Patent Publication Number: US-10788552-B2

Title: Method and control unit for operating a gradient coil device of an MRI system or another periodically stressed device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national phase application of International Application No. PCT/EP2017/069100, filed on Jul. 28, 2016, which claims the benefit of EP Application Serial No. 16181603.8 filed on Jul. 28, 2016 and is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a method for operating a gradient coil device of a magnetic resonance imaging system 
     The invention further relates to a corresponding control unit for operating a gradient coil device of a magnetic resonance imaging system and a corresponding magnetic resonance imaging system comprising a gradient coil device and a control unit. 
     BACKGROUND OF THE INVENTION 
     Document US 2015/0369888 A1 describes a method for operating a gradient coil device of a magnetic resonance imaging system including a modelling of vibrations of the gradient coil device with the aim to reduce acoustic noise during operation of said device. 
     Gradient coil devices of MRI systems (MRI: Magnetic Resonance Imaging) always have mechanical resonance frequencies in the MR relevant frequency bands. The corresponding mode shapes are excited by the gradient coils and the response is amplified. This can lead to failure by mechanical fatigue. This is known and mechanisms to avoid this are circumventing or reducing the excitation amplitude of the specific resonance frequencies of the MR scans. Further, an approach predict failure of gradient amplifies for magnetic resonance examination system is known from the international application WO2015/101556. This known approach relies on extracting fingerprints that are indicative for future failure by way of a neural network analysis and applies to gradient amplifier performance. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide (a) a method for operating a gradient coil device or other periodically stressed device, (b) a corresponding control unit (c) a corresponding MRI system for operating this device as well as (d) a corresponding computer program product to overcome the aforementioned challenges. 
     This object is achieved by the features of the independent claims. The dependent claims detail advantageous embodiments of the invention. 
     According to various embodiments of the invention the operating method comprises the following steps: (a) providing at least one first parameter of the device and/or at least one second parameter of the device; (b) performing damage calculation of an operation of said device by use of a mathematical model, which model is based on stress-cycle curves (S-N curves, also known as a Wöhler curves) or modified stress-cycle curves and uses the at least one first parameter and/or the at least one second parameter; and (c) determining second parameters for further operation of said device. In general, the gradient coil device can be a single gradient coil or a set of (x-, y-, z-) gradient coils as used in nearly all MR systems. 
     Material fatigue/mechanical fatigues are well characterised by the Wöhler curves or S-N curves. The S-N diagrams plot nominal stress amplitude S versus cycles to failure N. Part of the S-N curves show an approximate linear relation on log-log scale with slope −1/b, the Basquin relation (see for example document ‘Fatigue of Structures and Materials’ by Jaap Schijve, 2 nd  Edition 2009, Springer). The parameter b is material dependent. For example for copper fatigue b=6, but for general mechanical fatigue it is assumed here that b=4. In that case the allowable stress S in the power 4 (S 4 ) times the number of positive zero crossings N is constant: S 4 ·N=constant. This means that the damage potential of a stress (or velocity) at double amplitude is sixteen times as severe in contributing to mechanical fatigue. 
     Knowledge of material fatigue/mechanical fatigue and the dynamics of the device is built into the model to predict the cumulative damage potential of an operation period of said device. The model can then be used to predict the damage potential of that operation period for the device or it can be used to monitor cumulative damage build-up in such devices. This information can, e.g., be used to optimize MR scan protocols and to predict the necessity of predictive maintenance for gradient coil devices. Notably, known approaches of failure prediction do not assess metal fatigue. 
     The model uses at least one measured or calculated first parameter of the device based on velocities in the power b (corresponding to the Wöhler curve). According to the document ‘F. V. Hunt: “Stress and Strain Limits on the Attainable Velocity in Mechanical Systems”, JASA, 32(9) 1123-1128, 1960.’ velocity is the property that scales best with mechanical stress as function of frequency. The model can also use a different power than the power 4. For example using a power 2 makes the filters less sharp. 
     The model also uses at least one second parameter to count the number of positive zero-crossings. This information can be used to optimize MR scan protocols and to predict the necessity of predictive maintenance. 
     According to a preferred embodiment of the invention the at least one first parameter of the device is a transfer function/a plurality of transfer functions of the device preferably based on velocities in the power b. 
     According to another preferred embodiment of the invention the at least one second parameter of the device is the effective frequency of a scan protocol (taken from FFTs of the different gradient wave forms) and/or the total scan duration to count the number of positive zero-crossings. This information can be used to optimise MR scan protocols and to predict the necessity of predictive maintenance. 
