Apparatus for homogenizing a radio frequency magnetic field for magnetic resonance imaging - Patent Application 20070122963

Abstract

The invention relates to a device (2, 4) for homogenizing a radio frequency magnetic field for magnetic resonance imaging of a given wavelength radiated by a volume antenna, comprising at least one continuous metal track (8) having a total length of 50% to 75% of the wavelength of the radio frequency magnetic field, the metal track comprising two end segments extending in a direction parallel to each other and a main part extending between the two end segments and comprising several identical local deformations connected to each other in series by at least one at least partially straight connecting part (14) so ​​as to provide the homogenizing device (2, 4) with electric dipole properties, the homogenizing device having a fundamental frequency higher than the frequency corresponding to the wavelength of the radio frequency magnetic field.

Description

[Technical field]

[0001] This application relates to radio frequency magnetic field emission systems for magnetic resonance imaging techniques.

[0002] Magnetic resonance imaging (MRI) devices use a powerful magnet to generate a static main magnetic field and one or more transmit antennas to generate a radio frequency (RF) excitation field. The RF excitation field penetrates the imaging subject (e.g., a human body or part thereof) and interacts with atomic nuclei (e.g., protons) present in the imaging subject, exciting the nuclei. For this interaction to be optimal, the RF excitation field must resonate with the atomic nuclei, and to achieve this, the RF excitation field is emitted at a specific frequency, called the Larmor frequency of the atomic nuclei involved.

[0003] The Larmor frequency is an increasing function of the main magnetic field, and thus for a proton (i.e., hydrogen nucleus) it is approximately 64 MHz in a main magnetic field of 1.5 T and approximately 300 MHz in a magnetic field of 7 T. When the nuclei return to equilibrium, they emit an RF signal that is measured by the MRI scanner, providing the data necessary to reconstruct an image of the subject, known as an MRI image.

[0004] MRI machines are divided into "low field" and "high field" machines, with a main magnetic field of 1.5 to 3 T, and "ultra-high field" machines, with a main magnetic field of up to 7 T or more. The use of very strong magnetic fields allows for a significant increase in the signal-to-noise ratio (SNR) of the RF signal measurements used to provide MRI images, but this is accompanied by an increase in the Larmor frequency and therefore a reduction in the wavelength required for the RF excitation field.

[0005] Low and high field clinical devices are equipped with so-called body antennas in order to transmit an RF excitation field fairly uniformly throughout the body to the nuclei being investigated.

[0006] For very high field MRI devices, which require RF excitation fields with wavelengths too short (11 cm in the human body at 300 MHz) to be transmitted uniformly throughout the body by a single antenna, the use of a body antenna is no longer possible.

[0007] In that case, the strategy may consist of using antennas (referred to herein as "volumetric antennas") dedicated to specific parts of the body, such as the head, where a "birdcage" type volumetric antenna is used.

[0008] In this case, the volume antenna used ensures the transmission of the RF excitation field required for measurements in areas of reduced dimensions, but the above-mentioned inhomogeneity problems may remain despite the reduction in the imaged area, as may occur even with high-field MRI applied to the trunk (chest, abdomen or pelvis).

[0009] In order to improve the uniformity of the RF excitation field (and therefore the quality of the images obtained thanks to the excitation induced by the RF field), it is known to use a dielectric pad that is inserted between the volume antenna and the part of the body to be imaged, to distribute the RF excitation field uniformly over that part of the body.

[0010] Such dielectric pads are made of a solvent such as water and particles of a material with a higher dielectric constant, making them thinner and more comfortable to use. However, due to the evaporation of water and settling of particles, these dielectric pads have a relatively short service life. Their repeated handling during use (placement between a human body part and a volume antenna) accelerates the settling of particles and therefore shortens the service life of the dielectric pads.

[0011] EP 3550321 describes a dielectric pad with a different composition. It is a dielectric pad that includes a polar solvent, a dispensing agent and a dielectric compound. The purpose of this composition is to reduce the problem of particle settling and to improve the particle distribution in the dielectric pad. However, these cushions are very bulky, making them difficult to use in the potentially small space between the imaging device and the part of the body to be imaged. Such dielectric pads pose a risk of puncturing the patient or spilling the liquid, and therefore a health hazard.

