Abstract:
A method and a magnetic resonance tomography (MRT) system are provided. The MRT system includes a controller configured to store a transmit vector that is established on a local-coil-specific basis. The transmit vector, for a specific local coil, indicates with which amplitudes and phases, transmit elements of the local coil may be controlled by a transmit device. The controller is configured to initiate a patient-specific calibration measurement on a patient to generate patient-specific calibration data representing a field distribution. The controller is also configured to determine deviations in the patient-specific calibration data from the stored transmit vector established on a local-coil-specific basis. The patient-specific calibration data is generated in the patient-specific calibration measurement on the patient and represents a field distribution. An imaging MRT measurement is not allowed if deviations exceed a threshold value, but is otherwise performed and is monitored by a monitoring device.

Description:
[0001]    This application claims the benefit of DE 10 2014 215 531.2, filed on Aug. 6, 2014, which is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    The present embodiments relate to a magnetic resonance tomography system and a method for the operation thereof. 
         [0003]    Magnetic resonance devices (MRTs) for examining objects or patients using magnetic resonance tomography are known, for example, from DE 103 14 215 B4. 
       SUMMARY 
       [0004]    The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary. 
         [0005]    The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, a magnetic resonance tomography system and a method for the operation thereof are optimized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows elements of one embodiment of a magnetic resonance tomography (MRT); 
           [0007]      FIG. 2  shows acts of one embodiment of a method; and 
           [0008]      FIG. 3  shows a schematic and simplified representation of one embodiment of a magnetic resonance tomography (MRT) system. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]      FIG. 3  shows one embodiment of an imaging magnetic resonance device MRT  101  (e.g., included in a shielded room or Faraday cage F) including a hollow cylinder  102  having, for example, a tubular bore  103  into which a patient couch  104  bearing a body  105  (e.g., of an examination object such as a patient; with or without local coil arrangement  106 ) may be introduced in the direction of the arrow z so that images of the patient  105  may be generated by an imaging method. Disposed on the patient, for example, is a local coil arrangement  106  that may be used in a local region (e.g., a field of view (FOV)) of the MRT  101  to generate images of a subregion of the body  105  in the FOV. Signals of the local coil arrangement  106  may be evaluated (e.g., converted into images, stored or displayed) by an evaluation device (e.g., including elements  168 ,  115 ,  117 ,  119 ,  120 ,  121 , etc.) of the MRT  101  that may be connected to the local coil arrangement  106  (e.g., via coaxial cable or wirelessly ( 167 ), etc.). 
         [0010]    When a magnetic resonance device MRT  101  is used in order to examine a body  105  (e.g., an examination object or a patient) by magnetic resonance imaging, different magnetic fields that are coordinated with one another with the utmost precision in terms of temporal and spatial characteristics are radiated onto the body  105 . A strong magnet (e.g., a cryomagnet  107 ) in a measurement chamber having a, for example, tunnel-shaped bore  103  generates a strong static main magnetic field B 0  ranging, for example, from 0.2 Tesla to 3 Tesla or more. A body  105  that is to be examined, supported on a patient couch  104 , is moved into a region of the main magnetic field B 0  that is approximately homogeneous in the area of observation (e.g., FOV). The nuclear spins of atomic nuclei of the body  105  are excited via magnetic radio-frequency excitation pulses B 1  (x, y, z, t) that are emitted via a radio-frequency antenna (and/or a local coil arrangement if necessary) that is shown in  FIG. 3  in greatly simplified form as a body coil  108  (e.g., multipart body coil  108   a ,  108   b ,  108   c ). Radio-frequency excitation pulses are generated, for example, by a pulse generation unit  109  that is controlled by a pulse sequence control unit  110 . Following amplification by a radio-frequency amplifier  111 , the pulses are directed to the radio-frequency antenna  108  and/or local coil  106 . The radio-frequency system shown is indicated only schematically. In other embodiments, more than one pulse generation unit  109 , more than one radio-frequency amplifier  111 , and a plurality of radio-frequency antennas  108   a, b, c  are used in a magnetic resonance device  101 . 
