Abstract:
A multi-channel switching system ( 100 ) for an MRI gradient coil system is characterized in that the number of channels controlled by the power amplifiers is smaller than the number of switches and the number of channels controlled by the power amplifiers is smaller than the number of coil elements in the coil system. Current in each of the coil elements can be switched to flow in either a positive or negative direction or to bypass the respective coil element and power to the switch elements is delivered via a smaller amount of power lines using a power distribution system providing floating power to each of the switches. This allows to electrically connect matrix coil elements dynamically within a pulse sequence to generate dynamically switched magnetic field profiles and therefore reduce the number of gradient power amplifiers, gradient cables and power supplies needed.

Description:
[0001]    This application claims Paris convention priority from EP 14 155 823.9 filed Feb. 19, 2015, the entire disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a multi-channel switching system for a multi-channel gradient coil system for MRI (=Magnetic Resonance Imaging), comprising: a plurality of N switch  analog switches to connect a plurality of N element  coil elements, whereby said plurality of analog switches and coil elements forms a plurality of N channel  electrical channels each driven by a gradient power amplifier; a distribution board to generate control signals for each of the plurality of analog switches; a digital controller providing the command code to the distribution board through a communication bus; and a power delivery system to power each of N switch  analog switches. 
         [0003]    A multi-channel switching system of this type for shimming is known e.g. from Harris et al., “A new approach to shimming: the dynamically controlled adaptive current network”, MRM 71: 859-869, 2014. Such systems, however, require individual power supplies for each switch, which limits the scalability. 
         [0004]    The present invention relates generally to magnetic resonance imaging (MRI). It specifically relates to shimming and spatial encoding hardware for MRI. 
         [0005]    Magnetic resonance imaging (MRI) is a relative new technology compared with computed tomography (CT) and the first MR Image was published in 1973 by P. C. Lauterbur in “Image Formation by Induced Local Interactions: Examples of Employing Nuclear Magnetic Resonance”, Nature 242, 190491. It is primarily a medical imaging technique which most commonly used in radiology to visualize the structure and function of the body. It could provide detailed Images of the body in any plane. MRI provides much greater contrast between the different soft tissues of the body than CT does, making it especially useful in neurological, cardiovascular, and oncological imaging. It uses a powerful magnetic field to align the nuclear magnetization of hydrogen atoms in water in the body. Radio frequency fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body. 
         [0006]    An MRI system typically establishes a homogenous magnetic field, generally along a central axis of a subject undergoing an MRI procedure. This homogenous main magnetic field affects the magnetic properties of the subject to be imaged by aligning the nuclear spins, in atoms and molecules forming the body tissue. If the orientation of the nuclear spins is perturbed out of alignment, the nuclei attempt to realign their spins with the field. Perturbation of the orientation of the nuclear spins is typically caused by application of radio frequency (RF) pulses tuned to the Larmor frequency of the material of interest. During the realignment process, the nuclei precess about the direction of the main magnetic field and emit electromagnetic signals that may be detected by one or more RF detector coils placed on or around the subject. 
         [0007]    Magnetic resonance imaging employs temporally and spatially variable magnetic fields to encode position by affecting the local Larmor frequency of spins. Gradient coils typically used for that purpose generate spatial encoding magnetic fields (=SEMs) which are superimposed on the main magnetic field. This allows to choose the localization of the image slices and also to provide phase encoding and frequency encoding. This encoding permits identification of the origin of resonance signals during image reconstruction. The image quality and resolution depends significantly on the strength and how the applied encoding fields can be controlled. Control of the gradient coils is generally performed in accordance with pre-established protocols or sequences at events, called pulse sequences, permitting different types of contrast mechanisms to be imaged. 
         [0008]    Gradient coils are typically designed to generate spatial encoding magnetic fields in a linear fashion, i.e. constant linear gradient fields, along the three orthogonal directions X, Y and Z. Typically, a gradient coil operates with maximum currents of about few hundreds amperes and at maximum voltages in a range from about few hundreds volts to about few thousands volts. To achieve higher resolution of an image, stronger and faster gradient fields are needed. Therefore gradient coils need higher currents and voltages. However, this induces safety concerns due to peripheral nerve stimulation (=PNS) and increases the complexity and cost of the current sources which are referred to as gradient power amplifiers. Another drawback of linear gradients is the missing flexibility in terms of realizable field shapes. 
