Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
  • G01R33/34046—Volume type coils, e.g. bird-cage coils; Quadrature bird-cage coils; Circularly polarised coils
  • G01R33/34076—Birdcage coils
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
  • G01R33/34007—Manufacture of RF coils, e.g. using printed circuit board technology; additional hardware for providing mechanical support to the RF coil assembly or to part thereof, e.g. a support for moving the coil assembly relative to the remainder of the MR system
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
  • G01R33/3607—RF waveform generators, e.g. frequency generators, amplitude-, frequency- or phase modulators or shifters, pulse programmers, digital to analog converters for the RF signal, means for filtering or attenuating of the RF signal
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
  • G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
  • G01R33/3642—Mutual coupling or decoupling of multiple coils, e.g. decoupling of a receive coil from a transmission coil, or intentional coupling of RF coils, e.g. for RF magnetic field amplification
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
  • G01R33/5659—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the RF magnetic field, e.g. spatial inhomogeneities of the RF magnetic field

Definitions

• the present invention relates to Nuclear Magnetic Resonance (NMR) devices, as well as their applications such as Magnetic Resonance Imaging (MRI) for humans or animals, and Magnetic Resonance Spectroscopy (MRS).
• NMR Nuclear Magnetic Resonance
• MRI Magnetic Resonance Imaging
• MRS Magnetic Resonance Spectroscopy
• the invention is particularly applicable to high and ultra high frequency antennas at least one transmission channel may or may not be used for receiving the signal.
• Such antennas are used for the examination of part or all of the body of a patient in NMR devices and in particular in MRI imaging devices. These devices have the function of exciting the magnetic spins of certain atoms, for example the hydrogen atoms, of the sample placed inside the antenna at their Larmor frequency, and of collecting a radiofrequency signal resulting from 'a phenomenon of relaxation.
• the MRI apparatus comprises a magnet producing a static longitudinal magnetic field B0, and antennas having radiating elements, of variable shapes, providing either a transmitter function or a relaxation signal receiver function, or both functions. alternately. These antennas are arranged around the part of the body to be analyzed. In transmitter operation, these antennas receive an electrical excitation enabling them to produce a radiofrequency (RF) electromagnetic field having a transverse magnetic component B1, orthogonal to the B0 field. In receiver operation, they pick up an RF signal having the frequency of precession resonance or relaxation (also called Larmor frequency) of the nuclei of the atoms in the B0 magnetic field and having been momentarily subjected to the magnetic field B1.
• RF radiofrequency
• the magnetic spin moments of the nuclei of hydrogen atoms align in a direction parallel to the magnetic field B0.
• the magnetic spin moments of the nuclei of atoms deviate Gradually from the direction of the BO field to reach a flip angle noted FA (Flip Angle) with respect to this direction by describing a movement called "precession".
• the radiofrequency field B1 thus makes it possible to "switch" the magnetic spin moments of an angle FA with respect to the direction of the field BO.
• the antennas used for the examination of a part of the body and in particular of the head operate in close magnetic field, and therefore must be placed near the part of the body to be analyzed. It turns out that in this type of antenna, the part of the body placed near the antenna feedbacks on the radiofrequency field near the antenna.
• the human head has electromagnetic characteristics that can generate artifacts.
• the magnetic field intensities BO used in medical imaging are between 0.1 and 3 Tesla.
• the wavelength associated with the B1 field corresponding to the Larmor frequency for hydrogen remains large compared to the region to be analyzed. Artifacts tend to appear when this condition is no longer true, that is, when the region to be analyzed has large dimensions or when the wavelength associated with the B1 field is decreased.
• the corresponding Larmor frequency for hydrogen is 64 MHz, ie a wavelength of approximately 53 cm in water.
• this frequency is 128 MHz, ie wavelength of about 26 cm in the water.
• the frequency of Larmor reaches about 300 MHz, which corresponds to a wavelength of about 1 1 cm in the water.
