JP2016127952A - Magnet resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnet resonance imaging apparatus which can prevent image deterioration caused by phase gap of echo signals.SOLUTION: A magnet resonance imaging apparatus includes an execution part, a calculation part, and a correction part. The execution part executes pre-scanning for impressing a gradient magnet field for read out and a gradient magnet field in a slice direction like a pulse sequence for main scanning. The calculation part calculates a phase correction amount for correcting a phase difference in a plurality of echo signals collected in the main scanning by using data confined within a prescribed range in a plurality of echo signals collected in the pre-scanning. The correction part corrects the pulse sequence for the main scanning on the basis of the correction amount calculated by the calculation part.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to a magnetic resonance imaging apparatus.

  Conventionally, as an imaging method related to a magnetic resonance imaging apparatus, there is a fast spin echo (FSE) method. This FSE method is an imaging method that collects a plurality of echo signals called echo trains by sequentially applying a plurality of flop pulses after applying a flip pulse to a subject. Here, the flip pulse is an RF (Radio Frequency) pulse for exciting the nuclear spin in the subject. The flop pulse is an RF pulse for refocusing the phase of the nuclear spin.

  In the FSE method, a plurality of RF pulses are applied, and thus a stimulated echo is generated together with the spin echo. In some cases, the stimulated echo causes a phase shift in the collected echo signal. Such a phase shift of the echo signal causes image quality degradation such as uneven sensitivity, signal degradation, and ghost.

  In order to prevent such image quality degradation, in general, a pre-scan for measuring the phase difference generated in the echo signal is performed before the main scan, and the main scan is performed based on the phase difference measured by the pre-scan. The pulse sequence is corrected. In this case, for example, in the pre-scan, a pulse sequence for canceling the stimulated echo is executed, and only the spin echo is collected. The first and second echo signals of the spin echoes collected by the pre-scan are Fourier-transformed in the readout direction, and the 0th and 1st order echo signals between the 1st echo signal and the 2nd echo signal are obtained. A phase difference is calculated. Thereafter, a correction amount for correcting the phase shift in the readout direction and the slice selection direction is calculated from the calculated zeroth-order and first-order phase differences, and the pulse sequence for the main scan is calculated based on the calculated correction amount. Be changed.

  In the above technique, the phase shift generated in the phase encoding direction due to the influence of the eddy current caused by the phase encoding gradient magnetic field cannot be corrected, and the image quality deterioration may occur due to the phase shift.

US Pat. No. 6,369,568

  The problem to be solved by the present invention is to provide a magnetic resonance imaging apparatus that can prevent image degradation caused by a phase shift of an echo signal.

  The magnetic resonance imaging apparatus according to the embodiment includes an execution unit, a calculation unit, and a correction unit. The execution unit executes a pre-scan that applies the readout gradient magnetic field and the slice direction gradient magnetic field in the same manner as the pulse sequence for the main scan. The calculation unit corrects a phase difference in the plurality of echo signals collected in the main scan using data limited to a predetermined range among the plurality of echo signals collected by the pre-scan. A correction amount is calculated. The correction unit corrects the pulse sequence for the main scan based on the correction amount calculated by the calculation unit.

FIG. 1 is a diagram illustrating a configuration of the MRI apparatus according to the first embodiment. FIG. 2 is a functional block diagram showing a detailed configuration of the computer system shown in FIG. FIG. 3 is a diagram showing a pulse sequence for the main scan according to the first embodiment. FIG. 4 is a diagram showing a pre-scanning pulse sequence according to the first embodiment. FIG. 5 is a diagram (1) for explaining the correction of the phase shift performed by the sequence correction unit according to the first embodiment. FIG. 6 is a diagram (2) for explaining the phase shift correction performed by the sequence correction unit according to the first embodiment. FIG. 7 is a flowchart illustrating a processing procedure for phase shift correction performed by the MRI apparatus according to the first embodiment. FIG. 8 is a diagram for explaining a modification of the first embodiment. FIG. 9A is a diagram illustrating an example of an image obtained by a conventional pre-scan. FIG. 9B is a diagram illustrating an example of an image obtained by the pre-scan in the first embodiment. FIG. 10 is a diagram showing a second pulse sequence for the second prescan according to the second embodiment. FIG. 11 is a diagram for explaining the calculation of the phase shift performed by the sequence correction unit according to the second embodiment. FIG. 12 is a diagram for explaining a modification of the first and second embodiments. FIG. 13 is a diagram showing a pre-scanning pulse sequence according to the third embodiment. FIG. 14 is a diagram for explaining the collection of echo signals according to the third embodiment. FIG. 15 is a diagram showing a second pulse sequence for the second pre-scan according to the fourth embodiment. FIG. 16 is a diagram for explaining an MRI apparatus according to the fifth embodiment. FIG. 17 is a diagram for explaining an MRI apparatus according to the sixth embodiment. FIG. 18 is a diagram for explaining the MRI apparatus according to the seventh embodiment.

  Hereinafter, a magnetic resonance imaging apparatus according to an embodiment will be described in detail based on the drawings. In the embodiment described below, the magnetic resonance imaging apparatus is referred to as an MRI (Magnetic Resonance Imaging) apparatus.

(First embodiment)
FIG. 1 is a diagram illustrating a configuration of the MRI apparatus according to the first embodiment. As shown in FIG. 1, this MRI apparatus 100 includes a static magnetic field magnet 1, a gradient magnetic field coil 2, a gradient magnetic field power source 3, a bed 4, a bed control unit 5, a transmission RF coil 6, a transmission unit 7, a reception RF coil 8, A receiving unit 9, a sequence control unit 10, and a computer system 20 are provided.

  The static magnetic field magnet 1 is a magnet formed in a hollow cylindrical shape, and generates a uniform static magnetic field in an internal space. As the static magnetic field magnet 1, for example, a permanent magnet, a superconducting magnet or the like is used.

  The gradient magnetic field coil 2 is a coil formed in a hollow cylindrical shape, and is disposed inside the static magnetic field magnet 1. The gradient magnetic field coil 2 is formed by combining three coils corresponding to the X, Y, and Z axes orthogonal to each other, and these three coils are individually supplied with current from a gradient magnetic field power source 3 to be described later. In response, a gradient magnetic field whose magnetic field strength changes along the X, Y, and Z axes is generated. The Z-axis direction is the same direction as the static magnetic field. The gradient magnetic field power supply 3 supplies a current to the gradient magnetic field coil 2.