         [0009]    To overcome these limitations non-linearly varying SEMs have been introduced by EP 1 780 556 B1. Such gradient fields may overcome PNS limits and allow for parallel non-bijective image encoding or curved slice imaging (see e.g. EP 2 511 725 A1). For faster image acquisition more encoding fields than spatial dimensions (X, Y and Z) have been used (for example 4D-Rio, O-Space, see e.g. US 2012/0 286 783 A1). Multi element coil systems have been used to generate the SEMs needed for those applications. However, such systems still have a limited set of field shapes and need a dedicated gradient power amplifier per coil element. 
         [0010]    Susceptibility differences in the object to be imaged introduce field perturbations of the homogeneous main magnetic field. Compensation of these perturbations is commonly referred to as shimming. Shim coils used for these corrections usually aim to generate shim fields that are based on spherical volume harmonics. However, complex susceptibility differences present in the human body introduce perturbations that cannot be fully corrected for by spherical volume harmonics of lower orders. To overcome these limitations a set of multiple self-similar individual coil elements, each supplied by a separate current source has been introduced (by Christoph Juchem et al in “Magnetic field homogenization of the human prefrontal cortex with a set of localized electrical coils”, MRM 63:171-180, 2010). Although this approach gains flexibility in terms of realizable field shapes used for shimming, the number of individual current sources scales linearly with the number of coils. This increases cost and space needed. To significantly reduce the number of amplifiers while allowing to route currents along pre calculated paths a Dynamically Controlled Adaptive Current Network has been introduced (see Chad. T. Harris et al. in “A new approach to shimming: the dynamically controlled adaptive current network”, MRM 71: 859-869, 2014). 
         [0011]    The use of a multiple of individual coil elements to generate SEMs has been introduced for example in WO 2009/124873 A1. This allows for enhanced flexibility in terms of realizable field profiles of the SEMs. However, as with the set of localized electrical coils used for shimming the number of GPAs and the corresponding connection cables equals the number of individual coil elements. 
         [0012]    Switches to route currents in gradient setups have been presented in U.S. Pat. No. 6,157,280. However, the proposed switching method is either too slow to be changed within a pulse sequence (fluid actuated switch) or does not teach us on how the scaling constraint of requiring a separate power supply for each switch can be overcome. Furthermore, this patent did not describe a way of generating different field shapes with the same coil set up. 
         [0013]    The present invention presents a way to substantially overcome one or more disadvantages of the existing arrangements. 
         [0014]    An object of the present invention therefore is to provide a multi-channel switching system with the features defined initially, which allows to electrically connect matrix coil elements dynamically within a pulse sequence to generate dynamically switched magnetic field profiles, and therefore reduce the number of gradient power amplifiers, gradient cables and power supplies needed. 
       SUMMARY OF THE INVENTION 
       [0015]    This object is achieved by modifying such multi-channel switching system in that the number N channel  of channels controlled by the gradient power amplifiers is smaller than the number N switch  of analog switches N channel &lt;N switch , whereby the said switches ( 106 ) are connected in series, in parallel or in a bridge configuration, that the number N channel  of channels controlled by the gradient power amplifiers is smaller than the number N element  of coil elements in the multi-channel gradient coil system N channel &lt;N element , whereby current in every of the said coil elements can be switched to flow in either positive or negative direction or to bypass the respective coil element, and that the power to the plurality of N switch  elements is delivered via a smaller amount of N power  power lines, such that N power &lt;N switch  by means of a power distribution system providing floating power to each of the said switches. 
         [0016]    The present invention proposes that a multi-channel switching system for a multi-channel gradient coil system for Magnetic Resonance Imaging (MRI) and includes a plurality of analog switches, a plurality of individual coil elements, a digital controller and a distribution board. The proposed system is used to configure different combinations of gradient coil elements during pulse sequences for magnetic resonance imaging and shimming applications. A common power delivery system provides power to each analog switch. 
