RF receiver coil configuration for an NMR spectrometer

An RF receiver coil configuration for NMR spectrometers has at least two largely mutually orthogonal RF receiver coil systems of which at least one is made from superconducting material and is at least partially cooled to a cryogenic temperature lying far below room temperature, with each RF receiver coil system comprising at least one single coil arranged symmetrically about a sample the RF receiver coil systems having differing radial separations from the sample. The inner RF receiver coil system is made from a material having differing physical characteristics in dependence on the actual operating temperature than the material of the outer RF receiver coil system, with the materials and the geometries of the RF receiver coil systems being chosen to minimize the influence of the susceptibility of the RF receiver coil system on the homogeneity of the magnetic field in the vicinity of the sample. In this fashion, the high Q values of the resonator configuration achieved by the use of superconducting material with high electrical RF conductivity can be taken advantage of while, at the same time, avoiding the substantial interfering influences of the high susceptibility of the superconducting material.

BACKGROUND OF THE INVENTION 
The invention concerns an RF receiver coil configuration, in particular, 
for a high resolution nuclear magnetic resonance spectrometer having at 
least one RF receiver coil system for receiving nuclear magnetic resonance 
signals which is arranged about a sample normally at room temperature and 
within a homogeneous magnetic field, wherein the RF receiver coil system 
is at least partially cooled to a cryogenic temperature far below room 
temperature. 
Such an RF receiver coil configuration is, for example, known in the art 
from DE 40 13 111 C2. 
This type of RF receiver coil configuration is utilized particularly in 
high resolution NMR spectrometers having stringent requirements on the 
signal-to-noise ratio in signal detection. In order to increase the 
sensitivity of the receiver coil configuration in conventional devices, 
the RF receiver coil configuration is at least partially cooled to a 
cryogenic temperature lying far below room temperature. As a result, the 
electrical radio frequency (RF) conductivity of the coil is increased to 
improve the signal-to-noise ratio through reduced resonator loss. As a 
result, the resonator configuration has a particularly high Q which leads 
to a particularly high sensitivity. 
In addition to the resonator system Q of the receiver device, which is 
substantially determined by the electrical losses in the coils, the 
fill-factor (the spatial configuration between the coil and the sample) 
also plays an import role for the resulting resolution capability of the 
spectrometer. Despite the presence of cooling devices for the receiving 
coils, which cause an increase in the Q of the resonator system, a close 
coupling between the receiver coils and the sample and therefore a high 
fill-factor can nevertheless be achieved through indirect cooling 
techniques, at least in configurations having only one RF receiving coil 
system as described in DE 40 13 111 C2. 
In order to achieve a further improvement in the Q of the resonator and 
thereby in the resolution of the configuration, the above cited 
publication mentions the additional theoretical possibility of utilizing 
an RF receiver coil system made from superconducting material. This type 
of superconducting RF receiver coil system has, however, not actually been 
realized to date since superconducting coils are very difficult and 
expensive to produce and are particularly awkward when utilized for high 
resolution spectroscopy, since the very large magnetic field dependent 
susceptibility of conventional superconducting materials substantially 
degrades the required high homogeneity of the static magnetic field in the 
vicinity of the sample. The deterioration of field homogeneity at the 
measurement center of the apparatus leads, in turn, to a broadening of the 
lines and to a relative reduction in the peak values of the measured 
signal. The influence of the RF receiver coil configuration susceptibility 
is particularly troublesome for narrow lines in high resolution 
spectroscopy. 
For magnetic field strengths in the region of approximately 100 Gau.beta., 
the susceptibility of an ideal superconductor assumes a value of 
approximately -1. For higher field strengths in the vicinity of 10.sup.5 
Gau.beta., the susceptibility of the superconducting material is still on 
the order of 10.sup.-3. In contrast thereto, the susceptibility of copper 
in this field strength region is approximately 10.sup.-5. Due to its high 
susceptibility, a superconducting RF receiver coil with the same geometry 
therefore interferes with the field homogeneity by approximately two 
orders of magnitude more than a normally conducting resistive coil. 
The purpose of the present invention is therefore to introduce an RF 
receiver coil configuration of the above mentioned kind which takes 
advantage of the high Q of the resonator configuration achievable with 
superconducting material having a high electrical RF conductivity (as 
already theoretically proposed), but which also avoids the substantial 
practical disadvantages associated with the interfering effects of high 
superconducting material susceptibility on the homogeneity of the static 
magnetic field in the vicinity of the sample and therefore on the 
line-width of the received NMR signals. 
