Magnetic resonance imaging apparatus and control method thereof

Abstract

To realize, in contrast-enhanced MRI, increase in a visualization capability of a tissue which a contrast agent reaches, and decrease in overall imaging time.SOLUTION: An imaging unit of an MRI apparatus includes a gradient echo pulse sequence for acquiring a T1-weighted image including a fat saturation pulse, and a control unit controls the imaging unit to generate images of a plurality of phases having different arrival positions of a contrast agent by repeating the pulse sequence for a predetermined time from administration of the contrast agent to the subject. Here, a preparation pulse that suppresses a signal from a contrast agent present outside a target tissue (cell) is added prior to a pulse sequence, form a phase immediately before reaching the target tissue.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a magnetic resonance imaging apparatus (hereinafter abbreviated as an MRI apparatus), and more particularly to a control technique for an MRI apparatus in an MR examination using a contrast agent. [Background technology]

[0002] An MRI device is an imaging diagnostic device that collects and images signals generated by nuclear magnetic resonance from tissues under examination. By varying the pulse sequence and imaging conditions, it is possible to suppress signals from tissues not necessary for diagnosis and generate various enhanced images with different contrasts.

[0003] For example, in the diagnosis of hepatocellular carcinoma, there is a method to suppress the extracellular space of the liver using intravoxel incoherent motion imaging (IVIM) with a DWI (diffusion weighted) sequence. However, because the pulse sequence underlying IVIM is SE-EPI (spin echo echo planar), the TE (echo time) is extended, and the signal of the interstitial fluid increases due to the long T2 decay time of transverse magnetization, while the signal of the intracellular space decreases due to the short T2 decay time. This results in poor contrast in the resulting images, and low diagnostic capability.

[0004] In response to this, imaging techniques have been developed that improve diagnostic capabilities by combining contrast agents, taking advantage of differences in the absorption and retention time of contrast agents in tissue. For example, in the diagnosis of hepatocellular carcinoma, Gd-EOB-DTPA (gadolinium ethoxybenzyl diethylenetriamine pentaacetic Using a contrast agent such as EOB (extracted oxalate, hereafter abbreviated as EOB), continuous imaging (dynamic MRI) is performed from the time the contrast agent is administered until the time the contrast agent remains in the liver cells, and images are obtained that clearly distinguish between normal liver tissue and cancerous liver cells (Non-Patent Document 1).

[0005] This technique obtains images of each phase of the process from when the contrast agent is administered through the arteries, portal vein, and liver tissue. This provides detailed information about the cancer from images of the relatively early phase (arterial phase), in which the contrast agent is absorbed into the cancer from the arteries that nourish the cancer, and from images of the later phase (hepatocellular phase), in which the contrast agent travels through the arteries and portal vein to reach and reside in hepatocytes.

[0006] Furthermore, in MRI, there are several techniques for suppressing signals from tissues other than the target tissue, and these are also applied in contrast-enhanced MRI. For example, the technique described in Non-Patent Document 1 improves the visualization of liver cells by performing fat suppression during imaging. Furthermore, Non-Patent Document 2 discloses the use of MSDE (Motion-Sensitized Driven-Equilibrium) pulses to suppress arterial and portal vein signals in gallbladder imaging, where T2 weighting is important. However, the technique described in Non-Patent Document 2 is based on T2 weighted imaging and cannot be applied to liver cancer diagnosis, which involves tracking the movement of contrast agents from images of each phase as described above. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] “Pseudo-random Trajectory Scanning Suppresses Motion Artifacts on Gadoxetic Acid-enhanced Hepatobiliary-phase Magnetic Resonance Images” Yuko Nakamura et al, Magn. Reson. Med. Sci. doi:10.2463 / mrms.mp.2018-0174 [Non-patent document 2] “Balanced MR cholangiopancreatography with motion-sensitized driven-equilibrium (MSDE) preparation: feasibility of Gd-EOB-DTPA-enhanced biliary examination” Nakayama T. et al, Clin. Radiol. 2016 Dec. 71(12):1284-1288 Summary of the Invention [Problem to be solved by the invention]

[0008] Generally, it takes approximately 20 minutes for a contrast agent to be taken up by hepatocytes after administration and for the untaken contrast agent to be excreted. However, in patients with impaired liver function, the uptake and excretion of contrast agent takes longer, making it difficult to determine whether the patient is truly in the hepatocyte phase even after 20 minutes, which is generally considered the hepatocyte phase. If contrast agent is present but not taken up by hepatocytes, i.e., if contrast agent is present in the extracellular space of the liver, such as interstitial fluid and blood vessels, it is not possible to visualize tumors with high brightness, resulting in a decrease in diagnostic accuracy. Furthermore, imaging all subjects, including those with slow contrast agent uptake, up to the hepatocyte phase would take a long time.

[0009] The present invention aims to shorten the examination time, which increases depending on the arrival time of the contrast agent, in MRI using a contrast agent, and to provide images with excellent diagnostic ability in a relatively short time. [Means for solving the problem]

[0010] In order to solve the above problem, the present invention performs imaging by adding a pulse to suppress contrast agent signals in tissues around the target tissue (e.g., areas with movement such as the extracellular space of the liver) when imaging continuously across multiple phases after a predetermined phase has elapsed since the start of imaging.

