Abstract:
The invention relates to a device for synchronizing a magnetic resonance unit with the cardiac rhythm of a patient, comprising a number of electrodes for leading off electrocardiogram signals from the body of the patient and an evaluation unit for determining a characteristic trigger time within a cardiac rhythm period of the patient from the electrocardiogram signals. In order to provide a device, which allows reliable determination of characteristic trigger times within the cardiac rhythm period of the patient even with elevated magnetic field strengths and which operates at least largely independently of external magnetic fields, it is proposed that the evaluation unit is designed to compare electrocardiogram signals obtained before and after the introduction of the patient into the magnetic resonance unit, in order to determine an enlargement of at least one characteristic of the electrocardiogram signals due to the magnetic field.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority of German application No. 10 2006 053 613.4 filed Nov. 14, 2006, which is incorporated by reference herein in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to a device and a method for synchronizing a magnetic resonance unit with the cardiac rhythm of a patient by determining a characteristic trigger time within a cardiac rhythm period of the patient from the electrocardiogram signals of electrodes.  
       BACKGROUND OF THE INVENTION  
       [0003]     Devices are known from the field, which serve to synchronize a magnetic resonance unit with the cardiac rhythm of a patient. In addition to the electrodes of an electrocardiogram device these have an evaluation unit for determining a characteristic trigger time within a cardiac rhythm period of the patient from the electrocardiogram signals of the electrodes. The characteristic trigger time is determined by a simple trigger threshold within the rising or falling edge of the R wave of the electrocardiogram signal. Synchronization serves to prevent motion artifacts, which can be produced, if the magnetic resonance recording takes place in different phases within the periodic motion of the heartbeat.  
         [0004]     While the known methods and devices operate sufficiently reliably in the magnetic field-free space, in the magnetic resonance high field a significantly enlarged T wave can result in the ventricle part of the electrocardiogram signal compared with the field-free electrocardiogram due to the magnetohydrodynamic effect acting on the turbulences in the ventricle, so that the amplitude of the T wave can reach the trigger threshold. This significantly impedes automatic identification of the R wave.  
       SUMMARY OF THE INVENTION  
       [0005]     The object of the invention is in particular to provide a device and method, which allow reliable determination of characteristic trigger times within the cardiac rhythm period of the patient even with elevated magnetic field strengths.  
         [0006]     The object directed at the device is achieved by a device for synchronizing a magnetic resonance unit with the cardiac rhythm of a patient, comprising a number of electrodes for leading off electrocardiogram signals from the body of the patient and an evaluation unit for determining a characteristic trigger time within a cardiac rhythm period of the patient from the electrocardiogram signals, in which the evaluation unit is designed according to the invention to compare electrocardiogram signals obtained before and after the patient is introduced into the magnetic resonance unit, in order to determine an enlargement of at least one characteristic of the electrocardiogram signals due to the magnetic field. This allows the influence of the magnetic field of the magnetic resonance unit on the characteristics of the electrocardiogram signals to be determined and taken into account when synchronizing the magnetic resonance unit with the cardiac rhythm of a patient.  
         [0007]     The enlargement can also be used as an additional criterion for differentiating different characteristics of the electrocardiogram (ECG), so that diagnosis errors can be avoided. The influence of the magnetic field can largely be eliminated, thereby allowing the device to be independent of external magnetic fields. This ultimately ensures reliable determination of characteristic trigger times within the cardiac rhythm period of the patient even with elevated magnetic field strengths and makes it possible to prevent the characteristics being confused.  
         [0008]     Characteristics of the electrocardiogram signals in this context are all the characteristics of the time pattern of the electrocardiogram signals frequently used by those skilled in the art and able to be detected reliably by means of electronic data processing, in particular maxima, minima, gradients or waves. Specifically the characteristics considered are the P wave in the atrium part and the R wave and T wave in the ventricle part of the electrocardiogram signal. The electrode lead-offs can be bipolar or unipolar lead-offs. The inventive device can be deployed in conjunction with magnetic resonance and/or nuclear spin tomography. The evaluation unit can be an evaluation and trigger unit, which also carries out a triggering operation as well as evaluating a trigger time.  
