JP2012200557A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus capable of obtaining an image of good image quality even when a cardiac cycle varies.SOLUTION: The magnetic resonance imaging apparatus is characterized by including: a cardiac cycle prediction section for predicting the cardiac cycle when a pulse sequence for imaging is applied based on heart beat signals for a plurality of cardiac cycles before application of the pulse sequence for imaging; a delay time determination section for determining the delay time from a reference signal indicating the heart beat by using the predicted cardiac cycle; a data collection section for collecting magnetic resonance signals by applying the pulse sequence for imaging to a subject after the delay time from the reference signal; and an image generation section for generating an image from the collected magnetic resonance signals.

Description

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

  Conventionally, magnetic resonance imaging (MRI) using an electrocardiogram synchronous imaging method is known. In the field of magnetic resonance imaging, MRA (Magnetic Resonance Angiography) is known as a technique for obtaining a blood flow image. An MRA that does not use a contrast agent is called a non-contrast MRA. This non-contrast MRA is also usually performed using an electrocardiographic synchronization imaging method.

  In the electrocardiographic synchronization imaging method, an MR (Magnetic Resonance) signal is acquired in synchronization with a signal synchronized with a heartbeat (such as an ECG (Electro cardiogram) signal). For example, the MR signal is acquired at a timing delayed by a predetermined delay time from the position of the R wave in the ECG signal.

  On the other hand, in non-contrast-enhanced MRA, the contrast of the captured image varies depending on the blood flow velocity, so at what timing within the cardiac cycle the MR signal is acquired, that is, the delay from the reference signal such as the R wave to the start of MR signal acquisition How much time is set is very important.

  Therefore, in FS-FBI (Flow spoiled-Fresh Blood Imaging), which is one of the non-contrast MRA, a preliminary scan called an ECG-prep scan is performed before the main scan, and the ECG-prep scan can be obtained. There is a case in which an optimum delay time is determined from information and a main scan is performed using the optimum delay time (for example, Patent Document 1).

  In this method, a plurality of reconstructed images having different delay times within a cardiac cycle are generated by an ECG-prep scan, and a specific image is selected from these. As an image selection method, for example, there are a method of selecting visually, and a method of automatically selecting from an average pixel value of an image processed image as described in Patent Document 2. Then, the delay time corresponding to the selected specific image is determined as the optimal delay time, and the application timing of the pulse sequence for the main scan is determined using the optimal delay time. The optimal delay time here is the optimal delay time in the systole where the blood flow velocity is highest in the systole, the optimal delay time in the diastole where the blood flow velocity is lowest in the diastole, and the like.

  On the other hand, there is also a technique for obtaining the optimal delay time in systole or diastole using a calculation formula or a reference table from the cardiac cycle (interval between R wave and R wave) or heart rate (HR: Heart Rate). In this method, as long as the patient's cardiac cycle and heart rate can be measured, the optimal delay time can be obtained immediately from the calculation formula and the reference table, and therefore, it is not always necessary to perform the ECG-prep scan.

Japanese Patent Laid-Open No. 2002-200054 JP 2008-23317 A

  By the way, even in the same patient, the cardiac cycle or heart rate varies with time. Therefore, the average value is obtained by measuring the cardiac cycle and heart rate for a predetermined period before the start of the main scan, and the average value is input to a calculation formula or a reference table to obtain the optimum delay time in the systole or diastole. A method is also conceivable.

  However, even in this case, the cardiac cycle can change from when the cardiac cycle and the heart rate are measured in advance to when the actual scan is actually started. The cardiac cycle can also change during the period when the main scan is started and the MR signal is acquired. When the cardiac cycle changes, the optimal delay time obtained in advance from the calculation formula or the reference table using the cardiac cycle before the change is no longer an optimal delay time, and causes a reduction in image quality.

  Further, in an imaging method that suppresses the background signal using an inversion pulse, when the cardiac cycle changes and becomes shorter, the inversion pulse having an inversion time TI longer than the optimal delay time of the systole is In this case, an image having the expected image quality cannot be obtained.

  Therefore, there is a demand for a magnetic resonance imaging apparatus that can obtain an image with good image quality even when the cardiac cycle fluctuates.

  A magnetic resonance imaging apparatus according to an embodiment predicts a cardiac cycle when an imaging pulse sequence is applied from heartbeat signals for a plurality of cardiac cycles before application of the imaging pulse sequence, and the predicted heart A delay time determination unit that determines a delay time from a reference signal representing a heartbeat using a period, and data that collects a magnetic resonance signal by applying the imaging pulse sequence to the subject after the delay time from the reference signal A collection unit and an image generation unit that generates an image from the collected magnetic resonance signals are provided.

