KR20000015998A - Quadrature elliptical birdcage resonator for nmr - Google Patents

Abstract

PURPOSE: Disclosed is a quadrature elliptical bird-cage resonator for an NMR (Nuclear Magnetic Resonance). CONSTITUTION: Angular and current distributions for an elliptical birdcage resonator (54) are realized in a design that achieves constant electrical phase between adjacent legs of a coil (56). Equal peak currents in the driven legs of the orthogonal modes yields exact quadrature operation.

Description

Elliptical Orthogonal Drive Bird Cage Resonator and Elliptical Bird Cage Resonator

Applying rf excitation to the sample to take the resulting resonant signal is accomplished in the structure surrounding the sample, which may be a spiral coil, a saddle coin, a resonant cavity, or a bird cage resonator. The latter structure is an object of the present invention, and it is required to join a sample that is almost elliptical in cross section. The bird cage coil is itself a closed ladder circuit and the current flow around the coil is sinusoidally distributed. As a tuned rf circuit, it is used in nuclear magnetic resonance devices for one or both of rf excitation and signal detection functions.

Bird cage coils are fundamentally different in saddle coils, spirals, and geometries by their discrete structure. For bird cage coils, the phase shift should be distributed discretely around the circumference of the coil from zero to 2π (or 2πk, k being an integer). The phase shift of each element is quite frequency dependent, so that the bird cage coil is tuned to discrete frequencies to achieve the desired phase shift requirement.

Bird cage coils are particularly well suited for bulky samples encountered in medical imaging devices and in vivo analytical spectroscopy. Prior art bird cage coils were published in 1985 by Mag. Res., Vol. 63, 622-628, Hayes et al.

The bird cage structure can be regarded as a closed periodic structure itself. Periodic elements of the structure result in a phase shift where the sum must be several times 2π when the phase shift is summed over the closed loop. Geometrically, the resonator has a cylindrical symmetry and the rf current in the axial direction along the periphery of the structure is required to be proportional to sin kθ and / or cos kθ, where θ is the azimuth with respect to the cylindrical axis. In k = 1 mode, the most uniform lateral magnetic field is produced, as is commonly used in analytical NMR applications.

Measurement performance is limited in some ways by incomplete coupling components in the subject under investigation and the NMR transmitter / reception period. First, there is a limit to the sensitivity of the instrument performance when the weak resonant signal is not coupled enough to the receiver to exceed the natural noise. There is a limit to the signal-to-noise ratio, which is the result of inadequate occupancy factors, e. There is also a loss of precision due to heterogeneity in which the rf magnetic field is distributed throughout the magnitude of the response.

The binding component takes the form of an inducible structure that encloses the subject under investigation. Typically this may take the form of a cavity (eg frequency) or, more generally, a solenoid, saddle or bird cage geometry. Bird cage geometries are addressed in the present invention, which preferably employs an oval cross-section bird cage geometry that better matches the cross section of the human body for medical imaging purposes.

The oval bird cage cross section RF coil is known for use in medical imaging of the head and body of a human body. Proc. SMRM, Vinson, Martin, Griffith and Edward, p. 272 (1992); Proc. SMRM, 1342 (1993), Ly and Smith; Proc. SMRM, p. 4025 (1992), Kurezevsky, Pavlović, Steedley and Rawlins; Proc. See paper reported by ISMRM, p. 1411 (1996), Li et al.

The paper by Vinson et al. Describes a leg element in which segments formed by the central axis of the coil element and the elliptical cylinder (with its cross section) contain the same region, and the current is distributed sinusoidally between several legs with respect to the end ring of the coil. The array was taught.

Lie and Smith studied the B ₁ fields that could be obtained from elliptical coils with 16 elements (legs) spaced at equal intervals on the outer periphery increments. For ellipses with half-long axis A and half-axis B, the approximation formula for current density on the surface of the ellipse is to provide a magnetic field nearly parallel to the single axis.