[0012] Finally, it is known to use metasurfaces with negative permeability to focus RF fields like a magnetic lens. The metasurfaces can include metallic tracks that form magnetic dipoles that are excited by the magnetic component of the electromagnetic field radiated by a microstrip antenna. However, these metasurfaces are best suited for surface antennas and are not optimal for use in volumetric antennas, especially of the birdcage type.

[0013] The present invention describes a system comprising one or more devices for homogenizing an RF excitation field for an MRI apparatus, which makes it possible to overcome the above-mentioned difficulties.

[0014] The object of the present invention is a device for homogenizing a radio frequency magnetic field for magnetic resonance imaging of a given wavelength radiated by a volume antenna, comprising at least one continuous metal track having a total length of 50% to 75% of the wavelength of the radio frequency magnetic field, the metal track forming a pattern having a width of 4% to 10% of the wavelength of the radio frequency magnetic field and a height of 10% to 25% of the length of the radio frequency magnetic field, the metal track comprising two end segments extending in a direction parallel to each other and a main part extending between the two end segments and comprising several local deformations identical to each other and connected in series with each other by at least one at least partially straight connecting part so as to provide a homogenizing device with electric dipole properties, the homogenizing device having a fundamental frequency higher than the frequency corresponding to the wavelength of the radio frequency magnetic field.

[0015] The use of a homogenizing device as described above makes it possible to redistribute the high-frequency magnetic field emitted by the volume antenna of the MRI device, thus improving the homogeneity of its distribution in the part of the body being imaged. Greater homogeneity of the high-frequency magnetic field results in MRI images with better contrast.

[0016] The homogeneity device also does not interfere with the reception of RF signals generated by atomic nuclei within the body part being imaged by the receive channels of the surface antenna array of the MRI device, and the radio frequency magnetic field is not increased to levels that are harmful to the body.

[0017] The use of several local deformations as described above, connected in series by at least one at least partially straight section, makes it possible to achieve homogenization in one or more zones of the part of the body being imaged that are relatively far from the homogenization device itself. In other words, the homogenization device can be located in one area and still achieve optimal results in the remote area. As a result, the homogenization device can be attached to the volume antenna and still achieve the desired results. This avoids the need to handle the homogenization device every time an MRI scan is performed.

[0018] The use of several local deformations as described above connected in series according to the invention to form a continuous structure makes it possible to combine their electric dipole effects in order to improve the electric dipole behavior of the metal track and thus improve the homogenization of the high-frequency magnetic field by the homogenization device.

[0019] Finally, the geometry of the equalizer described above allows achieving an optimal compromise between the bandwidth of the equalizer itself and the radiation efficiency of the equalizer. In other words, by arranging the metal tracks as described above, the widest possible bandwidth for the equalizer is achieved (to ensure that the operating frequency overlaps with the bandwidth of the volume antenna) while maintaining a high radiation efficiency, some devices choosing to favor one or the other. For example, prior art devices can achieve a high quality factor, but with a narrow bandwidth (the quality factor is equal to the frequency at which the gain is at its highest divided by the bandwidth), resulting in an equalizer that is difficult to couple to the volume antenna and is very sensitive to the local environment. Conversely, if the quality factor is too low, the result is an equalizer with poor resonance and low compensation efficiency.

[0020] In this document we speak of the frequency of the radio frequency magnetic field, which corresponds to its wavelength and is related to the latter by the following formula: [Formula 1] f=c / λ where f is the frequency of the radio frequency magnetic field, λ is the wavelength of the radio frequency magnetic field, and c is the speed of light in a vacuum.

[0021] In this description, the frequency of the radio frequency magnetic field is the Larmor frequency used in MRI scanners.

[0022] According to this specification, the surface of the pattern is the surface of the area on which the metal tracks are engraved, for example a rectangular or square area.

