WO2003005053A1 - Method for correcting the intensity of a magnetic resonance image - Google Patents

Method for correcting the intensity of a magnetic resonance image Download PDF

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Publication number
WO2003005053A1
WO2003005053A1 PCT/DE2002/002343 DE0202343W WO03005053A1 WO 2003005053 A1 WO2003005053 A1 WO 2003005053A1 DE 0202343 W DE0202343 W DE 0202343W WO 03005053 A1 WO03005053 A1 WO 03005053A1
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magnetic resonance
resonance image
recorded
image
additional
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PCT/DE2002/002343
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German (de)
French (fr)
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Wilfried Landschütz
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Siemens Aktiengesellschaft
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/58—Calibration of imaging systems, e.g. using test probes, Phantoms; Calibration objects or fiducial markers such as active or passive RF coils surrounding an MR active material
    • G01R33/583—Calibration of signal excitation or detection systems, e.g. for optimal RF excitation power or frequency
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/24—Arrangements or instruments for measuring magnetic variables involving magnetic resonance for measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/246—Spatial mapping of the RF magnetic field B1
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
    • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
    • G01R33/341—Constructional details, e.g. resonators, specially adapted to MR comprising surface coils
    • G01R33/3415—Constructional details, e.g. resonators, specially adapted to MR comprising surface coils comprising arrays of sub-coils, i.e. phased-array coils with flexible receiver channels

Definitions

  • the present invention relates to a method for intensity correction of a magnetic resonance image of an object area formed from intensity values, which is recorded with one or more surface coils in a magnetic resonance system, in which an additional magnetic resonance image of the same object area is recorded with a whole-body coil and the intensity values of the magnetic resonance image of the surface coils Intensity values of the additional magnetic resonance image are corrected in order to achieve a homogeneous sensitivity distribution.
  • Magnetic resonance tomography is a known technique for obtaining images of the interior of a living examination object.
  • a basic field magnet generates a static, relatively homogeneous basic magnetic field. Rapidly switched gradient fields, which are generated by so-called gradient coils, are superimposed on this basic magnetic field during the acquisition of magnetic resonance images.
  • high-frequency transmission antennas high-frequency pulses for triggering magnetic resonance signals are radiated into the examination object. The magnetic resonance signals caused by these high-frequency pulses are picked up by high-frequency reception antennas.
  • the magnetic resonance images of the examined object area of the examination object are created on the basis of these magnetic resonance signals received with the receiving antennas. Each pixel in the
  • Magnetic resonance image corresponds to an intensity value of a received magnetic resonance signal of a small body volume.
  • Whole-body high-frequency antennas are generally used as high-frequency transmitting antennas, which also serve as receiving antennas.
  • Antennas are also known as so-called surface antennas or surface coils, which receive magnetic resonance signals from a relatively small area of the body.
  • These surface antennas are generally connected to the rest of the magnetic resonance tomography device via a flexible feed line and can be positioned close to the corresponding examination area.
  • the surface antennas have the advantage of a good signal-to-noise ratio compared to the whole-body antenna.
  • their disadvantage is a non-homogeneous reception sensitivity over the measurement volume.
  • the intensity of the received signal decreases with the distance to the signal source. This is noticeable in the recorded magnetic resonance images in that regions of the measurement volume located near the surface antenna appear with higher intensity than regions located further away.
  • a disadvantage of this method is the need to arrange opposite antenna systems between which the object area to be examined has to be positioned. This is complex and also not possible in every case.
  • Another method is from IC Carlsen et al., Reconstruction Algorithm for Images Obtained with Flexible Multi-Element Synergy Coils, Proceedings of the Society of Magnetic Resonance, Volume 2, August 6-12, 1994, San Francisco, USA known for the correction of magnetic resonance images.
  • two magnetic resonance images of the same object area are taken in succession before or after the actual measurement of the magnetic resonance image with the surface coils.
  • One image is created with the surface coils, the other with the whole body coil.
  • a correction factor is calculated from the two magnetic resonance images by dividing the correspondingly correlated intensity values, ie the intensity values at the same image positions, with which the magnetic resonance image or images of the actual measurement are corrected.
  • the object of the present invention is to provide a method for correcting the intensity of a magnetic resonance image of an object area recorded with surface coils, which method enables a reliable correction of the magnetic resonance image without additional antenna arrangements.
  • an additional magnetic resonance image of the same object area is also recorded with a whole-body coil.
  • the additional magnetic resonance image is recorded in counter sentence to the known methods of the prior art, however, simultaneously with the recording of the magnetic resonance image of the surface coil. Then the intensity values of the magnetic resonance image of the surface coil are corrected with intensity values of the additional magnetic resonance image.
