Abstract:
In a method and apparatus for determining a physiological activity signal in a subject using various movement sensors that each transmit a temporal movement signal, a physiological reference signal is determined from the various movement signals that best depicts a physiological movement of the subject, the movement sensor that generates the physiological reference signal is identified as a physiological reference sensor, at least one physiological addition signal is determined from the temporal movement signals that is similar to the physiological reference signal up to a limit, and the physiological reference signal and the at least one addition signal are added to form the physiological activity signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    The present invention concerns a method for determining a physiological activity signal in a subject using a system having several movement sensors. The invention further relates to the associated system for determining the physiological activity signal and a storage medium encoded with programming instructions for implementing such a method. 
         [0003]    Description of the Prior Art 
         [0004]    Knowledge of respiratory activity and heartbeat activity are of fundamental importance for imaging such as for an MR system or CT system, in order to be able to prevent motion artifacts because then recording of the measurement data in the MR or CT system can be synchronized with the heartbeat activity or respiratory movement by triggering. Heartbeat activity is usually measured by means of EKG detection, but this is time-consuming, because each EKG electrode has to be attached to the ribcage. Furthermore, methods for the detection of heartbeat and respiration with the use of radar sensors are known, as described in exemplary fashion in the following publications: 
         [0005]    1. “Monitoring respiratory and cardiac motion in CT using a continuous wave Doppler radar” (F. Pfanner, T. Allmendinger, T. Flohr, M. Kachelrieβ) 
         [0006]    2. “Cardiac and Respiratory Monitoring through Non-Invasive and Contactless Radar Technique” (M Varanini, PC Berardi, F Conforti, M Micalizzi, D Neglia, A Macerata) 
         [0007]    3. “Radar monitoring of heartbeats and respiration” (Øyvind Aardal) 
         [0008]    4. WIRELESS BIO-RADAR SENSOR FOR HEARTBEAT AND RESPIRATION DETECTION” (B.-J. Jang, S.-H. Wi and J.-G. Yook, M.-Q. Lee, K.-J. Lee) 
         [0009]    Furthermore, methods are known which detect the heartbeat (or EKG) with the aid of radar sensors: 
         [0010]    5. Microwave Doppler Radar for Heart Beat Detection Versus Electrocardiogram: A Validation Approach (Dany Obeid, Sawsan Sadek, Gheorghe Zaharia, Ghais El Zein) 
         [0011]    6. Non Contact Heart Monitoring (Lorenzo Scalise) 
         [0012]    7. Wearable Doppler Radar with Integrated Antenna for Patient Vital Sign Monitoring (Richard Ribón Fletcher, Sarang Kulkarni) 
         [0013]    8. A Comparison of Continuous Wave Doppler Radar to Impedance Cardiography for Analysis of Mechanical Cardiac activity (J. A J. Thijs, J. Muehlsteff, O. Such, R. Pinter, R. Elfring, C. H. Igney) 
         [0014]      FIG. 1  shows a diagrammatic view of how several transmitting and receiving antennas can be used, which have a fixed spatial arrangement. This system of the prior art has a radar unit  10  with several transmitting antennas  11 - 14  and a receiving antenna  15 . Likewise it is conceivable that the antennas are alternately transmitting and receiving antennas. Problematic with all these devices and methods is that respiratory signals and signals from heartbeat activity overlap and can only be separated with difficulty. This is because radar sensors detect movements and represent these movements as signals. As heartbeat and respiratory movement occur simultaneously, both activities also overlap in the resulting radar signal. When it is an uninterrupted respiratory signal that is of interest, the heartbeat activity that is additionally visible in the signal may be disturbing. Likewise, when it is the heartbeat signal that is of interest, the respiratory movement additionally visible in the signal can be disturbing. Both signals, heartbeat and respiratory movement, are therefore each available only in a disturbed fashion as a result of known radar arrangements. However, this means that use as a trigger signal for heartbeat activity or respiratory activity is barely or only imperfectly possible, leading to a deterioration in image quality. Furthermore, a disadvantage of radar arrangements according to the prior art is that due to their fixed position, they are not optimally positioned for all subjects, from large adults to children, from slim to fat patients, with the consequence that the quality of the signals generated fluctuates greatly depending on the patient being examined. 
