JP5689766B2 - Magnetic resonance imaging system - Google Patents

Description

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

  2. Description of the Related Art Conventionally, an ASL (Arterial Spin Labeling) method is known as a method for obtaining blood flow information without using a contrast agent by a magnetic resonance imaging apparatus. The ASL method magnetically labels blood flowing in a subject by applying an RF wave to the subject, and uses the labeled blood as a tracer to obtain a perfusion image. Is the method.

  Examples of the ASL method include a PASL (Pulsed ASL) method, a CASL (Continuous ASL) method, and a PCASL (Pulsed Continuous ASL) method. The PASL method is a method using a pulse wave (RF pulse), and the CASL method is a method using a continuous wave. The PCASL method is intended for practical use of the CASL method, and uses a large number of short RF pulses.

Edelmann RR et al, Radiology 192: 513-519 (1994) Tokunori Kimura: Non-invasive blood flow imaging using Modified STAR using asymmetric inversion slabs (ASTAR) method, Journal of Japanese Magnetism 2001; 20 (8), 374-385 Kwong KK, Chesler DA, koff RM, Donahue KM, et al, MR perfusion studies with T1-weighted echo planar imaging, Magn Reson Med 1995; 34: 878-887 Mani S et al. MRM 37: 898-905 (1997) Wells JA at al. In vivo hadamard encoded continuous arterial spin labeling (H-CASL). MRM 63: 1111-1118 (2010)

  The problem to be solved by the present invention is to provide a magnetic resonance imaging apparatus capable of reducing the remaining of SAR and blood vessel signals and improving the SNR of blood flow signals.

  The magnetic resonance imaging apparatus according to the embodiment includes an imaging condition setting unit, a data collection unit, an image reconstruction unit, and a fluid image generation unit. The imaging condition setting unit determines the tag thickness indicating the spatial thickness of the tag region to which the RF wave for labeling the fluid is applied according to the flow velocity of the fluid flowing in the subject, and the RF wave A tag condition including a tag interval indicating a time interval to be applied is set. The data collection unit applies the RF wave to the tag region a plurality of times at the tag interval set by the imaging condition setting unit, and the fluid flows after a predetermined waiting time has elapsed since the RF wave was applied. Collect magnetic resonance data of the flowing imaging region. The image reconstruction unit reconstructs an image from the magnetic resonance data collected by the data collection unit. The fluid image generation unit generates a fluid image from the image reconstructed by the image reconstruction unit.

FIG. 1 is a diagram illustrating a configuration example of an MRI apparatus according to the first embodiment. FIG. 2 is a diagram (1) for explaining the principle of the conventional ASL method. FIG. 3 is a diagram (2) for explaining the principle of the conventional ASL method. FIG. 4 is a diagram (3) for explaining the principle of the conventional ASL method. FIG. 5 is a diagram (4) for explaining the principle of the conventional ASL method. FIG. 6 is a diagram for explaining a conventional PASL method. FIG. 7 is a diagram illustrating an outline of the MT-PASL method executed by the MRI apparatus 100 according to the first embodiment. FIG. 8 is a diagram (1) for explaining blood labeling by the MT-PASL method according to the first embodiment. FIG. 9 is a diagram (2) for explaining the labeling of blood by the MT-PASL method according to the first embodiment. FIG. 10 is a diagram illustrating a configuration example of the MRI apparatus 100 according to the first embodiment. FIG. 11 is a diagram (1) illustrating the relationship between the tag interval and the AIF in the MT-PASL method according to the first embodiment. FIG. 12 is a diagram (2) illustrating the relationship between the tag interval and the AIF in the MT-PASL method according to the first embodiment. FIG. 13 is a diagram (3) illustrating the relationship between the tag interval and the AIF in the MT-PASL method according to the first embodiment. FIG. 14 is a flowchart illustrating the processing procedure of the MT-PASL method executed by the MRI apparatus 100 according to the first embodiment. FIG. 15 is a diagram (1) for explaining data collection when the tag thickness is fixed by the MT-PASL method according to the second embodiment. FIG. 16 is a diagram (2) for explaining data collection when the tag thickness is fixed by the MT-PASL method according to the second embodiment. FIG. 17 is a diagram (3) for explaining data collection when the tag thickness is fixed by the MT-PASL method according to the second embodiment. FIG. 18 is a diagram (1) for explaining data collection when the transit time according to the MT-PASL method according to the second embodiment is fixed. FIG. 19 is a diagram (2) for explaining data collection when the transit time according to the MT-PASL method according to the second embodiment is fixed. FIG. 20 is a diagram (3) for explaining data collection when the transit time according to the MT-PASL method according to the second embodiment is fixed. FIG. 21 is a diagram for explaining the improvement of the RF pulse profile in the MT-PASL method according to the second embodiment. FIG. 22 is a diagram (1) for explaining the HE-MT-MASL method according to the third embodiment. FIG. 23 is a diagram (2) for explaining the HE-MT-MASL method according to the third embodiment.

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

  The MRI apparatus according to the embodiment includes an imaging condition setting unit, a data collection unit, an image reconstruction unit, and a fluid image generation unit. The imaging condition setting unit determines the tag thickness indicating the spatial thickness of the tag region to which the RF wave for labeling the fluid is applied according to the flow velocity of the fluid flowing in the subject, and the RF wave A tag condition including a tag interval indicating a time interval to be applied is set. The data collection unit applies the RF wave to the tag region a plurality of times at the tag interval set by the imaging condition setting unit, and the fluid flows after a predetermined waiting time has elapsed since the RF wave was applied. Collect magnetic resonance data of the flowing imaging region. The image reconstruction unit reconstructs an image from the magnetic resonance data collected by the data collection unit. The fluid image generation unit generates a fluid image from the image reconstructed by the image reconstruction unit.

  In the following embodiment, an example in which the fluid is blood and the fluid image is a blood flow image will be described. However, the same embodiment can be applied even when the fluid is CSF (CcerebroSpinal fluid), for example. It is. In the embodiment described below, an example in which the blood flow image is a perfusion image will be described. For example, the same implementation is performed even when the blood flow image is a blood vessel image (MRA (MR Angio): angio image). The form is applicable.

(First embodiment)
First, a configuration example of the MRI apparatus according to the first embodiment will be described. FIG. 1 is a diagram illustrating a configuration example of an 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 sequencer 10, an ECG (Electrocardiogram) sensor 21, an ECG unit 22, and a computer system 30 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 36, and drives the couch 4 to move the top 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 MR (Magnetic Resonance) data based on the magnetic resonance signal output from the reception RF coil 8. Specifically, the receiving unit 9 generates MR data by digitally converting the magnetic resonance signal output from the reception RF coil 8. This MR data is obtained in the PE (Phase Encode) direction, RO (Read Out) direction, and SE (Slice Encode) direction by the above-described slice selection gradient magnetic field Gs, phase encoding gradient magnetic field Ge, and readout gradient magnetic field Gr. By associating the spatial frequency information, it is generated as data corresponding to the k space. When the MR data is generated, the receiving unit 9 transmits the MR data to the sequencer 10. The receiving unit 9 may be provided on the gantry device side including the static magnetic field magnet 1 and the gradient magnetic field coil 2.

  The sequencer 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 information transmitted from the computer system 30. Here, the sequence information refers to 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. Information defining the procedure for performing the scan, such as the timing at which the reception unit 9 detects the magnetic resonance signal.