[0011] That is, the MRI apparatus of the present invention includes an imaging unit that applies a radio frequency magnetic field and a gradient magnetic field to a subject, collects nuclear magnetic resonance signals generated from the subject, and generates an image of the subject, and a control unit that controls the imaging unit. The control unit controls the imaging unit to repeat a pulse sequence for a predetermined time after administration of a contrast agent to the subject, and generate images of multiple phases where the contrast agent reaches different positions. At this time, in some of the multiple phases, a preparation pulse that suppresses signals from the contrast agent existing outside the target tissue (cell) of the contrast agent is added prior to the pulse sequence. The pulse sequence executed by the imaging unit is, for example, a pulse sequence that acquires a T1-weighted image including a fat suppression pulse.

[0012] Furthermore, a control method for an MRI apparatus of the present invention is a control method for a magnetic resonance imaging apparatus that applies a radio frequency magnetic field and a gradient magnetic field to a subject in accordance with a pulse sequence, collects nuclear magnetic resonance signals generated from the subject, and generates an image of the subject, using a pulse sequence for acquiring a T1-weighted image including a fat suppression pulse, repeating the pulse sequence for a predetermined time after administration of a contrast agent to the subject, generating images of a plurality of phases where the contrast agent reaches different positions, and adding, prior to the pulse sequence, a preparation pulse in some of the plurality of phases that suppresses signals from the contrast agent present outside the target tissue (cell) of the contrast agent. [Effects of the Invention]

[0013] According to the present invention, signals from regions containing contrast agent present around the target tissue can be suppressed during a predetermined phase during which the contrast agent reaches the target tissue, thereby making it possible to obtain images in which the target tissue can be clearly distinguished from other tissues without waiting for imaging during the phase in which most of the contrast agent reaches the target tissue, thereby enabling images with excellent diagnostic capabilities to be obtained in a short time.

[0014] The present invention can be applied to contrast-enhanced MRI, for example, targeting hepatocytes, and can visualize the liver with high contrast, thereby improving the diagnostic ability for liver tumors. In this case, images with contrast equivalent to the hepatocyte phase can be obtained in the portal venous phase or equilibrium phase, thereby shortening the examination time. Furthermore, information useful for evaluating liver function can be obtained. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram showing the overall configuration of an MRI apparatus to which the present invention is applied. [Figure 2] FIG. 1 is a diagram showing an example of a gradient echo pulse sequence including a fat suppression pulse. [Figure 3] FIG. 10 shows a modified example of a fat saturation pulse. [Figure 4] FIG. 1 illustrates an embodiment of a contrast-enhanced MRI imaging procedure according to the present invention. [Figure 5] Diagram explaining phase changes associated with contrast agent movement [Figure 6] FIG. 1 shows an example of a pulse sequence used in the first embodiment. [Figure 7] Diagram showing an example of an MSDE pulse [Figure 8] FIG. 1 is a diagram showing the flow of imaging (control) in the first embodiment. [Figure 9] FIG. 10 is a diagram showing the imaging flow of the second embodiment. [Figure 10] Functional block diagram of a computer according to a second embodiment [Figure 11] FIG. 10 is a diagram showing an example of a UI displayed on a display device. [Figure 12] FIG. 10 is a diagram showing a processing flow in a computer according to a second embodiment. [Figure 13] FIG. 10 is a diagram showing an example of a result display on a display device. [Figure 14] FIG. 10 is a diagram showing the imaging flow in a modified example of the second embodiment. [Figure 15] FIG. 10 is a diagram showing an example of a pulse sequence used in the third embodiment. [Figure 16] FIG. 10 is a diagram illustrating spin labeling in embodiment 4. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the MRI apparatus of the present invention will be described with reference to the drawings.

[0017] First, the overall configuration of the MRI apparatus to which the present invention is applied will be described. <Configuration of MRI Apparatus> As shown in FIG. 1, the MRI apparatus 1 of the present embodiment includes a static magnetic field generation unit such as a static magnetic field coil 11 that generates a static magnetic field in a space where a subject is placed, a transmission high-frequency coil 12 (hereinafter simply referred to as a transmission coil) that transmits a high-frequency magnetic field pulse (RF pulse) to a measurement region of the subject, and a transmitter 16, a reception high-frequency coil 13 (hereinafter simply referred to as a reception coil) that receives a nuclear magnetic resonance signal generated from the subject, and a receiver 17, a gradient magnetic field coil 14 that applies a magnetic field gradient to the static magnetic field generated by the static magnetic field coil 11, and a gradient magnetic field power supply 15 that is its driving power supply, a sequence control device 18, and a computer 20. Each part of the MRI apparatus 1 except the computer 20 is collectively referred to as an imaging unit 10.

[0018] The MRI apparatus 1 has a vertical magnetic field method or a horizontal magnetic field method depending on the direction of the generated static magnetic field, and the static magnetic field coil 11 is adopted in various forms according to the method. The gradient magnetic field coil 14 is composed of a combination of a plurality of coils that generate gradient magnetic fields in three mutually orthogonal axial directions (x direction, y direction, z direction), and each is driven by a gradient magnetic field power supply 15. By applying a gradient magnetic field, position information can be added to the nuclear magnetic resonance signal generated from the subject.

