Patent Application
20090322330
1. Field of the Invention
The present invention relates to a MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to reduce influence of B1 inhomogenity by universal B1 shimming without depending on characteristic of each object.
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstruct an image based on NMR signals generated due to the excitation.
In recent years, B1 inhomogeneity becomes a problem along with making of the apparatus with a high magnetic field. The B1 inhomogeneity is also called RF magnetic field inhomogeneity. The B1 inhomogeneity is a phenomenon that a heterogeneity degree is increased due to attenuation of a RF pulse with a short wavelength in a living body and an echo signal becomes inhomogeneous due to the inhomogeneity of a RF pulse in a high magnetic field apparatus with a high resonant frequency. That is, in a high magnetic field MRI apparatus, a signal intensity distribution of a RF transmission pulse varies widely between a center and a peripheral part of an image area by influence of a difference in electric permittivity between objects and the like.
Therefore, B1 shimming is required for reducing an influence of B1 inhomogeneity. That is, if a correction to equalize a signal intensity distribution of a RF transmitted pulse is not performed, nonuniformity in sensitivity may be generated between the central part and on the peripheral part of an image area. For that problem, a research that transmitting the RF transmission pulse with a corrected amplitude and phase in an area to generate nonuniformity in sensitivity is effective for reducing an influence of B1 inhomogeneity is performed (for example, refer to G. McKinnon et al., “RF Shimming With a Conventional 3T Body Coil”, Proc. Intl. Soc. Mag. Reson. Med. 15 (2007), p 173, D. Weyers et al., “Shading Reduction at 3.0T using an Elliptical Drive”, Proc. Intl. Soc. Nag. Reson. Med. 14 (2006), p 2023 and J. Nistler et al., “Homogeneity Improvement Using A 2 Port Birdcage coil”, Proc. Intl. Soc. Mag. Reson. Med. 15 (2007), p 1063).
However, an area where sensitivity nonuniformity is generated by an influence of B1 inhomogeneity and an appropriate phase and amplitude of an RF transmission pulse vary depending on each characteristic such as a weight, a height, a volume, a water distribution, a fat distribution and a muscle distribution of an object. To the contrary, B1 shimming proposed conventionally is the method to obtain the phase and the amplitude of the transmission RF pulse appropriate for the characteristics of a specific object in advance and to transmit the transmission RF pulse having the determined phase and amplitude to the area where the sensitivity nonuniformity is generated depending on the characteristics of the object. Therefore, appropriate B1 shimming cannot be performed constantly since the sensitivity nonuniformity area and an appropriate amplitude and a phase of RF transmission pulse also change when the object changes. In other words, when the conventional method is used as it is, the amplitude and the phase of the transmission RF pulse are adjusted regarding the common sensitivity nonuniformity area determined in advance though the sensitivity nonuniformity area varies depending on each object.
Further, it is necessary for a user to obtain an appropriate phase and amplitude of transmission RF pulse and to perform adjustments of the phase and the amplitude of the transmission RF pulse manually and frequently every time the object changes.
The problem mentioned above hinders implementation of B1 shimming function in a high magnetic field apparatus. For that reason, developing a technology for universal and simple B1 shimming coping with changing the object is desired.
The present invention has been made in light of the conventional situations, and it is an object of the present invention to provide a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to reduce influence of B1 inhomogenity by universal B1 shimming without depending on characteristic of each object.
The present invention provides a magnetic resonance imaging apparatus comprising: an imaging condition acquisition unit configured to acquire at least one of an amplitude and a phase of a radio frequency transmission signal so as to reduce a deviation of data in at least one region of interest set in an object; and an imaging unit configured to acquire image data by imaging according to an imaging condition including at least the one of the amplitude and the phase, in an aspect to achieve the object.
The present invention also provides a magnetic resonance imaging apparatus comprising: an imaging condition acquisition unit configured to acquire at least one of an amplitude and a phase of a radio frequency transmission signal so as to reduce a deviation of data in at least one region of interest set in an object based on a radio frequency magnetic field distribution corresponding to at least one of a body shape of the object and a coil used for transmitting the radio frequency transmission signal; and an imaging unit configured to acquire image data by imaging according to an imaging condition including at least the one of the amplitude and the phase, in an aspect to achieve the object.
The present invention also provides a magnetic resonance imaging method comprising: acquiring at least one of an amplitude and a phase of a radio frequency transmission signal so as to reduce a deviation of data in at least one region of interest set in an object; and acquiring image data by imaging according to an imaging condition including at least the one of the amplitude and the phase, in an aspect to achieve the object.
The present invention also provides a magnetic resonance imaging method comprising: acquiring at least one of an amplitude and a phase of a radio frequency transmission signal so as to reduce a deviation of data in at least one region of interest set in an object based on a radio frequency magnetic field distribution corresponding to at least one of a body shape of the object and a coil used for transmitting the radio frequency transmission signal; and acquiring image data by imaging according to an imaging condition including at least the one of the amplitude and the phase, in an aspect to achieve the object.