  The sequencer 10 drives the gradient magnetic field power source 3, the transmission unit 7, and the reception unit 9 to scan the subject P. As a result, when the MR data is transmitted from the reception unit 9, the MR data is transmitted to the computer system 30. Forward.

  The ECG sensor 21 is attached to the body surface of the subject P and detects an ECG signal indicating the heartbeat, pulse wave, respiration, etc. of the subject P as an electrical signal. The ECG unit 22 performs a variety of processing including A / D conversion processing and delay processing on the ECG signal detected by the ECG sensor 21 to generate a gate signal, and transmits the generated gate signal to the sequencer 10.

  The computer system 30 performs overall control of the MRI apparatus 100. For example, the computer system 30 performs data collection, image reconstruction, and the like by driving the above-described units. The computer system 30 includes an interface unit 31, an image reconstruction unit 32, a storage unit 33, an input unit 34, a display unit 35, and a control unit 36.

  The interface unit 31 controls transmission / reception of various signals exchanged between the computer system 30 and the sequencer 10. For example, the interface unit 31 transmits sequence information to the sequencer 10 and receives MR data from the sequencer 10. When receiving the MR data, the interface unit 31 stores each MR data in the storage unit 33 for each subject P.

  The image reconstruction unit 32 performs post-processing, that is, reconstruction processing such as Fourier transform, on the MR data stored in the storage unit 33 to generate image data in which the inside of the subject P is depicted.

  The storage unit 33 stores MR data received by the interface unit 31, image data generated by the image reconstruction unit 32, and the like for each subject P.

  The input unit 34 receives various instructions and information input from the operator. As the input unit 34, 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 35 displays various information such as spectrum data or image data under the control of the control unit 36. As the display unit 35, a display device such as a liquid crystal display can be used.

  The control unit 36 includes a CPU, a memory, and the like (not shown) and performs overall control of the MRI apparatus 100. Specifically, the control unit 36 generates sequence information based on various instructions received from the operator via the input unit 34, and controls the scan by transmitting the generated sequence information to the sequencer 10. Or image reconstruction performed based on MR data sent from the sequencer 10 as a result of scanning.

  The configuration example of the MRI apparatus according to the first embodiment has been described above. Under such a configuration, the MRI apparatus 100 has a function of creating a perfusion image by the ASL method. The ASL method is a method of obtaining a perfusion image by magnetically labeling blood flowing in a subject by applying an RF wave to the subject and using the labeled blood as a tracer.

  Here, creation of a perfusion image by the conventional ASL method will be described. 2-5 is a figure for demonstrating the principle of the conventional ASL method. 2 (a) and 2 (b) each show a flow model of a blood vessel, “artery” indicates an artery, “arteriole” indicates an arteriole, “capillary” indicates a capillary, and “vein” "Indicates a vein, and" Tracer "indicates a tracer. “Flow: f” indicates the flow of blood flow, and the arrow indicates the direction of blood flow.

  As shown in FIG. 2 (a), blood containing oxygen and nutrients flows from the artery and exchanges with gas and metabolites in the capillary bed during the mean transit time (MTT). Later it is drained into the vein. Here, since blood in a relatively thick blood vessel does not contribute to perfusion, in order to perform accurate perfusion measurement, as shown in FIG. 2 (b), all tracers are in capillaries, and arteries and It is desirable that the vein does not have a tracer.

FIG. 3 (a) shows a flow model similar to FIG. 2, showing three types of capillary beds with different delay times Td (Transit delay) before the blood reaches the target tissue after labeling. ing. FIG. 3 (b) shows changes over time in the signal intensity in each of the three types of capillary beds shown in FIG. 3 (a) and changes over time in T1 relaxation of blood. Yes. Here, T1 relaxation D (t) of blood is expressed by a function D (t) = exp [−t / T1 _b ] of time t, where T1 _b is the longitudinal relaxation time of blood.

For example, as shown in FIG. 3 (a), innumerable tissues having different Td are gathered in a capillary bed in one organ such as the brain (A, B, etc. shown in FIG. 3 (a)). C). In general, the tracer is provided at a certain point upstream of the artery. For example, in ASL, the tracer is provided by applying RF waves to the carotid artery. As a result, the change in signal intensity in the tissue of the capillary bed rises after reaching t = Td by the tracer, and becomes a TIC (Time Intensity Curve: signal intensity curve) corresponding to the blood flow dynamics of the tissue. For example, as shown in FIG. 3B, the TIC rises to Td _A in the organization A, the TIC rises to Td _B in the organization B, and the TIC rises to Td _C in the organization C. In ASL, the longer the Td, the greater the attenuation of the signal due to T1 relaxation.

4 and 5 show a flow model similar to that of FIG. 3 and signal strength and T1 relaxation over time. 4 shows a case where the bolus width is short (see _Tbolus shown in FIG. 4A), and FIG. 5 shows a case where the bolus width is long (see _Tbolus shown in FIG. 5A). Show. 4 and 5, the bolus concentration (the size of the vertical axis of AIF (Arterial Input Function)) is assumed to be constant. In such a case, as shown in FIGS. 4B and 5B, at a certain time TI after labeling the blood, the tracer concentration (signal intensity) between the tissues having different Td is changed. The difference is reduced and the tracer concentration itself is increased. In other words, the longer the bolus width, the stronger the SNR and the larger the SNR.

  However, in the measurement of only the tracer concentration at a certain time TI, the measured signal value is not correctly proportional to the blood flow, and correction corresponding to Td is required for each tissue. In ASL, the longitudinal relaxation D (t) of blood depends only on time t. Therefore, if the measured signal value is corrected by 1 / D (t), the relaxation can be ignored. Further, if the size of the target tissue is the same or smaller than the voxel size, it can be considered that the average concentration of the tracer is measured even if the tracer concentration is not in an equilibrium state in the voxel. If the tracer bolus width is long enough to fill the capillaries in the tissue, the average concentration of the tracer will not exceed a certain value.

  The need for such an ASL method is that, without using a contrast medium, only a blood vessel bed suppresses a signal of a thick blood vessel and has a large SNR (Signal Noise Ratio) as much as possible to obtain quantitative blood flow information. I want to. " In addition, there is a need for “additionally” to obtain the blood flow form and dynamic information in the blood vessel.

  Conventionally, the ASL method is roughly divided into MRA (MR Angio) and MRP (MR Perfusion) depending on its application. MRA is a method for obtaining a blood vessel image, and MRP is a method for obtaining a perfusion image. In MRP, an image in which the SNR of the blood flow signal is much smaller than the background stationary tissue (1/1000 or less) is obtained as compared with MRA. Therefore, in MRP, a process for suppressing the background signal is performed, such as obtaining a difference between the tag image and the control image. Further, even when the difference is taken, the MTC effect is made the same between the control image and the tag image, or addition averaging is performed a plurality of times in a low matrix.

  The ASL method is roughly divided into a PASL method, a CASL method, and a PCASL method due to the difference in the label method using RF waves. The PASL method is a method using a pulse wave (RF pulse), and the CASL method is a method using a continuous wave. The PCASL method is intended for practical use of the CASL method, and uses a large number of short RF pulses.