[0019] In the illustrated example, the case where the transmission coil 12 and the reception coil 13 are separate is shown, but there may be a case where one coil that combines the functions of the transmission coil 12 and the reception coil 13 is used. The high-frequency magnetic field irradiated by the transmission coil 12 is generated by the transmitter 16. The nuclear magnetic resonance signal detected by the reception coil 13 is sent to the computer 20 through the receiver 17.

[0020] The sequence control device 18 controls the operations of the gradient magnetic field power supply 15, transmitter 16, and receiver 17, controls the timing of application of the gradient magnetic field and radio frequency magnetic field, and reception of nuclear magnetic resonance signals, and performs measurement. The control time chart is called a pulse sequence, and various pulse sequences that differ depending on the imaging method are stored in advance in a storage device or the like provided in the computer 20. Once the imaging method and imaging region are determined, the sequence control device 18 reads out a predetermined pulse sequence, calculates an imaging sequence to be used for imaging using imaging conditions set by the user, and controls the imaging unit 10 to collect nuclear magnetic resonance signals according to this imaging sequence.

[0021] The computer 20 is an information processing device equipped with a CPU, memory, storage device, etc., and has a function as a control unit that controls the operation of each part of the MRI apparatus via the sequence control device 18, and a function as a calculation unit that performs calculation processing on received echo signals. These functions are executed by the computer 20 reading a program stored in the storage device. However, some of the functions can also be realized by a programmable IC such as an ASIC or FPGA.

[0022] The computer 20 is connected to a display device 30, an input device 40, an external storage device 50, and the like. The display device 30 is an interface that displays results obtained by arithmetic processing to an operator. The input device 40 is an interface through which an operator inputs conditions, parameters, and the like necessary for the measurements and arithmetic processing performed in this embodiment. A user can input measurement parameters (imaging conditions) such as the number of echoes to be measured, the echo time TE, and the echo interval ITE via the input device 40. The external storage device 50, together with the storage device inside the computer 20, stores data used in various arithmetic processing executed by the computer 20, data obtained by the arithmetic processing, input conditions, parameters, and the like.

[0023] Next, a pulse sequence used for imaging by the MRI apparatus of this embodiment will be described.

[0024] In this embodiment, as an example, a T1-weighted gradient echo pulse sequence (FatSat-RSSG: Rf-Spoiled Steady state Gradient echo) including a fat suppression pulse (referred to as a FatSat pulse) is used. Fig. 2 shows an example of a 3D FatSat-RSSG. In Fig. 2, RF represents an RF pulse, and Gs, Gp, and Gr represent gradient magnetic field pulses in the slice direction, phase encoding direction, and readout direction, respectively.

[0025] In this pulse sequence, an RF pulse tuned to the magnetic resonance frequency of fat is applied as a FatSat pulse, and a spoiler gradient magnetic field pulse is applied to suppress signals from fat before starting an RF spoiled gradient echo pulse sequence. The RSSG sequence measures gradient echoes generated by the readout gradient magnetic field Gr while varying the strength of the phase encoding gradient magnetic field Gp. 3D measurement data is obtained by repeating this sequence while changing the slice gradient magnetic field Gs. This gradient echo pulse sequence produces T1-weighted images that emphasize the contrast of tissues with long longitudinal relaxation times T1.

[0026] In the example shown in Fig. 2, a FatSat pulse is inserted for each repetition of 3D-RSSG, but the FatSat pulse may be inserted intermittently, i.e., once every several times, instead of being used for each 3D-RSSG. Also, although an example is shown in which one RF pulse is used as the FatSat pulse, various pulses are known for FatSat, and there is no particular limitation as long as it functions as a fat suppression pulse, such as a FatSat pulse consisting of multiple RF pulses as shown in Fig. 3.

[0027] The MRI apparatus of this embodiment performs such fat-suppressed T1-weighted imaging continuously over a period of time including the period from administration of the contrast agent until the contrast agent reaches the target site, thereby continuously obtaining images of each phase. In this case, in a predetermined phase, a preparation pulse is added along with the FatSat pulse to suppress signals from the contrast agent present in tissue outside the target tissue, where the contrast agent is more likely to move. The preparation pulse suppresses contrast agent signals outside the target tissue, and there are several modes. Specific examples of the preparation pulse and the phases in which it is used will be described in detail in later embodiments.

[0028] Next, the processing flow of contrast MRI using the MRI apparatus configured as described above will be explained with reference to Fig. 4. Here, the explanation will be given taking as an example imaging used for diagnosing hepatocellular carcinoma.

[0029] A contrast agent is administered to the subject positioned in the examination space (S1), and imaging of a predetermined area, for example, the abdomen, is initiated and continuously performed (S2). Note that in Figure 4, imaging begins after the contrast agent is administered, but imaging may also begin before the contrast agent is administered. The pulse sequence used for imaging is essentially the same as the T1-weighted gradient echo pulse sequence (FatSat-RSSG) including fat suppression pulses shown in Figure 2. In contrast-enhanced MRI, this 3D imaging is continuously repeated from before the contrast agent is administered, suppressing signals from fat and obtaining signals from tissues whose signal intensity has been enhanced by the contrast agent.