• the antennas used have volumic resonant cavity type structures. This type of antenna, commonly called “bird cage” or TEM (Transverse Electric and Magnetic) includes:
• RF ports Two or four RF power ports (called “RF ports") interposed between all the transmission lines and the shield, to produce the radio frequency excitation of the resonant cavity and detect the NMR radio frequency signals.
• a shield may surround the set of transmission lines.
• a voluminal antenna separates internal volume from an external volume.
• the known voluminal antennas have the disadvantage of not functioning properly at high magnetic field values, typically above 3 Tesia, corresponding to a Larmor frequency of the order of 128 MHz, for cerebral imaging. Indeed, up to 3 Tesia, the corresponding Larmor frequency remains low and the defects of homogeneity of the field B1 in the object to be analyzed remain tolerable, which makes it possible to obtain an exploitable image of the studied region.
• artifacts can appear at these magnetic field values with large organs such as the pelvis.
• Network antennas have been developed for frequencies above 128 MHz. These antennas comprise a plurality of resonators, generally between 8 and 32, used as emitters and receivers, and which are distributed around the region to be analyzed. Each resonator comprises a specific control channel for transmitting and receiving the radiofrequency signal. Each resonator thus makes it possible to produce an image of the area opposite which it is located. The different images are then combined by algorithms to form the final image.
• This type of network antenna requires controlling each resonator by its own channel with an appropriate amplitude and phase, by means of a power amplifier for spatially controlling the excitation of protons in MRI around the region to be analyzed.
• This type of antenna associated with an active control of the homogeneity of the field B1 requires adjustments of tuning and impedance matching of each channel, which are difficult to achieve. The structure of these antennas and their use are complex, which entails high costs of installation and use.
• Each cushion makes it possible to modify the distribution of the field in the antenna.
• the materials used may be based on titanium oxide powder, such as CaTiO3 (ref [1]) or BaTiO3 (ref [4]), mixed in deionized water or with a high proportion of deuterium . It has also been proposed to use blocks of PZT (lead titanate and zirconium) (ref. [5]).
• PZT lead titanate and zirconium
• none of these materials is really satisfactory in the context of medical use. To have a sufficient effect on the distribution of the field, they must have a bulky volume and be placed against the body of the patient. The comfort of the patient is affected. Moreover, these materials generally have a relatively high cost and age rapidly. In addition, some materials like BaTiO3 are very toxic.
• a field B1 in a region of an object to be analyzed which has a homogeneous distribution using a conventional volumetric antenna, that is to say without active resonators individually controlled. It is also desirable to be able to adjust the distribution of the field in the analyzed region, depending on the nature of the latter, in order to obtain a homogeneous distribution of the field B1, or on the contrary, in order to avoid the presence of the B1 field in certain areas of the region to be analyzed. It is desirable to achieve this result without having to use a high dielectric constant material. It is also desirable to avoid having to directly contact a material against the body of the patient to be analyzed.
• Embodiments provide a method for controlling the distribution of a radio frequency magnetic field in a nuclear magnetic resonance imaging system, comprising the steps of: providing a volumic antenna in a permanent magnet providing a permanent magnetic field in accordance with a first axis, and feed the volume antenna by a radiofrequency signal, so that it generates a radiofrequency magnetic field rotating in a plane perpendicular to the first axis.
• the method comprises a step of placing an electromagnetic resonator having a resonance mode excited by the rotating magnetic field, the resonator being disposed at a position inside or outside the volumic antenna and at a distance from a region to be analyzed of an object to be arranged in the voluminal antenna, the resonance mode and the position of the resonator with respect to the volumetric antenna being adapted to adjust the intensity of the rotating magnetic field in a area of the region to be analyzed.
• the resonator resonance mode is adapted by modifying the structure, the geometry or a dimension of the resonator, or the nature of materials forming the resonator.
• the resonator is shaped to present several resonance modes.
• one of the following configurations is present: the resonator is formed by a single rod arranged parallel to the first axis, the rod being rectilinear or folded so as to form meanders, or folded so as to form a rectangular turn , and the resonator comprises a plurality of parallel conductive rods, interconnected in a matrix configuration, the rods being arranged parallel to the first axis and electromagnetically coupled together.