J _c (θ) = J ₀ cos (θ) / (B ² cos ² (θ) + A ² sin ² )

Is given by

Kufchevshik et al. Constructed an elliptical bird cage coil with the legs spaced apart by the same angle increments.

TECHNICAL FIELD The present invention relates to the field of NMR devices, and in particular, to NMR transmitter / receiver coupling to irradiated objects having a nearly elliptical cross section.

1 is a schematic view according to the present invention;

2A is a schematic diagram of a lowpass bird cage coil adapted for orthogonal operation.

FIG. 2B shows a simplified transmission line corresponding to FIG. 2A. FIG.

2C is a schematic diagram of a highpass elliptical bird cage coil adapted for orthogonal operation.

FIG. 2D shows a simplified transmission line corresponding to FIG. 2C. FIG.

FIG. 3A shows the continuous current density of Equation 1 for two orthogonal modes around an elliptic boundary of a 3: 2 ellipse, with sine and cosine curves for comparison.

FIG. 3B shows the (vertical) field homogeneity calculated for a conventional 3: 2 elliptical birdcage coil with cosine wave excitation. FIG.

FIG. 3C shows sinusoidal excitation, calculated (horizontal) field homogeneity for a conventional 3: 2 elliptical birdcage coil. FIG.

FIG. 4A shows (vertical) field homogeneity calculated for a 3: 2 elliptical birdcage coil in accordance with the present invention. FIG.

FIG. 4B shows calculated (horizontal) field homogeneity for a 3: 2 elliptical bird cage coil in accordance with the present invention. FIG.

5 illustrates a single mesh element of an elliptical bird cage coil of the present invention.

Figure 6 shows an elliptical cross section with a shield of the preferred confocal.

7 shows an image of a uniform density image with an image density distribution taken along a portion through the center plane of the image.

The starting point of the present invention is to obtain a continuous surface current distribution Ks (θ) on the surface of the elliptical cylinder, which will provide a uniform and perpendicular magnetic field for orthogonal operation inside the ellipsoid. Known conformal mapping techniques can be used to convert to simpler cases of cylindrical geometry. A discrete current distribution is then obtained, which gives an equivalent field distribution in the case of continuous currents. It is desirable that discrete current can be supported for 4M legs (M is an integer) to achieve orthogonal operation. The discrete case is further constrained by the situation of the same peak amplitude driving the quadrature mode so that a passive quadrature hybrid coupler can be used to provide the same power split port. By correlation, the analysis of the excitation of the bird cage coil from the rf current source is essentially the same as the reception of the signal introduced into the coil from the sample in the coil. In the present invention, the sample excitation function for the coil is understood to describe the same signal reception function as each other.

In the present invention, the discrete legs are unequally spaced on the elliptical periphery in the geometry plane and spaced at equal angular intervals of the electrical phase.

The physical background of the present invention is an NMR apparatus. An ideal example is shown in FIG. 1.

The magnet 10 having an inner diameter 11 provides a main magnetic field. In order to precisely control the magnetic field in time and direction, a magnetic field sensitization coil (not shown) is provided. These are driven by the sensitized power supplies 16, 18, and 20, respectively. In addition, other sensitizing coils (not shown) and power sources (not shown) may be needed to compensate for undesirable residual space heterogeneity in the basic magnetic field. The sample to be analyzed (hereinafter referred to as "sample") is placed in a magnetic field in the inner diameter 11 and the rf magnetic field is irradiated with the desired orthogonal relation with the magnetic field in the inner diameter 11 to investigate the rf power. This is achieved through the transmitter coil 12 inside the inner diameter 11. The resonant signal is induced in the receiver coil close to the sample in the inner diameter 11. The transmitter and receiver coils can be of the same structure or of separate structures.

