CN100526906C - Degenerate birdcage coil and transmit/receive apparatus and method for same - Google Patents

Abstract

A birdcage coil (16) used in conjunction with a magnetic resonance imaging apparatus includes a first conductive loop (81, 581), a second conductive loop (82, 582), and a plurality of first conductor rungs (80, 580) disposed between the first and second conductive loops. A third conductor (83, 83'', 583) is coupled to the second conductive loop at resonance frequencies, such as by second conductor rungs (84, 84'', 584). The birdcage coil also includes switches (590) for switching the birdcage coil at least among: 1) an RF transmit mode to operate as an RF transmit coil; and 2) an RF receive mode to operate as an RF receive coil.

Description

Degenerated cage coil and magnetic resonance imaging system and method using same

technical field

The present invention relates to Magnetic Resonance Imaging (MRI) techniques. The technique is particularly suitable for use in conjunction with a bore MRI scanner and will be described with particular reference to a bore MRI scanner. However, it should be understood that the invention may also be used in other similar applications and with other types of MRI scanners.

Background technique

Typically, in MRI, a substantially uniform, time-constant main magnetic field _B0 is established in an examination region in which the object to be imaged is placed. Through magnetic resonance radiofrequency excitation and manipulation, selected magnetic dipoles in the subject are tilted (via RF pulses) into a plane transverse to the main magnetic field region, allowing them to precess and resonate without being affected by magnetic resonance Under RF excitation and operation, these magnetic dipoles are aligned with the main magnetic field. The resonant dipoles can then decay or realign according to the main magnetic field to induce magnetic resonance (MR) echoes or MR signals. The various echoes making up the MR signal are encoded by the magnetic field gradients established in the main magnetic field. Raw data from the MRI scanner or device is collected into a matrix commonly referred to as K-space. Generally, each echo is sampled multiple times to generate a data line or row of data points in k-space. Finally, an image representation of the object is reconstructed from the k-space data using an inverse Fourier transform or other known transform.

Typically, when imaging a relatively large area, such as the torso, whole body radio frequency coils are employed in RF transmit and receive modes. The difference is that when imaging the head, neck, shoulders, or other relatively small areas, typically a local coil is used as the RF receive coil in conjunction with a whole body transmit coil. One reason for using a local coil is that the proximity of the local coil to the area being imaged improves the signal-to-noise ratio and resolution.

It is also beneficial to use local coils as RF transmissions in some procedures. One benefit is that the use of local coils during RF transmission reduces the specific absorption rate (SAR) and the effect of concomitant heat generation in the body compared to RF from body coils.

One type of local coil used in MRI scanners is known as a "birdcage" coil. So-called birdcage RF coils for receiving MR signals are known. See, eg, US Patent No. 4,680,548. Typically, a birdcage coil is cylindrical and contains two conductive end loops or rings connected by an even number of rods or axial conductors that divide the end rings into two arcs defined therebetween or paragraph. This structure makes this RF coil have a birdcage shape. Accordingly, the term "birdcage" often refers to this geometric shape.

Such birdcage coils typically have many different resonant frequencies. Each resonance frequency is associated with a different resonance mode. Typically, the birdcage coil is operated in a resonant mode selected to obtain within the coil: (i) as uniform a field distribution as possible (in the case of use as a transmitter coil); and/or, (ii) Constant sensitivity independent of position (in case of receiving coil). Position independence of sensitivity is advantageous, especially in a plane orthogonal to the coil axis, as it facilitates the reconstruction of the received image from the MR signal.

Referring to FIG. 2, the electrical characteristics of the birdcage coil are described, and FIG. 2 shows various resonance frequencies (ordinate) and resonance modes (abscissa). In principle, a birdcage coil can be viewed as having as many resonant modes or frequencies as there are half the number of pins or axial conductors connecting the end rings together. Thus, a birdcage with eight pins has four different resonant frequencies and four different resonant modes (with different field distributions or different position-dependent sensitivities). The resonant frequencies and resonant modes associated with a given ratio of the coil are interconnected by dashed curves. Assuming an eight-pin birdcage is built, the different birdcage coil types can be distinguished as follows:

a) The low-pass type is characterized in that the end ring has no capacitance, so that only the locally distributed inductance of a single arc or segment of the ring is active. There are four different resonant frequencies, the lowest resonant frequency is related to the resonant mode of uniform field distribution or sensitivity occurring in the area enclosed by the coil, therefore, it is recommended to use this mode. The field distributions of other resonant modes at higher resonant frequencies are less suitable and are therefore generally not used. See LP in Figure 2 for the curve of this type of coil.

b) A high-pass type is formed when there is capacitance only in the end loop, while the pins contain no capacitance and consist of continuous conductors so that only the locally distributed inductance of a single pin is active. This type also has four resonance frequencies with four resonance modes, however, the preferred resonance mode is at the highest frequency. See HP in Figure 2 for the curve of this type of coil.

c) Bandpass types have capacitance in both end rings and in the axial pins. As described in U.S. Patent No. 4680548, for example, "high-pass" and "low-pass" characteristics can be assigned to this type of bandpass, depending on whether the capacitance in the pin or in the end ring exhibits inductance (the other part of the respective The characteristic shown is capacitance).

For the curves of the above-mentioned band-pass type with low-pass characteristics see BP ₁ in Fig. 2, the preferred resonant mode (with locally uniform sensitivity) is at the lowest resonant frequency. A curve of the bandpass type with highpass characteristics see BPh in Figure 2, with the preferred resonance mode at the highest resonance frequency.

There are also coil arrays (also known as phased array coils, usually consisting of multiple overlapping planar sub-coils) that have a better signal-to-noise ratio than such birdcage coils. Accordingly, such coils are used to obtain the advantages associated with coil arrays in a birdcage geometry.