         [0017]    The switch system according to the present invention is particularly advantageous in a combination with a matrix gradient or shim coil. A plurality of coil elements forms a matrix coil wherein one or several sets of coil elements have the property of self-similarity. A plurality of analog switches allows connecting coil elements into channels which are controlled by a lower number of gradient power amplifiers (=GPAs). Aforementioned coil elements can be configured such that current flows in either positive or negative direction or can bypass selected elements. This allows for the generation of customized SEMs and for shim fields for MRI. 
         [0018]    The distribution board and analog switches are placed inside or close to the multi-channel gradient coil such that they are exposed to the main magnetic field thus reducing the number and length of connecting wires. Therefore a multiple of coil elements can be driven by a single amplifier and the total number of gradient amplifiers as well as feed through filters into the RF-shielded room can be made much smaller then the number of coil elements. The analog switches can handle high currents and can be switched at any time following a digital command. 
         [0019]    In preferred embodiments of the present invention, coil setting for individual channel combinations are sent via a digital interface from a computer to the digital controller ahead of experiment. The digital controller loads these parameters onto the distribution board via a digital bus system, preferably using an optical link. After download, the distribution board applies the code settings to the individual analog switches. The communication interface between digital controller and distribution board consists of data and clock connection. The number of analog switches depends on the number of coil elements and the required switching scheme. 
         [0020]    In a further embodiment of the invention, the distribution board directly produces control signals to control the metal-oxide-semiconductor field-effect transistors (MOSFETs) or insulated-gate bipolar transistors (IGBTs) or integrated gate-commutated thyristors (IGCTs) following the coil setting command. 
         [0021]    In a further embodiment of the invention, a bootstrap based driver circuit is used to drive a MOSFET or IGBT or IGCT. A timer circuit in combination with a negative voltage referenced current regulator charges the bootstrap capacitor periodically and generates the power for the floating driver. 
         [0022]    In another embodiment of the invention, a bipolar current regulator is used which has the advantages that they are compatible with bipolar GPAs and no current is injected into the driven SEM or shim system. 
         [0023]    In a further embodiment of the invention, the power supplying the electrical switching circuit is provided using an optical power delivery system comprising one or multiple light sources, a fiber optical transmission, a distribution system and a plurality of light to electricity transducers. 
         [0024]    In a preferred embodiment, the multi-channel switching system according to the present invention is designed to operate with a matrix coil comprising of one or several sets of self-similar coil elements. Therefore different magnetic field profiles can be generated by configuring the combination of the coil matrix. The matrix coil configured this way can be controlled by a low number of GPAs. 
         [0025]    In a further preferred embodiment, each of the plurality of analog switches is positioned in the immediate vicinity of the multi-channel gradient coil system and is exposed to the main magnetic field of an MRI system. Placing switches close to or into the main magnetic field allows for avoiding extra cables to connect or bypass each coil element. The length of connection wires between the respective element and the switching system can be reduced, which diminishes power losses and allows for faster switching due to lower inductively. 
         [0026]    It is further advantageous to position the distribution board in the immediate vicinity of the multi-channel gradient coil system and let it be exposed to the main magnetic field of an MRI system. The length of the connections to control each analog coil switch is reduced compared to placing the distribution board further away. If this board is placed outside the magnet room one feed through filter per switch has to be installed. In this way, the length of connecting wires of the control signals is reduced to improve the reliability of the system and to simplify RF shielding. 
         [0027]    In another embodiment of the invention, each of the plurality of analog switches is capable to dynamically control the combination of the matrix gradient coil elements during MR pulse sequences, in particular on a time scale of the lifetime of the MR signal, that is 1 to 100 ms. The dynamic re-configuration of the encoding fields allows for more flexible encoding strategies. Changing the configuration and therefore switching in between a pulse sequence allows for more realizable field shapes and for more freedom, e.g. for dynamic shimming (i.e. changing the shim field during a pulse sequence). 