       SUMMARY OF THE INVENTION 
       [0015]    An object of the present invention is to avoid the aforementioned disadvantages and to provide a method and a system in which, regardless of the person examined, physiological activities such as cardiac movement or respiratory movement can be clearly identified. 
         [0016]    According to a first aspect of the invention, a method for determining a physiological activity signal in a subject is implemented with the use of a system that has several movement sensors, which each transmit a temporal movement signal. The sensor that provides the best signal, i.e. for example best detects respiratory movement or cardiac movement, can be identified among the various movement sensors and the associated movement signals. If further signals are subsequently identified from the movement signals—the addition signals—which match the reference signal up to a limit or threshold value, a good activity signal can be obtained through the addition of the reference signal and the addition signals and can be used, for example, to trigger images in an imaging device such as a CT system or an MR system. 
         [0017]    The various movement sensors are preferably several radar sensors that can be installed in a couch of a medical imaging device. 
         [0018]    The physiological activity signal may contain a respiratory movement of the subject, in which case the physiological reference signal is a respiratory reference signal which is detected by a respiratory reference sensor. The physiological addition signal then contains a respiratory addition signal, and the respiratory reference signal and the respiratory addition signal are added to form a respiratory activity signal. 
         [0019]    Furthermore, the physiological activity signal may contain a cardiac movement of the subject, in which case the physiological reference signal is a cardiac reference signal which is detected by a cardiac reference sensor. The physiological addition signal may contain a cardiac addition signal, and the cardiac reference signal and the cardiac addition signal may be added to form a cardiac activity signal. 
         [0020]    The system can also detect both physiological signals simultaneously and generate both a respiratory activity signal and a cardiac activity signal. In doing so, it is possible, for example, that the movement sensors that generate neither the respiratory reference signal, nor the at least one respiratory addition signal, nor the cardiac reference signal, nor the cardiac addition signal, are deactivated. If only one of the two activity signals is to be detected, all sensors that do not detect the associated reference signal or addition signal can be deactivated. The movement sensors such as the radar sensors can all be controlled simultaneously so that they all transmit and receive simultaneously. However, it is also possible for all the radar sensors in a cycle to be only briefly activated so that only one radar sensor is ever active at any one time. It is also conceivable to activate, at any one time, only the radar sensors that are far enough away from each other geometrically to enable mutual interferences and disturbances to be largely avoided. However, radar sensors that cannot be assigned to any of the groups of sensors that detect respiratory activity or cardiac activity can be deactivated. 
         [0021]    To determine the respiratory reference signal or the cardiac reference signal, in each case it is possible to detect the signal with the greatest amplitude in a respiratory rate range or a cardiac rate range among the movement signals and in each case to use the signal with the greatest amplitude as the respiratory reference signal or the cardiac reference signal. The determination of the signal with the greatest amplitude can preferably take place in the respective frequency range in the respiratory rate range or cardiac rate range. 
         [0022]    Because the detected signals usually contain signals from both movement components, filtering is provided. For example, the temporal movement signals can be filtered with a first filter to generate respiratory movement signals in which the signal components induced by a cardiac movement are suppressed compared to signal components induced by a respiratory movement, wherein the respiratory reference signal is determined from the respiratory movement signals. 
         [0023]    Usually, respiratory rates are between 10 per minute and 30 per minute, i.e. between 0.16 Hz and 0.5 Hz. Thus, the heart signal components can be filtered out as the cardiac rate is usually between 35 per minute to 200 per minute, i.e. in a frequency range of 0.58 Hz to 3.33 Hz. To identify cardiac movement signals, the movement signals can be filtered using a second filter. The cardiac reference signal can then be determined from the cardiac movement signal components. 
         [0024]    In determining the cardiac addition signal or the respiratory addition signal, the signals from the other sensors which are similar to the respective reference signal are reviewed. This can be done with the use of a cross-correlation function or a cross-covariance function. 
