RU2795364C1 - Method for increasing the homogeneity of the radio frequency field of a mid-field magnetic resonance tomographic scanner and a coil for its implementation - Google Patents

Abstract

FIELD: medical equipment.

SUBSTANCE: group of inventions is related to a method for increasing the homogeneity of the radio frequency field of a sensor of a mid-field magnetic resonance tomographic scanner and a coil for its implementation. The method includes supplying an exciting electrical signal to parallel-connected turns of a radio frequency coil configured to transmit or pick up a signal from the turns of a radio frequency coil configured to receive, in which the exciting signal is applied or removed between the centres of the upper parts of the conductor loops, primarily to the extreme turns of the radio frequency coil or from them. The radio frequency coil of the sensor of a magnetic resonance tomographic scanner is a winding of electrically connected turns parallel to one another located on a hollow frame, oriented orthogonally relative to the longitudinal axis of the frame, distributed evenly. The ends of the turns are connected by conductors having an electrical connection with the equipment of a magnetic resonance tomographic scanner. The conductors are closed in loops on the outer side of the frame in such a way that the lower part of the loop connecting the turns is on the frame, the upper part is separated from it at a distance exceeding the gap between the parallel loops of the connecting conductors by more than 2 times. The conductors are interconnected in the middle of the upper part of the loop by an element for applying or removing a signal. The radio frequency coil of the magnetic resonance tomographic scanner sensor contains a winding of coils electrically connected in parallel to one another, having an electrical connection of conductors with the equipment of the magnetic resonance tomographic scanner and placed on a hollow frame, where the coils are oriented orthogonally relative to the longitudinal axis of the frame. The turns are evenly distributed, the conductors connecting them are closed in loops on the outside of the frame, while the lower part of the loop connecting the turns is on the frame. The upper part is separated from it at a distance exceeding the gap between the parallel loops of the connecting conductors by more than 2 times. The conductors are interconnected in the middle of the upper part of the loop by an element for applying or removing a signal.

EFFECT: optimization of the design of the inventive radio frequency coil of the sensor of a mid-field magnetic resonance tomographic scanner, including the relative position, dimensions and ratio of its elements, as well as a new method for supplying the exciting electrical signal to the turns of the transmitting radio frequency coil connected in parallel or picking up the signal from the turns of the receiving radio frequency coil, which leads to elimination of these shortcomings of analogues and prototype.

5 cl, 5 dwg

Description

The present technical solution relates to medical physics, namely to the receiving-transmitting system of a magnetic resonance tomograph (MRI), in which the main magnetic field is located orthogonally to the longitudinal axis of the sensor, to obtain a nuclear magnetic resonance signal with subsequent imaging. The proposed solution can be used to obtain a magnetic resonance image of a patient in a specialized mid-field MRI.

The uniformity of the NMR RF excitation field makes it possible to use various imaging techniques with minimization of the manifestation of artifacts associated with the inaccuracy of the rotation angle of the magnetization vector when exposed to RF pulses. The uniformity of the field "created" by the receiving circuit ensures the uniformity of the brightness and contrast of the obtained images over the entire working space of the sensor. The use of mid-field MRI setups with a field induction of 0.2-0.5 T sets specific requirements for the design of transmitting and receiving RF coils. Typically, either saddle coils or coils of a solenoid design are used. The disadvantage of the saddle coil is the impossibility of providing a high field uniformity far from the center of the observation area. The solenoidal coil has a large area of uniformity, but only with a significant number of series-connected turns, which in the case of medium fields creates too much inductance for operation at frequencies of 10-20 MHz. For such frequencies, with a coil size of about 200 mm, the acceptable number of turns is no more than 1-2. In this case, a parallel connection of the turns is used and the method of supplying the excitation signal to the turns of the coil or picking up the received signal becomes essential.

Known specialized receiving sensor for imaging magnetic resonance imaging of the forearm [US 5185577 A, 09.02.1993]. The sensor contains a saddle-shaped winding that is wound around a hollow body. The body has a tapering end section. The diameter of the transducer body increases towards the patient's hand in accordance with the area under examination, so that the spiral element can be located closer to the patient's forearm. The disadvantage of this solution is that the saddle-shaped coil does not allow achieving high uniformity in a large volume, in contrast to the solenoidal one.

A sensor with a conformal solenoid coil with parallel winding is described [US 5543710 A, 08/06/1996]. The shape of the coil is close to the area under study: arm, wrist and leg. The disadvantage of this solution is that evenly distributed conductors cannot create a uniform RF field in a large volume. Because of this, when diagnosing on an MRI scanner, it may be necessary to repeatedly move the coil. An inhomogeneous field, in addition, distorts the image, as a result of which the reliability of the diagnosis deteriorates.