For example, US Patent No. 6,043,658 (the '658 patent) describes a bandpass type of birdcage coil that incorporates capacitance in the axial pins and in both end rings. However, these capacitors are balanced so as not to exhibit high-pass or low-pass characteristics, but to seek a characteristic as shown by curve BP _d in Fig. 2, where all resonant modes appear or substantially converge to the same frequency. This property is referred to as "degeneration" in the paper "Hybrid birdcage resonators" in the Proceedings of the 11th Annual Meeting of the Society for Magnetic Resonance in Medicine in Troop (1992). Therefore, a band-pass type birdcage coil having a resonance frequency converging toward a common frequency (nominally called the degenerate frequency of the coil) is called a degenerate birdcage coil.

The '658 patent considers that, by proper balancing of the capacitors in the two end rings and the interconnecting axial pins, the other multiple resonances of a bandpass-type birdcage coil of a given ratio or size Frequency can degenerate to a single resonant frequency. At this degenerate resonance frequency, the individual adjacent grids that make up the birdcage are decoupled from each other. Due to this decoupling, the grids can function effectively or be operated as one coil array, and the MRI signals received from a single grid can be processed independently in a similar manner. Note that a single grid refers to two adjacent axial pins or conductors and the arcs or sections of two end rings or loops between them.

Preferably, according to the '658 patent, a common image can be reconstructed from the individual MR signals received from the respective individual grids, wherein the common image has a relatively high signal-to-noise ratio, especially near the grids in the coil. grid area. In other words, a single grid has position-dependent sensitivity, ie, signals from regions close to the grid have relatively high sensitivity, and signals from more distant regions have relatively low sensitivity. For example, when a single MR signal received from a single grid forms an overall image, a factor 2 to 4 higher signal-to-noise ratio can be obtained in the outer regions compared to other conventionally operated birdcage coils, whereas in its center then has essentially the same value as conventional coil operation.

Although the above patents are cited, further improvements are still needed. The importance of minimal coupling between eg the individual coils making up a phased array is well understood in the art. Similarly, degenerate birdcage coils also need to have more minimized coupling between meshes. In other words, greater isolation between cells is advantageous, and not only for immediately adjacent cells, but also for cells that are further away (i.e., the next adjacent cell or the cells near the adjacent cell) . Conventional degenerate birdcage coils typically provide no better than about 10 dB of insulation, and typically the grid is only effectively decoupled from its immediate neighbor grids. Additionally, other multiple resonant frequencies are required to converge or degenerate to within a small percentage of each other. In contrast, conventional degenerate birdcage coils may still have a frequency distribution that is not as small as desired. Typically, conventional degenerate birdcage coils can have a frequency range greater than 5% or greater in the resonant frequency. It is also worth noting that the difficulty lies in tuning degenerated birdcage coils to obtain the desired operating characteristics. It would be advantageous to develop a means for adjusting the coil more easily to address this difficulty. Accordingly, an additional degree of freedom is desirable and facilitates proper adjustment.

Degenerated birdcage coils are particularly relevant in parallel imaging techniques such as SENSE and SMASH imaging, among other applications. For example SENSE is based on the application of multiple RF coils and receivers. With multiple coil units, only a limited number of phase encoding steps are required to measure the same area of view with the same resolution: this reduces the acquisition time by equal to the number of coil units. However, unlike phased array coils, the coil units are not used to cover individual anatomical regions. Instead, it is used to measure the same area simultaneously, whereby the individual coil unit sensitivity is used to correct for the reduction in phase encoding steps.

Typically, sensitivity information is collected by transmitting RF signals to and receiving RF signals from the examination region, using alternately a first RF coil (eg, a body coil) and a second coil (eg, a degenerated local birdcage coil). While the first RF coil is receiving signals, the second coil is decoupled from the MR system. Similarly, when the second RF signal receives the signal, the first coil is decoupled. Sensitivity information about the second coil is obtained from the signals received by both coils and used to reconstruct an image from only the signals received by the second coil.

Contents of the invention

The present invention proposes a new and improved birdcage coil and technology which overcomes the above and other problems.

According to one aspect, the disclosed birdcage coil system is used in conjunction with a magnetic resonance imaging apparatus. The birdcage coil includes first and second conductive loops, and a plurality of first conductor bars are arranged between the first and second conductive loops electrically connected thereto. A switch arrangement is provided for switching the birdcage coil at least between an RF transmit mode and an RF receive mode. In RF receive mode, the birdcage coil is tuned for use as a phased array RF receive coil.

According to another aspect, a birdcage coil for use in conjunction with a magnetic resonance imaging device is disclosed. The birdcage coil comprises first and second conductive loops, a plurality of first conductor bars disposed between the first and second conductive coils and simultaneously electrically connected, and spaced from the second conductive loop and connected at radio frequency to the first The third conductor of the secondary circuit.

One advantage is reduced local absorption.

An additional advantage is the ability to perform parallel imaging techniques such as SENSE.

Another advantage is improved coil efficiency.

Yet another advantage is the improved insulation of the mesh of the degraded birdcage coil.

Further advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following description of the preferred embodiment.

Description of drawings

The invention may be implemented by means of parts and arrangements of parts, and in various processing operations and arrangements of processing operations. The drawings are only to illustrate the preferred embodiments and are not to be considered as limiting the invention.

FIG. 1 is a schematic diagram illustrating a magnetic resonance imaging apparatus.

Figure 2 is a diagram useful in describing the various types of operation of the birdcage coil.

Figure 3 is a schematic diagram depicting a birdcage RF coil.

Figure 4 is a schematic diagram of an arrangement for transmitting RF signals to a birdcage coil.

Figure 5 is a schematic diagram of an arrangement for feeding MR signals from a birdcage RF coil to a processing channel.

Figure 6 is a schematic diagram of another arrangement for feeding MR signals from a birdcage RF coil to a processing channel.

Figures 7A, 7B, 7C and 7D together form a plan view showing current flow in a birdcage RF coil.

FIG. 8 is a schematic diagram showing the connections of the birdcage coils of FIG. 3 used to provide a radio frequency receiving grid array.

FIG. 9 is a schematic diagram illustrating another exemplary embodiment of a birdcage RF coil.