     According to yet another preferred embodiment of the invention the damage calculation includes calculating at least one damage factor D. The damage calculation uses S-N curves (or modified S-N curves) to calculate the damage factor (or damage values). 
     The damage factor D based on the S-N curve preferably is given by the eq.: 
               D   =       ∑     x   ,   y   ,   z               ⁢       t   scan     ·     f     eff   ,   x   ,   y   ,   z       ·       (       G     x   ,   y   ,   z       ·     [     V   /     G     x   ,   y   ,   z         ]       )     b           ,         
wherein t scan  is the scan time of the protocol and f eff,x,y,z  (short form f eff ) is the ‘effective frequency’ of the x, y and z gradient spectra of the scan protocol, G x,y,z  are the frequency spectra of the x, y and z gradients of the scan protocol, and [V/G x,y,z ] are measured transfer functions as function of frequency of the gradient axes to the gradient coil velocity and −1/b is the slope of the straight approximation of the S-N curve on log-log scale. In connection with these embodiments of the invention the factor b preferably is in the rage of 2≤b≤6. For some particular embodiments of the invention the factor b is equal 4 (b=4).
 
     The corresponding damage Factor D of the (gradient coil) device can be calculated before an MR scan protocol is started, or it can be used to analyse scans that have already been executed in the past using log file information for example. 
     According to yet another preferred embodiment of the invention the method further comprises the step of (d) considering the determined parameters for further operation within the further operation of said device. The consideration of the parameters preferably is an automatic consideration of said parameters. 
     According to one preferred embodiment of the invention the at least one determined second parameter for further operation is a predicted material condition state of the device with respect to its material fatigue, which parameter limits the unrestricted use of the device after reaching this state. 
     According to another preferred embodiment of the invention the at least one determined second parameter for further operation is a set of second parameters for keeping a desired material condition state of the device with respect to its material fatigue within a given operating time of said device. 
     According to various embodiments of the invention the control unit for operating a gradient coil device of a magnetic resonance imaging system (MRI system) (which run into danger of material fatigue failure modes due to the mechanical vibrations) comprises a computer system having a processor device and a memory device, wherein a mathematical model is implemented in the computer system, which mathematical model is based on the stress-cycle curve or a modified stress-cycle curve and designed for performing damage calculation, especially designed for calculating at least one damage factor D, of an operation of the device by use of at least one first parameter and at least one second parameter of the device. The control unit can be used for performing the aforementioned operating method. 
     According to a preferred embodiment of the control unit according to the invention the at least one first parameter of the device is a transfer function/a plurality of transfer functions of the device. 
     According to another preferred embodiment of the control unit according to the invention the at least one second parameter of the device is the effective frequency of a scan protocol and/or the total scan duration to count the number of positive zero-crossings. 
     According to yet another preferred embodiment of the control unit according to the invention the control unit is further set up for determining second parameters for further operation of said device. 
     According to another preferred embodiment of the control unit according to the invention said control unit further comprises an output interface for an output of the determined second parameters for further operation of said device. 
     According to various embodiments of the invention, the magnetic resonance imaging system comprises a gradient coil device and an aforementioned control unit. 
     The invention further relates to a computer program product for executing the aforementioned method on a computer system, which computer system preferably is a computer system of a magnetic resonance imaging system. The approach of the present invention may also be applied to predict cumulative damage due to material fatigue in repetitive stressed devices in the field of magnetic resonance imaging other than the gradient coil. Examples may be the radio frequency (RF) body coil or a posterior RF coil array and their mountings to the system&#39;s support structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
       In the drawings: 
         FIG. 1  shows an MR-guided radiation therapy system with an MR imaging system according to a preferred embodiment of the invention; and 
         FIG. 2  shows a schematic view of a control unit for operating a gradient coil device of the magnetic resonance imaging system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows an embodiment of an MR-guided radiation therapy system  10 . The MR-guided radiation therapy system  10  comprises a LINAC  12  and a magnetic resonance imaging system (MRI system)  14  according to the invention. The LINAC  12  comprises a gantry  16  and a X-ray source  18 . The gantry  16  is for rotating the X-ray source  18  about an axis of gantry rotation  20 . Adjacent to the X-ray source  18  is an adjustable collimator  20 . The adjustable collimator  20  may for instance have adjustable plates for adjusting the beam profile of the X-ray source  18 . The adjustable collimator  20  may, for example, be a multi-leaf collimator. The magnetic resonance imaging system  14  comprises a magnet  22 . 