From [RU 2192165 C1, 11/10/2002] a sensor with a receiving radio frequency coil is known, designed for magnetic resonance imaging of the ankle. An increase in the signal-to-noise ratio is achieved by approaching the coils of the sensor to the object of study. This solution is aimed at diagnosing specific anatomical regions (ankle) and is unacceptable for diagnosing other parts of the human body: hand, elbow, knee.

The closest technical solution to the claimed one, chosen by the applicant as a prototype, is described in the patent [RU 2738132 C1, 08.12.2020]. It uses a receiving solenoid coil of 8 turns connected in parallel, the relative position of which is calculated and optimized using computer simulation of the generated fields. Based on the principle of symmetry, when using a coil as a transmitter, a radio frequency field will be created that approaches a uniform one. The disadvantage of the solution is that a small number of turns creates undulating field inhomogeneities in the observation area, especially when the control point approaches the coil turns. In addition, the radio frequency field is scattered over large distances between the turns and interacts with the elements of the magnetic system of the tomograph. This leads to a decrease in the quality factor of the circuit and an increase in noise, which requires additional shielding of the sensor.

The technical problem solved by the creation of the claimed invention is to reduce the inhomogeneity of the radio frequency field of the radio frequency coil operating in the transmit mode in the maximum observation area to ensure optimal excitation conditions for the spin system and to ensure uniform sensitivity of the radio frequency coil operating in the receive mode to signals from sources from different points of the observation area to obtain uniform brightness and contrast of images, as well as to reduce the scattering of the radio frequency field and its interaction with the elements of the MRI magnetic system to increase the quality factor of the radio frequency coils of the sensor and reduce interference.

The technical result consists in optimizing the design of the inventive RF coil of the mid-field MRI sensor, including the relative position, dimensions and ratios of its elements, as well as in a new method for supplying an exciting electrical signal to the turns of the transmitting RF coil connected in parallel or picking up the signal from the turns of the receiving RF coil, which leads to elimination of these shortcomings analogues and prototype.

The technical problem is solved, and the technical result is achieved by the claimed radio frequency coil of the mid-field MRI sensor, containing a winding of electrically connected turns in parallel to one another, having an electrical connection of conductors with MRI equipment and placed on a hollow frame, where the turns are oriented orthogonally relative to the longitudinal axis of the frame, the peculiarity of which is in that the turns are evenly distributed, the conductors connecting them are closed in loops on the outside of the frame in such a way that the lower part of the loop connecting the turns is on the frame, the upper part is separated from it at a distance exceeding the gap between the loops by more than 2 times , the conductors are interconnected in the middle of the upper part of the loop by an element for applying or removing a signal, and the mutual arrangement of the elements is optimized by calculation.

The proposed design of the radio frequency coil can be used for both transmitting and receiving coils, since the principle of symmetry applies to the propagation of electromagnetic fields, and the functions of the transmitting and receiving coils in the sensor can be either separated or combined in one coil using appropriate matching means. and signal decoupling.

The technical problem is also solved, and the technical result is achieved by the claimed method of increasing the homogeneity of the radio frequency field of the mid-field MRI sensor, including the supply of an exciting electrical signal to the turns of the transmitting radio frequency coil connected in parallel or picking up the signal from the turns of the receiving radio frequency coil, in which the excitation signal is applied or removed between the centers of the upper parts of the loops of conductors, when the signal is applied or removed, first of all, from the extreme turns of the radio frequency coil, and the radio frequency coil of the mid-field MRI sensor is the previously described radio frequency coil - a winding located on a hollow frame of electrically connected parallel to one another turns, oriented orthogonally relative to the longitudinal axes of the frame, the ends of the coils are connected by conductors that are electrically connected to the MRI equipment, the conductors are closed in loops on the outer side of the frame in such a way that the lower part of the loop connecting the coils is on the frame, and the upper part is separated from it at a certain distance exceeding the gap between loops more than 2 times, the conductors are interconnected in the middle of the upper part of the loop by an element for supplying or removing a signal, the turns are evenly distributed, and the mutual arrangement of the elements is optimized by calculation.

