Abstract:
An ultrasonic sensor according to the invention comprises at least one ultrasonic transducer, at least one resistor connected to the ultrasonic transducer and a housing accommodating the ultrasonic transducer and the resistor. The ultrasonic sensor is configured in such a way, that it is not or only slightly ferromagnetic, so that the ultrasonic sensor acts neutrally with respect to an external magnetic field (for example in an MRT).

Description:
FIELD OF THE INVENTION 
     The invention relates to an ultrasonic sensor for cardiotocography (CTG). In particular, the invention relates to a CTG ultrasonic sensor, which is usable in a magnetic resonance tomograph (MRT). Further, the invention relates to a cardiotocograph, i.e. a CTG device, having such an ultrasonic sensor as well as to a system with an MRT and a CTG device having such an ultrasonic sensor. 
     BACKGROUND OF THE INVENTION 
     Since the fetal heart is located within the uterus, the possibility of directly detecting the fetal heart frequency does not exist there, and therefore conventionally required electrocardiodiagram control while the patient holds his breath is not possible during the measurement. Therefore visualizing of anomalies of the heart and large vessels by MRI (magnetic resonance imaging) is not achievable. 
     For examination of a fetal heart by means of MRT, for example Manganaro et al., Prenat. Diagn. 2008, 28, 148-156, and Fogel et al., Fetal Diagn. Ther. 2005, 20, 475-480, describe True fast imaging with steady-state precision (True FISP) and real-time cine-MR-sequences to be used, whereby in these cases the procedure is carried out without control (triggering). Nijm et al., J. Magn. Reson. Imaging 2008, 28, 767-772 use self-gating (SG) algorithms for synchronization. Here among other things the low signal-to-noise ratio is limiting. Yamamura et al., Eur. Radiol. 2009, 19, 2383-90, use an invasive trigger system (Pulse wave triggering). All these methods have severe limitations, which either render their practical use on a human being impossible, or there is no yield of images of sufficient quality, which allow for evaluating anatomic structures and functional information. 
     Michel et al., American Journal of Roentgenologie 2003, 180, 1159-1164, come to the conclusion that fetal CTG during magnetic resonance tomography is feasible with modified standard equipment. However, it is also stated that, due to technical reasons, CTG monitoring while the patient is in the magnet is not possible (rather, measurements were made immediately after leaving the magnet). Accordingly, the study of Michel et al. provides no MRT images of the fetal heart. 
     The problem of interference between the CTG device and the MRT has been an unresolved problem for years. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the invention, an ultrasonic sensor for a cardiotocography device (CTG device) therefore comprises at least an ultrasonic transducer, at least one resistor which is connected to the ultrasonic transducer, and a housing. The housing accommodates the ultrasonic transducer and the resistor. The ultrasonic sensor is made of materials which are nonferromagnetic, so that the ultrasonic sensor acts neutrally with respect to an external magnetic field. Specifically this means that the resistor consists of a non-ferromagnetic material and that the resistor is connected to the ultrasonic transducer through a non-ferromagnetic wire. 
     The ultrasonic transducer is connected to the resistor through twisted wires. More precisely, a first terminal of the ultrasonic transducer can be connected to a signal conductor of a CTG electronic system of the CTG device, and a second terminal of the ultrasonic transducer is connected through the resistor with a ground terminal of the CTG electronic system, the two wires being twisted together. 
     Even when the ultrasonic sensor receives signals and itself is located within the magnetic field of an MRT, there is almost no disturbance or influence on the magnetic field of the MRT, so that imaging through the MRT is possible almost without any artefacts and with the desired resolution. By way of example, the ultrasonic sensor may influence imaging in its direct proximity, without this influence being relevant to the area of interest (e.g., the fetal heart). Further, disturbances of the operation of the ultrasonic sensor by the magnetic field of the MRT and by the high-frequency impulses emitted by the MRT are avoided. Accordingly, even during ongoing measurements, the ultrasonic sensor can be operated within the imaging area of the MRT. In particular, it is possible to record Doppler-sonography signals in the MRT during ongoing MRT measurements. 
     In this embodiment, it may be achieved that the ultrasonic sensor is on the one hand neutral with respect to the external magnetic field, i.e., generates no disturbances in the magnetic field. On the other hand, the ultrasonic sensor is, by virtue of the connection via twisted wires, also shielded from disturbances by the by far stronger external magnetic field. This good shielding allows for a complete functionality of the ultrasonic sensor both in the static magnetic field and during the ongoing MRT measurements with irradiation of high-frequency pulses. 
