Abstract: A low-noise preamplifier with an electronic amplifying element having an input (2) and an output (4), and with a first transformer (6) having a primary winding (5) and a secondary winding (8) is characterized in that the input (2) of the electronic amplifying element is the base of a transistor, the gate (G) of a field effect transistor (FET) or the grid of a vacuum tube, and the output (4) of the electronic amplifying element is the collector, the drain (D) or the anode, the output current (Ip) being passed from the electronic amplifying element through the primary winding (5) of the first transformer (6) to the output (7) of the preamplifier, wherein the secondary winding (8) of the first transformer (6) is connected to the input (2) of the electronic amplifying element. This preamplifier can be operated with extremely low noise and at any input impedance, in particular, at 50 ohms or 75 ohms.

Abstract: A low-noise preamplifier with an electronic amplifying element having an input (2) and an output (4), and with a first transformer (6) having a primary winding (5) and a secondary winding (8) is characterized in that the input (2) of the electronic amplifying element is the base of a transistor, the gate (G) of a field effect transistor (FET) or the grid of a vacuum tube, and the output (4) of the electronic amplifying element is the collector, the drain (D) or the anode, the output current (Ip) being passed from the electronic amplifying element through the primary winding (5) of the first transformer (6) to the output (7) of the preamplifier, wherein the secondary winding (8) of the first transformer (6) is connected to the input (2) of the electronic amplifying element. This preamplifier can be operated with extremely low noise and at any input impedance, in particular, at 50 ohms or 75 ohms.

Abstract: In a method and apparatus for the production of a nuclear magnetic resonance (NMR) tomography image with radio frequency (RF) signals from an extended measuring object (11) being detected by n surface coils (2a, 2b, 2c, 2d) of an NMR apparatus, whereby n>1, and the n RF signals are separately preamplified by n preamplifiers (3a, 3b, 3c, 3d) each assigned to one of the n surface coils (2a, 2b, 2c, 2d), and subsequently further amplified and detected in a phase sensitive fashion, as a result of which a low frequency signal is obtained in each case which is introduced to one of n low pass filters (5a, 5b, 5c, 5d) of an analog/digital converter (ADC) and whereby n partial images are produced in a computer (8) from the n signals obtained in this fashion and assembled into a single NMR tomography image of the measuring object (11) in a region of interest, the signals emanating from the n low pass filters (5a, 5b, 5c, 5d) are introduced to a single ADC (7) via a first multiplexing circuit (6) to be digitized and

Abstract: In a method for the generation of a nuclear magnetic resonance (NMR) spectrum from a measuring volume, which is exposed to a highly homogeneous stationary magnetic field B.sub.0, a radio frequency (RF)-pulse is irradiated in a first run into the measuring volume in order to excite the concerned nuclear spins; at least one magnetic gradient field G is switched, for example for selecting a certain slice from the measuring volume and after switching off the magnetic gradient field G, a first data set is recorded which contains the observable measuring signals from the measuring volume, subsequently the method is repeated in a second run with a magnetic gradient field -G which has the opposite polarity to the magnetic gradient field G, whereas the remaining experimental parameters of the first run are maintained, wherein after switching off the magnetic gradient field -G a second data set is recorded and added to the first data set.

Abstract: In particular with 3D tomography, it is necessary to acquire a large number of individual spectra. Therefore it is advisable to reduce the time needed for the acquisition of a single spectrum without loosing too much in signal-to-noise ratio. To a significant extent, this time is determined by the relaxation time of the spins. Prior to each excitation a significant quantity of these spins have to return into their equilibrium (z-direction of the homogeneous magnetic field) in order to create a usable signal with the next excitation. For spins with long relaxation times T.sub.2 this time can be reduced by a -90.degree. pulse 12 that coincides with the center of the last spin echo 9 with appropriate application of the gradient fields. This -90.degree. pulse returns the x-y magnetization that exists in the x-y plane into the z-direction. The diagnostic relevance can be significantly increased by such a procedure.

