Abstract:
A probe head for nuclear magnetic resonance measurements in an area of a surface of a measuring object is disclosed. The probe head comprises first means for generating a static magnetic field extending at least partially parallel to the surface, second means for generating a radio frequency magnetic field having components extending perpendicular to the surface, and third means for amplifying a radio frequency magnetic field effective within the measuring object. The third means are configured as an aperture and are located between the second means and the surface.

Description:
FIELD OF THE INVENTION 
   The invention, generally, is related to the field of nuclear magnetic resonance (NMR) measurements. 
   More specifically, the invention is related to the field of probe heads for such measurements, in particular to the field of so-called surface coil probe heads. 
   Still more specifically, the invention is related to a probe head for nuclear magnetic resonance measurements in the area of a surface of a measuring object, comprising first means for generating a static magnetic field extending at least partially parallel to the surface, second means for generating a radio frequency magnetic field having components extending perpendicular to the surface, and third means for amplifying the radio frequency magnetic field effective within the measuring object. 
   BACKGROUND OF THE INVENTION 
   For investigating areas of objects which are close to the object&#39;s surface it is well known in the art of NMR to use so-called surface coils. An example for such a coil is disclosed in US Patent Application Publications 2002/0089330 A1 and 2002/0084783 A1. These devices, having commercially become known under the trade name “NMR MOUSE®” utilize a U-shaped magnet system with permanent magnets. In the area of the gap between the magnet poles there exist components of the static magnetic field B 0  extending parallel to the surface of the magnets defined by the front surfaces of the magnet poles. In this area between the legs, i.e. within the gap, a radio frequency coil is positioned parallel to the magnet surfaces. The field lines of the radio frequency magnetic field B 1  generated by the coil have components extending perpendicular to the static magnetic field B 0 . The field lines of the static magnetic field B 0  and the field lines of the radio frequency magnetic field B 1 , therefore, intersect in an area above the surface and fulfil the one condition for the excitation of nuclear magnetic resonance and for the reception of nuclear magnetic resonance signals, resp., namely B 0 ×B 1 ≠0. This prior art apparatus operates within a relatively low frequency range of e.g. ν 0 =ω 0 /2π=15 MHz with ω 0 =γB 0 , where γ is the so-called gyromagnetic ratio. For such a frequency range the static magnetic field B 0  may be generated with permanent magnets. 
   If this prior art apparatus is placed on a surface of a measuring object under investigation, nuclear magnetic resonance signals may be generated and received in areas close to the surface. This method has been used for various applications like material science, the characterization of elastomers, quality control, for example in the rubber industry, explorative studies for curatorial problems and medical diagnostics. 
   Conventional apparatuses of this kind are characterized by their relatively limited sensitivity and their limited spatial resolution. As is generally known, the signal-to-noise ratio that may be expected in NMR measurements, depends on the number of nuclear spins contributing to the signal. In the case of the inhomogeneous magnetic fields of the present apparatus, the pulse bandwidth must, therefore, be considered as the decisive factor which, in conjunction with the spatial distribution of the static magnetic field (which is not constant in space) defines the sensitive volume (cf. Balibanu et al., J. Magn. Res., 145, (2000) pp. 246-258; Hürlimann, J. Magn. Res., 152, (2001), pp. 109-123). 
   The best measurements may, hence, be expected when, on the one hand, destructive interferences of the measuring signals from different sub-volumes of the sample within the sensitive volume are avoided, in which B 0 ×B 1 ≠0 and ω 0 =γB 0 , and, on the other hand, the bandwidth of the radio frequency pulses which is linked to the B 1  intensity, is large. The B 1  intensity and, likewise, the signal-to-noise ratio that may be expected within the stray field of the surface coil, depend superproportionally from the reciprocal value of the distance from the surface. 
   If, on the other hand, a surface area shall be measured with a high spatial resolution, e.g. in the mm range, then signals from adjacent areas must be suppressed. This may preferably be done by a spatial limitation of the radio frequency field, for example with micro coils. Insofar, the inherent inhomogeneity of the B 0  field is helpful. 
   A common approach for the imaging detection is the realization of spatial resolution by means of additional gradients, as are also used in 2D tomographs (Casanova et al., J. Magn. Res., 163, (2003) pp. 38-45). This approach, however, requires substantial design efforts and results in complex apparatuses which are difficult to operate and are, for example, inappropriate for mobile applications. 