The magnetic resonance imaging apparatus and the magnetic resonance imaging method according to the present invention as described above make it possible to reduce influence of B1 inhomogenity by universal B1 shimming without depending on characteristic of each object.
In the accompanying drawings:
A magnetic resonance imaging apparatus and a magnetic resonance imaging method according to embodiments of the present invention will be described with reference to the accompanying drawings.
A magnetic resonance imaging apparatus 20 includes a static field magnet 21 for generating a static magnetic field, a shim coil 22 arranged inside the static field magnet 21 which is cylinder-shaped, a gradient coil 23 and RF coils 24.
The magnetic resonance imaging apparatus 20 also includes a control system 25. The control system 25 includes a static magnetic field power supply 26, a gradient power supply 27, a shim coil power supply 28, a transmitter 29, a receiver 30, a sequence controller 31 and a computer 32. The gradient power supply 27 of the control system 25 includes an X-axis gradient power supply 27x, a Y-axis gradient power supply 27y and a Z-axis gradient power supply 27z. The computer 32 includes an input device 33, a display unit 34, a operation unit 35 and a storage unit 36.
The static field magnet 21 communicates with the static magnetic field power supply 26. The static magnetic field power supply 26 supplies electric current to the static field magnet 21 to get the function to generate a static magnetic field in a imaging region. The static field magnet 21 includes a superconductivity coil in many cases. The static field magnet 21 gets current from the static magnetic field power supply 26 which communicates with the static field magnet 21 at excitation. However, once excitation has been made, the static field magnet 21 is usually isolated from the static magnetic field power supply 26. The static field magnet 21 may include a permanent magnet which makes the static magnetic field power supply 26 unnecessary.
The static field magnet 21 has the cylinder-shaped shim coil 22 coaxially inside itself. The shim coil 22 communicates with the shim coil power supply 28. The shim coil power supply 28 supplies current to the shim coil 22 so that the static magnetic field becomes uniform.
The gradient coil 23 includes an X-axis gradient coil 23x, a Y-axis gradient coil 23y and a Z-axis gradient coil 23z. Each of the X-axis gradient coil 23x, the Y-axis gradient coil 23y and the Z-axis gradient coil 23z which is cylinder-shaped is arranged inside the static field magnet 21. The gradient coil 23 has also a bed 37 in the area formed inside it which is an imaging area. The bed 37 supports an object P. The RF coils 24 include a whole body coil (WBC: whole body coil), which is built in the gantry, for transmission and reception of RF signals and local coils, which are arranged around the bed 37 or the object P, for reception of RF signals.
The gradient coil 23 communicates with the gradient power supply 27. The X-axis gradient coil 23x, the Y-axis gradient coil 23y and the Z-axis gradient coil 23z of the gradient coil 23 communicate with the X-axis gradient power supply 27x, the Y-axis gradient power supply 27y and the Z-axis gradient power supply 27z of the gradient power supply 27 respectively.
The X-axis gradient power supply 27x, the Y-axis gradient power supply 27y and the Z-axis gradient power supply 27z supply currents to the X-axis gradient coil 23x, the Y-axis gradient coil 23y and the Z-axis gradient coil 23z respectively so as to generate gradient magnetic fields Gx, Gy and Gz in the X, Y and Z directions in the imaging area.
The RF coils 24 communicate with the transmitter 29 and/or the receiver 30. The transmission RF coil 24 has a function to transmit a RF signal given from the transmitter 29 to the object P. The reception RF coil 24 has a function to receive a MR signal generated due to an nuclear spin inside the object P which is excited by the RF signal to give to the receiver 30.
The sequence controller 31 of the control system 25 communicates with the gradient power supply 27, the transmitter 29 and the receiver 30. The sequence controller 31 has a function to storage sequence information describing control information needed in order to make the gradient power supply 27, the transmitter 29 and the receiver 30 drive and generate gradient magnetic fields Gx, Gy and Gz in the X, Y and Z directions and a RF signal by driving the gradient power supply 27, the transmitter 29 and the receiver 30 according to a predetermined sequence stored. The control information above-described includes motion control information, such as intensity, impression period and impression timing of the pulse electric current which should be impressed to the gradient power supply 27
The sequence controller 31 is also configured to give raw data to the computer 32. The raw data is complex data obtained through the detection of a NMR signal and A/D conversion to the NMR signal detected in the receiver 30.
The transmitter 29 has a function to give a RF signal to the RF coil 24 in accordance with control information provided from the sequence controller 31. The receiver 30 has a function to generate raw data which is digitized complex number data by detecting a MR signal given from the RF coil 24 and performing predetermined signal processing and A/D converting to the MR signal detected. The receiver 30 also has a function to give the generated raw data to the sequence controller 31.
