Abstract:
A motion sensor for detecting movements of a patient in an imaging medical system, in particular in a magnetic resonance tomography system has at least one HF resonator for emitting an HF signal fed into the resonator from an HF signal source and for receiving a response signal, and a detection circuit for detecting movements of the patient derived from the received signal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention concerns motion sensors and methods for detecting movements of a patient in a medical imaging system, in particular a magnetic resonance tomography system. 
         [0003]    2. Description of the Prior Art 
         [0004]    Examples of methods for detecting movements (in particular for respiratory detection) in a magnetic resonance tomography system are described in the following documents. 
         [0005]    The publication Buikman, Helzel, Roschmann: The Coil as a Sensitive Motion Detector for MRI-Magnetic-Resonance-Imaging, Vol. 6, Num. 3, 1988 belonging to Philips describes that the respiratory movement can be detected by measuring the reflection factor of the body coil of an MRT. 
         [0006]    DE 10 2014 209 488.7 by R. Rehner, A. Fackelmeier and S. Biber relates to “Respiratory detection by posterior high-frequency coils close to the body”, with the respiratory detection being described by a measurement of the reflection factor of a sensor coil under the patient. 
         [0007]    DE 19 2009 052 412 A1 relates to a “Measuring system for detecting the position of a moving organ”. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to optimize a medical device (in particular magnetic resonance tomography system) with respect to movement detection. 
         [0009]    In accordance with the invention, a motion sensor for detecting movements of a patient in an imaging medical system, in particular in a magnetic resonance tomography system has at least one HF resonator for emitting an HF signal fed into the resonator from an HF signal source and for receiving a response signal, and a detection circuit for detecting movements of the patient derived from the received signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows an exemplary embodiment of a scanning arrangement with an open LC resonator. 
           [0011]      FIG. 2  shows scanned changes in the transmission S 21  due to a body movement. 
           [0012]      FIG. 3  shows scanned temporal changes in the transmission S 21  due to breathing. 
           [0013]      FIG. 4  schematically shows an MRT system. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]      FIG. 4  shows (in particular in relation to the technical background as well) an imaging magnetic resonance scanner  101  (located in a shielded space or Faraday cage F) having a hollow cylinder  102  with a tubular space  103  in which an examination table  104  with a body  105 , for example of an examination object (such as a patient; with or without local coil arrangement  106 ) can be moved in the direction of the arrow z to generate scans of the patient  105  by execution of an imaging method. A local coil arrangement  106  is arranged on the patient  105  here, with which MRT scans of a section of the body  105  can be generated in the FoV (also called Field of View or FoV) in a local region. Signals of the local coil arrangement  106  can be evaluated by an evaluation device ( 168 ,  115 ,  117 ,  119 ,  120 ,  121  etc.) of the magnetic resonance scanner  101  that can be connected for example by coaxial cables or via radio ( 167 ), etc. to the local coil arrangement  106  (for example in converted into images, stored or displayed). 
         [0015]    To examine a body  105  (an examination object or a patient) by magnetic resonance imaging using a magnetic resonance device magnetic resonance scanner  101 , different magnetic fields that are matched as accurately as possible to each other in terms of their temporal and spatial characteristics are irradiated onto the body  105 . A strong magnet (often a cryomagnet  107 ) in a measuring booth having a tunnel-like opening  103  here generates a static strong main magnetic field B 0 , which amounts to, for example, 0.2 tesla to 3 tesla or more. A body  105  to be examined, positioned on an examination table  104 , is moved into a region of the main magnetic field B 0  that is substantially homogeneous in the field of observation FoV (also called “Field Of View” or “field of view”). The nuclear spins of atomic nuclei of the body  105  are excited by magnetic high-frequency excitation pulses B 1 ( x, y, z, t ) which are irradiated by a high-frequency antenna (and/or optionally a local coil arrangement) shown in very simplified form here as a (for example multi-part= 108   a ,  108   b ,  108   c ) body coil  108 . High-frequency excitation pulses are generated for example by a pulse-generating unit  109  which is controlled by a pulse sequence control unit  110 . After amplification by a high-frequency amplifier  111  they are led to the high-frequency antenna  108 . The high-frequency system shown here is only indicated schematically. More than one pulse-generating unit  109 , more than one high-frequency amplifier  111  and a plurality of high-frequency antennae  108   a, b, c  are potentially also used in one magnetic resonance device  101 . 
