Patent Publication Number: US-9897678-B2

Title: Magnetic resonance imaging data correction methods and systems

Description:
BACKGROUND 
     The subject matter disclosed herein relates generally to magnetic resonance imaging (MRI) systems and, more particularly, to systems and methods for performing diffusion weighted imaging (DWI) with an MRI system. 
     In general, magnetic resonance imaging (MRI) examinations are based on the interactions among a primary magnetic field, a radiofrequency (RF) magnetic field and time varying magnetic gradient fields with gyromagnetic material having nuclear spins within a subject of interest, such as a patient. Certain gyromagnetic materials, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of these nuclei can be influenced by manipulation of the fields to produce RF signals that can be detected, processed, and used to reconstruct a useful image. 
     Diffusion-weighted MRI techniques are known in the field of medical diagnosis and medical diagnostic imaging. For example, in some applications, MR DWI may be used as a non-contrast enhanced method for cancer imaging. In these applications, changes in DWI based diffusivity may correlate to the degree of response to cancer treatment, the diffusivity measured at baseline may be predictive of cancer treatment outcome, and so forth. 
     Conventional DWI techniques typically provide useful information about the diffusion properties of water in an organ of interest, but are associated with a variety of factors that may bias or distort the desired diffusivity measurement. For example, the accuracy and reproducibility of desired diffusion maps or coefficients may be affected by gradient non-linearity. For further example, errors may occur due to concomitant gradient fields (also commonly known as Maxwell fields) resulting from the applied diffusion gradient waveforms. Accordingly, there exists a need for improved systems and methods that address these drawbacks. 
     BRIEF DESCRIPTION 
     In one embodiment, a method of correcting magnetic resonance (MR) data includes receiving the MR data, and the MR data corresponds to diffusion weighted MR signals. The method also includes correcting errors present in the MR data due to non-uniformities in magnetic field gradients used to generate the diffusion weighted MR signals and correcting errors present in the MR data due to concomitant gradient fields present in the magnetic field gradients by using one or more gradient terms. At least one of the gradient terms is corrected based on the correction of errors present in the MR data due to the non-uniformities in the magnetic field gradients. 
     In another embodiment, a magnetic resonance (MR) system includes an imager having an MR magnet and being adapted to acquire diffusion weighted MR raw data. The system also includes a processor adapted to receive the diffusion weighted MR raw data, to perform a gradient non-linearity correction technique on the MR raw data to obtain corrected MR data, and to perform a concomitant field correction technique on the corrected MR data to produce processed MR data. One or more gradient terms used in the concomitant field correction technique is at least partially determined by the corrected MR data. 
     In another embodiment, a non-transitory computer readable medium encodes one or more executable routines, which, when executed by a processor, cause the processor to perform acts including receiving magnetic resonance (MR) data, wherein the MR data corresponds to diffusion weighted MR signals. The acts also include performing a first correction technique on the MR data to remove errors present in the MR data due to non-uniformities in magnetic field gradients used to generate the diffusion weighted MR signals and performing a spatially dependent second correction technique on the MR data to remove errors present in the MR data due to concomitant gradient fields present in the magnetic field gradients. The second correction technique is modified with one or more outputs produced by performing the first correction technique. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a diagrammatical illustration of an embodiment of a magnetic resonance (MR) imaging system configured to acquire diffusion weighted MR images in accordance with an aspect of the present disclosure; 
         FIG. 2  is a flow diagram illustrating an embodiment of a method that may be utilized to generate one or more corrected diffusion maps in accordance with an aspect of the present disclosure; 
         FIG. 3  is a flow diagram illustrating an embodiment of a method that may be utilized to correct one or more diffusion maps in accordance with an aspect of the present disclosure; 
         FIG. 4  illustrates an apparent diffusion coefficient map generated from raw MR data of an axial section of a phantom in accordance with an embodiment; 
         FIG. 5  illustrates a corrected apparent diffusion coefficient map generated from raw MR data of an axial section of a phantom in accordance with an embodiment; 
         FIG. 6  is an apparent diffusion coefficient plot illustrating non-corrected and corrected apparent diffusion coefficients acquired with a first imaging system; and 
         FIG. 7  is an apparent diffusion coefficient plot illustrating non-corrected and corrected apparent diffusion coefficients acquired with a second imaging system. 
     