The figure which shows the structural example of a magnetic resonance imaging apparatus. The functional block diagram of the computer of a magnetic resonance imaging apparatus. The figure which shows the example of a sequence of the data collection by the conventional electrocardiogram synchronization method as a comparative example with this embodiment. 5 is a flowchart showing a processing example of the magnetic resonance imaging apparatus according to the first embodiment. FIG. 3 is an operation explanatory diagram of the magnetic resonance imaging apparatus according to the first embodiment. Explanatory drawing of the method of calculating | requiring optimal delay time from a prediction cardiac cycle. The figure which shows the example of the data collection timing by 3D-high-speed SE method for obtaining the image of the artery and vein in a diastole favorable, and a pulse sequence. The figure which showed typically the volume data obtained by three-dimensional high-speed SE method. The figure which shows the operation | movement concept of FS-FBI method. The figure which shows the data acquisition sequence and pulse sequence for producing | generating a systolic image. The figure explaining operation | movement of the magnetic resonance imaging apparatus which concerns on 2nd Embodiment. 9 is a flowchart showing a processing example of a magnetic resonance imaging apparatus according to the second embodiment.

  An embodiment of a magnetic resonance imaging apparatus will be described with reference to the accompanying drawings.

(1) Configuration FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus 20 in the present embodiment. As shown in FIG. 1, the magnetic resonance imaging apparatus 1 includes a cylindrical static magnetic field magnet 22 that forms a static magnetic field, and a cylindrical shim coil 24 provided with the same axis inside the static magnetic field magnet 22. And a gradient magnetic field coil 26, an RF coil 28, a control system 30, and a bed 32 on which the subject P is placed.

  In the X-axis, Y-axis, and Z-axis that are orthogonal to each other in the apparatus coordinate system, when the axial direction of the static magnetic field magnet 22 and the shim coil 24 is the Z-axis direction and the vertical direction is the Y-axis direction, the bed 32 has its top plate Are arranged so that the direction perpendicular to the mounting surface is the Y-axis direction.

  The control system 30 includes a static magnetic field power supply 40, a shim coil power supply 42, a gradient magnetic field power supply 44, an RF transmitter 46, an RF receiver 48, a sequence controller 56, and a computer 58.

  The gradient magnetic field power supply 44 includes an X-axis gradient magnetic field power supply 44x, a Y-axis gradient magnetic field power supply 44y, and a Z-axis gradient magnetic field power supply 44z. The computer 58 includes an arithmetic device 60, an input device 62, a display device 64, and a storage device 66.

  The static magnetic field magnet 22 is connected to the static magnetic field power supply 40 and forms a static magnetic field in the imaging space by the current supplied from the static magnetic field power supply 40. The shim coil 24 is connected to a shim coil power source 42 and makes the static magnetic field uniform by a current supplied from the shim coil power source 42. The static magnetic field magnet 22 is often composed of a superconducting coil, and is connected to the static magnetic field power source 40 and supplied with current when excited, but after being excited, it is disconnected. Is common. The static magnetic field magnet 22 may be formed of a permanent magnet without providing the static magnetic field power supply 40.

  The gradient magnetic field coil 26 includes an X-axis gradient magnetic field coil 26 x, a Y-axis gradient magnetic field coil 26 y, and a Z-axis gradient magnetic field coil 26 z, and is formed in a cylindrical shape inside the static magnetic field magnet 22. The X-axis gradient magnetic field coil 26x, the Y-axis gradient magnetic field coil 26y, and the Z-axis gradient magnetic field coil 26z are connected to the X-axis gradient magnetic field power source 44x, the Y-axis gradient magnetic field power source 44y, and the Z-axis gradient magnetic field power source 44z, respectively.

  The X-axis gradient magnetic field power supply 44x, the Y-axis gradient magnetic field power supply 44y, and the Z-axis gradient magnetic field power supply 44z respectively supply the X-axis gradient magnetic field coil 26x, the Y-axis gradient magnetic field coil 26y, and the Z-axis gradient magnetic field coil 26z A gradient magnetic field Gx in the axial direction, a gradient magnetic field Gy in the Y-axis direction, and a gradient magnetic field Gz in the Z-axis direction are formed in the imaging space.

  That is, by synthesizing the gradient magnetic fields Gx, Gy, and Gz in the three axis directions of the apparatus coordinate system, the slice direction gradient magnetic field Gss (or the slice encode direction gradient magnetic field Gse), the phase encode direction gradient magnetic field Gpe as the logical axis, and Reading direction (frequency encoding direction) Each direction of the gradient magnetic field Gro can be arbitrarily set. Each gradient magnetic field in the slice direction, the phase encoding direction, and the readout direction is superimposed on the static magnetic field.