As shown in FIG. 1, rf power is provided from the transmitter 24, and modulated and modulated rf power pulses (amplitude or frequency or phase or a combination thereof) are output through the modulator 26 and supplied to the amplifier 31. After being amplified, it is sent through a multiplexer 27 to the transmitter coil 12 in the inner diameter 11. The transmitter and receiver coils obviously do not operate simultaneously in themselves. The same coil may be employed if necessary for both functions. Thus, the multiplexer 27 is provided to isolate the receiver from the transmitter. In the case of separate transmitter and receiver coils, component 27 is not strictly a multiplexer but will perform a similar isolation function that controls receiver operation.

The modulator 26 is controlled by a pulse programmer 29 which provides rf pulses of desired amplitude, duration and phase for the rf carrier at preselected time intervals. Pulse programmers may have hardware and / or software attributes. The pulse programmer controls the increase and decrease power supplies 16, 18, and 20 if increase or decrease is required. These increase and decrease power supplies may maintain a selected static increase and decrease in each increase and decrease coil if necessary.

The transient nuclear resonant waveform is processed by the receiver 28 and resolved into a phase quadrature via the phase detector 30. The time-domain signal phased out from the phase detector 30 is sent to the pre-transformer 32 and converted into the frequency domain according to the specific requirements of the process. Conversion of the analog resonant signal to digital form is typically performed through an analog-to-digital converter (ADC) structure that can be considered as a component of the phase detector 30 for the phase resolved signal.

In practice, it is understood that the pre-transformer 32 acts on storing (in storage 34) the phase resolved data. This reflects a common practice of averaging many waveforms that are time domain phase resolved to improve the signal to noise ratio. The conversion function is then applied to the averaged resulting waveform. The display device 36 operates on the data to represent the data obtained for the survey. The controller 38, most often including one or more computers, controls and correlates the operation of the device as a whole.

First, it is desirable to have a suitable continuous current distribution around the elliptical cylinder which will provide a uniform and perpendicular magnetic field within the elliptical cylinder. Resonance modes corresponding to phase shifts totaling 2πk and processing of quadruple, six-fold, and higher higher order multipole field patterns for k = 2, 3, ... are similar to those given below for k = 1. It is understood that it is subsequently obtained and generated. We will now limit to k = 1 mode, which provides a nearly homogeneous field that is most useful for magnetic resonance applications. Alternative embodiments may be used to obtain the results, but Beth's results (IEEE Transactions, Nuc. Sci. Vol. 14, pages 386-388, 1967) are straightforward. Consider an ellipse with the major axis m, the minor axis n, and the focal point at a = {m ² -n ² } ^1/2 and the eccentricity e = a / m. After conversion to standard polar coordinates the current density

K _z = B ₁ e ^jωt (m + n) / μ ₀ (-jmsinθ + ncosθ) {m ⁴ sin ² θ + n ⁴ con ² θ} ^-1/2

Obtained by Here, θ is the geometric angle measured from the long axis, and the real part of the equation represents a mode providing a field parallel to each of the long axis and the short axis. It is understood that vibration excitation is applied at the vacuum frequency ω and the physical quantity is given by the real part of this equation and the real part of equation (3). A 90 ° phase shift, represented by -j, was introduced between the sine and cosine modes to provide a circularly polarized field in the coil according to a common practice for circular bird cage coils. Spatial changes are shown in FIG. 3A for ellipses with an aspect ratio of 3: 2 with sinusoidal and cosine curves provided for comparison.

Discrete currents can flow through an array of N conductive legs to physically realize an elliptical cylinder. Therefore, you must specify the angular position with respect to the ellipse in which the leg carrying the current is placed and the current supported by it. Many discrete approximations for the continuous current model are possible. The resonant structure is limited by the requirement to support the two modes orthogonally and provide a nearly homogeneous lateral field. For a circular orthogonally driven bird cage coil, it requires N = 4M conductive legs at equal intervals around the circumference of the structure, and furthermore, the same peak drive currents for the two modes need to provide fields of the same magnitude. . Mapping this uniform angular spacing of a circular bird cage coil onto an elliptical surface will result in an uneven spacing for legs at the following angles.