     “Neutral” in this case may be considered to encompass that a possible disturbing field occurs only in a distance of up to 30 mm, measured perpendicularly with respect to the surface of for example the housing of the ultrasonic sensor. Preferably, a disturbing field occurs only up to a distance of 20 mm. Ideally, an external magnetic field is disturbed or influenced only in a range of distance between 0 and 12 mm. 
     According to an embodiment of the invention, the ultrasonic sensor has no circuit board connecting the ultrasonic transducer and the at least one resistor. In this way, an additional disturbing structure can be avoided. 
     According to an embodiment of the invention, the ultrasonic sensor has seven ultrasonic transducers and seven resistors, which are for example connected to each other through free wiring, preferably with twisted-pair wires. 
     The resistors as well as the wiring may consist of one or more non-ferromagnetic materials. In particular materials containing iron or nickel can be avoided in this way. 
     According to a further embodiment of the invention, the ultrasonic transducer, the resistor, as well as the twisted-wire wiring may be arranged on a non-ferromagnetic circuit board, which accordingly should be devoid of iron or nickel. Through such a circuit board, the automatable production may be facilitated. Additionally, a reliable arrangement of multiple elements can be ensured. 
     According to an embodiment of the invention, the ultrasonic transducer and the resistors in the housing of the ultrasonic sensor may be connected through a CTG cable with a CTG electronic system of the CTG device, which may be arranged remotely from the MRT. The CTG electronic system may be located a few meters from the MRT, for example in a separate room. According to an embodiment of the invention, the CTG cable between the ultrasonic sensor and the CTG electronic system can be longer than 5 m, for example 8 m. 
     In an embodiment of the invention, the CTG cable may be composed of a bipolar signal transmission core which has an inner shielding and an outer shielding, i.e., a double shielding. According to an embodiment, the inner shielding can be connected to a ground terminal of the CTG electronic system or a ground terminal of the CTG device. According to an embodiment, the outer shielding can be connected to a ground terminal of the MRT. 
     Further, the inner shielding may be connected to a housing shield which is located within the housing of the ultrasonic sensor. The housing shield can be a shield film of copper or a metallization of the inner surface of the housing. 
     As an additional shield the housing of the ultrasonic sensor may be completely metalized on the outside, e.g., treated with conductive silver. On the housing of the ultrasonic sensor, the additional shield or the conductive silver may be connected to the outer shielding of the CTG cable, whereby the outer shielding of the CTG cable in turn may be connected to the ground terminal of the MRT. 
     According to a further embodiment of the invention, at least one ferrite core, e.g. in the form of a ferrite ring, is arranged around the CTG cable. 
     According to a further embodiment of the invention, also wireless transmission can be provided. For example, by means of a transmitter/receiver unit, the ultrasonic sensor may perform wireless signal transmission between the ultrasonic sensor and the CTG electronic system. Further, it is also possible that the CTG electronic system is integrated in the housing of the ultrasonic sensor, and by means of a transmitter unit an output signal of the CTG electronic system, representing a heart rhythm, may be transmitted wirelessly to the MRT, which for example may use it as trigger signal for heart imaging. If the CTG electronic system is integrated with the ultrasonic sensor in the same housing, preferably also the CTG electronic system is realized with nonferromagnetic materials and uses a wiring with twisted-pair cables to avoid disturbances by dynamic electric and magnetic fields in operation of the MRT. Further, in this case the CTG electronic system may also comprise signal filters for suppressing frequencies of the high-frequency signals irradiated by the MRT. 
     Each of the above-described separate shielding measures provides for an improved compatibility of the ultrasonic sensor with the MRT. 
     According to a further embodiment of the invention, the ultrasonic sensor according to one or more of the above-mentioned embodiments may be used together with a CTG electronic system and an MRT, whereby the ultrasonic sensor is arranged within the magnetic field of the MRT and whereby the imaging of the MRT is controlled (triggered) by an output signal of the CTG electronic system (or a signal of the ultrasonic sensor). 