  Since the CASL method and the PCASL method are similar in nature, both are hereinafter collectively referred to as the CASL method. The PASL method and the CASL method are modeled as a difference in longitudinal magnetization density Mz (t) corresponding to an AIF (Arterial Input Function) that is a time function of the tracer concentration when labeling arterial blood.

  In the PASL method, the SNR of the blood flow signal is considered to be insufficient compared to the CASL method, but since the application of the RF pulse is one time, the application time of the RF pulse is as short as about 20 msec. Therefore, the PASL method has a tag efficiency that is the rate at which the blood passing through the range to which the RF pulse for labeling is applied is reversed, close to 100%, stable against flow velocity and magnetic field fluctuation, and compared with the SAR. Features such as small size.

In this PASL method, the tag thickness is set to be as large as 10 cm to ∞, and the rear end portion located on the upstream side of the labeled blood is sent from the front end portion located on the downstream side to reach the imaging region. Therefore, the PASL AIF has a shape accompanied by attenuation due to T1 relaxation of blood depending on the length of TI. Here, the application time T _tag in the AIF is determined from the _blood flow velocity V _blood and the _tag thickness D _tag . In the case of a steady flow, the passage time T _tag in the _tag is determined by the shorter of T _tag = D _tag / V _blood and T _tec , that is, T _tag = min [D _tag / V _blood , T _tec ]. T _tec is the time from when the RF pulse for labeling is applied until the saturation pulse for making the blood signal of the tag zero is applied.

  On the other hand, in the CASL method, since the amount of blood to be labeled (AIF time integral value) is large, the SNR of the ASL signal which is a blood flow signal is high. Therefore, in the CASL method, the waiting time TI from when a labeling RF wave is applied to when magnetic resonance data is collected can be set long, so that blood in a blood vessel with a slow flow rate hardly remains in the imaging region. Have

  The conventional PASL method and CASL method have been described above. However, it is known that these PASL method and CASL method have the following problems.

  First, in the PASL method, since the SNR is lowered, the TI cannot be set long. Therefore, in the PASL method, there is a problem that a blood vessel signal tends to remain in a blood vessel having a large Td because the blood flow velocity is slow or the flow path is long.

  On the other hand, the CASL method may not be practical due to restrictions of the RF amplifier and SAR. The PCASL method, which is an improved method of the CASL method, uses a pulse wave (RF wave), but the entire application time is as long as about 2 sec. Therefore, the CASL method has problems such as weak magnetic field inhomogeneity, tag efficiency varies depending on the flow velocity, and SAR is larger than the PASL method.

The CASL method has a larger SNR than the CASL method, but the TI is longer in the first half of the AIF (the portion labeled first within the time when the labeling RF wave is applied) than in the second half. , The probability of contributing to an ASL signal in a tissue with a large Td increases. However, the probability that the portion labeled in the second half cannot reach the imaging region and remains in the blood vessel increases. In CASL, in order to reduce this, the labeled blood is erased by increasing the waiting time T _pld (post labeling delay) after the RF wave for labeling is applied. However, there is no choice but to wait until T _pld elapses, resulting in wasted time. In this CASL, it is determined by T _tag = T _pld .

Further, when observing a dynamic change that is a temporal change in blood flow continuously, the CASL method has a long repetition time T _repeat, which is an interval from the application of the labeling RF wave to the data collection. The time efficiency is inferior compared with the PASL method. The CASL method may be used when continuously collecting data while changing the TI after applying a labeling RF wave once. However, in normal MRP, since the SNR of imaging is insufficient, addition averaging by applying a labeling RF wave a plurality of times is required. Therefore, the shorter the T _repeat, the shorter the total time.

  In addition, as a technique for improving temporal efficiency, a method (hereinafter referred to as HE method) applying Hadamard Encoding (HE) has been proposed in recent years. This HE method effectively uses the waiting time between TIs, efficiently collects RF waves for tag mode and control mode, collects a plurality of images, and adds and subtracts the collected images to calculate TI. This is a method for improving the SNR while reducing the acquisition time by creating a plurality of changed ASL images. This HE method has been proposed in combination with the CASL method.

  This HE method is applicable to both the PASL method and the CASL method. However, since many RF waves for labeling are required before imaging, the SAR is increased as compared with the method using one RF pulse. It is a method to do. Therefore, the HE method is compatible with the PASL method because the SAR is relatively large and a free time is generated between the RF pulses for labeling. In the PASL method, it is easy to add a plurality of mIR pulses for suppressing the background tissue at an arbitrary time in the free time between RF waves. Further, in the PASL method, since the SAR when the labeling RF wave is applied can be suppressed, the SAR allowed for imaging can be increased. That is, particularly when SSFP is used in a high magnetic field, the FA (Flip Angle) can be set large, and the SNR can be improved.

  As described above, the conventional PASL method has problems such as low SNR and that blood vessel signals are likely to remain in large blood vessels. Further, the conventional CASL method has problems such as a large SAR and a long tag (labeling pulse) application time compared to the PASL method.

  In response to such a problem, the MRI apparatus 100 according to the first embodiment reduces the remaining of SAR and blood vessel signals as compared with the CASL method while using the PASL method as a base, and the same as the CASL method is more than that. A data collection method capable of providing an SNR of a blood flow signal is performed.

  Specifically, the MRI apparatus 100 determines the tag thickness indicating the spatial thickness of the tag region to which the RF wave for labeling the blood is applied according to the flow velocity of the blood flowing in the subject. A tag condition including a tag interval indicating a time interval in which the RF wave is applied is set. Then, the MRI apparatus 100 applies the labeling RF wave to the tag region a plurality of times at the set tag interval, and MR of the imaging region where blood flows after a predetermined waiting time has elapsed since the RF wave was applied. Data is collected and an image is reconstructed from the collected MR data.

  The MRI apparatus 100 will be specifically described below. First, before describing the MRI apparatus 100 according to the first embodiment, a conventional PASL method will be described. FIG. 6 is a diagram for explaining a conventional PASL method. FIG. 6 shows an example of a single-tag-PASL (ST-PASL) method in which an RF pulse for tag mode is applied once and data is collected after a waiting time TI has elapsed since the application of the RF pulse. ing. 6A shows an example of an RF pulse application range in the ST-PASL method, and FIG. 6B shows an example of a pulse sequence in the ST-PASL method.

  In the PASL method, tag mode data collection for generating a tag image and control mode data collection for generating a control image are executed. For example, in the tag mode, as shown in FIG. 6, first, the saturation pulse SAT1 is applied to the region 42 including the imaging region 41, and then the tag mode 43 is set in the tag region 43 set upstream of the imaging region 41. RF pulse tag RF is applied.

Thereafter, at a time point before TInss2 from the start of data collection (imaging shown in FIG. 6B), the region 45 including the tag region 43 set upstream of the imaging region 41 and the control region 44 set downstream is A region non-selective IR pulse nssIR2 is applied. Subsequently, the saturation pulse SAT2 is applied to the region 46 including the tag region 43 set upstream of the imaging region 41 after a predetermined time T _tec has elapsed since the RF pulse tag RF was applied. Here, the saturation pulse SAT2 is used to reduce a signal in a thick blood vessel in the imaging region 41.

  Thereafter, at a time point before TInss1 from the start of data collection, the region non-selective IR pulse nssIR1 is applied to the region 45 including the tag region 43 set upstream of the imaging region 41 and the control region 44 set downstream. The Here, the region non-selective IR pulses nssIR1 and nssIR2 are used to selectively suppress signal intensities of a plurality of different tissues such as brain white matter and gray matter by reversing longitudinal magnetization at a predetermined timing. . Data collection from the imaging region 41 is started when the waiting time TI has elapsed since the tag mode RF pulse was applied.