         [0028]    Above said embodiment of the invention can be further improved, when a plurality of analog switches is capable to change its state whenever no current is flowing through the respective switch. This allows to reduce switching losses in the analog switches and to guarantee that the current waveform provided by the GPA will not be distorted by the analog switches. In some cases it might be even impossible to realize the change of the state of the switches while high currents are present. 
         [0029]    In another preferred embodiment of the invention, each of the plurality of analog switches is provided with sufficient power using the power delivery system for the period of an arbitrary MR pulse sequence. This is very essential for practical imaging applications and allows for continuous measurements without timing limits. Without such a system, there are hard timing limits for a pulse sequence. 
         [0030]    Also preferred is an embodiment of the invention, wherein each of the plurality of analog switches is powered by one or a plurality of power supplies using a periodic rechargeable bootstrap circuit providing floating power for each of the plurality of analog switches. The advantage of the bootstrapping circuit is that it provides a continuous floating power to the switches, while maintaining a sufficient isolation between them. This is a practical solution for the above mentioned timing limits. The circuit to generate floating power allows for reducing the minimum number of required power supplies to at least one. Without this feature, one power supply is required for each of the analog switches. 
         [0031]    In a further embodiment of the invention, each of the plurality of analog switches is powered by an optical power delivery system comprising a light source and a plurality of light-to-electricity convertors to provide floating power for each of the plurality of analog switches. Such a setup allows for a power delivery with a reduced risk of RF interference and improved isolation between the switches. This is another solution to supply floating power. The possibility for RF-interferences is reduced as well, if less conductive paths get into the magnet room. 
         [0032]    Further preferred is an embodiment of the multi-channel switching system according to the present invention which is designed suitable for spatial encoding. Spatial encoding using such a system is possible if high currents can be switched. 
         [0033]    This allows for novel nonlinear encoding strategies, e.g. extensions of PatLoc encoding. It is to be noted, that imaging applications require operation modes with high currents and voltages applied to the multi-channel gradient coil system, which puts stringent requirements on the isolation between the switches and their power supplies. 
         [0034]    Another preferred embodiment of the invention is the multi channel system being designed suitable for magnetic field shimming. Better accuracy of shim fields can be achieved compared to high order spherical harmonic shimming (typically 2nd or incomplete 3rd order on human-size systems), while reducing the number of shim amplifiers needed. This further has a potential for a combination with the encoding system making dedicated shim coils redundant. 
         [0035]    In yet another preferred embodiment of the invention, the switch is realized by either metal-oxide-semiconductor MOSFETs (=field-effect transistors or IGBTs (=insulated-gate bipolar transistors) or IGCTs (=integrated gate-commutated thyristors). Such devices allow higher switching speed then mechanical or fluid or pneumatically actuated switches known previously. 
         [0036]    It can be further advantageous if the entire system for imaging or shimming is connected to a single power amplifier. This reduces cost and space needed for additional amplifiers and simplifies the system architecture. 
         [0037]    These, as well as other objects and advantages of this invention can be better understood and appreciated through careful study of the following detailed description of presently preferred exemplary embodiments of this invention in conjunction with the accompanying drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0038]      FIG. 1  shows a block diagram of a multi-channel switching system, in accordance with aspects of the presented technique; 
           [0039]      FIG. 2  shows a block diagram of an exemplary distribution board of the multi-channel switching system of  FIG. 1 , in accordance with aspects of the presented technique; 
           [0040]      FIG. 3  shows a diagrammatical illustration of an exemplary analog switch of the multi-channel switching system of  FIG. 1 , in accordance with aspects of the presented technique; 
           [0041]      FIG. 4  shows a diagrammatical illustration of an exemplary analog switch element of  FIG. 3 , in accordance with aspects of the presented technique; 
           [0042]      FIG. 5  shows a diagrammatical illustration of an alternative exemplary analog switch element of  FIG. 3 , in accordance with aspects of the presented technique; 
           [0043]      FIG. 6  shows a diagrammatical illustration of analog switches of  FIG. 3  interconnects with matrix coil elements are in accordance with aspects of the presented technique; and 
           [0044]      FIG. 7  shows a block diagram illustrating a gradient system of a magnetic resonance imaging system employing the multi-channel switching system of  FIG. 1 , in accordance with aspects of the presented technique. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0045]    A multi-channel switching system for a multi-channel gradient coil system for Magnetic Resonance Imaging (MRI) is proposed. An embodiment of a switching system includes a plurality of analog switches, distribution board, digital controller and a power delivery system for each switch. The plurality of analog switches is placed in the immediate vicinity of coil elements inside or close to the magnet bore. The distribution board is also located inside or close to the magnet bore. The switching system is used to control a combination of matrix coil elements, such that many coil elements can be driven by a lower number of amplifiers than coil elements and the total number of gradient amplifiers required driving a matrix gradient coil can be reduced. 