As shown in
The amplitude control unit 31A, the phase control unit 31B and the signal generator 31C in the sequence controller 31 are configured to be controllable respectively by control signals from the computer 32. The amplitude control unit 31A has a function to adjust an amplitude of I signal by providing a control signal to the amplitude-phase balancer 29B in the transmitter 29. The phase control unit 31B has a function to adjust a phase of I signal by providing a control signal to the amplitude-phase balancer 29B in the transmitter 29. The signal generator 31C has a function to generate a RF signal and output the RF signal to the 90-degree signal distributor 29A in the transmitter 29.
The amplitude-phase balancer 29B and the transmission pre-amplifier 29C for Q signal are connected in parallel to the 90-degree signal distributor 29A in the transmitter 29. The transmission pre-amplifier 29C for I signal, the power amplifier 29D and the isolator 29E are connected in series subsequently to the amplitude-phase balancer 298. The power amplifier 29D for Q signal and the isolator 29E are connected in series subsequently to the transmission pre-amplifier 29C for Q signal. The 50Ω terminations 29F are respectively connected to the respective isolators for I signal and Q signal.
The 90-degree signal distributor 29A has a function to distribute a RF signal obtained from the signal generator 31C to an I signal with the 0-degree phase and a Q signal with the 90-degree phase and to output the distributed Q signal to the corresponding transmission pre-amplifier 29C while output the distributed I signal to the amplitude-phase balancer 29B. The transmission pre-amplifier 29C has a function to set an amplitude and a phase of I signal to predetermined values according to the control signals from the amplitude control unit 31A and the phase control unit 31B and to output the I signal after adjustment to the corresponding transmission pre-amplifier 29C. The respective transmission pre-amplifiers 29C and the respective power amplifiers 29D respectively have a function to amplify signals. The respective isolators 29E respectively have a function to insulate an input signal and an output signal serially for prevention of entering of a signal and protection of a circuit by the 50Ω terminations 29F.
The transmission/reception selector switches 38A in the RF front-end 38 have function to switch between a circuit connection configuration for transmission of a RF signal and a circuit connection configuration for reception of an echo signal by switching the I signal transmission/reception channel 38B between the isolator 29E for I signal side and the 90-degree phase compositor 38D side and the Q signal transmission/reception channel 38C between the isolator 29E for Q signal side and the 90-degree phase compositor 38D side respectively. The I signal transmission/reception channel 38B is configured to transmit an I signal and is connected to the coil element 24A to receive an echo signal corresponding to the I signal. The Q signal transmission/reception channel 38C is configured to transmit a Q signal and is connected to the coil element 24B to receive an echo signal corresponding to the Q signal. The 90-degree phase compositor 38D has a function to obtain a single echo signal by performing phase correction to cancel the 90-degree phase difference on echo signals respectively output from the I signal transmission/reception channel 38B and the Q signal transmission/reception channel 38C and combining the echo signals after the phase correction and to output the reception echo signal to the reception pre-amplifier 38F. The reception pre-amplifier 38F has a function to amplify the echo signal combined in the 90-degree phase compositor 38D and to output the amplified echo signal to the receiver 30.
The computer 32 gets various functions by the operation unit 35 executing some programs stored in the storage unit 36 of the computer 32. Alternatively, some specific circuits having various functions may be provided with the magnetic resonance imaging apparatus 20 instead of using some of the programs.
The computer 32 functions as an imaging condition setting unit 40, a sequence controller control unit 41, a k-space database 42, an image reconstruction unit 43, an image database 44, an image processing unit 45 and an amplitude-phase balance database 46. The imaging condition setting unit 40 has a ROI setting part 40A and an amplitude-phase setting part 40B.
The imaging condition setting unit 40 has a function to set an imaging condition including a pulse sequence based on instruction from the input device 33 and to provide the set imaging condition to the sequence controller control unit 41. For that purpose, the imaging condition setting unit 40 has a function to display screen information for setting of an imaging condition on the display unit 34.
The ROI setting unit 40A in the imaging condition setting unit 40 has a function to set single or plural regions of interest (ROI). For that purpose, the ROI setting unit 40A is configured to refer to the image database 44 as needed.
The amplitude-phase setting unit 40B has a function to determine an absolute or relative value of either or both of a phase and an amplitude of a RF transmission signal for excitation of an object P so as to reduce a variation in signal intensity between pieces of data acquired from respective ROIs set in the ROI setting unit 40A and/or a variation in data in a single ROI. For that purpose, the amplitude-phase setting unit 40B is configured to refer to the image database 44. Thus, the amplitude-phase setting unit 403 is configured to set the determined phase and amplitude of the RF transmission signal as an imaging condition for an imaging scan.