  On the other hand, in the control mode, as shown in FIG. 6, first, the saturation pulse SAT1 is applied to the area 42 including the imaging area 41, and then the control area 44 set downstream of the imaging area 41 is used for the control mode. RF pulse control IR is applied.

  Thereafter, similarly to the tag mode, the region non-selective IR pulse nssIR2, the saturation pulse SAT2, and the region non-selective IR pulse nssIR1 are sequentially applied. Then, data collection from the imaging region 41 is started when TInss1 has elapsed since the application of the region non-selective IR pulse nssIR1.

  The MRI apparatus 100 according to the first embodiment executes an imaging method in which a labeling RF pulse is applied a plurality of times for one data collection, based on the above-described PASL method. Hereinafter, the imaging method executed by the MRI apparatus 100 is referred to as a Multi-Tag-PASL (MT-PASL) method.

  FIG. 7 is a diagram illustrating an outline of the MT-PASL method executed by the MRI apparatus 100 according to the first embodiment. 7A shows AIF and tissue TIC in data collection by the conventional ST-PASL method, and FIG. 7B shows AIF and tissue TIC in data collection by the MT-PASL method. Yes.

  As shown in FIG. 7A, in the conventional ST-PASL method, a labeling RF pulse (tag / control RF) 51 is applied once, and a waiting time TI has elapsed after the RF pulse 51 is applied. Later, data collection is performed. As a result, the AIF 61 and the tissue TIC 71 shown in FIG.

On the other hand, when the MT-PASL method is executed, the MRI apparatus 100 according to the first embodiment, for example, as shown in FIG. Is applied at a tag interval _Tint , and data collection is performed after a waiting time TI has elapsed since the first RF pulse 52 was applied. In FIG. 7B, AIF 62 and tissue TIC 72 correspond to the RF pulse 52, AIF 63 and tissue TIC 73 correspond to the RF pulse 53, and AIF 64 and tissue TIC 74 correspond to the RF pulse 54.

Here, for example, the MRI apparatus 100 sets tag conditions including the tag thickness D _tag and the tag interval T _int according to the flow velocity of blood flowing in the subject. Specifically, the MRI apparatus 100 sets at least one of the _tag thickness D _tag and the tag interval T _int so that the transit time T _tag required for blood to pass through the tag region and the tag interval T _int become the same. Set. As a result, blood labeled with the tag thickness D _tag at each tag interval T _int is spatially continuous without a gap.

8 and 9 are diagrams for explaining blood labeling by the MT-PASL method according to the first embodiment. FIG. 8 shows an example of the spatial distribution of blood to be labeled when the labeling RF pulse is applied three times in the MT-PASL method. As shown in FIGS. 8A to 8C, for example, at t = 0, t = T _int , and t = 2T _int , an RF pulse is applied to the tag region 81 with the tag thickness being D _tag. To do. In this case, as shown in FIG. 8 (d), the blood flowing out of the tag region 81 at t = 3T _int is labeled without any gap in space, and as a result, the range of 3D _tag is labeled. Will be.

On the other hand, FIG. 9 shows an example of a spatial distribution of blood to be labeled when a labeling RF pulse is applied once in the ST-PASL method. As shown in FIG. 9A, for example, when an RF pulse is applied to the tag region 82 having a tag thickness of 3D _tag at t = 0, as shown in FIG. 9B, The blood that flows out of the tag region 82 at t = 3T _int will be labeled in the 3D _tag range. That is, the range of blood that is labeled when an RF pulse is applied three times with a tag thickness of D _tag in the MT-PASL method is the blood that is labeled when the tag thickness is 3D _tag in the ST-PASL method. It becomes the same as the range.

  As a result, as shown in FIG. 7C, the entire AIF 65 in the MT-PASL method is a combination of AIFs 62 to 64, and the time integral value of the AIF compared to the AIF 61 in the ST-PASL method. Becomes larger. Further, the entire tissue TIC75 in the MT-PASL method is a combination of the tissues TIC72 to 74, and the SNR of the tissue ASL signal increases. Therefore, according to the MRI apparatus 100 according to the first embodiment, while remaining based on the PASL method, the remaining of the SAR and the blood vessel signal is reduced as compared with the CASL method, and the blood flow signal equal to or more than the CASL method is used. SNR can be provided.

  Next, a configuration example of the MRI apparatus 100 described above will be described. FIG. 10 is a diagram illustrating a configuration example of the MRI apparatus 100 according to the first embodiment. 10 shows the sequencer 10 and the computer system 30 shown in FIG. 1, and among the functional units of the computer system 30, an interface unit 31, an image reconstruction unit 32, a storage unit 33, an input unit 34, A display unit 35 and a control unit 36 are shown.

  As illustrated in FIG. 10, the storage unit 33 includes an imaging parameter storage unit 33a, an MR data storage unit 33b, and an image data storage unit 33c.

  The imaging parameter storage unit 33a stores various imaging parameters necessary for setting imaging conditions for obtaining a blood flow image. The MR data storage unit 33b stores MR data received from the sequencer 10 via the interface unit 31. The image data storage unit 33c stores the image reconstructed from the MR data by the image reconstruction unit 32.

  The control unit 36 includes an imaging condition setting unit 36a, a data collection unit 36b, a blood flow image generation unit 36c, and a display control unit 36d.

  The imaging condition setting unit 36a sets imaging conditions based on various instructions received from the operator via the input unit 34 and imaging parameters stored in the imaging parameter storage unit 33a. Further, the imaging condition setting unit 36a, when imaging by the MT-PASL method, is applied with a tag to which an RF wave for labeling the blood is applied according to the flow velocity of the blood flowing in the subject. A tag condition including a tag thickness indicating a spatial thickness of the region and a tag interval indicating a time interval in which the RF wave is applied is set.

In the first embodiment, an example will be described in which the imaging condition setting unit 36a sets the tag condition by regarding the blood flow as a steady flow when imaging by the MT-PASL method is performed. Assuming that the blood flow is a steady flow, assuming that the _blood flow velocity is V _blood and the tag thickness is D _tag , T _tag is the time required for blood to pass through the tag region, T _tag = D _tag / V represented by _blood .

  Here, in order to maximize the tag efficiency, it is desirable to optimally set the tag conditions including the tag thickness and the tag interval according to the blood flow rate. Therefore, the imaging condition setting unit 36a avoids interference so that blood that is sequentially labeled is not labeled more than once in the direction of blood flow when imaging by the MT-PASL method is performed, and labeling is performed. The tag condition is set so that no gap is generated between the blood.

Specifically, the imaging condition setting unit 36a sets the _tag thickness D _tag and the tag interval T _int so that the transit time T _tag required for blood to pass through the tag region and the tag interval T _int are the same. To do. That is, the imaging condition setting unit 36a sets the tag interval T _int so as to satisfy Equation 1 shown below.

