JP2007029250A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To shorten photographing time while reducing body movement artifacts by improving the acquisition efficiency of echo signals for image reconstitution in a photographing sequence in a photographing method using a navigation sequence together. <P>SOLUTION: In the magnetic resonance imaging apparatus, a pulse sequence has a body movement detection sequence for detecting the body movement position of a subject and a photographing sequence for acquiring the image of the subject. A control means repeatedly executes the body movement detection sequence, a breathing stop instruction to the subject through a transmission means based on body movement information detected by the body movement detection sequence, the photographing sequence, and the instruction of breathing restart to the subject through the transmission means, also detects the body movement position by continuing the body movement detection sequence even after the breathing stop instruction, and the photographing sequence is executed when the body movement position is within a prescribed range. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. Activating the imaging sequence, especially when performing a pulse sequence (hereinafter referred to as imaging sequence) for image creation after a pulse sequence (hereinafter referred to as navigating sequence) that monitors the respiratory cycle The present invention relates to a technique for controlling timing.

  The MRI device measures NMR signals (echo signals) generated by the spins of the subject, especially the tissues of the human body, and forms the shape and function of the head, abdomen, limbs, etc. in two or three dimensions. It is a device that images. In imaging, the echo signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured echo signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In imaging of the chest and abdomen using this MRI apparatus, body movement caused by respiration causes artifacts in the image (hereinafter referred to as “body movement artifact”), and causes significant image quality degradation. Therefore, various methods for preventing body movement artifacts have been proposed as follows.

  Method 1 is a method of performing imaging while holding the breath. In the case of Method 1, the navigation sequence is not performed. Although body movement artifacts due to breathing do not occur in order to hold the breath, it is necessary to complete imaging within a finite breath-holding time, and thus there may be a limitation in spatial resolution and SN ratio.

  Method 2 is a method using a navigation sequence. In a typical navigating sequence, an echo signal from an area including the diaphragm is acquired as a navigating signal, and a positional change of the diaphragm is tracked to detect a respiratory cycle. In order to reduce body motion artifacts, an imaging sequence is controlled based on the detection result of the respiratory cycle, and an image is reconstructed using only echo signals acquired in a specific period within the respiratory cycle. The execution sequence of the navigation sequence and the imaging sequence by the control is roughly divided into a method 2-1, a method 2-2, and a method 2-3 described below.

(1) Method 2-1 This method is a method that is often applied to cardiac imaging. First, an R wave of the electrocardiogram is detected, and a navigation sequence is executed after a first time from the R wave to obtain a navigation signal. Next, the imaging sequence is executed after a second time (> first time) from the R wave, and an echo signal for image reconstruction is acquired. The echo signal acquired in the imaging sequence is determined to be used / discarded for image reconstruction using the navigation signal acquired immediately before. That is, the echo signal acquired in the imaging sequence is used for image reconstruction only when the position of the diaphragm detected by the navigation signal is included in a predetermined gate window. The navigation sequence and the imaging sequence are continuously executed within one heartbeat in synchronization with the R wave of the electrocardiogram, and are repeated until acquisition of an echo signal necessary for image reconstruction is completed. This method is disclosed in, for example, (Patent Document 1).

(2) Method 2-2 In this method, the navigation sequence is first executed to monitor the position of the diaphragm, and when it is determined that the position of the diaphragm is included in the gate window, the start of imaging is instructed. A shooting sequence is executed. After the imaging sequence is executed for a predetermined period, a predetermined pause period is provided and no sequence is executed. Thereafter, the navigation sequence-shooting sequence-pause period is repeated until acquisition of echo signals necessary for image reconstruction is completed. This method is disclosed in, for example, (Non-Patent Document 1).

(3) Method 2-3 In this method, a navigation sequence is first executed to monitor the position of the diaphragm. When the diaphragm reaches a predetermined position, a message “HOLD” is notified to the subject, and breath holding is instructed. An imaging sequence is executed during this breath holding period. After the imaging sequence is executed for a predetermined period, the above procedure is repeated until a set of echo signals necessary for image reconstruction is measured. This method is disclosed in, for example, (Non-Patent Document 2).

Method 2-2 and method 2-3 are based on the point that the imaging sequence starts after the navigation sequence detects that the diaphragm is included in the predetermined range, and the echo signal acquired in the imaging sequence. On the other hand, it is different from Method 2-1 in that determination of use / discard is unnecessary. The method 2-3 is different from both the method 2-1 and the method 2-2 in that the imaging sequence is performed while holding the breath.
JP 2004-57226 A Journal of Magnetic Resonance Imaging, 21, 576-582 (2005) Magnetic Resonance in Medicine, 34, 11-16 (1995)

In particular, in Method 2-3, since the process of guiding the diaphragm position into the predetermined gate window depends on the subject, it is uncertain that the diaphragm position is guided into the predetermined gate window. Have
Therefore, the present invention has been made to solve the above-described problems, and an object of the present invention is to perform imaging while ensuring that the body movement position is within a predetermined range.