  The RF transmitter 46 generates an RF pulse (RF current pulse) with a Larmor frequency for causing nuclear magnetic resonance based on the control information input from the sequence controller 56 and transmits this to the RF coil 28 for transmission. To do. The RF coil 28 includes a whole body coil (WBC) for receiving and transmitting RF pulses built in the gantry, a local coil for receiving RF pulses provided in the vicinity of the bed 32 or the subject P, and the like. is there. The transmission RF coil 28 receives an RF pulse from the RF transmitter 46 and transmits it to the subject P. The receiving RF coil 28 receives an MR signal (high frequency signal) generated by exciting the nuclear spin inside the subject P by the RF pulse, and this MR signal is detected by the RF receiver 48. .

  The RF receiver 48 performs various signal processing such as pre-amplification, intermediate frequency conversion, phase detection, low-frequency amplification, and filtering on the detected MR signal, and then performs A / D (analog to digital) conversion. Then, raw data (raw data) which is digitized complex data is generated. The RF receiver 48 outputs the generated raw data of the MR signal to the sequence controller 56.

  The sequence controller 56 drives the gradient magnetic field power supply 44, the RF transmitter 46, and the RF receiver 48 according to a predetermined sequence, thereby generating the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, the Z-axis gradient magnetic field Gz, and the RF pulse. generate. In addition, the sequence controller 56 receives the raw data (raw data) of the MR signal output from the RF receiver 48 and outputs it to the computer 58.

  The computer 58 includes an arithmetic unit (MPU) 60, an input device 62, a display device 64, and a storage device 66. The input device 62 is an input device such as a keyboard or a mouse, and the display device 64 is a display device configured from a liquid crystal panel or the like. The input device 62 and the display device 64 constitute a user interface.

  The ECG unit 36 acquires an ECG signal (electrocardiogram signal) from the subject P, inputs it to the sequence controller 56, and transmits it to the arithmetic device 60. Instead of an ECG signal that represents pulsation as heartbeat information, a pulse wave synchronization (PPG) signal that represents pulsation as pulse wave information can also be acquired. The PPG signal is, for example, a signal obtained by detecting a fingertip pulse wave as an optical signal. When acquiring the PPG signal, a PPG signal detection unit is provided.

  The sequence controller 56 stores control information necessary for driving the gradient magnetic field power supply 44, the RF transmitter 46, and the RF receiver 48 in accordance with a command from the arithmetic device 60. The control information here is, for example, sequence information describing operation control information such as the intensity, application time, and application timing of the pulse current to be applied to the gradient magnetic field power supply 44.

  The computing device 60 of the computer 58 realizes various functions according to the program stored in the storage device 66 in addition to the control of the system controller 56 and the system control of the entire magnetic resonance imaging apparatus 1. Note that these various functions may be realized by providing a specific circuit in the magnetic resonance imaging apparatus 20 without depending on a program.

  FIG. 2 is a functional block diagram of the computer 58 shown in FIG.

  The computer 58 includes an imaging condition setting unit 100, a sequence controller control unit 101, an image generation unit 102 (including the k-space database 103), an image database 104, an image processing unit 105, a display control unit 106, and a cardiac cycle prediction unit 107 according to programs. , Function as the delay time determination unit 108 and the like.

  The imaging condition setting unit 100 sets imaging conditions including a pulse sequence based on instruction information from the input device 62, a delay time determined by a delay time determination unit 108 to be described later, and the set imaging conditions are set by a sequence controller. This is given to the control unit 101. The sequence controller control unit 101 sets the pulse sequence set by the imaging condition setting unit 100 in the sequence controller 56.

  The data acquisition unit is configured by the imaging condition setting unit 100, the sequence controller control unit 101, the sequence controller 56, the RF transmitter 46, the RF coil 28, the RF receiver 48, and the imaging conditions set by the imaging condition setting unit 100. Based on this, magnetic resonance signals (MR signals) from the subject P are collected as raw data.

  The image generation unit 102 arranges the raw data input from the RF receiver 48 through the sequence controller 56 as k-space data in the k-space database 103, while taking in the k-space data from the k-space database 103 and applies the Fourier data to this Image data is generated by performing image reconstruction processing including conversion. Then, the generated image data is stored in the image database 104.

  The image processing unit 105 reads necessary image data from the image database 104, and performs image processing such as difference processing and display processing such as MIP processing to generate image data for display, and the generated image data Is stored in the storage device 66.

  The display control unit 106 performs control for displaying image data for display stored in the storage device 66 and characters and images for various user interfaces on the display device 64.

  The cardiac cycle prediction unit 107 predicts a cardiac cycle when the imaging pulse sequence is applied from heartbeat signals for a plurality of cardiac cycles before the imaging pulse sequence is applied. The heartbeat signal is an ECG signal of the subject P output from the ECG unit 36, a pulse wave synchronization signal (PPG signal), or the like.

