JP5575695B2 - MRI equipment - Google Patents

Description

  The present invention relates to an MRI apparatus that images a blood flow without using an MR contrast agent, and in particular, by using an electrocardiogram synchronization method, an effect due to inflow of blood flow (that is, to a fresh blood flow imaging region). The present invention relates to an MRI apparatus capable of performing MRA (MA angiography) in which the effect of increasing the MR signal value with the inflow of the inflow: the inflow effect) is maximally useful.

  Magnetic resonance imaging is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited with a high-frequency signal of its Larmor frequency, and an image is reconstructed from an MR signal generated by the excitation. .

  In the field of magnetic resonance imaging, when an blood flow image of a lung field or abdomen is obtained, clinically, MR angiography for performing angiography by administering an MR contrast agent to a subject has begun to be performed. However, this contrast-enhanced MR angiography method requires an invasive treatment for administering a contrast agent, and above all, the burden on the patient's mental and physical strength is great. Also, the inspection cost is high. Furthermore, the contrast agent may not be administered depending on the patient's constitution.

  When a contrast agent cannot be administered or is not administered, a time-of-flight (TOF) method or the like is known as an alternative method (see, for example, Patent Document 1).

  This time-of-flight method is a technique that uses the effect of a flow such as blood flow. In general, the flow effect is caused by one of two properties of a moving spin. One is that the spin simply moves its position, and the second depends on the phase shift of the transverse magnetization caused by the movement of the spin in the gradient magnetic field. Among these, the method based on the former position movement is the TOF method.

JP-A-8-103429

  Incidentally, as an example of imaging using the conventional TOF method, three-dimensional imaging is performed by applying the TOF method to the head. However, in the blood flow image obtained by this, the contrast between the blood flow and the substantial part is not yet sufficient, and there is a demand from the clinical site to improve this contrast and increase the resolution. It had been.

  Therefore, even if the blood flow can be imaged without administering a contrast medium, and the blood flow changes, the contrast between the blood flow and the substantial part is maximized by taking advantage of the inflow effect of the blood flow. There is a demand for an MRI apparatus capable of obtaining an MRA image with improved image quality.

The MRI apparatus of the embodiment performs a preparatory two-dimensional scan on a part of the desired region in the MRI apparatus configured to perform three-dimensional imaging by an electrocardiographic synchronization method on a desired region of a subject. Preparation scanning means for acquiring in advance a plurality of preparation images having different delay times from the reference wave, and analyzing the signal intensity distribution of the blood flow portion of the plurality of preparation images, and three-dimensional imaging by the electrocardiographic synchronization method An optimum delay time setting means for setting a delay time used for the scan for the use, and a three-dimensional imaging scan by the electrocardiographic synchronization method with the delay time set by the optimum delay time setting means to collect echo signals The imaging scanning means and the collected echo signals are arranged in the k space, and the reconstruction processing is applied to the echo signals arranged in the k space. Comprising image generating means for generating an MRA image without using an MR contrast agent, wherein the plurality of preparatory images is a two-dimensional phase image (2D-PS), and wherein the.

The functional block diagram which shows the structural example of the MRI apparatus which concerns on embodiment and modification of this invention. The figure explaining the time context of the ECG-prep scan and imaging scan in embodiment and modification. 4 is a schematic flowchart illustrating an ECG-prep scan procedure executed by the host computer in the embodiment. The timing chart which illustrates the relationship between an ECG-prep scan and the data arrangement | positioning to k space. The timing chart explaining the pulse train of ECG-prep scan. 3 is a schematic flowchart of an imaging scan executed by a host computer. 3 is a schematic flowchart of an imaging scan executed by a sequencer. The timing chart which illustrates the relationship between an imaging scan and the data arrangement | positioning to k space. The timing chart explaining the pulse train of an imaging scan. The figure explaining the three-dimensional imaging area of an imaging scan. The timing chart explaining the pulse train of a two-dimensional imaging scan. The figure explaining the concept of the scroll method applicable to data collection and arrangement | positioning. A timing chart of a pulse sequence using a blank pulse for an imaging scan. The timing chart of the pulse sequence which used the some division | segmentation MT pulse for the imaging scan. 6 is a schematic flowchart showing control of the start timing of an imaging scan.