[0023] According to other optional features of the homogenization device, taken alone or in combination, -The local deformations are regularly spaced, - the end segments are aligned, -The pattern is formed by several quadratic Hilbert curves connected in series, several quadratic Koch curves connected in series, or several quadratic Minkowski curves connected in series. Among other things, these shapes allow the structure to resonate like an electric dipole at the Larmor frequency. The use of such curves makes it possible to fold a metal track of a given length while still maintaining these resonant properties, - quadratic Hilbert curves, quadratic Koch curves, or quadratic Minkowski curves are distributed in an alternating right / left pattern along a number of aligned at least partially straight line connected segments; at least one at least partially linear connection portion includes a cross-shaped pattern, the presence of the cross-shaped pattern improving the performance of the uniformizing device, -The patterns are 4-10 centimeters wide and 10-25 centimeters high. These are the optimal size intervals for a static main magnetic field strength of 7T. -The metal tracks are placed on a dielectric substrate, which maintains the mechanical structure of the metal tracks. - The dielectric substrate is 0.1 to 1 millimeter thick. This results in a thin, flexible structure that is easy to install, especially for antennas with complex geometries. the substrate has a dielectric loss factor of less than 0.05, evaluated at a frequency corresponding to the wavelength of the high frequency magnetic field. This optimizes the efficiency of the homogenizer by limiting the absorption of the high frequency magnetic field by the substrate. In this specification, the dielectric loss factor (or loss angle) is a dimensionless quantity with a meaning known to those skilled in the microwave field. For dielectric materials, it is approximately equal to the ratio between the imaginary and real parts of the complex permittivity of the material. The dielectric loss factor depends in particular on the frequency of the high frequency magnetic field considered.

[0024] Another object of the invention is an assembly formed by a volume antenna adapted to radiate a high frequency magnetic field and a homogenization device according to the invention.

[0025] Advantageously, the volumetric antenna is a birdcage type antenna configured to be positioned around the region of the body to be imaged. [Brief description of the drawings]

[0026] The invention will be better understood on reading the following description, given purely by way of example, and on referring to the accompanying drawings, in which: [Figure 1] 1 shows two examples of homogenization devices according to the invention. [Diagram 2] 1 shows some examples of metal tracks according to the invention. [Diagram 3] 4 shows simulation results illustrating the distribution of the radio frequency excitation field with an example of a homogenizing device according to the invention. [Figure 4] Experimental results are presented showing the distribution of the radio frequency excitation field using an example homogenizer implementing a double-sided configuration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] It should be noted that in this specification, some elements are not drawn to scale for reasons of visibility. Moreover, for reasons of visibility, only some of the elements having the same reference numbers are referenced.

[0028] Figures 1 and 2 show examples of devices 2 and 4 according to the invention. These homogenizing devices are designed for use in radio frequency magnetic field (hereinafter referred to as "RF field") emission systems for magnetic resonance imaging (MRI) machines. A head 6 is shown diagrammatically to show the placement and order of magnitude of dimensions of the homogenizing devices 2 and 4 relative to the body part.

[0029] The radiation system comprises one or more RF field emitting volume antennas and one or more RF field homogenizers, such a system being configured to fit around a part of the body to be imaged (e.g. head 6) and to radiate a homogenous RF field at a given frequency in this part of the body in order to excite atomic nuclei therein.

[0030] The part of the body to be imaged may be placed on a support, around which the volume antenna is placed. The positioning may be performed by sliding the volume antenna along a positioning slide.

[0031] The frequency of the RF field used to excite the nuclei of the body part being imaged, called the Larmor frequency, depends on the type of nuclei being excited and the main magnetic field of the MRI apparatus. The Larmor frequency may be, for example, about 300 MHz for hydrogen nuclei in a main magnetic field of about 7 T. The wavelength of the RF field used may therefore be, for example, about 1 meter.

[0032] Figure 1 shows two examples of devices for homogenizing a radio frequency magnetic field according to the invention, also known as RF field "homogenizing pads". In these examples, the devices comprise a metal track 8 arranged on a dielectric substrate 10. As will be explained in more detail later, the metal track 8 comprises two end segments extending in a direction parallel to each other (the end segments are aligned with each other in the example shown in Figure 2) and a main part extending between the two end segments and including several (at least two) local deformations identical to each other, arranged at regular intervals in the example described and connected in series to each other by at least one at least partially straight connecting part 14 so as to provide the homogenizing device with electric dipole properties.

[0033] Each local deformation is contained in a single two-dimensional plane that contains the entire structure that forms it, and forms an open curve that does not intersect with itself or any other local deformation. The curve of a local deformation may form an angle or may be a curved segment.