• the resonance mode of the resonator is adapted by adjusting the length of the resonator along the first axis or by modifying the matrix configuration of the rods.
• the resonator is disposed in front of a power supply port of the antenna and shaped to increase or decrease the locally rotating magnetic field in an area of the region to be analyzed close to the resonator or in an area of the region. to be analyzed located opposite the resonator with respect to a center of the region to be analyzed.
• the method comprises steps of placing a plurality of electromagnetic resonators each having an own resonance mode excited by the rotating magnetic field, at a respective position inside or outside the volumetric antenna and remote from the region to be analyzed, the resonance mode and the position of each of the resonators being adapted to adjust the intensity of the rotating magnetic field in an area of the region to be analyzed.
• Embodiments may also relate to an antenna system for a nuclear magnetic resonance imaging system, comprising: a voluminal antenna including a port for receiving a radio frequency signal to generate a radiofrequency magnetic field within the antenna turning in a plane.
• the antenna system comprises an electromagnetic resonator having a resonance mode excited by the rotating magnetic field, the resonator being disposed at a position inside or outside the voluminal antenna and at distance of a region to be analyzed from an object disposed in the volumetric antenna, the resonance mode and the position of the resonator relative to the volumetric antenna being adapted to adjust the intensity of the magnetic field rotating in an area of the region to be analyzed.
• the resonator has one of the following configurations: the resonator has a periodic structure formed of a juxtaposition of elementary cells, each elementary cell comprising at least two distinct materials, the resonator comprises several parallel conductive rods, distributed in an n ⁇ n matrix configuration, where n is an integer greater than 0, and embedded in a dielectric material, the resonator being disposed inside or outside the antenna so that the rods are perpendicular to the plane the resonator is formed by a single rod disposed perpendicularly to the plane, the rod being rectilinear or folded so as to form meanders, or folded on itself so as to form an elongate coil extending perpendicularly to the plane.
• the antenna system comprises a plurality of resonators disposed inside or outside the antenna perpendicular to the plane.
• the resonator comprises 2 x 2 rods, the rods having a diameter of between 0.2 and 1.2 mm, and spaced apart by 1 to 3 cm, the resonator being disposed more than 2 cm from the region to be analyzed.
• the resonator is configured to present a resonance mode centered on a frequency of the rotating magnetic field.
• the voluminal antenna is of the high-pass birdcage type comprising 16 bars connecting two rings to each other, each ring portion of the two rings, between two bars, comprising a capacitor.
• Embodiments may also relate to a nuclear magnetic resonance imaging system, comprising a voluminal antenna as defined above, disposed in a permanent magnet providing a permanent magnetic field along an axis perpendicular to the plane.
• the permanent magnetic field produced by the permanent magnet is 7 T
• the voluminal antenna comprising bars interconnecting two rings, each ring portion of the two rings, between two bars, comprising a capacitor, the bars having a length of between 23 and 27 cm, the rings of the voluminal antenna having a diameter of between 24 and 28 cm, and capacitors having a capacitance of between 2 and 6 pF.
• FIG. 1 schematically represents an MRI apparatus
• FIG. 2 is a diagrammatic perspective view of an example of a bird cage type volumic antenna
• FIG. 3 diagrammatically represents the axial aerial in axial view, according to one embodiment
• FIG. 3A represents a detail in axial section of a resonator placed in the voluminal antenna, according to one embodiment
• FIG. 4 diagrammatically shows the voluminal antenna in axial section along a vertical plane, according to one embodiment
• FIGS. 5A to 5E are images in axial section along a vertical plane of the distribution of the field B1 in an object to be analyzed placed in the voluminal antenna
• FIG. 6A represents curves of variation of the intensity of the magnetic field B1 along a line OY, with and without a resonator, inside the antenna,
• FIG. 6B represents a variation curve of the relative difference between the two curves of FIG. 6A
• FIG. 7A represents curves of variation of the intensity of the magnetic field B1 along a line OZ inside the antenna, with and without resonator,
• FIG. 7B represents a variation curve of the relative difference between the two curves of FIG. 7A
• FIGS. 8A, 8B are cross-sectional images along a vertical plane of the distribution of the field B1 in the voluminal antenna, in the presence of an object to be analyzed placed in the latter, respectively without and with a resonator according to another mode.