     According to a further embodiment of the invention, a system for imaging of a heart, in particular a fetal heart, is provided, which comprises an ultrasonic sensor, a CTG electronic system or a CTG device, and an MRT, whereby the CTG electronic system is adapted to provide, on the basis of oscillation detection by the ultrasonic transducer, a signal for controlling imaging by the MRT. 
     For this purpose, the CTG electronic system may repeatedly send special pulse chains (bursts) to the sensor, which irradiates these as ultrasonic sound. Then the respective echo of the bursts may be received and interpreted by the CTG electronic system, whereby the interpretation in this application may be based on runtime differences (Doppler effect), from which then the heart frequency can be calculated. 
     According to a further embodiment of the invention, the signal from the ultrasonic sensor to the CTG device or to an active CTG electronic system of the CTG device may also be wirelessly transmitted. For this purpose, the ultrasonic sensor may comprise a transmitter/receiver unit, which is arranged close to the ultrasonic sensor, but outside the MRT, so that wireless transmission between this transmitter/receiver unit and the CTG electronic system may be performed. 
     The above-described aspects and further aspects, features and advantages of embodiments of the invention may also be learned from the examples of embodiments, which in the following will be described with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an ultrasonic sensor according to an embodiment of the invention. 
         FIG. 2  is an enlarged illustration of detail X in  FIG. 1 . 
         FIG. 3  is a schematic illustration of a system with CTG and MRT according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a schematic illustration of an ultrasonic sensor  10  with a CTG cable  20  according to an embodiment of the invention. The ultrasonic sensor provides a trigger signal for improved imaging by an MRT. 
     The ultrasonic sensor  10  has a housing  17  in which an ultrasonic transducer  12  and resistors  14  are located. By way of example, in  FIG. 1  an ultrasonic transducer  12  and a resistor  14  are connected to each other through a twisted wiring  13  (detail X). For the sake of overview, it was refrained from depicting the twisted wirings between the other ultrasonic transducers and the corresponding resistors. 
     Further, a shield film  16  is provided within the housing  17 . In  FIG. 1 , the shield film  16  is only indicated schematically. It is noted that the shield film may be configured in such a way that it covers all ultrasonic transducers  12  and also the resistors  14  and the wirings  13  within the housing  17 . 
     The exterior of the housing  17  is completely metalized, for example treated with conductive silver  18 . Also in this case, the conductive silver  18  is only partially and schematically indicated in  FIG. 1 . It is noted that the conductive silver  18  may cover the complete outer surface of the housing  17 . 
     A CTG cable  20  extends from the housing  17  of the ultrasonic sensor  10 . The CTG cable  20  is composed of a core  22 , an inner shielding  24 , and an outer shielding  26 . Within the ultrasonic sensor, the inner core  22  is connected to the wiring  13  of the resistors  14  and the ultrasonic transducers  12 , so that the signal from the ultrasonic transducers can be conducted from the ultrasonic sensors to a CTG electronic system. The core of the CTG cable  20  may have a bipolar configuration. 
     On the side of the ultrasonic sensor, the inner shielding  24  extends into the housing  17  and, in the housing, is connected to the shield film  16 . The inner shielding  24  is provided over the entire length of the cable  20  and, at its other end, is connected to the ground terminal of the CTG electronic system. In this way, the shield film in the housing  17  of the ultrasonic sensor is connected to the ground terminal of the CTG electronic system. 
     The outer shielding  26  of the CTG cable  20  is not provided over the entire length of the CTG cable. For example, 1.5 m of the CTG cable starting from the ultrasonic sensor  10  may be formed with the outer shielding  26 . This additional outer shielding is connected to the ground terminal of the MRT and, on the ultrasonic sensor, connected to the conductive silver  18  on the outside of the housing  17 . 
       FIG. 2  shows detail X in  FIG. 1  in an enlarged illustration. Via twisted wiring  13 , the ultrasonic transducer  12  is connected on the one hand to the core  22  of the CTG cable as signal conductor and on the other hand via a resistor  14  to the inner shielding  24  of the CTG cable. The resistor  14  forms a part of a block which is composed of seven SMD-resistors, whereby these seven resistors have a common resistor contatct  23  to the shielding  24  and each a respective free contact. 
     One of the two wires  13  coming from the ultrasonic transducer  12  is connected to a free contact of a resistor  14 , and the other of the two wires is connected to the core  22 , whereby signal contact point  21  is formed in such a way that all ultrasonic transducers may be connected to this contact point. 