[0030] Contrast agents administered to a subject (intravenously) travel through arteries to the target tissue, where they are then taken up by the target tissue. If the contrast agent is taken up by the target tissue but is also present in the surrounding tissue (tissues that diffuse and flow) (in an equilibrium state), it is not possible to visualize only the target tissue with high brightness.

[0031] Therefore, a basic imaging sequence is repeated to obtain images until a predetermined time has elapsed since the administration of the contrast agent, and then a preparation pulse is applied to suppress signals outside the target tissue after the predetermined time has elapsed (S3, S4). This imaging including the preparation pulse can be continued even after the contrast agent has passed the equilibrium state and been taken up by the target tissue (S4).

[0032] After the contrast agent is taken up into the target tissue and a predetermined time has elapsed (S5), the examination is terminated (S6). The imaging time after the contrast agent is taken up into the target tissue can be the same as that of conventional contrast-enhanced MR, but in this imaging procedure, the target tissue can be visualized with high brightness by the time the contrast agent is taken up into the target tissue from its equilibrium state, so the imaging time can be shorter, or it can even be omitted. When imaging the abdomen, breath-hold imaging (intermittent imaging) may be performed. In such cases, positional deviations are corrected between breath-hold imaging sessions.

[0033] As described above, according to this embodiment, by suppressing signals from tissues with flow or diffusion, such as blood vessels, around the target tissue during the equilibrium state until the contrast agent is taken up by the target tissue, it is possible to improve the imaging ability of the target tissue and provide advanced diagnostic support information. Furthermore, even if the ability of the target tissue to take up the contrast agent differs between subjects, it is possible to obtain an image of the target tissue with high brightness without requiring a long time to image the target tissue.

[0034] A specific embodiment will be described below, taking as an example a case where the purpose of imaging is imaging for diagnosing hepatocellular carcinoma.

[0035] <Embodiment 1> This embodiment is characterized by the use of an MSDE (Motion-Sensitized Driven-Equilibrium) pulse as a preparation pulse. The MSDE pulse is used to suppress signals from tissues with flow, and it is known that the MSDE pulse is used in combination with T2*-weighted imaging (Non-Patent Document 2). However, this embodiment uses this MSDE pulse in a predetermined phase of consecutive contrast-enhanced MR imaging to accentuate the difference in signal intensity between phases, and the use (control) of the MSDE pulse and its purpose are completely different from those known in the past.

[0036] First, we will explain the distribution and signal changes of the contrast agent (EOB) during diagnostic imaging of hepatocellular carcinoma from the time the contrast agent is administered until it is taken up by the target tissue (hepatocytes). As shown in Figure 5, imaging phases vary depending on the movement of the contrast agent: arterial phase, portal venous phase, equilibrium phase, and hepatocellular phase. Because the liver is nourished by the portal vein and the tumor is nourished by arteries, the signal intensity of the tumor is high during the arterial phase. The contrast agent that reaches the tumor is gradually excreted, and the signal intensity of the tumor decreases as the imaging progresses through the portal venous phase, equilibrium phase, and hepatocellular phase. On the other hand, normal hepatocytes have the ability to take up EOB, so the signal intensity increases as the imaging progresses through the portal venous phase, equilibrium phase, and hepatocellular phase.

[0037] Diagnosis of hepatocellular carcinoma using contrast-enhanced MR utilizes the fact that the signal intensity of normal liver cells and that of the tumor change inversely depending on the phase to visualize the tumor. However, during the equilibrium and hepatocellular phases, EOB may be distributed throughout the interstitial fluid, blood vessels, and cells, and signals from the interstitial fluid and blood vessels in the extracellular space of the liver cause a relative decrease in signal intensity during the hepatocellular phase, reducing diagnostic ability.

[0038] In this embodiment, attention is paid to the fact that the degree of freedom of diffusion of EOB taken up into cells is lower compared to EOB present in interstitial fluid and blood vessels, FatSat For the +RSSG sequence, an MSDE pulse is applied as a pre-pulse to suppress the signal value of the extracellular space of the liver.

[0039] An example of a pulse sequence including an MSDE pulse is shown in Fig. 6. As shown in Fig. 6, an MSDE pulse and a FatSat pulse are applied prior to an RSSG sequence.

[0040] Various methods for MSDE have been proposed, and any of them can be adopted. As an example, Figure 7 shows a composite pulse combining RF pulse groups and MPG (Motion Probing Gradient) pulses. The upper panel of Figure 7 shows three RF pulse groups, while the lower panel shows five RF pulse groups. With this type of MSDE, contrast can be adjusted by adjusting the applied MPG pulse intensity and the interpulse spacing indicated by PreTE in the figure. Increasing the number of RF pulses has the disadvantage of lowering the SNR and changing the contrast, but it also has the advantage of being robust against nonuniform static magnetic field strength and irradiation intensity. The appropriate method should be selected depending on the target to be suppressed, other imaging conditions, etc. This can be selected by the user via the UI (described later) or can be determined by the device itself.