  • the magnetic resonance images of the surface coil can be corrected reliably. This eliminates a complicated calculation for oblique layers, as is required with other theoretical correction methods, and no prior knowledge of the coil position is necessary. Furthermore, no additional antenna arrangements are required for the correction of the magnetic resonance image.
  • the simultaneous measurement of the magnetic resonance image with the surface coils and the additional magnetic resonance image with the whole-body coil ensures that the correction data relate to the identical object area. Furthermore, there is no need for additional measurements before or after the actual recording of the magnetic resonance image or images.
  • Magnetic resonance image is obtained by multiplying the intensity values by corresponding correction values assigned to the spatial position of the intensity values in the magnetic resonance image.
  • the correction values are formed from a division of the intensity values of the additional magnetic resonance image and the intensity values of the magnetic resonance image of the surface coil assigned according to the image position. If necessary, the measurement data can be subjected to pre-filtering to reduce the noise.
  • the corrected magnetic resonance image corresponds to the magnetic resonance image originally recorded with the surface coils. intensity values were multiplied by the correction factors.
  • the simultaneous measurement with surface coils and the whole body coil can take place in that in addition to the selected receiving channels of the surface coils, the whole body coil is also switched to receive for each measurement. In this way, the correction data can be recorded in parallel.
  • the simultaneous connection of the whole-body coil to the surface coils is not possible due to an undesired coupling of the coils, the simultaneous recording of the additional magnetic resonance image can be realized by the following preferred embodiment of the present method.
  • additional k-space lines are generated by the surface coils, which are not detected with the surface coils but with the whole-body coil.
  • the measurement is carried out by switching the reception between the surface coils and the whole body coil in such a way that the lines specified for the desired resolution of the image are recorded with the surface coils and the additional lines with the whole body coil.
  • the additional lines are preferably generated or recorded at the same k-space position as lines that are scanned with the surface coils. This can be implemented in a simple manner by means of a suitable control sequence for controlling the phase-coding gradient coil.
  • the additional magnetic resonance image It is not necessary to provide the same number of lines for the additional magnetic resonance image as for the magnetic resonance image of the surface coils. Rather, a significantly lower resolution is sufficient for the additional Magnetic resonance image of the whole body coil.
  • the additional lines are preferably recorded in the middle of k-space. Due to the possibility of recording the additional magnetic resonance image with a lower resolution, the measurement time increase when using the present method is small. In the case of multi-slice measurements, the correction data can be interpolated in the slice direction, so that a complete correction data record does not have to be recorded for each slice recorded.
  • FIG. 1 shows an example of the basic structure of a magnetic resonance tomography system with which the present method can be carried out
  • Fig. 2 shows an example of the processing of the data to produce a corrected image.
  • Figure 1 shows schematically a sectional view through a magnetic resonance imaging device.
  • a basic field magnet 1 a gradient coil system 2
  • a high-frequency transmitting antenna 3 a surface receiving coil 4
  • a surface receiving coil 4 a surface receiving coil 4
  • a patient 5 can be seen on a patient couch 6, which represents the examination object.
  • one or more high-frequency pulses for generating magnetic resonance signals are radiated into the body of the person 5 via the high-frequency transmission antenna 3 designed as a whole-body coil.
  • the magnetic resonance signals are recorded with the surface coil 4 and displayed in the form of a two-dimensional magnetic resonance image.
  • the image elements of the image are arranged in a right-angled matrix, each pixel corresponding to an intensity value measured with the surface coils 4.
  • FIG. 2 shows an example in which the additional magnetic resonance image was obtained by generating and scanning additional lines during the recording of the original image by the surface coils.
  • the figure shows schematically on the left side the measurement data of the surface coil in a k-space 7, which comprises the complete columns and rows of the scan.
  • the additional lines 8 scanned with the whole-body coil can be seen as a section from k-space. These additional lines are located in the middle of k-space at the same position as the lines scanned with the surface coil. With an original image size of 256 lines x 256 columns, for example, it is sufficient to scan only the middle 32 lines in k-space with the whole-body coil.
  • the measurement data of the whole-body coil and the data of the surface coil assigned to the corresponding k-space section are each subjected to an FFT with zerofilling for the transformation to the size of the original image, and the data values corresponding to the image position are divided by one another in order to obtain correction factors. To correct the original data of the surface coil, these are multiplied by the determined correction values, so that a corrected image 9 is produced.
  • the correction data sets resulting from the division are subjected to masking or filtering 10 before the multiplication with the original data, in order not to amplify the existing image noise due to the correction.
  • all image areas in the correction data record that only contain noise can be set to the value 1 by masking.