In order to solve the above problems, the MRI apparatus of the present invention is configured as follows. That is,
Means for generating a static magnetic field, means for generating a gradient magnetic field, means for generating a high-frequency magnetic field, means for detecting a nuclear magnetic resonance signal generated from the subject, and the subject using the nuclear magnetic resonance signal Controlling the gradient magnetic field generating means, the high-frequency magnetic field generating means, and the signal detecting means based on a predetermined pulse sequence, means for reconstructing the image of the image, means for transmitting predetermined information to the subject, And a means for controlling the transmission means, wherein the pulse sequence includes a body motion detection sequence for detecting a body motion position of the subject and an imaging sequence for acquiring an image of the subject. And the control means includes the body movement detection sequence and a breath holding instruction to the subject via the transmission means based on the body movement information detected by the body movement detection sequence. The imaging sequence and the instruction to resume breathing to the subject via the transmission means are repeatedly executed, and the body movement detection sequence is continued after the breath holding instruction to detect the body movement position. The imaging sequence is executed when the body movement position is within a predetermined range.

  Preferably, the control means detects the body movement position by executing the body movement detection sequence even after the imaging sequence, and after the value derived from the fluctuation of the body movement position falls within a predetermined range, Performs breath holding instructions. Furthermore, when the value derived from the fluctuation of the body movement position falls outside the predetermined range, the nuclear magnetic resonance signal acquired in the immediately preceding imaging sequence may be discarded. Alternatively, the execution period of the imaging sequence may be shortened.

Preferably, the predetermined range is determined based on a change in the body movement position detected by the body movement detection sequence executed under free breathing of the subject.
Preferably, the part for detecting the body movement position by the body movement detection sequence includes a diaphragm.

  Preferably, the control unit controls a parameter of the imaging sequence based on a change in the body movement position detected by the body movement detection sequence. Preferably, this parameter is an imaging sequence execution period or a slice selection frequency.

  Preferably, the control means acquires zero-encoded data at a constant time interval in the imaging sequence, detects the body movement position using the obtained plurality of zero-encoded data, and determines the position of the position. The nuclear magnetic resonance signal acquired during a period within a predetermined range is used for image reconstruction.

  Preferably, means for generating a static magnetic field, means for generating a gradient magnetic field, means for generating a high-frequency magnetic field, means for detecting a nuclear magnetic resonance signal generated from a subject, and the nuclear magnetic resonance signal Means for reconstructing an image of the subject, means for transmitting predetermined information to the subject, the gradient magnetic field generating means, the high-frequency magnetic field generating means, and the signal detecting means based on a predetermined pulse sequence And means for controlling the transmission means, and means for detecting the body movement position of the subject, the control means when the body movement position falls within a predetermined range, the information A breath-holding instruction is transmitted to the subject via the transmission means, and thereafter measurement of the nuclear magnetic resonance signal is started. A means for generating an alarm when a motion is detected may be provided.

  According to the MRI apparatus of the present invention, it is possible to perform imaging while ensuring that the body movement position is within a predetermined range. As a result, body motion artifacts are reduced. Further, since the acquisition efficiency of the echo signal for image reconstruction in the imaging sequence is improved, the imaging time is shortened.

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus to which the present invention is applied will be described with reference to FIG. 101 is a magnet that generates a static magnetic field, 102 is a subject such as a patient, 103 is a bed on which the subject 102 is placed, 104 is a high-frequency magnetic field coil that detects an echo signal by applying a high-frequency magnetic field to the subject 102, 105, 106 107 are gradient magnetic field generating coils for generating gradient magnetic fields in the X, Y, and Z directions, respectively. Reference numeral 108 denotes a high-frequency magnetic field power source for supplying power to the high-frequency magnetic field coil 104, and reference numerals 109, 110, and 111 denote gradient magnetic field power sources for supplying current to the gradient magnetic field generating coils 105, 106, and 107, respectively. A sequencer 116 transmits commands to peripheral devices such as the gradient magnetic field power sources 109, 110, 111, the high frequency magnetic field power source 108, the synthesizer 112, the modulator 113, the amplifier 114, and the receiver 115 to control the operation of the MRI apparatus. Reference numeral 117 denotes a storage medium for storing data such as photographing conditions. Reference numeral 118 denotes a computer that performs image reconstruction with reference to the echo signal input from the receiver 115 and data in the storage medium 117. A display 119 displays the result of image reconstruction performed by the computer 118. The calculator 118 is provided with input means such as a mouse and a keyboard, and inputs instructions from the operator. The computer 118 controls the entire MRI apparatus based on this instruction.

  Further, if necessary, the subject 102 is attached with an external sensor 122 for detecting the heartbeat and respiratory motion, and the heartbeat information and respiratory motion information of the subject 102 via the biological signal detection unit 121. Is input to the sequencer 116. The sequencer 116 performs imaging timing control of the subject 102 based on the biological information.