T _int = D _tag / V _blood (Expression 1)

11 to 13 are diagrams illustrating the relationship between the tag interval and the AIF in the MT-PASL method according to the first embodiment. FIG. 11 shows a case where T _int = T _tag , (a) shows the spatial distribution of the blood to be labeled, and (b) shows the application timing of the RF pulse for labeling. (C) shows the AIF at the position of the arrow 83 shown in (a). FIG. 12 shows a case where T _int > T _tag , (a) shows the spatial distribution of the blood to be labeled, and (b) shows the RF pulse for labeling. The application timing is shown, and (c) shows the AIF at the position of the arrow 84 shown in (a). FIG. 13 shows a case where T _int <T _tag , (a) shows the spatial distribution of the blood to be labeled, and (b) shows the RF pulse for labeling. The application timing is shown, and (c) shows the AIF at the position of the arrow 85 shown in (a).

As described above, by setting the tag thickness D _tag and the tag interval T _int so that the passage time T _tag and the tag interval T _int are the same, blood flows as shown in FIG. In the direction, blood labeled with the tag thickness D _tag at every tag interval T _int is spatially continuous without a gap. As a result, as shown in FIG. 11C, the magnetization is relaxed in the individual AIFs corresponding to the respective RF pulses, but in the entire AIF, the AIFs are arranged without gaps along the time axis. The time integral value of AIF increases.

When T _int > T _tag , as shown in FIG. 12A, blood that is not spatially labeled is produced, so as shown in FIG. A gap is also generated between the corresponding AIFs. Therefore, the time integral value of the AIF becomes smaller than when T _int = T _tag, and the tag efficiency is lowered.

When T _int <T _tag , as shown in FIG. 13 (a), the nuclear spin included in a part of the blood once labeled is inverted again by the next RF pulse. Will be. As a result, the value of ΔMz is subtracted from M0 after having been decreased by M0 × exp (−t / T _int ) from M0 due to relaxation. As a result, as shown in FIG. 13C, the AIF corresponding to each RF pulse remains as a small positive value without ΔMz being 0 when T _int <t <T _tag . However, the time integral value of the AIF is smaller than when T _int = T _tag, and the tag efficiency is lowered.

  Note that the imaging condition setting unit 36a uses, for example, a standard blood flow velocity determined in advance for each type of blood vessel (blood vessel to which a labeling pulse is applied) in the tag portion as the blood flow velocity. Alternatively, for example, the imaging condition setting unit 36a may use the blood flow velocity measured by the pre-scan performed before the main scan as the blood flow velocity. In this case, the MRI apparatus 100 measures the blood flow rate, for example, by executing the Phase Contrast (PC) method in the pre-scan.

  Returning to the description of FIG. 10, the data collection unit 36 b generates sequence information based on the imaging conditions set by the imaging condition setting unit 36 a and transmits the generated sequence information to the sequencer 10 via the interface unit 31. To do. In addition, the data collection unit 36b stores the MR data received from the sequencer 10 via the interface unit 31 in the MR data storage unit 33b.

  In addition, when imaging by the MT-PASL method is performed, the data collection unit 36b applies the labeling RF wave to the tag region a plurality of times at the tag interval set by the imaging condition setting unit 36a, and the RF wave MR data of an imaging region in which blood flows after a predetermined waiting time TI has elapsed since the application of, is collected.

  Specifically, the data collection unit 36b applies tag mode RF pulses to the tag region a plurality of times at the tag interval set by the imaging condition setting unit 36a in the tag mode in the above-described data collection by the PASL method. Then, MR data of the imaging region is collected after a waiting time TI has elapsed since the RF wave was applied. Further, in the control mode, the data collection unit 36b applies the RF pulse for the control mode to the control region a plurality of times at the tag interval set by the imaging condition setting unit 36a, and waits for the waiting time TI after the RF wave is applied. After the elapse of time, MR data of the imaging region is collected.

For example, when the data collection unit 36b applies an RF pulse to the tag region 81 with a tag thickness of D _{tag at} t = 0, t = T _int and t = 2T _int , as shown in FIG. In addition, the blood flowing out of the tag region 81 at t = 3T _int is labeled without any spatial gap, and as a result, the range of 3D _tag is labeled.

  The image reconstruction unit 32 reconstructs a plurality of images corresponding to a plurality of different waiting times TI from the MR data collected by the data collection unit 36b. Specifically, the image reconstruction unit 32 reconstructs a tag image from MR data collected in the tag mode, and reconstructs a control image from the MR data collected in the control mode.

  The blood flow image generation unit 36 c generates a blood flow image from the image reconstructed by the image reconstruction unit 32. Specifically, the blood flow image generation unit 36c reads the tag image and control image reconstructed by the image reconstruction unit 32 from the image data storage unit 33c for each same TI, and reads the read tag image and control image. Is generated as a blood flow image. The blood flow image generation unit 36c stores the generated blood flow image in an internal memory or the like.

  The display control unit 36d causes the display unit 35 to display the blood flow image generated by the blood flow image generation unit 36c. Specifically, when the blood flow image is stored in an internal memory or the like by the blood flow image generation unit 36c, the display control unit 36d reads out the stored blood flow image and displays it on the display unit 35.

  Next, a processing procedure of the MT-PASL method executed by the MRI apparatus 100 according to the first embodiment will be described. FIG. 14 is a flowchart illustrating the processing procedure of the MT-PASL method executed by the MRI apparatus 100 according to the first embodiment. As illustrated in FIG. 14, when the MRI apparatus 100 receives an imaging start instruction by the MT-PASL method from the operator via the input unit 34 (step S11, Yes), the MRI apparatus 100 performs the following processing.

  First, the imaging condition setting unit 36a determines the tag thickness indicating the spatial thickness of the tag region to which the RF wave for labeling the blood is applied according to the flow velocity of the blood flowing in the subject, A tag condition including a tag interval indicating a time interval in which the RF wave is applied is set (step S12).

  Subsequently, the data collection unit 36b applies the tagging RF wave to the tag region a plurality of times at the tag interval set by the imaging condition setting unit 36a, and a predetermined waiting time TI has elapsed since the RF wave was applied. After that, MR data of the imaging region through which blood flows is collected (step S13).

  Subsequently, the image reconstruction unit 32 reconstructs the tag image and the control image from the MR data collected by the data collection unit 36b (step S14). Thereafter, the blood flow image generation unit 36c generates a difference image between the tag image reconstructed by the image reconstruction unit 32 and the control image as a blood flow image (step S15). Then, the display control unit 36d displays the blood flow image generated by the blood flow image generation unit 36c on the display unit 35 (step S16).

  As described above, in the MRI apparatus 100 according to the first embodiment, the imaging condition setting unit 36a applies an RF wave for labeling the blood according to the flow velocity of the blood flowing in the subject. A tag condition including a tag thickness indicating a spatial thickness of the tag region and a tag interval indicating a time interval in which the RF wave is applied is set. In addition, after the data collection unit 36b applies the labeling RF wave to the tag region a plurality of times at the tag interval set by the imaging condition setting unit 36a, and after a predetermined waiting time has elapsed since the RF wave was applied. MR data of the imaging region through which blood flows is collected. Then, the image reconstruction unit 32 reconstructs an image from the MR data collected by the data collection unit 36b. Therefore, according to the first embodiment, while the PASL method is used as a base, the remaining of the SAR and the blood vessel signal is reduced as compared with the CASL method, and the SNR of the blood flow signal that is higher than that of the CASL method is provided. be able to.