         [0009]     In an advantageous refinement of the invention the device comprises at least three electrodes for leading off electrocardiogram signals from the body of the patient. It is possible to obtain detailed information about cardiac activity and it is possible in particular to generate a three-dimensional vector cardiogram from the signals. The direction, in which the external magnetic field predominantly distorts the three-dimensional vector cardiogram, can be at least largely eliminated by means of a simple projection operation. The different possible views or projections of a three-dimensional vector cardiogram can provide information, which allows clear differentiation of the different characteristics. The selection of a view or projection, in which magnetohydrodynamic effects only have a minor influence, can easily be facilitated.  
         [0010]     In a further refinement of the invention it is proposed that the evaluation unit is designed, to determine the characteristic trigger time as a function of the result of the comparison of the electrocardiogram signals before and after the introduction of the patient into the magnetic resonance unit, to select one characteristic of the electrocardiogram signal from a number of characteristics of the electrocardiogram signal. From the changes in the magnetic field-dependent characteristics it is possible to determine their type. In particular the T wave, whose amplitude increases significantly due to magnetohydrodynamic effects when a strong magnetic field is applied, can be differentiated from the R wave, which is largely independent of the magnetic field.  
         [0011]     If the evaluation unit is designed to generate a number of vector cardiograms with a smaller dimension from a higher dimension vector cardiogram by means of projection operations, it is possible to reduce the influence of the magnetic field in a particularly simple manner, by selecting the projection direction so that it is as parallel as possible to the direction, in which the magnetic field predominantly acts on the three-dimensional vector cardiogram.  
         [0012]     The influence of the magnetic field can hereby be kept small, if the evaluation unit is provided to select the vector cardiogram showing the smallest changes when the electrocardiogram signals before and after the introduction of the patient into the magnetic resonance unit are compared from the number of vector cardiograms with a smaller dimension for synchronizing the magnetic resonance unit with the cardiac rhythm of the patient.  
         [0013]     The evaluation unit is advantageously designed to select one channel or group of channels of the electrocardiogram signals for determining the characteristic trigger time as a function of the result of the comparison of the electrocardiogram signals before and after the introduction of the patient into the magnetic resonance unit. It is then possible to reduce the influence of the magnetic field in a particularly simple manner. For example it is possible to select only that channel or those channels, which show the smallest changes when the electrocardiogram signals before and after the introduction of the patient into the magnetic resonance unit are compared.  
         [0014]     In the specific instance where the influence of the magnetic resonance unit on all the channels of the electrocardiogram signals is approximately equal however all the channels for determining the characteristic trigger time can be selected.  
         [0015]     The parameter used for the magnetic field-dependent changes can be the amplitude, the square of the difference between amplitude patterns before and after the introduction of the patient or another parameter that appears expedient to the person skilled in the art.  
         [0016]     The evaluation unit is hereby particularly advantageously designed in such a manner that it selects the channel or group of channels for determining the characteristic of the electrocardiogram signals, whose enlargement due to the magnetic field, in particular in respect of the T wave, is the smallest.  
         [0017]     It is also proposed that the evaluation unit should be designed, to determine an enlargement of the characteristic of the electrocardiogram signals configured as a T wave, to compare the height of the T wave of the electrocardiogram signals before and after the introduction of the patient into the magnetic resonance unit. The height of the maximum of the T wave is a parameter that is simple to capture, which is extremely dependent on the magnetic field due to the magnetohydrodynamic effects, and which is therefore particularly suitable as a parameter for identifying the T wave.  
         [0018]     Alternatively or additionally the evaluation unit for determining an enlargement of the characteristic of the electrocardiogram signals configured as a T wave can compare a gradient of the T wave of the electrocardiogram signals before and after the introduction of the patient into the magnetic resonance unit. The gradient is similarly magnetic field-dependent and easy to detect and can be determined in the rising edge or the falling edge. In order to obtain a reliable and reproducible value, the gradient is particularly advantageously determined at the turning point of the respective edge.  