θ _p = tan ^-1 (n / m tan φ _p )

φ _p = 2πp / N + φ ₀

Here, phi _p is an electric phase angle, and p = 0, 1, 2, ..., N-1 are integer multiples of 4 supporting orthogonal operation. The offset angle φ ₀ = 0 causes the reds of the finished bird cage coil to be on-axis, while φ ₀ = π / N has a window at these positions. The first case is useful when using an electrical bond to the leg (s) and the second case is useful when an electrical bond is used to the ring member. Either configuration is suitable for inductive coupling. One skilled in the art will know the efficacy of the different values of the offset angle and offset method as another alternative. Thus, the legs are not evenly spaced apart and are not equidistant in the peripheral distance increments with respect to the ellipse in polar coordinates and the electrical phase angles are evenly distributed. Consider this system in elliptical cylindrical coordinates, as shown in FIG. 6 and described by the ellipsoid and hyperbola of the confocal to specify the displacement and angle units u and v, respectively. Elliptical coil is u = ln b, where Is described. The electrical phase angle variable φ in the circular bird cage coil can be explained by mapping from this ellipse to the variable v. Thus, the legs are spaced at equal phase intervals and at equal intervals when the ellipse is viewed in natural coordinates. This is shown in Figure 6 which shows the legs on the ellipse are arranged at the intersection with the hyperbolic v = {0, 2π / N, 4π / N ...}. Where the phase angle is uniformly incremented, the spacing of the legs is very important for linear and orthogonal motion.

Equation 1 is integrated from the phase angles φ _p −π / N to φ _p + π / N to obtain an equation for the current supported by the P-th leg.

I _p = ^{2Ce jωt} (m + n) sinπ / N (-j sinφ _p + cos φ _p )

Note that the same peak current is described in each quadrature mode.

It is necessary to provide equal phase spacing between the legs in order to generate the desired current distribution with respect to the ellipse. In an actual coil, legs of finite length are connected by reactive ring conductors to form transmission lines. Achieving the same phase spacing is equivalent to mapping the transmission line characteristics of a circular bird cage coil to an elliptical geometry. Circuit component transformations that affect this are easily derived.

The cross section of the elliptical transmission line is shown in Fig. 5, and the elements are designated by the network number p, since they vary by the elliptic circumferential position, ie the magnetic inductance of the network and its mutual inductance for other networks depend on their exponents. The behavior of the bird cage coil is governed by the mesh self-inductance and the closest adjacent coupling, so adding two sets of trim capacitors placed in the ring and legs, as well as equalizing them around the ellipse, is commonly used. The desired component transformation is achieved within the nearest adjacent approximation. The capacitor is therefore shown in ring and leg positions. In the following description, we will consider a lowpass bird cage structure that is easily extended to highpass, bandpass and bandstop forms and has legs at the center of the x-axis for the sake of limitation, although the present invention includes them. The flux coupling between adjacent networks cannot be changed once the coil geometry is selected, but their effective mutual inductance M _{p, q} can be changed by changing the effective inductance L _p of the shared leg. Since mutual inductance changes twice as fast as the effective magnetic inductance dM _{p, q} = 2dL _p , M _{p, q} has a small reactance

The in-leg trim capacitor is added to make it equal to the smallest coupling M _N-1,0 . Conceptually, these capacitors are part of the leg inductance, but are physically coupled to the normal leg capacitor C2 ₀ to provide one leg capacity.

Can be provided.

Returning to the network inductance, denote the magnetic inductance of the p-th network as M _{p, p} and the effective magnetic inductance M _0,0 of the smallest network is the trim capacitor in the leg that is shared with the = 1 network. Note that it is reduced by the presence of no leg-and-trim capacitors. In other words,

Larger net magnetic inductance can be reduced to M _0,0 ^(eff) by adding trim capacitance C1 to each ring portion, typically without a lowpass coil. The magnetic inductance of these networks is driven by their mutual inductance trim capacitors.

Reduced However, the two ring trimmers only need to compensate for the effective inductance difference.