  Here, the gradient magnetic fields of the X, Y, and Z axes generated by the gradient magnetic field coil 2 correspond to, for example, the slice selection gradient magnetic field Gs, the phase encoding gradient magnetic field Ge, and the readout gradient magnetic field Gr, respectively. The slice selection gradient magnetic field Gs is used to arbitrarily determine an imaging section. The phase encoding gradient magnetic field Ge is used to change the phase of the magnetic resonance signal in accordance with the spatial position. The readout gradient magnetic field Gr is used for changing the frequency of the magnetic resonance signal in accordance with the spatial position.

  The couch 4 includes a couchtop 4a on which the subject P is placed. Under the control of a couch controller 5 described later, the couchtop 4a is placed in the cavity of the gradient magnetic field coil 2 with the subject P placed thereon. Insert into (imaging port). Usually, the bed 4 is installed such that the longitudinal direction is parallel to the central axis of the static magnetic field magnet 1. The couch controller 5 is a device that controls the couch 4 under the control of the controller 26, and drives the couch 4 to move the couchtop 4a in the longitudinal direction and the vertical direction.

  The transmission RF coil 6 is arranged inside the gradient magnetic field coil 2 and receives a high frequency pulse from the transmission unit 7 to generate a high frequency magnetic field. The transmission unit 7 transmits a high-frequency pulse corresponding to the Larmor frequency to the transmission RF coil 6. The reception RF coil 8 is disposed inside the gradient coil 2 and receives a magnetic resonance signal radiated from the subject P due to the influence of the high-frequency magnetic field. When receiving the magnetic resonance signal, the reception RF coil 8 outputs the magnetic resonance signal to the receiving unit 9.

  The receiving unit 9 generates magnetic resonance (MR) signal data based on the magnetic resonance signal output from the reception RF coil 8. The receiver 9 generates MR signal data by digitally converting the magnetic resonance signal output from the reception RF coil 8. The MR signal data is associated with spatial frequency information in the phase encoding direction, the readout direction, and the slice encoding direction by the above-described slice selection gradient magnetic field Gs, phase encoding gradient magnetic field Ge, and readout gradient magnetic field Gr. And placed in k-space. When the MR signal data is generated, the receiving unit 9 transmits the MR signal data to the sequence control unit 10.

  The sequence control unit 10 scans the subject P by driving the gradient magnetic field power source 3, the transmission unit 7, and the reception unit 9 based on the sequence execution data transmitted from the computer system 20. Here, the sequence execution data includes the strength of the power supplied from the gradient magnetic field power supply 3 to the gradient magnetic field coil 2 and the timing of supplying the power, the strength of the RF signal transmitted from the transmitter 7 to the transmission RF coil 6 and the RF signal. This is information defining a pulse sequence indicating a procedure for performing a scan of the subject P, such as a transmission timing and a timing at which the receiving unit 9 detects a magnetic resonance signal. When the MR signal data is transmitted from the receiving unit 9 after driving the gradient magnetic field power source 3, the transmitting unit 7, and the receiving unit 9 based on the sequence execution data, the sequence control unit 10 calculates the MR signal data. Transfer to system 20.

  The computer system 20 performs overall control of the MRI apparatus 100. For example, the computer system 20 performs scanning of the subject P, image reconstruction, and the like by driving each unit included in the MRI apparatus 100. The computer system 20 includes an interface unit 21, an image reconstruction unit 22, a storage unit 23, an input unit 24, a display unit 25, and a control unit 26.

  The interface unit 21 controls input / output of various signals exchanged with the sequence control unit 10. For example, the interface unit 21 transmits sequence execution data to the sequence control unit 10 and receives MR signal data from the sequence control unit 10. When receiving MR signal data, the interface unit 21 stores each MR signal data in the storage unit 23 for each subject P.

  The image reconstruction unit 22 performs post-processing, that is, reconstruction processing such as Fourier transform, on the MR signal data stored in the storage unit 23, thereby obtaining spectrum data or images of desired nuclear spins in the subject P. Generate data.

  The storage unit 23 stores various data and various programs necessary for processing executed by the control unit 26 described later. For example, the storage unit 23 stores the MR signal data received by the interface unit 21 and the spectrum data and image data generated by the image reconstruction unit 22 for each subject P.

  The input unit 24 receives various instructions and information input from the operator. As the input unit 24, a pointing device such as a mouse or a trackball, a selection device such as a mode change switch, or an input device such as a keyboard can be used as appropriate.

  The display unit 25 displays various types of information such as spectrum data or image data under the control of the control unit 26. As the display unit 25, a display device such as a liquid crystal display can be used.

  The control unit 26 includes a CPU (Central Processing Unit), a memory, and the like (not shown), and performs overall control of the MRI apparatus 100. For example, the control unit 26 generates various sequence execution data based on the imaging conditions input from the operator via the input unit 24 and transmits the generated sequence execution data to the sequence control unit 10 to perform scanning. To control. In addition, when MR signal data is sent from the sequence control unit 10 as a result of scanning, the control unit 26 controls the image reconstruction unit 22 to reconstruct an image based on the MR signal data.

  The configuration of the MRI apparatus 100 according to the first embodiment has been described above. Under such a configuration, the MRI apparatus 100 performs pre-scanning. Here, in the pre-scan, the readout gradient magnetic field and slice-direction gradient magnetic field are applied in the same manner as the pulse sequence for the main scan, and the phase is the same as in the pulse sequence for the main scan until the echo for calculating the correction amount. An encoding gradient magnetic field is applied. Then, the MRI apparatus 100 calculates the amount of phase shift as the correction amount from the phase difference in the plurality of echo signals collected by the prescan. The MRI apparatus 100 corrects the pulse sequence for the main scan based on the correction amount calculated by the correction amount calculation unit 26c described later.

  That is, the MRI apparatus 100 according to the first embodiment performs pre-scan that applies a phase encoding gradient magnetic field, and based on the phase shift between the echo signals obtained by the pre-scan, Calculate the correction amount of the pulse sequence. Therefore, according to the first embodiment, in addition to the shift in the readout direction and the slice direction, it is possible to prevent image degradation caused by the 0th-order phase shift due to the phase encoding gradient magnetic field. Hereinafter, functions of the MRI apparatus 100 will be described in detail.