         [0019]     Even though in principle the selection of the characteristic or the location of the R wave can also be effected on the basis of the time patterns of the electrode signals, this process can be significantly simplified, if the evaluation unit is designed to generate at least a two- or multi-dimensional vector cardiogram from the electrocardiogram signals. The two- or multi-dimensional vector cardiogram has the advantage of clarity.  
         [0020]     If the evaluation unit is designed, to determine the characteristic trigger time within a cardiac rhythm period of the patient as a function of the comparison of the ECG signals, to select a characteristic of the electrocardiogram signals, which shows a smaller enlargement due to the magnetic field than at least one other characteristic, it is possible to achieve a device that is largely independent of the external magnetic field of the magnetic resonance unit.  
         [0021]     The object directed at the method is achieved by a method for synchronizing a magnetic resonance unit with the cardiac rhythm of a patient by determining a characteristic trigger time within a cardiac rhythm period of the patient from the electrocardiogram signals of electrodes, in which according to the invention, to determine an enlargement of at least one characteristic of the electrocardiogram signals due to the magnetic field before and after the introduction of the patient into the magnetic resonance unit, electrocardiogram signals obtained are compared. It is possible to determine the influence of the magnetic field of the magnetic resonance unit on the characteristics of the electrocardiogram signals and take it into account when synchronizing the magnetic resonance unit with the cardiac rhythm of a patient. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The invention is described in more detail with reference to exemplary embodiments shown in the drawings, in which:  
         [0023]      FIG. 1  shows a schematic diagram of a magnetic resonance unit with a device for synchronizing the magnetic resonance unit with the cardiac rhythm of a patient,  
         [0024]      FIG. 2  shows an electrocardiogram signal with different characteristics,  
         [0025]      FIG. 3  shows a two-dimensional vector cardiogram with a T wave and an R wave,  
         [0026]      FIG. 4   a  shows a first two-dimensional projection of a three-dimensional vector cardiogram and  
         [0027]      FIG. 4   b  shows a second two-dimensional projection of a three-dimensional vector cardiogram. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 1  shows a schematic diagram of a device for synchronizing a magnetic resonance unit  2  with the cardiac rhythm of a patient  4 . The device comprises a number of electrodes  6 ,  8 ,  10  for leading off electrocardiogram signals  12  ( FIG. 2 ) from the body of the patient  4  and an evaluation unit  14  for determining a characteristic trigger time  22  ( FIG. 2 ) within a cardiac rhythm period of the patient  4  from the electrocardiogram signals  12  of the electrodes  6 ,  8 ,  10 . The evaluation unit  14  is configured as a single piece with the control unit of the magnetic resonance unit  2 .  
         [0029]     The control unit uses the trigger signals to synchronize the recording of magnetic resonance images by the magnetic resonance unit  2  with the cardiac rhythm of the patient  4 , thus avoiding motion artifacts, by acquiring different recordings respectively in the same phase of the cardiac rhythm period.  
         [0030]     The evaluation unit  14  is designed by a suitable program, whose function is described in more detail below, to determine an enlargement of at least one characteristic of the electrocardiogram signals  12  due to the magnetic field, to compare the electrocardiogram signals  12  before and after the introduction of the patient  4  into the magnetic resonance unit  2 .  
         [0031]     In the exemplary embodiment shown in  FIG. 1  the device comprises three electrodes  6 ,  8 ,  10  for leading off electrocardiogram signals  12  from the body of the patient  4 . The three electrodes  6 ,  8 ,  10  are attached in such a manner that the lead-offs are as perpendicular as possible to each other. The three electrodes  6 ,  8 ,  10  are used to pick off voltages, which are assigned respectively to a channel. The evaluation unit  14  picks off the voltages present at the electrodes  6 ,  8 ,  10  directly by way of cables  16 ,  18 ,  20  in each instance.  