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  • Physics & Mathematics (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)

Abstract

The invention relates to a method for correcting the intensity of a magnetic resonance image of an object area formed from intensity values, said image being recorded using one or more surface coils in a magnetic resonance system. According to said method, an additional magnetic resonance image of the same object area is recorded using a whole-body coil and the intensity values of the magnetic resonance image of the surface coils are corrected by intensity values of the additional magnetic resonance image, in order to obtain a homogeneous sensitivity distribution. According to the inventive method, the additional magnetic resonance image is recorded simultaneously with the magnetic resonance image of the surface coil(s).

Description

Beschreibungdescription
Verfahren zur Intensitätskorrektur eines MagnetresonanzbildesMethod for correcting the intensity of a magnetic resonance image
Die vorliegende Erfindung betrifft ein Verfahren zur Intensitätskorrektur eines aus Intensitätswerten gebildeten Magnetresonanzbildes eines Objektbereiches, das mit einer oder mehreren Oberflächenspulen in einer Magnetresonanzanlage aufgenommen wird, bei dem ein zusätzliches Magnetresonanzbild des gleichen Objektbereiches mit einer Ganzkörperspule aufgenommen und die Intensitätswerte des Magnetresonanzbildes der 0- berflächenspulen mit Intensitätswerten des zusätzlichen Magnetresonanzbildes korrigiert werden, um eine homogene Empfindlichkeitsverteilung zu erreichen.The present invention relates to a method for intensity correction of a magnetic resonance image of an object area formed from intensity values, which is recorded with one or more surface coils in a magnetic resonance system, in which an additional magnetic resonance image of the same object area is recorded with a whole-body coil and the intensity values of the magnetic resonance image of the surface coils Intensity values of the additional magnetic resonance image are corrected in order to achieve a homogeneous sensitivity distribution.
Die Magnetresonanztomographie ist eine bekannte Technik zur Gewinnung von Bildern des Körperinneren eines lebenden Untersuchungsobjekts. Zur Durchführung der Magnetresonanztomographie erzeugt ein Grundfeldmagnet ein statisches relativ homo- genes Grundmagnetfeld. Diesem Grundmagnetfeld werden während der Aufnahme von Magnetresonanzbildern schnell geschaltete Gradientenfelder überlagert, die von sog. Gradientenspulen erzeugt werden. Mit Hochfrequenzsendeantennen werden Hochfrequenzpulse zur Auslösung von Magnetresonanzsignalen in das Untersuchungsobjekt eingestrahlt. Die mit diesen Hochfrequenzpulsen hervorgerufenen Magnetresonanzsignale werden von Hochfrequenzempfangsantennen aufgenommen. Die Magnetresonanzbilder des untersuchten Objektbereiches des Untersuchungsobjektes werden auf Basis dieser mit den Empfangsantennen emp- fangenen Magnetresonanzsignale erstellt. Jeder Bildpunkt imMagnetic resonance tomography is a known technique for obtaining images of the interior of a living examination object. To carry out magnetic resonance tomography, a basic field magnet generates a static, relatively homogeneous basic magnetic field. Rapidly switched gradient fields, which are generated by so-called gradient coils, are superimposed on this basic magnetic field during the acquisition of magnetic resonance images. With high-frequency transmission antennas, high-frequency pulses for triggering magnetic resonance signals are radiated into the examination object. The magnetic resonance signals caused by these high-frequency pulses are picked up by high-frequency reception antennas. The magnetic resonance images of the examined object area of the examination object are created on the basis of these magnetic resonance signals received with the receiving antennas. Each pixel in the
Magnetresonanzbild entspricht dabei einem Intensitätswert eines empfangenen Magnetresonanzsignals eines kleinen Körpervolumens .Magnetic resonance image corresponds to an intensity value of a received magnetic resonance signal of a small body volume.
Als Hochfrequenzsendeantennen werden in der Regel Ganzkörperhochfrequenzantennen eingesetzt, die gleichzeitig auch als Empfangsantennen dienen. Neben diesen Ganzkörperhochfrequenz- antennen sind auch sog. Oberflächenantennen bzw. Oberflächenspulen bekannt, die Magnetresonanzsignale aus einem verhältnismäßig kleinen Körperbereich empfangen. Diese Oberflächenantennen sind in der Regel über eine flexible Zuleitung mit dem übrigen Magnetresonanztomographiegerät verbunden und lassen sich nahe am entsprechenden Untersuchungsbereich positionieren. Die Oberflächenantennen weisen den Vorteil eines guten Signal/Rausch-Verhältnisses im Vergleich zur Ganzkörperantenne auf. Ihr Nachteil besteht jedoch in einer nicht- homogenen Empfangsempfindlichkeit über das Messvolumen. DieWhole-body high-frequency antennas are generally used as high-frequency transmitting antennas, which also serve as receiving antennas. In addition to these whole body high frequency Antennas are also known as so-called surface antennas or surface coils, which receive magnetic resonance signals from a relatively small area of the body. These surface antennas are generally connected to the rest of the magnetic resonance tomography device via a flexible feed line and can be positioned close to the corresponding examination area. The surface antennas have the advantage of a good signal-to-noise ratio compared to the whole-body antenna. However, their disadvantage is a non-homogeneous reception sensitivity over the measurement volume. The
Intensität des Empfangssignals nimmt mit dem Abstand zur Signalquelle ab. Dies macht sich in den aufgenommenen Magnetresonanzbildern dadurch bemerkbar, dass nahe der Oberflächenantenne gelegene Bereiche des Messvolumens mit höherer Intensi- tat erscheinen als weiter entfernt gelegene Bereiche.The intensity of the received signal decreases with the distance to the signal source. This is noticeable in the recorded magnetic resonance images in that regions of the measurement volume located near the surface antenna appear with higher intensity than regions located further away.