  The delay time determination unit 108 uses the cardiac cycle predicted by the cardiac cycle prediction unit 107 to determine a delay time from the reference signal (eg, R wave) representing the heartbeat to the start of application of the imaging pulse sequence.

  More specific operations of the cardiac cycle prediction unit 107 and the delay time determination unit 108 will be described later.

(2) 1st Embodiment FIG. 3: is a figure which shows the example of a sequence of the data collection by the conventional electrocardiogram synchronous imaging method as a comparative example with this embodiment. In the conventional ECG-synchronous imaging method, as shown in FIG. 3A, the patient's ECG waveform is acquired in advance before the start of imaging, and the interval of the reference signal (usually R wave) of the ECG waveform An average value T _AVE is obtained. Then, from this average value T _AVE, to determine the optimum delay time Tdfix until start of the application of the imaging pulse sequence.

  On the other hand, at the time of imaging, an R wave signal is acquired from the patient as shown in FIG. 3B, and imaging synchronized with this R wave is performed. At this time, each R wave is delayed by the previously determined optimum delay time Tdfix (fixed value) and an imaging pulse sequence is applied to the patient to collect MR signals (FIG. 3 (c)). .

  The optimum delay time Tdfix varies depending on the imaging purpose, the imaging target region, and the like. In non-contrast MRA for obtaining a blood flow image, the optimum delay time Tdfix differs depending on blood flow conditions such as blood flow velocity. For example, the optimum delay time Tdfix in the systole at which the blood flow velocity is assumed to be the highest and the optimum delay time Tdfix in the diastole at which the blood flow velocity is assumed to be the lowest are naturally different values.

  On the other hand, as described above, the heart cycle of a person is not constant even for a healthy person and varies with time. Therefore, the cardiac cycle acquired in advance for determining the optimum delay time Tdfix and the cardiac cycle when imaging is actually performed are not necessarily the same. The cardiac cycle also varies during imaging. When the cardiac cycle fluctuates, the blood flow conditions in the delay time differ even if the delay time is the same from the R wave. For example, even when the optimal delay time Tdfix is determined so that the blood flow velocity in the diastole is the lowest, if the cardiac cycle changes, the blood flow velocity at the optimal delay time Tdfix is no longer the lowest, and “optimum” This is not a delay time, and data is collected in a state different from the intended blood flow condition.

  The present embodiment solves such inconvenience, and employs a method of determining the optimum time dynamically by changing the cardiac cycle. The operation will be described with reference to the flowchart of FIG. 4 and FIG.

  FIG. 5A shows an example in which the interval (Tn-N to Tn + 2) of each R wave (Rn-N wave to Rn + 3 wave) fluctuates gradually as an example in which the cardiac cycle fluctuates. Is illustrated.

  In step ST10 of FIG. 4, N cardiac cycles before the target Rn wave (that is, the R wave whose optimum delay time Tdopt (n) is to be determined) are acquired. In the example of FIG. 5A, N cardiac cycles (Tn-N to Tn-1) immediately before the Rn wave are acquired.

  In step ST11, the cardiac cycle corresponding to the Rn wave, that is, the interval Tpn between the Rn wave and the Rn + 1 wave is predicted from the acquired N cardiac cycles. Most simply, a simple average value of N cardiac cycles (Tn-N to Tn-1) can be used as the predicted cardiac cycle Tpn, but is not limited thereto. For example, a weighted average in which the weight of the cardiac cycle near the target Rn wave is set larger than the weight of the cardiac cycle far from the Rn wave may be used. Further, the predicted cardiac cycle Tpn of the Rn wave may be obtained from the N cardiac cycles (Tn-N to Tn-1) immediately before the Rn wave using a known prediction filter or the like.

  In step ST12, the optimum delay time Tdopt (n) from the target Rn wave is calculated from the predicted cardiac cycle Tpn. This calculation may be obtained by substituting the predicted cardiac cycle Tpn into the calculation formula as shown in FIG. 6 (a), or is associated with the predicted cardiac cycle Tpn as shown in FIG. 6 (b). You may obtain | require from a reference table.

  For example, the optimal systolic delay time Tds opt (n) can be obtained from a calculation formula represented by the function fs of the predicted cardiac cycle Tpn. This function fs is obtained in advance as an approximate expression obtained by using the least square method or the like from a large number of measurement data indicating the relationship between the delay time from the R wave and the cardiac cycle so that the blood flow velocity in the systole is maximized. be able to.

  Similarly, the optimal delay time Tdd opt (n) in the diastolic period can be obtained from the calculation formula represented by the function fd of the predicted cardiac cycle Tpn. This function fd can be obtained in advance as an approximate expression from a large number of measurement data indicating the relationship between the delay time from the R wave and the cardiac cycle so that the blood flow velocity in the diastole is minimized. .

