Magnetic resonance imaging device and method for acquiring magnetic resonance images

Abstract

The invention relates to a magnetic resonance imaging device (1) comprising a radio frequency assembly (7) configured to transmit and receive radio frequency signals, the radio frequency assembly being characterized by a natural bandwidth and a natural resonant frequency and comprising a radio frequency coil (8) intended to transmit and receive radio frequency signals, and a tunable circuit (9) associated with the radio frequency coil (8) and configured to make it possible to adjust the equivalent impedance of the radio frequency assembly (7) within a given impedance range, the adjustment of the equivalent impedance making it possible to adjust a resonant frequency, referred to as tuning frequency, and a bandwidth, referred to as tuning bandwidth, of the radio frequency assembly (7), the tuning frequency and the tuning band each being included in the natural bandwidth such that the radio frequency assembly (7) has a quality factor higher than the quality factor of the radio frequency coil (8) alone.

Description

[Technical Field]

[0001] The present invention relates to the field of magnetic resonance imaging. More particularly, the present invention relates to a magnetic resonance imaging device, and in particular to a magnetic resonance imaging device provided with a radio frequency assembly equipped with a transmit / receive radio frequency coil. More particularly, the radio frequency assembly according to the invention is also provided with a radio frequency coil having a narrow bandwidth and means for adjusting the resonant frequency of the radio frequency assembly in a range of operating frequencies that is wider than the bandwidth of the coil. The proposed configuration thus makes it possible to improve the quality of images when a relatively small static magnetic field must be taken into account.

[0002] The present invention is particularly advantageous when considering portable magnetic resonance imaging devices. [Background technology]

[0003] Magnetic resonance imaging (MRI) is now widely used for non-invasive imaging of the interior of the body, in particular the human body. In particular, magnetic resonance imaging makes it possible to probe the hydrogen nuclei of water molecules that form part of the body being examined, in particular their nuclear spin.

[0004] In this respect, the MRI device is provided with a magnet intended to impose a static magnetic field (called the "main magnetic field") on the body, under the influence of which the nuclear spins associated with the hydrogen nuclei contained in the water molecules forming part of this body become polarized.

[0005] Specifically, the magnetic moments associated with these spins are preferentially aligned along an axis called the z-axis, which is determined by the orientation of the main magnetic field to produce the magnetization of the body.

[0006] The MRI device also includes gradient coils configured to produce small-amplitude, spatially varying magnetic fields when current is applied to them. More specifically, the gradient coils are aligned parallel to the main magnetic field and are designed to produce magnetic field components whose amplitudes vary linearly with position along one of the x, y, or z axes (each pair of x, y, and z axes is perpendicular).

[0007] In this way, the combined effect of the magnetic fields imposed by the gradient coils makes it possible to spatially encode each of the body locations intended to be probed.

[0008] The MRI device also includes at least one radiofrequency (RF) coil intended to function as a RF transceiver, specifically configured to emit RF energy pulses at a frequency equal to or close to the resonance frequency of hydrogen atomic nuclei spins, which RF energy pulses are at least partially absorbed by these atomic nuclei.

[0009] As soon as the RF emission is discontinued, the nuclear spins relax to return to their initial energy state and then emit RF signals that can be collected by at least one RF coil, which are then processed using a computer and reconstruction algorithms to obtain an image of the body.

[0010] Generally, a main magnetic field comprised between 1.5 Tesla and 3 Tesla makes it possible to achieve a relatively reasonable signal-to-noise ratio, and consequently to form images of the human body of sufficient quality for durations of the order of one minute or more.

[0011] However, there are situations in which it is not possible to implement a main magnetic field of such strength, such as portable MRI devices, which generally contain permanent magnets or electromagnets of limited capacity and cannot impose a main magnetic field having a strength greater than 60 mT, or even greater than 200 mT, without adversely affecting the mass or volume of the MRI device under consideration.

[0012] This limitation on the main magnetic field strength directly affects the performance of MRI equipment. Specifically, the quality of images obtained using such MRI devices can be significantly degraded by an unfavorable signal-to-noise ratio, which in part reflects the significant reduction in magnetization present in tissue.