  FIG. 2 is a functional block diagram showing a detailed configuration of the computer system 20 shown in FIG. FIG. 2 shows the interface unit 21, the storage unit 23, and the control unit 26 among the functional units included in the computer system 20.

  The storage unit 23 includes a sequence execution data storage unit 23a and an MR signal data storage unit 23b. The sequence execution data storage unit 23a stores sequence execution data generated by an imaging condition setting unit 26a described later. The MR signal data storage unit 23b stores the MR signal data received by the interface unit 21.

  The control unit 26 includes an imaging condition setting unit 26a, a pre-scan execution unit 26b, a correction amount calculation unit 26c, a sequence correction unit 26d, and a main scan execution unit 26e.

  The imaging condition setting unit 26a generates sequence execution data defining a pulse sequence used for imaging based on the imaging condition input by the operator via the input unit 24. For example, when an FSE imaging condition is input by the operator, the imaging condition setting unit 26a generates sequence execution data for each of the pulse sequence for main scanning and the pulse sequence for prescan described below. To do.

  FIG. 3 is a diagram showing a pulse sequence for the main scan according to the first embodiment. In FIG. 3, “RF” indicates a timing at which an excitation flip pulse and a refocus flop pulse are applied. “Gss” indicates the application timing and intensity of the slice selection gradient magnetic field, “Gro” indicates the application timing and intensity of the readout gradient magnetic field, and “Gpe” indicates the phase encoding gradient magnetic field. The application timing and intensity are shown. In FIG. 3, only a pulse sequence related to one slice selection is shown, and illustration of slice encoding is omitted. “ETS (Echo Train Spacing)” indicates an echo interval.

  As shown in FIG. 3, the pulse sequence for the main scan is a general FSE method pulse sequence. As shown in FIG. 3, the pulse sequence for the main scan includes a plurality of flop pulses flo1, flo2,... Flo9, flo10, flo11,. Echo signals Echo1, Echo2,... Echo9, Echo10, Echo11. Note that the pulse sequence shown in FIG. 3 is an example when the gradient signal for phase encoding is set to zero by the echo signal Echo10 collected tenth.

  FIG. 4 is a diagram showing a pre-scanning pulse sequence according to the first embodiment. The pre-scanning pulse sequence is similar to the main-scanning pulse sequence shown in FIG. 3 by applying the readout gradient magnetic field and slice-direction gradient magnetic field, and before the echo for calculating the correction amount, for the main scanning. Similar to the pulse sequence, a phase encoding gradient magnetic field is applied.

  For example, as shown in FIG. 4, the pre-scan pulse sequence is applied in the main scan pulse sequence by applying the readout gradient magnetic field and the slice selection gradient magnetic field in the same manner as the main scan pulse sequence. At least one of the phase encoding gradient magnetic fields is applied up to an echo near the center of the k-space. In other words, this pulse sequence is obtained by applying a phase encoding gradient magnetic field to the vicinity of the contrast TE in the main scanning pulse sequence and removing the phase encoding gradient magnetic field after the contrast TE. FIG. 4 shows an example in which Echo 10 corresponds to the center of k-space.

  For example, the pre-scan pulse sequence is applied with a readout gradient magnetic field and a slice selection gradient magnetic field in the same manner as the main scan pulse sequence, and among the plurality of phase encodings used in the main scan pulse sequence. A gradient magnetic field for phase encoding in the vicinity of the average intensity is applied.

  Further, in this pulse sequence, the stimulated echo is canceled and only the spin echo is collected. For example, the method described in US Pat. No. 5,818,229 can be used. In this method, the echo signal of the first shot collected while changing the phase of the flop pulse for refocusing to π, π, π, π..., Π, −π, π, −π. By adding the echo signals of the second shot collected while changing, only the spin echo component is extracted. Alternatively, only the stimulated echo component may be extracted by subtracting the echo signal of the second shot from the echo signal of the first shot, and the extracted stimulated echo component may be used instead of the spin echo component.

  Here, the pulse sequence for pre-scan requires, for example, 256/19 = 13 shots to fill the k-space when the phase encoding Matrix is 256 and 19 echoes are collected. In this case, the pre-scanning pulse sequence includes a phase encoding gradient magnetic field applied in a shot having an average phase encoding gradient magnetic field intensity (or a central shot (7 shots when the phase encoding is sequentially filled)). It is set to apply.

  Returning to the description of FIG. 2, the pre-scan execution unit 26 b executes a pre-scan pulse sequence.

  Specifically, when pre-scan sequence execution data is generated by the imaging condition setting unit 26a, the pre-scan execution unit 26b first reads the sequence execution data from the sequence execution data storage unit 23a. The prescan execution unit 26b transmits the read sequence execution data to the sequence control unit 10 via the interface unit 21, thereby executing prescan. For example, the prescan execution unit 26b executes prescan by transmitting sequence execution data defining the pulse sequence shown in FIG. 4 to the sequence control unit 10.

  The correction amount calculation unit 26c calculates the amount of phase shift as the correction amount from the phase differences in the plurality of echo signals collected by the prescan. The correction amount calculation unit 26c calculates a phase shift (0th order and 1st order) caused by a readout gradient magnetic field, a slice selection gradient magnetic field, and a phase encoding gradient from phase differences in a plurality of echo signals collected by pre-scanning. The amount of zero-order phase shift caused by the magnetic field is calculated as a correction amount.

  Specifically, the correction amount calculation unit 26c reads MR signal data related to echo signals collected by the prescan from the MR signal data storage unit 23b after the prescan is executed by the prescan execution unit 26b. Thereafter, the correction amount calculation unit 26c calculates a first-order phase difference after Fourier-transforming each read MR signal data in the readout direction. This primary phase difference is the primary phase difference of the readout gradient magnetic field.

  The phase difference for each echo generated by the FSE method includes a linear phase difference that is a linear function of the location and a phase difference that does not depend on location caused by a mismatch in coil arrangement. A phase difference having no place dependency is called a zeroth-order phase difference. A zero-order phase difference is calculated by correcting the first-order phase calculated for each MR signal data and obtaining a phase average. This 0th-order phase difference includes the 0th-order phase difference of the gradient magnetic field for readout in addition to the 0th-order phase difference of the gradient magnetic field for readout and the 0th-order and 1st-order phase differences of the gradient magnetic field for slice selection. The effect of.