  In addition, instead of these formulas, a systolic reference table in which the cardiac cycle and the optimal delay time in systole are associated, and a diastole reference in which the cardiac cycle and the optimal delay time in diastole are associated. The optimum delay times Tds opt and Tdd opt can also be obtained from the table.

  When the optimum delay time Td opt (n) for the target R wave (Rn) is calculated in step ST12, the application timing of the imaging pulse sequence is determined based on the optimum delay time Td opt (n). . Then, a predetermined imaging pulse sequence is applied to the subject after the optimum delay time Td opt (n) from the target R wave (Rn), and MR signals from the subject are collected (step ST13).

  When the MR signal acquisition is continued, the process returns to step ST10, and the optimum delay time Tdopt (n + 1) is calculated in the same manner for the next target R wave (Rn + 1 in the example of FIG. 5). . When all the MR signals are collected, an image is reconstructed from the collected MR signals.

  Thus, in the magnetic resonance imaging apparatus 20 according to the present embodiment, for each target R wave, the predicted cardiac cycle Tp is sequentially updated based on the previous (preferably immediately preceding) N cardiac cycle data, and updated. The optimum delay time Td opt is calculated based on the latest predicted cardiac cycle Tp. Since the calculated optimal delay time Td opt reflects the fluctuation of the cardiac cycle, MR signals with substantially the same blood flow conditions (blood flow velocity, etc.) are always collected even if the cardiac cycle fluctuates over time. can do. As a result, the target image originally intended by the user can be generated with the originally intended quality.

(3) Application example of the first embodiment (part 1)
The first embodiment described above can be applied to various MR imaging based on the electrocardiogram synchronous imaging method, and is not limited to a specific imaging method or a specific pulse sequence. The following MRA data acquisition sequence and pulse sequence based on the three-dimensional high-speed SE method are merely examples of the present embodiment.

  FIGS. 7A and 7B are diagrams illustrating an example of data collection timing and a pulse sequence by a 3D-fast SE (Fast Spin Echo) method for obtaining good images of arteries and veins in the diastole. .

In this imaging method, as shown in FIG. 7B, each pulse sequence is composed of one excitation pulse and a plurality of subsequent refocus pulses. After each refocusing pulse is gradient G _PE of different phase encoding amount are applied, by application of subsequent readout gradient magnetic field G _RO, MR signal (echo signal) is acquired. Data of a plurality of phase encode lines can be collected by one excitation pulse. In this example, one slice corresponds to one excitation pulse, that is, one pulse sequence shown in FIG. 7B. Phase encode line data is collected.

On the other hand, for one excitation pulse and the refocusing pulse that follows, that is, for one pulse sequence, a gradient magnetic field G _SE of one and the same slice encoding amount is applied. Then, as shown in FIG. 7 (a), the slice encoding amount for each pulse sequence is set as follows: SE (n), SE (n), SE (n + m), SE (n + m + 1) ) And SE (n + m + 2), volume data for a plurality of slices is collected and 3D-MR imaging is performed.

  Since one slice of data is collected by one excitation pulse, the length of one pulse sequence is relatively long. That is, data collection for one slice is performed across a plurality of R waves (two R waves in the example of FIG. 7) in one pulse sequence. On the other hand, as shown in FIG. 7, the data collection order (so-called centric order) is provided in which the data of the phase encode line with zero encoding amount (so-called k0 data) is provided at the head of the pulse sequence. For this reason, the main structure and main contrast of the reconstructed image are almost determined at the position of k0 data, that is, the start position of each pulse sequence, and the influence of the length of the pulse sequence is small.

  On the other hand, since the main configuration and main contrast of an image are determined by collected data in the vicinity of the start position of the pulse sequence, selection and setting of the delay time from each R wave to the start of each pulse sequence is extremely important. In non-contrast MRA, this delay time is selected and set in order to obtain an image of blood flow conditions such as a desired blood flow velocity.

  However, as described above, when the cardiac cycle varies, the blood flow velocity and the like change even with the same delay time, and an image with a desired blood flow condition cannot be obtained.

  On the other hand, in the present embodiment, as shown from the upper stage to the lower stage in FIG. 7A, even if the heart cycle varies from a short cardiac cycle to a long cardiac cycle, the cardiac cycle varies for each target R wave. Since the reflected optimum delay time Td opt is obtained, MR signals under substantially the same blood flow condition (blood flow velocity, etc.) can be collected at all times.

  This means that the three-dimensional high-speed SE method according to the present embodiment can always generate a blood flow image under the same blood flow condition in the slice encoding direction.

  FIG. 8 is a diagram schematically showing volume data obtained by the three-dimensional high-speed SE method. In the conventional three-dimensional high-speed SE method, the pulse sequence for collecting each slice data is always started from the R wave with the same delay time. Therefore, when the cardiac cycle changes, the blood flow condition varies from slice to slice. A three-dimensional image was reconstructed based on MR signals collected under different blood flow conditions.