The conversion is completed in these ways. Also note that the numerically calculated ring trim capacity idea for the highpass coil is known from the prior art. Finally, note that the closest contiguous bond of adjacent networks is governed by the two legs shared legs. Therefore, the change in the nearest adjacent coupling is small and can be neglected for ellipticals with a small eccentricity, so the leg trim capacitor can be eliminated in these cases.

As defined in the preceding three paragraphs, the reactance of the lowpass elliptical coil is the nominal parameters C2 ₀ , M _0,0 ^(eff) and M _N-1, which define the equivalent cylindrical bird cage within the nearest adjacent approximation framework _. converted to _p . For practical purposes, elliptical bird cages were converted into circular cages electrically. The effect of the neglected change in mesh coupling is a very small fluctuation of this circular coil. Electrical component conversion is consistent with that described earlier, whereby equalizing the active component values around the ellipse makes the electrical phase shift between any adjacent legs constant and C2 ₀ makes the total phase shift around the periphery equal to 2π. will be. For elliptical highpass and bandpass coils, corresponding procedures and results are obtained.

A particular key aspect of the present invention is the specific structure of the transmitter or receiver coil 11. Physically, the lowpass bird cage coil of the prior art may be identical to the perspective view of FIG. 2A, and electrically identical to the network of FIG. 2B. Coaxial expansion, black longitudinal member A _i is a series LC component whose series inductance L ₂ is distributed throughout the conductor length, or longitudinal bar. Each adjacent pair of bars A _i and A _i-1 are joined to form a parallel array of bars A _i through an annular coupling element B _i containing the inductance of the coupling element. Illustrated coupling points 40, 42, 44, 46 are shown to suggest specific coupling configurations. Although not shown, the trim capacitor is as described above.

For a lossless coil, the same peak magnitude current flows in the driven leg of each quadrature mode.

3B and 3C are lateral field contour diagrams calculated for each lateral rectangular field mode, with aspect ratio m: n 3: 3, with prior art coil leg at equal interval displacement along the circumference. Normalized to field strength at the center of the constructed discrete 16-pole ellipse. In FIG. 3B the current distribution is cosine wave and in FIG. 3C the current distribution is sinusoidal. In each case the contour spacing represents a variation of approximately 5% from the field strength normalized to the center of the figure.

4A and 4B show the calculated field contour diagrams corresponding to FIGS. 3B and 3C for the present invention, wherein the coil legs are arranged at equal intervals of the electrical phase according to equations 2a and 2b, and the current distribution is Is given by 3. Improved homogeneity is evident.

Bird cage coils are usually conductively shielded, and the elliptical shield 54, confocal with the elliptical cross section of the coil 56, has been found to theoretically have an advantage in that it retains the best multipolar symmetry. In practice, however, the variations that can be introduced due to the presence of any geometry shield can be compensated using the method described above to maintain the desired constant phase shift for each section of the transmission line.

The operation of the present invention is shown in FIG. 7, where a phase of uniform density is formed by using a bird cage coil orthogonally driven in a conventional magnetic resonance imaging apparatus. The image size distributed along the part taken through the middle plane of the image shows an almost uniform profile. Grayscale indicators are included on the right side of the image to approximate the density. The slight incompleteness of field homogeneity at the top of the semi-short axis is due to the change in the reactance of the leg at θ = π / 2 introduced when the operating frequency of the coil is slightly increased to the frequency of the imaging device. The axis scale is in centimeters.

For conductive elliptical cylindrical samples enclosed in an elliptical bird cage, the uniform long axis and short axis fields may be expected to be not the same for the two modes. This behavior actually occurs when the conductive elliptical cylindrical sample is in any coil structure that provides a truly uniform magnetic field, including a circular bird cage coil.