  Here, the correction amount calculation unit 26c calculates the first-order and zero-order phase differences in the echo signals collected by the prescan. This difference becomes a phase shift caused by the readout gradient magnetic field, the slice selection gradient magnetic field, and the phase encoding gradient magnetic field.

  For example, assume that the pulse sequence shown in FIG. 4 is executed. In that case, the correction amount calculation unit 26c calculates the phase difference p1 between the tenth echo signal Echo10 and the eleventh echo signal Echo11 shown in FIG. Then, the correction amount calculation unit 26c calculates the phase shift p1 as the correction amount from the calculated phase difference.

  Here, although the correction amount calculation unit 26c uses the tenth echo signal and the eleventh echo signal for calculation of the correction amount, a plurality of 12th and subsequent echoes may be used. For example, the correction amount calculation unit 26c sets the 10th and 11th phase differences of the pre-scan pulse sequence as p1, the 12th and 11th phase differences as p1_2, and the 12th and 13th phase differences as p1_3. The phase shift generated in the phase encoding direction by the encoding gradient magnetic field may be (p1 + p1_2 + p1_3) / 3.

  The sequence correction unit 26d corrects the pulse sequence for the main scan based on the phase shift calculated by the correction amount calculation unit 26c. Specifically, when the phase shift is calculated by the correction amount calculation unit 26c, the sequence correction unit 26d performs the main scan sequence stored in the sequence execution data storage unit 23a based on the calculated phase shift. Correct the execution data.

  Here, the sequence correction unit 26d corrects the pulse sequence for the main scan so that the phase shift calculated by the correction amount calculation unit 26c becomes zero. At this time, for example, with respect to the primary phase difference, the sequence correction unit 26d changes the pulse sequence for the main scan so that the correction gradient magnetic field is applied between the flip pulse and the flop pulse. The sequence correction unit 26d may add a correction gradient magnetic field before and after the readout gradient magnetic field in the pulse sequence for the main scan. Further, for example, the sequence correction unit 26d changes the phase of the flop pulse so that the phase shift becomes zero with respect to the zero-order phase difference.

  In this way, the sequence correction unit 26d corrects the pulse sequence for main scanning so that the phase shift observed in the pre-scan becomes zero, whereby the 0th order of the readout gradient magnetic field and the slice selection gradient magnetic field. An image that is not affected by the first-order eddy current and the zero-order eddy current of the phase encoding gradient magnetic field can be obtained.

  5 and 6 are diagrams for explaining the correction of the phase shift performed by the sequence correction unit 26d. FIG. 5 shows an example of the phase shift in the phase encoding direction. In FIG. 5, a solid line 61 represents the zero-order phase shift in the phase encoding direction for each echo signal, and a broken line 62 represents the intensity of the phase encoding gradient magnetic field. FIG. 6 shows a k-space in which the echo signals shown in FIG. 5 are arranged. In FIG. 6, the horizontal axis represents the phase encoding direction.

  Here, the zero-order phase shift in the phase encoding direction depends on the arrangement of echo signals in the k space. The example illustrated in FIG. 5 illustrates a case where the collected echo signals are sequentially arranged in the phase encoding direction with respect to the k space. The numbers shown in FIG. 6 represent the order of the collected echo signals. When the pre-scan is performed using the pulse sequence shown in FIG. 4, the position corresponding to the echo signal Echo10 collected tenth (the position of the arrow shown in FIG. 6), that is, near the center of the k space. The zero-order phase shift of the arranged echo signal is corrected. Here, a case where correction is performed with reference to the echo signal Echo10 collected tenth will be described, but the reference echo signal may be an arbitrary echo signal.

  The main scan execution unit 26e executes the main scan using the pulse sequence for main scan corrected by the sequence correction unit 26d. Specifically, when the sequence execution data for main scan is corrected by the sequence correction unit 26d, the pre-scan execution unit 26b reads the corrected sequence execution data from the sequence execution data storage unit 23a. Then, the main scan execution unit 26e transmits the read sequence execution data to the sequence control unit 10 via the interface unit 21, thereby executing the main scan.

  Next, a processing procedure for phase shift correction performed by the MRI apparatus 100 according to the first embodiment will be described. FIG. 7 is a flowchart illustrating a processing procedure for phase shift correction performed by the MRI apparatus 100 according to the first embodiment.

  As shown in FIG. 7, in the MRI apparatus 100 according to the first embodiment, when the operator gives an instruction to start imaging (Yes in step S <b> 101), the imaging condition setting unit 26 a uses the input unit 24. An input of imaging conditions is received from the operator (step S102).

  Subsequently, the imaging condition setting unit 26a generates main scan and pre-scan sequence execution data based on the imaging conditions input by the operator (step S103).

  For example, the imaging condition setting unit 26a generates sequence execution data defining the pulse sequence shown in FIG. 3 as sequence execution data for the main scan. For example, the imaging condition setting unit 26a generates sequence execution data defining the pulse sequence illustrated in FIG.

  Subsequently, the prescan execution unit 26b executes prescan based on the sequence execution data of the pulse sequence generated by the imaging condition setting unit 26a (step S104).

  Subsequently, the correction amount calculation unit 26c calculates the amount of phase shift as the correction amount from the phase differences in the plurality of echo signals collected by each prescan (step S106). Thereafter, the sequence correction unit 26d corrects the sequence execution data for the main scan based on the correction amount calculated by the correction amount calculation unit 26c (step S106).

  Subsequently, the main scan execution unit 26e executes the main scan based on the sequence execution data for main scan corrected by the sequence correction unit 26d (step S107). Then, the image reconstruction unit 22 reconstructs an image from the MR signal data collected by the main scan (step S108).

