Abstract:
An magnetic resonance examination system comprises an RF arrangement with an RF antenna to acquire magnetic resonance signals from an object to be examined, and a motion sensing arrangement to detect motion information of the object. The motion sensing arrangement is provided with one or more RF antenna motion sensors mounted on the RF antenna and one or more object motion sensors to be attached to the object. In an example the motion sensors are integrated devices having motion sensitivity along three independent axes.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention pertains to a magnetic resonance examination system with a motion sensing arrangement to detect motion information of the object to be examined. 
       BACKGROUND OF THE INVENTION 
       [0002]    Such a magnetic resonance examination system is known from the international application WO2011/033422. 
         [0003]    The known magnetic resonance examination system includes a motion sensor, e.g. a compression sensor, an acceleration sensor or a position sensor. The motion sensor generates motion signals that can be used for motion correction in an MR imaging procedure. Optionally, several motion sensors may be integrated into an RF coil unit of the magnetic resonance examination system. Motion information collected by different sensors may be combined in a motion model that is used for motion compensation. In one embodiment the RF coil unit with the integrated motion sensor is positioned directly on the body of a patient to be examined in order to detect motion of the body in the region from which the magnetic resonance signals are acquired. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the invention is to provide a magnetic resonance examination system with a motion detection arrangement that provides motion information at a higher accuracy. 
         [0005]    This object is achieved by a magnetic resonance examination system according to the invention comprising 
         [0006]    an RF arrangement with an RF antenna to acquire magnetic resonance signals from an object to be examined, 
         [0007]    a motion sensing arrangement to detect motion information of the object, 
         [0008]    a data processor to reconstruct an magnetic resonance image from the magnetic resonance signals, 
         [0009]    the RF arrangement being coupled to the data processor to apply the magnetic resonance signals to the data processor, 
         [0010]    the motion sensing arrangement being coupled to the data processor to apply the detected motion information to the data processing unit, wherein 
         [0011]    the motion sensing arrangement is provided with 
         [0012]    one or more RF antenna motion sensors mounted on the RF antenna and 
         [0013]    one or more object motion sensors to be attached to the object. 
         [0014]    The motion sensing arrangement comprises both RF antenna motion sensors mounted on the RF antenna as well as object motion sensors to be attached to the object. Notably, the object motion sensors are to be placed on or to be attached to the body of the patient to be examined. The RF antenna motion sensors provide information on the motion of the RF antenna. The object motion sensors provide information on the motion of the object. Thus, the RF antenna motion sensors and the object motion sensors together provide information on the relative position of the RF antenna and the object. This allows accurate determination of the motion of both the object and the RF antenna even when the RF antenna and the object each move in different manners. Thus, motion is accurately determined also when the RF antenna does not rigidly move with the motion of the object. This occurs when the RF antenna is for example an RF receiver coil which is deformable or semi-rigid which does not precisely conform to the motion of the patient to be examined, but also does not remain fixedly mounted to the structure of the magnetic resonance examination system. 
         [0015]    The motion sensing arrangement further includes an arithmetic unit to compute the motion of both the RF antenna and the object on the basis of the detected motion by the RF antenna motion sensors and the object motion detection sensors, respectively. Thus, the motion information is computed by the arithmetic unit. 
         [0016]    The RF arrangement has an output coupled to the data processor to apply the received magnetic resonance signals to the data processor. The data processor is provided with a motion correction module. The motion correction module corrects the magnetic resonance signals for the detected motion. The data processor further includes a reconstruction module to reconstruct an magnetic resonance image on the basis of the magnetic resonance signals, e.g. by way of a fast-Fourier transform (FFT). These motion-corrected magnetic resonance signals are then fed into a reconstructor which reconstructs the magnetic resonance image from the motion-corrected magnetic resonance signals. This reconstructed magnetic resonance image has no or only few residual motion artefacts. Alternatively, the magnetic resonance signals can be fed into the reconstructor which reconstructs the magnetic resonance image from the magnetic resonance signals. The motion correction module is arranged to receive the magnetic resonance image in which motion artefacts occur and on the basis of the motion information corrects the received magnetic resonance image to the reconstructed magnetic resonance image in which no or only few residual motion artefacts occur. 
         [0017]    These and other aspects of the invention will be further elaborated with reference to the embodiments defined in the dependent Claims. 
