the magnetic energy in that material.Similarly like electric field.If we released the magnetic field then aligned magnetic dipoles got scattered again and tilted themselves in different directions finally became resonant.In addition those magnetic dipoles while reverting back,oscillates back&forth and produced secondary waves which combined out of phase with the applied magnetic field and produced an overall slowing response which we called a magnetic response of a material.From all above discussion regarding material's dielectric and magnetic response,metamaterial were then designed and fabricated by the analysis of Lorentz oscillator model.The model stated the equation of motion which resulted in dielectric and magnetic properties of material in the form of Lorentz parameters.Those parameters included plasma frequencies of a material {switching frequency},damping factor which was responsible of signal{applied wave} loss and resonant frequency {restoring force}.Then those parameters were plotted to retrieve the real&imaginary parts of relative permittivity&permeability of the material and then analyzed the Lorentz response of their resonant behavior at required working frequency.It was observed that restoring force of material restored the magnitude of incoming evanescent wave while it passed through it.Then results were verified by exploiting Drude Model in which restoring force was omitted as mentioned in Lorentz oscillator model which resulted in the free movement of electrons in the material finally gave very negative dielectric constant below the plasma frequency.In consequence,very lossy imaginary part yielded in un-restored magnitude of incoming evanescent wave while it passed through the material in the absence of restoring force.Study of human pathologies and acquisition of anatomical images without any surgical intervention inside human body was possible because of magnetic resonance imaging(MRI),which was the keystone technique to characterize the psychology and neurochemistry of human body.However for clinical trials,the study and cure of human diseases were followed by medical investigations of different animal anatomies.By employing different imaging techniques to animal anatomical models during their clinical trials yielded in exceptional image acquisition without any surgical invasion in the model,which resulted in noninvasive technique as compared to surgical invasion and opened the possibility to study human pathologies more precisely.The principle of function in MRI scanners was the induction of magnetic Lorentz force which was defined as static magnetic field(B0)on the current carrying gradient coils due to applied electric field.At Larmor frequency,which was the frequency at which the proton of an atom perturbed from its neutral state and resulted in absorption of energy by the nucleus of the of proton contained atoms in the body.Absorption of energy depended upon the strength of external oscillating static magnetic field(B0)by MRI scanner and gyro-magnetic ratio.The gyro-magnetic ratio was the ratio of static magnetic field(B0)strength in Tesla(T)of the MRI scanner and frequency of angular momentum {spin} of proton particle of an atom inside body.The spin of the proton was defined in two states namely longitudinal magnetization in which,the proton was polarized due to applied field(B0)finally regarded as enthalpy of a spin(T1).The other form of spin was due to generation of transient transverse magnetic field(B1)at the receiver of MRI scanned coils due to switched off of(B0)which reversed the proton's spin in 180°out of phase and finally regarded as entropy of a spin(T2).Furthermore,the imaging capability of MRI scanner was rated and analyzed according to its slew rate,rise time,flat top,field of view,spin packet,echo spacing,echo time,reception time and GRE sequence.One of the application areas for metamaterials is that of MRI.Theoretically and experimentally proved that metamaterials when used with MRI systems,improved image resolution,image acquisition time and image quality inside the human body.Metamaterial lenses for MRI imaging have been designed using e.g.,capacitively loaded split ring resonators(CLSRR),edge coupled/broadside coupled split ring resonators(EC/BC-SRR),magneto-inductive(MI)wave guides,metallic arrays metamaterials for endoscopy.The main idea was to enhance the magnetic field intensity and SNR at region of interest(ROI)inside the body.It was depended upon B1 which was mainly governed by transmit/receive MR coils of MRI system.B1 perturb ROI's protons from equilibrium which yielded in image acquisition at receiver MR coil.Scientists introduced surface coils/volume coils,complex disposition of resonators and multi transmit/receive antenna arrays in a way to reduced SAR(specific absorption ratio),signal losses and increase of homogeneity.But the proposed ideas were much complex in design and costly solution.Our desgned metamaterials for MRI scanners are defined in below mentioned projects.