SUMMARY OF THE INVENTION 
This purpose is achieved in accordance with the invention in that at least 
two RF receiver coil systems are provided for which are preferentially 
orthogonal to each other, with at least one being constructed from 
superconducting material, wherein each RF receiver coil system comprises 
at least one single coil arranged symmetrically about the sample, the RF 
receiver coil systems having differing radial separations from the sample, 
and the inner RF receiver coil system comprises a material exhibiting 
differing operating temperature dependent characteristics than the 
material of the outer RF receiver coil system, with the materials and the 
geometries of the RF receiver coil system being chosen to minimize the 
overall influence of the susceptibility of the RF receiver coil 
configuration from all RF receiver coils of the configuration at the 
cryogenic operating temperature of the superconducting RF receiver coil 
system on the homogeneity of the magnetic field in the vicinity of the 
sample. 
RF receiver coil configurations having more than one RF receiver coil 
system are per se known in the art for NMR measurements on a plurality of 
differing nuclear species, with the RF receiver coil system being 
orthogonally arranged relative to the magnetic fields received and thereby 
being RF magnetically decoupled. Multi-coil configurations for 
multi-nuclear measurements are in existence having two combined 
transmitter and receiver coil systems with which, for example, protons and 
.sup.13 C-nuclear signals can be measured. Other conventional 
configurations provide for a second receiver coil system for locking the 
measuring signal onto a deuterium line. Neither of these conventional 
multi-coil configurations provides for either a cooling of the RF receiver 
coil system to cryogenic temperatures or the utilization of 
superconducting material for the coils. For this reason, the above 
mentioned problems of the negative influence of the high susceptibility of 
superconducting coil materials on the field homogeneity in the vicinity of 
the sample and therefore the substantial deterioration of the resolution 
capability of the NMR spectrometer do not occur with these conventional 
multi-coil configurations. 
The RF receiver coil system which is in closest spatial proximity to the 
sample naturally has the largest influence on the homogeneity of the 
static magnetic field at the sample location. Therefore, in a particularly 
preferred embodiment of the RF receiver coil configuration in accordance 
with the invention, at least the innermost RF receiver coil system has a 
structure which compensates the overall external susceptibility of the 
system. The compensation therefore takes place locally through appropriate 
choice of the coil materials and an appropriate relative geometric 
arrangement. Since the outer RF receiver coil system located more distant 
from the sample has a substantially reduced effect at least with respect 
to homogeneity interferences caused by susceptibility, it is, for example, 
possible for the external systems to only have coarse susceptibility 
compensation or none at all without leading to substantial line-broadening 
and resolution losses. In addition, the outer coil systems produce longer 
wavelength interferences due to their larger spatial dimensions which 
primarily produce lower order magnetic field gradients. These, in turn, 
are easier to compensate for with conventional shim-systems than the short 
wavelength interferences from the innermost coils which produce magnetic 
field gradients of higher order. 
In an improvement in this embodiment, the inner RF receiver coil system is 
made from electrically resistive material. Such a material exhibits, as 
discussed above, a susceptibility which is substantially less than that of 
superconducting material so that susceptibility compensation is easier 
than with a superconducting coil. The susceptibility compensation of a 
resistive material is, in addition, a technical problem which has been 
long solved so that conventional methods can be utilized. In contrast 
thereto, the superconducting RF receiver coil system at larger radial 
separation from the sample can be designed without susceptibility 
compensation since its larger separation causes a reduced influence of the 
susceptibility on the homogeneity at the sample location. The worsened 
fill-factor of the RF receiver coil system due to its reduced coupling can 
be compensated for or even more than compensated for by means of the 
higher resonator Q associated with the high electrical conductivity of the 
superconducting material. 
In a concrete embodiment of such a susceptibility compensated 
configuration, the inner RF receiver coil system can be constructed from 
copper Wire whose external susceptibility is compensated for with a 
platinum core. 
In an alternative embodiment, the inner RF receiver coil system can be 
superconducting. Due to the close spatial separation to the sample, such a 
system has a particularly high fill-factor which, in conjunction with the 
extremely low losses in the resonator system due to the high RF 
conductivity of the superconductor, leads to a particularly high 
sensitivity of the inner RF receiver coil system and therefore to an 
excellent signal-to-noise ratio. In contrast thereto, the outer RF 
receiver coil system can, for example, be resistive and without 
susceptibility compensation to be more economical and easier to construct. 
In a further improvement in this embodiment, all RF receiver coil systems 
can be superconducting. In this manner, all receiver coil systems are 
particularly sensitive which leads to a very high signal-to-noise ratio 
for the outer systems as well so that the reduced fill-factor caused by 
the larger radial separation from the sample is compensated for or more 
than compensated for. 