  As described above, in the first embodiment, the pre-scan execution unit 26b executes pre-scan. Here, in the pre-scan, the readout gradient magnetic field and slice-direction gradient magnetic field are applied in the same manner as the pulse sequence for the main scan, and the phase is the same as in the pulse sequence for the main scan until the echo for calculating the correction amount. An encoding gradient magnetic field is applied. For example, in the pre-scan, the readout gradient magnetic field and the slice selection gradient magnetic field are applied in the same manner as the pulse sequence for the main scan, and at least one of the phase encoding gradient magnetic fields applied in the pulse sequence for the main scan is applied. This applies up to the echo near the center of k-space. Then, the correction amount calculation unit 26c determines the phase shift (0th order and 1st order) caused by the readout gradient magnetic field, the slice selection gradient magnetic field, and the phase encoding from the phase differences in the plurality of echo signals collected by the prescan. The amount of zero-order phase shift caused by the gradient magnetic field is calculated as a correction amount. The sequence correction unit 26d corrects the pulse sequence for the main scan based on the correction amount calculated by the correction amount calculation unit 26c. Therefore, according to the first embodiment, it is possible to prevent image quality degradation caused by the gradient magnetic field for readout, the 0th and 1st eddy currents of the gradient magnetic field for slice selection, and the 0th eddy current of the gradient magnetic field for phase encoding. it can.

  In the first embodiment, the pre-scanning pulse sequence is the same as the period in which the phase encoding gradient magnetic field becomes zero in the main scan, or before the echo signal collected in the vicinity of the period. In this manner, a phase encoding gradient magnetic field is applied in the same manner as the pulse sequence for use. Therefore, according to the first embodiment, the phase shift of the echo signal arranged in the vicinity of the center of the k space that contributes most to the image quality is corrected, so that the image quality of the image generated by the MRI apparatus 100 is further improved. be able to.

  As a modification of the first embodiment, the prescan execution unit 26b may perform the prescan a plurality of times. For example, the pre-scan execution unit 26b applies the readout gradient magnetic field and the slice direction gradient magnetic field in the same manner as the main scan pulse sequence, and performs the same operation as the main scan pulse sequence up to the echo before calculating the correction amount. And a second pre-scan using a second pulse sequence that applies a phase encoding gradient magnetic field different from the phase encoding gradient magnetic field applied in the first pre-scan. Perform a pre-scan.

  For example, the prescan execution unit 26b applies the phase encoding gradient magnetic field of the shot that applies the maximum phase encoding gradient magnetic field in the head echo among the plurality of shots as the first prescan, and performs the second prescan. As described above, the phase encoding gradient magnetic field of the shot in which the minimum phase encoding gradient magnetic field is applied in the head echo is applied.

  FIG. 8 is a diagram for explaining a modification of the first embodiment. FIG. 8 shows a phase shift from the first echo signal (Echo1) to the fourth echo signal (Echo4) among the phase shifts in the phase encoding direction shown in FIG. Similar to FIG. 5, the solid line 61 represents the zero-order phase shift in the phase encoding direction for each echo signal, and the broken line 62 represents the intensity of the phase encoding gradient magnetic field.

  For example, the example shown in FIG. 8 shows a case where a plurality of shots a, b, c,..., K, l, m are used. Further, for example, as shown in FIG. 8, the maximum phase encoding gradient magnetic field is set in the first echo signal (Echo1) among the plurality of shots a, b, c,..., K, l, m. The shot to be applied is shot a, and the shot to which the minimum phase encoding gradient magnetic field is applied is shot m. In this case, the pre-scan execution unit 26b applies the phase encoding of the shot a (see the triangular mark shown in FIG. 8) as the first pre-scan, and the phase of the shot m as the second pre-scan. Apply encoding (see black triangles shown in FIG. 8).

  In this case, for example, the correction amount calculation unit 26c sets the phase difference of the first prescan as p1, the phase difference of the second prescan as p2, p1 as the correction amount for the shot a, and p2 as the shot m. The correction amount for the shot in the middle may be obtained by interpolation. Specifically, when the number of shots is N, the correction amount for the i-th shot with the largest phase encoding gradient magnetic field is p1 + (p2−p1) * (i−1) / (N−1).

  FIG. 9A is a diagram illustrating an example of an image obtained by a conventional pre-scan. FIG. 9B is a diagram illustrating an example of an image obtained by the pre-scan in the first embodiment. FIG. 9A is an example of an image acquired by pre-scanning without conventional phase encoding. FIG. 9B is an example of an image collected by the prescan of the first embodiment. As shown in these figures, in the first embodiment, it can be seen that the signal degradation due to the zero-order phase shift due to the phase encoding gradient magnetic field is improved.

(Second Embodiment)
In the first embodiment, the case of correcting the phase shift of the echo signal arranged near the center of the k space has been described, but the embodiment of the MRI apparatus 100 is not limited to this. Therefore, in the following, as a second embodiment, a case will be described in which phase shifts of a plurality of echo signals are corrected. The configuration of the MRI apparatus according to the second embodiment is basically the same as that shown in FIGS. 1 and 2, but the imaging condition setting unit 26a, the pre-scan execution unit 26b, the correction amount calculation unit 26c, The processing performed by the sequence correction unit 26d is different from that of the first embodiment.

  In the second embodiment, the imaging condition setting unit 26a will be described below in addition to the pulse sequence for main scanning and the first pulse sequence for first prescan (pulse sequence shown in FIG. 4). Further, sequence execution data is created for the second pre-scanning pulse sequence.

  FIG. 10 is a diagram showing a second pulse sequence for the second prescan according to the second embodiment. The second pulse sequence applies a phase encoding gradient magnetic field corresponding to the number of echoes different from the phase encoding gradient magnetic field applied in the first prescan. For example, as shown in FIG. 10, the second pulse sequence applies the same phase encoding gradient magnetic field as the pulse sequence for the main scan for the number of echoes smaller than that of the first prescan. The readout gradient magnetic field and the slice selection gradient magnetic field are the same as those in the first pre-scan.

  For example, as shown in FIG. 10, the second pulse sequence is set to apply the phase encoding gradient magnetic field up to the third echo signal.

  In the second embodiment, the prescan executing unit 26b further executes a second prescan using the second pulse sequence shown in FIG. 10 in addition to the first prescan.

  In the second embodiment, the correction amount calculation unit 26c calculates the first phase shift from a plurality of echo signals collected by the first prescan. In addition, the correction amount calculation unit 26c calculates a second phase shift from the phase differences in the plurality of echo signals collected by the second prescan. Then, the correction amount calculation unit 26c calculates correction amounts related to a plurality of echo signals from the calculated first phase shift and second phase shift.