(a)Notable properties of compact and thin metamaterial(MM)lens that could be use to enhance image quality of 0.35-T MRI systems.The lens structure was based on a three-dimensional array of split-ring resonators(SRRs)having a thickness of only 2.5 mm corresponding the side length of the structure.By including parametric elements(capacitors,inductors)with SRRs,the required resonance at very low working frequency for 0.35-T MRI of 14.88 MHz was achieved.It concluded in the resonant enhancement of magnetic field(B)and SNR(signal-to-noise ratio)inside body(phantom)and improved the efficiency of MR(magnetic resonance)coil.(b)Notable properties of zero permeability(0 ?)split ring resonators(SRR)metamaterial(MM)magnetic reflector which could distort and reject uniform RF(radio frequency)magnetic field for 1.5-T MRI systems.The design was composed of PCB(printed circuit board)slabs containing parametric elements(capacitors,inductors)and attached with SRRs with total compact thickness of 5 mm only.Furthermore the reflector,when used with MRI scanner at optimized position uniformly redistribute and enhance the magnetic field while keeping low specific absorption ratio(SAR),low electric field and power dissipation,by improving SNR(signal to noise ratio)at the scanned region of human body which in result prevented from tissue heating of human body and significantly minimizes the damaging effects of RF energy absorption in human tissue.(c)Notable properties of compact/thin broadside coupled(BC)split ring resonator(SRR)metamaterial(MM)lens that could be use to enhance image quality of 1.5-T MRI systems.We analyzed two strongly coupled BC-SRR copper arrays attached on PCB(printed circuit board)and loaded with parametric elements(capacitor&inductor).The significance of design was its compact thickness of 3.2 mm,its tunability at different working frequencies due to parametric elements and it placed no restrictions on MR coil designing as proposed in previous work.The technique combining parametric elements,copper loops and SRRs had not been used before for such lower working frequency.In addition the designed lens improved the mutual inductance between BC-SRR arrays and finally restored the amplitude of magnetic field(B)at the considered area e.g.phantom and enhances the image quality.(d)Notable properties of unique combination of multi-circular hybridized surface coils which could be used as hybridized magnetic metamaterial hat(HMMH).HMMH not only strengthened the uniformity of radio frequency(RF)rotational symmetry around its axis but also improved the signal-to noise ratio(SNR)for rat's brain imaging at 7-T MRI.We analyzed a periodic array of strongly coupled circular copper coils attached on circular coil shaped printed circuit board(PCB)substrate.In the design,some copper coils were inspired by the slot cavity loaded with parametric elements(capacitor&inductor).In addition,coils in the form of HMMH exploited the advantages of the hybrid modes which exhibited better and deeper RF sensitivity into the region of interest(ROI)as compared to single loop RF coil by exciting two eigen modes simultaneously which resulted in homogenized magnetic field(B-field)and enhanced SNR at ROI.(e)Notable properties of unique combination of multi-circular hybridized surface copper coils as tunable hybridized magnetic metamaterial hat(THMMH).In THMMH,some of coils were inspired by the slot cavity loaded with parametric elements(capacitor)and parallel merger of sinusoidal steady state current source(Is),which was applied externally to THMMH.We introduced the significance of IS,as it changed the balance of forces at nanoscale by engaging Ampere's forces into the design's circuit thus changed its optical properties and finally yielded in tunable,reconfigurable magnetic metamaterial hat.Furthermore,we performed the efficiency comparison between THMMH and un-tunable HMMH on rat's brain imaging at 300 MHz for 7-T MRI applications.We observed that THMMH not only strengthened the uniformity of radio frequency(RF)rotational symmetry around its axis but also exploited the advantages of the hybrid modes which exhibited better and deeper RF sensitivity into the region of interest(ROI)at rat's brain by exciting two eigen modes simultaneously which resulted in homogenized magnetic field(B-field)and enhanced signal-to-noise ratio(SNR)at ROI in comparison with un-tunable HMMH.But we believe that there is still much hidden potential which should be uncovered regarding metamaterials for MRI applications.