Especially, when the magnetic resonance imaging apparatus 20 is a high magnetic field apparatus, a signal intensity of a RF transmission signal changes every imaging part by an influence of so-called B1 inhomogeneity. That is, the signal intensity of the RF transmission signal becomes inhomogeneous spatially. The spatial inhomogeneity of the RF transmission signal also changes depending on characteristics including a weight, a height, a volume, a distribution of water, a distribution of fat, a distribution of muscle, a body shape and an age of an object. Therefore, a variation in intensity between acquired signals changes every ROI, every position in a ROI and every characteristic of an object by an influence of B1 inhomogeneity.
For that reason, the amplitude-phase setting unit 403 determines an appropriate phase and/or amplitude of a RF transmission signal so as to reduce a variation in signal intensity between pieces of data acquired in respective ROIs and/or a variation in data in a single ROI in order to reduce an influence of B1 inhomogeneity. A single ROI or plural ROIs that become a target for reducing a variation in signal intensity can be set manually based on instruction information from the input device 33 or automatically in the ROI setting unit 40A according to a predetermined condition. For example, a ROI setting mode for setting a ROI manually by user's operation of the input device 33 and a ROI setting mode in which a ROI is set automatically can be provided.
In the case of setting a ROI, it is necessary to determine a region for setting the ROI. The region for setting the ROI can be stored as a default value in the ROI setting unit 40A and can be also determined based on data acquired by a preparation scan for positioning to determine the region for setting the ROI automatically or manually.
In
On the other hand, ideal values of the signal intensities of the image data in the case of assuming that there is no influence of B1 inhomogeneity take a constant value within the range where the human body equivalent phantom exists as shown in the dashed line in
The region for setting a ROI can be set with reference to the signal intensities of the image data in a range where the signal intensities are larger than or equal to a certain threshold value to be considered that the human body equivalent phantom exists. As described above, an existence region of an object P such as a human body equivalent phantom can be extracted as a region for setting a ROI. A user can set the region for setting a ROI manually by displaying a profile of the signal intensities of the image data as shown in
Note that, the threshold can be defined as an absolute value to a signal intensity. Alternatively, a low signal part to be considered as a background noise region is extracted automatically from the image data and the threshold can be also defined as a relative value to the low signal part.
A ROI can be also set manually or automatically. An overwrap rate or an overwrap amount between adjacent ROIs and a ratio of an area of a ROI to an area of the ROI setting region are listed in addition to the number, locations and shapes of ROIs as conditions for setting ROIs. If adjacent ROIs are made overwrap, continuous image data can be obtained by an imaging scan. Therefore, plural ROIs overwrapped for a preparation scan can be set according to a ROI for an imaging scan. Parameters for setting ROIs can be stored as default values in the ROI setting unit 40A and be changed by operation of the input device 33.
As shown in
However, as shown in
Methods for minimizing a variation in signal intensity in a ROI and between ROIs include a method for defining an index indicating a variation in signal intensity to obtain such an amplitude and a phase of a RF transmission signal that the defined index becomes minimum and set the obtained amplitude and phase as an imaging condition and a method for obtaining such an amplitude and a phase of a RF transmission signal that a more homogeneous B1 distribution can be obtained and set the obtained amplitude and phase as an imaging condition.
First, the method to obtain such an amplitude and a phase of a RF transmission signal that an index representing a variation in signal intensity becomes minimum will be described.
For example, as shown in
D=Ia1−(Ib1+Ib2+Ib3+ . . . +Ibi)/i (1)
wherein Ia1 is a representative value of signal intensity for the ROI A1 and Ibk (1≦k≦i) is a representative value of signal intensity for each ROI Bk.
A representative value of signal intensity can be an average value, an intermediate value, a maximum value or a minimum value of signal intensities in each ROI. The representative value such as an average value and an intermediate value may be calculated with eliminating a singular value and/or an error value.
Specifically, as shown in equation (1), a difference between the representative value of signal intensities regarding the ROI A1 in the central part with relatively-high signal intensities and the average value of signal intensities regarding the ROI Bk on the peripheral part with relatively-low signal intensities can be defined as a deviation D in signal intensity of data among plural ROIS. Note that, a deviation D in signal intensity of data among ROIs can be defined arbitrarily according to conditions such as the number and/or shapes of the ROls or an imaging purpose without using on equation (1). Similarly, a deviation in signal intensity of data in a single ROI can be also defined as equation (1) by setting plural regions and/or points in the ROI.
Note that, the values measured automatically in the ROI setting unit 40A can be used as the signal intensities in each ROI. However, the signal intensities in each ROI can be also measured automatically in the amplitude-phase setting unit 40B.
When a deviation D in signal intensity of data among plural ROIs or in a single ROI is obtained as a value as mentioned above, such an amplitude and a phase of RF transmission signal that the deviation D becomes minimum can be calculated by an arbitrary search method. The representative methods for searching such an amplitude and a phase of RF transmission signal that the deviation D becomes minimum are lattice search algorithm and bisection method. Both lattice search algorithm and bisection method can be used as needed.