         [0025]    The various movement sensors can be divided into at least two separate subgroups, wherein the respective subgroup in which the cardiac reference sensor and the respiratory reference sensor are arranged is identified. If the two reference sensors are in the same subgroup, the movement sensors of the at least one other subgroup can be deactivated. As it is possible that the subject is moved into the imaging device feet-first or head-first and in a face-down or supine position, the movement sensors like the radar sensors should cover a large spatial area. This can take place by arranging various sensors in physically separated subsections. If the cardiac reference sensor and the respiratory reference sensor are now in the same subsection, then it is unlikely that meaningful signals are still detected in another subsection which is then covered, for example, by the feet, enabling these sensors to be deactivated. 
         [0026]    Likewise, it is possible that a position of the respiratory reference sensor within the various movement sensors is determined or the position of the cardiac reference sensor. To determine the addition signals which are combined with the respective reference signal, movement sensors can then preferably be considered which are immediately adjacent to the respective reference sensor, as it highly probable that the sensors adjacent to the reference sensors also provide relatively good signals, while sensors which are very far away from the respective reference sensor can probably contribute little to the addition signals. 
         [0027]    The invention also encompasses the associated system with the various movement sensors and an evaluation computer that evaluates the various signals as described above. 
         [0028]    The invention also encompasses an electronically readable data storage medium encoded with programming instructions that, when the storage medium is loaded into a computer, causes the computer to implement the method according to the invention when the programming instructions are executed by the computer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  shows a sensor arrangement for detecting respiratory and cardiac movements according to the prior art. 
           [0030]      FIG. 2  shows a system with which cardiac activity and/or respiratory activity can be inventively identified. 
           [0031]      FIG. 3  is a flowchart of the steps which are performed by an evaluation computer of the system from  FIG. 2  to determine a cardiac activity signal. 
           [0032]      FIG. 4  is a flowchart of the steps to determine a respiratory activity signal. 
           [0033]      FIG. 5  shows an example of a radar sensor signal in which the signal of the cardiac activity outweighs the respiratory activity. 
           [0034]      FIG. 6  shows an example of a radar sensor signal in which the respiratory activity outweighs the signal of the cardiac activity. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    A system or a method is described with which a respiratory signal or a signal of the cardiac activity is robustly identifiable.  FIG. 2  shows such a system  100 , which can be implemented in a patient couch  200 . The system has a multiplicity of radar sensors  110   a  and  110   b  which in the example shown are arranged in two different subsections  120   a  and  120   b . Both the subgroups of the radar sensors  110  in the subsections  120   a  and  120   b  are physically separated from each other. Both subsections can, for example, be 50 cm long and 40 cm wide but other proportions are also conceivable. Altogether, the two subsections are dimensioned such that a respiratory signal or a cardiac signal can be detected in a subject who can be moved into an imaging device  300  (shown only in a diagrammatic view) either feet-first, i.e. with the feet in the direction of the edge A, or head-first. The couch  100  may be part of the imaging device. The imaging device  300  may be an MR system or CT system of a known type. As the subject can either be moved into the imaging device  300  in a supine position or face-down, the subsections  120   a  and  120   b  must be designed such that the cardiac movement can be detected in both a face-down and a supine position. The ribcage of the subject can therefore be located either in the subsection  120   a  or in the subsection  120   b.  The two subsections  120   a  and  120   b  are preferably arranged symmetrically to one another so that a face-down and a supine position or a position in which the subject is moved feet-first or head-first into the imaging device can be covered. However, more than two subsections  120   a  and  120   b  can also be provided. Each subsection  120   a,    120   b  can in turn be divided into three subsections, each with a subsection for the detection of respiration and two subsections for the detection of cardiac activity depending on a face-down or supine position. Radar sensors can be used as sensors; alternatively coils can also be used that are designed such that bodies in the vicinity of the coils generate another magnetic field in the coils. When using radar sensors, in principle they can all be controlled simultaneously so that they all transmit and receive simultaneously. However, care should be taken so that the radar signals transmitted by one radar sensor are not received by another radar sensor. It is therefore conceivable for all radar sensors in a very fast cycle to each be activated only briefly, so that at any one time only one radar sensor is ever briefly active. Furthermore, it is possible that, at any one time, only radar sensors are activated that are far enough away from each other so as to geometrically exclude mutual disturbances. At any one time, therefore, only one subgroup is active. This active subgroup can be, for example, a group of sensors in the subsection  120   a,  and another group of sensors in the subsection  120   b,  or various groups in a subsection that are far enough away from each other for a radar sensor of one subgroup not to receive the reflection signal that has been sent by a radar sensor of another subgroup. All the radar sensors can be briefly active in succession until the sequence recommences. In doing so, depending on the position of the subject, the radar sensors are now dynamically assigned to a group for the detection of respiration or to a group for the detection of cardiac activity, wherein the sensors which are not assigned to either of the two groups can be deactivated. 