         [0032]     To determine the characteristic trigger time  22  ( FIG. 2 ), the evaluation unit  14  selects one characteristic of the electrocardiogram signal  12  from a number of characteristics of the electrocardiogram signal  12  as a function of the result of the comparison of the electrocardiogram signals before and after the introduction of the patient  4  into the magnetic resonance unit  2 .  
         [0033]      FIG. 2  shows a typical electrocardiogram signal  12  of a patient  4  in the magnetic field-free space. The electrocardiogram is made up of an atrium part  24  and a ventricle part  26 . The atrium part  24  is assigned to the contraction of the atrium and has a P wave  44  as its characteristic. The ventricle part  26  starts with a so-called QRS phase  28 , the primary characteristic of which is the central R wave  30 , which is framed by a preceding Q wave  32  with reverse polarization and a subsequent S wave  34  with reverse polarization. The ventricle part  26  also comprises a T phase  36  with a T wave  38  and a U phase  40  with a U wave  42 . Without any magnetic field the amplitude of the T wave  38  is around ⅙ to ⅔ of the amplitude of the R wave  30  and the amplitude of the Q wave  32  is less than ¼ of the amplitude of the R wave  30 . The magnitude of the amplitude of the R wave  30  is around 1 mV. In comparison the amplitude of the P wave  44  is below 0.25 mV.  
         [0034]     In  FIG. 2  an enlarged T wave  38 ′, as shown after the introduction of the patient into the magnetic resonance unit  2 , is marked with a broken line. On the right next to the actual electrocardiogram signal  12  is a calibration wave with a height of 1 mV.  
         [0035]     The evaluation unit  14  uses the detected electrocardiogram signals  12  to calculate a three-dimensional vector cardiogram, with the signal of each channel or each of the three electrodes  6 ,  8 ,  10  forming a component of a three-dimensional vector. In the three-dimensional vector cardiogram every characteristic of the electrocardiogram signal  12  has a correspondence in a loop in the resulting curve, which the point in the cardiac rhythm of the patient  4  defined by the three-dimensional vector periodically passes through.  
         [0036]     In a further refinement of the invention the evaluation unit  14  carries out projection operations to generate a number of vector cardiograms with a smaller dimension from one higher dimension vector cardiogram. By way of example  FIG. 3  shows a two-dimensional vector cardiogram, generated by means of a projection operation onto the X-Y plane from the three-dimensional vector cardiogram. In  FIG. 3  the larger loop, which appears dotted due to the high speed of motion of the point, represents the R wave  30 , while the smaller, narrow loop represents the T wave  38 .  
         [0037]     In the three-dimensional vector cardiogram the differences between the R wave  30  and the T wave  38  are clearly identifiable, also because the T wave  38  increases significantly on application of the high field of the magnetic resonance unit  2 .  
         [0038]     To determine the characteristic trigger time  22  within a cardiac rhythm period of the patient  4 , the evaluation unit  14  selects one characteristic of the electrocardiogram signals  12 , the R wave  30 , which shows a smaller enlargement due to the magnetic field than the T wave  38 . This selection is made by selecting a specific loop in the three-dimensional vector cardiogram. Since the T loop responds in a more sensitive manner to the magnetic field than the R wave  30 , the characteristic thus selected corresponds reliably to the R wave  30 .  
         [0039]     To determine the characteristic trigger time  22 , the evaluation unit  14  specifies a point or region  44  defined by angle and distance from source or by three coordinate values on the loop corresponding to the R wave  30 . If the vector moving on the loop reaches this region  44 , the trigger is activated. Activation of the trigger determines the characteristic trigger time  22 .  
         [0040]     The evaluation unit  14  selects from the three two-dimensional vector cardiograms ( FIG. 3, 4   a ,  4   b ) for synchronizing the magnetic resonance unit  2  with the cardiac rhythm of the patient  4  obtained by the projections onto the coordinate planes the vector cardiogram which shows the smallest changes when the electrocardiogram signals  12  before and after the introduction of the patient  4  into the magnetic resonance unit  2  are compared. The selected two-dimensional vector cardiogram ( FIG. 3 ) is assigned to the projection plane, which is at the largest angle to the longitudinal axis of the loop of the three-dimensional vector cardiogram, which maps the T wave  38 .  