Zur Verminderung dieser Problematik ist aus der US 5,712,567 A ein Verfahren zur Korrektur der Empfangscharakteristik einer Antennenanordnung bekannt, bei dem zwei sich gegenüber- liegende Antennenanordnungen eingesetzt werden. Der zu untersuchende Objektbereich befindet sich zwischen den beiden Antennenanordnungen. Mit beiden Antennen werden Magnetresonanzbilder aufgenommen, aus denen ein geometrisches Mittel der entsprechend korrelierten Intensitätswerte gebildet wird. Das korrigierte Magnetresonanzbild setzt sich aus diesen gemit- telten Intensitätswerten zusammen.To reduce this problem, a method for correcting the reception characteristic of an antenna arrangement is known from US Pat. No. 5,712,567 A, in which two antenna arrangements located opposite one another are used. The object area to be examined is located between the two antenna arrangements. Magnetic resonance images are recorded with both antennas, from which a geometric mean of the correspondingly correlated intensity values is formed. The corrected magnetic resonance image is composed of these averaged intensity values.
Ein Nachteil dieses Verfahrens besteht jedoch in der Notwendigkeit der Anordnung sich gegenüberliegender Antennensyste- me, zwischen denen der zu untersuchende Objektbereich positioniert werden muss. Dies ist aufwendig und zudem nicht in jedem Falle möglich.A disadvantage of this method, however, is the need to arrange opposite antenna systems between which the object area to be examined has to be positioned. This is complex and also not possible in every case.
Aus I. C. Carlsen et al., Reconstruction Algorithm for Images Obtained with Flexible Multi-Element Synergy Coils, Proceed- ings of the Society of Magnetic Resonance, Volume 2, August 6 - 12, 1994, San Francisco, USA, ist ein weiteres Verfahren zur Korrektur von Magnetresonanzbildern bekannt. Bei diesem Verfahren werden - vor oder nach der eigentlichen Messung des Magnetresonanzbildes mit den Oberflächenspulen - nacheinander zwei Magnetresonanzbilder des gleichen Objektbereiches aufge- nommen. Das eine Bild wird mit den Oberflächenspulen, das andere mit der Ganzkörperspule erzeugt. Aus beiden Magnetresonanzbildern wird durch Division der entsprechend korrelierten Intensitätswerte, d. h. der Intensitätswerte an den gleichen Bildpositionen, ein Korrekturfaktor errechnet, mit dem das oder die Magnetresonanzbilder der eigentlichen Messung korrigiert werden.Another method is from IC Carlsen et al., Reconstruction Algorithm for Images Obtained with Flexible Multi-Element Synergy Coils, Proceedings of the Society of Magnetic Resonance, Volume 2, August 6-12, 1994, San Francisco, USA known for the correction of magnetic resonance images. In this method, two magnetic resonance images of the same object area are taken in succession before or after the actual measurement of the magnetic resonance image with the surface coils. One image is created with the surface coils, the other with the whole body coil. A correction factor is calculated from the two magnetic resonance images by dividing the correspondingly correlated intensity values, ie the intensity values at the same image positions, with which the magnetic resonance image or images of the actual measurement are corrected.
Bei diesem Verfahren entstehen jedoch unter anderem dadurch Probleme, dass die Korrekturdaten aus zwei aufeinanderfolgen- den Messungen berechnet werden. Durch eine Veränderung derProblems with this method arise, among other things, from the fact that the correction data are calculated from two successive measurements. By changing the
Position des Untersuchungsobjektes, bspw. aufgrund der Atemtätigkeit der zu untersuchenden Person, während der Aufnahme der Korrekturmessungen kann es dabei zu fehlerhaften Korrekturwerten kommen.Position of the examination object, for example due to the breathing activity of the person to be examined, during the recording of the correction measurements, incorrect correction values may result.
