Patent Application
20070167737
This application claims priority of German application No. 10 2005 027 438.2 filed Jun. 14, 2005, which is incorporated by reference herein in its entirety.
The invention relates to a method for ECG-triggering a measuring sequence of a magnetic resonance device, in which ECG signals of a patient are detected over two channels.
It is necessary for magnetic resonance tomography (MR) to detect the ECG signals of the patients in order to synchronize the triggering of the MR measuring sequences to the heart beat of the patient. Likewise, the ECG signal detected during an MR examination allows the information about the current heart phase to be acquired. If the ECG signals and the triggering of the measuring sequence are not synchronized, there is a danger that the MR images contain movement artifacts.
In practice however there are difficulties associated with the detection of the ECG signals, since the electrical and magnetic fields acting during the MR sequences significantly interfere in the ECG electronics circuit, whereby the reliable determination of the heart phase is impaired. Aside from these undesired interferences in the ECG electronics circuit, the so-called magnetohydrodynamic effect occurs in the case of a higher magnetic fluid densities, which results in a superelevation of the T wave of the heart beat. The different phases of the heart cycle are indicated in the electrocardiography using letters, for instance by the sequence P-Q-R-S-T. Here the R wave shows the greatest deflection. It is the center of reference of the triggering and must therefore be reliably determined.
A method for ECG triggering a measuring sequence in a magnetic resonance device is known from U.S. Pat. No. 6,070,097. However, the ECG signals of a patient are detected there via a single channel.
The detection of two ECG channels has been proposed in WO 99/04688, a vector display in a coordinate system being derived therefrom. The R wave of the heart cycle should be able to be taken from this display. It is however doubtful whether this method is satisfactorily reliable as this vector projection is dependent on many influences, for instance it changes if the patient holds his/her breath.
The problem underlying the invention is thus to specify a method for triggering a measuring sequence in a magnetic resonance device, which functions reliably and trouble-free.
To solve this problem, provision is made in accordance with the invention with a method of the type mentioned at the outset that the triggering of the measuring sequence occurs when the ECG signal of at least one channel, said ECG signal being subjected to a digital signal processing, exceeds a threshold value, and at the same time the derivation of the ECG signal or the norm of the derivation of the ECG signal with two channels lies within a defined interval.
With the method according to the invention, a triggering, in other words a release of the measuring sequence of the magnetic resonance device, only takes place when a number of conditions are fulfilled at the same time. In accordance with the invention, the two channels for the ECG signals are evaluated separately. A triggering only takes place when a defined threshold value is exceeded in at least one channel. The derivation of the ECG signal or its norm must lie within a defined interval or value range and is used as a further criterion. The triggering of the measuring sequence is only carried out when this condition is fulfilled for both channels.
In accordance with the invention, the criteria determination is carried out in parallel so as to minimize delay times. In accordance with the invention, the parallel extraction of the criteria determination can be almost arbitrarily expanded by further criteria. In this case, the triggering is the result of a special criteria weighting.
There can be provision in accordance with the invention for the digital signal processing of the ECG signals to comprise the following steps: a low-pass filtering which is adapted to the MR-specific disturbances, and/or calculation of a derivation norm function and/or calculating the threshold value by means of an interpolator. In this case, the derivation norm function can be formed on the basis of the first or second derivation of the ECG signal or a combination thereof.
With the method according to the invention, a lower and an upper limit value can be determined in each instance for both channels, said limit values defining the interval. An especially highly reliable method is produced if the limit values defining the interval are determined in a learning phase, whilst none or at the most negligible disturbances occur during the detection of the ECG signals. It is particularly beneficial if the patient is positioned outside a patient tunnel of the magnetic resonance device during the learning phase, as the signal detection is not influenced in this case by the magnets of the magnetic resonance device. An even higher reliability can be achieved if a further learning phase is carried out prior to another magnetic resonance examination. This can also be carried out in the patient tunnel. This enables changes occurring in the meantime, which can be caused for instance by the administration of a drug, to be compensated for.
The invention further relates to a magnetic resonance device. The magnetic resonance device according to the invention is designed to implement the described method.
Further advantages and details of the invention are explained on the basis of an exemplary embodiment with reference to the figures. The figures are schematic representations in which;
A shown in
As shown in
The limit values of the interval determined in the initial learning phase are taken into account during the MR examination carried out in connection therewith. The limit values or a subset thereof are optionally adjusted or updated during further learning phases in the patient tunnel.
The digital signal processing for the second channel 2 is carried out in a similar way to the first channel. The signal is likewise processed in two separate branches, so as to achieve the upper and the limit value for the second channel.
The input data of the first channel 1 reaches a low pass filter 9 in the upper branch and subsequently arrives at a derivation norm creator 10. The value determined by the derivation norm creator 10 is compared in a comparator 11 with a threshold value supplied by a threshold value creator 12. The course of this digital signal processing corresponds in each instance to the lower branch of the learning phase shown in
This same signal processing with a low pass filter 13, a derivation norm creator 14, a threshold value creator 15 and a comparator 16 is carried out for the lower branch of the second channel 2.
In the respective other branch of the first and of the second channel 1, 2, a derivation creator 17 and/or 18 is used, which supplies the derivation or the norm of the derivation of the respective ECG signal. Finally, an examination is carried out for both channels separately in the method step 19 and 20 as to whether the value supplied by the derivation creator 17, 18 lies within the defined interval. So that this condition is fulfilled, the value must lie between the lower and the upper limit value, which were determined in the preceding learning phase. Both branches are linked to one another by an AND function 21, in other words the method is continued only when the derivations of both channels 1, 2 lie within their respective reliable value ranges.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 027 438.2 | Jun 2005 | DE | national |