• FIGS. 9A, 9B show schematically the voluminal antenna, respectively in axial view and in axial section along a vertical plane, associated with a resonator according to another embodiment,
• FIG. 10 is a cross-sectional image along a vertical plane of the distribution of the field B1 in the voluminal antenna, in the presence of an object to be analyzed placed in the latter, and the resonator of FIGS. 9A, 9B,
• FIGS. 11A, 1B show diagrammatically the voluminal antenna, respectively in axial view and in axial section along a vertical plane, associated with a resonator according to another embodiment
• FIG. 12 is an image in axial section along a vertical plane of the distribution of the field B1 in the voluminal antenna, in the presence of an object to be analyzed placed in the latter, and of the resonator of FIGS. 1 1 A, 1 1 B
• FIG. 13 is a cross-sectional image along a vertical plane of the distribution of the field B1 in the voluminal antenna, in the presence of an object to be analyzed placed in the latter, with the resonator of FIG. 2A placed at the Outside of the antenna, according to another embodiment
• FIGS. 14 and 15 show schematically the voluminal antenna, in axial view, associated with several resonators according to another embodiment.
• FIG. 1 represents an MRI apparatus 10.
• the MRI apparatus comprises a magnet 11 which may be of cylindrical shape, in which a patient to be analyzed is placed.
• the magnet 1 1 has a Z axis which is generally oriented horizontally.
• the magnet 1 1 comprises a coil 12 which generates inside the magnet 1 1 a longitudinal magnetic field B0, oriented along the Z axis.
• the MRI apparatus also comprises a voluminal type antenna 1 arranged at the center of the magnet 1 1. inside the magnet 1 1, around a region to be analyzed of an object such as the body of the patient.
• the antenna 1 is configured to generate an oscillating or rotating magnetic field, of radiofrequency (RF) type, which is transverse in an XY plane perpendicular to the Z axis.
• RF radiofrequency
• the antenna 1 is connected to circuits of FIG. TRX radiofrequency transmission / reception which provide the antenna 1 with an RF signal enabling it to generate the field B1.
• the TRX circuits receive from the antenna 1 or another antenna (not shown) nuclear magnetic resonance (NMR) signals that can be used to generate images.
• the device IRM 10 also includes a power supply circuit PS for supplying the coil 12, and a processing unit DPU which controls the circuits TRX and PS, and which receives the NMR signals provided by the circuit TRX.
• the DPU processing unit processes the NMR signals to generate images that can be displayed on a DSP display screen.
• FIGS. 2 to 4 show an example of a high-pass "bird cage" voluminal antenna 1 used in the MRI apparatus.
• the PA region to be analyzed of an object for example the head of a patient, is disposed in the antenna 1.
• the antenna 1 has a generally cylindrical shape having a center point O.
• the antenna 1 comprises axial bars 2 extending along the Z axis between two rings 3, 3 'located in a plane parallel to the XY plane.
• the bars 2 are uniformly distributed around the PA region.
• Each part of the rings 3, 3 'between two bars 2 comprises a capacitor C making it possible to couple together two of the bars 2.
• the antenna 1 is powered by two or more ports P1, P2 through which the circuit TRX supplies the RF signal .
• P1, P2 the circuit TRX supplies the RF signal .
• the ports P1, P2 are disposed on one of the rings 3, the port 1 being disposed on the axis PX, and the port P2 being disposed on the axis PY (the point P being in the center of the ring 3), so as to feed the antenna in quadrature.
• the antenna comprises 16 bars 2, and therefore 32 capacitors C.