  Embodiments according to the present invention will be described below.

(First embodiment)
A first embodiment will be described with reference to FIGS.

  A schematic configuration of an MRI (magnetic resonance imaging) apparatus according to this embodiment is shown in FIG.

  The MRI apparatus includes a bed unit on which the subject P is placed, a static magnetic field generation unit that generates a static magnetic field, a gradient magnetic field generation unit for adding position information to the static magnetic field, a transmission / reception unit that transmits and receives high-frequency signals, A control / arithmetic unit responsible for overall system control and image reconstruction, and an electrocardiogram measurement unit that measures an ECG signal as a signal representing the cardiac time phase of the subject P are provided.

  The static magnetic field generation unit includes, for example, a superconducting magnet 1 and a static magnetic field power supply 2 that supplies current to the magnet 1, and an axial direction of a cylindrical opening (diagnostic space) into which the subject P is loosely inserted. A static magnetic field H0 is generated in the (Z-axis direction). In addition, the shim coil 14 is provided in this magnet part. The shim coil 14 is supplied with a current for homogenizing a static magnetic field from a shim coil power supply 15 under the control of a host computer to be described later. The couch portion can removably insert the top plate on which the subject P is placed into the opening of the magnet 1.

  The gradient magnetic field generator includes a gradient magnetic field coil unit 3 incorporated in the magnet 1. The gradient coil unit 3 includes three sets (types) of x, y, and z coils 3x to 3z for generating gradient magnetic fields in the X-axis direction, the Y-axis direction, and the Z-axis direction orthogonal to each other. The gradient magnetic field unit also includes a gradient magnetic field power supply 4 that supplies current to the x, y, and z coils 3x to 3z. The gradient magnetic field power supply 4 supplies a pulse current for generating gradient magnetic fields to the x, y, z coils 3x to 3z under the control of a sequencer 5 described later.

  By controlling the pulse current supplied to the x, y, z coils 3x to 3z from the gradient magnetic field power source 4, the gradient magnetic fields in the three axes (X axis, Y axis, Z axis) directions which are physical axes are synthesized. The logical axis direction composed of the slice direction gradient magnetic field GS, the phase encode direction gradient magnetic field GE, and the readout direction (frequency encode direction) gradient magnetic field GR orthogonal to each other can be arbitrarily set and changed. Each gradient magnetic field in the slice direction, the phase encoding direction, and the readout direction is superimposed on the static magnetic field H0.

  The transmission / reception unit includes an RF coil 7 disposed in the vicinity of the subject P in the imaging space in the magnet 1, and a transmitter 8T and a receiver 8R connected to the coil 7. The transmitter 8T and the receiver 8R operate under the control of the sequencer 5 described later. With this operation, the transmitter 8T supplies the RF coil 7 with an RF current pulse having a Larmor frequency for exciting nuclear magnetic resonance (NMR). The receiver 8R takes in the MR signal (high frequency signal) received by the RF coil 7 and performs various signal processing such as preamplification, intermediate frequency conversion, phase detection, low frequency amplification, filtering, etc. Digital data (original data) of MR signal is generated by / D conversion.

  Further, the control / arithmetic unit includes a sequencer (also called a sequence controller) 5, a host computer 6, an arithmetic unit 10, a storage unit 11, a display device 12, an input device 13, and a sound generator 16. Among these, the host computer 6 has a function of instructing the pulse sequence information to the sequencer 5 according to the stored software procedure (not shown) and supervising the operation of the entire apparatus.