Lattice search algorithm is the method to obtain a deviation Ddata group in signal intensity among ROIs or in a ROI in case where a phase and an amplitude of RF transmission signal are changed two-dimensionally, i.e., a phase and an amplitude are set to parameters, and search such an amplitude and a phase of RF transmission signal that a deviation D becomes minimum based on the deviation Ddata group. Therefore, in the case of using lattice search algorithm, a preparation scan is performed with respectively changing a phase and an amplitude of RF transmission signal. The imaging condition of the preparation scan with changing a phase and an amplitude of RF transmission signal respectively is set in the imaging condition setting unit 40. Changing the phase or the amplitude of RF transmission signal per slice section gradually at a predetermined interval as multi slice imaging leads to shortening of an imaging time, considering a relaxation time and the like. In addition, sharing the preparation scan for acquiring data for extracting the contour of the object P described above and the preparation scan with respectively changing the phase and the amplitude of RF transmission signal into a single scan also leads to shortening of the imaging time.
The amplitude and the phase of RF transmission signal set as an imaging condition in the imaging condition setting unit 40 are output to the amplitude control unit 31A and the phase control unit 31B in the sequence controller 31 through the sequence controller control unit 41 respectively. Thus, the amplitude and the phase of a I signal in a RF transmission signal can be adjusted to the set values respectively under the control of the amplitude-phase balancer 29B by the amplitude control unit 31A and the phase control unit 31B.
In
For example, it is assumed that a preparation scan is performed with changing the ratio of the amplitude Aq of the Q signal to the amplitude Ai of the I signal at a regular interval in the Aqmax:Aimin to Aqmin:Aimax range by controlling the amplitude Ai of the I signal while the phase Φi of the I signal is changed from 90 degrees in the +X to −Y range at a regular interval with keeping the phase Φq of the Q signal 0 degree. Note that, it is reported that 4:1 is limit in Aqmax:Aimin and 1:4 is limit in Aqmin:Aimax respectively. Then, a deviation Ddata(Aq/Ai, Φi) in signal intensity according to the amplitude ratio Aq/Ai between the Q signal and the I signal and the phase Φi of the I signal is obtained on the respective lattice points in the two-dimensional space as shown in
Here, the minimum deviation Ddata_min among the deviations Ddata on the lattice points is searched. Next, the points surrounding the lattice point with the minimum deviation Ddata_min are extracted. Then, it can be regarded that the point showing the minimum deviation Dmin in signal intensity exists in the range surrounded in the surrounding points of the lattice point with the minimum deviation Ddata_min. Accordingly, the minimum deviation Dmin and the corresponding point, i.e., the amplitude ratio Aqopt/Aiopt between the Q signal and the I signal and the phase Φi=90+ΔΦopt of the I signal can be estimated by an interpolating processing using the respective deviations Ddata on the surrounding points of the lattice point with the minimum deviation Ddata_min. The interpolation processing may be arbitrary one such as a linear interpolation, a spline interpolation or a second order interpolation and a user can also select one from these interpolation methods described above. For example, a three-dimensional spline interpolation is practical.
Note that, the minimum deviation Dmin, the amplitude ratio Aqopt/Aiopt between the corresponding Q signal and I signal and the phase Φi=90+ΔΦopt of the I signal can be also obtained by fitting with an arbitrary-order-function using the deviations Ddata on all the lattice points. However, as described above, fitting using only the surrounding points of the lattice point with the minimum deviation Ddata_min in a narrow range can make a closer approximation to improve a calculation accuracy.
On the other hand, bisection method is the method to search such an amplitude and a phase of RF transmission signal that a deviation D becomes minimum by repeating determination of which of deviations D1 and D2 in signal intensities, between ROIs or in a ROI, of two pieces of image data acquired with changing either an amplitude or a phase of RF transmission signal is smaller and acquisition of two pieces of image data with changing either the amplitude or the phase of the RF transmission signal into values near a value corresponding to the smaller deviation again. Consequently, in the case of using bisection method, a preparation scan for acquiring image data with changing either a phase or an amplitude of RF transmission signal and the processing to determine which of the deviations D1 and D2 in signal intensities between ROIs of two pieces of image data is smaller are performed repeatedly.
In
Then, with setting the amplitude of RF transmission signal into two amplitudes equivalent to the two points (3A1#A2)/4 and (A1+3A2)/4 in both sides of the midpoint (A1+A2)/2 by a preparation scan, image data is acquired. Next, respective deviations D((3A1+A2)/4) and D((A1+3A2)/4) in signal intensities of pieces of image data corresponding to the two points (3A1+A2)/4 and (A1+3A2)/4 respectively are calculated. Next, the deviation D((3A1+A2)/4) is compared with the deviation D((A1+3A2)/4) to specify which deviation D(A) is smaller. Then, it can be thought that the amplitude Aopt corresponding to the minimum deviation Dmin exists in the smaller deviation D(A) side of the midpoint (A1+A2)/2 of the two base points A1 and A2.