• one or more electromagnetic resonators are disposed inside or outside of the antenna 1, to adjust the distribution of the field B1 in the region to be analyzed PA, the resonators being electrically isolated from the antenna.
• each resonator is configured to present an own resonance mode excited by the field B1 generated by the antenna 1.
• each resonator behaves like a passive secondary antenna, under the effect of the field B1.
• Each EMR resonator is disposed at a minimum distance h from the object to be analyzed.
• the structure, the geometry and the position of each EMR resonator with respect to the antenna 1 are chosen according to the effect to be obtained on the distribution of the field B1 in the antenna 1, and therefore as a function of the frequency of the field B1.
• the effect obtained on the distribution of the field B1 can be a uniformization of the field B1 in the region to be analyzed, an increase or a decrease of the field B1 located in a zone to proximity of the EMR resonator, or an increase or decrease in the field located in an area opposite the resonator with respect to the center O of the region to be analyzed or of the antenna 1. This last effect allowing a remote action on the distribution of the field B1, is particularly useful when the space available near the area where to act is insufficient to house a resonator.
• Each EMR resonator may for example be made of a metamaterial, and thus have a periodic structure formed by the juxtaposition of elementary cells, also called "meta-atoms".
• Each elementary cell consists of one or more materials, and has small dimensions relative to the wavelength of the field B1. This condition is considered realized if the dimensions of the elementary cells are less than or equal to 50% of the wavelength of the field B1.
• a single EMR resonator is disposed in the antenna 1.
• Figure 3A shows a section of the EMR resonator.
• the EMR resonator comprises four rods T of circular section, oriented along the Z axis, and arranged at the vertices of a square, each rod T being disposed at a distance d from two others of the four rods T.
• Each rod T can thus be considered as an electric dipole forming a resonator electromagnetically coupled with the other resonators each formed by one of the other rods T.
• the rods are distant from each other a distance of less than half the wavelength of the B1 field in the medium of the object to be analyzed (usually water). When the field B0 is 7 T, this wavelength is of the order of 10 cm in the water.
• the EMR resonator is disposed in the antenna 1 opposite the port P1 (between the port P1 and the PA region to be analyzed).
• the resonator comprises 4 conductive T rods, for example copper, and embedded in a dielectric material, such as polystyrene.
• the rods T may have a diameter of between 0.3 and 1.5 mm, and a length of between 30 and 90 cm, depending on the desired effect on the distribution of the B1 field.
• the distance d between the rods T can be between 1 and 3 cm.
• the distance h is between 1.5 cm and 15 cm.
• the antenna is adapted to receive a human head.
• the distance d between the rods represents for example 3 to 13% of the inside diameter of the antenna.
• FIGS. 5A to 5E Simulations of the distribution of the B1 field were made by placing in the antenna 1 an object simulating a human head and having similar electromagnetic properties and placing to the right of the simulated head along the Z axis.
• a four-stage EMR resonator stems having the section described above, and having different lengths. These measurements (in dB) are presented in FIGS. 5A to 5E.
• the measurements of FIG. 5A were obtained without resonator.
• the measurements of FIGS. 5B to 5E were obtained with a resonator of length 40, 50, 60 and 80 cm, respectively. These measurements show that the length of the EMR resonator (along the Z axis) influences the distribution of the B1 field in the region to be analyzed PA.
• the EMR resonator has a resonance mode that depends on its geometry and in particular on its length.
• the distribution of the field B1 is not significantly modified when the length of the EMR resonator is less than 30 cm.
• the length of the EMR resonator is between 35 and 45 cm (FIG. 5B)
• an area of the region PA to be analyzed close to the EMR resonator has an increased field value B1.
• a resonance mode of the EMR resonator is excited by the field B1.
• the length of the EMR resonator is between 45 and 55 cm (FIG.
• the position of the EMR resonator with respect to the antenna 1 has an influence on the distribution of the field B1 in the antenna.
• the distribution of the B1 field can also be modified by changing the length of the resonator (along the Z axis). However, this change in length modifies the resonance mode (s) of the EMR resonator.