(Second Embodiment)
Next, a second embodiment will be described. In the first embodiment, an example has been described in which the imaging condition setting unit 36a sets the tag condition by regarding the blood flow as a steady flow when imaging by the MT-PASL method is performed. In the second embodiment, an example in which a tag condition is set with a blood flow as a pulsating flow (a flow that is not a steady flow) will be described.

Also in the second embodiment, the imaging condition setting unit 36a sets at least one of the tag thickness and the tag interval so that the passage time T _tag required for blood to pass through the tag region and the tag interval T _int are the same. Set. Specifically, in the second embodiment, the imaging condition setting unit 36a sets the blood flow velocity at time t to V _blood (t), the tag thickness to D _tag , and the tag interval to T _int. At least one of D _tag and T _int is set so as to satisfy the above.

For example, when the blood flow is a pulsatile flow, the imaging condition setting unit 36a dynamically sets the tag interval T _int so that the tag thickness D _tag is constant according to the change in the blood flow velocity. Set. Specifically, the imaging condition setting unit 36a adjusts the tag interval T _int according to the _blood flow velocity V _blood (t) so as to satisfy Expression 2 after setting the _tag thickness D _tag to a fixed value. To set. This means that the imaging condition setting unit 36a varies the tag interval T _int according to the change in the blood flow velocity.

15 to 17 are diagrams for explaining data collection when the _tag thickness D _tag is fixed by the MT-PASL method according to the second embodiment. FIG. 15A shows the change over time of the ECG signal detected by the ECG sensor 21, and FIG. 15B shows the change over time of the _blood flow velocity V _blood (t). ing. FIG. 15C shows the application timing of the labeling RF pulse, and FIG. 15D shows the AIF corresponding to the arrow 86 shown in FIG. . FIGS. 16A to 16C show the spatial distribution of blood labeled when the _tag thickness D _tag is fixed in the MT-PASL method, and FIG. 17 shows the ST-PASL. 2 shows an example of the spatial distribution of blood labeled in the law.

  In the second embodiment, the imaging condition setting unit 36a controls the execution of the sequence of the MT-PASL method based on the heartbeat detected by the ECG sensor 21. For example, as shown in FIGS. 15A to 15C, the imaging condition setting unit 36a sets the number of RF pulses applied in one MT-PASL method sequence to three, and every heartbeat. Tag conditions and imaging conditions are set so as to execute a single MT-PASL method sequence. As a result, as shown in FIG. 16, the labeled blood is arranged with no gap in the same tag thickness.

  Then, as shown in FIG. 15 (d), the AIFs are arranged at different time intervals for each RF pulse. For example, the AIF in MT-PASL (see the solid line shown in FIG. 15 (d)) is the AIF when a labeling RF pulse is applied once in the ST-PASL method as shown in FIG. Compared to the broken line shown in FIG.

The imaging condition setting unit 36a measures the blood flow velocity V _blood (t) in real time with a blood flow meter such as an ultrasonic blood flow meter, and dynamically uses the measured flow velocity to dynamically set the tag interval T _int. Set. Alternatively, for example, the imaging condition setting unit 36a uses a standard blood flow rate determined in advance for each blood vessel type of the tag unit as the _blood flow rate V _blood (t). Alternatively, for example, the imaging condition setting unit 36a may use the blood flow velocity measured by the pre-scan performed before the main scan as the _blood flow velocity V _blood (t). In this case, the MRI apparatus 100 measures the blood flow rate, for example, by executing the Phase Contrast (PC) method in the pre-scan.

When using a standard flow rate or a flow rate measured by pre-scanning, for example, an RF pulse applied within one heartbeat using a flow rate of one heartbeat time (T _RR ) measured before the main scan. And the tag intervals T _int (t1), T _int (t2),... T _int (tn) are determined in advance and stored in the storage unit 33 or the like. Then, the imaging condition setting unit 36a estimates the flow velocity at a desired phase from the delay time from the R wave while synchronizing the gate signal with the R wave based on the heartbeat detected by the ECG sensor 21. Imaging conditions are dynamically set so that one MT-PASL method sequence is executed for each heartbeat. Since the blood flow rate changes continuously, for example, the imaging condition setting unit 36a may use an average flow rate during each T _int . Further, _TRR may fluctuate during measurement of the same object, but the timing of the R wave is always measured, and even if there is some fluctuation until the next R wave, no major problem occurs.

Alternatively, for example, when the blood flow is a pulsatile flow, the imaging condition setting unit 36a determines that the passage time T _tag required for the blood to pass through the tag region is constant according to the change in the blood flow velocity. The tag thickness D _tag may be set dynamically so that Specifically, the imaging condition setting unit 36a adjusts the _tag thickness D _tag in accordance with the _blood flow velocity V _blood (t) so as to satisfy Equation 2 after setting the passage time T _tag to a fixed value. To set. This means that the imaging condition setting unit 36a varies the _tag thickness D _tag according to a change in blood flow velocity.

18 to 20 are diagrams for explaining data collection when the transit time T _tag is fixed by the MT-PASL method according to the second embodiment. 18A shows the change with time of the ECG signal detected by the ECG sensor 21, and FIG. 18B shows the change with time of the blood flow velocity V _blood (t). ing. 18C shows the application timing of the labeling RF pulse, and FIG. 18D shows the AIF corresponding to the arrow 87 shown in FIG. 19C. . FIGS. 19A to 19C show the spatial distribution of blood labeled when the transit time T _tag is fixed in the MT-PASL method, and FIG. 20 shows the ST-PASL. 2 shows an example of the spatial distribution of blood labeled in the law.

In the second embodiment, the imaging condition setting unit 36a controls the execution of the sequence of the MT-PASL method based on the heartbeat detected by the ECG sensor 21. For example, as shown in FIGS. 18A to 18C, the imaging condition setting unit 36a sets the number of RF pulses applied in one MT-PASL method to three, and every heartbeat. Tag conditions and imaging conditions are set so as to execute a single MT-PASL method sequence. As a result, as shown in FIG. 19, the labeled blood has no gap in space with tag thicknesses D _tag (t ₁ ), D _tag (t ₂ ), and D _tag (t ₃ ) proportional to the flow rate. Will be lined up.

Then, as shown in FIG. 18D, the AIFs are arranged at the same time interval for each RF pulse. For example, the AIF in MT-PASL (see the solid line shown in FIG. 18 (d)) is the AIF when the labeling RF pulse is applied once in the ST-PASL method as shown in FIG. Compared to the broken line shown in FIG. _Moreover, AIF time width and space thickness between _{T RR} is the same as the case of fixing the TaguAtsu _{D tag.}

The imaging condition setting unit 36a measures the blood flow velocity V _blood (t) in real time with a blood flow meter such as an ultrasonic blood flow meter, and dynamically uses the measured flow velocity to dynamically set the tag interval T _int. Set. Alternatively, for example, the imaging condition setting unit 36a uses a standard blood flow rate determined in advance for each blood vessel type of the tag unit as the _blood flow rate V _blood (t). Alternatively, for example, the imaging condition setting unit 36a may use the blood flow velocity measured by the pre-scan performed before the main scan as the _blood flow velocity V _blood (t). In this case, the MRI apparatus 100 measures the blood flow rate, for example, by executing the Phase Contrast (PC) method in the pre-scan.