         [0016]    The magnetic resonance scanner  101  also has gradient coils  112   x ,  112   y ,  112   z  with which magnetic gradient fields B G (x, y, z, t) can be irradiated during a scan for selective slice excitation and for spatial encoding of the scan signal. The gradient coils  112   x ,  112   y ,  112   z  are controlled by a gradient coil control unit  114  (and optionally by amplifiers Vx, Vy, Vz) which, like the pulse-generating unit  109 , is also connected to the pulse sequence control unit  110 . 
         [0017]    Signals emitted by the excited nuclear spins (of the atomic nuclei in the examination object) are received by the body coil  108  and/or at least one local coil arrangement  106 , amplified by associated high-frequency pre-amplifier  116  and processed further by a receiving unit  117  and digitized. The recorded scan data is digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix with assigned values by means of a multi-dimensional Fourier transformation. 
         [0018]    For a coil which can be operated in both transmitting and receiving modes, such as e.g. the body coil  108  or a local coil  106 , the correct signal forwarding is regulated by an upstream duplexer  118 . 
         [0019]    From the scan data an image processing unit  119  generates an image which is displayed for a user and/or stored in a memory unit  121  via a control console  120 . A central arithmetic unit  122  controls the individual system components. 
         [0020]    In MR tomography images with a high signal-to-noise ratio (SNR) are currently usually recorded using what are known as local coil arrangements (coils, local coils). These are antenna systems which are provided in the immediate vicinity on top of (anterior) or below (posterior) or on or in the body  105 . During an MR scan the excited nuclei induce a voltage in the individual antennae of the local coil and this is then amplified using a low-noise pre-amplifier (for example LNA, Preamp) and is finally forwarded to the electronic receiving device. What are known as high field systems (1.5 T-12 T or more) are used to improve the signal-to-noise ratio even in high-resolution images. If more individual antennae can be connected to an MR receiving system than there are receivers, a switch matrix for example (sometimes also called an RCCS) is installed between receiving antennae and receivers. This routes the instantaneously active receiving channels (usually those which are located precisely in the field of view of the magnet) to the existing receivers. As a result it is possible to connect more coil elements than there are receivers since in the case of whole-body coverage only the coils which are located in the FoV or in the homogeneity volume of the magnet have to be read. 
         [0021]    Generally an MR antenna system, which can be formed by one antenna element or, as an array coil, of multiple antenna elements (in particular coil elements), is designated a local coil arrangement  106 . These individual antenna elements are designed, for example, as loop antennae (loops), butterfly, flex coils or saddle coils. A local coil arrangement includes for example coil elements, a pre-amplifier, further electronic devices (sheath wave traps, etc.), a housing, supports and usually a cable with connectors by which it is connected to the MRT system. A receiver  168 , provided on the system, filters and digitizes a signal received by a local coil  106 , for example via radio, etc., and passes the data to a digital signal processor which usually derives an image or a spectrum from the data obtained by a scan and makes it available to the user for example for subsequent diagnosis by him, and/or storage. 
         [0022]      FIG. 1-4  shows some details of exemplary embodiments of the invention. 
         [0023]    Undesirable patient movement during image recording can cause pronounced image artifacts in the case of medical imaging. Undesirable movement artifacts of this kind in the chest or abdominal region can be also be caused by breathing. For this reason it may be expedient to detect the patient breathing and to synchronize the (MRT) image recording with the respiratory cycle. This may be helpful in particular for magnetic resonance tomography (MRT) because the image recording (MR sequence) can last a few minutes and not every patient can hold their breath for that long. 
         [0024]      FIG. 1  shows an embodiment of the invention with a placement of one more HF resonator(s) (resonant circuits) HF-Res in the vicinity of the patient  105  in the chest region BrB and/or abdominal region BaB. The HF resonator HF-Res (and/or motion sensor AS with electronic evaluation device) can be integrated, for example, in the examination table  104  of the magnetic resonance scanner  101 . An HF signal from an HF generator (HF-Si-Srs) is coupled, for example, by a coupling coil Cpl-In (or capacitively by a coupling capacitor or by another type of coupling) into the HF resonator HF-Res. The HF signal generated in the HF generator HF-Si-Srs, and coupled into the HF resonator HF-Res is decoupled (by Cpl-out) from the HF resonator again (inductively or capacitively by, for example, a coupling capacitor or by another type of coupling) and is filtered and detected by the, for example narrowband, HF detector (HF-Filt and/or HF-Dtect). 