         [0011]    The magnetic resonance device  101  also includes gradient coils  112   x,    112   y ,  112   z,  by which magnetic gradient fields B G  (x, y, z, t) are radiated in the course of a measurement in order to provoke selective slice excitation and for position encoding of the measurement signal. The gradient coils  112   x,    112   y,    112   z  are controlled by a gradient coil control unit  114  (and if appropriate, via amplifiers Vx, Vy, Vz) that, like the pulse generation unit  109 , are connected to the pulse sequence control unit  110 . 
         [0012]    Signals emitted by the excited nuclear spins (e.g., of the atomic nuclei in the examination object) are received by the body coil  108   a, b, c  and/or at least one local coil arrangement  106 , are amplified by assigned radio-frequency preamplifiers  116 , and are further processed and digitized by a receiving unit  117 . The recorded measurement data is digitized and stored in the form of complex numeric values in a k-space matrix. Using a multidimensional Fourier transformation, an associated MR image may be reconstructed from the k-space matrix populated with values. 
         [0013]    For a coil that may be operated in both transmit and receive mode (e.g., the body coil  108  or a local coil  106 ), correct signal forwarding is regulated by an upstream duplexer  118 . 
         [0014]    From the measurement data, an image processing unit  119  generates an image that is displayed to a user via an operator console  120  and/or stored in a memory unit  121 . A central computer unit  122  controls the individual system components. 
         [0015]    In MR tomography, images having a high signal-to-noise ratio (SNR) may be acquired by local coil arrangements (e.g., coils, local coils). These are antenna systems that are mounted in direct proximity on (e.g., anterior) or below (e.g., posterior), on, or in the body  105 . In the course of an MR measurement, the excited nuclei induce a voltage in the individual antennas of the local coil. The induced voltage is amplified by a low-noise preamplifier (e.g., LNA, preamp) and forwarded to the receive electronics. High-field systems (e.g., 1.5 T-12 T or more) are used to improve the signal-to-noise ratio, even with high-resolution images. If more individual antennas may be connected to an MR receive system than there are receivers present, a switching matrix (e.g., referred to or implemented as RCCS), for example, is incorporated between receive antennas and receivers. This routes the currently active receive channels (e.g., the receive channels currently lying in the FOV of the magnet) to the receivers present. This enables more coil elements to be connected than there are receivers available, since in the case of whole-body coverage, only the coils that are located in the FOV or in the homogeneity volume of the magnet may be read out. 
         [0016]    The term local coil arrangement  106  serves generally to describe, for example, an antenna system that may include, for example, one antenna element or a plurality of antenna elements (e.g., coil elements) configured as an array coil. These individual antenna elements are embodied, for example, as loop antennas (e.g., loops), butterfly coils, flex coils or saddle coils. A local coil arrangement includes, for example, coil elements, a preamplifier, further electronics (e.g., standing wave traps, etc.), a housing, supports, and may include a cable with plug-type connector by which the local coil arrangement is connected to the MRT system. A receiver  168  mounted on the MRT system side filters and digitizes a signal received, for example, wirelessly, etc. by a local coil  106  and passes the data to a digital signal processing device that may derive an image or a spectrum from the data acquired by a measurement and makes the derived image or spectrum available to the user (e.g., for subsequent diagnosis by the user and/or for storage in a memory). 
         [0017]    For signal excitation in magnetic resonance (MR) scanners of the latest generation, multiple transmit channels and elements (e.g., in one or more local coils) are used simultaneously in accordance with known methods. This may also be the parallel transmit (pTX) method. Safe operation for the patient may only be provided if the specific absorption rate (SAR) dose is known in terms of temporal and spatial characteristics. 