The general design of the radio frequency coil of the sensor is shown in figure 1. The radio frequency coil is a number of identical, evenly and densely located on a hollow frame 1 of a cylindrical shape made of dielectric material perpendicular to the axis of the cylinder turns 2. Each turn 2 is a strip of sheet metal with high conductivity, attached to the frame 1 and covering it almost 360 degrees, but not closed by the ends. The sheet material was chosen in order to increase the quality factor of the RF coil due to the larger surface area of the turns, which reduces the effective resistance to high-frequency currents. The ends of the turns 2 on each side are interconnected in parallel by strips of the same material along the generatrix of the cylindrical frame - connecting conductors 3. The length of the connecting conductors 3 is more than twice the length of the cylindrical frame 1, so that the ends of the connecting conductors 3 can be bent at the edges frame 1 outward (up) and closed in loops (figure 2) so that the upper edge of the loops of the connecting conductors 3 is separated from the surface of the frame 1 at a certain distance (loop height). Loops of parallel connecting conductors 3 are connected to each other in the upper part 4 in the middle of the connecting conductors 3 (in the center of the frame 1) by an element 5 for applying or removing a signal, which is a matching circuit consisting of capacitors and diodes used to tune the RF coil circuit to resonance at the operating frequency of the sensor and ensure efficient power transfer from the MRI transmitter. For the receiving RF coil, the circuit provides suppression of the signal from the transmitting RF coil at the time of the excitation pulse and maximum resonant gain when the NMR signal is received.

The fields were simulated using a computer application for three-dimensional electrodynamic simulation "CSTStudioSuiteStudentEdition", Computer Simulation Technology, Germany. A variant of low-frequency modeling was used to obtain the spatial distribution of the magnetic component of the radio-frequency field inside the radio-frequency coil. Optimization was carried out by multiple calculations with the assignment of various design parameters of the RF coil, such as the width of the turn, the gap between the turns, the width of the connecting conductor and the loop, the gap between the loops of the signal supply. Based on the calculation results, graphs of the distribution of the field strength along straight lines parallel to the axis of the radio-frequency coil at various distances from the center were plotted. According to the graphs, the option with the best results in terms of homogeneity was selected. The simulation was performed for several variants of RF coil diameters, the results of optimizing the ratio of the height of the loop and the gap between the loops are the same.

Using computer simulation, the optimal parameters of the gaps and height of the loops of the connecting conductors 3 and other design parameters of the radio frequency coil were obtained with dimensions of 220 × 195 mm (length and diameter of the frame):

• The gap between parallel looped connecting conductors 3 is no more than 2 mm, since a larger gap worsens the field uniformity (the field in the middle of the radio frequency coil increases) and increases scattering;

• The height of the loop of connecting conductors 3, including the thickness of the tape - from 4 to 10 mm, is limited by the permissible dimensions of the sensor, a smaller size worsens the field uniformity similarly to the effect of the gap size;

• Width of turns 2 and connecting conductors 3 - 8 mm;

• Thickness of turns 2 and connecting conductors 3 - 0.2 mm (chosen based on the thickness of the skin layer of the electromagnetic field at the operating frequency);

• Gap between coils 2 - 2 mm.

The simulation was also performed for other diameters of the RF coil - 130 and 90 mm, the results of optimizing the ratio of the height of the loop and the gap between the loops are the same.

The inventive method is carried out as follows.

The input signal is applied to the inventive radio frequency coil between the middle of the upper side 4 of the loops of the connecting conductors 3 through the element 5 for applying or removing the signal (figure 3). With the proposed design of the radio frequency coil, the current is primarily transmitted to the turns located at the ends of the RF coil, as a result of which the turns located closer to the edge receive more current and create a stronger field than the turns in the center of the RF coil, which have high inductance due to mutual influence. This compensates for the decrease in field strength at the edges caused by the decrease in effective turn density towards the ends of the RF coil.

When the sensor is exposed to gradient pulses from the MRI magnetic system, Foucault currents arise in the turns of radio-frequency coils, which limit the rate of change of gradient fields. From this point of view, the width of the turns should be as small as possible in order to minimize the resulting ring currents. At the same time, a decrease in the width and gap between the turns and an increase in the number of turns increase the technological costs and labor intensity of manufacturing RF coils. Modeling for various parameters of the width of the turns showed that with the specified width (8 mm), the effect of the radio frequency coil on the gradient pulses is insignificant.

The distribution of currents is also affected by the dimensions and gaps in the design of the loops, since the currents in the loops at radio frequencies create interacting fields. With the stated ratio of the height of the loop to the gap between the loops of the connecting conductors 3, which is more than two, the fields inside the loops are mutually compensated and do not significantly affect the distribution of currents in the turns of the radio frequency coil.

The figure 4 shows the graphs of the intensity of the longitudinal component of the magnetic field in units of A/m when excited by a current of 1 A at a frequency of 17.5 MHz on the displacement along the cylinder at various distances from its axis (0, 50, 70 mm) in the vertical plane according to the results of computer simulation for the model of the previously indicated dimensions. With displacements in the horizontal plane, the results are practically the same. As we can see, at an average displacement from the center, the field strength along the axis is actually constant over a considerable distance, which provides optimal conditions for the excitation of the spin system in the volume of the observed object. With displacements in the horizontal plane, the results are practically the same.