The case of unequal loading by the orthogonal mode can be addressed in this embodiment by the relative rotation of the excitation field with respect to the sample. For example, if the coil is driven by coupling RF current to a driven leg and the number of legs is an integer multiple of eight, driving the leg at 45 ° electrical phase angle with respect to the major and minor axis of the elliptical load will characterize the sample. It can have the same loading effect of the two modes from the same linear combination of. In this orientation, each mode will experience the same linear combination of horizontal and vertical loading. Otherwise proper field orientation may be achieved by rotation of the distribution pattern of the legs as given above so that the legs are at each of the desired driven points.

While the invention has been described with reference to specific embodiments and examples, other modifications and changes will occur to those skilled in the art as taught above. Within the scope of the appended claims, it should be understood that the invention may be practiced otherwise than as specifically described and that many modifications are consistent with the claims. In the claims, the means function portion is intended to include equivalent structures as well as structural and structural equivalents as set forth herein when performing the recited functions.

Claims (12)

In the elliptical quadrature driven bird cage resonator, (a) N leg members disposed parallel to a common axis and distributed around an elliptical boundary surrounding the axis, wherein N comprises a multiple of 4, and the elliptical boundary has a half major axis m and a half minor axis n, The leg members, θ _p = tan ⁻¹ {(n / m) tanφ _p }, φ _p = 2πp / N + φ ₀ , φ ₀ = 0 or π / N, the N leg members disposed at a given angle with respect to the pth leg member, and the adjacent leg-to-leg electric phase angles are constant at 2π / N; (b) reactive elements connecting the adjacent leg members, the leg members and the reactive elements forming a transmission line disposed on a surface transverse to the axis defined by the boundary line, The reactive elements may have a corresponding size I _p = Ce ^jωt (m + n) sinπ / N (-cosφ _p + jsinφ _p ) and C is adapted to distribute electrical current to each of the leg members so that it is delivered by each of the leg members being constant. Said reactive elements comprising an electrical current divider, (c) Elliptical orthogonal drive bird cage resonator used to communicate with each of the RF current sources in the same peak amplitude and orthogonal relationship and comprising two leg members having a phase angle difference of 90 °. The elliptical orthogonal drive bird cage resonator of claim 1, wherein the two leg members are substantially aligned with respective elliptical long and short axes. The elliptical orthogonal drive bird cage resonator of claim 1, wherein the two leg members are aligned at an electrical phase angle of 45 ° substantially at each elliptical long and short axis. The elliptical orthogonal drive bird cage resonator of claim 1, wherein the two leg members communicate with the RF current sources via inductive coupling to the current sources. The elliptical orthogonal drive bird cage resonator of claim 1, wherein the two leg members communicate with the RF current sources via a non-inductive coupling to the current sources. The elliptical quadrature drive bird cage resonator of claim 1, wherein the two reactive elements communicate with the RF current sources via inductive coupling to the current sources. 2. The elliptical orthogonal drive bird cage resonator of claim 1, wherein the two reactive elements communicate with the RF current source via a non-inductive coupling to the current sources. In the elliptical bird cage resonator, (a) a plurality of leg members parallel to a common axis, distributed around an elliptic boundary line surrounding the axis, the elliptical boundary line having an aspect ratio m / n; (b) reactive elements connecting the adjacent leg members, the leg members and the reactive elements forming a transmission line disposed on a surface transverse to the axis defined by the boundary line, supporting rf current And said leg has an rf phase, said legs including reactive elements disposed at angular intervals corresponding to a constant angular increment in the phase angle of said rf current. 9. The elliptical bird cage resonator of claim 8, wherein the reactive elements comprise a trim capacitor of a selected value to achieve substantially the same electrical phase angle between adjacent legs. 9. The elliptical cage resonator of claim 8, wherein said leg members comprise trim capacitors of a selected value to achieve substantially the same electrical phase angle between adjacent legs. 9. The elliptical bird cage resonator of claim 8, further comprising a conductive shield enclosed in the transverse direction and spaced apart from the resonator. The elliptical bird cage resonator of claim 8, wherein the conductive shield shares a focal point with the elliptical boundary.