    
    
     DETAILED DESCRIPTION 
     As described in more detail below, provided herein are systems and methods for performing diffusion weighted imaging (DWI) using magnetic resonance imaging (MRI) systems. More specifically, various embodiments provided herein may employ correction methods that correct for errors present in MR data due to non-uniformities in magnetic field gradients used to generate diffusion weighted MR signals and/or errors present in the MR data due to concomitant gradient fields (also known as Maxwell fields) present in the magnetic field gradients. As such, presently disclosed embodiments provide for a combined gradient nonlinearity correction (GNC) and concomitant field correction (CFC). In certain embodiments, the CFC may be retrospective with respect to MR data setup and collection and one or more features of the CFC may be corrected with one or more features of the GNC. For example, in one embodiment, one or more gradient terms utilized in the CFC may be corrected with results of the GNC, thereby rendering the CFC spatially dependent. These and other features of presently disclosed embodiments are described in more detail below. 
     The implementations described herein may be performed by a magnetic resonance imaging (MRI) system, wherein specific imaging routines are initiated by a user (e.g., a radiologist). For example, the implementations described herein may be applicable to a variety of types of diffusion acquisition schemes known to those skilled in the art. For further example, the disclosed embodiments may be utilized with DWI, or any other desired type of diffusion based MRI. 
     Further, the MRI system may perform data acquisition, data construction, image reconstruction/synthesis, and image processing. Accordingly, referring to  FIG. 1 , a magnetic resonance imaging system  10  is illustrated schematically as including a scanner  12 , a scanner control circuit  14 , and a system control circuitry  16 . System  10  additionally includes remote access and storage systems or devices as picture archiving and communication systems (PACS)  18 , or other devices, such as teleradiology equipment, so that data acquired by the system  10  may be accessed on-site or off-site. While the MRI system  10  may include any suitable scanner or detector, in the illustrated embodiment, the system  10  includes a full body scanner  12  having a housing  20  through which a bore  22  is formed. A table  24  is moveable into the bore  22  to permit a patient  26  to be positioned therein for imaging selected anatomy within the patient  26 . The selected anatomy may be imaged by a combination of patient positioning, selected excitation of certain gyromagnetic nuclei within the patient  26 , and by using certain features for receiving data from the excited nuclei as they spin and precess, as described below. 
     Scanner  12  includes a series of associated coils for producing controlled magnetic fields for exciting the gyromagnetic material within the anatomy of the subject being imaged. Specifically, a primary magnet coil  28  is provided for generating a primary magnetic field generally aligned with the bore  22 . A series of gradient coils  30 ,  32 , and  34  permit controlled magnetic gradient fields to be generated for positional encoding of certain of the gyromagnetic nuclei within the patient  26  during examination sequences. A radio frequency (RF) coil  36  is provided, and is configured to generate radio frequency pulses for exciting the certain gyromagnetic nuclei within the patient. In addition to the coils that may be local to the scanner  12 , the system  10  also includes a set of receiving coils  38  (e.g., a phased array of coils) configured for placement proximal to (e.g., against) the patient  26 . The receiving coils  38  may have any geometry, including both enclosed and single-sided geometries. 
     As an example, the receiving coils  38  can include cervical/thoracic/lumbar (CTL) coils, head coils, single-sided spine coils, and so forth. Generally, the receiving coils  38  are placed close to or on top of the patient  26  so as to receive the weak RF signals (weak relative to the transmitted pulses generated by the scanner coils) that are generated by certain of the gyromagnetic nuclei within the patient  26  as they return to their relaxed state. The receiving coils  38  may be switched off so as not to receive or resonate with the transmit pulses generated by the scanner coils, and may be switched on so as to receive or resonate with the RF signals generated by the relaxing gyromagnetic nuclei. 
     The various coils of system  10  are controlled by external circuitry to generate the desired field and pulses, and to read emissions from the gyromagnetic material in a controlled manner. In the illustrated embodiment, a main power supply  40  provides power to the primary field coil  28 . A driver circuit  42  is provided for pulsing the gradient field coils  30 ,  32 , and  34 . Such a circuit may include amplification and control circuitry for supplying current to the coils as defined by digitized pulse sequences output by the scanner control circuit  14 . Another control circuit  44  is provided for regulating operation of the RF coil  36 . Circuit  44  includes a switching device for alternating between the active and inactive modes of operation, wherein the RF coil  36  transmits and does not transmit signals, respectively. Circuit  44  also includes amplification circuitry for generating the RF pulses. Similarly, the receiving coils  38  are connected to switch  46  that is capable of switching the receiving coils  38  between receiving and non-receiving modes such that the receiving coils  38  resonate with the RF signals produced by relaxing gyromagnetic nuclei from within the patient  26  while in the receiving state, and they do not resonate with RF energy from the transmitting coils (i.e., coil  36 ) so as to prevent undesirable operation while in the non-receiving state. Additionally, a receiving circuit  48  is provided for receiving the data detected by the receiving coils  38 , and may include one or more multiplexing and/or amplification circuits. 