  In contrast, in the three-dimensional high-speed SE method according to the present embodiment, even when the cardiac cycle varies, the optimum delay time Td opt that reflects the variation of the cardiac cycle is obtained for each pulse sequence. MR signals under the same blood flow conditions (blood flow velocity, etc.) can be collected for any slice, and a high-quality three-dimensional blood flow image can be generated.

(4) Application example of the first embodiment (part 2)
FIG. 9 is a diagram illustrating an operation concept of the FS-FBI (Flow Spoiled Fresh Blood Imaging) method. The FS-FBI method is one of three-dimensional non-contrast MRA based on an electrocardiogram synchronous imaging method as described in Patent Document 1 and the like. In the FS-FBI method, differential processing is performed between a systolic image IMsys obtained by reconstructing MR signals collected in the systole and a diastole image IMdis obtained by reconstructing MR signals collected in the diastole.

The systolic image IMsys is pulsed at the delay time from the R wave where the MR signal from the artery is the lowest, that is, the blood flow velocity is the highest in the systole and the flow void effect is greatest. A sequence is applied to generate the image. As a result, in the systolic image IMsys, an image in which the signal strength of the artery is suppressed while the signal strength of the vein is retained is obtained. In order to further promote the flow void effect, as shown at the left end of FIG. 9, for example, a dephase pulse may be added to both ends of the lead-out pulse _GRO body. By adding a dephase pulse, the flow void effect is promoted, and the arterial signal can be further suppressed.

On the other hand, the diastolic image IMdis has a blood flow condition in which both MR signals of the arteries and veins are retained, that is, the blood flow velocity of the arteries and veins is the lowest and the flow void effect is minimized in the diastole. A pulse sequence is applied at a delay time from the R wave, and the image is generated. As a result, in the diastolic image IMsys, an image in which the signal intensities of both the artery and the vein are maintained is obtained. In order to further reduce the flow void effect, as shown at the right end of FIG. 9, for example, a rephase pulse may be added to both ends of the lead-out pulse _GRO body. By adding the rephase pulse, the flow void effect is further suppressed, and the signal strength of the artery and vein can be increased.

And veins retained systolic image IMsys artery is suppressed, by the difference between arteries and veins both diastole the held image IMdis, is possible to generate an arterial image IM _AR that only arteries is held it can. Moreover, by further subtracting the arterial image IM _AR and diastole image IMdis, veins only can further generate a vein image held.

The data acquisition sequence and pulse sequence for generating the diastolic image IMdis have already been shown in FIG. On the other hand, FIG. 10 is a diagram showing a data acquisition sequence and a pulse sequence for generating the systolic image IMsys. The first difference between FIG. 7 and FIG. 10 is that the optimum delay time is set to the diastole in FIG. 7 whereas it is set to the systole in FIG. The second difference is that in FIG. 7 (diastolic phase), rephase pulses are added to both ends of the readout pulse _GRO , whereas in FIG. 10 (systolic phase), both ends of the readout pulse _GRO are added. The dephasing pulse is added.

  As shown in FIGS. 7 and 10, the FS-FBI method to which the present embodiment is applied has an optimum delay time Td opt in which the variation of the cardiac cycle is reflected for each pulse sequence even when the cardiac cycle varies. Therefore, in both the systolic image IMsys and the diastolic image IMdis, MR signals of the same blood flow conditions (blood flow velocity, etc.) can be collected for any slice, and high quality 3 A dimensional blood flow image can be generated.

(5) Second Embodiment FIG. 11 is a diagram for explaining the operation of the magnetic resonance imaging apparatus 20 according to the second embodiment, and FIG. 12 is a flowchart showing a processing example of the operation.

  The pulse sequence for imaging used in the magnetic resonance imaging apparatus 20 according to the second embodiment is a pulse sequence including a pre-pulse (preparation pulse) and an excitation pulse following the pre-pulse. Here, the pre-pulse is an RF pulse applied before the excitation pulse for background suppression and other purposes. The type and number of prepulses are not particularly limited. Examples of the pre-pulse include an inversion pulse, a tagging pulse, an MT pulse (Magnetization Transfer pulse), and a saturation pulse.

  The time between the prepulse and the excitation pulse varies depending on the purpose of the prepulse. For example, in the case of an inversion pulse, the inversion time TI (Inversion time) (the time between the inversion pulse and the excitation pulse) is set to the time until the longitudinal magnetization of the background returns from 180 ° to 0 °, so that the background Is suppressed.