         [0046]    Referring to  FIG. 1 , a block illustration of an exemplary multi-channel switching system  100  for use in a matrix gradient coil system for MRI, in accordance with aspects of the presented technique, is depicted. The multi-channel switching system  100  is illustrated, including a digital controller  102 , a distribution board  104 , and a plurality of analog switches  106 . The function of each component will be described in greater detail with reference to  FIGS. 1-6 . The digital controller  102  is used to define a combination of matrix coil elements. The distribution board  104  is used to provide control signals to a plurality of analog switches  106 . The plurality of analog switches  106  is used to configure the combination of matrix gradient coil elements and deliver high current gradient waveforms to matrix gradient coil elements, following a command of digital controller  102 . 
         [0047]    In the embodiment illustrated in  FIG. 1 , the digital controller  102  receives data via communication bus  110 . The data through communication bus  110  may be received from an external source, such as a computer (not shown in  FIG. 1 ). Data received from communication bus  110  are employed to generate the gated clock  112  and the serial data stream  114  to distribution board  104 . Thereafter, the gated clock  112  and serial data  114  are used to send the command signal to distribution board  104  and control the work modes of each of the plurality of analog switches  106 . 
         [0048]      FIG. 2  is a block diagram of one embodiment  200  of the exemplary distribution board. The distribution board  200  is implemented by a complex programmable logic device (CPLD) or by a field programmable gate array (FPGA). The distribution board  200  includes decoder  202  and command generator  204 . Decoder  202  receives the clock  210  and serial data  212  from digital controller  102 , decodes this data and generates command data  214 . Command generator  204  generates control signal  216  to control the plurality of analog switch  106 . Two optical fiber cables may be used to avoid noise interference to transmit clock  210  and serial data  212 . Clock  210  may be activated only at the communication period to minimize the induced radio frequency noise during the MR signal acquisition periods. 
         [0049]      FIG. 3  is a schematic representation of one embodiment  300  of one analog switch  106  in  FIG. 1  connected to a matrix coil element  310 . In the presently contemplated configuration, the embodiment  302  of analog switch  106  as a bridged switch includes four switches S 1  ( 312 ), S 2  ( 314 ), S 3  ( 316 ) and S 4  ( 318 ), where the bridges are connected in series. The analog switch  302  may work in three different modes. One mode called positive current mode is used to control switches S 2  ( 314 ) and S 4  ( 318 ) in on-state and switches S 1  ( 312 ) and S 3  ( 316 ) in off-state; one mode called negative current mode is used to control switches S 3  ( 316 ) and S 1  ( 312 ) in on-state, and switches S 2  ( 314 ) and S 4  ( 318 ) in off-state; the last mode called bypass mode may be used to control switches S 3  ( 316 ) and S 4  ( 318 ) in on-state and switches S 1  ( 312 ) and S 2  ( 314 ) in off-state, or to control switches S 2  ( 314 ) and S 1  ( 312 ) in on-state and switches S 3  ( 316 ) and S 4  ( 318 ) in off-state. There are three different corresponding modes to control current waveforms through the coil element  310 . 