[0041] In Figure 6, the MSDE pulse and FatSat pulse may be applied once for multiple RSSGs rather than each time RSSG is repeated. The MSDE pulse may be applied for all phase encoding acquisitions, but the imaging time may be shortened by applying it only to the low region of k-space, which strongly contributes to contrast, and not to the high region.

[0042] In addition, although the MSDE pulse is applied before the FatSat pulse in FIG. 6, this order may be reversed. When MSDE is applied before FatSat, the signal suppression ability decreases but the fat suppression ability increases. Conversely, when FatSat is applied before MSDE, the fat suppression ability decreases but the signal suppression ability increases. Depending on whether signal suppression or fat suppression is prioritized, the order may be user selectable or fixed to either one. Allowing the user to set the order increases the degree of freedom for the user. On the other hand, fixing the order reduces the burden on the user and improves the workflow.

[0043] The amount of MSDE applied may be determined internally by the system based on the imaging conditions, may be explicitly set by the operator, or the application rate may be determined by prior measurement.

[0044] An example of a GUI displayed on the display device when MSDE is added is shown in Fig. 8. In this example, the computer (controller) 20 causes the display device 30 to display a condition setting screen 800 related to MSDE as an imaging condition setting screen set by the user in, for example, a hepatocellular carcinoma diagnostic test using contrast-enhanced MRI.

[0045] The condition setting screen 800 displays a button for indicating whether a suppression pulse is required, blocks 801 to 805 for receiving information such as the phase in which a pulse to suppress EOB signals in the hepatic extracellular space is to be added, the type of pulse, the degree of suppression (intensity of the suppression pulse), and the position at which the suppression pulse is to be inserted into the pulse sequence. Although the display format is not limited to that shown in the figure, the user can specify, via the GUI of blocks 801 to 803, in which phase the suppression pulse is to be used (e.g., only the equilibrium phase, or both the equilibrium phase and the hepatic phase), whether to use MSDE as the suppression pulse or a pulse other than the one described below, and the intensity (degree of suppression) of the suppression pulse when used. Furthermore, for 804, if a fat suppression pulse is used, the user can select whether to add the pulse before or after the fat suppression pulse. For 805, the user can select whether to add the pulse only to a portion of the k-space data (e.g., the low-frequency region) (in which case, the data in the high-frequency region is shared with the data from the phase in which it was acquired to form an image) or to add the pulse to the entire k-space data. While FIG. 8 shows a GUI for selecting high, medium, or low for the suppression pulse intensity, the user may also specify the value of the applied intensity directly.

[0046] These conditions may be determined by presenting pre-set conditions in the device and allowing the user to approve or change them.

[0047] Next, the imaging procedure of this embodiment will be described with reference to Fig. 9. In Fig. 9, steps with the same content as in Fig. 4 are designated by the same reference numerals.

[0048] First, a contrast agent (EOB) is administered intravenously to the subject, and imaging begins (S1). During arterial and portal venous phase imaging, the FatSat+RSSG sequence shown in Figure 2 is executed without applying MSDE (S21, S22). When the portal venous phase transitions to the equilibrium phase, an MSDE pulse is added to perform imaging while suppressing signals from the extracellular space of hepatocytes (S41).

[0049] The point at which the transition to the equilibrium phase has occurred may be determined within the system based on empirical time, and imaging with MSDE added may be automatically initiated. Alternatively, in dynamic imaging, image reconstruction is also performed continuously, and images of each phase are displayed on the display device 30, so the user may determine the transition time from the images.

[0050] The subsequent hepatic phase is also imaged using the FatSat+RSSG sequence with MSDE added (S42). Computer (arithmetic unit) 20 performs image reconstruction using the k-space data acquired by executing the pulse sequence and sequentially displays the images on display device 30. However, if MSDE is applied only during data acquisition in the low-frequency region of k-space data, image reconstruction is performed for data in the high-frequency region using data acquired in phases without MSDE. Because the low-frequency region of k-space determines the image contrast, an image that demonstrates the effect of MSDE can be obtained even when MSDE is added only to the low-frequency region of k-space. Furthermore, by limiting data acquisition during MSDE addition to a portion of the k-space data (the low-frequency region), imaging time can be shortened. After that, after a predetermined time has elapsed, the test is terminated (S6).

[0051] According to this embodiment, applying MSDE during the equilibrium phase allows for the acquisition of images in which signals due to EOB present in the extracellular space are suppressed, thereby improving diagnostic performance. Furthermore, the hepatocellular phase is generally imaged approximately 20 minutes after administration of a contrast agent. However, in people with impaired liver function, it takes time for EOB to be taken up and excreted, making it difficult to determine whether the patient is truly in the hepatocellular phase even after 20 minutes, which is generally considered to be the hepatocellular phase. However, applying MSDE allows for the acquisition of images in which signals due to EOB present in the extracellular space are suppressed, thereby improving diagnostic performance.

[0052] <Embodiment 2> In the first embodiment, imaging including application of MSDE was performed in the equilibrium phase, but in the present embodiment, imaging including application of MSDE and imaging without application of MSDE are performed, and the necessity of imaging in the hepatocyte phase is determined based on the results of comparing the two.