For that reason, acquisition of pieces of image data corresponding to two different amplitudes and comparison determination processing described above are performed with setting the two points A1 or A2 and (A1+A2)/2 of both sides, between which the point (3A1+A2)/4 or (A1+3A2)/4 corresponding to the value of the smaller deviation D(A) is a midpoint, as new basic points. The minimum deviation Dmin and the corresponding amplitude Aopt can be obtained by repeating the image data acquisition and the comparison determination processing mentioned above.
Note that, when the search for the amplitude progresses to a certain extent, it is preferable that the search for the phase is performed by a similar method. The phase Φopt and the amplitude Aopt of RF transmission signal to minimize the deviation D can be obtained by repeating the searches for the amplitude and the phase mutually with reflecting the search results. For example, in this case, it leads to reduction of an imaging period to prepare plural sets each consisting of a sequence for multi slice imaging that transmits RF transmission signals having mutual different amplitudes to acquire two pieces of slice image data and a sequence for multi slice imaging that transmits RF transmission signals having mutual different phases to acquire two pieces of slice image data, perform calculation and comparison determination processing of the deviation D in intervals between multi slice imagings and reflect the processing result on the subsequent multi slice imaging. It is thought that the minimum deviation Dmin and the corresponding amplitude Aopt and phase Φopt can be obtained with a practicable accuracy when the multi slice imaging is repeated approximate four times respectively for the amplitude and the phase.
Next, the method that obtains the amplitude and the phase of RF transmission signal so as to form more homogeneous B1 distribution and sets the obtained amplitude and phase as an imaging condition will be described.
In this case, a B1 distribution (RF magnetic field distribution) is acquired in advance by a preparation scan with respectively changing the phase and the amplitude of RF transmission signal. The B1 distribution can be acquired before installation of the apparatus or each imaging by a known method. For example, B1 distribution component can be extracted by removing components due to influences other than the B1 distribution with a calculation between respective signals acquired by applying a 30-degree RF pulse and a 60-degree RF pulse. When the B1 distribution is obtained, the object P may be a phantom. However, it is preferable that the object P is a human body in view of accuracy. The B1 distribution varies depending on a body shape including a weight and a height of an object P, RF coils 24 used for transmitting a RF transmission signal and an imaging region. For that reason, it is preferable in view of accuracy to obtain B1 distributions for respective body shapes of objects P, respective RF coils 24 to be used and respective imaging regions.
The B1 distribution obtained as mentioned above has a characteristic similar to that of a signal intensity distribution. Then, the optimum phase Φopt and amplitude Aopt of RF transmission signal can be obtained based on the B1 distribution. That is, the amplitude and the phase of RF transmission signal to obtain more homogeneous B1 distribution corresponding to an imaging condition including a body shape of an object P can be set as an imaging condition.
Thus far, the method to obtain the phase Φopt and the amplitude Aopt of RF transmission signal by which the deviation in signal intensity becomes minimum based on data acquired by a preparation scan and the method to obtain the phase Φopt and the amplitude Aopt of RF transmission signal so as to obtain the most homogeneous B1 distribution are described. However, the optimum phases Φopt and amplitudes Aopt of RF transmission signal, which reduce an influence of B1 inhomogeneity, previously obtained corresponding to respective imaging conditions such as characteristics of objects P, imaging parts and/or characteristics of RF coils 24 may be stored in the computer 32 to form a database so that an appropriate phase Φopt and an appropriate amplitude Aopt of RF transmission signal can be selected and used according to an imaging condition
For that purpose, the optimum phases Φopt and amplitudes Aopt of RF transmission signal according to imaging conditions such as characteristics of objects P, imaging parts and/or characteristics of RF coils 24 are stored in the data amplitude-phase balance database 46 in the computer 32. The characteristics of an object P include a body shape such as a weight and a height of the object P, a volume, a distribution of water, a distribution of fat, a distribution of muscle, a body frame and an age. The characteristics of a RF coil 24 include the number of surface coils used for transmission of a RF signal, a size, a shape and a type.
The phase Φopt and the amplitude Aopt of RF transmission signal for each imaging condition obtained in the amplitude-phase setting unit 40B by the method described above with performing a preparation scan in the past can be written and stored in the data amplitude-phase balance database 46. In this case, an appropriate phase Φopt and an appropriate amplitude Aopt of RF transmission signal vary into different values for each apparatus. Alternatively, without the above-described method with a preparation scan, an appropriate phase Φopt and amplitude Aopt of RF transmission signal for each imaging condition estimated or obtained in advance by an another method can be also stored in the data amplitude-phase balance database 46 before installation of the apparatus. In this case, the phase Φopt and the amplitude Aopt of RF transmission signal can be also set to values according to a type of the magnetic resonance imaging apparatus 20. However, an appropriate phase Φopt and an appropriate amplitude Aopt of RF transmission signal may be also obtained every imaging and stored in the data amplitude-phase balance database 46 from the perspective of accuracy improvement.