         [0018]    A pTX system enables phase and amplitude of each transmit element to be freely set, so that the calculation of the SAR limits is a complex task. Depending on the phase length and amplitude, SAR hotspots may occur in the tissue. The SAR hotspots are influenced by the individual patient physiology (e.g., cysts or tumors). 
         [0019]    In the case of known single-channel transmit systems, a “K-factor” is determined based on a finite element (FE) simulation and validation measurements. This describes the SAR dose per time and amplitude unit (e.g., in the worst case, in the SAR hotspot of the local coil). Taking into consideration a safety margin (e.g., at least a factor of 2), the usable RF power is restricted by the k-factor to the legally prescribed limit (e.g., lookahead or online supervision). 
         [0020]    In pTX systems, an analog method, in which the safety margin is scaled with the number of the channels, is known. This takes account of the risk of a worst case overlay of the SAR hotspots of the transmit elements in the patient. A disadvantage is that the potential advantages of the pTX system may not be exploited because of the high safety margins. 
         [0021]    A further method (e.g., virtual observation points (VOP) method), in which the SAR limits are predicted by the use of a parameterizable patient model, is known. The model is determined by a series of FE-based simulation calculations. For this, the SAR hotspots in the patient tissue are calculated (e.g., several models with different size, sex, age, etc.) based on the electromagnetic fields that are generated by the transmit elements. One advantage is that the necessary safety margins may turn out to be significantly less compared to the K-factor approach (and do not increase linearly with the number of transmit elements). Since the influence of each transmit element may be represented independently, the SAR in the patient may be calculated with the model for each linear combination of transmit amplitudes and phases (e.g., prior to the measurement and also during the measurement by comparison with measured transmitted and reflected RF amplitudes and phases). 
         [0022]    One disadvantage may be the complexity of the method and the associated problems of providing evidence of safe clinical operation. 
         [0023]    Movements by the patient during the measurement, the patient&#39;s anatomical deviations from a precalculated standard model, and thermal effects in the TX/RX path may lead to a change in the actual SAR hotspots compared to the theoretically assumed hotspots. The greater the number of transmit elements, the stronger such effects may be. In the context of a potential clinical certification, these aspects and others may be analyzed and used for a safe solution. 
         [0024]    According to embodiments, a local-coil-specific restriction on the amplitude and phase space of the transmit elements (e.g., antennas TX 1 , TX 2 , TX 3 ) of at least one local coil  106  and/or body coil  108   a, b, c  is introduced for a clinical application (e.g., in the form of a transmit vector B 1 _H, which for a local coil  106 , indicates which amplitude/phase combinations the controller  110  of the local coil  106  may send). 
         [0025]      FIG. 1  shows in simplified fashion one embodiment of an MRT controller  110  that imposes transmit signals (e.g., with permissible amplitude/phase combinations) on a plurality of antennas TX 1 , TX 2 , TX 3  of a local coil (e.g., also usable for receipt) using an amplifier  111  (e.g., FRPA). The transmit signals and/or return signals transmitted to the antennas TX 1 , TX 2 , TX 3  are monitored by a monitoring device K_SUP. 
         [0026]    As  FIG. 2  makes clear, the transmit vector B 1 _H is determined for each local coil  106 , for example, initially based on an FE simulation (act S 1 ) and/or a calibration measurement (e.g., on a patient  105  to be examined in the MRT by MRT “measurement”) in act S 2 , and indicates permitted amplitude ratios and phases for the MRT imaging. 
         [0027]    A patient-specific calibration measurement is carried out (act S 6 ) on a patient  105  to be examined (e.g., prior to MRT imaging on the patient) using a conservatively calculated SAR dose. A wide variety of methods may be provided for determining the SAR dose on the patient  105  (e.g., microwave thermometry, consideration of phases and amplitudes of forward and return transmit power to/from antennas of the at least one local coil, etc.). 