Die Aufgabe der vorliegenden Erfindung besteht darin, ein Verfahren zur Intensitätskorrektur eines mit Oberflächenspulen aufgenommenen Magnetresonanzbildes eines Objektbereiches anzugeben, das ohne zusätzliche Antennenanordnungen eine zu- verlässige Korrektur des Magnetresonanzbildes ermöglicht.The object of the present invention is to provide a method for correcting the intensity of a magnetic resonance image of an object area recorded with surface coils, which method enables a reliable correction of the magnetic resonance image without additional antenna arrangements.
Die Aufgabe wird mit dem Verfahren gemäß Patentanspruch 1 gelöst. Vorteilhafte Ausgestaltungen des Verfahrens sind Gegenstand der Unteransprüche.The object is achieved with the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims.
Bei dem vorliegenden Verfahren zur Intensitätskorrektur eines aus Intensitätswerten gebildeten Magnetresonanzbildes eines Objektbereiches, das mit einer oder mehreren Oberflächenspulen in einer Magnetresonanzanlage aufgenommen wird, wird e- benfalls ein zusätzliches Magnetresonanzbild des gleichen Objektbereiches mit einer Ganzkörperspule aufgenommen. Die Aufnahme des zusätzlichen Magnetresonanzbildes erfolgt im Gegen- satz zu den bekannten Verfahren des Standes der Technik jedoch gleichzeitig mit der Aufnahme des Magnetresonanzbildes der Oberflächenspule. Anschließend werden die Intensitätswerte des Magnetresonanzbildes der Oberflächenspule mit Intensi- tätswerten des zusätzlichen Magnetresonanzbildes korrigiert.In the present method for intensity correction of a magnetic resonance image of an object area formed from intensity values, which is recorded with one or more surface coils in a magnetic resonance system, an additional magnetic resonance image of the same object area is also recorded with a whole-body coil. The additional magnetic resonance image is recorded in counter sentence to the known methods of the prior art, however, simultaneously with the recording of the magnetic resonance image of the surface coil. Then the intensity values of the magnetic resonance image of the surface coil are corrected with intensity values of the additional magnetic resonance image.
Durch die Aufnahme des zusätzlichen Magnetresonanzbildes mit der Ganzkörperspule, die im Gegensatz zu den Oberflächenspulen eine sehr gute Homogenität der Empfangsempfindlichkeit aufweist, können die Magnetresonanzbilder der Oberflächenspule zuverlässig korrigiert werden. Dadurch entfällt eine komplizierte Berechnung bei obliquen Schichten, wie dies bei anderen theoretischen Korrekturverfahren erforderlich ist, und es ist keinerlei Vorwissen über die Spulenposition notwendig. Weiterhin sind keinerlei zusätzliche Antennenanordnungen für die Korrektur des Magnetresonanzbildes erforderlich. Durch die gleichzeitige Messung des Magnetresonanzbildes mit den Oberflächenspulen und des zusätzlichen Magnetresonanzbildes mit der Ganzkörperspule wird gewährleistet, dass sich die Korrekturdaten auf den identischen Objektbereich beziehen. Weiterhin entfällt die Notwendigkeit zusätzlicher Messungen vor oder nach der eigentlichen Aufnahme des oder der Magnetresonanzbilder.By recording the additional magnetic resonance image with the whole-body coil, which, in contrast to the surface coils, has a very good homogeneity of the reception sensitivity, the magnetic resonance images of the surface coil can be corrected reliably. This eliminates a complicated calculation for oblique layers, as is required with other theoretical correction methods, and no prior knowledge of the coil position is necessary. Furthermore, no additional antenna arrangements are required for the correction of the magnetic resonance image. The simultaneous measurement of the magnetic resonance image with the surface coils and the additional magnetic resonance image with the whole-body coil ensures that the correction data relate to the identical object area. Furthermore, there is no need for additional measurements before or after the actual recording of the magnetic resonance image or images.
Die Korrektur des mit den Oberflächenspulen aufgenommenenThe correction of that recorded with the surface coils
Magnetresonanzbildes erfolgt durch Multiplikation der Intensitätswerte mit entsprechenden, der räumlichen Position der Intensitätswerte im Magnetresonanzbild zugeordneten Korrekturwerten. Die Korrekturwerte werden aus einer Division der Intensitätswerte des zusätzlichen Magnetresonanzbildes und der entsprechend der Bildposition zugeordneten Intensitätswerte des Magnetresonanzbildes der Oberflächenspule gebildet. Die Messdaten können hierbei gegebenenfalls einer Vorfilterung zur Verminderung des Rauschens unterzogen werden. Das korrigierte Magnetresonanzbild entspricht dem mit den Oberflächenspulen original aufgenommenen Magnetresonanzbild, des- sen Intensitätswerte mit den Korrekturfaktoren multipliziert wurden.Magnetic resonance image is obtained by multiplying the intensity values by corresponding correction values assigned to the spatial position of the intensity values in the magnetic resonance image. The correction values are formed from a division of the intensity values of the additional magnetic resonance image and the intensity values of the magnetic resonance image of the surface coil assigned according to the image position. If necessary, the measurement data can be subjected to pre-filtering to reduce the noise. The corrected magnetic resonance image corresponds to the magnetic resonance image originally recorded with the surface coils. intensity values were multiplied by the correction factors.