     It should be noted that presently disclosed embodiments may enable a decoupling of the desired diffusion-weighted gradient waveforms utilized to probe the patient  26  and the CFC. For example, by utilizing a retrospective CFC instead of a prospective CFC, the patient  26  may be probed as desired (i.e., without a manipulation of the gradient waveform), and the CFC may be applied to the MR data after acquisition. In some embodiments, the CFC may be enhanced by utilizing the GNC to correct one or more gradient terms in the CFC. 
     In the illustrated embodiment, scanner control circuit  14  includes an interface circuit  50  for outputting signals for driving the gradient field coils  30 ,  32 ,  34  and the RF coil  36 . Additionally, interface circuit  50  receives the data representative of the magnetic resonance signals produced in examination sequences from the receiving circuitry  48  and/or the receiving coils  38 . The interface circuit  50  is operatively connected to a control circuit  52 . The control circuit  52  executes the commands for driving the circuit  42  and circuit  44  based on defined protocols selected via system control circuit  16 . Control circuit  52  also serves to provide timing signals to the switch  46  so as to synchronize the transmission and reception of RF energy. Further, control circuit  52  receives the magnetic resonance signals and may perform subsequent processing before transmitting the data to system control circuit  16 . Scanner control circuit  14  also includes one or more memory circuits  54 , which store configuration parameters, pulse sequence descriptions, examination results, and so forth, during operation. The memory circuits  54 , in certain embodiments, may store instructions for implementing at least a portion of the image processing techniques described herein. 
     Interface circuit  56  is coupled to the control circuit  52  for exchanging data between scanner control circuit  14  and system control circuit  16 . Such data may include selection of specific examination sequences to be performed, configuration parameters of these sequences, and acquired data, which may be transmitted in raw or processed form from scanner control circuit  14  for subsequent processing, storage, transmission and display. 
     An interface circuit  58  of the system control circuit  16  receives data from the scanner control circuit  14  and transmits data and commands back to the scanner control circuit  14 . The interface circuit  58  is coupled to a control circuit  60 , which may include one or more processing circuits in a multi-purpose or application specific computer or workstation. Control circuit  60  is coupled to a memory circuit  62 , which stores programming code for operation of the MRI system  10  and, in some configurations, the image data for later reconstruction, display and transmission. An additional interface circuit  64  may be provided for exchanging image data, configuration parameters, and so forth with external system components such as remote access and storage devices  18 . Finally, the system control circuit  60  may include various peripheral devices for facilitating operator interface and for producing hard copies of the reconstructed images. In the illustrated embodiment, these peripherals include a printer  66 , a monitor  68 , and user interface  70  including devices such as a keyboard or a mouse. 
     It should be noted that subsequent to the acquisitions described herein, the system  10  may simply store the acquired data for later access locally and/or remotely, for example in a memory circuit (e.g., memory  56 ,  62 ). Thus, when accessed locally and/or remotely, the acquired data may be manipulated by one or more processors contained within an application-specific or general-purpose computer. The one or more processors may access the acquired data and execute routines stored on one or more non-transitory, machine readable media collectively storing instructions for performing methods including the image processing, correction, and reconstruction methods described herein. 
     Further, it should be noted that the MRI system  10  may be utilized to implement a variety of suitable diffusion acquisition schemes and to correct the acquired MR data in accordance with the embodiments described herein. For example, the MRI system  10  may be utilized to perform a DWI scan. In such embodiments, in operation, the MRI system  10  is utilized to acquire MR data of the patient  26  and to subsequently process the data to reduce or remove errors present in the data. For example, the MR data may be processed to reduce or remove errors present due to non-uniformities (e.g., nonlinearities) in the magnetic field gradients used to generate the diffusion weighted MR signals. For further example, the MR data may be corrected for errors present in the MR data due to concomitant gradient fields. In certain embodiments, the corrected MR data may be utilized to generate one or more diffusion maps, such as an apparent diffusion coefficient (ADC) map, a fractional anisotropy (FA) map, a relative anisotropy (RA) map, a mean diffusivity (MD) map, or any other desired map. One such method for generating a map corresponding to the imaged tissue is provided in method  72  shown in  FIG. 2 . 
     The method  72  includes receiving the diffusion weighted MR raw data (block  74 ) and performing a first correction of the MR raw data for the contribution of errors introduced by gradient non-uniformities, such as gradient nonlinearities (block  76 ). In accordance with one embodiment, the equations and steps for gradient nonlinearity correction (GNC) for diffusion imaging are adapted here for completeness. The gradient field map tensor, Λ(r) relates the idealized gradient vector g=[g X  g Y  g Z ] T  to the spatially-varying, actual gradient vector g′(r)=[g′ X (r) g′ Y (r) g′ Z (r)] T , described as a function of its physical location in magnet coordinates, r=[X Y Z] T . Λ(r) is defined by the spatial derivative of the B 0 -field contributed by each of the three gradient axes, B′ X (r), B′ Y (r) and B′ Z (r) relative to the nominal gradient amplitudes G X , G Y  and G Z : 
     