  While executing a main program (not shown), the MRI apparatus first performs a preparation scan (hereinafter referred to as an ECG-prep scan), and then performs an imaging scan (hereinafter referred to as an imaging scan). A two-stage scanning method is used. In the ECG-prep scan, a preparation pulse sequence is executed using the ECG signal, and the acquisition start timing at the cardiac time phase of echo data to be arranged in the low frequency region of the k space is determined optimally in the subsequent imaging scan. Here, the acquisition start timing is determined as a delay time from the peak value of the R wave of the ECG signal. In the imaging scan, an imaging pulse sequence based on an electrocardiographic synchronization method including a scan synchronized with the timing determined in this manner is executed.

  The sequencer 5 includes a CPU and a memory, stores pulse sequence information sent from the host computer 6, controls the operations of the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R according to this information, The digital data of the MR signal output from the receiver 8R is once inputted and transferred to the arithmetic unit 10. Here, the pulse sequence information is all information necessary for operating the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R according to a series of pulse sequences, for example, x, y, z coils 3x to 3z. Includes information on the intensity, application time, application timing, and the like of the pulse current applied to.

  The pulse sequence is a two-dimensional (2D) scan or a three-dimensional scan (3D), and the pulse train is in various forms such as a segmented FFE (high-speed FE) method and an FE method. Can be used.

  Further, the arithmetic unit 10 inputs the digital data (also referred to as original data or raw data) output from the receiver 8R through the sequencer 5, and the digital data is input to the k space (also referred to as Fourier space or frequency space) by its internal memory. Data is arranged, and this data is subjected to two-dimensional or three-dimensional Fourier transform for each set to reconstruct image data in real space. The arithmetic unit is also capable of executing data composition processing and difference arithmetic processing as necessary. This synthesis processing includes processing for adding each pixel, maximum value projection (MIP) processing, and the like. As another example of the above synthesis process, the axes of a plurality of frames may be aligned in Fourier space, and the original data may be synthesized into one frame of original data. The addition processing includes simple addition processing, addition averaging processing, weighted addition processing, and the like.

  The storage unit 11 can store not only the reconstructed image data but also the image data that has been subjected to the above-described combining process and difference process. The display 12 is used for displaying a reconstructed image, for example. Also, parameter information desired by the surgeon, scan conditions, pulse sequences, information relating to image synthesis and difference calculation, and the like can be input to the host computer 6 via the input unit 13.

  The voice generator 16 can issue a breath holding start message and a breath holding end message as voice when instructed by the host computer 6.

  Further, the electrocardiogram measurement unit attaches to the body surface of the subject and detects an ECG signal as an electrical signal, and performs various processing including digitization processing on the sensor signal to perform the host computer 6 and the sequencer. And an ECG unit 18 for outputting to the PC. This ECG measurement signal is used by the sequencer 5 when executing an ECG-prep scan and an imaging scan. Thereby, the synchronization timing for the ECG synchronization method can be set optimally, and data can be collected by performing an ECG synchronization imaging scan based on the synchronization timing.

  Subsequently, the ECG-prep scan will be described with reference to FIGS.

  While executing a predetermined main program (not shown), the host computer 6 starts execution of the ECG-prep scan process shown in FIG. 3 in response to a command from the input device 13, for example.

  First, the host computer 6 reads scan conditions and parameter information for executing the ECG-prep scan from the input device 13 (step S1 in the figure). The scan condition includes a scan type, a pulse sequence, a phase encoding direction, and the like. The parameter information includes a delay time TDL that determines the timing within the cardiac phase. These parameters can be arbitrarily set by the operator.

  Next, the host computer 6 sends message data to the voice generator 16 to cause the subject (patient) to give a breath holding command such as “please hold your breath” (step S2). This breath holding is preferably performed from the viewpoint of suppressing the body movement of the subject during the execution of the ECG-prep scan. However, in some cases, the ECG-prep scan is executed without performing the breath holding. May be.

  Thereafter, the host computer 6 starts reading the ECG signal (step S3).

  Next, the count value of the software timer T that measures the elapsed time from the peak value of the R wave as the reference wave in the ECG signal is cleared (T = 0) (step S4).

  Then, the host computer 6 determines whether or not an R wave peak value appears in the ECG signal (step S5). This determination is continued until the R wave peak value appears.