  FIG. 11A is a diagram showing the application timing of a conventional pulse sequence including an R wave and a pre-pulse (inverted pulse in the example of FIG. 11) for comparison with the second embodiment. In the conventional example, since the application timing of the inversion pulse is determined based on the Rn wave, the delay time from the Rn wave to the excitation pulse can be set only to the inversion time TI or more. Therefore, when the optimum delay time Td opt for imaging under a desired blood flow condition is shorter than the inversion time TI, the delay time from the Rn wave to the excitation pulse cannot be set to the optimum delay time Td opt. It was. Further, if the delay time is set to the optimum delay time Tdopt, the inversion time TI has to be shortened.

  On the other hand, in the magnetic resonance imaging apparatus 20 according to the second embodiment, as shown in FIG. 11B, the optimum delay time Td opt from the reference signal (Rn wave) to the excitation pulse is a pre-pulse (for example, When the time is shorter than the time between the inversion pulse) and the excitation pulse (for example, the inversion time), the pre-pulse (inversion) starts from the reference signal (Rn-1 wave) immediately before the reference signal (Rn wave) immediately before the excitation pulse. A delay time Td until the pulse) is determined using the predicted cardiac cycle Tp, and an imaging pulse sequence starting from the inversion pulse after the delay time Td from the previous reference signal (Rn-1 wave) is applied to the subject. A magnetic resonance signal is collected.

  A more specific procedure will be described with reference to the flowchart of FIG.

  The inversion time TI is set in the imaging condition setting unit 100 as one of imaging conditions (step ST20). When the preparation for imaging is completed, N cardiac cycles before the target R wave (Rn wave) (for example, N cardiac cycles immediately before the Rn-1 wave) are acquired from the electrocardiogram signal from the patient. Then, the predicted cardiac cycle Tp is calculated from the collected N cardiac cycles (steps ST21 and ST22). Further, the optimum delay time Td opt from the target R wave (Rn wave) is calculated from the predicted cardiac cycle Tp (step ST23). The processing from step ST21 to step ST23 is the same as the processing from step ST10 to step ST12 in the first embodiment (FIG. 4). However, the predicted cardiac cycle Tp in the first embodiment is a predicted value of the cardiac cycle between the Rn wave and the Rn + 1 wave, whereas the predicted cardiac cycle Tp in the second embodiment is It is a predicted value of the cardiac cycle between the Rn wave and the Rn + 1 wave, and is also a predicted value of the cardiac cycle between the Rn-1 wave and the Rn wave.

  In step ST24, it is determined whether or not the calculated optimum delay time Tdopt is shorter than the inversion time TI. When the optimum delay time Td opt is longer than the inversion time TI (Td opt ≧ TI) (NO in step ST24), a pulse sequence starting with the inversion pulse after the delay time (Td opt-TI) from the target Rn wave. Is applied.

On the other hand, when the optimum delay time Tdopt is shorter than the inversion time TI (Tdopt <TI) (YES in step ST24), the delay time Td from the previous Rn-1 wave is expressed by the following equation. Calculate (step ST26).
Td = Tp- (TI-Tdopt)

  Then, as shown in FIG. 11 (b), after a delay time Td from the previous Rn-1 wave, a pulse sequence beginning with an inversion pulse is applied to the subject to collect MR signals (step ST27). Then, an image is reconstructed from the collected MR signals (step ST28).

  According to the magnetic resonance imaging apparatus 20 according to the second embodiment, it is possible to apply a pulse sequence including an inversion pulse before the target Rn wave, so that a predetermined inversion time is secured, The excitation pulse can be brought close to the Rn wave.

  Even when the cardiac cycle becomes shorter than the expected value due to the fluctuation of the cardiac cycle, and as a result, the optimum delay time Td opt obtained by the calculation formula or the like becomes shorter than the inversion time TI, the predetermined inversion time and the optimum delay It is possible to apply a pulse sequence in which both the times Td opt are secured.

  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.

20 Magnetic Resonance Imaging Device 58 Computer 60 Arithmetic Unit 56 Sequence Controller 100 Imaging Condition Setting Unit 101 Sequence Controller Control Unit 102 Image Generation Unit 105 Image Processing Unit 107 Heart Cycle Prediction Unit 108 Delay Time Determination Unit

Claims (16)

