Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2002-02-19
2004-03-02
Lefkowitz, Edward (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06700372
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for generating measurement signals in magnetic fields which are present in the area around an NMR-MOUSE apparatus and whose changes have to be measured. Herein, a time-constant magnetic polarization field B
0
is generated by magnets, for example electromagnets or permanent magnets, and a magnetic measurement field B
1
is generated by means of a high frequency oscillation circuit in a pulsed manner and the echo signals S generated in the surrounding medium are measured. The echo signals can be measured by the NMR-MOUSE apparatus in a time-dependent manner after a change of the magnetic field by one or several signal impulses generated by the NMR-MOUSE apparatus in each case after an echo time t
E
. The measurement signal is generated in the magnetic field around the NMR-MOUSE apparatus where the components of the two magnetic field B
0
and B
1
extend normally to each other, that is, both fields have mutually perpendicular components.
The NMR-MOUSE apparatus (Nuclear Magnetic Resonance MObile Universal Surface Explorer) is a mobile measuring apparatus for nuclear magnetic resonance wherein the magnetic field generated and the measuring range provided by the measurement apparatus are disposed in the area around the apparatus. The NMR-MOUSE apparatus, which will simply be called “NMR-MOUSE”, is therefore very suitable for determining data from the surrounding medium. It is used for the examination of spatial structures: It is possible to analyze therewith crystalline or glassy materials, as well as soft materials such as elastomers with regard to their molecular dynamics. Also, liquids as well as biological materials can be analyzed, see for example, G. Eidmann et al. “The NMR-MOUSE, a mobile universal surface explorer”, Journal of Magnetic Resonance, 1996, pages 104/109, or A. Guthausen et al., “NMR-Bildgebung und Material Forschung” (NMR Imaging and material Research), Chemie in unserer Zeit, 1998, p.73/82. The time-constant static magnetic polarization field B
0
is usually generated with the NMR-MOUSE by one or several permanent magnets. The pulsed magnetic measuring field B
1
, is the magnetic component of a high frequency field, which is formed by a coil as part of an electric oscillation circuit, wherein the coil usually serves also as the receiver coil for the echo signals to be measured. With the NMR-MOUSE, spatially homogenous magnetic fields are not necessary for the polarization of the nuclear magnetization in the permanent magnetic polarization field B
0
and for the generation and detection of the measuring signals.
NMR-MOUSE apparatus can therefore be small and relatively inexpensive in comparison with normal NMR apparatus. With the NMR-MOUSE, the form and size of the surrounding volume, which is nuclear magnetically implied in the surrounding field and which is to be detected by measuring the echo signals, on one hand, the orthogonal components of both magnetic fields B
0
and B
1
and, on the other hand, the specific bandwidth of the magnetic excitation of the material to be examined are defined. The pattern of the magnetic field lines can be changed by the dimensioning and the arrangement of the permanent magnets and of the coil of the electric high frequency oscillation circuit.
The spatial measuring range in the surrounding medium to be examined is variable three-dimensionally by displacement of the NMR-MOUSE, by deformation of the magnetic fields by additional windings and by changing the high frequency field. It is a disadvantage of the conventional apparatus of this type that the detection of the echo signals, which are generated in the surrounding medium by the transmitter signals, is very time consuming. There is an insufficient spatial resolution within the measuring range and the signals to be measured have too little contrast for distinguishing different material properties.
As it is known from DE 195 11 835 C2, the measuring time can be shortened by scanning the measuring range with frequency-selective high frequency pulses while utilizing the given constant magnetic field. In this way, a rapid measurement value yield is obtained, however the contrast achievable is insufficient for the representation of the medium to be examined, particularly for material examinations, where the requirements are very high.
It is the object of the present invention to generate, in time-constant inhomogeneous magnetic fields, measurement signals, which make it possible to detect in the measurement field several spatial points with a problem-specific contrast in a single measuring sweep.
SUMMARY OF THE INVENTION
In a method for generating measurement signals in time constant inhomogeneous magnetic fields, which are produced by a NMR-MOUSE apparatus in the surrounding medium, a static magnetic polarization field B
0
and a pulsed or oscillating magnetic polarization field B
1
are generated such that echo signals S occur which are measured, and an additional pulsed magnetic field B
z
is provided which affects the echo signals S so as to generate contrasts for the identification of the measuring locations where the echo signals originate.
With the additional magnetic fields, changes of the complete magnetic field in different spatial directions are obtained. If the high frequency impulses follow one another with a time spacing t
Ec
, the additional magnetic fields are generated for the spatial coding only within the first half echo time t
Ec
/2. The echo signals S formed thereby are subsequently called up several times with a time spacing t
Ec
to provide thereby for contrast. In this way several spatial points can be measured in a single measuring sweep for the detection of the surrounding volume, which is called a “multiplex advantage”. Furthermore, the echo signals S are re-focussed several times with the impulses of the high-frequency oscillation circuit for generating contrasts. In addition to the multiplex advantage in the surrounding space, a multiplex advantage is also provided for the contrast measurements by weighting with typical NMR parameters (for example, by transverse relaxation time). For the examination of the surrounding field to be measured therefore not only a high number of measuring points can be scanned within a measuring time unit but the measuring points are determined at the same time with increased contrast so that, by generating the pulsed additional magnetic fields, a substantial qualitative improvement is achieved with respect to the analysis with the NMR-MOUSE as compared with conventional measuring methods.
Preferably, the pulsed additional magnetic fields are time-dependent in such a way that the polarity of the additional magnetic field reverses within the echo time t
Ec
.
For generating the high frequency signals in the pulsed magnetic measuring field B
1
, expediently the high frequency excitation according to the method of Carr, Purcell, Meiboon, and Gill, that is, the CPMG method, is used for the nuclear magnetization (see Guthausen et al. “Analysis of Materials by Surface NMR via MOUSE”, J. Magn. Reson. 130, 1998 pages 1/7). Also, other known NMR-echo procedures for contrast variation may be utilized and may be appropriate, whereby echo signals can be generated whose amplitudes are, to a large extent, independent of the inhomogeneity of an existing permanent magnetic field. The influence on the measuring signal resulting from the inhomogeneous static magnetic field as it is present in the polarization field B
0
of the NMR-MOUSE is eliminated in this way. The effect of the additional magnetic field B
2
on the echo signal S can therefore be determined with little interference.
A variation in the solution of the object stated above based on the method described resides in the generation of oscillation-modulated additional magnetic fields B
z
(+) (for example, oscillation modulated gradient fields) and the timing t
E
of the high frequency impulses, and the modulation of the oscillating additional magnetic field are timed relative to each other in such a way that th
Blümich Bernhard
Blümler Peter
Bach Klaus J.
Lefkowitz Edward
Vargas Dixomara
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