(First embodiment)
Next, a first embodiment of the MRI apparatus of the present invention will be described. In the present embodiment, the diaphragm position is monitored using a navigation sequence, and in the case where imaging is performed by guiding the subject to a breath holding state, it is determined whether or not the diaphragm position is included in a predetermined range. is there.
An example of this embodiment will be described with reference to FIGS. FIG. 2 is a flowchart showing an operation procedure and a data processing procedure in the present embodiment. The operator guides the subject into the MRI apparatus, executes positioning imaging, determines the imaging site, and then performs imaging (hereinafter, main imaging) by the following processing steps.

First, an overall outline of the processing procedure in the main photographing will be described, and then details of main processing steps will be described. The overall outline of the processing procedure in the main photographing is as follows. That is,
In step S201, in the main photographing, a navigation sequence is first executed under free breathing.

In step S202, the operator sets a gate window (GW) based on the profile of the diaphragm position detected from the navigation signal. For example, this profile is displayed on the display 119, and the operator refers to this display and sets a gate window on the profile via the input means. The profile means, for example, a graph shown in FIG. 3, in which the vertical axis indicates the diaphragm position and the horizontal axis indicates time.
After the gate window is set, the computer 118 of the MRI apparatus performs internal processing described below. These internal processes are performed, for example, when the computer 118 reads and executes the corresponding program stored in the storage medium 117.

In step S203, the average respiration cycle, the average displacement amount of the diaphragm, and the average profile are derived in the computer 118 using the navigation signal acquired before the gate window setting. These calculation results are stored on the storage medium 117 in the MRI apparatus.
In step S204, an intersection point between the average profile and the gate window position is derived from the derived average profile and the set gate window position, and then the diaphragm position Ps at a time point that is a predetermined time Ts earlier than the time point of the intersection point is obtained. Derived.

In step S205, processing for instructing the subject to hold his / her breath when the diaphragm position sequentially detected by the navigation sequence substantially coincides with the Ps is registered in the sequencer 116.
In step S206, since the navigation sequence is continuously executed in parallel with the internal processing, when the diaphragm position Ps is detected in the latest profile detected by the navigation sequence, that is, the diaphragm position is the Ps. Is approximately instructed to hold the breath to the subject.

  In step S207, when the latest profile detected by the navigation sequence after the breath holding instruction is in the gate window, that is, the diaphragm position detected from the navigation signal acquired after the breath holding instruction is included in the gate window. If so, the shooting sequence is executed. The shooting sequence is executed for a period specified in advance by shooting conditions.

In step S208, the process for instructing the subject to hold his / her breath is deleted from the sequencer 116, and the process for instructing the subject to resume breathing is registered in the sequencer 116.
In step S209, after the imaging sequence ends, the subject is instructed to resume breathing.
In step S210, it is confirmed whether all echo signals necessary for image reconstruction have been acquired. If the acquisition is completed, resumption of breathing is instructed to complete the main imaging, and if not completed, the process proceeds to step S211.

In step S211, the navigation sequence is resumed.
In step S212, the process of instructing the subject to resume breathing is deleted from the control content, and the presentation of the instruction to resume breathing is terminated. Then, the process returns to step S205. Note that before returning to step S205, a pause period may be provided in which the subject breathes for a certain number of times or for a certain time.

  In steps S205 to S212, the detection of the diaphragm position / respiration cycle by the navigation sequence and the acquisition of the echo signal by the imaging sequence are repeated until all the echo signals necessary for the image reconstruction are acquired.

Next, details of the following main processing steps will be described.
First, the average respiration cycle, the average diaphragm displacement, and the average profile derivation process executed in step S203 will be described in detail with reference to FIG. FIG. 4 shows a profile of the diaphragm position, and a profile of about 5 cycles is measured. The maximum and minimum values are derived from this profile, and the average value M is calculated from both values. Next, the intersection between the profile and the average value M and the distance d between the average value M and the gate window are derived. The derived distance d is used in step S204. The start of the navigation sequence is used as a reference time, and numbers such as t1, t2, and t3 are assigned to each intersection. Here, a time difference between the odd-numbered intersections, for example, t3-t1 and t5-t3 is defined as a respiratory cycle. In FIG. 4, the first respiratory cycle is t3-t1, and the second respiratory cycle is t5-t3. Hereinafter, the nth respiratory cycle is expressed as Tn. Next, the maximum value and the minimum value in the nth breathing cycle are derived, and this difference is set as the displacement amount An. In FIG. 4, the maximum value and the minimum value of the first respiratory cycle are shown as black circles, and the displacement A1 is shown. Next, the average respiration cycle Tm and the average displacement amount Am are derived using the following formula (1), respectively.
Tm = (T1 + T2 + T3 + ... + Tn) / n Am = (A1 + A2 + A3 + ... + An) / n (1)

 Thereafter, an average profile is derived. An example of this procedure is shown in FIG. First, since the 1st to nth respiratory cycles are different (left in FIG. 5), T1 to Tn are normalized and matched with the average respiratory cycle Tm (center in FIG. 5). Each profile is subjected to linear interpolation processing and the like. Thereafter, the newly created 1st to nth respiratory profiles are averaged, and this is used as the average profile (right of FIG. 5). Note that the above-described method for deriving the average respiratory cycle, average displacement amount, and average profile describes a typical method in step S203, and does not limit the derivation method to be applied.