         [0050]    Referring to  FIG. 4 , a diagrammatical representation  400  of one embodiment of one analog switch element of the analog switch  106  of  FIG. 1  is depicted. In a presently contemplated configuration, the analog switch element  400  includes a current regulator  402 , a timer  404 , a high side driver  406 , a bootstrap diode  412 , a current limiting resistor  414 , a bootstrap capacitor  416 , a current limiting resistor  418 , a Zener diode  420  and two power MOSFETs  422 . The high side driver  406  receives control signal  410  at the input terminal to control the “on” and “off” states of two MOSFETs  422 . A bootstrap circuit is used to supply the current as a substitute for an isolated supply. The bootstrap circuit consists of bootstrap diode  412 , bootstrap capacitor  416  and current limiting resistor  414 . The bootstrap capacitor  416  supplies the gate charge when two MOSFETs  422  are turned on. Since a capacitor is used as substitute for an isolated power supply, its supply capability is limited. For the given component parameter a maximal duration of the reliable supply Tmax can be calculated and measured. The timer  404  in combination with the current regulator  402  charges the bootstrap capacitor  416  periodically with a period Tp&lt;Tmax and provides the power supply for the output of high side driver  406 . Therefore the analog switch element  400  may be accurately controlled during an indefinite period of time. The current regulator  402  may provide bipolar current. A current limiting resistor  418  may be used between high side driver  406  and two MOSFETs  422 . A Zener diode  420  may be used to protect the two MOSFETs  422  under overvoltage condition. It is also possible to use the optical power delivery system to replace the bootstrap circuit. 
         [0051]    The detail is shown in  FIG. 5 . The laser transmitter  540  transmits high power light into the optical fiber  504 , then photovoltaic power receiver  502  converts the laser light into electrical power to supply the floating power. 
         [0052]      FIG. 6  is a diagrammatical representation  600  of one exemplary embodiment of a plurality of analog switches (shown in  FIG. 3 ) which interconnects matrix coil elements.  FIG. 6  shows how to connect in serial and in parallel between nine (3*3) coil elements and nine (3*3) analog switches  300  in bridge configuration (see  FIG. 3 ) and eight (2*4) additional single switches. This embodiment is particularly flexible and may achieve many different current flow patterns through the matrix coil. 
         [0053]      FIG. 7  is a block diagram  700  illustrating a gradient system for MRI that includes the exemplary multi-channel switching system  100  (see  FIG. 1 ), in accordance with aspects of the present technique. The gradient system for MRI  700  is illustrated diagrammatically, including digital controller  720 , gradient amplifiers  718 , filters  716 , matrix gradient coils  712 , multi-channel switches  714  and Computer  702 . MRI bore magnet  710  is placed in a separate RF shielded room  704 . The matrix coils  712  are positioned in the center of magnet bore. The multi-channel switches  714  are placed in the immediate vicinity of the matrix coil elements inside or close to the magnet bore. The corresponding MR-scanner may be of any suitable type of field strength, including scanners varying from 0.5 Tesla to 7 Tesla and beyond. 
         [0054]    The matrix coils  712  include conductive wires, bars or plates that are wound or cut to form a coil structure that generates a gradient filed superimposed over the primary magnetic field. The matrix coils  712  in combination with multi-channel switches  714  may form many different configurations to generate desired non-linear or linear fields for imaging or shimming. The configuration of matrix coils  712  may be dynamically adapted during pulse sequences. The matrix coils  712  are driven by gradient power amplifiers  718  to obtain precisely controlled magnetic field amplitudes. The field strength may vary over a predefined field of view, typically with positive and negative polarity. RF filters  716  are located between gradient amplifiers  718  and matrix coils  712  to eliminate interference noise from outside the shielded room  704 . The matrix coils  712  are controlled by digital controller  720  to generate the desired magnetic field. The predefined current waveforms and all command information are downloaded from PC  702 . As one embodiment shown, the command of multi-channel switches  714  is supplied by the digital controller  720 . The command signal includes clock  724  and serial data  722 . The communication may be implemented by optical fiber connections to eliminate interference noise or safety concerns due to increased voltages. 
         [0055]    The methods and systems described hereinabove aid in dynamically generating the desired magnetic field in a gradient system. The dynamically adapted magnetic field is implemented by multi-channel switches to control different configuration of matrix coil elements during pulse sequences. Therefore, many coil elements are driven by a single gradient amplifier. The total number of gradient amplifiers can be dramatically reduced for matrix coil configuration. 
         [0056]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.