FIG. 10 is a schematic diagram illustrating another exemplary embodiment of a birdcage RF coil.

FIG. 11 is a schematic diagram illustrating another exemplary embodiment of a birdcage RF coil.

Figure 12 is a plan view of a birdcage coil similar to the coil of Figure 3, where each coil loop is connected to a separate transmit/receive switch.

FIG. 13 is a schematic diagram of one of the transmit/receive switches of FIG. 12 .

preferred embodiment

Referring to Figure 1, an MRI scanner is depicted. A main field controller 10 controls a main field magnet 12, such as a superconducting or resistive magnet. The main magnet 12 is controlled to generate a substantially uniform, stable main magnetic field region B ₀ having a desired strength, such as |B _{0 |} =3.0 Tesla, in the examination zone or region 14, where the subject placed in the inspection area. As shown, the inspection zone 14 has a cylindrical inner diameter, but other shapes such as an open area inspection zone may also be used. Typically, B ₀ is set in the direction of the Z axis, which is the longitudinal center axis of the examination zone 14 .

Pulse sequences are stored in sequence memory 32 and executed by sequence controller 30 . A sequence controller 30 controls the gradient pulse amplifier 20 and controls the first and second RF transmitters 24, 25 as required. More specifically, sequence controller 30 controls gradient pulse amplifier 20 and the transmitter to generate a plurality of MRI pulse sequences that generate and gradient-encode (i.e., slice, phase, and/or frequency-encode) ultimately received and sampled MR signal.

Gradient pulse amplifier 20 applies current pulses to selected gradient coils 22 to create magnetic field gradients along orthogonal axes to examination region 14 . These magnetic fields extend substantially in the same direction as B ₀ , but have gradients in the x, y and/or z directions, respectively.

A digital radio frequency transmitter 24 transmits radio frequency pulses or pulse packets to a first RF coil 26, such as a whole body birdcage RF coil, to transmit RF pulses to the examination region. Typical RF pulses consist of a group of immediately adjacent burst segments combined with each other, and any applied gradients effect a selected magnetic resonance operation. The RF pulses establish a _B1 magnetic field in the examination region 14 at the Larmor frequency determined by the field strength and the gyromagnetic constant of the magnetic dipole of interest. This frequency is typically about 63.8 MHz for a magnetic field having a strength of 1.5 Tesla, about 128 MHz for a magnetic field having a strength of 3.0 Tesla, and so on. The RF pulse is used to flood, excite resonance, demagnetize, refocus resonance, or otherwise manipulate resonance in a selected portion of the examination region.

In the examination region a second RF coil 16 is arranged and optionally controllable by a second RF transmitter 25 to selectively generate RF pulses in the examination region 14 . As described in FIG. 1, the second RF coil 16 is a birdcage head coil. Like the first RF coil 26, the RF pulses establish a _B1 magnetic field in the examination region.

Regardless of the RF transmitter used, a typical RF pulse consists of a group of immediately adjacent burst segments combined with each other, and any applied gradients effect a selected magnetic resonance operation. Optionally employed MRI pulse sequences generate MR signals for two-dimensional (2D) or three-dimensional (3D) imaging. The optionally used MRI pulse sequence contains any number of single-echo or multi-echo imaging sequences. Imaging can be performed using any of the existing MRI techniques such as field echo (FE) imaging, spin echo (SE) imaging, echo planar imaging (EPI), echo volume imaging, gradient and spin echo imaging (GSE ), fast spinning echo (FSE) imaging, single-camera FSE imaging, three-dimensional volumetric FSB imaging, parallel imaging techniques (eg, sensitivity encoding (SENSE), simultaneous acquisition of spatial harmonics (SMASH), etc.), and/or similar techniques. In the case of 2D imaging, the region of interest represents a 2D cross-sectional slice through the object being imaged. In the case of 3D imaging, the region of interest represents the 3D volume of the object being imaged.

The magnetic resonance signals generated from the examination region 14 are received by RF receive coils which are placed close to the examination region. Optionally, the RF receiving coil may be a human body RF coil 26 or a local RF coil 16 . A first receiver 38 receives magnetic resonance signals from the whole body RF coil 26 and demodulates the signals to a plurality of data lines. If the receiver is analog, a first analog-to-digital converter 40 converts each data line into digital form. Optionally, the analog-to-digital converter is placed between the radio frequency receiving coil 26 and the receiver 38 for a digital receiver.

A second receiver 34 receives the magnetic resonance signal from the local RF coil 16 and demodulates the signal to a plurality of data lines. If receiver 34 is analog, local analog-to-digital converter 40 converts each data line from the local receiver to digital form. Also, the analog-to-digital converter may be located upstream of the digital receiver. The data lines from the receivers 38 , 34 are set and stored in the data memory 42 .

It will be appreciated that the local coil 16 is subdivided into a plurality of sections or grids (eg 8), each section being connected to a second receiver 34 . Thus, the second receiver 34 may comprise a plurality of receivers and associated analog-to-digital converters, eg each grid of local coils 16 comprises a receiver and associated analog-to-digital converter. Alternatively, a single multi-channel receiver can be used.

A reconstruction processor applies an inverse Fourier transform or other suitable reconstruction algorithm to reconstruct the data lines into an image representation. The image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The electronic representation is then stored in image memory 52, where the image can be selectively accessed by video processor 54, which renders slices, projections, or other portions of the representation to a display (e.g., providing The monitor 56) of the human-readable display of the composite image is converted into a suitable form.

Referring to Figure 3, the local RF coil 16 is shown in more detail. The local RF coil 16 can be used as a transmit/receive birdcage coil and thus can act as a quadrature transmit coil and a phased array receive coil. As shown, the phased array consists of eight individual coils or grids. Additionally, the local RF coil 16 can be decoupled from the MR system for sequences where only the body RF coil 26 is used as a receive coil.