  Next, the derivation of the diaphragm position Ps executed in step S204 will be described in detail. In step S203, since the average profile and the average value M and the position difference d are derived, first, the position of the gate window is calculated from the average value M and the position difference d. The intersection of the average profile and the gate window is then determined. The diaphragm position before the predetermined time Ts from the intersection is the diaphragm position Ps to be derived. The predetermined time Ts corresponds to a delay time from the start of breath holding instruction control within the MRI apparatus until the subject's breath holding is completed, and is composed of a system response time and a subject response time. The Here, the system response time is the time from when the sequencer 116 issues a breath-hold instruction command to when the instruction is presented to the subject by a device such as an auto voice, for example. Is the time from when the subject recognizes the instruction until the subject performs breath holding.

  Next, FIG. 6 shows a timing chart of the navigation sequence and the shooting sequence in steps S206 and S207. In the present embodiment, the navigation sequence is continuously executed even after the breath holding is instructed in step S206, and the imaging sequence is executed after confirming that the diaphragm position is within the gate window in step S207.

  As described above, as a procedure for determining whether or not the imaging sequence can be executed, the process of step S207 for determining whether the diaphragm is included in the gate window in the breath-holding state is different from the method 2-3. .

  Next, hardware related to the breath holding / resuming instruction will be described. Voice and images can be used for instructions for breath holding and resuming breathing. For the voice instruction, for example, an auto voice function capable of registering the instruction contents and order in advance can be used. An instruction using an image or visual information is performed by, for example, displaying an instruction content on the panel in an MRI apparatus in which a fluoroscopic panel or a screen is provided to reduce a feeling of pressure on the subject. Can do. When the chest of the subject is compressed with an air band to reduce respiratory motion, air may be injected into or exhausted from the air band in place of the breath holding / resuming respiration instructions described above. Alternatively, the breath holding / resume resuming instruction and the air control to the air band may be performed in parallel.

  As described above, according to the present embodiment, the subject is instructed to hold his / her breath so that the diaphragm position is in the predetermined gate window, and the diaphragm position is in the predetermined gate window. Is confirmed and the shooting sequence is executed. As a result, it is possible to improve the acquisition efficiency of the echo signal for image reconstruction by the imaging sequence. As a result, the photographing time can be shortened.

(Second Embodiment)
Next, a second embodiment of the MRI apparatus of the present invention will be described. In addition to the first embodiment described above, the present embodiment further confirms the stability of the breathing motion of the subject at the end of the imaging sequence, and restarts the imaging sequence after the breathing motion is stabilized. is there. If breath holding is performed while the breathing motion is unstable, breathing may start unexpectedly during execution of the shooting sequence, and in such a case, the measured echo signal must be discarded. Therefore, the acquisition efficiency of the echo signal is reduced. Preventing such a situation is one of the objects of the present embodiment.

  An example of this embodiment will be described with reference to FIG. FIG. 7 is a flowchart of the present example, in which processing step S213 is newly added to the flowchart showing the operation procedure and data processing procedure of the example of the first embodiment shown in FIG. Hereinafter, only process step S213 different from the flowchart of FIG. 2 will be described in detail, and description of the same process steps as those of FIG. 2 will be omitted.

After step S212, the process proceeds to step S213 without returning to step S205.
In step S213, the latest breathing cycle is derived from the latest profile detected by the navigation sequence after resuming breathing, and the latest breathing cycle is compared with the average breathing cycle derived in step S203. If the absolute value of the difference between these cycles is larger than a predetermined threshold, the detection of the latest profile and the derivation of the respiratory cycle by the navigation sequence are continued. If it is within the threshold, the process returns to step S205, and the above-described steps S205 to S213 are repeated until all echo signals necessary for image reconstruction are acquired.

  That is, in step S213, after the imaging sequence is completed, the latest breathing cycle after resuming breathing is compared with the average breathing cycle derived in step S203, and whether or not the next breath holding instruction is possible is determined. For example, in FIG. 4, since the first and second breaths after the end of the imaging sequence have a short breathing cycle and do not satisfy the condition of step S213, the process does not return to step S205. At the time of the third breath satisfying the condition of the breathing cycle, the process returns to step S205, and the imaging sequence under the breath holding is followed. Note that, similarly to the other processing steps, this processing step is also performed by the computer 118 reading and executing the corresponding program stored in the storage medium 117, for example.