  If YES in step S5, that is, if an R wave peak value appears, the host computer further counts whether the delay time TDL (= α1, α2,..., Αn has elapsed from the R wave peak value or not by the timer T. The delay times TDL = α1, α2,..., Αn are set to α1 <α2,..., <Αn as shown in FIG. A plurality of time phases (here, the number of time phases m = 1 to 5; each time phase is composed of a plurality of scans) forming each of the segmented segments (number of segments n = 1 to a predetermined number) Here, it is a time value that defines the start timing of the number of scans p = 1 to 4. The determination in step S6 is also continued until the set delay time TDL (= α1, α2,..., Αn) elapses. The

  When the determination in step S6 is YES, that is, when it is recognized that the delay time TDL (= α1α2,..., Αn) has arrived, the host computer 6 instructs the sequencer 5 to perform, for example, a two-dimensional segmented FFE method. (Hereinafter, referred to as seg.FFE method) is instructed to execute a scan based on the number of time phases m for each segment number n and the number of scans p constituting each time phase (scan S7).

  This seg. The FFE method is a method of obtaining a lump of continuous data called a segment between RR waves of an ECG signal. This seg. The ECG-prep scan based on the FFE method is performed based on a method called “single slice multiphase”. According to this “single slice multiphase” method, echo data of a plurality of time phases can be collected at a time for a single slice determined by the gradient magnetic field for slice GS and the RF frequency. For this reason, in the present embodiment, as shown in FIG. 4 for example, the slice of the head is scanned by the ECG-prep scan, and the slice thickness is determined so that the imaging target blood vessel enters the slice.

  In this embodiment, the pulse sequence used for the ECG-prep scan is a two-dimensional scan, and that in an imaging scan described later is a three-dimensional scan. The purpose of the ECG-prep scan is not to obtain the image itself, but it is only necessary to find a cardiac time phase in which the blood flow portion is depicted with the highest signal value when performing the ECG synchronization method. A scan is sufficient. By executing the ECG-prep scan as a two-dimensional scan, the time for the ECG-prep scan can be shortened.

  However, it is desirable that the pulse sequences used for the ECG-prep scan and the imaging scan are of the same type. FFE method. If the type of pulse sequence changes, the image quality changes even in the same scan region, and this is natural. Further, it is desirable that the echo times TE (see TE = TE1 in FIG. 5 and FIG. 7 described later) of the pulse sequences used for the ECG-prep scan and the imaging scan are the same. Thereby, the dephase component of the spin of blood flow can be made substantially the same.

  Note that this ECG-prep scan may be performed by a two-dimensional (2D) scan, for example, when the imaging scan (main scan) is a three-dimensional (3D) method, or a three-dimensional scan adapted to the region of the imaging scan. You may go on.

  On the other hand, regarding the correspondence between the ECG-prep scan and the ECG signal, as shown in FIG. 4 and FIG. 5 as an example, five time phases m for each cardiac cycle become delay times TDL = α1 to α5 from the R wave. It is set to almost the corresponding value. For this reason, when the ECG-prep scan is executed, under the control of the sequencer 5, as shown in FIG. 5, four scans are executed for each time phase based on the FFE method. An echo signal is collected.

  As shown in FIG. 4, the k-space is divided into four regions in the phase encoding direction. Then, by appropriately setting the phase encoding amount by the phase encoding direction gradient magnetic field pulse GE, the echo data of four scans in the constant time phase m in each cardiac cycle fill one two-dimensional k-space. It has become. At this time, four echo signals collected at the time phase of m = 1 in the segment n = 1 are at the head phase encode position in each of the four divided regions, and then when m = 1 in the segment n = 2. The four echo signals collected in phase are sequentially arranged at the next phase encoding position in each of the four divided regions.

  Next, the host computer 6 determines whether or not scanning for the predetermined number of segments n has been completed. If NO, the process returns to step S4 and repeats the above-described processing (step S8).