The amplitude-phase setting unit 40B is configured to refer to the data amplitude-phase balance database 46, read the phase Φopt and the amplitude Aopt of RF transmission signal corresponding to an imaging condition such as a characteristic of an object P, an imaging part and a characteristic of the RF coil 24 input from the input device 33 from the data amplitude-phase balance database 46 and set the read phase Φopt and amplitude Aopt as an imaging condition for an imaging scan.
Note that, instead of the phase Φopt and the amplitude Aopt of RF transmission signal, a function or a table representing B1 distributions corresponding to respective imaging conditions may be stored in the data amplitude-phase balance database 46 and the amplitude-phase setting unit 40B may be configured to set the optimum phase Φopt and amplitude Aopt of RF transmission signal based on the B1 distribution corresponding to the imaging condition.
Then, other functions of the computer 32 will be described.
The sequence controller control unit 41 has a function for controlling the driving of the sequence controller 31 by giving an imaging condition including a pulse sequence, acquired from the imaging condition setting unit 40, to the sequence controller 31 based on information from the input device 33 or another element. In addition, the sequence controller control unit 41 has a function for receiving raw data from the sequence controller 31 and arranging the raw data to k space formed in the k-space database 42. Therefore, the k-space database 42 stores the raw data generated by the receiver 30 as k space data.
The image reconstruction unit 43 has a function for reconstructing image data of the object P by capturing the k-space data from the k-space database 42 and performing image reconstruction processing including FT (Fourier transformation) of the k-space data, and writing the generated image data to the image database 44. Therefore, the image database 44 stores the image data reconstructed in the image reconstruction unit 43.
The image processing unit 45 has a function for generating two-dimensional image data for displaying by performing necessary image processing to image data read form the image database 44 and displaying the generated image data on the display unit 34.
Then, the operation and action of a magnetic resonance imaging apparatus 20 will be described.
Here, a case of setting plural ROIs and determining an amplitude and a phase of RF transmission signal so that a deviation in signal intensity between the ROIs becomes minimum will be described. Also in a case where an amplitude and a phase of RF transmission signal are determined so that a deviation in signal intensity in a single ROI becomes minimum or a homogeneous B1 distribution is obtained, imaging can be performed by a similar flow.
First, in step S1, plural ROIs are created manually or automatically in the ROI setting unit 40A in the computer 32. In the case of creating the ROIs automatically, a preparation scan is performed and image data is obtained by a signal processing and an image reconstruction processing described below. Then, a region for setting the ROIs is extracted with a threshold processing of signal intensities of the image data.
Next, in Step S2, the data amplitude-phase balance database 46 is referred to by the amplitude-phase setting unit 40B and it is determined whether an appropriate phase and an appropriate amplitude of RS transmission signal corresponding to the imaging condition such as a characteristic of the object P and the ROIs are stored in the data amplitude-phase balance database 46 or not.
When an appropriate phase and an appropriate amplitude of RF transmission signal are not stored in the data amplitude-phase balance database 46, a preparation scan with changing an amplitude and a phase of RF transmission signal is performed in Step S3. Note that, when an instruction to calculate an appropriate phase and an appropriate amplitude of RF transmission signal with a preparation scan is input to the amplitude-phase setting unit 40B from the input device 33, the preparation scan is performed in Step S3 even where an appropriate phase and an appropriate amplitude of RF transmission signal are stored in the data amplitude-phase balance database 46.
For that purpose, an imaging condition for the preparation scan with changing an amplitude and a phase of a RF transmission signal is set by the imaging condition setting unit 40. Then, data acquisition is performed according to the set imaging condition.
That is, the object P is set to the bed 37 in advance, and a static magnetic field is generated at an imaging area of the magnet 21 (a superconducting magnet) for static magnetic field excited by the static-magnetic-field power supply 26. Further, the shim-coil power supply 28 supplies current to the shim coil 22, thereby uniformizing the static magnetic field generated at the imaging area.
Then, the input device 33 sends instruction of starting the preparation scan to the sequence controller control unit 41. The sequence controller control unit 41 supplies the imaging condition with changing an amplitude and a phase of a RF transmission signal received from the imaging condition setting unit 40 to the sequence controller 31. Therefore, the sequence controller 31 drives the gradient power supply 27, the transmitter 29, and the receiver 30 in accordance with the imaging condition received from the sequence controller control unit 41, thereby generating a gradient magnetic field at the imaging area having the set object P, and further generating RF signals from the RF coil 24.