     It is also possible to use permanent or resistive magnets. The use of different types of magnets is also possible for instance it is also possible to use both a split cylindrical magnet and a so called open magnet. A split cylindrical magnet is similar to a standard cylindrical magnet, except that the cryostat has been split into two sections to allow access to the iso-plane of the magnet, such magnets may for instance be used in conjunction with charged particle beam therapy. An open magnet has two magnet sections, one above the other with a space in-between that is large enough to receive a subject: the arrangement of the two sections area similar to that of a Helmholtz coil. Open magnets are popular, because the subject is less confined. Inside the cryostat of the cylindrical magnet there is a collection of superconducting coils. The magnet  22  shown in this embodiment is a standard cylindrical superconducting magnet. The magnet  22  has a cryostat  24  with superconducting coils  26  within it. The magnet  22  has a bore  28 . Within the bore  28  of the cylindrical magnet  22  there is an imaging zone where the magnetic field is strong and uniform enough to perform magnetic resonance imaging. 
     Within the bore  28  of the magnet  22  is a magnetic field gradient coil device  30  for acquisition of magnetic resonance data to spatially encode magnetic spins within an imaging zone of the magnet. The magnetic field gradient coil device  30  is connected to a magnetic field gradient coil power supply  32 . The magnetic field gradient coil device  30  is intended to be representative, to allow radiation to pass through without being attenuated it will normally be a split-coil design. Typically, magnetic field gradient coils contain three separate sets of coils for spatially encoding in three orthogonal spatial directions. The magnetic field gradient power supply  32  supplies current to the magnetic field gradient coils  30 . The current supplied to the magnetic field coils  30  is controlled as a function of time and may be ramped or pulsed. 
     There is an antenna device  34  connected to a transceiver  36 , which device  34  comprises at least one MR imaging antenna, each with a corresponding antenna loop. The device  34  is adjacent to an imaging zone  38  of the magnet  22 . The imaging zone  38  has a region of high magnetic field and homogeneity which is sufficient for performing magnetic resonance imaging. The device  34  may be for manipulating the orientations of magnetic spins within the imaging zone and for receiving radio transmissions from spins also within the imaging zone. The antenna device  34  may also be referred to as an antenna or channel. The device  34  is intended to also represent a dedicated transmit antenna and a dedicated receive antenna. Likewise, the transceiver may also represent a separate transmitter and receivers. 
     Also within the bore  28  of the magnet  22  is a subject support  40  for supporting a subject  42 . The subject support  40  may be positioned by a mechanical positioning system  44 . Within the subject  42  there is a target zone  46 . An axis of gantry rotation  48  is coaxial in this particular embodiment with the cylindrical axis of the magnet  22 . The subject support  40  has been positioned such that the target zone  46  lies on the axis  48  of gantry rotation. The X-ray source  18  is shown as generating a radiation beam  50  which passes through the collimator  20  and through the target zone  46 . As the radiation source  18  is rotated about the axis  48  the target zone  46  will always be targeted by the radiation beam  50 . The radiation beam  50  passes through the cryostat  24  of the magnet. The magnetic field gradient coil device  30  has a gap  52  which separate the magnetic field gradient coil device  30  into two sections. The gap  52  reduced attenuation of the radiation beam  50  by the magnetic field gradient coil device  30 . In an alternative embodiment a split or open magnet design is used to reduce the attenuation of the X-ray beam by the magnet  22 . The device  34  can be seen as being attached to the inside of the bore of the magnet  22  (not shown). 
     The transceiver  36 , the magnetic field gradient coil power supply  32  and the mechanical positioning system  44  are all shown as being connected to a hardware interface  54  of a computer system of a control unit  56 . The computer system of said control unit  56  is shown as further comprising a processor  58  for executing machine executable instructions and for controlling the operation and function of the MR-guided radiation therapy system  10 . The hardware interface  54  enables the processor  58  to interact with and control the MR-guided radiation therapy system  10 . The processor  58  is shown as further being connected to a user interface  60 , computer storage  62 , and computer memory  64 . 
     The computer storage  62  contains a treatment plan and an X-ray transmission model of the antenna device  34 . The X-ray transmission model may comprise the location of sensitive components of the device  34  and also the X-ray transmission properties of the antenna device  34 . The computer storage  62  further contains a pulse sequence. A pulse sequence as used herein is a set of commands used to control various components of the magnetic resonance imaging system  14  to acquire magnetic resonance data. The computer storage  62  contains magnetic resonance data that was acquired using the magnetic resonance imaging system  14 . 