   In contrast thereto it is much simpler and more cost effective in such cases to use very small coils for achieving a high spatial resolution with a high filling factor and, hence, high sensitivity. For a series resonant circuit, the quality factor Q is proportional to ω 0 L/R, such that a small inductivity L seems to be of little advantage. However, in the field of NMR the quality factor Q does not really set limits at low frequencies because, first, there are known resonant circuit concepts at hand bringing L and R into a range that is acceptable for the experiment, second, there is sufficient power available, and, third, a finite pulse length is necessary for the definition of the carrier frequency ω 0 . 
   In an article “NMR microscope” published in the internet journal “spectroscopy-NOW/Resonants”, 8, (2005), John Wiley &amp; Sons (www.spectroscopynow.com/Spy/basehtml/SpyH/1,1181,5-5-7-0-89587-ezine-0-2,00.html) a so-called NMR microscope for medical diagnostic applications is disclosed. This microscope uses a tubular magnet system being configured by a tubular direct current magnet having coils at one terminal end thereof for generating an arc-shaped constant magnetic field B 0 . A funnel-like radio frequency antenna is positioned within the central longitudinal opening of the magnet system. The antenna consists of a plurality of capacitive/inductive rings of stepped diameter which are arranged at an axial distance with respect to each other. The antenna is tapered in the direction towards the object under investigation. Such an antenna is also disclosed in WO 2004/083883 A1 for a measuring wavelength of 1 m, corresponding to a measuring frequency of 300 MHz. The configuration of the antenna shall effect a focussing of the radio frequency magnetic field B 1  into the object under investigation which, for a spatial resolution of 10 μm shall result in an enhancement of the sensitivity by a factor of 100. 
   This prior art apparatus has the disadvantage that it is complex in its design and has large dimensions. It is, further, dimensioned for a radio frequency range in which the required constant magnetic field B 0  may no more be generated by simple permanent magnets but an electromagnet system is required instead. Accordingly, the prior art apparatus may solely be used in a laboratory environment. Mobile field applications, in particular under confined spatial circumstances, are impossible. The conditions underlying conventional NMR within a homogeneous field B 0  as disclosed in WO 2004/083883 A1 are, therefore, not transferable to the present invention. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object underlying the invention, to improve a probe head of the type specified at the outset, such that the afore-mentioned disadvantages, in particular those related to technically complicated designs, are avoided. In particular, a probe head design shall be made available having small dimensions, a robust design, a simple operation, and which may be manufactured at low cost. 
   This object is achieved by the present invention with a probe head for nuclear magnetic resonance measurements in the area of a surface of a measuring object, comprising first means for generating a static magnetic field extending at least partially parallel to the surface, second means for generating a radio frequency magnetic field having components extending perpendicular to the surface, and third means for amplifying the radio frequency magnetic field effective within the measuring object, characterized in that the third means are configured as an aperture and are located between the second means and the surface. 
   The object underlying the invention is, thus, entirely solved. 
   The invention provides a probe head with an extremely simple and flat design being based on prior art probe heads with permanent magnet systems as discussed above with reference to US Patent Application Publications 2002/0089330 A1 and 2002/0084783 A1 and which are commercially available in the same design. As compared to these prior art systems only minor modifications are necessary such that the robust and simple design of these prior art apparatuses are preserved. The probe head of the present invention may, therefore be used under the same rough operational conditions in a field, also under confined spatial conditions. 
   Preferably, the aperture comprises a first opening, and a slot extending away from the first opening. 
   This measure has the advantage that eddy currents within the aperture are suppressed. 
   In a preferred embodiment of the invention, the second means are configured as a planar coil. 
   This measure has the advantage that a flat design is possible. 
   Insofar, it is particularly preferred when the planar coil is configured as a loop-shaped conductive coating on a first substrate. 
   This measure has the advantage that the planar coil may be manufactured in a simple technological manner. 
   The same holds true when the aperture is configured as a planar conductive coating on a second substrate, the coating being provided with the first opening. 
   In the cases mentioned before a particularly good effect is achieved when the first substrate and the second substrate are configured as one and the same substrate, wherein the planar coil and the aperture are located on opposite surfaces thereof. The distance between the coil and the aperture as well as the dielectric constant of the substrate are of particular importance insofar. 