Die gleichzeitige Messung mit Oberflächenspulen und der Ganzkörperspule kann dadurch erfolgen, dass bei jeder Messung neben den angewählten Empfangskanälen der Oberflächenspulen zusätzlich die Ganzkörperspule auf Empfang geschaltet wird. Auf diese Weise lassen sich die Korrekturdaten parallel aufnehmen.The simultaneous measurement with surface coils and the whole body coil can take place in that in addition to the selected receiving channels of the surface coils, the whole body coil is also switched to receive for each measurement. In this way, the correction data can be recorded in parallel.
Sollte das gleichzeitige Zuschalten der Ganzkörperspule zu den Oberflächenspulen aufgrund einer unerwünschten Kopplung der Spulen nicht möglich sein, so kann die gleichzeitige Aufnahme des zusätzlichen Magnetresonanzbildes durch folgende bevorzugte Ausführungsform des vorliegenden Verfahrens realisiert werden.If the simultaneous connection of the whole-body coil to the surface coils is not possible due to an undesired coupling of the coils, the simultaneous recording of the additional magnetic resonance image can be realized by the following preferred embodiment of the present method.
Bei dieser Ausführungsform werden bei der Messung bzw. Aufnahme des Magnetresonanzbildes durch die Oberflächenspulen zusätzliche k-Raum-Zeilen erzeugt, die nicht mit den Oberflächenspulen, sondern mit der Ganzkörperspule erfasst werden. Die Messung erfolgt hierbei durch Umschalten des Empfanges zwischen den Oberflächenspulen und der Ganzkörperspule derart, dass die für die gewünschte Auflösung des Bildes vorge- gebenen Zeilen mit den Oberflächenspulen und die zusätzlichen Zeilen mit der Ganzkörperspule aufgenommen werden.In this embodiment, when the magnetic resonance image is measured or recorded, additional k-space lines are generated by the surface coils, which are not detected with the surface coils but with the whole-body coil. The measurement is carried out by switching the reception between the surface coils and the whole body coil in such a way that the lines specified for the desired resolution of the image are recorded with the surface coils and the additional lines with the whole body coil.
Die zusätzlichen Zeilen werden vorzugsweise an der gleichen k-Raum-Position erzeugt bzw. aufgenommen wie Zeilen die mit den Oberflächenspulen abgetastet werden. Durch eine geeignete Steuersequenz zur Ansteuerung der phasenkodierenden Gradientenspule kann dies auf einfache Weise realisiert werden.The additional lines are preferably generated or recorded at the same k-space position as lines that are scanned with the surface coils. This can be implemented in a simple manner by means of a suitable control sequence for controlling the phase-coding gradient coil.
Hierbei ist es nicht erforderlich, für das zusätzliche Mag- netresonanzbild die gleiche Zeilenanzahl wie für das Magnetresonanzbild der Oberflächenspulen vorzusehen. Vielmehr reicht eine deutlich geringere Auflösung für das zusätzliche Magnetresonanzbild der Ganzkörperspule aus. Die zusätzlichen Zeilen werden hierbei vorzugsweise in der Mitte des k-Raumes aufgenommen. Durch die Möglichkeit der Aufnahme des zusätzlichen Magnetresonanzbildes mit geringerer Auflösung ist der Messzeitzuwachs bei Einsatz des vorliegenden Verfahrens gering. Bei Mehrschichtmessungen kann gegebenenfalls in Schichtrichtung eine Interpolation der Korrekturdaten vorgenommen werden, so dass nicht für jede aufgenommene Schicht ein kompletter Korrekturdatensatz aufgenommen werden muss.It is not necessary to provide the same number of lines for the additional magnetic resonance image as for the magnetic resonance image of the surface coils. Rather, a significantly lower resolution is sufficient for the additional Magnetic resonance image of the whole body coil. The additional lines are preferably recorded in the middle of k-space. Due to the possibility of recording the additional magnetic resonance image with a lower resolution, the measurement time increase when using the present method is small. In the case of multi-slice measurements, the correction data can be interpolated in the slice direction, so that a complete correction data record does not have to be recorded for each slice recorded.
Die vorliegende Erfindung wird nachfolgend anhand eines Ausführungsbeispiels in Verbindung mit den Zeichnungen nochmals kurz erläutert. Hierbei zeigen.The present invention is briefly explained again below using an exemplary embodiment in conjunction with the drawings. Show here.