As shown, the local birdcage coil 16 is created so as to be generally axisymmetric with respect to an axis of symmetry extending in the z-direction and to comprise a first , second and third radio frequency conductors or loops 81, 82 and 83. In the illustrated embodiment, the loop is substantially equal in diameter. The first and second conductor loops 81 and 82 are electrically interconnected by the first conductor bar 80 . As shown, there are eight rods 80, but other even numbers of similar conductors could be used. Each bar 80 contains a first bar capacitance C ₁ . The first and second loops 81 and 82 are divided into sections or arcs defined between their respective connections to the rod 80 . Each portion of the first conductor loop 81 includes a tuning capacitor C ₄ and a matching capacitor C ₅ . Similarly, each portion of the second conductor loop 82 contains a second loop capacitance C ₂ . Two adjacent bars 80 and the parts of the loops 81 and 82 between the bars define a single grid.

In the illustrated embodiment, the end conductor is a loop. However, other terminal conductors, such as terminal caps or disks, other capacitive or feedback circuits and the like may also be used.

The third loop 83 is electrically interconnected with the loop 82 through the second conductor bar 84 . As shown, there are eight second conductor bars 84, however other even numbers of similar conductors may be used. The second rods 84 are equal in number to the first rods 80 and they are connected to the second loop 82 at substantially the same position as the first conductor 80 . Optionally, the first and second conductor bars 80 and 84 form a single substantially rectilinear conductive unit electrically intersecting the loop 82, the ends of which are electrically connected to the first and third conductors 81 and 83, respectively. Each second conductor bar 84 contains a second bar capacitor _C3 . Optionally placed in the third conductor 83 is an eddy current capacitor C ₆ to access the gradient eddy current established in the coil 16 .

Turning to FIG. 4 , an end view of the first conductor loop 81 is shown, along with the individual first conductor bars 80 ₁ , 80 ₂ , ... 80 ₈ on this page for orthogonal emission, the second (i.e. partial ) RF transmitter 25 is connected to the quadrature splitter for generating two RF transmit signals having a substantially 90 degree phase relationship. The two RF signals are transmitted to the local RF coil 16 via first and second coaxial cables 220, 225, as shown in FIG. A first coaxial cable 220 is coupled to a first half-wavelength coaxial cable 200 whose two feed points 201 , 202 on the coil 81 are connected to the local RF coil 16 . These two feed points are electrically connected to the first conductive rods ₈₀₂ and ₈₀₆ . The second coaxial cable 225 is electrically coupled to the second half-wavelength coaxial cable 210 which is electrically connected to the local RF coil 16 at feed points 211 and 212 . These latter two feed points are electrically connected to the first conductor bars ₈₀₄ and ₈₀₈ . Although a four point feed arrangement is shown in Figure 4, only two feed points may be employed, or alternatively more than four feed points may be implemented.

Figure 5 shows how the received RF signal is obtained from a single grid. At the same time, an end view of the first conductor loop 81 is shown. Each capacitor _C5 acts as a matching capacitor, and a balun 300 is connected across it. The balun is a further suitable shape of a butterfly balun or balun. Note that _the C capacitor is not shown in Figure 5 for clarity and only two output devices are shown. It should be understood that other _C5 capacitors have similar output devices.

Each balun is mounted to a high impedance amplifier or preamplifier 310 via a capacitor _C8 that provides preamplifier isolation. Since each capacitance _C5 is only associated with a respective grid, the voltage across a particular _C5 capacitance is indicative of the MR signal induced by that grid. The MR signal derived from the _C5 capacitance is then applied to a second (ie local) RF receiver via a respective amplifier 310 .

Referring to Figure 6, an alternative feed arrangement is shown which advantageously reduces the number of processing channels 320 used. That is to say, the number of processing channels used in the embodiment of FIG. 5 is equal to the number of grids used in the coil, while the number of processing channels 320 used in the embodiment of FIG. 6 is less than the number of grids used in the coil 16. . Note that the C capacitor is not shown in Figure 6 for clarity, and _only the feed arrangement for the two _C capacitors is shown. It should be understood that other _C5 capacitors have similar feed arrangements.

Figure 6 shows an embodiment in which a pair of processing channels 320 are combined so that only half the number of processing channels 320 used in the embodiment of Figure 5 is used. However more processing channels 320 may also be combined in a similar manner.

As shown in FIG. 6 , the MR signals from two adjacent grids are applied through respective amplifiers 310 to the input of a phase shifter 330 (eg a combined line). Combining line 330 adds the signals appearing at its two inputs with an adjustable phase shift. Subsequently, the output signal of combining line 330 is processed by processing channel 320 . Images formed from MR data from a single processing channel 320 have the best achievable signal-to-noise ratio in this region, the location of which depends on the phase used to combine the MR signals from the two grids. Thus, by changing the phase position, this optimal value can optionally be shifted to a preferred region.

With continued reference to FIGS. 3-6 and to FIG. 7 in more detail, the wiring of the local birdcage coil 16 is shown in plan view. It will be appreciated that in operation, the coil is formed about the longitudinal axis such that points A, B, C and D on the left of FIG. 7 are connected to points A, B, C and D on the right of FIG. 7, thus forming a generally cylindrical Bird cage coil16.

In addition to having a virtually position dependent sensitivity (such as a degenerated birdcage coil), it may be desirable to have a coil that has a substantially uniform sensitivity in the examination region 14, such as can be obtained with conventional birdcage coils. To this end, the capacitance C1 in the first conductor bar 80 and the first and second conductor loops 81, 82 can optionally be switched by switching means, for example a controllable switch such as a PIN diode D1 or the like, which can The controlled switches are arranged in parallel with or bridge the respective capacitors so that the same capacitors are short-circuited when the switches are closed. By selectively closing or opening respective switches, birdcage coils with different electrical characteristics can be obtained. Accordingly, it is possible to switch between different types of birdcage coils (ie, transmit, receive, low-pass, high-pass, band-pass or degenerate, or decouple) without exchanging physical coils.