   This measure has the advantage that minimum dimensions of the coil and of the aperture may be obtained in a direction perpendicular to the surface of the object under investigation. This extremely flat design guarantees that the measuring objects may be analysed even very close to the surface at relatively high B 1  intensities. 
   In embodiments of the invention, the loop-shaped conductive coating is provided with a second opening being essentially square-shaped, wherein, preferably, the first opening is also essentially square-shaped. It is, insofar, possible, to adapt the geometry to the particular requirements of an investigation or experiment. Modifications of the B 0  geometry may likewise be taken into account. 
   This shape of the openings has turned out to be of particular advantage, considering the conventional B 0  field geometry and the spatial resolution to be attained. 
   It is, further, preferred, when the first and the second openings are arranged coaxially. 
   Practical tests of embodiments of the probe head according to the present invention have shown that the first and the second opening should have an area ratio in the range of between 1:2 and 1:6, preferably of about 1:4. For example, at a frequency of the radio frequency magnetic field B 1  of about 15 MHz the first opening may have dimensions of 2×2 mm and the second opening may have dimensions of 4×4 mm. 
   In further embodiments of the invention, the first means are configured as a U-shaped magnet system, wherein the second and the third means are located between legs of the U. The magnet system may comprise two legs configured as permanent magnets interconnected by a yoke. Other geometries of the design are, of course, possible depending on the particular requirements of an application, and are within the scope of the present invention. 
   Preferably, the second means are located within an electrically conductive housing having a side facing the surface, the third means being arranged within the side. 
   Thereby, the radio frequency assembly is shielded against spurious signals from the environment, and an unwanted coupling with the magnet system, possibly time-varying couplings, are suppressed. 
   Preferably, means for radio frequency matching and tuning are provided within the housing, which, therefore, are likewise shielded. 
   This compact and well-defined design allows to maintain a certain setting of the tuning over extended periods of time, wherein the tuning remains unaffected by external influences. 
   It is possible to provide a plurality of exchangeable housings with second and third means. 
   By doing so, second and third means of different design and tuning may be used depending on the particular requirements of an application, in particular second and third means having different sensitivities or measuring depths, while the same first means are used. 
   Further advantages will become apparent from the description and the attached drawing. 
   It goes without saying that the features mentioned before and those that will be explained hereinafter may not only be used in the particularly given combination but also in other combinations or alone without leaving the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
       FIG. 1  shows a schematic side elevational view of a probe head according to the prior art; 
       FIG. 2  shows an illustration, similar to that of  FIG. 1 , however, for an embodiment of the present invention; 
       FIG. 3  on an enlarged scale shows a top plan view of a planar coil, as may be used for the probe head of  FIG. 2 ; 
       FIG. 4  on an enlarged scale shows a top plan view of an aperture, as may be used for the probe head of  FIG. 2 ; and shows experimentally obtained two-dimensional spatial dependencies of the measuring signal intensity of a small sample object (1 mm 3  natural caoutchouc) in a 1 mm raster within a plane directly adjacent the surface: a) with aperture, and b) without aperture. 
       FIG. 5a , as an example, shows the experimentally determined two-dimensional spatial dependance of the measuring signal amplitude of a small sample object (1 mm 3  natural caoutchouc) in a 1 mm raster within a plane directly at the surface with an aperture; 
       FIG. 5b , as an example, shows the experimentally determined two-dimensional spatial dependance of the measuring signal amplitude of a small sample object (1 mm 3  natural caoutchouc) in a 1 mm raster within a plane directly at the surface without an aperture. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1  reference numeral  10  as a whole designates a probe head for measurements close to the surface by means of NMR, according to the prior art. A U-shaped magnet system  14  is provided symmetrically to an axis  12 . Magnet system  14  has legs having free surfaces which configure contact surfaces  16  of probe head  10 . Probe head  10  may be applied with these contact surfaces  16  on a surface  18  of a measuring object, with surface  18  extending perpendicular to axis  12 . 
   Field lines  20  of the constant magnetic field B 0  generated by magnet system  14  exit under right angles from contact surfaces  16 , and bridge the gap between the legs with an arc. In the area of axis  12  the extend perpendicular to the latter. 