Fig. 1 ein Beispiel für den prinzipiellen Aufbau einer Magnetresonanztomographieanlage, mit der das vorliegende Verfahren durchführbar ist; und1 shows an example of the basic structure of a magnetic resonance tomography system with which the present method can be carried out; and
Fig. 2 ein Beispiel für die Verarbeitung der Daten zur Erzeugung eines korrigierten Bildes.Fig. 2 shows an example of the processing of the data to produce a corrected image.
Figur 1 zeigt schematisch ein Schnittbild durch ein Magnetresonanztomographiegerät. In der Figur sind nur die wesentlichen Bauteile des Geräts, ein Grundfeldmagnet 1, ein Gradientenspulensystem 2, eine Hochfrequenzsendeantenne 3 sowie eine Oberflächenempfangsspule 4 dargestellt. Weiterhin ist ein Patient 5 auf einer Patientenliege 6 zu erkennen, der das Untersuchungsobjekt darstellt.Figure 1 shows schematically a sectional view through a magnetic resonance imaging device. In the figure, only the essential components of the device, a basic field magnet 1, a gradient coil system 2, a high-frequency transmitting antenna 3 and a surface receiving coil 4 are shown. Furthermore, a patient 5 can be seen on a patient couch 6, which represents the examination object.
Bei der Messung wird über die als Ganzkörperspule ausgebilde- te Hochfrequenzsendeantenne 3 ein oder mehrere Hochfrequenzimpulse zur Erzeugung von Magnetresonanzsignalen im Körper der Person 5 eingestrahlt. Die Magnetresonanzsignale werden mit der Oberflächenspule 4 erfasst und in Form eines zweidi- mensionalen Magnetresonanzbildes dargestellt. Die Bildelemen- te des Bildes sind hierbei in einer rechtwinkligen Matrix angeordnet, wobei jeder Bildpunkt einem mit den Oberflächenspulen 4 gemessenen Intentsitätswert entspricht. Figur 2 zeigt ein Beispiel, bei dem das zusätzliche Magnetresonanzbild durch Erzeugung und Abtastung zusätzlicher Zeilen während der Aufnahme des Originalbildes durch die Oberflächenspulen erhalten wurde. Die Figur zeigt auf der linken Seite schematisch die Messdaten der Oberflächenspule in einem k-Raum 7, der die vollständigen Spalten und Zeilen der Abtastung umfasst. Auf der rechten Seite sind die mit der Ganzkörperspule abgetasteten zusätzlichen Zeilen 8 als Ausschnitt aus dem k-Raum zu erkennen. Diese zusätzlichen Zeilen befin- den sich in der Mitte des k-Raums an der gleichen Position, an der auch die mit der Oberflächenspule abgetasteten Zeilen liegen. So ist es bei einer Originalbildgröße von 256 Zeilen x 256 Spalten beispielsweise ausreichend, mit der Ganzkörperspule zusätzlich nur die mittleren 32 Zeilen im k-Raum abzu- tasten.During the measurement, one or more high-frequency pulses for generating magnetic resonance signals are radiated into the body of the person 5 via the high-frequency transmission antenna 3 designed as a whole-body coil. The magnetic resonance signals are recorded with the surface coil 4 and displayed in the form of a two-dimensional magnetic resonance image. The image elements of the image are arranged in a right-angled matrix, each pixel corresponding to an intensity value measured with the surface coils 4. FIG. 2 shows an example in which the additional magnetic resonance image was obtained by generating and scanning additional lines during the recording of the original image by the surface coils. The figure shows schematically on the left side the measurement data of the surface coil in a k-space 7, which comprises the complete columns and rows of the scan. On the right side, the additional lines 8 scanned with the whole-body coil can be seen as a section from k-space. These additional lines are located in the middle of k-space at the same position as the lines scanned with the surface coil. With an original image size of 256 lines x 256 columns, for example, it is sufficient to scan only the middle 32 lines in k-space with the whole-body coil.
Die Messdaten der Ganzkörperspule und die dem entsprechenden k-Raum-Ausschnitt zugeordneten Daten der Oberflächenspule werden zur Hochtransformation auf die Größe des Originalbildes jeweils einer FFT mit Zerofilling unterzogen und die je- weils aufgrund der Bildposition korrespondierenden Datenwerte durcheinander dividiert, um Korrekturfaktoren zu erhalten. Zur Korrektur der Originaldaten der Oberflächenspule werden diese mit den ermittelten Korrekturwerten multipliziert, so dass ein korrigiertes Bild 9 entsteht.The measurement data of the whole-body coil and the data of the surface coil assigned to the corresponding k-space section are each subjected to an FFT with zerofilling for the transformation to the size of the original image, and the data values corresponding to the image position are divided by one another in order to obtain correction factors. To correct the original data of the surface coil, these are multiplied by the determined correction values, so that a corrected image 9 is produced.