       
         
           
             
               
                 
                   
                     
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     When the idealized b-matrix ( 20 ) b is known, GN effects can be applied directly to obtain the actual b-matrix, b′. With that, the signal S i  for the i th  diffusion acquisition relative to its non-diffusion-encoded reference signal S 0  can be expressed as a Frobenius inner product 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     However, in addition to performing the GNC, the method  72  also calls for correcting the raw MR data for the effect of concomitant gradient fields by using gradient terms corrected with the GNC (block  78 ) before generating one or more diffusion maps with the corrected data (block  80 ). That is, in certain embodiments, it may be desired or necessary to correct for concomitant field effects resulting from diffusion-gradient waveforms whose magnetic moments do not cancel out. This may occur, for example, in the dual-spin-echo (DSE) diffusion preparation, but may not occur in the single spin-echo (SSE) diffusion preparation. 
     More specifically, in diffusion-weighted MRI acquisition, a diffusion preparation sequence (or gradient waveforms) is required, which imparts the diffusion weighting on the acquired MRI signal. Typically, the gradient waveforms are placed symmetrically beside the refocusing radiofrequency pulse, imparting no net phase accrual. A common diffusion preparation that uses symmetrical waveforms is known as SSE. Waveforms that are asymmetric include DSE, also known as twice-refocused spin echo (TSE), and may include other novel waveforms that may not currently be known to those skilled in the art. These asymmetric waveforms have a net phase accrual that occurs over a cumulative duration (τ), which result in a concomitant field effect that in turn results in a bias in the obtained diffusion signal. Using a linear sinc approximation, this concomitant field effect can be calculated as a function of τ. 
     It should be noted that in some embodiments, a full-correction may be performed that utilizes changes in the gradient waveforms to correct both k-space and signal bias effects. However, in the embodiment described below, only the signal bias correction that may be performed retrospectively together with GNC is incorporated. Using the known relation between the spatially-varying concomitant field, B c , and the applied gradient amplitudes, the lowest-order terms obtained from the expansion of Maxwell&#39;s equations are 
     