     The computer storage  62  is further shown as containing a magnetic resonance image that was reconstructed from the magnetic resonance data. The computer storage  62  is further shown as containing an image registration of the magnetic resonance image. The image registration registers the location of the image relative to the magnetic resonance imaging system  14  and the LINAC  12 . The computer storage  62  is further shown as containing the location of the target zone  46 . This was identified in the magnetic resonance image. The computer storage  62  is further shown as containing control signals. The control signals are control signals which are used to control the LINAC  12  to irradiate the target zone  46 . 
     The computer memory  64  is shown as containing a control module. The control module contains computer-executable code which enables the processor  58  to control the operation and function of the medical apparatus  10 . For instance, the control module may use the pulse sequence to acquire the magnetic resonance data. The control module may also use the control signals to control the LINAC  12 . The computer memory  64  is further shown as containing a treatment plan modification module. The treatment plan modification module modifies the treatment plan using the information contained in the X-ray transmission model. The computer memory  64  is shown as further containing an image reconstruction module. The image reconstruction module contains code which enables the processor  58  to reconstruct the magnetic resonance image from the magnetic resonance data. 
     The computer memory  64  is shown as further containing an image registration module. The image registration module contains code which enables the processor  58  to generate the image registration in the location of the target zone  46  using the magnetic resonance image. The computer memory  64  is shown as further containing a target zone location module. The target zone location module contains code which enables the processor  58  to generate the location of the target zone  46  using the image registration. The computer memory  64  is further shown as containing a control signal generation module. The control signal generation module contains code which enables the processor  58  to generate the control signals from the treatment plan and the location of the target zone. The treatment plan after it has been modified in accordance with the X-ray transmission module is used. 
       FIG. 2  shows a schematic view of the control unit  56  for operating a gradient coil device  30  of the magnetic resonance imaging system  14  shown in  FIG. 1 . The control unit  56  comprises the computer system having the processor  58  as well as the computer memory  64  and the computer storage  62 , wherein a mathematical model  66  is implemented in the computer system of the control unit  56 . The mathematical model is based on the known stress-cycle curve (S-N curve, also known as a Wöhler curve) or a modified stress-cycle curve and designed for calculating damage factors D of an operation of the device  30  by use of the at least one first parameter  68  and the at least one second parameter  70 ,  72  of the device  30 ; wherein the control unit  56  is further set up for determining second parameters for further operation of said device  30 . The damage factor D is 
     
       
         
           
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     The damage Factor can be calculated before an MR scan protocol is started, or it can be used to analyze scans that already have been executed in the past using log file information for example. 
     The model  66  uses measured transfer functions  68  of the gradient coil device  30  based on velocities in the power 4. The velocity is the property that scales best with mechanical stress as function of frequency. 
     General mechanical fatigue S-N curves have a power of 4. This means that the damage potential of a stress (or velocity) at double amplitude is 16 times as severe in contributing to mechanical fatigue. 
     The model  66  also uses the effective frequency of a scan protocol (taken from FFTs of the different gradient wave forms)  70 ,  72  and the total scan duration to count the number of positive zero-crossings. This information can be used to optimize MR scan protocols and to predict the necessity of predictive maintenance. 
     Ways of Implementation or Usage: 
     Predictive maintenance: Use the model  66  on systems in the installed base to monitor and potential mechanical damage build-up. 
     Circumvent or minimize damage build-up, increase reliability: Optimize a scan before it is executed, use the model to steer scan parameters such as slew rate, repetition time etc. in a direction that causes less damage to the gradient coil device  30  or the MRI system  14 . 
     Alternatives: 
     Use a different power than the power 4 (known from the Wöhler curves of the materials used). For example a modified stress-cycle curve using a power 2, which makes the filters less sharp or a power 6 for pure copper, with sharper filters. 
     Use a different power than the power 4 (known from the Wöhler curves). For example a power 2 provides a good measure to reduce acoustic noise radiation. The far field radiated acoustic power of a body in general scales with the surface averaged velocity in the power 2. 
     Introduce peak widening to incorporate variation in resonance frequencies. Typically +/−2.5% widening of peaks is enough. 
     Use calculated transfer functions based on stresses instead of measured velocities. 
     Apply this approach to other devices than a gradient coil device  30  which can have fatigue failure modes due to mechanical vibrations (other repetitive stressed devices). 
     Leave out the scan duration and effective frequency in the model  66 . In this way no cumulative damage potential over multiple scans can be calculated, but a single scan can be optimized for damage potential or can be forbidden to be executed because the potential for damage is too large in the long run. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.