To illustrate the invention, a sensor was made with the inventive RF coil. Cylindrical frame 1, made of dielectric material, in this embodiment of the invention was made of polystyrene 220 mm long with a circular cross section with a diameter of 195 mm. On the cylindrical frame 1 tightly to each other there are 22 coils 2 covering the frame 1 almost 360 degrees, but not closed into a ring, 8 mm wide with a gap between them of 2 mm. The ends of the turns 2 on each side are interconnected by connecting conductors 3 8 mm wide located parallel to the generatrix of the cylindrical frame 1, closed in loops in such a way that the upper edge of the loops of the connecting conductors 4 is spaced from the surface of the frame 1 at a distance of 5 mm (loop height). The gap between the conductors of the loops is 2 mm. Coils 2, connecting conductors 3 and loops 4 are made of sheet electrical copper. The thickness of the copper tape for the coils and conductors is chosen based on the size of the skin layer at an operating frequency of 17.5 MHz and is 0.2 mm. It is necessary to choose a material thickness of at least two skin-layer values, taking into account that the current flows through the conductor from all sides.

The figure 5 shows the results of an experimental test on the layout of the transmitting RF coil. The measurements were carried out using a probe in the form of a coreless winding placed inside the sensor when a radio frequency signal was applied to the loops of the transmitting radio frequency coil. The dependences of the amplitude of the detected signal in millivolts on the position of the probe along the axis at different distances from it are given. As you can see, the measurement results generally correspond to the calculations, distortions can be caused by the influence of the environment and the lead wires of the probe.

Thus, the proposed method for increasing the homogeneity of the radio frequency field of a mid-field MRI sensor, characterized by the position of the power supply and signal pickup element, as well as the design of the radio frequency coil of the sensor, provide better uniformity of the radio frequency field created by the transmitting radio frequency coil, as well as the constancy of the sensitivity of the receiving circuit over the sensor observation area. In addition, the tight arrangement of the turns of the RF coil ensures minimal field leakage through the turn-to-turn gaps and good shielding from interference from the MRI magnetic system. Also, the invention is directed to expanding the arsenal of devices for imaging in magnetic resonance imaging.

Claims (5)

1. A method for increasing the homogeneity of the radio frequency field of a sensor of a magnetic resonance tomograph, including supplying an excitation electrical signal to the turns of a radio frequency coil connected in parallel, configured to transmit, or picking up a signal from the turns of the radio frequency coil, configured to receive, in which the excitation signal is applied or removed between the centers of the upper parts of the loops of conductors, thus, first of all, to the extreme turns of the radio frequency coil or from them, and the radio frequency coil of the sensor of a magnetic resonance tomograph is a winding located on a hollow frame of electrically connected parallel to one another turns, oriented orthogonally relative to the longitudinal axis frame, evenly distributed, the ends of the turns are connected by conductors having an electrical connection with the equipment of a magnetic resonance imaging scanner, the conductors are closed in loops on the outside of the frame in such a way that the lower part of the loop connecting the turns is on the frame, the upper part is separated from it at a distance, exceeding the gap between parallel loops of connecting conductors by more than 2 times, the conductors are connected to each other in the middle of the upper part of the loop by an element for applying or removing a signal. 2. A radio frequency coil of a magnetic resonance tomograph sensor, containing a winding of turns electrically connected in parallel to one another, having an electrical connection of conductors with the equipment of a magnetic resonance tomograph and placed on a hollow frame, where the turns are oriented orthogonally relative to the longitudinal axis of the frame, characterized in that the turns are evenly distributed, the conductors connecting them are closed in loops on the outside of the frame in such a way that the lower part of the loop connecting the turns is located on the frame, the upper part is separated from it at a distance exceeding the gap between the parallel loops of the connecting conductors by more than 2 times, the conductors interconnected in the middle of the upper part of the loop by an element for applying or removing a signal. 3. The radio frequency coil of the magnetic resonance scanner according to claim 2, characterized in that the radio frequency coil is configured to perform the function of a transmitting coil. 4. The radio frequency coil of the magnetic resonance scanner according to claim 2, characterized in that the radio frequency coil is configured to perform the function of a receiving coil. 5. The radio frequency coil of the magnetic resonance tomograph sensor according to claim 2, characterized in that the radio frequency coil is configured to perform the functions of a transmitting and receiving coil at the same time.

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