  As described above, as a procedure for determining whether or not the imaging sequence can be executed, in step S207 for determining whether or not the diaphragm is included in the gate window in the breath-holding state described in the example of the first embodiment. In addition, Step S213 in which imaging is permitted in a state where physiological factors such as respiration and heartbeat of the subject are stable is implemented in a point different from the method 2-3.

  Next, a comparison between the latest respiratory cycle and the average respiratory cycle in step S213 will be described. The purpose of this processing is to prevent an immediate execution of the imaging sequence because breathing is disturbed immediately after the breath holding is completed. To determine respiratory stability, the latest respiratory cycle and the average respiratory cycle are compared. For example, the criterion for determining respiratory stability may be that the latest respiratory cycle is within ± 10% of the average respiratory cycle. Furthermore, a plurality of conditions may be set such that four consecutive heartbeats satisfy the determination criterion.

  As described above, the example using the respiratory cycle as the determination criterion has been described, but the heart rate can also be used as the determination criterion. For example, the heart rate during execution of the navigation sequence immediately after the start of the main photographing is measured, the average heart rate is stored on the storage medium 117, and the latest heart rate immediately after the end of the breath-holding photographing sequence and the average heart rate are obtained. By comparing, it is also possible to determine the stability of the heartbeat. It is of course possible to carry out imaging by combining the determination of the stability of the respiratory cycle and the determination of the stability of the heartbeat. By performing step S213 as described above, it is possible to stably perform breath holding measurement.

  As described above, according to the present embodiment, since the imaging sequence by breath holding is executed in a state where the respiratory cycle of the subject is stable, imaging is performed with more certainty that the diaphragm position is within the predetermined gate window. A sequence can be performed. As a result, the burden on the subject can be reduced, and breath holding imaging can be stably performed. As a result, the acquisition efficiency of the echo signal for image reconstruction can be further improved, and the photographing time can be further shortened.

(Processes that can be added)
Next, an additional process or operation that can be used in combination with at least one of the first embodiment or the second embodiment described above and further increases the improvement in the execution efficiency of the imaging sequence and the improvement in image quality due to the reduction of body motion artifacts. The procedure is described below.

  In the first additional process, the navigation sequence is performed at least during the first breath-holding period with respect to the breath-holding performed when the imaging sequence illustrated in FIG. 4 is executed. That is, as described above, the navigation sequence is executed under free breathing immediately after the start of the main shooting, and then, as the additional processing, the navigation is performed at least after the first breath holding instruction and during the breath holding period. The sequence continues. FIG. 8 shows an example of a sequence when this addition process is performed.

  By monitoring the position of the diaphragm by continuing the navigation sequence after the breath-hold instruction, the time required for the diaphragm position fluctuation to decrease from the breath-hold instruction and the duration of the period during which the diaphragm position fluctuation is small And can get. The time required from the breath holding instruction until the diaphragm position fluctuation becomes small can be, for example, the predetermined time Ts in step S204, and the duration of the period when the diaphragm position fluctuation is small is, for example, that of the imaging sequence It can be an execution period, or this value can be used in optimizing the number of echo signals acquired during the breath holding period.

  According to this additional process, it is possible to more precisely control the start timing and duration of the imaging sequence, so that the execution efficiency of the imaging sequence can be further improved.

  The second additional processing is acquired during the last breath holding period when the first detected diaphragm position is not included in the gate window in the navigation sequence executed following the end of the imaging sequence. This is a procedure for discarding an echo signal. An example of this addition processing is shown in the flowchart of FIG. (a) is an example added to the example of the first embodiment shown in FIG. 2, and (b) is an example added to the example of the second embodiment shown in FIG. In (a), the procedure up to step S211 is the same as in FIG. After step S212, the process proceeds to step S20A to be newly added without returning to step S205.

  In this step S20A, the adoption / discard of the echo signal acquired in the imaging sequence in S207 is determined based on the diaphragm position obtained from the navigation sequence immediately after the imaging sequence. After step S20A, the process returns to step S205. Similarly, in (b), the procedure up to step S212 is the same as in FIG. The process proceeds to step S20A to be newly added after step S212. After step S20A, the process proceeds to step S213.

  As a further additional process, as shown in FIG. 10, after the echo signal is discarded in step S20A, step S20B is performed to reduce the number of echo signals acquired in one breath-holding imaging sequence. (a) is an example in which step S20B is added to FIG. 9 (a), and (b) is an example in which step S20B is added to FIG. 9 (b).

More specific contents of step S20B
(1) Change in the number of echo signals acquired in one breath-holding sequence
(2) Update of the total shooting time. By performing step S20B, the breath holding period per one time can be shortened. This makes it possible to continue imaging under appropriate conditions even when the period during which breathing can be stopped due to fatigue of the subject is shortened.
According to this additional process, the echo signal acquired when the diaphragm position is outside the gate window is discarded, and the number of echo signals acquired corresponding to the degree of fatigue of the subject is adjusted. Can be further reduced to further improve the image quality.