[0013] One object of the invention is to propose a magnetic resonance imaging device, which implements an advantageously low strength main magnetic field, provided with a radio frequency assembly which makes it possible to improve the signal-to-noise ratio and consequently the quality of the images. Summary of the Invention

[0014] The present invention relates to a magnetic resonance imaging device comprising a radio frequency assembly adapted to transmit and receive radio frequency signals, the assembly comprising: a radio frequency coil characterized by a natural bandwidth and a natural resonant frequency and intended to transmit and receive radio frequency signals; a tunable circuit associated with the radio frequency coil and configured to make it possible to adjust the equivalent impedance of the radio frequency assembly within a given impedance range, the adjustment of the equivalent impedance making it possible to adjust a resonant frequency, called the tuning frequency, within a frequency range called the operating frequencies of the radio frequency assembly, the range of the operating frequencies being greater than the range of the intrinsic bandwidth; - adjusting means configured to instruct the tunable circuit to dynamically adjust the equivalent impedance during acquisition of an image by the imaging device.

[0015] According to one embodiment, the tuning means (9A) is configured to allow radio frequency transmission at a given frequency, called the Larmor frequency, and reception of radio frequency signals whose tuning frequency is dynamically tuned within an operating range.

[0016] According to one embodiment, the radio frequency coil comprises a capacitor, referred to as the main segmented capacitor.

[0017] According to one embodiment, the tunable circuit comprises at least two components arranged according to an L-shaped topology, the at least two components being combined with each other in the tunable circuit to generate a reactance, one and / or the other of these two components being tunable to allow adjustment of the equivalent impedance of the radio frequency assembly, advantageously the two components comprising two capacitors, or two inductors, or one capacitor and one inductor.

[0018] According to one embodiment, the tunable circuit comprises two inputs and two outputs, the two inputs, respectively called the first input and the second input, intended to be powered by a generator of current pulses, and the two outputs, respectively called the first output and the second output, each connected to one of the ends of the radio frequency coil.

[0019] According to one embodiment, the radio frequency assembly comprises two branches, namely a first branch and a second branch, connected in parallel at the level of the first input section and the second input section, respectively, the first branch comprising a radio frequency coil and one of two components connected in series, and the second branch comprising the other of the two components.

[0020] According to one embodiment, the radio frequency assembly further comprises means for generating radio frequency pulses, the means for generating radio frequency pulses being adapted to impose a circulation of current pulses in the radio frequency coil via the tunable circuit.

[0021] According to one embodiment, the imaging device comprises radio frequency processing means, the radio frequency processing means being adapted to process radio frequency signals that can be received by the radio frequency coil.

[0022] According to one embodiment, the imaging device comprises a magnet defining a bore in which a radio frequency coil is placed, the interior of the radio frequency coil forming a zone called the analysis zone, the magnet imposing a static magnetic field.

[0023] According to one embodiment, the magnet is a permanent magnet, advantageously capable of generating a static magnetic field of less than 100 mT, even more advantageously less than 50 mT.

[0024] According to one embodiment, the device also comprises gradient coils intended to spatially encode each of the positions of the analysis zone, which, in combination with the static magnetic field, is intended to associate each of the positions with a resonant frequency, called the natural frequency, for the spins of hydrogen nuclei likely to be located at said positions.

[0025] According to one embodiment, the tuning frequency can cover all of the natural frequencies of hydrogen nuclear spins likely to be present at each location in the analysis zone by tuning the equivalent impedance.

[0026] The invention also relates to a method for obtaining an image of a body by magnetic resonance using an imaging device of the invention, the method comprising the steps of: a) exposing a body disposed within a radio frequency coil to a static magnetic field; b) imposing spatial encoding on the body using gradient coils, which expose the body to gradient magnetic fields that are added to the static magnetic field to form a resultant magnetic field in order to associate with each body position a resonant frequency called the natural frequency of the spins of hydrogen nuclei, all of which natural frequencies are spread over the operating range; c) transmitting radio frequency signals using a radio frequency coil to excite hydrogen nuclear spins throughout the body at locations that are spatially encoded by the gradient coil; d) measuring echoes of hydrogen nuclear spins emitted by at least some of the body locations subjected to spatial encoding by the gradient coil, the measuring including dynamic adjustment of the equivalent impedance of the assembly formed by the tunable circuit and the radio frequency coil.