         [0018]    In a preferred embodiment of the invention, the motion sensors are provided with integrated circuit devices having motion sensitivity in one or more, notably two or three, independent, axes. The integrated circuit is based on micro-electronics with smallest details of less than 100 nm, or even in nano-electonics. Motion detection can be realized e.g. with MEMS (micro-electro-mechanical switches) as well. For example the independent axes are mutually transverse or orthogonal. Notably micro/nano-electronic devices that are sensitive to roll and pitch rotations can be employed in combination with micro/nano-electronic devices that are sensitive to yaw rotations. Thus, on the basis of miniature electronic devices sensing of motion along three or even nine axes, i.e. in a volume, is achieved. 
         [0019]    In another embodiment of the invention, the RF antenna is formed by an RF receiving coil that has an structure of electrically conductive coil elements, e.g. forming one or more coil loops, which can pick up magnetic flux of magnetic resonance signals. The RF antenna motion sensors, e.g. comprising micro/nano-electronic devices are mounted rigidly to or even integrated in the structure of the RF receiving coil. It does not really matter exactly where the RF antenna motion sensors are located in or on the structure of the RF receiving coil, as long as the connection is rigid, i.e. loop and sensor follow the same trajectory during motion. In this way the RF antenna motion sensors accurately sense the movement, position and shape of the coil. 
         [0020]    In a further embodiment the RF receiving coil is provided with flexible receiver elements to pick-up magnetic resonance signals. The RF antenna motion sensors are integrated in the flexible receiver elements. This enables to track the deformation of the flexible receiver elements in a direct manner. For example, the RF receiving coil has a semi-rigid part with coil elements and RF antenna motion sensors tracking the coil motion and a flexible part with coil elements and object motion sensors on top of the patient following patient motion. Notably, the motion signals output by the motion sensors which are mounted in or on an individual flexible receiver element represent directly the shape of the flexible receiver element. Successive sensing by the motion sensors of the flexible receiver element enables to track changes of the shape of that receiver element, i.e. to track the deformation of the flexible receiver element. In a semi rigid coil one RF antenna sensor at one of the sides of the RF receiving coils would already bring improvement. Typically during breathing in the centre of the patient&#39;s chest is lifted, lifting the centre of the coil (but remain at the same relative distance). The edges of the coil however move away from the patient body, i.e. distance and element orientation changes. Typically motion detection is required with respect to three axis: movement in x-direction, movement in y-direction and roll over the z-axis. (z-axis is in the longitudinal direction of the magnet of the magnetic resonance examination system). If coil elements become very flexible, like ‘non-elastic wearable’ sensors at various positions are required, in practice four RF antenna motion sensors on the RF receiving coil achieves good results. 
         [0021]    These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter and with reference to the accompanying drawing wherein 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  shows diagrammatically a magnetic resonance imaging system in which the invention is used. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0023]      FIG. 1  shows diagrammatically a magnetic resonance imaging system in which the invention is used. The magnetic resonance imaging system includes a set of main coils  10  whereby the steady, uniform magnetic field is generated. The main coils are constructed, for example in such a manner that they enclose a tunnel-shaped examination space. The patient to be examined is placed on a patient carrier ( 14 ) which is slid into this tunnel-shaped examination space. The magnetic resonance imaging system also includes a number of gradient coils  11 ,  12  whereby magnetic fields exhibiting spatial variations, notably in the form of temporary gradients in individual directions, are generated so as to be superposed on the uniform magnetic field. The gradient coils  11 ,  12  are connected to a gradient control  21  which includes one or more gradient amplifier and a controllable power supply unit. The gradient coils  11 ,  12  are energised by application of an electric current by means of the power supply unit  21 ; to this end the power supply unit is fitted with electronic gradient amplification circuit that applies the electric current to the gradient coils so as to generate gradient pulses (also termed ‘gradient waveforms’) of appropriate temporal shape The strength, direction and duration of the gradients are controlled by control of the power supply unit. The magnetic resonance imaging system also includes transmission and receiving coils  13 ,  16  for generating the RF excitation pulses and for picking up the magnetic resonance signals, respectively. The transmission coil  13  is preferably constructed as a body coil  13  whereby (a part of) the object to be examined can be enclosed. The body coil is usually arranged in the magnetic resonance imaging system in such a manner that the patient  30  to be examined is enclosed by the body coil  13  when he or she is arranged in the magnetic resonance imaging system. The body coil  13  acts as a transmission antenna for the transmission of the RF excitation pulses and RF refocusing pulses. Preferably, the body coil  13  involves a spatially uniform intensity distribution of the transmitted RF pulses (RFS). The same coil or antenna is usually used alternately as the transmission coil and the receiving coil. Furthermore, the transmission and receiving coil is usually shaped as a coil, but other geometries where the transmission and receiving coil acts as a transmission and receiving antenna for RF electromagnetic signals are also feasible. The transmission and receiving coil  13  is connected to an electronic transmission and receiving circuit  15 . 