[0053] For this reason, the computer 20 that controls and performs calculations on the MRI apparatus 1 includes an image comparison unit (comparison unit) 23 in addition to an image reconstruction unit 21, as shown in Fig. 10. It may also include an assistance information generation unit 27 that calculates diagnostic assistance information using the results of the image comparison unit 23. Among the functions of the computer 20, the function of controlling the imaging unit 10, the display device, etc. is shown as a control unit 25. The procedure of this embodiment will be described below with reference to the flow in Fig. 11. In Fig. 11, the same steps as in Fig. 9 are designated by the same reference numerals, and duplicated explanations will be omitted.

[0054] In this embodiment, similarly to the first embodiment, after the administration of the contrast agent (S1), the MSDE is not applied in the imaging of the arterial phase and the portal vein phase. FatSat + Execute the RSSG sequence (S21, S22).

[0055] In the subsequent equilibrium phase imaging, the MSDE was not applied (MSDE_off) at predetermined intervals. FatSat +RSSG sequence and MSDE applied (MSDE_on) FatSatThe RSSG sequence is repeatedly executed (S411). The repetition may be performed by repeating the application and non-application of voltage in a nested manner during imaging, or by imaging either with or without voltage, and then imaging the other, and repeating the process. Either method may be employed. The former method is advantageous in that it is less susceptible to the influence of body movement.

[0056] Comparing the MSDE_off image with the MSDE_on image reveals that the signal suppression effect of the MSDE is evident immediately after the start of equilibrium phase imaging, resulting in a large difference in signal intensity. However, as time passes and EOB is taken up into hepatocytes, the suppression effect of the MSDE application disappears as a difference in brightness. By displaying a comparison image of the MSDE_off and MSDE_on images on the display device 30, the user can confirm the status of EOB uptake into hepatocytes (S412) and determine whether subsequent hepatic phase imaging is necessary (S413). For example, if the signal intensity no longer changes and it is determined that EOB has been largely expelled from the extracellular space, an image of the hepatic phase has been obtained, and imaging is terminated without further hepatic phase imaging (S6).

[0057] The decision as to whether or not to capture an image of the user based on the displayed image may be automatically made within the computer 20. In this case, the processing flow within the computer 20 is shown in Fig. 12. In this processing, steps S411 and S412 in Fig. 11 are replaced with steps S51 to S54 in Fig. 12.

[0058] First, the image reconstruction unit 21 reconstructs an image using echo signals collected by executing a pulse sequence, and the image comparison unit 23 compares the image obtained with MSDE_off with the image obtained with MSDE_on, and calculates the rate of decrease in signal intensity (pixel value) (S51). The rate of decrease in signal intensity can be calculated, for example, by the following equation (1).

number

[0059] As mentioned above, as time passes and EOB is taken up by hepatocytes, the suppression effect of applying MSDE no longer appears as a difference in brightness, and the rate of signal intensity decrease decreases.

[0060] The control unit 25 displays the signal decline rate calculated by the image comparison unit 23 on the display device 30 (S52). An example display is shown in FIG. 13. In this display example, a map (calculated image) 1301 showing the signal decline rate for each pixel is displayed together with a scale bar 1302 showing the signal decline rate. The user can determine whether or not imaging of the next hepatic phase is necessary by viewing this result (S53). The user may be configured to accept the result of this determination. However, if the determination is to be made automatically, a threshold value for the signal decline rate is set in advance, and the control unit 25 determines whether or not imaging is necessary based on this threshold value (S54). That is, if the signal decline rate is lower than the threshold, it is considered that an image depicting the hepatic phase at high brightness has already been obtained, and the examination is terminated without imaging of the hepatic phase (S6). On the other hand, if the signal decline rate is equal to or greater than the threshold, imaging of the hepatic phase is continued with MSDE_on (S42). Even in hepatocellular phase imaging, turning MSDE_on can provide images in which the signal from EOB present in the extracellular space of the hepatocytes is suppressed, improving diagnostic ability.

[0061] In this embodiment, as in the first embodiment, user settings regarding suppression pulses may be accepted via a GUI such as that shown in Fig. 8, thereby improving the degree of freedom for the user. Also, the order of the MSDE pulse and the FatSat pulse may be fixed, which simplifies the examination flow and shortens the time required.

[0062] As described above, according to this embodiment, in addition to the same effects as in embodiment 1, two types of imaging, MSDE_off and MSDE_on, are performed in the equilibrium phase, and the necessity of imaging in the hepatocellular phase is determined by comparing these images. This makes it possible to shorten the examination time if imaging in the hepatocellular phase is not required. Generally, the hepatocellular phase is said to occur 20 minutes after contrast agent administration, but imaging can be completed before that, resulting in a significant reduction in the examination time.

[0063] <Modification of the second embodiment> In this modification, imaging with MSDE_off and imaging with MSDE_on are performed in the equilibrium phase as in the second embodiment, but in this modification, comparative imaging with MSDE_off / on is also performed in the hepatocellular phase. The results of the comparative imaging in the hepatocellular phase can be used to evaluate hepatocellular function.