[0027] According to one embodiment, the spatial encoding imposed by the gradient coils is reflected in the resultant magnetic field by a decomposition into slices, called motion slices, which are themselves subdivided into mutually parallel motion lines along which the resultant magnetic field varies.

[0028] According to one embodiment, spin echo measurements are performed one operating line at a time.

[0029] According to one embodiment, the spin echoes likely to be measured along the line of operation cover a frequency range wider than the bandwidth of the radio frequency coil, and measurements along the line of operation are performed by dynamically adjusting the equivalent impedance of the assembly formed by the tunable circuit and the radio frequency coil to collect all of the spin echoes associated with the line of operation.

[0030] According to one embodiment, the frequency range associated with the spin echo of one line is at least five times larger, advantageously ten times larger, than the characteristic bandwidth of the radio frequency coil. [Brief explanation of the drawings]

[0031] Other features and advantages of the present invention will become apparent from the detailed description that proceeds with reference to the accompanying drawings. [Figure 1] FIG. 1 is a schematic exploded view of a magnetic resonance imaging device. [Figure 2] FIG. 2 is a schematic diagram of a radio frequency assembly in accordance with an aspect of the present invention. [Figure 3] FIG. 3 is a graphical representation of the response of a radio frequency assembly according to an aspect of the present invention, including a radio frequency coil; specifically, FIG. 3 shows the characteristics of the assembly (i.e., the reflection coefficient in dB on the vertical axis) for different adjustments of the resonant frequency in a 30 kHz operating range (the horizontal axis represents frequency in MHz); more specifically, FIG. 3 shows four intensity profiles for four different settings of the tunable circuit (curves "A," "B," "C," and "D"). [Figure 4] FIG. 4 shows a radio frequency assembly in which the first branch is formed by the series connection of a radio frequency coil and a capacitor C1, and the second branch is formed by an inductor L1. [Figure 5] FIG. 5 shows a radio frequency assembly in which the first branch is formed by the series connection of a radio frequency coil and an inductor L1, and the second branch is formed by a capacitor C1. [Figure 6] FIG. 6 shows a radio frequency assembly in which the first branch is formed by the series connection of a radio frequency coil and a capacitor C2, and the second branch is formed by a capacitor C1. [Figure 7] FIG. 7 shows the effect of the quality factor of the radio frequency coil on the obtained images, where image (1) and image (2) are obtained using an imaging device with a radio frequency coil having a standard quality factor and a high quality factor (greater than the standard quality factor), respectively. DETAILED DESCRIPTION OF THE INVENTION

[0032] The present invention relates to a magnetic resonance imaging device comprising a radio frequency assembly configured to transmit and receive radio frequency signals.

[0033] In particular, the radio frequency assembly comprises a radio frequency coil, characterized by a natural bandwidth and a natural resonant frequency, and intended to transmit and receive radio frequency signals.

[0034] The radio frequency assembly also comprises a tunable circuit associated with the radio frequency coil and configured to enable tuning of the equivalent impedance of the radio frequency assembly within a given impedance range, the tuning of the equivalent impedance enabling tuning of a resonant frequency, referred to as tuning frequency, within a frequency range, referred to as the operating range of the radio frequency assembly, in this respect the range of the operating range being greater than the range of the intrinsic bandwidth.

[0035] The radio frequency assembly further comprises an adjusting means configured to instruct the tunable circuit to dynamically adjust the equivalent impedance during acquisition of an image by the imaging device.