   A radio frequency coil  22  is located between the legs of magnet system  14  in the area of axis  12 . Radio frequency coil  22  is connected to a terminal  26  via a line  24  and a matching and tuning unit  25 . Radio frequency coil  24  is positioned such that field lines  28  of a radio frequency magnetic field B 1  generated by it extend essentially parallel to axis  12  in the area of surface  18 . A limited spatial area close to surface  18  and within the measuring object is thus generated in which field lines  20  of field B 0  intersect field lines  28  of field B 1  under right angles. In this area the condition for exciting nuclear magnetic resonance and for receiving nuclear magnetic resonance signals, resp., is maximally fulfilled. 
     FIG. 2  shows an apparatus similar to that of  FIG. 1 , however, for an embodiment of the present invention. 
   A probe head  30  with an axis  32  comprises a magnet system  34 , contact surfaces  36  of which contact a surface  38  of a measuring object under investigation. The field lines of constant magnetic field B 0  generated by magnet system  34  are designated  40 . A radio frequency coil  42  is positioned within an electrically conductive housing  81  between legs of magnet system  34 , and is connected with a terminal  46  via a line  44  and a matching and tuning unit  45 . The field lines of radio frequency field B 1 * generated by coil  42  are designated  48 . 
   In  FIG. 2  a brickwork wall structure  50  is shown as an example for a measuring object. Wall structure  50  is provided with a fresco painting  52 . Depending on how close probe head  30  is approached to fresco painting  52  (arrow  54 ) or depending on the particular design of probe head  30 , spatially resolved measurements may be made on fresco painting  52  for curatorial purposes. 
   Magnet system  34  is of a design comprising a yoke  56  made from soft iron and two permanent magnets  58   a  and  58   b  of oppositely directed polarity which configure the two legs of magnet system  34 . 
   The main distinction as compared to prior art probe head  10  of  FIG. 1  is that probe head  30  has an element between radio frequency coil  42  and surface  38  which modifies field lines  48 , i.e. also modifies radio frequency field B 1 *. This element is configured as a hole or aperture  60 . Aperture  60  has a first opening  62  being smaller than a second opening  64  of radio frequency coil  42 . Openings  62  and  64  are preferably arranged coaxially along axis  32 . 
     FIGS. 3 and 4  on an enlarged scale show details of radio frequency coil  42  ( FIG. 3 ) and of aperture  60  ( FIG. 4 ). 
   In an embodiment of the invention radio frequency coil  42  as well as aperture  60  are located on a substrate  70  and  76 , resp., substrates  70  and  76  being, for example, made from the same material as is used for printed circuit boards. Radio frequency coil  42  and aperture  60  are preferably manufactured by appropriately coating substrates  70  and  76 . As an alternative, they can be arranged on opposite surfaces of one and the same substrate for achieving a still flatter design. 
   Radio frequency coil  42  is configured by a loop-shaped, electrically conductive coating  72 , for example a copper coating, on substrate  70 . Loop-shaped coating  72  ends in two terminals  74   a  and  74   b . Second opening  64  is preferably square shaped. 
   Aperture  60  is likewise configured by a plane, electrically conductive coating  78  on substrate  76 . Coating  78 , for avoiding eddy currents, is provided with a slot  80  extending away from first opening  62  to the periphery of coating  78 . Preferably, first opening  62  is also square shaped. 
   Preferably, coatings  72  and  78  are provided with rounded corners. 
   The area ratio between first opening  62  and second opening  64 , preferably, is in the range of between 1:2 and 1:6, still more preferably at about 1:4. 
   In a practical embodiment of the invention, designed for a radio frequency of 15 MHz, a length L of substrates  70  and  76  is, for example, 53 mm, and a width B is 19 mm. For this embodiment, first opening  62  has dimensions c×d of 2×2 mm, and second opening has dimensions a×b of 4×4 mm. Second opening  64 , hence, is smaller as in prior art apparatuses according to  FIG. 1  having no aperture, in which for the same frequency range the opening of radio frequency coil  22  is, for example 7×16 mm. The measuring depth, i.e. the distance from surface  38 , at which NMR measurements are conducted, is about 2 mm for the above described embodiment of the invention. 
   For the above described embodiment,  FIG. 5 , as an example, shows the experimentally determined two-dimensional spatial dependence of the measuring signal amplitude of a small sample object (1 mm 3  natural caoutchouc) in a 1 mm raster within a plane directly at the surface: a) with aperture, and b) without aperture. As one can easily see, there is a considerable enhancement in signal amplitude in a) as compared to b), and an improved localization within the central area. 
   The NMR experiments were conducted in a manner as known per se, namely by using pulse sequences of field B 1 .