  For this reason, when YES is determined in step S8, the scan for the predetermined number n of segments is completed. The echo signals related to these scans are received and processed by the receiver 8R, and sent as echo data to the arithmetic unit 10 via the sequencer. The echo data is filled in a total of five sets of two-dimensional k-spaces KS1 to KS5 corresponding to the number of time phases m = 5 in each segment n by the arithmetic unit 10. The echo data filling the five sets of k spaces KS1 to KS5 can be regarded as echo data groups collected almost in synchronization with the delay times TDL = α1 to α5 from the R wave, respectively.

  When the determination of YES is made in step S8, since the ECG-prep scan is completed, the host computer 6 causes the voice generator 16 to output a breath holding release command (step S9). As a result, a voice-holding voice message is output from the voice generator 16 with the content of “breathing is fine”, for example.

  When the above-described processing is sequentially executed, echo data for 5 frames with TDL = 50 msec, 150 msec, 250 msec, 350 msec, and 450 msec are prepared in the arithmetic unit 10 respectively. . Therefore, these echo data are reconstructed into real space images by the arithmetic unit 10 and stored in the storage unit 11. For example, in response to an operation signal from the input device 13, the host computer 6 reads out the reconstructed image from the storage unit 11 and sequentially displays it as an MRA image.

  Therefore, the operator can determine the MRA image in which the blood flow portion is depicted with the highest signal by visually observing these cine display images.

  That is, since the plurality of MRA images obtained by the ECG-prep scan have different delay times TDL from the R wave, the inflow effect when the pulsating blood flow flows into the head slice region is the most. The larger the value, the higher the signal value of the blood flow of the reconstructed MRA image. For this reason, by specifying an image in which the blood flow portion of the image is the clearest and high signal, an optimum delay time TDL, that is, a fresh inflow (blood flow) that is not saturated under the execution of the ECG synchronization method. It is possible to determine the synchronization timing that can be reliably captured.

  Note that the process of determining the synchronization timing of the electrocardiographic synchronization method from the plurality of MRA images is not necessarily limited to the visual observation of the image of the operator. An ROI is set on the blood flow portion on the display image, and the ROI is set. It may be performed by an automated process such as analyzing the signal intensity distribution in the signal and comparing the analysis results. In addition, if there are multiple images that are considered optimal as a result of visual observation or automation processing, that is, there are multiple delay times that are considered optimal, new processing such as interpolation is performed between these delay times. Time (synchronization timing) may be calculated.

  The synchronization timing optimally set through the ECG-prep scan is reflected in the subsequent imaging scan based on the electrocardiographic synchronization method.

  Subsequently, an imaging scan based on the electrocardiogram synchronization method will be described with reference to FIGS.

  The host computer 6 executes the processing shown in FIG. 6 in response to the operation information from the input device 13 while executing a predetermined main program (not shown).

  Specifically, the host computer 6 first reads the optimum delay time TDL of the electrocardiographic synchronization method determined through the execution of the ECG-prep scan described above (FIG. 6, step S20). This time value can be read by receiving an input from the operator via the input device 13 or by automatically reading the optimum delay time TDL stored in the memory in the storage unit 11 into the work area. You may do it.

  Next, the host computer 6 scans the scanning conditions (phase encoding direction, image size, number of scans, waiting time between scans, type of pulse sequence according to the scan site, etc.) designated by the operator from the input device 13 and the image processing method. (Addition processing or maximum value projection (MIP) processing, etc. In the case of addition processing, any of simple addition, addition averaging processing, weighted addition processing, etc.) is input and those including the optimum delay time TDL are input. The information is processed into control data, and the control data is output to the sequencer 5 and the arithmetic unit 10 (step S21).

  Next, when the host computer 6 can determine that there is a notification of preparation completion before the imaging scan (step S22), it outputs a breath holding start command to the sound generator 14 at step S23 (step S23). As a result, the voice generator 14 issues a voice message of “Please hold your breath” in the same manner as in the ECG-prep scan, and the patient who has heard this will stop breathing.

  Thereafter, the host computer 6 instructs the sequencer 5 to start an imaging scan (see step S27 and the process of FIG. 7).