When using a standard flow rate or a flow rate measured by pre-scanning, for example, an RF pulse applied within one heartbeat using a flow rate of one heartbeat time (T _RR ) measured before the main scan. And the _tag thicknesses D _tag (t1), D _tag (t2),... D _tag (tn) are determined in advance and stored in the storage unit 33 or the like. Then, the imaging condition setting unit 36a estimates the flow velocity at a desired phase from the delay time from the R wave while synchronizing the gate signal with the R wave based on the heartbeat detected by the ECG sensor 21. Imaging conditions are dynamically set so that one MT-PASL method sequence is executed for each heartbeat. Since the blood flow rate changes continuously, for example, the imaging condition setting unit 36a may use an average flow rate during each T _int . Further, _TRR may fluctuate during measurement of the same object, but the timing of the R wave is always measured, and even if there is some fluctuation until the next R wave, no major problem occurs.

  As described above, in the MRI apparatus 100 according to the second embodiment, the imaging condition setting unit 36a causes the tag thickness and the tag interval so that the passage time and the tag interval required for blood to pass through the tag region are the same. Set at least one of the tag intervals. For example, the imaging condition setting unit 36a dynamically sets the tag thickness so that the passage time is constant. Alternatively, the imaging condition setting unit 36a dynamically sets the tag interval so that the tag thickness is constant. In any case, even if MT-PASL under pulsating flow, AIF without a time gap is generated, and tag efficiency can be improved.

For example, if the total time during which the labeling RF pulse is applied is the same, the D _tag is decreased or the T _int is shortened to increase the number of application of the RF pulse, thereby reducing attenuation due to AIF. Since the slab space characteristic (profile) of the RF pulse becomes closer to a rectangle, the tag efficiency is improved.

  FIG. 21 is a diagram for explaining the improvement of the RF pulse profile in the MT-PASL method according to the second embodiment. In FIG. 21, (a) shows the RF pulse profile in the ST-PASL method, and (b) shows the RF pulse profile in the MT-PASL method.

  As shown in FIG. 21, in the MT-PASL method, a slab in the spatial axis (z) direction is applied even when the same RF pulse is used as compared with the ST-PASL method by applying a labeling RF pulse a plurality of times. Spatial profile is improved.

  Here, for example, in the case of an RF pulse in which the nuclear spin falls by 180 ° at the maximum, for a plurality of RF pulses, the portions of adjacent RF pulses that are 180 ° or less are overlapped at every half-width 90 ° position. It is desirable to make it. As a result, when two RF pulses are applied, the portion between these RF pulses is about 180 ° in total, so that the nuclear spin falls by 180 °. That is, the half-value width FWHM of the RF pulse profile is improved to a magnification of the number of RF pulses.

Note that the labeling RF pulse used in the MT-PASL method does not necessarily have good slab excitation characteristics. Even when there is a flow in the Z-axis direction, if the tag condition is optimized with a uniform plug flow and the relaxation is ignored, it becomes the same as the stationary portion. In the case of laminar flow or turbulent flow, the flows are mixed with each other and may be somewhat different. However, if the transition part that reaches 0 ° to 180 ° is short and T _int is sufficiently short (˜100 ms), an error will occur. Can be ignored.

  Further, since the tag thickness is defined by the half-value width FWHM of a commonly used RF pulse profile, if the FWHM of a plurality of labeling RF pulses is overlapped with no gap in the flow velocity direction, 180 ° between the slabs. The degradation of tag efficiency in the following part can be ignored. On the other hand, if the slab space characteristics are equal, MT-PASL can equalize the SAR and reduce the maximum RF power restriction by using an RF pulse with a low peak value and a poor profile.

(Third embodiment)
Next, a third embodiment will be described. In the third embodiment, various modifications related to the above-described first and third embodiments will be described.

For example, regarding the timing of applying the labeling RF pulse, in the ASL of the head, the flow velocity becomes maximum in the systole of the heart in the vicinity of the carotid artery where the labeling RF pulse is applied. Therefore, for example, the imaging condition setting unit 36a applies the labeling RF wave during a period including the systole of the heart where the flow rate of the blood flow becomes the maximum speed based on the heartbeat detected by the ECG sensor 21. The imaging conditions may be set in. When the tag thickness is thin or the flow velocity is high and T _tag is shorter than one heartbeat cycle, V _blood can be regarded as constant within T _tag , so it can be approximated to steady flow, and in that case the flow velocity is high Is more convenient because T _int can be shortened.

For example, when the heart rate cycle is usually around 1 sec and V _blood is 30 cm / sec, which is averaged in the carotid artery, D _tag = 10 to 20 cm, which is a normal tag thickness, and T _tag = 0.33 to 0.67 sec And below heartbeat cycle. If on the basis of the heart beat match the application timing of RF pulse for labeling the _systole, since _{V blood} is _{large, T int} may even shorter. For example, if D _tag = 10 cm and V _blood = 100 cm, T _int = 0.1 sec. Thus, if T _int under the optimum condition is equal to or less than the cardiac cycle, it is effective to apply a plurality of RF pulses from the systole.

It is also possible to set D _tag itself so that T _int is an integral multiple (usually 1 time) of the cardiac cycle. In particular, if the SAR is a problem is one way to maximize the tags efficiency while increasing the T _int. For example, if D _tag = 100 cm and V _blood = 100 cm / sec, T _int = 1 sec, which is almost the heartbeat cycle. However, since the T1 of blood is actually about 1.5 sec, it is desirable that the number of times of applying the labeling RF pulse at the heartbeat cycle interval is about two times.

  Here, in the MT-PASL method, even if the application timing of the RF pulse is not necessarily controlled based on the heartbeat, the tag condition is set to the optimum condition for the average flow velocity (nominal value or measurement value) of the target blood vessel. Compared with the ST-PASL method that does not synchronize with the electrocardiogram, an improvement in tag efficiency can be expected.

  Note that the optimum tag condition setting method described here is applicable not only to the MT-PASL method but also to the HE-MT-PASL method described later. Further, since it is a feature related to the application of the RF pulse for labeling, it can be applied independently of the imaging unit. That is, a single TI having a single imaging unit or a plurality of multi TIs can be applied.

In addition, the T _tag control method at the time of the flow rate fluctuation described here is useful for improving the tag efficiency in the Multi-Tag (MT), but also in the Single-Tag (ST) ST-PASL, the tag has a flow rate of multiple times. It is applicable to make Ttag constant when it fluctuates every time, and thus to make the flow rate of tag blood constant.

  When a plurality of blood vessels with different flow rates exist in the tag portion, even if determined by a single flow rate (time distribution), the other blood vessels do not match. If the subject to be labeled is the cervix, the main arteries going to the brain tissue are the internal carotid artery, the external carotid artery and the vertebral artery, and the former two have a slower flow rate than the latter. In that case, it is better to match with the average flow velocity or with the carotid artery with a large blood flow, but the difference becomes less as the tag thickness of the MT-PASL method becomes thinner.

  The MT-PASL method described in the above embodiment can also be applied to the Hadamard Encoding (HE) method. As for the HE method, for example, a method combined with the CASL method is proposed by Wells JA at al. In vivo hadamard encoded continuous arterial spin labeling (H-CASL). MRM 63: 1111-1118 (2010). Below, the HE-MT-MASL method which is MT-PASL method using HE method is demonstrated.