Optional werden die sich aus der Division ergebenden Korrekturdatensätze vor der Multiplikation mit den Originaldaten einer Maskierung oder Filterung 10 unterzogen, um das vorhandene Bildrauschen durch die Korrektur nicht zu verstärken. So können beispielsweise alle Bildbereiche im Korrekturdatensatz, die nur Rauschen enthalten, durch die Maskierung auf den Wert 1 gesetzt werden. Optionally, the correction data sets resulting from the division are subjected to masking or filtering 10 before the multiplication with the original data, in order not to amplify the existing image noise due to the correction. For example, all image areas in the correction data record that only contain noise can be set to the value 1 by masking.

Claims

Patentansprüche claims
1. Verfahren zur Intensitätskorrektur eines aus Intensitätswerten gebildeten Magnetresonanzbildes eines Objektbereiches, das mit einer oder mehreren Oberflächenspulen (4) in einer Magnetresonanz-Anlage aufgenommen wird, bei dem ein zusätzliches Magnetresonanzbild des gleichen Objektbereiches mit einer Ganzkörperspule (3) aufgenommen und die Intensitätswerte des Magnetresonanzbildes der Oberflächenspule (n) (4) mit In- tensitätswerten des zusätzlichen Magnetresonanzbildes korrigiert werden, um eine homogene Empfindlichkeitsverteilung zu erreichen, d a d u r c h g e k e n n z e i c h n e t , dass das zusätzliche Magnetresonanzbild gleichzeitig mit dem Magnetreso- nanzbild der Oberflächenspule (n) (4) aufgenommen wird.1. Method for correcting the intensity of a magnetic resonance image of an object area formed from intensity values, which is recorded with one or more surface coils (4) in a magnetic resonance system, in which an additional magnetic resonance image of the same object area is recorded with a whole-body coil (3) and the intensity values of the magnetic resonance image of the surface coil (s) (4) are corrected with intensity values of the additional magnetic resonance image in order to achieve a homogeneous sensitivity distribution, characterized in that the additional magnetic resonance image is recorded simultaneously with the magnetic resonance image of the surface coil (s) (4).
2. Verfahren nach Anspruch 1, d a d u r c h g e k e n n z e i c h n e t , dass zur gleichzeitigen Aufnahme des zusätzlichen Magnetresonanzbildes zusätzliche Zeilen (8) erzeugt und während der Aufnahme des Magnetresonanzbildes durch die Oberflächenspule (n) (4) durch Umschalten des Empfanges von der/den Oberflächenspule (n) (4) auf die Ganzkörperspule (3) aufgenommen werden.2. The method according to claim 1, characterized in that for the simultaneous recording of the additional magnetic resonance image additional lines (8) are generated and during the recording of the magnetic resonance image by the surface coil (s) (4) by switching the reception from the surface coil (s) (s) ( 4) on the whole body coil (3).
3. Verfahren nach Anspruch 1, d a d u r c h g e k e n n z e i c h n e t , dass die Ganzkörperspule (3) während der Aufnahme des Magnetresonanz- Bildes durch die Oberflächenspule (n) (4) gleichzeitig mit der/den Oberflächenspule (n) (4) auf Empfang geschaltet wird, um das zusätzliche Magnetresonanzbild aufzunehmen.3. The method according to claim 1, characterized in that the whole-body coil (3) during the recording of the magnetic resonance image through the surface coil (s) (4) simultaneously with the / the surface coil (s) (4) is switched to receive the record additional magnetic resonance image.
4. Verfahren nach einem der Ansprüche 1 oder 2, d a d u r c h g e k e n n z e i c h n e t , dass die zusätzlichen Zeilen (8) in der Mitte des k-Raumes (7) aufgenom- men werden. 4. The method according to any one of claims 1 or 2, characterized in that the additional lines (8) in the middle of the k-space (7) are recorded.
5. Verfahren nach einem der Ansprüche 1 bis 4, d a d u r c h g e k e n n z e i c h n e t , dass das zusätzliche Magnetresonanzbild mit geringerer Auflösung als das Magnetresonanzbild der Oberflächenspule (n) (4) aufgenommen wird.5. The method according to any one of claims 1 to 4, so that the additional magnetic resonance image is recorded with a lower resolution than the magnetic resonance image of the surface coil (s) (4).
6. Magnetresonanz-Anlage mit einer Steuereinheit, die zur Durchführung des Verfahrens nach einem der vorangehenden Ansprüche ausgebildet ist. 6. Magnetic resonance system with a control unit which is designed to carry out the method according to one of the preceding claims.
PCT/DE2002/002343 2001-07-04 2002-06-26 Method for correcting the intensity of a magnetic resonance image WO2003005053A1 (en)

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