Abstract: A nuclear magnetic resonance tomograph comprises a magnet surrounding a measuring space for receiving an object under test, e.g. a human body. A coil is provided for generating a high-frequency magnetic field within the measuring space. The coil is fed by a transmitter comprising a stage for adjusting the amplitude of a high-frequency current generated by the transmitter. A high-frequency magnetic field sensor is arranged at a predetermined calibration location outside the measuring space for measuring the high-frequency magnetic field strength prevailing at the predetermined calibration location. For calibrating the high-frequency magnetic field strength during a tomography measurement, the coil with the object under test inside is subjected to a test current and the resulting high-frequency magnetic field amplitude is sensed.

Abstract: In nuclear spin tomography disturbances are encountered as a result of the insertion of the gradient fields, which disturbances are due to the generation of eddy currents and lead to field distortions making themselves felt, among other things, in the rapid decay of the echo signals of spin echo pulse sequences, i.e. in disturbances of the rephasing conditions. According to the invention, the rephasing conditions required for generating spin echo impulse sequences can be established either by varying the value of the static magnetic field between the excitation pulse and the next following rf pulse of a spin echo pulse sequence, or else by adjusting the interval between the excitation pulse and the next following rf pulse to a value different from half the interval between the subsequent rf pulses.

Abstract: The use of longer 180.degree. pulse sequences for the generation of spin echos is made possible according to the invention during NMR tomography using the 2d-Fourier transformation process in that the phase coding gradient between each pair of 180.degree. pulses is switched on in such a manner that the dephasing caused between each pair of 180.degree. pulses by the phase coding gradient is twice as great as the dephasing caused before the beginning of the first 180.degree. pulse by the phase coding gradient. The mirror images which appear otherwise when using the known 180.degree. pulse sequences, e.g. the Carr-Purcell-Gill-Meiboom pulse sequence, are avoided by this measure.

Abstract: For recording the nuclear magnetic resonance in selected areas of a body for the purpose of representing body cross-sections in the form of images (NMR tomography), wherein the nuclear spins of a selected type present in the area of one plane of the body are subjected to a superimposed gradient field varying in the selected cross-sectional plane, and selectively excited, the excitation of the nuclear spins is effected by application of a pulse sequence which induces the nuclear spins to supply at one stage a number of chronologically successive induction signals. The induction signals corresponding to each other in chronolical sequence are then processed to obtain separate sets of image signals so that by one single measurement a sequence of cross-sectional images is obtained which differ from each other by the intensity of the excited nuclear spins which varies according to the relaxation times T.sub.1 or T.sub.2.

Abstract: The use of longer 180.degree. pulse sequences for the generation of spin echos is made possible according to the invention during NMR tomography using the 2d-Fourier transformation process in that the phase position and, thus, the sense of rotation of the 180.degree. pulses is reversed after every two pulses. The mirror images which appear otherwise when using the known 180.degree. pulse sequences, e.g. the Carr-Purcell-Gill-Meiboom pulse sequence, are avoided by this measure.

Abstract: For the purpose of limiting NMR spectroscopy to a selected area of a body the said area is prepared by applying first a homogeneous magnetic field passing through the entire body and superimposing thereafter upon the said magnetic field a first magnetic field of identical orientation whose strength varies in a first sense (first field gradient), exciting thereafter all the selected nuclear spins present within the body and in addition, selectively, only the nuclear spins present in a first body plane containing the volume element and extending vertically to the said first field gradient so that the selected nuclear spins contained in this body plane are returned to the direction of the homogeneous magnetic field while the nuclear spins outside this plane receive an orientation differing from the sense of the homogeneous magnetic field.

Abstract: In NMR tomography, the nuclear spins of a selected type present in a selected area of a body are selectively excited by applying first a selection gradient field and exciting thereafter the nuclear spins by a HF signal which initially effects a rotation of the nuclear spins situated in a selected plane by an angle .alpha. of less than 90.degree., then a rotation of all selected nuclear spins by 90.degree. and finally again a rotation of the nuclear spins contained in the selected plane by an angle of 90.degree.-.alpha.. As a result thereof, the nuclear spins contained in the selected area have resumed at the end of this excitation their original orientation determined by a homogeneous magnetic field, while nuclear spins located outside this area lie within a plane extending almost vertically thereto. Thereafter, a measuring gradient field may be switched on without disturbing the orientation of the nuclear spins contained in the selected area.