  In the HE-MT-MASL method, for example, the data collection unit 36b collects data by efficiently arranging RF pulses for tag mode and RF pulses for control mode at predetermined time intervals, and collects images. The HE-MTPASL method for obtaining a plurality of ASL images with different waiting times TI is added / subtracted. According to this method, the SNR can be further improved as compared with the ST-SI method and the ST-MI method.

22 and 23 are diagrams for explaining the HE-MT-MASL method according to the third embodiment. In the HE-MT-PASL method, for example, the number N of combinations of different waiting times TI is set to 3, 5, 7, 15..., That is, N = 2 ⁿ⁻¹ (n is a natural number). For example, FIG. 22 shows an example in which the number of combinations of different waiting times TI is N = 7 (n = 2).

  Note that among the plurality of RF pulses (tag / control RF) shown in FIG. 22, the ones with a left-down diagonal line are tag mode RF pulses, and the one with a right-down diagonal line is a control mode. RF pulse for use. As shown in FIG. 22, for example, when N = 7, combinations of N + 1 = 8 types of pulse sequences having different RF pulse orders are used. Note that TI (i) may be an arbitrary time interval as long as it is the same in each pulse sequence of k = 1 to 8.

  Here, Sk is a complex signal of data collected in each pulse sequence of k = 1 to 8, and a quadruple difference signal S {TI (i)} in each TI (i), i = 1 to 7 is It is obtained by the following formula.

    S {TI (i)} = 4 [S {TI (i)}-Scont {TI (i)}]

  That is, when i = 1 to 7, S {TI (1)} to S {TI (7)} are obtained by the following equations.

S {TI (1)} = S1-S2 + S3-S4 + S5-S6 + S7-S8
S {TI (2)} = S1 + S2-S3-S4 + S5 + S6-S7-S8
S {TI (3)} = S1-S2-S3 + S4 + S5-S6-S7 + S8
S {TI (4)} = S1 + S2 + S3 + S4-S5-S6-S7-S8
S {TI (5)} = S1-S2 + S3-S4 + S5-S6 + S7-S8
S {TI (6)} = S1 + S2-S3-S4-S5-S6 + S7 + S8
S {TI (7)} = S1-S2-S3 + S4-S5 + S6 + S7-S8

  The SNR in this case is sqrt [(N + 1) / 2] times equivalent to NAQ = (N + 1) / 2 addition of the difference per one time. Here, NAQ = sqrt (4) = 2 times corresponding to the addition of 4 times. That is, finally, as a result of the synthesis operation, as shown in FIG. 23, the RF pulse from which the TI is separated and the imaging unit are arranged, and a differential image having a double SNR is generated.

Also, _assuming that one repeat time is T _repeat , the imaging time is N sets of TIs.
(N + 1) * T _repeat . On the other hand, in the case where data collection is performed every time an RF pulse for labeling is applied once, the imaging time is 2 * (N + 1) / 2 * N * T _repeat = N (N + 1) for N sets of TIs. ) * T _repeat . Therefore, in the HE-MT-SI method, the imaging time ratio is N + 1 / N (N + 1) = 1 / N, compared to the case where data collection is performed every time a labeling RF pulse is applied once. In other words, when N = 7, a difference image with a different SNR TI is obtained in 1/7 time, equivalent to the case where data collection is performed each time an RF pulse for labeling is applied once. Become.

  In addition, in a general ST-PASL method in ASL MR Perfusion (ASL-MRP), several averages are performed in order to secure a sufficient SNR. On the other hand, according to the HE-MT-PASL method described above, the same data can be obtained only by one acquisition, and an image equivalent to the average of several stages of TI can be obtained. It can be shortened effectively. The RF pulse for the control mode may be applied only when MTC is a problem for MRP. Therefore, when MTC is not a problem for MRA of blood vessels, the RF pulse for control mode may be omitted.

  The example using the HE-MT-PASL method has been described above. However, in this method, there are restrictions on tag conditions such as control of the tag interval according to the blood flow rate. Further, in the HE method, when the blood flow rate fluctuates with time, for example, when used under pulsatile flow, when the blood flow rate is not the same at the timing of the RF pulse at the same time, the image is added or subtracted. In particular, when subtracting, the fluid signal labeled at an unnecessary timing may not disappear as intended. This is the same whether combined with PASL or combined with CASL.

  Therefore, for example, the first RF pulse may be applied using the R wave in the heartbeat gate signal as a trigger signal so that RF pulses at the same time are performed in the same cardiac phase.

  Furthermore, in the PASL method, in order to maximize the tag efficiency even in a steady flow, it is desirable to control the tag conditions such as the tag thickness and the tag interval according to the blood flow rate. Therefore, for example, the data collection unit 36b may set the tag thickness, the tag interval, and the number of RF pulses so that the tag efficiency is maximized based on the flow velocity, diameter, or flow rate of the target blood vessel. In this case, for example, the flow velocity, diameter, and flow rate of the target blood vessel are stored in advance in the apparatus for each type of blood vessel based on statistical data and the like. Further, for example, the flow velocity of the target blood vessel may be obtained dynamically by the phase contrast MRA method or the like. For such optimization of tag conditions, the MT-PASL method using a plurality of labeling pulses described in the above embodiment can be applied as it is.

  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.

DESCRIPTION OF SYMBOLS 100 MRI (magnetic resonance imaging) apparatus 30 Computer system 32 Image reconstruction part 36 Control part 36a Imaging condition setting part 36b Data collection part 36c Blood flow image generation part

Claims (7)

A tag thickness indicating a spatial thickness of a tag region to which an RF wave for labeling the fluid is applied and a time interval at which the RF wave is applied according to the flow velocity of the fluid flowing in the subject. An imaging condition setting unit for setting a tag condition including a tag interval to be shown;
The RF wave is applied to the tag region a plurality of times at the tag interval set by the imaging condition setting unit, and the magnetism of the imaging region where the fluid flows after a predetermined waiting time has elapsed since the RF wave was applied. A data collection unit for collecting resonance data;
An image reconstruction unit for reconstructing an image from the magnetic resonance data collected by the data collection unit;
A fluid image generation unit that generates a fluid image from the image reconstructed by the image reconstruction unit;
A magnetic resonance imaging apparatus comprising:   The imaging condition setting unit sets at least one of the tag thickness and the tag interval so that a passage time required for the fluid to pass through the tag region and the tag interval are the same. The magnetic resonance imaging apparatus according to claim 1.   When the fluid flow is a pulsating flow, the imaging condition setting unit dynamically sets the tag thickness so that the passage time is constant according to a change in the flow velocity of the fluid. The magnetic resonance imaging apparatus according to claim 2.   When the fluid flow is a pulsating flow, the imaging condition setting unit dynamically sets the tag interval so that the tag thickness is constant according to a change in the flow velocity of the fluid. The magnetic resonance imaging apparatus according to claim 2.   The imaging condition setting unit uses, as the fluid flow velocity, a standard fluid flow velocity determined in advance for each type of flow tube of the tag unit. The magnetic resonance imaging apparatus described.   5. The magnetism according to claim 1, wherein the imaging condition setting unit uses a fluid flow rate measured by a pre-scan performed before a main scan as the fluid flow rate. Resonance imaging device. A detection unit for detecting a heartbeat of the subject;
The imaging condition setting unit sets the imaging condition based on the heartbeat detected by the detection unit so that the RF wave is applied in a period including a systole of the heart where the flow velocity of the fluid reaches a maximum speed. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus.

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