• This resonance mode can also be modified by changing the matrix configuration of the rods T (for example by changing the spacing of the rods along the X axis and / or the Y axis), or by changing the diameter of the rods T. or by changing the materials forming the EMR resonator.
• FIG. 6A shows two curves C1, C2 of variation of the intensity of the field B1, along the straight line OX, respectively without and with the resonator EMR in front of the port P1, the resonator having a length of 40 cm.
• the curve C2 has been shifted upwardly so as to compensate for the decrease in the intensity of the field B1 in the antenna 1, due to the presence of the EMR resonator in front of the port P1.
• FIG. 6B represents a curve C3 obtained by calculating a relative percentage difference between the curves C1 and C2.
• Curve C3 shows that the resonator modifies the distribution of the field B1, by increasing it in certain zones by 10%, and up to 22% in the vicinity of the EMR resonator, in the case where the power loss of the field B1, due at the presence of the resonator has been compensated.
• FIG. 7A represents two curves C4, C5 of variations of the field B1 along the straight line OZ.
• FIG. 7B represents a curve C6 of variation of a relative percentage difference between the curves C4 and C5.
• the comparison of the curves C4 and C5 makes it possible to observe that the resonator EMR in front of the port P1 modifies the distribution of the field B1 (+ or - 10%) without increasing it in a central zone where it presents locally a maximum value.
• the presence of one or more EMR resonators in or around the antenna 1 does not significantly increase the specific absorption rate SAR (Specification Absorption Rate), representative of the transmission of energy at the region to be analyzed PA, even if one compensates for a possible loss of power in the region to be analyzed, resulting from the presence of the EMR resonator in or around the antenna 1.
• SAR Specific Absorption Rate
• the resonator is disposed between the shield and the antenna or inside the antenna. The position of each resonator is determined according to the areas where the field B1 is to be decreased or increased.
• the present invention is capable of various alternative embodiments and various applications.
• the invention also applies to a voluminal antenna of the "bird cage" low-pass type, that is to say in which the capacitors C are arranged not on the rings 3, 3 'but on the Bars 2.
• the invention also applies to a band-pass antenna in which capacitors are arranged both on the rings 3, 3 'and on the bars 2.
• the resonator may have other shapes and be realized in various other ways.
• the resonator can be realized by example by etching a conductive layer deposited on a wafer in an insulating material.
• the resonator may be in the form of a U-ring or split ring resonator (SRR), an omega, a Jerusalem cross, platelets, or more of these coupled elements.
• SRR split ring resonator
• FIG. 8A represents an image of the distribution in the voluminal antenna 1 of the component of the measured field B1 to form MRI images, in the presence of an object to be analyzed PA placed in the latter, in the absence of a resonator.
• Figure 8A highlights regions of more intense field inside the antenna 1 along the bars of the antenna and decreasing closer to the object to be analyzed. It can also be observed the presence of a relatively intense field in a central region of the object to be analyzed PA and decreasing towards the periphery of the object, with a slight increase in the vicinity of the periphery of the object, inside and outside the latter
• FIG. 8B represents an image of the distribution of the field B1 in the voluminal antenna 1, in the presence of an object to be analyzed PA placed in the latter, the voluminal antenna being associated with a resonator consisting of a single rod. Straight T placed axially in the antenna.
• FIG. 8A showing the distribution of the field B1 in the volumetric antenna 1, without a resonator
• FIG. 8B shows a local increase of the field B1 around the rod T inside the antenna, and in a central region of the object to be analyzed PA.
• the field is measured in Vs / m 2 .
• the rod has a diameter of 1 mm (+/- 20%) and a length equal to twice the length of the antenna (+/- 20%).
• FIGS. 9A, 9B show the voluminal antenna 1 associated with an ER1 resonator of the SRR type, in the form of a split ring, having a rectangular shape, the ends of the split ring being folded towards the inside of the ring for form a gap.
• the resonator ER1 is disposed in the antenna 1 in the OXZ plane, the longest sides of the resonator being oriented along the Z axis of the antenna.