[0036] By associating the tunable circuit with the adjustment means, it is possible to consider radio frequency coils with a bandwidth narrower than the operating range. This latter consideration makes it possible to implement radio frequency coils whose quality factor is greater than that of coils normally considered and which are generally required to have a bandwidth that at least covers the operating range. In this way, the principles of the present invention make it possible to improve the quality of images obtained by magnetic resonance imaging devices and to envisage the implementation of main magnetic fields with low amplitudes, in particular less than 100 mT, or even less than 50 mT. According to these conditions for the main magnetic field, the quality factor of the radio frequency coil is advantageously between 90 and 120 (the quality factor of the coil is defined as the ratio of the resonant frequency of the coil to its bandwidth).

[0037] Thus, FIG. 1 is a schematic diagram of a magnetic resonance imaging device 1 according to the invention.

[0038] The imaging device 1 comprises a magnet, in particular a permanent magnet 2. The permanent magnet 2 may in particular extend along an extension axis z.

[0039] More specifically, the permanent magnet 2 defines a bore 3 opening through a first opening 4 and a second opening 5 opposite each other along the axis of elongation z.

[0040] In this respect, the permanent magnet 2 is arranged to allow the insertion of a body, more particularly a human body, into the bore 3 through the first opening 4 along the axis of elongation z.

[0041] The permanent magnet 2 is more particularly configured to impose, in a zone of the bore 3 called the analysis zone, a static magnetic field oriented along an axis perpendicular to the elongation axis z.

[0042] In this respect, the permanent magnet 2 may comprise an assembly of elementary magnets, in particular arranged in a series of Halbach rings. Document EP 3368914 B1 shows an example thereof. However, the invention is not limited to the configuration described therein.

[0043] By way of example, the permanent magnet 2 is configured to impose a static magnetic field having an amplitude of less than 100 mT, advantageously less than 65 mT, and even more advantageously less than or equal to 50 mT.

[0044] The imaging system 1 also comprises a set of gradient coils 6. The gradient coils 6 are particularly configured to produce a small amplitude, spatially varying magnetic field when a current is applied to them.

[0045] More specifically, the gradient coils 6 are aligned parallel to the static magnetic field and are designed to produce magnetic field components whose amplitudes vary linearly with position along one of the axes x, y or z (axes x, y and z form an orthogonal reference frame).

[0046] The combined effect of the magnetic fields imposed by the gradient coils 6 thus makes it possible to spatially encode the signals present in the bore 3 and coming from the body intended to be explored. The spatial encoding is manifested in particular by a change in the resonance energy of the nuclear spin of the hydrogen nuclei contained in the body intended to be explored and present in the analysis zone. In other words, the nuclear spin of the hydrogen nuclei is subjected to different magnetic fields at different positions.

[0047] The imaging system 1 further comprises a radio frequency assembly 7 .

[0048] 2, the radio frequency assembly 7 comprises a radio frequency coil 8. The radio frequency coil 8 is specifically disposed within the bore 3 and at least partially defines an analysis zone. The radio frequency coil 8 is further configured to accommodate the body intended to be probed.

[0049] The radio frequency coil 8 may also comprise a capacitor called the main segmented capacitor.

[0050] The radio frequency coil 8 is characterized by a natural resonant frequency f i and a natural bandwidth Δf i . In this respect, these two properties make it possible to quantify the quality factor Q i of the radio frequency coil, which corresponds in particular to the ratio of the natural resonant frequency f i to the natural bandwidth Δf i .

[0051] The radio frequency assembly 7 according to the invention further comprises a tunable circuit 9 .

[0052] More particularly, a tunable circuit 9 is associated with the radio frequency coil 8 and is configured to allow adjustment of the equivalent impedance of the radio frequency assembly within a given impedance range.

[0053] It will be understood that a tunable circuit according to aspects of the present invention has a variable impedance as a function of conditions imposed on it. For example, as this aspect will become more apparent in the remainder of this disclosure, a tunable circuit may comprise electronic components whose impedance can be adjusted and / or set.

[0054] In particular, the adjustment of the equivalent impedance makes it possible to adjust the resonant frequency, called the adjustment frequency, of the radio frequency assembly 7 in a frequency range, called the operating range.

[0055] Specifically, the range of the operating range is greater than the natural bandwidth range.