The third additional process is a process for performing a simple body motion monitor when the imaging sequence is executed. Specifically, in the imaging sequence executed under breath holding, the zero-encoding signal is acquired by periodically setting the gradient magnetic field intensity in the phase encoding direction to zero. It is possible to detect a displacement in the readout direction from projection data obtained by Fourier transforming the zero-encoded signal. An algorithm for detecting a positional deviation amount from projection data is described in, for example, (Patent Document 2).
Japanese Patent No. 2932175

  The third additional process can minimize a decrease in the execution efficiency of the imaging sequence in the second additional process. In other words, in the second additional process, all echo signals acquired during the immediately preceding breath-holding period are discarded, whereas in the third additional process, the positional deviation in the readout direction is small due to the projection data. The echo signal determined to be used for image reconstruction, and only the echo signal determined to have a large positional deviation is discarded.

  An example of this addition processing is shown in FIG. As shown in FIG. 4, the imaging sequence is executed when the diaphragm position is included in the gate window. The imaging sequence is roughly divided into acquisition of projection data and acquisition of image reconstruction echo signals. The projection data is acquired, for example, immediately after the start of the imaging sequence. Thereafter, an image reconstruction echo signal is acquired. An example of a sequence for acquiring projection data and an image reconstruction echo signal is shown on the left and right sides of the lower part of FIG. These sequences are SSFP-type sequences in which the applied amount of the gradient magnetic field is made zero for all axes within the repetition time (TR). In particular, the sequence for acquiring the left projection data is a case where the phase encoding gradient magnetic field is zero in the sequence for acquiring the right image reconstruction echo signal.

  Immediately after execution of the imaging sequence under breath holding, zero-encoded data is acquired, and projection data is created from that data. The projection data acquired immediately after the start of the shooting sequence is held as reference data. After the acquisition of zero-encoded data, echo signals are acquired according to a predetermined phase encoding order. After a predetermined time (shown as Tpro in FIG. 11), zero-encoded data is acquired again. The projection data and the reference data are compared to calculate the amount of positional deviation in the lead-out direction. When the amount of positional deviation is small, an echo signal acquired before acquisition of zero-encoded data is used for image reconstruction.

Note that the algorithm for deriving the amount of misalignment described in (Patent Document 2) is often applied mainly to projection data in which a stationary part is an object to be imaged. On the other hand, the imaging target of the imaging sequence is a chest region including the heart in particular. Therefore, in the case where a stationary part and a part with movement are mixed, in the projection data, masking the heart and its neighboring region, or weighting the data of the chest body table, (Patent Document 2) described This algorithm should be applied.
According to this additional processing, since the selection of the echo signal acquired in the breath-holding imaging sequence is determined corresponding to the amount of positional deviation of the diaphragm, it is possible to further improve the execution efficiency of the imaging sequence.

The fourth additional process is a process for changing a condition such as a breath holding instruction in response to the diaphragm position drift. The position of the diaphragm has been reported as an example in which position fluctuation occurs in a long period other than the fluctuation due to the respiratory cycle, and such long period fluctuation is generally referred to as drift. In this additional processing, the position of the gate window is changed when drift occurs. Along with this position change, the diaphragm position Ps that satisfies the condition of the breath holding instruction is also changed. In FIG. 12, the left side shows the diaphragm position profile immediately after the start of imaging, and the right side shows the same profile after drift. As shown in this figure, when the position of the gate window is changed, the coordinate Ps of the diaphragm position is moved by the moving distance of the gate window and set to the new coordinate Ps ′ of the diaphragm position. This position Ps ′ becomes a new diaphragm position used for determination of the breath holding instruction.
According to this additional process, the position of the gate window is changed in response to the drift of the diaphragm position, so that the execution efficiency of the imaging sequence can be further increased.

The fifth additional process is a process of changing the slice position of the imaging sequence during each breath holding period based on the diaphragm position immediately before the start of the imaging sequence. As an example of this additional processing, for example, (Non-Patent Document 3) is known.
Magnetic Resonance in Medicine, 37,148-152 (1977)

  FIG. 13 shows a flowchart of an example of this addition process. Of the following processing steps, step S20C is part of step S207 that is performed the first time in FIG. 2, and steps S20D to S20F are the second and subsequent times within the repetition of steps S205 to S212 in FIG. It is added as part of S207 to be implemented.

In step S20C, the diaphragm position P1 acquired immediately before the first imaging sequence under breath holding is derived and stored as the reference position Pr.
In step S20D, the diaphragm position P2 acquired immediately before the second imaging sequence under breath holding is derived.
In step S20E, the algorithm of (Non-patent Document 3) is applied to the difference value P2-Pr from Pr to change the slice selection frequency in the second imaging sequence. In addition to the slice selection frequency, the reception phase of the echo signal may be changed.
In step S20F, the second imaging sequence under breath holding is executed.