Correspondingly, within each first conductor bar 80, the first bar PIN diode D1 is arranged in parallel with the capacitor C1. As shown, the PIN diodes in adjacent bars are arranged in opposite directions relative to each other. Within each segment of the first conductor loop 81 is a matching PIN diode D5 arranged in parallel with a matching capacitor C5. The first and second half wavelength coaxial cables 200, 210 are connected to their respective feed points 201, 202, 211, 212 through a transmit PIN diode D6 and a transmit matching capacitor C9. The launch coaxial cable is grounded to the base ring 85 as shown. A second eddy current capacitor C7 is arranged in the base ring 85 to handle the eddy current in the coil. The switching device switches between transmit, receive and decoupled modes based on the DC bias applied to the cage voltage input V1 and the preamplifier voltage input V2.

In one embodiment of the local birdcage RF coil 16, the rod conductors 80, 84 and conductors or loops 81, 82, 83 comprise a cylindrical support unit having a width of about 1.0 cm and a thickness of about 0.009 cm and arranged on a cylindrical support unit suitable for electrical insulation. on the copper strip. In a particular application for anatomical imaging, such as imaging the head, the length of the first rod 80 is about 12 cm and the diameter of the conductor loop is about 25.0 cm. In other applications, the length of the first rod conductor 80 may be about 24.5 cm, and the diameter of the loop may be about 23.0 cm. The second rod conductor 84 may be approximately 2.0 cm in length, and may optionally be less than 1.0 cm in length. In any case, the dimensions indicated here are examples only and may be suitably changed to different dimensions to suit a particular application.

Capacitors are chosen to obtain the desired level of attenuation of the resonant frequency and isolation between meshes. In one embodiment, the resonant frequencies of the coils 16 converge or decay to within less than 5% of each other. In another embodiment, the resonant frequencies of the coils 16 converge or decay to within less than 2% of each other. By adding a preamplifier decoupling device, the grid insulation can be greater than 20dB.

For example, in one experiment using the birdcage coil 16 that created the copper tape dimensions described above, with eight first bar conductors 80 of length 24.5 cm, conductors 81, 82 and 83 of loop diameter 23.4 cm, and eight lengths of The second bar conductor 84 of 2.0ch, and C1=45.8pf, C2=56pf, C3=4.7pf, and C4+C5=56pf, the measured normal mode frequency range is about 980kHz, the center frequency is about 65MHz, and The isolation achieved with preamplifier decoupling is greater than 18dB. Accordingly, it has been found that the addition of a third coil or conductor 84 in the manner provided can support improved mesh insulation and resonant frequency attenuation. Further, a third loop or conductor 83 provides an additional degree of freedom for tuning the coil.

Note that capacitors C4 and C5 are combined in this experimental example as a single first loop capacitor. Optionally, in practice, they are combined. However when the two are separated, the C5 capacitor is used as a matching capacitor to feed the receiver channel 320, and the C4 capacitor is used as a means of trimming the coil.

In operation, a main magnetic field B ₀ is generated in the examination region 14 in which the object to be examined is placed. Applying a series of RF and magnetic field gradient pulses to invert or excite magnetic couple spins, induce MRI, refocus MRI, manipulate MRI, spatially and otherwise encode MRI, saturate spins and the like to generate MRI pulse sequences . More specifically, gradient pulse amplifier 20 applies current pulses to selected or pairs of gradient coils 22 to create magnetic field gradients along the x, y, and z axes of examination region 14 . An integral RF transmitter 24 drives an integral RF transmit coil 26 to transmit RF pulses or pulse packets into the examination region 14 . Optionally, local RF transmitter 25 drives local RF coil 16 to transmit RF pulses or pulse packets into examination region 14 in a similar manner.

In accordance with the RF transmissions described above, in a first mode of operation, the local birdcage coil 16 transmits RF pulses into the examination region. When the local RF coil is applied in transmit mode, the coil is operated as a quadrature transmit coil. In this transmit mode, the cage voltage input V1 and the preamplifier voltage input V2 are applied with a negative potential. This negative potential at V1 and V2 causes emitter PIN diode D6, matching PIN diode D5, first bar PIN diode D1 and pre-pin diode D7 to be forward biased. The resulting coil structure is a four-point fed Qualcomm birdcage. The birdcage feed is connected to the coil through transmit PIN diode D6. The matching diode D5 short-circuits the receiving adjustment inductance coil L5 and the matching capacitor C5, as shown in FIG. 7 . Doing so allows the birdcage to be tuned when the guard level is applied to the preamplifier 310 . The first leg of PIN diode D1 shorts the first leg of capacitor C1 making the coil a high-pass birdcage. Preamplifier diode D7 shorts the input of preamplifier 310, providing another level of protection for the preamplifier. The third conductor or loop 83 (supporting the degenerate birdcage mode) has negligible effect in this mode. RF choke PRF and DC blocking capacitor C-DCBlock are used to ensure proper DC bias. Quadrature splitter 230 provides two transmit signals at ninety degrees to excite the birdcage in quadrature.

Regardless of the MRI pulse sequence and technique employed, the sequence controller 30 coordinates the multiple MRI pulse sequences that generate the MR signals that are ultimately received from the examination region. Each echo of the received MR signal is received by the first RF coil 26 or the local RF coil 16 for the selected sequence.

Where a local RF coil 16 is used to receive the signal from the examination region, the local coil operates in the second mode of operation. In this mode, the local RF coil 16 is used as a phased array receive coil comprising, for example, eight loop coils or grids. The DC bias at the cage voltage input V1 and the preamplifier voltage input V2 is positive, thus reverse biasing all diodes. In this mode, the correct reactance is selected in the second stick capacitor C3, the second loop capacitor C2, the first stick capacitor C1, the tuning capacitor + the receiving tuning inductor L5 and the matching capacitor C4 to generate a degenerated eight-channel bird cage. Turning off the transmit PIN diode D6 completely insulates the transmit part from the birdcage. All eight loop coils are fully insulated and can be used to separate receivers, as shown in Figure 5, for optimal SENSE or other parallel processing imaging. Optionally, the signal can be combined into any number from one to eight at the preamplifier output, as shown in FIG. 6 .