[0034] The presence of two end segments, made at regular intervals and surrounding some local deformations with respect to symmetry around the center of the pattern, makes it possible to obtain a maximum amplitude in the center of the pattern and therefore to have a centralized magnetic field (indeed, although the current in the end segments is low, an increase in the resonant frequency at the local deformations is observed).

[0035] The metal track 8 is continuous and its total length (by "total length" of the metal track 8 we mean the length of the fully deployed track) is preferably 50%-75% of the wavelength of the RF field used by the MRI device. In practice, the aim is to ensure that the metal track occupies a surface area large enough to cover the part of the body to be imaged.

[0036] The metal tracks 8 form a pattern with a width L of 4%-10% of the RF field wavelength and a height H of 10%-25% of the RF field length. Figure 2 shows the height H and width L of the metal tracks in an example of a metal track according to the invention. These are the distance between the two most distant points in the longitudinal direction of the metal track and the distance between the two most distant points in the lateral direction of the metal track, respectively. For example, if the wavelength of electromagnetic waves in a vacuum is equal to 1 meter, the width L of the pattern is 4 cm-10 cm and the height H is 10 cm-25 cm. These spacings are optimal for a volume antenna used for brain MRI at 7T.

[0037] In particular, this dimension allows the surface to cover the portion of the body being imaged, such as the brain or pelvic region, and in particular, when the device is applied to brain MRI, the dimension ensures that the device is effective in the lateral or temporal lobes, areas where the RF field is generally not very present when a birdcage antenna is used without a homogenizing device.

[0038] In some cases, it may be necessary to image an area larger than the surface covered by a single metal track 8. Therefore, the number of metal tracks 8 present on the dielectric substrate 10 may vary. Each of the metal tracks 8 has dimensions within the ranges given above.

[0039] Preferably, the metal track 8 forms (i.e. a pattern is formed) several second-order Hilbert curves 16, preferably six second-order Hilbert curves, connected in series as described above, according to the mathematical definition. This shape allows the structure to resonate like an electric dipole at the Larmor frequency. The use of Hilbert curves makes it possible to fold a metal track of a given total length while still maintaining these resonant properties. In fact, the use of Hilbert curve geometry means that a metal track of a given total length can be folded in such a way as to reduce the two-dimensional space it occupies on a surface while retaining the same total length (corresponding to the unrolled length). In particular, this allows the metal track according to the invention to interact with bulk antennas that radiate RF fields of wavelengths larger than the dimensions of the surface occupied by the metal track.

[0040] The second-order Hilbert curves 16 can be distributed in a right / left alternating manner along several aligned at least partially straight connection portions 14, and the cross patterns 18 can be arranged in the at least partially straight connection portions 14. In FIG. 1, the left / right alternation is shown for both the equalizers 2 and 4, the equalizer 2 being equipped with a cross pattern 18 at each connection portion 14 between two second-order Hilbert curves 16, the cross pattern 18 improving the performance of the equalizer 2. The presence of symmetry helps to give the equalizer 2 or 4 the properties of an electric dipole that can interact with a volume antenna.

[0041] Instead of several quadratic Hilbert curves, the local deformations can be formed by quadratic Koch curves 16' or quadratic Minkowski curves 16", with the same effect as above. The number of curves, their arrangement or the presence of a cross pattern can be the same as described for the quadratic Hilbert curve 16.

[0042] The three examples of curves shown above represent preferential but not limiting cases of the possibilities for creating patterns according to the invention.

[0043] The metal tracks 2 or 4 can be placed, for example printed, on a dielectric substrate 10. This maintains the mechanical structure of the metal tracks. This dielectric substrate 10 is large enough to contain the metal tracks 2 or 4. Its thickness can be 0.1 to 1 mm, for example 0.5 mm. This results in a thin, flexible structure that is easy to install, especially for antennas with complex geometries.

[0044] The dielectric substrate 10 may have a dielectric loss factor of less than 0.05, evaluated at a frequency corresponding to the wavelength of the high frequency magnetic field. This optimizes the efficiency of the homogenizer by limiting the absorption of the high frequency magnetic field by the substrate. In this specification, the dielectric loss factor (or loss angle) is a dimensionless quantity having a meaning known to those skilled in the microwave field. For dielectric materials, it is approximately equal to the ratio between the imaginary and real parts of the complex permittivity of the material. The dielectric loss factor depends, inter alia, on the frequency of the high frequency magnetic field considered.