• the resonator is disposed at a distance h from the object to be analyzed PA.
• the resonator ER1 is split substantially in the middle of one of the two long sides of the rectangular shape, for example the one furthest away from the object PA to be analyzed.
• the resonator may also be split substantially in the middle of the two long sides of the rectangular resonator.
• FIG. 10 represents an image of the distribution in the voluminal antenna 1 of the component of the field B1 (in Vs / m 2 ) measured to form MRI images, in the presence of an object to be analyzed PA placed in the latter, the resonator ER1 being placed in the volumetric antenna 1 as shown in FIGS. 9A, 9B.
• FIG. 8A showing the distribution of the field B1 in the voluminal antenna 1, without a resonator
• FIG. 10 shows an increase of the field B1 in the right half of the antenna (in the figure), the resonator ER1 being located in the plane delimiting the right and left parts of the antenna 1.
• the resonator ER1 is produced using a conducting wire having a diameter of 1 mm (within + or -10%) or a conductive track formed on an insulating substrate having a width of 1 mm (within + or - 20%).
• the ring formed by the resonator ER1 has a width of 3% (within + or -20%) of the diameter of the antenna 1, and a length equal to 90% of the length of the antenna (at + or - 20%).
• FIGS. 11A, 11B show the voluminal antenna 1 associated with a rod-shaped resonator ER2 describing meanders or crenellations.
• the resonator ER2 is disposed in the antenna 1 in the OXZ plane, the longest sides of the resonator being oriented along the Z axis of the antenna.
• the resonator is disposed at a distance h from the object to be analyzed PA.
• FIG. 12 represents an image of the distribution in the voluminal antenna 1 of the component of the field B1 (in Vs / m 2 ) measured to form MRI images, in the presence of an object to be analyzed PA placed in the latter, the voluminal antenna being associated with the resonator ER2.
• FIG. 8A shows the distribution of the field B1 in the volumetric antenna 1, without a resonator
• the resonator ER2 shows a local increase of the field B1 between the resonator ER2 and the part of the nearest antenna of the resonator, and a local decrease of the field B1 in the object to be analyzed PA, in a region close to the position of the resonator ER2 and in the opposite region of the object PA (along the axis OX). It can also be observed a local increase of the field B1 in the object PA, in lateral regions (along the axis OY).
• the resonator ER2 is produced using a conducting wire having a diameter of 1 mm (within ⁇ 20%) or a conductive track formed on an insulating substrate having a width of 1 mm. (within + or - 20%).
• the ER2 resonator has a meander width and a meander width of 0.8% (within + or -20%) of the inside diameter of the antenna 1, and a length equal to one and a half times the length of the antenna ( to + or - 20%).
• FIG. 13 represents an image of the distribution in the volume antenna 1 of the component of the field B1 (in Vs / m 2 ) in the presence of an object to be analyzed PA placed in the latter, the voluminal antenna being associated with the EMR resonator with four rods (FIG. 2A), placed outside the antenna (between the antenna and an SH shielding)
• the EMR resonator is placed along an axis passing through the center O of the antenna 1, oriented at an angle of + 135 ° with respect to an origin defined by the axis OX
• FIG. 13A shows a slight local decrease of the field B1 in the object to be analyzed PA at the periphery of the latter.
• FIG. 14 and 15 show the volumetric antenna 1 and the object to be analyzed PA disposed in the antenna.
• the antenna 1 is associated with four four-rod EMR resonators T (FIG. 3A), placed in the antenna, and oriented parallel to the longitudinal axis Z of the antenna, namely two resonators respectively of either side of the object PA along the axis OX, and two resonators respectively on either side of the object PA along the axis OY.
• the voluminal antenna 1 is associated with six four-rod EMR resonators T (FIG. 3A), placed in the antenna and oriented parallel to the longitudinal axis Z of the antenna, namely two pairs of resonators. respectively on either side of the object PA, along the axis OX, and two resonators respectively on either side of the object PA, along the axis OY.
• EMR resonators T FIG. 3A

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• 2016
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