摘要:
A magnetic resonance examination system is disclosed comprising a field probe system to measure the magnetic field distribution of the main magnetic field and gradient magnetic field. The measurements are made in an earlier configuration and yield the resultant magnetic field due to gradient switching or external causes. From the measured resultant magnetic field the response relation is derived and stored in the memory. The response relation from the memory is available for compensating activation of the gradient fields or correction in reconstruction for the response relation in reconstruction. This compensation or correction can be carried-out in a current configuration. Thus is the current configuration to field probes are needed.
摘要:
A method for compensating magnetic noise in a spatial volume in which two concurrently operating compensation loops are provided comprising: a closed compensation loop for magnetic noise fields outside the spatial volume and inside the electromagnetically environment; an open compensation loop for magnetic noise fields in the spatial volume; said two compensation loops generating each one a magnetic noise compensation field; said two compensation fields concurrently provide for compensation of the magnetic noise field in the spatial volume.
摘要:
A method of determining an actual gradient impulse response function during execution of a magnetic resonance (MR) imaging or spectroscopy sequence, wherein a main magnetic field is generated in a sample region of an MR apparatus by means of a main magnet and wherein superimposed time dependent gradient fields and radiofrequency fields in a first RF band are generated in the sample region according to a first MR sequence for forming images or spectra, the gradient fields being generated by gradient forming means of the MR apparatus operated according to a gradient forming sequence part of said first MR sequence, the MR apparatus further comprising at least one magnetic field probe. The method comprises the steps of: - operating said at least one magnetic field probe according to a second sequence, whereby a time dependent probe signal is repeatedly acquired from each magnetic field probe during a probe acquisition time window, thus providing at least one time dependent probe signal; - obtaining said actual gradient impulse response function by calculating an impulse response function from said at least one time dependent probe signal and from said gradient forming sequence part.
摘要:
Eine Vorrichtung, die einen HF-Generator (29), ein NMR-Sende- (20) und Empfangssystem ( 21) sowie einen ersten Regelkreis (28) umfasst, mit dem die Frequenz f RF des HF-Generators mit der Resonanzfrequenz f 0 einer NMR-Linie synchronisiert wird, wobei aus dem Signal des HF-Generators eine Folge von Anregungsimpulsen (EX) der Repetitionsfrequenz f m erzeugt wird, mit der Kernspins einer bestimmten Resonanzfrequenz einer zugehörigen NMR-Linie quasi-kontinuierlich (CW) angeregt werden, und in den Zeiten zwischen den Anregungsimpulsen das NMR-Signal empfangen wird (AQ), wobei die Periodenzeit 1/f m viel kleiner als die Relaxationszeit der NMR-Linie gewählt wird, vorzugsweise kürzer als 1/10 der Relaxationszeit, und das in den Niederfrequenz-Bereich hinunter gemischte NMR-Signal U D dazu benutzt wird, mit Hilfe des ersten Regelkreises den Wert der Sendefrequenz (=Frequenzlock) oder den Wert des B 0 -Feldes (=Feldlock) so zu regeln, dass Frequenz und Phase des HF-Generators und der NMR-Linie möglichst genau übereinstimmen, ist dadurch gekennzeichnet, dass ein Phasenschieber (22), der die Hochfrequenzphase des NMR-Sende- und Empfangssystems im ersten Regelkreis um den Wert Δϕ dreht, von einem zweiten Regelkreis (27) gesteuert wird, dessen Eingangssignal aus einer Signalextraktionsstufe (25, 26) kommt, die den sägezahnförmigen AC-Anteil des Signals U D extrahiert und der dieses Eingangssignal auf Null regelt. Damit lässt sich der Einfluss der HF-Phase auf den Synchronisationsvorgang berücksichtigen, um dadurch die Genauigkeit, mit der auf die Resonanzfrequenz einer NMR-Linie synchronisiert wird, zu verbessern.
摘要:
Eine Vorrichtung zum Kompensieren von Feldstörungen in Magnetfeldern von Elektromagneten mit hoher Feldhomogenität, insbesondere zur Stabilisierung des H0-Feldes eines MR-Messsystems, umfassend, mindestens einen Felddetektor (31) zum Erfassen von Störsignalen (U in ), mindestens einen Regelkreis zum Bearbeiten der erfassten Störsignale (U in ), und mindestens eine Kompensationsspule (34), an welche die erfassten und bearbeiteten Ausgangsstörsignale (U out ) übertragen werden und welche ein Korrekturfeld zur Kompensation der Störsignale (U in ) erzeugt, ist dadurch gekennzeichnet, dass mindestens einer der Regelkreise ein multiselektives Filtersystem (35) umfasst, welches ein oder mehrere parallel geschaltete selektive Filterelemente aufweist, deren Mittelfrequenzen entweder einmalig oder adaptiv auf die Frequenzwerte der zu kompensierenden Störsignale (U in ) abstimmbar sind, wobei die Ausgänge dieser Filterelemente mit mindestens einer der Kompensationsspulen (34) verbunden sind. Die erfindungsgemäße Vorrichtung erlaubt eine selektive Erfassung von periodischen Feldstörungen mit hohen Frequenzkomponenten, während ein Großteil des vorhandenen Rauschens ausgeblendet und somit das SINO erhöht wird.
摘要:
The invention enables to monitor a magnetic field drift of a magnetic resonance imaging apparatus on the basis of the magnetic resonance signals, which are acquired during magnetic resonance image data acquisition, such as by single shot EPI or by a gradient echo sequence. The phases of at least two magnetic resonance signals are acquired an echo time after the corresponding RF excitations. This corresponds to the central k-space line, which has frequency encoding but no phase encoding. The difference of two consecutive phase measurements, which are acquired at a certain time interval provides the shift of the resonance frequency. This enables monitoring of the shift of the resonance frequency and compensation of the magnetic field drift.
摘要:
The apparatus includes a magnet system (1) for generating a steady magnetic field in a measuring space (35), which steady magnetic field is oriented mainly parallel to one of the axes of an orthogonal coordinate system, a gradient system (2) with gradient coil systems (3) and with control means (7, 9) for supplying the gradient coil systems with excitation currents with a predetermined variation in time. The gradient coil systems (3) include a number of linear gradient coil systems, each of which is arranged to generate a main gradient field which is dependent on the location in the measuring space (35) in such a manner that a magnetic field formed by superposition of one of the main gradient fields on the steady magnetic field can be described, as a function of the coordinates of the coordinate system, as a series containing first-order terms and higher-order terms, the first-order terms having predetermined coefficients which are equal to zero for two of the coordinates. The gradient system (2) is arranged so that at least a part of the second-order terms of the series describing the magnetic field formed by the superposition of the magnetic fields generated by the gradient system on the steady magnetic field have predetermined coefficients. The undesirable effects of concomitant gradient fields can thus be counteracted.
摘要:
An MRI magnetic field generator so constructed as to enhance a heat efficiency by reducing variations in temperature without a sacrifice in magnetic field uniformity and to be able to control a permanent magnet temperature with high accuracy, comprising temperature controlling heaters (10, 11) embedded and incorporated in base yolks (3, 3) which form magnetic path forming members, wherein, when the temperature controlling heaters (10, 11) are heated by a temperature controller according to temperatures detected by a temperature sensor (12), permanent magnets (5, 5) disposed in the vicinities of the base yolks are heated efficiently to thereby provide an excellent control follow-up and wherein heat from heaters is conducted through the base yolks to directly reach the permanent magnets to thereby permit very efficient heat control without heat loss due to heat dissipation to the outside.