Specifically, a RF signal generated in the signal generator 31C in the sequence controller 31 is output to the 90-degree signal distributor 29A in the transmitter 29 and distributed to an I signal and a Q signal in the 90-degree signal distributor 29A. An amplitude and a phase of the I signal are adjusted in the amplitude-phase balancer 29S according to the control signals from the amplitude control unit 31A and the phase control unit 31B. Then, the I signal and the Q signal are output to the corresponding coil elements 24A and 24B from the I signal transmission/reception channel 38B and the Q signal transmission/reception channel 38C through the transmission pre-amplifiers 29C, the power amplifiers 29D, the isolators 29E and the transmission/reception selector switches 38A respectively. Consequently, a RF transmission signal with an adjusted amplitude and phase are transmitted toward the object P from the coil elements 24A and 24B.
Consequently, the coil elements 24A and 24B of the RF coil 24 receive NMR signals generated due to nuclear magnetic resonance in the object P. Then, the receiver 30 receives the NMR signals from the RF coil 24 via the transmission/reception selector switch 38A, the 90-degree phase compositor 38D and the a reception pre-amplifier 38F, and generates raw data which is digital data of NMR signals by A/D conversion subsequently to necessary signal processing. The receiver 30 supplies the generated raw data to the sequence controller 31. The sequence controller 31 supplies the raw data to the sequence controller control unit 41. The sequence controller control unit 41 arranges the raw data as k-space data to the k space formed in the k-space database 42.
Subsequently, the image reconstruction unit 44 reads the k-space data from the k-space database 42 and reconstructs image data. The generated image data is written in the image database 44. Consequently, pieces of image data corresponding to the mutually different amplitudes and phases of the RF transmission signal with regard to each of the ROIs are stored in the image database 44.
Next, in step S4, the amplitude-phase setting unit 40B in the computer 32 obtains a deviation in representative value of signal intensities between the ROIs as an index indicating a variation in signal intensity of the image data between the ROIs according to a predetermined definition. Then, such an amplitude and a phase of RF transmission signal that the deviation of the plural pieces of image data corresponding to mutually different amplitudes and phases of RF transmission signal becomes minimum are calculated by an arbitrary search method based on the deviation.
Next, in step S5, the amplitude-phase setting unit 40B sets the calculated amplitude and phase of RF transmission signal as an imaging condition for an imaging scan.
On the other hand, when it is determined that the appropriate phase and amplitude of RF transmission signal corresponding to the imaging condition such as the characteristic of the object P and the ROIs are stored in the data amplitude-phase balance database 46 in step S2, the amplitude-phase setting unit 40B sets the corresponding appropriate phase and amplitude of RF transmission signal as the imaging condition for the imaging scan in step S6.
Subsequently, in step S7, an imaging scan is performed with the phase and amplitude of the RF transmission signal set as the imaging condition. The flow of the imaging scan is similarly to that of the preparation scan.
Next, in step S8, an image reconstruction processing is performed. Consequently, the image data, acquired by the imaging scan, on which an influence of B1 inhomogeneity is reduced by B1 shimming with appropriate adjustment of the phase and amplitude of RF transmission signal is stored in the image database 44. Then, the image processing unit 45 reads the image data form the image database 44 and performs necessary image processing, thereby generating two-dimensional image data for display. The generated two-dimensional image data is displayed on the display unit 34.
That is, the magnetic resonance imaging apparatus 20 mentioned above is an apparatus to change an amplitude and/or a phase of a RF transmission signal transmitted from the RF coil 24 for reducing an influence of B1 inhomogeneity. Especially, it is possible to set a desired single ROI or desired plural ROIs in the magnetic resonance imaging apparatus 20. Then, for example, an amplitude and/or a phase of a RF transmission signal are determined so that a variation in signal intensity of data in a ROI and/or between the ROIs is reduced. Alternatively, an amplitude and/or a phase of a RF transmission signal are determined so that a more homogeneous B1 distribution is obtained.
Consequently, according to the magnetic resonance imaging apparatus 20, flexibility for setting a region to be a correction target of a variation in signal intensity can be improved and operability regarding a B1 shimming target region can be improved. In addition, the optimum amplitude and phase of RF transmission signal adaptively to an imaging condition such as a characteristic of an object P are obtained easily and B1 shimming can be performed. Consequently, it is possible to transmit NMR signals to the data processing system from the RF coil 24 without deteriorating the NMR signals, and therefore, B1 shimming function can be implemented to the apparatus.
Note that, in the embodiment described above, a deviation in signal intensity of pieces of image data corresponding to respective ROIs is used as an indicator of variation. However, a deviation in signal intensity of pieces of k-space data corresponding to respective ROIs also may be used as an indicator of variation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-167784 | Jun 2008 | JP | national |
| 2009-106627 | Apr 2009 | JP | national |