A cardiac cycle prediction unit that predicts a cardiac cycle at the time of applying an imaging pulse sequence from heartbeat signals for a plurality of cardiac cycles before application of the imaging pulse sequence;
A delay time determination unit that determines an optimal delay time from a reference signal representing a heartbeat using the predicted cardiac cycle;
A data collection unit that collects a magnetic resonance signal by applying the imaging pulse sequence to a subject after the optimum delay time from the reference signal;
An image generator for generating an image from the collected magnetic resonance signals;
A magnetic resonance imaging apparatus comprising: The cardiac cycle prediction unit sets an average value of intervals of the plurality of heartbeat signals immediately before application of the imaging pulse sequence as the predicted cardiac cycle.
The magnetic resonance imaging apparatus according to claim 1. The delay time determination unit includes:
The optimal delay time to which the imaging pulse sequence is to be applied is determined from the predicted cardiac cycle using a calculation formula or a reference table.
The magnetic resonance imaging apparatus according to claim 1. The delay time determination unit determines the first optimum delay time in the systole and the second delay time in the diastole using the calculation formula or the reference table,
The data collection unit collects a first magnetic resonance signal by applying a first imaging pulse sequence after the first optimum delay time from the reference signal, and collects the second optimum delay time from the reference signal. Applying a second imaging pulse sequence later to collect a second magnetic resonance signal;
The image generation unit generates a first image from the first magnetic resonance signal, and generates a second image from the second magnetic resonance signal;
The magnetic resonance imaging apparatus according to claim 3. The imaging pulse sequence is a pulse sequence for collecting magnetic resonance signals for a plurality of phase encodes by one excitation pulse.
The magnetic resonance imaging apparatus according to claim 4. The collection period of magnetic resonance signals for the plurality of phase encodings spans a plurality of cardiac cycles,
The magnetic resonance imaging apparatus according to claim 5. The image generation unit generates a artery image by subtracting the first image from the second image,
The magnetic resonance imaging apparatus according to claim 4, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. In the case of collecting a three-dimensional magnetic resonance signal by applying a plurality of the imaging pulse sequences together with a plurality of slice encodings corresponding to each of the imaging pulse sequences,
The cardiac cycle prediction unit obtains an average value of a plurality of intervals between the heartbeat signals immediately before each application with respect to the plurality of imaging pulse sequences described above, and the predicted cardiac cycle corresponding to each imaging pulse sequence age,
The delay time determining unit determines an optimum delay time to which each imaging pulse sequence is to be applied from the predicted cardiac cycle corresponding to each imaging pulse sequence using a calculation formula or a reference table. ,
The data collection unit applies each imaging pulse sequence to the subject after each optimum delay time from the reference signal, and collects the three-dimensional magnetic resonance signal,
The image generation unit generates a three-dimensional image by performing a three-dimensional inverse Fourier transform on the collected three-dimensional magnetic resonance signal;
The magnetic resonance imaging apparatus according to claim 1. The delay time determination unit determines the first optimal delay time in the systole and the second delay time in the diastole using the calculation formula or the reference table for each imaging pulse sequence. ,
The data collection unit collects a first magnetic resonance signal by applying a first imaging pulse sequence after the first optimum delay time from the reference signal for each imaging pulse sequence, Applying a second imaging pulse sequence after the second optimum delay time from a reference signal to collect a second magnetic resonance signal;
The image generation unit generates a first three-dimensional image from the first magnetic resonance signal, and generates a second three-dimensional image from the second magnetic resonance signal;
The magnetic resonance imaging apparatus according to claim 8. Each imaging pulse sequence is a pulse sequence for collecting magnetic resonance signals for a plurality of phase encodings by one excitation pulse.
The magnetic resonance imaging apparatus according to claim 9. The collection period of magnetic resonance signals for the plurality of phase encodings spans a plurality of cardiac cycles,
The magnetic resonance imaging apparatus according to claim 10. The image generation unit generates a artery image by subtracting the first image from the second image,
The magnetic resonance imaging apparatus according to claim 8, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. The reference signal representing the heartbeat is an R wave of an electrocardiogram signal, and the cardiac cycle is an interval between adjacent R waves.
The magnetic resonance imaging apparatus according to claim 1. The imaging pulse sequence is a pulse sequence including a pre-pulse and an excitation pulse applied after the pre-pulse,
When the predetermined optimum delay time from the reference signal to the excitation pulse is shorter than the time between the pre-pulse and the excitation pulse, the delay time determining unit determines 1 of the reference signal immediately before the excitation pulse. Determining the delay time from the previous reference signal to the pre-pulse using the predicted cardiac cycle;
The data collection unit collects magnetic resonance signals by applying the imaging pulse sequence to a subject after the determined delay time from the previous reference signal;
The magnetic resonance imaging apparatus according to claim 1. The pre-pulse is an inversion pulse, and the time between the pre-pulse and the excitation pulse is an inversion time.
The magnetic resonance imaging apparatus according to claim 14. The optimal delay time from the reference signal to the excitation pulse is _Tdopt , the inversion time is TI, the delay time from the previous reference signal to the prepulse is _Td , and the predicted cardiac cycle is Tp. When
The delay time determination unit, _{T dopt} <TI, _{when, T} d = Tp- as _{(TI-T dopt),} determines the delay time _{T d,}
The magnetic resonance imaging apparatus according to claim 15.

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