[0056] Finally, the imaging device further comprises adjusting means 9A configured to instruct the tunable circuit to dynamically adjust the equivalent impedance during acquisition of an image by the imaging device.

[0057] The adjusting means 9A may comprise any digital device capable of implementing commands for the tunable circuit to dynamically adjust the equivalent impedance during acquisition of images by the imaging device.

[0058] The implementation of the tunable circuit 9 and the adjustment means 9A makes it possible to consider radio frequency coils with a relatively high quality factor and more particularly associated with a bandwidth much lower than the operating range. In this respect, the radio frequency coil 8 may, according to the aspects of the invention, have a bandwidth of less than 15 kHz, advantageously less than 10 kHz.

[0059] FIG. 3 is a graph illustrating tuning of the resonant frequency of the radio frequency assembly 7. In the illustrated embodiment, the radio frequency coil 8 has a natural bandwidth much lower than the operating range, which is approximately 30 kHz. Curves "A," "B," "C," and "D" represent the resonant profiles of the radio frequency coil of the radio frequency assembly for four different impedance tunings of the tunable circuit. More specifically, each of these four profiles has a resonant frequency within the interval defined by the operating range. It will be appreciated that by continuously varying the impedance of the assembly, it is possible to cover all of the frequencies included in the operating range.

[0060] Thus, during operation, the body is introduced inside the radio frequency coil 8. This body is then subjected to a magnetic field resulting from the sum of the static magnetic field generated by the magnet 2 and the gradient magnetic fields generated by the gradient coil 6.

[0061] The resultant magnetic field, which varies as a function of the coordinates x, y, and z defined by the reference coordinate system (x, y, z), specifically allows spatial encoding of the signals coming from each of the body positions (x, y, z) intended to be probed, thereby making it possible to impose a specific resultant magnetic field on each of these positions. The resultant magnetic field at a given position determines the resonant frequency of the spins of hydrogen nuclei that are subjected to this resultant magnetic field at this point. In other words, the spatial encoding makes it possible to associate each position with a resonant frequency, called the natural frequency, of the spins of hydrogen nuclei at that position. The spatial encoding is reflected by a decomposition of the resultant magnetic field into "slices" (perpendicular to the z-axis), called operating slices, which themselves are decomposed into parallel lines, called operating lines, along which the resultant magnetic field advantageously varies linearly. Each operating line thus defines a range of natural frequencies, called the operating range, that covers all the resonant frequencies of the spins of hydrogen nuclei belonging to that operating line.

[0062] During the measurement, the radio frequency assembly 7 is specifically tuned to transmit a radio frequency signal at a given frequency, called the Larmor frequency of the hydrogen cores corresponding to a given operating line, which is then followed by emission at that Larmor frequency so as to be absorbed by all hydrogen nuclei of the operating line of interest.

[0063] As soon as the RF transmission is interrupted, the nuclear spins of the target line of motion relax to return to their initial energy state and then emit RF signals that can be collected by the radio frequency coil 8. In order to collect all these signals, the adjustment means 9A imposes a dynamic change of the resonant frequency of the radio frequency assembly in the operating range of the target line of motion.

[0064] This process can then be repeated as many times as necessary to explore each of the operating lines.

[0065] In this manner, the radio frequency assembly 7 according to aspects of the present invention allows for covering a wide frequency range while allowing for radio frequency coils having a bandwidth lower than the operating range of interest. In other words, dynamic adjustment of the resonant frequency allows for allowing for radio frequency coils associated with much larger bandwidths, specifically having quality factors greater than the quality factor of the coils comprising the operating range.

[0066] According to one embodiment, the tunable circuit 9 may comprise at least two components arranged according to an L-shaped topology, the at least two components being combined with each other in the tunable circuit to generate a reactance, one and / or the other of these two components being tunable to allow adjustment of the equivalent impedance of the radio frequency assembly.

[0067] Advantageously, the two components include two capacitors, or two inductors, or one capacitor and one inductor.