       FIG. 3  shows a schematic illustration of a system for imaging of a heart, in particular a fetal heart, according to an embodiment of the invention. The ultrasonic sensor  10  is connected via the CTG cable  20  with a CTG electronic system  30 . Also here in  FIG. 3 , it is indicated that the inner shielding  24  of the cable  20  is connected to the ground terminal  32  of the CTG electronic system  30 . Further, it is illustrated that the outer shielding  26  is connected to the ground terminal  72  of the MRT  70 . Further, it is illustrated that a ferrite core  28 , in the form of a ferrite ring, is arranged around the CTG cable  20 . 
     On the CTG electronic system  30 , an illumination field in the front plate may blink in the heart rhythm, as in a conventional CTG device. To utilize this blinking illumination field, an optocoupler  34  may be used, which generates an electronic signal, representing a heart rhythm, from the signal of the illumination field. Alternatively, it is possible to use a CTG device which provides an electric output signal representing a heart rhythm. Finally, the CTG electronic system  30  may also be integrated in the same housing with the ultrasonic sensor  10 , e.g. in the form of a handheld device. In this case, also a wireless transmission of the output signal of the CTG electronic system  30  may be transmitted to the MRT  70 , e.g. via radio signals, infrared signals or acoustic signals. 
     In the illustrated example, the electronic signal of the optocoupler  34  is forwarded via a cable  36  to an electronic circuit  40 , which converts the signal into a ECG-like, very low impedance signal. Via coaxial cable  50 , on which a further ferrite core  52  may be provided, this converted signal is forwarded to a further electronic circuit  60 , which accomplishes signal level adaptation. 
     The resulting conditioned signal may now be used by the MRT  70  as control signal (trigger signal) for heart imaging. In this way, MRT visualizations of the heart of a patient  80  may be performed, which are always recorded at the same point of time in a heart cycle, so that anatomic structures of the heart may be visualized at very high resolution. By means of the system according to the illustrated embodiment of the invention, this is in particular also possible for a heart of an unborn child in the mother&#39;s womb. It is noted that also the course of heart movement may be visualized, whereby for this purpose the control signal may determine, relative to the heart cycle, a progressing point of time for imaging. 
     It is to be understood that various modifications are possible in the illustrated embodiments. For example, the ultrasonic sensor and the CTG electronic system do not need to be provided as separate components, but may be integrated in the same housing, e.g., the housing  17  of the ultrasonic sensor  10  as illustrated in  FIG. 1 . For example, the ultrasonic sensor and the CTG electronic system may be combined in a handheld device or compact device. Also the required hardware and software for signal generation and conditioning could then be implemented in this compact device, which may be configured in an MRT compatible manner, similar to the ultrasonic sensor. The MRT compatible compact device may for example be placed on the patient above the object the be examined, and the measured signals may be wirelessly forwarded to the MRT. A special CTG cable, e.g. with ground shunt at the MRT device, may then be dispensed with. 
     Further, it is to be understood that the concepts as described herein offer advantages in a plurality of application fields. Examples of such application fields are:
         Generation of a trigger signal which represents the heart frequency of adults, children or fetuses in utero. This trigger signal may be used for heart and vessel imaging in the MRT. The triggered cardiovascular fetal MRT imaging, which is enabled in this way, delivers valuable information for the further therapeutic action in case of fetal malformations. The MRT allows for a precise anatomic visualization of the heart (including foramen ovale) and additionally functional conclusions, such as for example the ejection fraction. Thus, the degree of a cardiac malformation may be determined already in utero for planning subsequent surgical interventions. Apart from application in prenatal diagnostics, the system may also replace the ECG control in the examination of adults. It may then be utilized in a beneficial manner for examination of adults, if the conventional ECG electrodes generally cause additional efforts (shaving the chest) and problems (falling off of the self-adhesive electrodes) and are impossible to be applied in some cases (e.g. with patients having pleural effusions, pericardial effusions, adiposity).   Monitoring the heart frequency of fetuses, children, adults, and thus a vital function, during the MRT measurement: Beside the cardiac triggering, the MRT compatible CTG device described herein is also applicable for continuous monitoring of fetuses during MRT examination. This is of clinical relevance because often high-risk pregnancies are examined in MRT. Of course such monitoring may also be performed on children or adults.