In case the first RF coil 26 is used to receive the signal from the examination region, the local coil operates in the third mode of operation. In this mode, the local RF coil 16 is decoupled from the MR system, since the coupling of the local RF coil 16 to the MR system is only required when the first RF coil 26 is used to receive RF signals from the examination region 14 . In this mode, the DC bias at the cage voltage input V1 is positive and the DC bias at the preamplifier voltage input V2 is negative. Accordingly, all cage diodes are in receive or degeneration mode, while preamplifier diode D7 is biased. Preamplifier capacitor C8 is selected such that the combination of matching capacitor C5, coaxial balun 300 and preamplifier capacitor C8 results in a high impedance at matching capacitor C5. This decouples the local RF coil 16 allowing RF reception by the first RF coil 26 .

Regardless of which coil is used as the RF receive coil, the received MR signal is sampled multiple times by the associated RF receiver 38, 36 and analog-to-digital converter 40, 36 resulting in a data line or sampled data point representing each echo array of . This raw MR signal is then loaded into the MR data memory or buffer 42 . The MR data store represents a data matrix commonly referred to as k-space. Based on the specific gradient coding for each echo, the corresponding MR data are mapped or assigned a position in k-space.

A reconstruction processor 50 reconstructs this data into an image representation by applying an inverse Fourier transform or other suitable reconstruction algorithm. The electronic representation is then stored in image memory 52, and the images in image memory 52 may be selectively accessed by video processor 54, which renders slices, projections, or other portions of the representation to a display (e.g., to provide The monitor 56) of the human-readable display of the composite image is converted to a suitable format.

The coil 16 has been described based on an embodiment comprising eight conductors 80, 84, ie eight grids. However, it is also possible to use a different even number of conductors or grids, eg 6 or 32. It is also advantageous to add capacitance in sections or arcs of the third coil or conductor 83, especially when the number of grids is between 8 and 16. Similarly, a fourth or more conductive loops may advantageously be added to the third loop 83 in a similar manner, especially if the mesh number is around 32. A fourth or more additional conductive loops may be added to either end of the coil 16 . However, due to physical constraints, it is preferable to add a fourth or more conductive loops to the same end as the third conductor 83, so that the conductor 81 is free to feed the MR signal to the processing channel. The introduction of a fourth or additional loop, while increasing complexity, also has the beneficial effect of adding more degrees of freedom for coil tuning.

Referring to FIG. 8 , it shows a schematic diagram of the connection of the coil 16 as the receiving coil array. MR signals are received by coil 16 . The coil 16 is subdivided into sections or grids (eight in the illustrated coil 16 ), each section being connected to a separate signal processing channel 400 , shown by dashed lines in the arrangement of FIG. 8 . In each processing channel 400, received MR signals are demodulated, decoded and digitized to generate MR signals representative of the nuclear magnetic distribution in the area covered by the respective grid. The respective MR signals are reconstructed and/or combined by the reconstruction device 402 to generate an image representing the distribution of nuclear magnetic dipoles in the object and/or space enclosed by the coil 16, eg by inverse Fourier transform and/or other suitable algorithms. After reconstruction, the image is formatted for display on a display 404, such as a video monitor or other human viewable display or output device capable of selectively providing the composite image or a description of the image. Optionally, the formatted image may be a surface perspective view, a 3D volume, a 2D cutaway view, or other visual representation of the object. Each processing channel 400 optionally includes a receiver for demodulating, decoding and/or digitizing the MR signals detected by its corresponding grid. Alternatively, a single multi-channel receiver is applied.

Referring to Figure 9, an alternative embodiment of a degenerate birdcage coil 16' is shown. In this embodiment, the second conductor bar 84 and its associated capacitance are omitted. In other words, the third conductor loop 83 is not electrically connected to the conductor loop 82 by a physical power connection in the coil 16'. Instead, the third conductor loop 83 is arranged as close as possible to the conductor loop 82 so as to be capacitively and/or inductively coupled there. This capacitive/inductive coupling replaces the second conductor bar 84 and its associated capacitance. It should also be understood that when third loop 83 is shown alongside loop 82 , third loop 83 may optionally have a diameter slightly smaller or larger than loop 82 so that loop 83 may be arranged concentrically with loop 82 . If placed inside loop 82, for example, conductor 83 may be a loop or a disc.

Fig. 10 shows another embodiment of the degenerate birdcage coil 16", wherein the second conductor bar is retained, and the capacitance C1 of the conductor bar 84" is omitted, and wherein an inductance coil having an inductance L1 has been added to the second conductor bar. In the part of the three-conductor loop 83".

To achieve a similar effect, in another embodiment of the degenerate birdcage coil 16"', the third loop 83 is completely omitted and the L1 inductive coil is arranged in parallel with the C2 capacitor, as shown in Figure 11. However, the third loop is omitted 83 removes the mutual coupling of the tertiary loop and the second loop 82 and adversely affects the additional degrees of freedom for coil tuning resulting from the addition of the tertiary loop.

12 and 13, degenerate birdcage coil 516 is similar to birdcage coil 16, and this degenerate birdcage coil 516 comprises first conductor bar 580, first, second and third conductor or loop 581,582,583 and second Conductor rod 584. As in the birdcage coil 16, the reactance of the conductor bars 580, 584 and conductor loops 581, 581 is selected so that the birdcage coil 516 is a degenerate coil in which the grid defined by the connected portions of adjacent bars 580 and loops 581, 582 Basically decoupled from at least the nearest and next closest neighbor meshes. Each grid is operatively connected to a separate transmit/receive (T/R) switch 590 for operating that grid. The coefficients of the coil 516 rod, coil and shunt capacitances C-Rung, C-Ring and C-Shunt are selected to produce high loop-to-loop isolation at the operating frequency. Applying individual T/R switches 590 to each grid allows independent control of the current flow in different grids. The degenerate coils are substantially decoupled from the nearest neighbor mesh. The use of the third conductor loop 583 provides substantially improved mesh insulation compared to a two loop degenerate birdcage coil. The third return conductor 583 substantially reduces sub-proximate radio frequency coupling. The improved insulation provides better efficiency and improved control of current flow in a single grid compared to degenerate two-circuit birdcage coils.