[0068] The remainder of this disclosure is limited to a tunable circuit formed by a capacitor C1 and an inductor L1. However, those skilled in the art can adapt this description to consider a tunable circuit formed by either two inductors or two capacitors. As an example, those skilled in the art could consider two components, each formed by capacitors C1 and C2 (as illustrated in FIG. 6), or each formed by an inductor (not shown).

[0069] According to a particularly advantageous embodiment, the tunable circuit 9 comprises an inductor L1 and a capacitor C1 and has an L-shaped topology (FIGS. 4 and 5). In particular, the tunable circuit 9 can be formed by an integrated circuit comprising the inductor L1 and the capacitor C1.

[0070] More specifically, one and / or the other of inductor L1 and capacitor C1 is tunable.

[0071] Thus, according to a first variant, it is possible to consider a tunable capacitor C1, in other words, the equivalent impedance of the radio frequency assembly can be adjusted by changing the capacitance of the capacitor C1.

[0072] According to a second variant, it is possible to consider an adjustable inductor L1: in other words, the equivalent impedance of the radio frequency assembly can be adjusted by changing the inductance of the inductor L1.

[0073] Furthermore, according to this advantageous embodiment, the tunable circuit 9 comprises two inputs and two outputs. In particular, the two inputs, respectively called the first input E1 and the second input E2, are intended to be powered by a generator of current pulses, and the two outputs, respectively called the first output S1 and the second output S2, are each connected to one of the ends of the radio frequency coil 8. One and / or the other of the second input E2 and the second output S2 can be grounded.

[0074] More specifically, the radio frequency assembly 7 comprises two branches, namely a first branch and a second branch, connected in parallel to a first input E1 and a second input E2, respectively. The first branch comprises a radio frequency coil 8 and either an inductor L1 or a capacitor C1, connected in series, and the second branch comprises the other of the inductor L1 and the capacitor C1.

[0075] More specifically, FIG. 4 shows a radio frequency assembly in which a first branch is formed by the series connection of a radio frequency coil 8 and a capacitor C1, and a second branch is formed by an inductor L1.

[0076] Similarly, FIG. 5 shows a radio frequency assembly in which a first branch is formed by the series connection of a radio frequency coil 8 and an inductor L1, and a second branch is formed by a capacitor C1.

[0077] The magnetic resonance imaging device may further comprise radio frequency pulse generating means 10 adapted to impose a circulation of current pulses in the radio frequency coil 8 via a tunable circuit 9. Advantageously, the radio frequency pulse generating means 10 may also be configured to control the tunable circuit and thus allow adjustment of the equivalent impedance. However, the invention is not limited to this aspect and a person skilled in the art may, based on his general knowledge, consider any other means, for example a digital controller, for adjusting the equivalent impedance.

[0078] The magnetic resonance imaging device may further comprise radio frequency processing means 11 adapted to process radio frequency signals that the radio frequency coil 8 can receive.

[0079] More particularly, the radio frequency pulse generating means 10 may also be implemented to power the gradient coils 6 in order to spatially encode each of the body positions likely to be present within the bore 3.

[0080] The imaging device 1 may further comprise an interface 12 providing a link between the pulse generating means 10 and the gradient coil 6 .

[0081] The adjusting means 9A, the pulse generating means 10, the radio frequency processing means 11 and the interface 12 can be controlled by a control unit 13, for example a computer.

[0082] The conditioning means 9A, the pulse generating means 10, the radio frequency processing means 11 and the interface 12 can be integrated into a control console of the imaging device.

[0083] Further, in accordance with an aspect of the present invention, the radio frequency coil 8 may be sized to define a receiving bore (for receiving a body or body part) having a length of 50 cm and a diameter of 27 cm.

[0084] The imaging device according to the invention can advantageously be implemented in a portable imaging system, for example having a mass of less than 100 kg (in particular equal to 75 kg).

[0085] Finally, Figure 7 shows the effect of the quality factor of the radio frequency coil on the obtained image. More specifically, images (1) and (2) are obtained using an imaging device equipped with a radio frequency coil having a standard quality factor and a high quality factor (greater than the standard quality factor), respectively. Image (2) reveals an artifact associated with a high quality factor, which results in spatial variation of the signal-to-noise ratio along the reading axis. This artifact is detrimental to the clinical interpretation of the image. The objective of the present invention is to maintain the signal-to-noise ratio throughout the image.