         [0041]      FIGS. 4   a  and  4   b  show the two further two-dimensional vector cardiograms, obtained in addition to the selected vector cardiogram shown in  FIG. 3  by projection onto the coordinate planes from the three-dimensional vector cardiogram.  FIG. 4   a  here shows the projection onto the sagittal plane or the X-Z plane and  FIG. 4   a  shows the projection onto the cranial plane or the Y-Z plane. Once the suitable projection has been selected, the actual trigger process takes place by evaluating the movement of the vector determined by the electrode signals in the two-dimensional diagram. The region  44  or point defining the trigger threshold is projected onto the two-dimensional plane with the path curve.  
         [0042]     In alternative refinements of the invention it is possible to obtain two-dimensional vector cardiograms by projections onto further projection planes that are different from the coordinate planes. The projection plane can be selected by forming the minimum of the suitably selected parameter for changes in the electrocardiogram signals  12 .  
         [0043]     In a further alternative refinement of the invention the evaluation unit  14  is designed to select one channel or a group of channels of the electrocardiogram signals  12  for determining the characteristic trigger time  22  as a function of the result of the comparison of the electrocardiogram signals  12  before and after the introduction of the patient  4  into the magnetic resonance unit  2 . Selection of the channel replaces selection of the projection plane, with selection of a projection plane from the three coordinate planes naturally corresponding to a selection of a pair of channels.  
         [0044]     If the device comprises more than 3 electrodes  6 ,  8 ,  10 , such that a higher dimension vector cardiogram is generated in a higher dimension vector space, projection onto a corresponding hyperplane or lower space can be achieved by channel selection. For example it is possible to obtain four three-dimensional vector cardiograms and/or six two-dimensional vector cardiograms by means of projections from one four-dimensional vector cardiogram, etc.  
         [0045]     The evaluation unit  14  is hereby programmed in such a manner that it selects the channel or group of channels for determining the characteristic of the electrocardiogram signals  12 , whose enlargement due to the magnetic field is smallest. The maximum difference between or the mean, optionally squared difference between the signals before and after the introduction of the patient  4  into the magnetic resonance unit  2  can for example be selected as the parameter for the enlargement due to the magnetic field.  
         [0046]     Since the T wave  38  responds in a particularly sensitive manner to the external magnetic field, when the channel or group of channels is being selected, the data processing algorithm of the evaluation unit  14  concentrates in particular on the response of the T wave  38 . This is achieved in that the evaluation unit  14  for determining an enlargement of the T wave  38  compares the height  46  of the same in the electrocardiogram signals  12  before the introduction of the patient  4  and the height  46 ′ after the introduction of the patient  4  into the magnetic resonance unit  2  or forms the difference between the two determined heights  46 ,  46 ′.  
         [0047]     Alternatively or additionally in a development of the invention to determine an enlargement of the T wave  38  an increase in the T wave  38  of the electrocardiogram signals  12  before and after the introduction of the patient  4  into the magnetic resonance unit  2  is compared or the difference between the two determined increases is formed. To this end the evaluation unit  14  first determines the turning point  50  of the rising edge  48  of the T wave  38  and then determines the rise in the turning point  50 .  
         [0048]     Ultimately the device described above implements a method for synchronizing a magnetic resonance unit  2  with the cardiac rhythm of a patient  4  by determining a characteristic trigger time  22  within a cardiac rhythm period of the patient  4  from the electrocardiogram signals  12  of electrodes  6 ,  8 ,  10 .  
         [0049]     In the inventive method, to determine an enlargement of at least one characteristic of the electrocardiogram signals  12 , the electrocardiogram signals  12  before and after the introduction of the patient  4  into the magnetic resonance unit  2  are compared.