The change of the imaging condition as described above is applied to the imaging sequence under full breath holding, and the echo signal is measured.
According to this additional process, since the slice position is changed in response to a change in the diaphragm position, it is possible to further reduce body movement artifacts and improve image quality.

  Heretofore, the additional processing that can be used in combination with at least one of the first embodiment or the second embodiment has been described. These additional treatments can be used either alone or in combination of at least two. By using the additional processing together, it is possible to further improve the execution efficiency of the imaging sequence and improve the image quality by reducing body motion artifacts.

(Third embodiment)
Next, a third embodiment of the MRI apparatus of the present invention will be described. In the present embodiment, an external sensor that detects the respiratory motion of the subject is provided to monitor the respiratory motion, and imaging is performed during a period when it is determined to be a breath-holding state.
A first example of this embodiment will be described with reference to the flowchart shown in FIG.
In step S1401, prior to imaging of the subject, an external sensor 122 that detects the respiratory motion of the subject is attached to the subject.

In step S1402, when the operator presses a shooting start button (not shown) provided in the computer 118, a shooting start command is sent from the computer 118 to the sequencer 116.
In step S1403, the sequencer 116 that has received the imaging start command sends a command to start monitoring the respiratory motion of the subject to the biological signal detection unit 121, and starts the respiratory motion monitoring of the subject by the respiratory motion detection sensor 122. . The monitored respiratory motion waveform is displayed on the display unit 119, for example.

  In step S1404, a predetermined time after the monitoring of respiratory motion is started, the operator gives an instruction to hold the breath to the subject. The breath-holding instruction may be automatically issued after a predetermined time from the start of respiratory motion monitoring by the MRI apparatus using a medium such as sound or light. Since the human respiratory cycle is about 1 to 3 seconds, it is desirable that about 5 to 10 seconds after the start of respiratory motion monitoring when automatically instructing to stop breathing.

FIG. 15 shows an example of the respiratory motion waveform of the monitored subject. The example of FIG. 15 is a case where the respiratory motion detection sensor 122 is attached to the subject and the movement of the chest wall or the abdominal wall is monitored. A curve 1501 is a waveform representing the movement of the chest wall or abdominal wall due to respiration (hereinafter referred to as a respiratory motion waveform).
During a period in which the subject is breathing freely, the respiratory motion waveform changes in vibration as indicated by 1504. In such a breathing motion state, when a breath holding instruction is issued at the timing 1502, the subject holds the breath several seconds after the instruction. When the subject holds his / her breath, the chest wall or abdominal wall almost does not move, so the respiratory motion waveform becomes almost flat as indicated by 1503 in FIG.

  In step S1405, the respiratory motion monitoring is continued, and a time point at which the respiratory motion waveform hardly changes is detected. The criterion for determining that the respiratory motion waveform has hardly changed is, for example, when the movement of the chest wall or abdominal wall is monitored, and the time when the fluctuation amplitude of the waveform is less than 3 mm lasts 0.5 seconds or longer. it can.

In step S1406, it is determined whether the respiratory motion waveform has hardly changed. If it is determined that there is almost no change, the process proceeds to step S1407, and if it is determined that there is still a change, the process proceeds to step S1405.
In step S1407, the sequencer 116 starts a predetermined shooting sequence. When shooting is completed, normal image processing is performed and an image is displayed on the display unit 119.

  Next, a second example of the present embodiment will be described. In the present embodiment, when the respiratory motion is monitored by an external sensor, it is utilized that the respiratory motion can be continuously monitored in parallel with the imaging.

  When the sequencer 116 detects that the respiratory motion waveform is disturbed while performing imaging, an alarm indicating that the breathing is disturbed is displayed on the display unit 119. By displaying the alarm, the operator can know that the subject's breathing is disturbed during imaging. When an artifact occurs in the acquired image, it can be determined whether or not the artifact is due to breathing disturbance.

  In addition, as shown in FIG. 15, when the disturbance of the respiratory motion waveform 1501 ′ occurs temporarily, the sequencer 116 discards the echo signal acquired during the period when the respiratory motion waveform is disturbed, and the respiratory motion waveform is stable. Then, the echo signal can be acquired again. As a result, artifacts due to breathing disturbance can be suppressed.

  Detection of the disturbance of the respiratory motion waveform 1501 ′ can be determined, for example, by providing a width 1601 with respect to the respiratory motion waveform at the time of breath holding and if the respiratory motion waveform is outside the range of 1601, breathing is disturbed. For example, when the movement of the chest wall or the abdominal wall is monitored, the width 1601 can be about 3 mm. Alternatively, the determination may be made based on the magnitude of the first derivative of the respiratory motion waveform 1501 '.