[0064] The flow of the imaging procedure according to this modified example is shown in FIG. Leave The same steps as in Figure 11 are denoted by the same reference numerals, and redundant explanations will be omitted. In this modified example, as in the second embodiment, the necessity of imaging in the hepatocellular phase is determined based on the results of comparative imaging with MSDE off / on in the equilibrium phase (S411 to S413), and this determination may be made either by the user or automatically by the device.

[0065] In this modified example, when the control unit 25 determines that imaging of the hepatocyte phase is necessary, it repeats imaging with MSDE off and imaging with MSDE on during this imaging as well (S421), and the image comparison unit 23 uses the images obtained from the two imaging sessions to calculate the signal reduction rate using the above-mentioned formula (1) (S422).

[0066] A small change in the signal decline rate is considered to indicate a low EOB uptake capacity of hepatocytes. The image comparison unit 23 displays the change in the signal decline rate as shown in FIG. 13 on the display device 30, allowing the user to confirm the value of the signal decline rate or its change, which can be used as a diagnostic index. Alternatively, qualitative liver function assessments corresponding to the signal decline rates may be registered in advance, and the support information generation unit 27 may create diagnostic support information based on the registered information and display it together.

[0067] In the above explanation, the signal reduction rate obtained in the hepatocellular phase is presented as an indicator for determining liver function, but the signal reduction rate obtained in the equilibrium phase may also be used in combination with or alone as an indicator for determining liver function.

[0068] According to this modification, in addition to the effect of the second embodiment, it is possible to present information that is effective for diagnosing liver function to the user.

[0069] <Embodiment 3> In the first and second embodiments, the MSDE pulse is used as a pulse for suppressing signals from EOB present outside the hepatocytes in imaging of the equilibrium phase and the hepatocyte phase, but a pre-pulse other than the MSDE pulse can be used as long as it suppresses signals from tissues with flow or diffusion. In this embodiment, an MPG (Motion Probing Gradient) pulse is used as such a pre-pulse.

[0070] The MPG pulse is a strong gradient magnetic field applied in diffusion-weighted imaging (DWI) to generate a signal difference between spins in tissues undergoing diffusion, such as perfusion, and stationary spins. In this embodiment, the MPG pulse is used to suppress signals from EOB in the extrahepatic space during specific phases of continuous imaging, namely, the equilibrium phase until the contrast agent reaches the hepatocytes and, if necessary, the hepatocyte phase.

[0071] Figure 15 shows a pulse sequence with an MPG pulse added. As shown, after the RF pulse is applied, bipolar MPG pulses are applied in three axes. As with MSDE, the MPG pulse may be applied for each TR, or only when acquiring data from the center of k-space. The b-value, which is an index of the magnitude of the MPG pulse, may be a single b-value for each time phase, including the equilibrium phase and the hepatocellular phase, or imaging may be performed with multiple b-values. When imaging with multiple b-values ​​is performed, perfusion information can also be obtained, as with IVIM.

[0072] Note that applying an MPG pulse extends the TE, resulting in a worse contrast than when applying an MSDE. It may be possible to select between MSDE and MPG pulses depending on whether contrast is prioritized or perfusion information is to be obtained. Furthermore, similar to the first and second embodiments, data in the high frequency range of k-space may be shared between each time phase to shorten the imaging time. Although not shown, such user selection can be accepted via a GUI such as that shown in FIG. 8.

[0073] In this embodiment, too, the MSDE pulse of embodiment 1 may be replaced with an MPG pulse, and comparative imaging with and without an MPG pulse may be performed in the equilibrium phase or hepatocellular phase, as in embodiment 2 and its modifications, thereby achieving the same effects as embodiment 2, such as improved diagnostic performance, shortening the examination time, and enabling evaluation of liver function from the signal decline rate. Furthermore, according to this embodiment, by using MPG pulses with multiple b-values, it is possible to obtain perfusion information that cannot be obtained with MSDE.

[0074] <Embodiment 4> The above-described embodiments are all examples in which a pre-pulse for suppressing signals from a contrast agent present in the hepatic extracellular space is added to a pulse sequence for acquiring signals from a target region, but the present embodiment uses ASL (Arterial Spin Labeling) to suppress signals from the hepatic extracellular space.

[0075] As shown in Figure 16, ASL is a technique in which an area 1602 adjacent to a target area 1601 is pre-excited, and the signal is reduced by the MT effect of labeled blood flowing into the target area from the adjacent area.Known labeling techniques include the PASL method, which uses an inversion pulse, and the pCASL method, which continuously irradiates RF and determines the blood spin as a result, and either of these may be used.

[0076] In this embodiment, as in the first and second embodiments, arterial and portal vein phase imaging is performed using a gradient echo pulse sequence including FatSat as shown in Figure 2. However, in equilibrium phase imaging, the hepatic artery and portal vein are spin-labeled and then imaged after a predetermined delay time PLD (Post Labeling Delay). As a result, the signal intensity of the liver parenchyma (the portion of the liver excluding the extracellular space) is reduced due to the magnetization transfer (MT) effect of the spin-labeled blood. This creates a difference in signal intensity between the liver cells and the extracellular space, allowing signals from the liver cells to be extracted, excluding signals from the extracellular space.