         [0036]    As described in detail below, the signals of the individual groups are each added to generate a cardiac activity signal  155  or a respiratory activity signal  156  which can be supplied by an evaluation unit  150 , subsequently described in more detail, to an imaging device  300 . 
         [0037]    How the cardiac activity signal or the respiratory activity signal can be generated and how the respiratory and cardiac signals can be separated is explained below with reference to  FIGS. 3 and 4 . 
         [0038]    In a learning phase, as noted above the various radar sensors can be individually controlled, either in succession or simultaneously, depending on the respective spacing of the sensors. In connection with  FIG. 3 , the determination of a cardiac activity signal is explained with the use of the movement signals that are detected by the radar sensors  110   a  and  110   b.  The movement signals are detected in step S 30 . Then the signals can be filtered with a high pass filter or band pass filter to pass all the signal components in the typical cardiac rate range of 35 per minute to 200 per minute, i.e. between approx. 0.58 Hz and 3.3 Hz. As a result of this filtering, cardiac movement signals that essentially contain the signal components on the basis of the cardiac movement are thus generated in step S 31 . Then the radar signal that detects the cardiac activity with the highest amplitude is determined from the filtered signals. This determination can take place in the time range of the signal, as well as in the frequency range, wherein the highest peak or the highest amplitude in the frequency range is decisive. This corresponds to the step S 32  of  FIG. 3  of the determination of the cardiac reference signal. With reference to  FIG. 2 , this may be the sensor with the reference character  110   h,  for example. In the presence of a total of N movement sensors, all the additional N−1 sensors can then be assessed with regard to the similarity of the signals to the reference signal of the reference sensor  110   h.  This can be done, for example, with the use of a cross-correlation, wherein all the signals filtered in the cardiac rate range can be assessed for similarity, or only signals from sensors in the vicinity which are at a predetermined distance from the reference sensor. In the example shown in  FIG. 2 , it may be useful, for example, not to take the sensors of the subsection  120   b  into consideration if the cardiac reference sensor  110   h  is located in the subsection  120   a.  The cross-correlation function is as follows: 
         [0000]        R   XY ( k )=Σ n=−∞   ∞   x ( n )· y ( n+k )   (1)
 
         [0039]    The cross-correlation function R XY  describes the similarity between two temporal signals, namely the signals X and Y, as a function of the time n. This determination of the cardiac addition signals takes place in step S 33  with reference to  FIG. 3 . In this step S 33 , all the signals which have an adequate relationship to the cardiac reference signal are taken into consideration, adequate here meaning similar up to a limit or threshold value. This limit or threshold value may also be selected and altered by a user of the system of  FIG. 2  as a function of the quality of the respective signals. Thus only signals with adequate similarity are taken into consideration, so that signals with a poor signal-to-noise ratio are not taken into consideration. Then the cardiac reference signal can be added to the cardiac addition signals to form the cardiac activity signal, as shown in step S 34  of  FIG. 3 . 