       
         
           
             
               
                 
                   
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     The spatially-varying signal bias from concomitant field effects is due to intra-slice signal dephasing from spatially-varying Maxwell fields. In addition to B c , other contributing factors to signal dephasing include the duration of accumulated magnetic moment τ, the slice thickness w, the normal vector to the imaging plane {circumflex over (r)} s , and b. If the intra-slice dephasing is linear in the slice direction, the concomitant-field-corrected signal S′ i  for the i th  diffusion measurement is related to the acquired signal S i  by the relation: 
     
       
         
           
             
               
                 
                   
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     With the incorporation of gradient nonlinearity into CFC, many of the constant terms of equation 6, in addition to B C , will also become spatially-dependent. Hence, the complete equation with both GNC and CFC in a DSE acquisition is: 
     
       
         
           
             
               
                 
                   
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     As shown, equation 6 shows a retrospective CFC that may be performed on MR data after acquisition, and equation 7 shows a combined GNC and CFC correction having retrospective CFC that is further corrected with GNC. That is, as shown in equation 7, utilizing the GNC in the disclosed manner may enhance the CFC by rendering the CFC spatially dependent. For example, in the embodiment of equation 7, the terms S′ i , w, {circumflex over (r)} s , and ∇B C  become spatially dependent when the retrospective CFC method represented in equation 6 is further modified with the information obtained via GNC. In this way, one or more of the gradient terms used in the concomitant field correction portion of equation 7 is at least partially determined by the GNC correction. 
     It should be noted that presently disclosed embodiments, such as the illustrated method  72 , enable the novel combination of GNC and CFC type corrections. For example, the CFC is cascaded to the processing pipeline following the GNC for the b-matrix, and the gradient terms used in CFC are corrected with GNC as described above. These features enable a retrospective combined GNC and CFC method that provides further accuracy than that provided with an uncorrected system or a system only corrected with GNC. Further, as compared to systems that employ a prospective CFC, embodiments of the disclosed combined methods do not require gradient waveform manipulation and can be performed on previously acquired data, thus offering additional advantages. In these ways, presently disclosed embodiments resolve both gradient nonlinearity and concomitant field effects retrospectively, which enables existing images to be corrected to improve spatial accuracy, same-scanner reproducibility, and inter-scanner reproducibility of diffusion imaging. This feature may offer advantages, for example, in medical applications in which diffusion metrics are used as biomarkers for diseases such as cancer and stroke. 
     It should be noted that in some embodiments the presently disclosed combined GNC and CFC correction method may be applied not to the raw MR data as in the method  72  of  FIG. 2 , but instead to a diffusion map obtained from the raw MR data. For example, in a method  82  illustrated in  FIG. 3 , the diffusion weighted MR raw data is received (block  84 ), for example by a controller or a processor, and one or more diffusion maps are generated based on the received data (block  86 ). Subsequently, the first correction for errors introduced by gradient non-uniformities is performed on the diffusion maps (block  88 ) followed by the second correction for concomitant field effects (block  90 ). However, in some instances, performing the retrospective combined GNC and CFC method on the diffusion maps instead of the raw MR data may result in the use of scalar data instead of vector data, thus giving rise to a different result. Further, it should be noted that although the first and second corrections are represented as separate blocks  76  and  78  in  FIG. 2  and blocks  86  and  88  in  FIG. 3 , it should be understood that in certain embodiments, the retrospective combined GNC and CFC method may be applied such that the GNC and CFC corrections are performed concurrently, for example, by implementing equation 7. 