         [0024]    It is to be noted that it is alternatively possible to use separate receiving and/or transmission coils  16 . For example, surface coils  16  can be used as receiving and/or transmission coils. Such surface coils have a high sensitivity in a comparatively small volume. The receiving coils, such as the surface coils, are connected to a demodulator  24  and the received magnetic resonance signals (MS) are demodulated by means of the demodulator  24 . The demodulated magnetic resonance signals (DMS) are applied to a reconstruction unit. The receiving coil is connected to a preamplifier  23 . The preamplifier  23  amplifies the RF resonance signal (MS) received by the receiving coil  16  and the amplified RF resonance signal is applied to a demodulator  24 . The demodulator  24  demodulates the amplified RF resonance signal. The demodulated resonance signal contains the actual information concerning the local spin densities in the part of the object to be imaged. Furthermore, the transmission and receiving circuit  15  is connected to a modulator  22 . The modulator  22  and the transmission and receiving circuit  15  activate the transmission coil  13  so as to transmit the RF excitation and refocusing pulses 
         [0025]    The reconstruction unit derives one or more image signals from the demodulated magnetic resonance signals (DMS), which image signals represent the image information of the imaged part of the object to be examined. The reconstruction unit  25  in practice is constructed preferably as a digital image processing unit  25  which is programmed so as to derive from the demodulated magnetic resonance signals the image signals which represent the image information of the part of the object to be imaged. The signal on the output of the reconstruction monitor  26 , so that the monitor can display the magnetic resonance image. It is alternatively possible to store the signal from the reconstruction unit  25  in a buffer unit  27  while awaiting further processing. 
         [0026]    The magnetic resonance imaging system according to the invention is also provided with a control unit  20 , for example in the form of a computer which includes a (micro)processor. The control unit  20  controls the execution of the RF excitations and the application of the temporary gradient fields. To this end, the computer program according to the invention is loaded, for example, into the control unit  20  and the reconstruction unit  25 . 
         [0027]    Further, one or more object motion sensors  41  are provided that can be placed on or attached to the body of the patient to be examined. The signals from these object motion sensors are applied to a motion detection and correction module  33  that is incorporated in the control unit  20 . The motion detection and correction module  33  computes the motion correction for the magnetic resonance image from the received signals from the object motion sensors. Also RF antenna motion sensors  42  are provided on or integrated in the surface coils  16 . The signals from the RF antenna motion sensors  42  area also applied to the motion detection and correction module  33 . The motion detection and correction module computes the motion correction also on the basis of the motion and deformation of the surface coils  16 . The signals form, the RF antenna motion detection represent the position and form (which can be deformed) of the electrically conducting receiver loop of the RF surface coil  16 . Preferably, in this embodiment the surface coils are flexible, deformable RF surface coils. The signals from the object motion sensors  41  represent the position and shape of the part of the body of the patient to be examined to which the object motion sensors are mounted. Further, the motion detection and correction module is configured to compute the motion of the patient to be examined relative to the surface coils  16  and the RF body coil  13 . That is, the motion detection and correction module computes the motion of the patient to be examined relative to the movement of the surface coils, e.g. as they move with the movement of the patient to be examined. The motion correction computed by the motion detection and correction module is applied to the reconstuctor that achieves that the reconstructed image is corrected for motion of the patient to be examined as well as motion and deformation of the surface coils  16 . 
         [0028]    Preferably, the object motion sensors as well as the RF antenna motion sensors are implemented as nano-motion sensors. for example 2-axis gyro devices or yaw axis devices may be employed.