An embodiment of the RF receiver configuration in accordance with the 
invention is particularly preferred in which at least one superconducting 
RF receiver coil system is provided for, wherein the coils comprise a thin 
superconducting layer on the order of several microns in thickness 
introduced onto a substrate. In this fashion, the amount of 
superconducting material utilized is substantially reduced leading to a 
large reduction in the negative influences of the susceptibility of the RF 
receiver coil system on the field homogeneity at the sample location and 
thereby on the line-width and on the resolution capability of the NMR 
spectrometer. This type of thin superconducting layer having thicknesses 
between approximately one and approximately ten microns, corresponding 
largely to the RF skin-depth, can, for example, be introduced onto the 
substrate with the assistance of conventional epitaxial techniques if high 
temperature superconducting materials are utilized. 
In particular, an RF receiver coil system of this type can also be 
configured as a bird-cage resonator evaporated onto a support structure. 
In particular, in the event that the layered superconducting system forms 
the inner RF receiver coil system, the extremely small spatial extent of 
the superconducting layer allows for a particularly high fill-factor due 
to the extremely good RF coupling of the coil system to the sample. 
In an improvement of this embodiment the substrate upon which the 
electrically conducting layer is deposited can be a low-loss dielectric 
having high heat conductivity at low temperatures. In particular, in 
conjunction with an indirect cooling of the superconducting RF receiver 
coil system, the substrate can, for example, comprise sapphire or quartz 
tubes. Such a configuration not only leads to a substantial reduction of 
ohmic losses in the receiver coil system, but also achieves a substantial 
reduction in the capacitive losses in the RF resonator system (consisting 
of the receiver coil and the capacitive tuning). 
In another embodiment, a metal block made, in particular, from copper or 
aluminum is in heat conducting contact with the substrate to form a cooled 
platform. This facilitates a particularly simple indirect cooling which is 
particularly advantageous when the relevant coil system is in vacuum, 
since the configuration can be made to be very compact. The cooled 
platform thereby serves for heat conduction to the cryogenic medium. 
Embodiments of the RF receiver coil configuration in accordance with the 
invention are preferred with which at least a part of the RF receiver coil 
is indirectly cooled, since this is-easily realized in vacuum for the 
geometric reasons mentioned above. 
In other embodiments, at least a part of the RF receiver coil can be 
directly cooled to facilitate a more efficient heat conduction away from 
the coils. 
Finally, an embodiment is particularly advantageous with which the RF 
receiver coil system is in vacuum about the sample. In this fashion, a 
particularly simple thermal isolation of the cooled coils from the room 
temperature sample can be realized. Only one single Dewar vessel wall, 
rather than a double-walled configuration, is thereby required so that the 
RF receiver coil system can be arranged in closer spatial proximity to the 
sample to thereby achieve an improved fill-factor for the resonator 
configuration. 
The invention is described and explained more closely below with reference 
to the embodiments in the drawing and the related description. The 
features which can be extracted from the description and the drawing can 
be utilized in embodiments of the invention individually or collectively 
in arbitrary combination.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The RF receiver coil configuration shown in FIG. 1, which is, in 
particular, utilized in high resolution nuclear magnetic resonance 
spectrometers, has a sample 1 in a homogeneous field of a magnet 2 
surrounded by two RF receiver coil systems and arranged largely orthogonal 
with respect to each other, each of which comprises individual coils 3a, 
3b and 4a, 4b respectively arranged symmetrically about the sample 1. The 
RF receiver coil systems 3a, 3b; 4a, 4b exhibit differing radial 
separations from the sample 1 to facilitate an orthogonal spatial 
configuration. 
In order to achieve as low an ohmic loss as possible and a particularly 
good signal-to-noise ratio for this resonator configuration, both RF 
receiver coil systems in the example shown are given the shape of bent 
tubes having a cryogenic fluid 5 flowing through them to thereby be cooled 
from within. In other embodiments, the cooling can take place indirectly 
or be restricted to only one part of the coil. At least one of the RF 
receiver coil systems 3a, 3b; 4a, 4b and possibly both systems are 
constructed from superconducting material with the inner RF receiver coil 
system 3a, 3b consisting of a different material than the outer RF 
receiver coil-system 4a, 4b. The materials utilized have differing 
physical properties in dependence on the operating temperature and are 
chosen according to the associated geometry of the RF receiver coil system 
to minimize the influence of the susceptibility of the RF receiver coil 
configuration resulting from all RF receiver coils of the configuration at 
the cryogenic operating temperature of the superconducting RF receiver 
coil system on the homogeneity of the magnetic field in the region of 
sample 1. 