  For example, assume that the first pulse sequence shown in FIG. 4 and the second pulse sequence shown in FIG. 10 are executed. In this case, the correction amount calculation unit 26c outputs the phase difference p1 between the tenth echo signal Echo10 and the eleventh echo signal Echo11 shown in FIG. 4 and the fifth echo signal Echo5 shown in FIG. And a phase difference p2 between the first echo signal Echo4 and the fourth echo signal Echo4.

  Further, the correction amount calculation unit 26c calculates p1 as the first phase shift. Further, the correction amount calculating unit 26c calculates p2 as the second phase shift. Here, p1 that is the first phase shift is the phase shift dif10 in the tenth echo signal Echo10. The second phase shift p2 is the phase shift dif4 in the fourth echo signal Echo4.

  Then, the correction amount calculation unit 26c calculates a correction amount related to the echo signals other than the fourth and tenth echo signals from the first phase shift and the second phase shift. For example, in the main scan, when the gradient magnetic field for phase encoding becomes zero in the 10th echo signal Echo10 collected, the phase shift in the echo signal gradually increases until Echo10, and gradually increases beyond Echo10. Decrease. From this, for example, it can be assumed that the phase shift dif16 in the 16th echo signal is equal to the phase shift dif4 in the fourth echo signal Echo4.

  Therefore, the correction amount calculation unit 26c uses the echo signal in which the phase encoding gradient magnetic field is zero in the main scan as a reference, and the echo collected in the second half from the phase shift in the echo signal collected in the first half of the echo signal. Estimate the phase shift in the signal. Furthermore, the correction amount calculation unit 26c calculates phase shifts in a plurality of echo signals by linearly interpolating the phase shifts in the calculated echo signals.

  FIG. 11 is a diagram for explaining the calculation of the phase shift performed by the sequence correction unit 26d according to the second embodiment. As illustrated in FIG. 11, for example, the sequence correction unit 26d estimates the phase shift dif16 in the 16th echo signal Echo16 from the phase shift dif4 in the fourth echo signal Echo4 with reference to the 10th echo signal Echo10. . Thereafter, the correction amount calculation unit 26c calculates phase shifts in a plurality of echo signals by linearly interpolating the phase shift dif4 in the echo signal Echo4, the phase shift dif10 in the echo signal Echo10, and the dif16 in the echo signal Echo16.

  In the second embodiment, the sequence correction unit 26d uses the gradient field in the phase encoding direction applied to each echo signal in the pulse sequence for the main scan based on the phase shift calculated by the correction amount calculation unit 26c. Correct the intensity. At this time, for example, the sequence correction unit 26d changes the intensity of the gradient magnetic field for phase encoding applied by each echo signal so that the phase shift in each echo signal becomes zero. Alternatively, the sequence correction unit 26d may change the intensity of the rewind gradient magnetic field so that the phase shift in each echo signal becomes zero.

  As described above, in the second embodiment, the pre-scan execution unit 26b further executes the second pre-scan. Here, the second pre-scan applies a phase encoding gradient magnetic field corresponding to the number of echoes different from the phase encoding gradient magnetic field applied in the first pre-scan. For example, in the second pre-scan, a phase encoding gradient magnetic field similar to the pulse sequence for the main scan is applied for the number of echoes smaller than that in the first pre-scan. In addition, the correction amount calculation unit 26c calculates the second phase shift from the phase differences in the plurality of echo signals collected by the first prescan and the phase differences in the plurality of echo signals collected by the second prescan. Then, phase shifts relating to a plurality of echo signals are calculated from the calculated first phase shift and second phase shift. Then, the sequence correction unit 26d corrects the pulse sequence for the main scan based on the phase shift calculated for each echo signal by the correction amount calculation unit 26c. Therefore, according to the second embodiment, since the phase shift of each echo signal collected by the main scan is corrected, image degradation can be prevented with higher accuracy.

  In the second embodiment, the echo collected in the second half is detected from the phase shift in the echo signal collected in the first half of the echo signal with reference to the echo signal in which the phase encoding gradient magnetic field becomes zero in the main scan. We decided to estimate the phase shift in the signal. On the other hand, by performing the pre-scan once more, the phase shift in the echo signal collected in the latter half of the echo signal in which the gradient magnetic field for phase encoding becomes zero in the main scan may be measured. As a result, image degradation caused by a phase shift in the phase encoding direction can be prevented with higher accuracy.

  In the first and second embodiments described above, the case where echo signals are arranged in all areas of the k space by one acquisition has been described. On the other hand, for example, the k space may be divided into a plurality of regions in the phase encoding direction, and a plurality of echo signals may be grouped and collected by a plurality of collections. In such a case, the phase shift may be calculated for each group, and the pulse sequence for the main scan may be corrected for each group.

  FIG. 12 is a diagram for explaining a modification of the first and second embodiments. FIG. 12 shows the k-space along the phase encoding direction, and shows an example when the k-space is divided into three regions. In addition, the arrow shown in FIG. 12 has shown the center of k space. Here, for example, it is assumed that echo signals are collected by being grouped into two groups Gr1 and Gr2. In that case, for example, as shown in FIG. 12, the echo signals of the group Gr1 are sequentially arranged in the phase encoding direction with respect to the middle region. In addition, the echo signals collected in the first half of the echo signals of the group Gr2 are sequentially arranged with respect to one of the regions on both sides, and the echo signal of the group Gr2 is disposed with respect to the other region. The echo signals collected in the latter half are arranged in order.

  In such a case, for example, the correction amount calculation unit 26c may calculate the correction amount for each group. In this case, the sequence correction unit 26d corrects the pulse sequence for the main scan for each group. For example, the correction amount calculation unit 26c calculates the correction amount for any one group, and the sequence correction unit 26d corrects the pulse sequence for the main scan based on the same correction amount for all the groups. May be.

(Third embodiment)
Next, as a third embodiment, an example of echo signal collection and correction when the center of the k space is the head echo will be described. In the third embodiment, the pre-scan execution unit 26b applies at least one phase encoding gradient magnetic field as the pre-scan. FIG. 13 is a diagram showing a pre-scanning pulse sequence according to the third embodiment. As shown in FIG. 13, for example, the pre-scan execution unit 26b applies the phase encoding gradient magnetic field to the second echo signal, and uses the third and subsequent echo signals for calculating the correction amount.

