RF coil for magic angle spinning probe

Details RF coil for magic angle spinning probe RF coil for magic angle spinning probe

2002-10-15

2004-10-19

Gutierrez, Diego (Department: 2859)

Electricity: measuring and testing

Particle precession resonance

Spectrometer components

C324S307000

Reexamination Certificate

active

06806713

ABSTRACT:

FIELD OF THE INVENTION
This invention is in the technical field of nuclear magnetic resonance (NMR) and more particularly relates to an NMR probe capable of generating an optimized RF magnetic field orthogonal to a polarizing field for study of a sample rotating about an axis oriented at a specified angle from the direction of the polarizing field.
BACKGROUND OF THE INVENTION
It has been known in the analysis by magnetic resonance to rotate a sample at a high speed &ohgr;
s
in a uniform magnetic polarizing field around a sample rotation axis directed along a selected angle &thgr; from the direction of this polarizing field (B
0
) to average over dipolar couplings in the sample and to average over spatial inhomogeneities of the sample. The selected angle is frequently the so-called magic angle, which is defined as the zero of the function 3 cos
2
&thgr;−1, or about 54° 44′.
In order to achieve a desired distribution of RF magnetic field (B
1
) over the volume of the sample, it has been known to provide a solenoidal coil with the coil former support structure oriented on the axis of rotation of the sample (or the sample container). Consider FIG.
1
. The RF magnetic field B
1
s
generated by such a solenoidal coil (represented here by resonator
8
) is in the direction of the axis of rotation
9
but it is the component of this field perpendicular to the polarizing field B
0
that is of importance in NMR applications, that is, the projection of B
1
s
on the x-y plane. For simplicity, let the rotation axis be in the z-y plane and let the angle between the direction of the polarizing field B
0
, and the axis of rotation (solenoid axis) be &thgr; and the RF phase for the solenoid is &rgr;
s
. It follows that
B
1
s
B
1
s
sin &thgr; cos(&ohgr;
t+&rgr;
s
)
Y+B
1
s
cos &thgr; cos(&ohgr;
t+&rgr;
s
)
Z
The effective field component due to the solenoid is limited to the projection onto the x-y plane and thus will be
B
1
s
effective
=B
1
s
sin &thgr; cos(&ohgr;
t+&rgr;
s
)
Y
  (Equ.1)
If &thgr; is the magic angle, the effective field will be about 0.816B
1
for such prior art.
It is desired to increase the B
1
field available for manipulation of nuclear spins and to increase the signal-to-noise ratio for resonance detection when an axially symmetric probe coil is inclined with respect to B
0
. It is known to produce an RF field at a small angle with respect to the solenoidal axis of a solenoidal RF coil by tilting the approximate plane (of a current loop) to the solenoidal axis. The B
1
s
effective
vector resulting from this known arrangement is approximately inclined by the tilt angle with respect to the axis of the coil support. However, the vector is smeared over a cone (cone-angle equal to the tilt angle) in accordance with the distribution of normals to the non-coplanar surface enclosed by the current “loop”.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide more effective RF coils for a spinning probe which may be oriented at a selected angle with respect to the static field and in particular, at the magic angle.
Saddle coil and birdcage coil geometries each produce an RF magnetic field in the plane perpendicular to their geometric symmetry axis which may be identified with the solenoidal/sample rotation axis.
A birdcage coil having a pair of angularly displaced RF ports, each tuned to the same resonance frequency produces (on excitation) a plane polarized RF magnetic field. Where the two amplitudes are equal, the angular displacement is 90° and the phase difference is &pgr;/2, the polarization will be circular in the median plane of the birdcage coil. (The present invention is not limited to quadrature mode/circular polarization; in some cases, elliptical polarization may be desirable, in order to produce circular polarization projected onto the x-y plane). For simplicity, reference to circular polarization is intended to comprehend elliptical polarization and quadrature mode is representative of multi-mode coils in general.) A quadrature birdcage coil disposed with its axis oriented at &thgr; with respect to a polarizing field B
0
along Z is again identifiable with resonator
8
of FIG.
1
. The analysis is simplified if the rotation axis is again assumed to lie in the z-y plane and the two (quadrature) modes are of equal amplitude and characterized by phases &rgr;
B
and &rgr;
B
+&pgr;/2.
B
1
B
=B
1
B
cos(&ohgr;
t+&rgr;
B
)
X
(first mode)+[
B
1
B
cos &thgr; cos(&ohgr;
t+&rgr;
B
+&pgr;/2)
Y+B
1
B
sin &thgr; sin(&ohgr;
t+&rgr;
B
+&pgr;/2)
Z
](second mode)
Noting that only the X and Y contribute to NMR excitation phenomena, one has
B
1
B
effective
=B
1
B
cos(&ohgr;
t+&rgr;
B
)
X+B
1
B
cos &thgr; cos(&ohgr;
t+&rgr;
B
+&pgr;/2)
Y
  (Equ. 2)
and these two terms are identified with the RF magnetic field components (oscillating in the plane P). The plane of polarization for the birdcage coil is that plane orthogonal to its cylindrical axis and will be referenced where appropriate as the P plane. Relaxing the condition of equal amplitudes for the two modes, one obtains an elliptically polarized wave in the plane P. With an appropriate choice of these amplitudes the elliptical polarization on the plane P will be projected onto the x-y plane as a circularly polarized wave, as discussed below.
A saddle coil may be disposed with its symmetry axis directed at an angle to B
0
. A single saddle coil produces linear polarization in the plane transverse to the inductive members of the coil, that is, transverse to the symmetry axis of the coil. The saddle coil may be rotated about its geometric axis to orient the polarization axis in a plane orthogonal to B
0
whereby the RF magnetic field is optimized for a desired sample rotation axis orientation.
Plane polarized or linearly polarized RF fields may be vectorialy added to a solenoidal field to produce a resultant RF magnetic vector exhibiting a greater projection on the plane orthogonal to the uniform field B
0
as compared to the solenoidal field component alone. The greater RF field projection on the x-y plane, orthogonal to B
0
, is more efficacious for NMR excitation and the same features which allow this greater coupling to the sample on excitation also promotes a closer coupling to the resonant de-excitation of the sample.
A spinning NMR probe according to this invention may be characterized as comprising not only a container for containing a sample, and means for rotating the container around an axis of rotation which makes a specified non-zero angle (such as the magic angle) but also a saddle coil resonator or a multi-mode resonator disposed around the sample and arranged along the axis of rotation of the sample container and means for exciting the saddle coil or multi-mode resonator to thereby provide a resultant B
1
field having a major component perpendicular to the axis of rotation. In most familiar NMR applications the multi-mode resonator is represented by a quadrature coil and reference to quadrature coils throughout this work should be understood to include more general multi-mode coils where applicable. The quadrature coil may be a paraxial birdcage coil with rungs extending parallel to its central axis. Where such a coil is employed with quadrature detection/excitation, instead of a solenoid coil according to the prior art, the B
1
(P) field generated thereby is perpendicular to its symmetry axis(also the axis of rotation of the sample container). The B
1
(P) field of the present invention is characterized by a rotating vector (circularly or elliptically polarized field) rotating in the plane P transverse to the inductors of the quadrature coil.
In certain embodiments of the present invention, the vector B
1
(P) rotating in plane P or oscillating along an axis in plane P, couples to another vector B
1
(S), e.g., an axial field tuned to the same resonance frequency with a selected phase difference to B
1
(P) to produce a resultant B
1
. Assume tha

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