       FIGS. 4-7  illustrate experimental results obtained for an ice-bath phantom and processed in accordance with a presently disclosed embodiment. More specifically,  FIG. 4  illustrates a control ADC map  92  for an axial section of a phantom having five circular water regions with an assumed true ADC of approximately 1100 μm 2 /sec positioned 11 cm right of isocenter in a 55-cm patient bore system. The control ADC map  92  is representative of a map that has neither been corrected for errors present in the raw MR data due to non-linearities in the magnetic field gradients nor for concomitant gradient field errors. Further, arrows  94 ,  96 ,  98 ,  100 , and  102  point to the locations of five cylindrical tubes filled with distilled water that were oriented longitudinally in the MRI scanner during data acquisition. 
       FIG. 5  illustrates an ADC map  104  generated with the same data used to generate the control ADC map  92  but which reflects a retrospective combined GNC and CFC correction performed in accordance with equation 7. To measure ADC when generating each of these maps  92  and  104 , five circular regions of 5 mm diameter were manually placed on each axial image slice, one for each of the five tubes of water. Further, only the 10 central slices of the phantom were used to avoid gross errors and echo planar imaging distortion. Additionally, in the analysis shown in the plots  106  and  122  of  FIGS. 6 and 7 , the mean ADC for each region of interest (ROI) was calculated. 
       FIG. 6  illustrates ADC value plots  112 ,  114 , and  116  generated from imaging acquisitions performed on the phantom with the phantom positioned 11 cm right of the magnet isocenter, 11 cm left of the magnet isocenter, and at the magnet isocenter, respectively, in a 55-cm bore MRI system. Each plot shows ADC values lying along the ADC axis  108  for each of the correction methods lying along the correction method axis  110 . Further, dashed lines  118  and  120  indicate ±5% offsets from the assumed true ADC of 1100 μm 2 /sec. 
     As shown in the plots  112 ,  114 , and  116 , the ADC values obtained without applying any correction are farther from the assumed true ADC than the ADC values obtained when a traditional GNC correction is applied. However, when the retrospective combined GNC and CFC method outlined in equation 7 is applied, the ADC values are even closer to the assumed true ADC value. That is, by applying an embodiment of the retrospective GNC and CFC method disclosed herein, statistically significant improvements in the ADC values are obtained at all three positions within the imaging system. 
       FIG. 7  illustrates ADC value plots  126 ,  128 , and  130  generated from imaging acquisitions performed on the phantom with the phantom positioned 11 cm right of the magnet isocenter, 11 cm left of the magnet isocenter, and at the magnet isocenter, respectively, in a 60-cm bore MRI system in which the extent of the original ADC value error was reduced. Here again, each plot shows ADC values lying along the ADC axis  124  for each of the correction methods. 
     As shown in the plots  126 ,  128 , and  130 , the ADC values obtained without applying any correction or when applying just the GNC correction are farther from the assumed true ADC than the ADC values obtained when an embodiment of the disclosed combined GNC and CFC correction is applied. Here again, the benefits of applying an embodiment of the retrospective GNC and CFC method disclosed herein can be seen insomuch as the determined ADC value is closer to the true ADC value when the combined GNC and CFC method is applied. Specifically, in these experimental results, the combination of GNC with a DSE pulse sequence and a retrospective concomitant field correction was found to reduce ADC error due to spatial variance from 9.5% to 1.8% (55 cm bore system) and from 4.2% to 1.8% (60 cm bore system). 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.