         [0028]    From calibration data obtained with the calibration measurement S 6  (e.g., referred to as ‘B 1  map’ or with reference character ‘B 1 _Map’ in  FIG. 2 ), the transmit vector B 1 _H (e.g., for MRT imaging on the patient) may be calculated/optimized such that a B 1  excitation that is as homogeneous as possible may take place in the coil volume (e.g., volume within a local coil  106 ). In addition, the relative deviations of the transmit vector B 1 _H from the simulated reference value are limited in order to limit resulting overshoots of the SAR (e.g., anatomical deviations may potentially be detectable at this point in the method). The result of these calculations is the vector space V_B 1 _H. 
         [0029]    For example, for magnetic field strengths &gt;3T, such a field homogenization in the imaging volume is an important prerequisite for a clinically acceptable operation of the MR scanner  101 . 
         [0030]    Because of the overall homogeneous excitation in the imaging volume (e.g., FOV), the calculation of the SAR dose is simplified. Essentially, the classic K-factor approach may be used. For example, the SAR hotspots, calculated in act S 3 , are calculated based on an FE model of the local coil  106 , with a patient model, in the homogeneous excitation mode (‘CP mode’). Then, if necessary, SAR hotspots for B 1 _H are calculated for possible deviations (act S 4  in  FIG. 2 ) such as a different possible position of the patient  105  in the MRT  101  and/or for a different possible head size of a patient  105 , and a K-factor determined in this way is passed (S 5 ) to an SAR monitoring device S 5 . 
         [0031]    Prior to the start of an imaging measurement (e.g., S 10 ; MRT imaging on the patient), the monitoring vector V_B 1 _H (e.g., may indicate in which range f 01 &lt;f&lt;f 11 , f 02 &lt;f&lt;f 1 &lt;f 12  angles f and in which ranges |A 1 /A 2 |&lt;a 1 , |A 2 /A 3 |&lt;a 2  amplitudes A 1 , A 2 , A 3  and/or amplitude ratios a 1 , a 2  are contained) is passed to the non-measurement-system-dependent monitoring component K_SUP (shown in simplified form in  FIG. 1 ), as indicated in  FIG. 2  in act S 9 . 
         [0032]    The monitoring device K_SUP includes, for example, measuring devices for measuring transmitted and reflected RF power of all transmit elements TX 1 , TX 2 , TX 3 , etc. with complete phase accuracy. In an embodiment, the monitoring device K_SUP may be part of an MR receiver in a controller  117  and/or in a local coil  106 . In a further embodiment, the monitoring device K_SUP may be part of an autonomous measuring unit with directional couplers and the necessary evaluation circuit or software. 
         [0033]    During, for example, a calibration measurement (S 6 ) and/or a total imaging measurement SMR (e.g., examination of a patient with a magnetic resonance tomography device), a monitoring device K_SUP (or each monitoring device if there are several such devices) checks (e.g., in accordance with reference character S 7  in  FIG. 2 ) at suitable time intervals (e.g., 10 μs) whether the measured amplitude ratios (e.g., |A 1 /A 2 |, |A 2 /A 3 | etc.; transmitted) and phases (e.g., f 01 &lt;f&lt;f 11 , f 02 &lt;f&lt;f 1 &lt;&lt;f 12 ) of the transmit elements TX 1 , TX 2 , TX 3  lie in the permitted range (e.g., f 01 &lt;f&lt;f 11 , f 02 &lt;f&lt;f 1 &lt;f 12 , |A 1 /A 2 |&lt;a 1 , |A 2 /A 3 |&lt;a 2  etc.). In the event of an error, designated by step S 8  in  FIG. 2  (e.g., because the patient moves a lot or because of erroneous or impermissible RF pulses), the monitoring unit K_SUP sends an interruption signal to the MR controller  117 / 110 . As an additional error criterion, the reflected power may optionally be used on a connection line AL 1 , AL 2 , AL 3  of an antenna TX 1 , TX 2 , TX 3 . 
         [0034]    The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification. 
         [0035]    While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.