[0045] According to the present invention, the homogenizing device 2 or 4 has a fundamental frequency higher than the frequency corresponding to the wavelength of the RF field, for example higher than the Larmor frequency used by the MRI device (for example 300 MHz for a 7T ultra-high field MRI device, or 125 MHz for a 3T high field MRI device). This prevents the presence of the human body from lowering the fundamental frequency below the RF field frequency when the device is in use. This allows the device to homogenize the RF field without resonating with the RF field.

[0046] FIG. 3 shows simulation results for the distribution of the RF excitation field for an example apparatus according to the present disclosure.

[0047] In particular, the simulation models the distribution of a 300 MHz RF field (corresponding to that used in a 7T ultra-high field MRI machine) in the case of brain imaging.

[0048] The simulation uses parameters of an orthogonal birdcage antenna with one (middle row) or two (right row) equalizers 2 or 4 (equalizer 2 is referenced in FIG. 3, but could also be reference 4).

[0049] The head model used is the SAM (Special Anthropomorphic Mannequin) phantom model conventionally used in this type of simulation: the SAM model is a dummy with standardized properties close to those of the human body (permittivity: 42, conductivity: 0.99 S / m and density 1000 kg / m3).

[0050] Simulations are performed using CST Microwave Studio® simulation software to evaluate the RF magnetic field distribution and the Specific Absorption Rate (SAR). SAR (expressed as W / kg) quantifies the amount of electromagnetic power absorbed by human body tissue and then dissipated in the form of heat. This amount is typically used by those skilled in the art to evaluate safety standards for the use of radiation devices on patients.

[0051] In particular, FIG. 3 shows a simulation of the RF field distribution in the brain in three cases: - Without homogenizer (left column). - A single homogenizer was used, in this example homogenizer 2 or 4 placed on one side of the right temporal region of the brain (middle row). - Two homogenizers 2 or 4 (right row) were used.

[0052] The metal track 8 has dimensions within the ranges mentioned above.

[0053] For each of the three configurations, the upper mid-coronal plane (22, 22', 22") and the lower mid-sagittal plane (24, 24', 24") are shown. The areas outlined in black indicate regions of interest 26 that correspond to different brain regions.

[0054] For example, the areas represented by arrows 28 show examples of shadow regions where the RF field is very weak due to a lack of uniformity in the RF field. In accordance with one or more embodiments, the device of the present invention aims to eliminate these shadow regions 28 by homogenizing the distribution of the RF field over the part of the body being imaged.

[0055] FIG. 3 shows that the addition of a homogenization device according to the present invention eliminates the shadow region 28 in the brain by reintroducing the signal through homogenization of the RF excitation field (this can be clearly seen by comparing the mid-sagittal planes 24 and 24'' as well as the mid-coronal planes 22 and 22'').

[0056] Furthermore, the uniformity of the RF field in the cerebellar region is not unduly affected by the presence of the uniformizers 2 or 4. This aspect is noteworthy in contrast to known pads in the prior art that are based on dielectric materials.

[0057] The statistical average results of the RF field values ​​in different brain regions for different device configurations are shown in Table 1 below.

[0058] [Table 1]

[0059] They show a clear improvement in the mean value of the RF field amplitude in the temporal regions (+45% for the unilateral configuration and +56% for the bilateral configuration).

[0060] The electromagnetic power absorbed in the phantom model brain can also be calculated using CST Microwave Studio® simulation software. The quantity of interest is the Specific Absorption Rate (SAR), whose value can be averaged over the entire volume of a part of the body (global value) or is the maximum value in a volume equal to 10 g of tissue (local value).

[0061] Table 2 below shows simulated SAR results for different configurations of devices according to different embodiments of the present invention. The results in Table 2 show that the local SAR increases by about 45% for the double-sided configuration.

[0062] This effect can be offset by a small increase in the distance between the device and the head, which is a compromise between signal increase in the shadow region and local SAR.

[0063] Table 2 below shows that the local SAR for brain imaging remains low even when the antenna and imaging region are in close proximity.