Referring to Fig. 12, the mark "connect to R[]", where the range of [] is 1-8, indicates that it is connected to the radio frequency signal receivers R1-R8 for receiving magnetic resonance signals. Similarly, the notation "connect to T[ ]", where [ ] ranges from 1-8, indicates RF signal transmitters T1-T8 for selectively exciting a selected grid of RF coils 516 .

FIG. 13 schematically shows details of one of the T/R switches 590 . During the transmit cycle, a switched bias voltage 600 is applied to one end of the inductive coil 601 to operatively connect the transmitter input 602 to the RF shield and thus to the matching capacitance C-Match of the coupling coil grid of the birdcage coil 516 . During the receive cycle, switching bias 600 is turned on such that PIN diode 606 isolates transmitter input 602 from well 604, and the magnetic resonance signal picked up by the coupled coil grid of birdcage coil 516 passes through the quarter-wavelength transmit line 610 , 612 are transmitted to a preamplifier 614 which produces a conditioning signal that is fed to a receiver coupling 616 .

Utilizing a degenerate birdcage coil 516 with high insulation to individual grids provided by first, second and third conductor loops 581, 582 and 583, together with a T/R switch 590 operating each individual grid individually, Coil 516 may be operated in various modes. For example, coil 516 can be used as a volume resonator by appropriate relative phase adjustment of the transmitter signal. In high magnetic fields (for example in the case of B ₀ - 3.0 Telsa or higher), radiation losses can be reduced by individually controlling the coil mesh section. The degenerate coil 516 may also be used as a coil array for transmitting radio frequency pulses or for detecting magnetic resonance. For example, the coil grid may be used as a transmit or receive sensitivity encoding (SENSE) coil array, as a parallel imaging coil array, or the like.

Although circular conductor loops are described, other shapes are also contemplated, such as polygonal (with as many angles as there are conductors) or oval. The conductor loop can optionally also be closed and one of the grids can be omitted. In addition, although specific capacitors and/or capacitances, and inductive coils and/or inductors are mentioned, it should be understood that similar reactance units, distributed reactance units or equivalent circuits with similar reactances may be properly substituted. Furthermore, it is conceivable for the third conductor loop to have a different diameter or other cross-sectional dimensions than the second conductor loop, so that one loop can be placed inside the other loop. In such an arrangement, the second and third conductor loops may be coplanar or may be spaced apart along the axis of the coil. It is further contemplated that the third conductor may be an end cap or end plate.

The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to those who have read and understood the foregoing. It should be realized that the present invention is deemed to embrace all such modifications and alterations as come within the scope of the appended claims or their equivalents.

Claims (28)

1. birdcage coil system of using in conjunction with MR imaging apparatus, this birdcage coil system comprises:

By the birdcage coil of tuning degeneration (16,16 ', 16 ", 16 " ', 516), this coil comprises:

First galvanic circle (81,581) is divided into a plurality of parts, each part be defined in they separately with the corresponding connection of first stub (80,580) between, and each part all comprises matching capacitance (C5),

Second galvanic circle (82,582) and

Place between first and second galvanic circles and a plurality of first stubs (80,580) that are connected with the described first and second galvanic circle electric power; And

Be used at least the RF emission mode and wherein this birdcage coil be suitable for the conversion equipment of this birdcage coil of conversion between the RF receiving mode of making phased array RF receiving coil, this conversion equipment comprises:

Be connected, be used to this coil to present a plurality of RF emission feed point (201,202,211,212) that RF transmits with first galvanic circle (81) electric power;

Be used for a plurality of emission PIN diode (D6) that are coupled with this feed point are presented in the RF emission;

A plurality of RF that cross-over connection matching capacitance (C5) is arranged receive output point;

With the parallel a plurality of coupling PIN diode (D5) of arranging of this matching capacitance (C5); With

Be connected, be used for this emission and coupling P IN diode are applied a plurality of cage DC bias voltages inputs (V1) of bias voltage with matched diodes electric power with this emission.

2. the birdcage coil system described in claim 1, wherein:

This birdcage coil is suitable for and makes quadrature RF transmitting coil in this emission mode.

3. the birdcage coil system described in claim 1, wherein:

This birdcage coil is suitable for and makes phased array RF transmitting coil in this emission mode.

4. the birdcage coil system described in claim 1, wherein

In emission mode, described emission and coupling PIN diode are by cage DC bias voltage (V1) forward bias, thereby this emission PIN diode connects lead to this birdcage coil for providing from transmitting of RF transmitter (25), and this coupling PIN diode makes this matching capacitance short circuit and avoid the output of RF signal.

5. the birdcage coil system described in claim 4, wherein:

In this receiving mode, described emission and coupling PIN diode are by cage DC bias voltage (V1) reverse biased, thereby this emission PIN diode disconnects being connected of this birdcage coil and this RF transmitter (25), and should mate the output that PIN diode allows the RF signal.

6. the birdcage coil system described in claim 5, wherein this conversion equipment further comprises:

A plurality of prime amplifier PIN diode (D7), wherein each prime amplifier PIN diode (D7) all connects with corresponding RF receiving preamplifier (310) electric power;

The a plurality of prime amplifier DC bias voltages inputs (V2) that are connected with this prime amplifier PIN diode electric power are used to this prime amplifier PIN diode to apply bias voltage and make this RF receiving preamplifier (310) short circuit.

7. the birdcage coil system described in claim 6, wherein:

This conversion equipment further is applicable to this birdcage coil is transformed into the decoupling zero pattern, in this decoupling zero pattern, this cage DC bias voltage (V1) is that this emission (D6) and coupling PIN diode (D5) apply reverse deflection voltage, and this prime amplifier PIN diode (D7) of this prime amplifier DC bias voltage (V2) forward direction deflection, thereby this RF coil and this magnetic resonance device decoupling zero.