         [0025]    Properties WW (such as here in particular quality) of the HF resonator are affected here by the breathing of the patient and consequently the transmission (S 21 ) of the HF signal via the HF resonator also changes (i.e. the difference S 21 diff between S 21 - e  in the case of a patient who has inhaled compared to S 21 - a  in the case of a patient who has exhaled). This arrangement enables measurement of the transmission losses of the HF resonator HF-Res, as is shown in  FIG. 1 . 
         [0026]    The HF resonator HF-Res is constructed here in such a way that the induced electromagnetic field HF-Si of the HF resonator HF-Res penetrates into the body of the patient  105  enabling an interaction WW between the HF resonator HF-Res and the body of the patient  105 . A body movement AB (as well as for example a respiratory movement or a shift in the anatomy inside the body which also may not be outwardly visible) then causes a change in the loaded quality of the HF resonator HF-Res. 
         [0027]    This can be detected, for example as shown in  FIG. 2 , as a change S 21 diff (as a difference in the transmission S- 21 - e  when the body of the patient has inhaled and S 21 - a  when the body has exhaled) in the transmission S 21 [dB] as a function of the frequency. 
         [0028]    Respiratory movements AB of the patient  105  can also be measured as temporal changes S 21 -diff, shown in  FIG. 3 , in the transmission S 21 - e -S 21 - a  (before and after inhaling) over the course of time t[s]. For an MRT application it is advantageous if the resonance frequency of the HF resonator HF-Res differs from the MR frequency of the magnetic resonance magnetic resonance scanner  101  so that little, or optimally no, mutual interference can occur between the motion sensor or respiratory sensor AS and MR imaging. 
         [0029]    If the dimensions of the used HF resonator HF-Res are much smaller than the used wavelength of the scan signal, then the HF resonator HF-Res behaves, for example, like an inductive or capacitive sensor and not (or rarely) like an MRT imaging antenna ( 106 ), because work is carried out in the inductive (or capacitive) near field. The irradiated HF energy is consequently minimized. 
         [0030]    Embodiments of the invention can be sensitive to respiration because the scanned transmission losses of an HF resonator HF-Res are dependent on the air-tissue distribution; the muscle-tissue can have large HF losses compared to the inhaled air. 
         [0031]    A transmission scan does not require a directional coupler like a scan of the reflection factor, and can be less sensitive to mismatches in a scanning section. 
         [0032]    Transmission scans according to the invention can be carried out in a large dynamic range and can be less ambiguous (zero crossing and change in sign) than a scan of the reflection factor. 
         [0033]    An HF resonator HF-Res can work with negligible irradiation in a manner similar to an inductive sensor, insofar as only the near field is required for operation. 
         [0034]    Contactless scanning can be enabled. The respiratory sensor does not have to be placed on the body of a patient  105  and secured. 
         [0035]    Embodiments of the invention can have a negligible effect on the SAR balance (in contrast to MR navigators), insofar as the used HF signals can have a low power (of for example 1 mW). 
         [0036]    A motion sensor AS can be integrated in an examination table  104 . In this way the body spacing is always the same and, compared to other methods, no new, visible cabling is necessary. The spacing between a motion sensor AS and a patient  105  may be very constant in the case of a motion sensor AS integrated in an examination table  104 , in particular compared to the alternative of placing the sensor above the patient behind the bore cladding. 
         [0037]    Those skilled in the art know how a transmission scan could be carried out, for example, also from www.wikipedia.de. In principle S parameters, such as, for example, the forward transmission factor S 21 , are scanned with the use of network analyzers as a function of the frequency, see for example network analyzers illustrated in de.wikipedia.org/wiki/Netzwerkanalysator#mediaviewer/File:Vna3.png, although any other network analyzers may also be considered. 
         [0038]      FIG. 1  shows an example of an HF Filter HF-Filt and an HF detector HF-Detect for detecting an HF signal. 
         [0039]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.