[0086] The invention also relates to a method for obtaining images of a body via magnetic resonance using the imaging device 1.

[0087] In detail, the method comprises the following steps: a) exposing a body disposed within a radio frequency coil to a static magnetic field; b) imposing spatial encoding on the body using gradient coils, which expose the body to gradient magnetic fields that are added to the static magnetic field to form a resultant magnetic field in order to associate each body position with a resonant frequency called the natural frequency of the spins of hydrogen nuclei, all of which natural frequencies are spread over the operating range; c) transmitting radio frequency signals using a radio frequency coil to excite hydrogen nuclear spins throughout the body at locations that are spatially encoded by the gradient coil; d) measuring echoes of hydrogen nuclear spins emitted by at least some of the body locations subjected to spatial encoding by the gradient coil, the measuring including dynamic adjustment of the equivalent impedance of the assembly formed by the tunable circuit and the radio frequency coil.

[0088] Advantageously, the spatial encoding imposed by the gradient coils is reflected in the resultant magnetic field by a decomposition into slices, called motion slices, which are themselves subdivided into mutually parallel motion lines along which the resultant magnetic field varies.

[0089] More advantageously, spin echo measurements are performed one operating line at a time.

[0090] Specifically, the spin echoes likely to be measured along the line of operation cover a frequency range wider than the bandwidth of the radio frequency coil, and measurements along the line of operation are performed by dynamically adjusting the equivalent impedance of the assembly formed by the tunable circuit and the radio frequency coil so as to collect all spin echoes associated with the line of operation.

[0091] The frequency range associated with the spin echo of one line may be at least five times larger, advantageously ten times larger, than the characteristic bandwidth of the radio frequency coil.

[0092] Step c) can also be carried out by considering the dynamic adjustment of the equivalent impedance of the assembly formed by the tunable circuit and the radio frequency coil.

[0093] Alternatively, dynamic adjustment may not be implemented during step c) so as to collectively export multiple regions of the body.

[0094] Naturally, the invention is not limited to the described embodiments, and variant embodiments can be envisaged without departing from the scope of the invention as defined by the claims.

Claims

1. 1. A magnetic resonance imaging device comprising a radio frequency assembly configured to transmit and receive radio frequency signals, The radio frequency assembly includes: a magnet defining a bore; a radio frequency coil placed in said bore, the interior of which forms a zone called the analysis zone in which said magnet imposes a static magnetic field, said radio frequency coil characterized in terms of a natural bandwidth and a natural resonant frequency, and intended to transmit and receive radio frequency signals; a tunable circuit associated with said radio frequency coil and configured to make it possible to adjust the equivalent impedance of said radio frequency assembly within a given impedance range, said adjustment of said equivalent impedance making it possible to adjust a resonant frequency, called tuning frequency, within a frequency range called operating frequencies of said radio frequency assembly, said range of operating frequencies being greater than the range of said intrinsic bandwidth; - adjusting means adapted to instruct said tunable circuit to dynamically adjust said equivalent impedance during acquisition of an image by said magnetic resonance imaging device; - radio frequency processing means, said radio frequency processing means being adapted to process radio frequency signals capable of being received by said radio frequency coil; - a magnetic resonance imaging device comprising a gradient coil intended to spatially encode each of the positions of the analysis zone, said spatial encoding being intended, in combination with the static magnetic field, to associate each of said positions with a resonant frequency, called the natural frequency, for the spins of hydrogen atoms nuclei likely to be located at said positions.

2. 2. The magnetic resonance imaging device of claim 1, wherein the tuning means is configured to enable radio frequency transmission at a given frequency, called the Larmor frequency, and reception of radio frequency signals in which the tuning frequency is dynamically tuned within an operating range.