         [0040]    Similarly, the respiratory activity signal can be generated, as explained below with reference to  FIG. 4 . The movement signals detected by the sensors  110   a,    110   b  are recorded (S 40 ). Then the signals are filtered with a low pass filter or band pass filter to pass all the signal components in the typical respiratory frequency range between 0.16 Hz and 0.5 Hz, or to suppress the other signal or frequency components. This is with reference to step S 41  in  FIG. 4 : determination of the respiratory movement signal for the individual sensors. In a step S 42  the respiratory reference signal is then determined, wherein in turn either in the time or frequency range the highest amplitude in the time or frequency range which forms the respiratory reference signal is established. In step S 43 , the respiratory addition signals that are sufficiently similar to the respiratory reference signal are then determined. All N−1 signals of the radar sensors filtered in the respiratory frequency range can in turn be correlated with the signal of the respiratory reference sensor filtered in the respiratory frequency range using the above equation (1) where K=0. The signals that have a greater similarity than a limit or threshold value can then be used as respiratory addition signals to obtain the activity signal in step S 44 . 
         [0041]    Instead of cross-correlation, another cross-covariance function can be used as shown below in equation (2). 
         [0000]        G   XY ( k )=Σ n=−∞   ∞ [ x ( n )−μ X ]·[γ( n+k )−μ Y ]  (2)
 
         [0042]    With the cross-covariance function, mean-adjusted signals are used, wherein the mean-adjusted signals are totaled to determine the respective activity signal. Altogether, for equation (1) and (2) only a finite number of samples is added to determine the cross-correlation or cross-covariance function. 
         [0043]    In another embodiment, not only K=0 is used for the evaluation of the equation as a number, but a range which, for example, corresponds to half a second. The adequate relationship then to be measured is then determined according to the maximum peak occurring in the band of n used. This takes into account that the signals of the radar sensors may have a small phase delay or lag between them which may be caused by the movement sequences in the body, or by various transmission delays in the hardware used. The addition in step S 34  or in step S 44  must be accordingly corrected by this k for each radar sensor. 
         [0044]    The processing steps shown with regard to  FIGS. 3 and 4  may be performed in an evaluation computer  150  shown in  FIG. 2 , which has at least one processor  151  and one memory  152 . The evaluation computer receives the movement signals via the input-output unit  153 , via which it also controls the sensors  110 . The evaluation computer  150  can finally transmit the respiratory activity signal  155  and/or the cardiac activity signal  156  to the imaging device  300 . The components of the evaluation computer  150  can be designed as hardware components, as software components or as a combination of the two. Both the filters for the generation of the respective addition signals can be provided as separate units, or the associated functions can be performed by the processor unit  151 . 
         [0045]      FIG. 5 , for example, shows a radar sensor signal in which the signal of cardiac activity, which has a higher frequency, outweighs the respiratory activity. In selecting the respective sensors, which may provide an addition signal which is sufficiently similar to the respective reference signal, not all the signals need to be assessed by all the sensors of  FIG. 2 . In the example described, the two reference sensors are located in one of the two subsections  120   a ,  120   b.  For example, with sensor signals from the subsection  120   b,  it is impossible to assess at all whether they create addition signals as it is highly unlikely that a sensor in the subsection  120   b  still supplies cardiac movement signals with an acceptable signal-to-noise ratio. Furthermore, subgroups  121  or  122  can be created as a function of the position of the respective reference sensor. Subgroup  121  may, for example, create the subgroup of sensors, the signals of which are only or are first assessed for similarity. Subgroup  122  may create the group of sensors in which respiratory addition sensors are sought. In  FIG. 5  the amplitude or the peak  51  of cardiac activity is higher than the amplitude or the peak  52 .  FIG. 6  shows a frequency range of a sensor signal in which the peak of the respiratory signal  61  is higher than the peak  62  of the cardiac signal. The sensor, the signal of which is shown in  FIG. 6 , could for example be located under the other half of the ribcage but not far from the sternum.  FIGS. 5 and 6  also show clearly that in principle the two signals can be separated from one another by appropriate filters such as low pass in relation to band pass or high pass. 
         [0046]    The aforementioned processing steps can be performed by the evaluation unit  150  or its processor  151 , wherein programs may be found in the storage unit  152  which perform the aforementioned steps during execution by the processor. The system or method described supplies automatic signals for any position of the subject without the attachment of sensors to the subject. These signals describe the cardiac or respiratory movement well and can thus be used to trigger imaging. 
         [0047]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the Applicant to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the Applicant&#39;s contribution to the art.