[0064] [Table 2]

[0065] Experimental tests showing the effect of the device on the distribution of a 300 MHz RF field in a brain MRI context were also carried out to verify the homogenizing function of the device according to the invention.

[0066] The measurements shown in Figure 4 are the results of an in vivo test. The homogenization device 2 or 4 is positioned as in the test shown in Figure 3.

[0067] In these experimental tests, the antenna used is a quadrature birdcage antenna with transmit / receive channels intended for imaging the brain in a 7T ultra-high field MRI device. A metal track 8 is printed, the dimensions of which correspond to those mentioned above.

[0068] FIG. 4 shows experimental results of RF excitation field distribution using an example apparatus implementing a one-sided configuration in accordance with the present invention.

[0069] In particular, FIG. 4 shows the results of measuring the flip angle (a quantity proportional to the RF field amplitude) in the brain phantom model described above for different measurement configurations. -No homogenizer (top row 30). - Two homogenizers (middle stage 32) were used.

[0070] The third row 34 of FIG. 4 shows a comparison between the two rows mentioned above to visualize the relative signal gain between the two configurations.

[0071] For each measurement configuration, results are presented in sagittal (left column), coronal (middle column) and axial (right column) sections.

[0072] The results show a highly positive effect of the device on RF field amplitude, with an increase of approximately 50%-60% in each temporal lobe.

[0073] References 2, 4: Equalization device 6:Head 8: Metal track 10: Substrate 14: Straight connection part 16: Quadratic Hilbert curve 16': Quadratic Koch curve 18': Quadratic Minkowski curve 18:Cross pattern 22, 22', 22”: midcoronal plane 24, 24', 24”: midsagittal plane 26: Region of interest 28: Shadow area 30: Measured without homogenizing device 32: Measurement using a homogenizing device 34:Comparing measurements L: Pattern width H: Pattern height

Claims

1. 1. A device (2, 4) for homogenizing a radio frequency magnetic field for magnetic resonance imaging of a given wavelength emitted by a volume antenna, comprising at least one continuous metal track (8) having a total length of 50% to 75% of the wavelength of the radio frequency magnetic field, said metal track (8) forming a pattern having a width area (L) of 4% to 10% of the wavelength of the radio frequency magnetic field and a height (H) of 10% to 25% of the length of the radio frequency magnetic field, said metal track comprising two end segments extending in a direction parallel to each other and a main part extending between said two end segments and comprising several identical local deformations connected to each other in series by at least one at least partially straight connecting part (14) so ​​as to provide a homogenizing device (2, 4) with electric dipole properties, said homogenizing device having a fundamental frequency higher than the frequency corresponding to the wavelength of the radio frequency magnetic field.

2. 2. The homogenizing device (2, 4) according to claim 1, wherein the pattern is formed by a plurality of second-order Hilbert curves (16) connected in series with each other, by several second-order Koch curves (16') connected in series with each other, or by several second-order Minkowski curves (16'') connected in series with each other.

3. 3. The homogenization device (2, 4) according to claim 2, wherein the quadratic Hilbert curve (16), quadratic Koch curve (16') or quadratic Minkowski curve (16") are distributed alternately right / left along several aligned at least partially straight-line connected portions (14).

4. The equalization device (2, 4) according to any one of claims 1 to 3, wherein at least one at least partially straight connecting portion (14) comprises a cross-shaped pattern (18).

5. The homogenizing device (2, 4) according to any one of claims 1 to 3, wherein the pattern has a width (L) of between 4 and 10 centimeters and a height (H) of between 10 and 25 centimeters.

6. 2. The homogenizing device (2, 4) according to claim 1, wherein the metal tracks (8) are arranged on a dielectric substrate (10).

7. The homogenizing device (2, 4) according to claim 6, wherein the dielectric substrate has a thickness of between 0.1 millimeter and 1 millimeter.

8. 8. The homogenizing device (2, 4) according to claim 6 or 7, wherein the substrate comprises a dielectric loss factor, evaluated at the frequency corresponding to the wavelength of the radio frequency magnetic field, of less than 0.

05.

9. Assembly formed by a volume antenna adapted to emit a high frequency magnetic field and a homogenizing device (2, 4) according to claim 1.

10. The assembly of claim 9 , wherein the volumetric antenna is a birdcage antenna configured to be placed around a region of the body to be imaged.

Images