  FIG. 14 is a diagram for explaining the collection of echo signals according to the third embodiment. As shown in FIG. 14, normally, when the center of the k space is the head echo, the echo signal is collected by dividing the k space into two Gr. In such a case, the pre-scan execution unit 26b applies a representative phase encoding gradient magnetic field of Gr1 as the first pre-scan and uses a representative phase encoding of Gr2 as the second pre-scan. Apply a gradient magnetic field. Then, the correction amount calculation unit 26c calculates the correction amount from the phase difference of the plurality of echoes in each prescan, as in the first and second embodiments.

  In addition, the pulse sequence used in the pre-scan similarly applies a gradient magnetic field such as a pre-pulse used in the main scan.

(Fourth embodiment)
Next, as a fourth embodiment, an example will be described in which the first to third embodiments are used in combination with a pre-scan that takes into account a phase shift due to slice encoding of a 3D acquisition sequence. FIG. 15 is a diagram showing a second pulse sequence for the second pre-scan according to the fourth embodiment. In the fourth embodiment, the pre-scan execution unit 26b performs the first pre-scan using the first pulse sequence shown in FIG. 4 and the second pre-scan using the second pulse sequence shown in FIG. Run a scan.

  Here, for example, as in the first embodiment, the first pulse sequence applies a typical phase encoding gradient magnetic field of the main scan up to an echo near the center of the k space. The second pulse sequence applies at least one slice encoding gradient magnetic field for the main scan in addition to the phase encoding gradient magnetic field applied in the first prescan. For example, the second pulse sequence applies a typical slice encoding gradient magnetic field among the slice encoding gradient magnetic fields applied in the pulse sequence for the main scan.

  In the fourth embodiment, the correction amount calculation unit 26c calculates the phase difference in the plurality of echo signals collected by the first prescan and the phase difference in the plurality of echo signals collected by the second prescan. Therefore, the phase shift generated in the slice encoding direction by the slice encoding gradient magnetic field is calculated as a correction amount. The sequence correction unit 26d corrects the pulse sequence for the main scan based on the correction amount calculated by the correction amount calculation unit 26c. Therefore, according to the fourth embodiment, image degradation caused by a phase shift in the slice encoding direction can also be prevented.

  For example, when the number of slice encodes (the number of slices) is 64, the slice encode steps are −32 to 31. For example, in the second pulse sequence, −32 slice encodes are applied and the first prescan is performed. If the phase difference (primary or zeroth order) between Echo10 and Echo11 is s1, and the phase difference between Echo10 and Echo11 in the second prescan is s2, the phase difference due to the i-th slice encoding is (s2-s1) * (- i) It is obtained by / 32. By correcting the phase of the 180-degree pulse, it is possible to correct a phase shift caused by slice encoding. In addition, the accuracy can be improved by increasing the number of pre-scans (for example, applying +31 slice encoding).

  As described above, according to the first to fourth embodiments, it is possible to prevent image degradation caused by the zero-order phase shift of the phase encoding gradient magnetic field.

(Fifth embodiment)
Next, a fifth embodiment will be described. According to the first to fourth embodiments, it is possible to prevent image degradation caused by the zero-order phase shift of the phase encoding gradient magnetic field, but it is used for imaging by applying the phase encoding gradient magnetic field by pre-scanning. Depending on the RF coil and imaging area to be obtained, the phase correction amount cannot be obtained correctly, and the image quality may be deteriorated.

  FIG. 16 is a diagram for explaining an MRI apparatus according to the fifth embodiment. FIG. 16 shows an example of imaging a lumbar sagittal section with the Z-axis direction as the phase encoding direction. In the example shown in FIG. 16, a phased array coil (PAC) is used as the RF coil, and four coil elements included in the PAC coil are arranged in the Z-axis direction. In this example, when a phase encoding gradient magnetic field is applied as in the above embodiment, a primary phase shift occurs in the Z direction. However, in the above embodiment, since sampling is not performed in the phase encoding direction, the primary phase shift in the phase encoding direction cannot be calculated and corrected.

  Here, when the phase correction amount is obtained for each RF coil by the conventional method, as shown in FIG. 16, in the channels ch1 to ch4 connected to the four coil elements, the zero-order phase shifts p1 to p4 are respectively detected. Desired. In this case, for example, there are methods such as obtaining the zero-order phase shift amount to be corrected by the average of p1 to p4, or adopting the average weighted with the signal intensity of the coil element. However, for example, when p1 is -170 degrees and p3 is 178 degrees near 180 degrees or -182 degrees, Unwrap processing cannot be performed appropriately. In addition, depending on the type of RF coil, there are cases where the phase cannot be calculated correctly, and the method for obtaining the phase for each coil element may not always be performed appropriately.

  On the other hand, instead of obtaining the phase for each coil element, there is a method of using the data of the coil element in which the maximum signal is detected. However, in this method, when ch1 or ch4 detects the maximum signal, the image quality in the vicinity of the coil element is improved. However, when the signal is separated from the coil element, the image quality is extremely deteriorated.

  In order to solve such a problem, in the fifth embodiment, the MRI apparatus 100 adds the data of all the coil elements to calculate one phase correction amount. Specifically, the correction amount calculation unit 26c adds a plurality of echo signals collected through a plurality of elements by pre-scanning as described below, and outputs the position in the plurality of echo signals collected in the main scan. One phase correction amount for correcting the phase difference is calculated.

For example, as the phase correction amount to be calculated, the primary (tilt) phase angle (rad) is θ _1, and the zeroth (offset) phase angle (rad) of the i-th slice is θ _{0, i} . The number of readout points is N, the number of echoes is M, the number of slices is NS (0 ≦ i <NS), the number of PAC channels is L (0 ≦ l <L), and the prescan data is P (x, i, l) (however, complex data after 1D Fourier transform: 0 ≦ x <N) If the number of points for obtaining the primary phase (for example, 4 points) is dxx, θ ₁ and θ _{0, i} are as follows: It is calculated | required by Formula (1) and Formula (2) which show.