The susceptibility compensation is generally accomplished locally, i.e. the 
corresponding coils are each individually externally susceptibility 
compensated. It is preferred when at least the inner RF receiver coil 
system 3a, 3b is susceptibility compensated. Both the resistive as well as 
the superconducting RF receiver coil systems of the configuration can each 
be susceptibility compensated. It is also possible for all RF receiver 
coil systems to be superconducting. When the inner RF receiver coil system 
3a, 3b is superconducting at least this coil should be susceptibility 
compensated, since it is in closest spatial proximity to the sample 1 and 
the interfering influence of its susceptibility on the homogeneity of the 
magnetic field at the sample location is particularly large. 
On the other hand, a hybrid configuration can also be advantageous with 
which a preferentially susceptibility compensated resistive coil system is 
utilized as the inner RF receiver coil system 3a, 3b, and no or only 
coarse susceptibility compensation is necessary for the outer RF receiver 
coil system 4, 4b to substantially reduce the manufacturing difficulties 
and thereby the costs of the apparatus. The susceptibility compensated 
resistive inner RF receiver coil system can, in a conventional fashion, be 
constructed from susceptibility compensated copper wire having a platinum 
core. 
It is preferred when the RF receiver coil systems 3a, 3b; 4a, 4b are 
arranged within a vacuum 6 so that only one single separation wall 7 is 
necessary for the thermal decoupling with respect to the sample 1, which 
is normally located at room temperature, to facilitate a substantially 
reduced spatial separation between the inner RF receiver coil system 3a, 
3b, and the sample 1 for effecting a particularly good RF coupling of the 
resonator system and thereby a particularly high fill-factor. 
FIG. 2 shows an RF receiver coil configuration in a schematic spatial 
representation. For reasons of clarity, the inner RF receiver coil system 
13a, 13b is shown axially displaced above the outer RF receiver coil 
system 14a, 14b. In the operating state of the apparatus, the inner RF 
receiver coil system 13a, 13b is inserted in the direction of arrow 10 
into the outer RF receiver coil system 14a, 14b and the two RF receiver 
coil systems are arranged largely orthogonal to each other for 
facilitating a maximum decoupling of the received NMR signals. 
In the RF receiver coil configuration in accordance with the invention, the 
RF receiver coils can be simultaneously utilized as RF transmitter coils 
to effect a compact receiver configuration and to substantially simply its 
geometry. 
The superconducting RF receiver coil system preferentially comprises a thin 
superconducting layer with a thickness of a few microns introduced onto a 
substrate, for example, a sapphire or quartz tube. In embodiments not 
represented in the drawing, the substrate can be in direct heat-conducting 
contact with a metal block made, in particular, from copper or aluminum to 
form therewith a cooled platform with which the superconducting layer is 
directly cooled in a particularly compact manner. 
A compensation to approximately 1% of the susceptibility of the copper is 
usually effected when utilizing resistive copper wire in RF receiver coil 
configurations. Since, as mentioned above, commercially available 
superconducting materials have a susceptibility which is two orders of 
magnitude larger than that of copper even at high magnetic field strengths 
and cryogenic temperatures, in the event of a straightforward thickness 
compensation the superconducting coil would have to have a volume four 
orders of magnitude less than that of the resistive coil in order to 
equally compensate for the damaging influence of the susceptibility on the 
homogeneity of the magnetic field at the sample location. 
Conventional wire thicknesses utilized in RF receiver coil systems are 
about 0.2 to 0.3 mm. In contrast thereto, when utilizing thin 
superconducting layers on the order of 0.005 mm in thickness, the 
interfering effects of the susceptibility are approximately one order of 
magnitude lower due to the volume reduction. Expressed in another way, in 
order to achieve the same susceptibility influence on the homogeneity of 
the magnetic field at the sample location it is sufficient to utilize a 
thin superconducting layer as an RF receiver coil system having only a 
susceptibility compensation of 10% of the susceptibility of copper, which 
is an order of magnitude less than the usual compensation for copper wire 
to approximately 1% of the susceptibility of copper. Viewed in another 
way, the utilization of the above mentioned thin superconducting layer 
allows for the complete elimination of susceptibility compensation for the 
superconducting RF receiver coil system, and only in the event that 
conditions are otherwise the same as in a resistive configuration, the 
influence of the susceptibility on the magnetic field homogeneity at the 
sample location is increased by approximately an order of magnitude. This 
is completely acceptable in measurements of broad NMR lines, although not 
for measurements in high resolution NMR.