3. The magnetic resonance imaging device of claim 1 , wherein the radio frequency coil comprises a capacitor.

4. 2. The magnetic resonance imaging device of claim 1, wherein the tunable circuit comprises at least two components arranged according to an L-shaped topology, the at least two components being combined with each other in the tunable circuit to generate a reactance, one and / or the other of the two components being tunable to allow adjustment of the equivalent impedance of the radio frequency assembly, and advantageously the two components include two capacitors, or two inductors, or one capacitor and one inductor.

5. 5. The magnetic resonance imaging device of claim 4, wherein the tunable circuit has two inputs and two outputs, the two inputs, called first and second inputs respectively, intended to be powered by a generator of current pulses, and the two outputs, called first and second outputs respectively, each connected to one of the ends of the radio frequency coil.

6. 6. The magnetic resonance imaging device of claim 5, wherein the radio frequency assembly comprises two branches, i.e., a first branch and a second branch, connected in parallel to the first input portion and the second input portion, respectively, the first branch comprising the radio frequency coil and one of the two components connected in series, and the second branch comprising the other of the two components.

7. 2. The magnetic resonance imaging device of claim 1, wherein the radio frequency assembly further comprises a radio frequency pulse generating means adapted to impose a circulation of current pulses in the radio frequency coil via the tunable circuit.

8. 2. A magnetic resonance imaging device according to claim 1, wherein the magnet is a permanent magnet, advantageously capable of generating a static magnetic field of less than 100 mT, even more advantageously less than 50 mT.

9. 9. The magnetic resonance imaging device of claim 8, further comprising gradient coils intended to spatially encode each of the positions in the analysis zone, the spatial encoding, in combination with the static magnetic field, intended to associate each of the positions with a resonant frequency, called the natural frequency, for hydrogen atomic nuclear spins likely to be located at said positions.

10. 10. The magnetic resonance imaging device of claim 9, wherein the adjusted frequency can cover all of the natural frequencies of the hydrogen atomic nuclear spins that are likely to be present at each of the positions in the analysis zone by adjusting the equivalent impedance.

11. A method for acquiring magnetic resonance images using a magnetic resonance imaging device according to any one of claims 1 to 10, said method comprising the steps of: a) exposing a body disposed within the radio frequency coil to the static magnetic field; b) imposing spatial encoding on the body using the gradient coils, which expose the body to gradient magnetic fields that are added to the static magnetic field to form a resultant magnetic field in order to associate with each of the body positions a resonant frequency called the natural frequency of the hydrogen nuclear spins, all of which natural frequencies are spread over an operating range; c) transmitting radio frequency signals using the radio frequency coil to excite the hydrogen nuclear spins throughout the body at locations that are spatially encoded by the gradient coil; d) measuring echoes of hydrogen nuclear spins emitted by at least some of the locations of the body that are subjected to the spatial encoding by the gradient coil, said measuring comprising dynamically adjusting the equivalent impedance of the radio frequency assembly formed by the tunable circuit and the radio frequency coil.

12. 12. The method for acquiring magnetic resonance images according to claim 11, wherein the spatial encoding imposed by the gradient coils is reflected in the resultant magnetic field by a decomposition into slices, called motion slices, which are themselves subdivided into mutually parallel motion lines along which the resultant magnetic field varies.

13. 13. A method for acquiring magnetic resonance images as claimed in claim 12, wherein said measuring of echoes of hydrogen nuclear spins is performed one line of motion at a time.

14. 14. The method for acquiring magnetic resonance images as described in claim 13, wherein the echoes of hydrogen nuclear spins likely to be measured along the line of operation cover a frequency range wider than the bandwidth of the radio frequency coil, and the measurements along the line of operation are performed by dynamically adjusting the equivalent impedance of the radio frequency assembly formed by the tunable circuit and the radio frequency coil so as to collect all of the echoes of hydrogen nuclear spins associated with the line of operation.

15. 15. A method for acquiring magnetic resonance images according to claim 14, wherein the frequency range associated with echoes of the hydrogen nuclear spins of one line is at least five times greater, advantageously ten times greater, than the characteristic bandwidth of the radio frequency coil.

Images