The correction amount calculation unit 26c calculates θ ₁ and θ _{0, i} as phase correction amounts using the above equations (1) and (2). According to this method, it is possible to obtain an average correction amount within an imaging target that is not affected by a difference in SNR (Signal-to-Noise Ratio) and a phase difference for each coil element. This method is also effective for a conventional prescan that does not apply a phase encoding gradient magnetic field.

  According to the fifth embodiment, the phase correction amount can be stably obtained regardless of the coil element, and the image quality deterioration can be prevented.

(Sixth embodiment)
Next, a sixth embodiment will be described. In the fifth embodiment, all data in the lead-out direction is used for phase calculation. However, for example, data used for phase calculation may be narrowed down to data in an FOV (Field Of View: imaging area) designated by the operator.

  FIG. 17 is a diagram for explaining an MRI apparatus according to the sixth embodiment. FIG. 17 shows an example in which an axial cross section of the wrist is imaged with the X-axis direction as the lead-out direction. In FIG. 17, a range indicated by a solid line indicates an FOV designated by the operator, and a range indicated by a dotted line is an FOV that takes into account NoWrap.

  Here, NoWrap is a FOV wider than the FOV set by the operator at the time of shooting plan is set as a developed FOV, and after generating a developed FOV image by unfolding processing, the image of the FOV set by the operator This is a function of reducing aliasing artifacts by cutting out the image from the expanded FOV image and displaying it.

  As shown in FIG. 17, for example, a non-linear phase shift may occur at a position away from the center in the X-axis direction (see the lower diagram in FIG. 17). In such a case, if all the data in the readout direction is used, data outside the FOV specified by the operator also contributes to the phase calculation, and the phase correction amount cannot be obtained accurately.

  In order to solve such a problem, in the sixth embodiment, the MRI apparatus 100 limits the calculation target area in the lead-out direction to the FOV specified by the operator. Specifically, the correction amount calculation unit 26c uses the data in the FOV designated by the operator in the readout direction among the plurality of echo signals collected by the pre-scan as described below. The amount of phase correction for correcting the phase difference in the plurality of echo signals collected in step S is calculated.

  For example, assuming that the FOV specified by the operator is Fu, the FOV taking into account NoWrap is Fn, the lead-out calculation start point number is Nstart, and the lead-out calculation end point number is Nend, Nstart is expressed by the following formula ( 3) and the equation (4).

      Nstart = N * (1-Nu / Nf) / 2 Formula (3)

      Nend = N−Nstart (4)

  After calculating Nstart and Nend-1 using Equation (3) and Equation (4), the correction amount calculation unit 26c adds x in Equation (1) and Equation (2) from Nstart to Nend-1. Do. According to this method, it is possible to remove the influence of data outside the FOV in calculating the phase correction amount when the phase is different between the inside and the outside of the FOV designated by the operator. This method is also effective for conventional pre-scanning.

  According to the sixth embodiment, the phase correction amount can be obtained stably regardless of the imaging region, and image quality deterioration can be prevented.

(Seventh embodiment)
Next, a seventh embodiment will be described. In the sixth embodiment, the data in the readout direction is narrowed down to the data in the FOV designated by the operator when calculating the phase. However, for example, when the lead-out direction is the Z-axis direction, it may be affected by non-uniform static magnetic fields. Therefore, when calculating the phase, the data in the readout direction may be narrowed down to data within a range where the influence of the static magnetic field inhomogeneity is small.

  FIG. 18 is a diagram for explaining the MRI apparatus according to the seventh embodiment. FIG. 18 shows an example of imaging the abdomen with the Z-axis direction as the readout direction. A nonlinear phase shift occurs at a position away from the center in the Z-axis direction (see the middle diagram in FIG. 18), and the signal strength may increase due to the effect of magnetic field inhomogeneity (the lower diagram in FIG. 18). See). In such a case, if all the data in the lead-out direction is used, the data at a position away from the center in the Z-axis direction has a strong influence on the phase calculation, and the correction amount cannot be obtained accurately.

  In order to solve such a problem, in the seventh embodiment, the MRI apparatus 100 limits the calculation target area in the lead direction to at least the FOV in the Z-axis direction compensated by the system. Specifically, the correction amount calculation unit 26c uses a plurality of echo signals collected by the pre-scan to use a plurality of data collected in the main scan using data within a range in which at least the static magnetic field uniformity is compensated. A phase correction amount for correcting the phase difference in the echo signal is calculated. For example, the correction amount calculation unit 26c calculates the phase correction amount by using data of a half region (region surrounded by a dotted line in FIG. 18) of the FOV in the Z-axis direction compensated by the system. According to this method, the influence of the static magnetic field inhomogeneity in the phase correction amount calculation can be removed. This method is also effective for conventional pre-scanning.

  According to the seventh embodiment, the phase correction amount can be obtained stably without depending on the static magnetic field inhomogeneity, and image quality deterioration can be prevented.

  As described above, according to the first to seventh embodiments, it is possible to prevent image degradation caused by a phase shift of an echo signal.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

100 MRI system (magnetic resonance imaging system)
20 computer system 26 control unit 26b pre-scan execution unit 26c correction amount calculation unit 26d sequence correction unit

Claims (4)

An execution unit that executes a pre-scan that applies a readout gradient magnetic field and a slice direction gradient magnetic field in the same manner as the pulse sequence for the main scan;
A phase correction amount for correcting a phase difference in a plurality of echo signals collected in the main scan is calculated using data limited to a predetermined range among a plurality of echo signals collected by the pre-scan. A calculating unit to
A magnetic resonance imaging apparatus comprising: a correction unit that corrects the pulse sequence for the main scan based on the correction amount calculated by the calculation unit.   The magnetic resonance imaging apparatus according to claim 1, wherein the calculation unit calculates the phase correction amount by using data in an imaging region designated in a readout direction as the limited data.   The magnetic resonance imaging apparatus according to claim 1, wherein the calculation unit calculates the phase correction amount using data within a range in which at least static magnetic field uniformity is compensated as the limited data. An RF coil having a plurality of element coils;
The calculation unit adds a plurality of echo signals collected through the plurality of elements by the pre-scan and corrects a phase difference in the plurality of echo signals collected by the main scan. Calculate the quantity,
The magnetic resonance imaging apparatus according to claim 1.