8. the birdcage coil system described in claim 1 further comprises:

Three conductor RF-coupled (83,83 ", 583) with second galvanic circle (82,582).

9. the birdcage coil system described in claim 8, wherein this birdcage coil is a degenerate birdcage coil, this degenerate birdcage coil has defined a plurality of grids, this grid at least with close on most and time meshes decoupling zero, and this conversion equipment comprises:

Be connected a plurality of transmit/receive switch (590) with a plurality of decoupling zero grids of independent operation with this grid.

10. the birdcage coil system described in claim 9, wherein each this transmit/receive switch (590) comprises:

Connect this transmit/receive switch (590) and with the grid of the corresponding decoupling zero grid of this transmit/receive switch (590) coupling (C-match);

The receiving circuit (610,612,614) that links to each other with this grid coupling; With

Transmitter input (602); With

Be used for selectively this transmitter being imported the device (606) that (602) and this grid are of coupled connections.

11. a birdcage coil that uses in any the described birdcage coil system as claim 1-10, this birdcage coil comprises:

First galvanic circle (81,581);

Second galvanic circle (82,582);

A plurality of first stubs (80,580) that are arranged between first and second galvanic circles and are connected with the first and second galvanic circle electric power; With

Separate with second galvanic circle (82,582) and to arrange and three galvanic circle RF-coupled (83,83 ", 583) with second galvanic circle.

12. the birdcage coil described in claim 11, wherein:

First and second galvanic circles (81,82,581,582) are arranged in the opposite end of first stub (80,580), described a plurality of first stub is connected with the first and second galvanic circle electric power, thereby described first and second galvanic circles have first and second parts between the electric power of the first adjacent stub connects, and a pair of adjacent first stub and defining a grid with the first and second galvanic circle parts between this electric power to first stub is connected; And

This birdcage further comprises:

(C1 C-Rung) is arranged in first stub in the first stub reactance;

(C5, C-Tune C-Match) are arranged in the first galvanic circle part in the first galvanic circle reactance; And

(C2 C-Ring) is arranged in the second galvanic circle part in the second galvanic circle reactance.

13. the birdcage coil described in claim 12 further comprises:

A plurality of second stubs (84 that between second galvanic circle and the 3rd galvanic circle, extend, 84 "; 584); thereby the 3rd galvanic circle (83; 83 ", 583) be arranged in and first galvanic circle (81,581) side of the second relative galvanic circle, described a plurality of second stub (84,84 "; 584) be connected with the second galvanic circle electric power and described a plurality of second stub (84,84 ", 584) thus being connected described the 3rd galvanic circle with the 3rd galvanic circle electric power has part between the electric power of adjacent second stub connects.

14. the birdcage coil described in claim 13, wherein:

Second stub (84,84 ", 584) identical with the number of first stub (80,580), and first stub is connected to second galvanic circle (82,582) with second stub at identical position electric power.

15. the birdcage coil described in claim 14, wherein first stub and second stub (80,84,84 "; 580; 584) form single stub, this single stub has the electrical interconnection with second galvanic circle (82,582); and the end of this single stub respectively electric power be connected to first galvanic circle (81; 581) and the 3rd galvanic circle (83,83 ", 583).

16. the birdcage coil described in claim 15 further comprises:

Be arranged in the second stub reactance in second stub (84,84 ", 584) (C3, C-Shunt).

17. the birdcage coil described in claim 16, wherein:

The first stub reactance, the second stub reactance, the first galvanic circle reactance and the second galvanic circle reactance comprise the first stub electric capacity, the second stub electric capacity, the first galvanic circle electric capacity and the second galvanic circle electric capacity respectively.

18. the birdcage coil described in claim 11 further comprises:

Be arranged in second with the 3rd galvanic circle and with second a plurality of second stubs that are connected with the 3rd galvanic circle electric power (84,84 ", 584).

19. the birdcage coil described in claim 18, the wherein chosen inferior meshes of coming this birdcage coil of decoupling zero of at least one in the reactance of the reactance of the 3rd galvanic circle (83,583) and a plurality of second stub (84,584).

20. the birdcage coil described in claim 11, wherein second galvanic circle and the 3rd galvanic circle the two one of place these two another inside.

21. the birdcage coil described in claim 20, wherein second galvanic circle and the 3rd galvanic circle coplane.

22. the birdcage coil described in claim 11, wherein the 3rd galvanic circle (83) and second galvanic circle (82) induction coupling.

23. the birdcage coil described in claim 22, wherein the 3rd galvanic circle (83 ") comprise the chosen inductance that comes the inferior meshes in this birdcage coil of decoupling zero.

24. the birdcage coil described in claim 11, thereby the reactance of the reactance of the reactance of wherein said stub, galvanic circle and conductor is to have to close on most and the degenerate birdcage coil of inferior meshes decoupling zero by so selected this birdcage coil.

25. the birdcage coil described in claim 11, wherein this birdcage coil is the degenerate birdcage coil that has defined a plurality of grids, and from closing on most and time meshes decoupling zero, this birdcage coil further comprises these a plurality of grids at least:

The a plurality of transmit/receive switch (590) that link to each other with these grids come a plurality of decoupling zero grids of independent operation as in radiofrequency launcher and the radio frequency receiver at least one.

26. birdcage coil as claimed in claim 25, wherein this transmit/receive switch (590) is suitable for and operates this birdcage coil to be used as one of the following at least:

Volume resonator,

Emission SENSE coil array,

Receive the SENSE coil array and

The parallel imaging coil array.

27. a magnetic resonance method comprises:

The time-invariant magnetic field of the birdcage coil inside of claim 11 is passed in generation; With

Birdcage coil in one of following at least pattern operational rights requirement 11:

The volume resonator pattern,

Emission SENSE pattern,

Receive the SENSE pattern,

The parallel imaging pattern.

28. a magnetic resonance imaging system comprises:

The main magnet (12) of rise time stationary magnetic field;

On time-invariant magnetic field, apply the magnetic field gradient coils (22) of selected magnetic field gradient selectively; With

Birdcage coil system as claimed in claim 1.

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