  As described above, according to the present embodiment, it is confirmed that the body movement position has stopped at a predetermined position by monitoring the respiratory movement by providing an external sensor for detecting the respiratory movement of the subject. Is implemented. In addition, since it is not necessary to use a navigation sequence together, it is possible to suppress an increase in shooting time. In addition, since the respiratory motion can be detected in real time, the start of the imaging sequence can be optimized. Further, since the state of the respiratory motion and the disturbance thereof can be monitored in parallel with the imaging sequence, the remeasurement based on the disorder of the respiratory motion can be minimized and the extension of the imaging time due to the remeasurement can be minimized.

  The present invention described above is particularly effective when imaging a region where image quality deterioration due to respiratory body movement is expected. For example, when a diffusion-weighted image is taken on a target site of a chest or abdominal organ such as the liver, it is necessary to improve the SN ratio by addition averaging. Inevitably, the imaging time becomes longer, and artifacts due to respiratory movement are inevitable. In addition, the navigation sequence shown in the prior art is difficult to apply because the excitation region of the navigation sequence becomes a band-like artifact on the diffusion weighted image. The present invention solves the above problems and can simultaneously realize a reduction in photographing time and an improvement in image quality. In addition, since the position of the heart changes with breathing, the present invention is effective for cardiac imaging in general.

MRI equipment diagram. 5 is a flowchart showing a processing procedure of an example of the first embodiment of the present invention. The figure which shows an example of the profile of a diaphragm position. The timing chart of the diaphragm navigation sequence and imaging | photography sequence of this invention. The figure which shows an average respiration cycle and the diaphragm average displacement derivation | leading-out process. The figure which shows an average profile derivation process. The flowchart which shows the process sequence of an example of the 2nd Embodiment of this invention. It is an example of the sequence of this invention, and is a sequence diagram in the case of implementing a navigation sequence under breath holding. An example of a data processing procedure when the diaphragm position is outside the gate window in the first navigation sequence after the imaging sequence. Another example of data processing procedure when the diaphragm position is outside the gate window in the first navigation sequence after the imaging sequence. The figure which shows an example of a sequence in the case of implementing a simple body movement monitor at the time of an imaging | photography sequence. The figure which shows the example which changed the gate window position. 10 is a flowchart illustrating an additional procedure for changing a slice position in an imaging sequence. 9 is a flowchart showing a processing procedure of an example of a third embodiment of the present invention. An example of a respiratory motion waveform monitor. An example of a respiratory motion waveform monitor during measurement.

Explanation of symbols

  101 Static magnetic field generating magnet, 102 Imaging object, 103 bed, 104 High frequency magnetic field coil, 105 X direction gradient magnetic field coil, 106 Y direction gradient magnetic field coil, 107 Z direction gradient magnetic field coil, 108 High frequency magnetic field power supply, 109 X direction gradient magnetic field coil , 110 Y-direction gradient coil, 111 Z-direction gradient coil, 112 synthesizer, 113 modulator, 114 amplifier, 115 receiver, 116 sequencer, 117 storage medium, 118 computer, 119 display

Claims (3)

Means for generating a static magnetic field, means for generating a gradient magnetic field, means for generating a high-frequency magnetic field, means for detecting a nuclear magnetic resonance signal generated from the subject, and the subject using the nuclear magnetic resonance signal Controlling the gradient magnetic field generating means, the high-frequency magnetic field generating means, and the signal detecting means based on a predetermined pulse sequence, means for reconstructing the image of the image, means for transmitting predetermined information to the subject, And means for controlling the transmission means,
The pulse sequence has a body motion detection sequence for detecting a body motion position of the subject, and an imaging sequence for acquiring an image of the subject,
The control means includes the body movement detection sequence, a breath holding instruction to the subject via the transmission means based on body movement information detected by the body movement detection sequence, the imaging sequence, and the transmission In a magnetic resonance imaging apparatus that repeatedly executes respiration resumption instruction to the subject via means,
The control means continues to detect the body movement position after the breath holding instruction, and detects the body movement position, and executes the imaging sequence when the body movement position is within a predetermined range. Magnetic resonance imaging device. The magnetic resonance imaging apparatus according to claim 1.
The control means executes the body movement detection sequence after the imaging sequence to detect a body movement position, and after the value derived from the fluctuation of the body movement position falls within a predetermined range, the breath holding instruction The magnetic resonance imaging apparatus is characterized in that the transmission is performed. Means for generating a static magnetic field, means for generating a gradient magnetic field, means for generating a high-frequency magnetic field, means for detecting a nuclear magnetic resonance signal generated from the subject, and the subject using the nuclear magnetic resonance signal Controlling the gradient magnetic field generating means, the high-frequency magnetic field generating means, and the signal detecting means based on a predetermined pulse sequence, means for reconstructing the image of the image, means for transmitting predetermined information to the subject, A magnetic resonance imaging apparatus comprising: means for controlling the transmission means; and means for detecting a body movement position of the subject.
The control means transmits a breath holding instruction to the subject via the information transmission means when the body movement position falls within a predetermined range, and thereafter starts measuring the nuclear magnetic resonance signal. Magnetic resonance imaging apparatus.

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