[0077] However, because the signal intensity reduction due to the MT effect of spin labeling is only a slight 1-2%, it is also possible to acquire an image without spin labeling as a reference image and output the difference image. Using the difference image, it is possible to obtain images depicting hepatocytes and the extracellular space with different contrasts. Therefore, images depicting hepatocytes at high brightness can provide information that enables tumor detection, and images depicting the extracellular space at high brightness can also provide perfusion information.

[0078] In this embodiment, comparative imaging with spin labeling turned on / off may be performed during imaging of the equilibrium phase to determine whether imaging of the subsequent hepatocyte phase is necessary, or, as in the other embodiments, comparative imaging may be performed when imaging of the hepatocyte phase to obtain information on liver function.

[0079] As a result, in this embodiment, as in the embodiment using a pre-pulse such as an MSDE or MPG pulse, effects such as improved diagnostic ability, evaluation of liver function, and shortened examination time can be obtained. Also, in this embodiment, perfusion information can be obtained. [Explanation of symbols]

[0080] 1: MRI device, 10: imaging unit, 20: computer, 21: image reconstruction unit, 23: image comparison unit (comparison unit), 25: control unit

Claims

1. an imaging unit that applies a radio frequency magnetic field and a gradient magnetic field to a subject, collects nuclear magnetic resonance signals generated from the subject, and generates an image of the subject; and a control unit that controls the imaging unit; the imaging unit includes a pulse sequence for acquiring a T1 weighted image including a fat suppression pulse, the control unit controls the imaging unit to repeat the pulse sequence for a predetermined time from administration of the contrast agent to the subject, and generate images of a plurality of phases where the contrast agent reaches different positions; In this case, a preparation pulse that suppresses signals from the contrast agent present outside the target tissue (cell) of the contrast agent is added to the pulse sequence prior to the pulse sequence in some of the multiple phases.

2. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus according to claim 1, wherein the imaging unit applies the preparation pulse either before or after the fat suppression pulse.

3. 3. The magnetic resonance imaging apparatus according to claim 2, The magnetic resonance imaging apparatus is characterized in that the imaging unit uses an MSDE pulse or a bipolar MPG pulse as the preparation pulse.

4. 2. The magnetic resonance imaging apparatus according to claim 1, The magnetic resonance imaging apparatus is characterized in that the imaging unit applies a spin labeling pulse as the preparation pulse to a region outside the imaging region.

5. 2. The magnetic resonance imaging apparatus according to claim 1, A magnetic resonance imaging apparatus characterized in that the pulse sequence for acquiring the T1 weighted image is a 2D or 3D RF spoiled gradient echo sequence.

6. 2. The magnetic resonance imaging apparatus according to claim 1, the imaging unit executes, in the part of phases, a first pulse sequence in which the preparation pulse is added and a second pulse sequence in which the preparation pulse is not added, and comprises a comparison unit that compares signals acquired by the first pulse sequence with signals acquired by the second pulse sequence.

7. 7. The magnetic resonance imaging apparatus according to claim 6, The magnetic resonance imaging apparatus according to claim 1, wherein the control unit determines whether to continue imaging of the next phase by the imaging unit depending on the result of the comparison unit.

8. 7. The magnetic resonance imaging apparatus according to claim 6, The magnetic resonance imaging apparatus further comprises an assistance information generating unit that generates diagnostic assistance information for the subject based on the result of the comparison unit.

9. 7. The magnetic resonance imaging apparatus according to claim 6, The magnetic resonance imaging apparatus is characterized in that the control unit causes a display device to display at least one of the results of the comparison unit and diagnostic support information.

10. 2. The magnetic resonance imaging apparatus according to claim 1, a user interface unit configured to accept user settings for at least one of the type of the preparation pulse, the signal suppression intensity by the preparation pulse, and the position at which the preparation pulse is added in the repetition of a pulse sequence;

11. 2. The magnetic resonance imaging apparatus according to claim 1, 10. A magnetic resonance imaging apparatus, comprising: a target tissue for the contrast agent to reach is a liver; and wherein the plurality of phases include an arterial phase, a portal venous phase, an equilibrium phase, and a hepatocyte phase.

12. 1. A control method for a magnetic resonance imaging apparatus that applies a radio frequency magnetic field and a gradient magnetic field to a subject in accordance with a pulse sequence, collects nuclear magnetic resonance signals generated from the subject, and generates an image of the subject, comprising: using a pulse sequence for acquiring a T1 weighted image including a fat suppression pulse, repeating the pulse sequence for a predetermined time from administration of a contrast agent to the subject, thereby generating images of a plurality of phases at which the contrast agent reaches different positions; a control method for a magnetic resonance imaging apparatus, comprising: adding, prior to the pulse sequence, a preparation pulse for suppressing signals from the contrast agent present outside a target tissue (cell) of the contrast agent in some of the plurality of phases.

13. 13. A method for controlling a magnetic resonance imaging apparatus according to claim 12, comprising: a control method for a magnetic resonance imaging apparatus, wherein the preparation pulse is added when acquiring a portion of k-space data constituted by the nuclear magnetic resonance signals in the repetition of the pulse sequence.

Images