  FIG. 8 shows an outline of the pulse sequence of this imaging scan, the temporal relationship with the blood flow signal intensity (beat) for each cardiac cycle, and an example of the arrangement of echo signals, and FIG. 9 shows a detailed example of the pulse train of this pulse sequence. Shown in

  Note that the processing of the echo signals collected by this imaging scan is the same as in the ECG-prep scan described above. That is, the echo signal is received by the RF coil 7, and then sent to the receiver 8R for reception processing. The processed signal is sent as echo data to the arithmetic unit 10 via the sequencer 5 and is arranged in the three-dimensional k space according to the slice encoding amount and the phase encoding amount. For this reason, in the following description, detailed description from reception of the echo signal to arrangement is omitted.

  As shown in FIGS. 8 and 9, the imaging scan is performed using a three-dimensional seg. Based on the FFE method, for example, the three-dimensional region Rima of the head is implemented as an imaging region (see FIG. 10). This three-dimensional seg. In the FFE method, as shown in FIGS. 8 and 9, for each cardiac cycle, a plurality of segmented field echo collection scans (segment n) and MT ( Magnetic transfer (pulse): Pmt, CHESS (chemical shift selective) pulse: Pches, and spoiler pulses: SPs, SPr, SPe are used. The three-dimensional seg. The echo time TE = TE1 in the pulse train of the FFE method is set to be the same as that of the ECG-prep scan described above.

  Specifically, when the sequencer 5 receives a scan start command, the sequencer 5 executes a first group scan S1 based on several FFE methods at a predetermined timing after the appearance of the R wave for each cardiac cycle (FIG. 7). Steps S24-1 to 24-5). As shown in FIG. 8, the phase encoding GE1 is set so that the echo signals collected by the scan S1 of the first half group are arranged in the high frequency region RH1 with respect to the slice encoding GE2 having a three-dimensional k space (FIG. 8). 9).

  In the example of FIG. 8, the high-frequency region RH1 and the high-frequency region RH2 at the target position are arranged in the center of the phase encode amount = 0 from the top and bottom of the 128 phase encode amounts divided into 8 regions by 16 lines. Refers to the three areas heading. For this reason, the remaining two divided regions located at the center in the phase encoding direction are set as low frequency regions RL important for image contrast.

  For the scan S1 of the first half group, the echo signal collected in the first scan is set in the first divided region of the high-frequency region RH1 for the slice encode amount in the k space by setting the value of the phase encode amount SE1. The echo signal collected in the second scan is in the second divided region of the high frequency region RH1 in the same space, and the echo signal collected in the third scan is in the third divided region of the high frequency region RH1 in the same space. Each is arranged. Moreover, the echo signal collected by the first cardiac cycle scan in the first group scan S1 is arranged at the position of the first phase encoding amount in each of the three divided regions of the high-frequency region RH1, and hereinafter the cardiac cycle. Each is sequentially arranged at the position of the next phase encoding amount.

  Also, in each cardiac cycle, MT pulse: Pmt, CHESS pulse: Pches, and spoiler pulses: SPs, SPr, SPe are sequentially applied at appropriate timing following the first group scan S1 (step S24- in FIG. 7). 6-24-11). The MT pulse is applied to cause an MT (Magnetic Transfer) effect and suppress a signal value from a substantial part. The CHESS pulse is applied for fat suppression. Furthermore, the spoiler pulse is applied in order to sufficiently dephase the phase of the nuclear spin so that the application of the MT pulse or the CHESS pulse does not affect the subsequent echo signal acquisition. Here, the spoiler pulse is applied in three directions: a slice direction, a phase encoding direction, and a reading direction.

  Thereafter, at an appropriate timing, the scan S2 of the latter half group is seg. This is executed by the FFE method (see FIG. 7, steps S24-12 to 24-13 and FIG. 9). This scan S2 is for collecting echo signals arranged in the low frequency region RL and the remaining high frequency region RH2 located at the center of the k space.

  In particular, the start timing of the first scan executed in the latter half group scan S2 is made to coincide with the delay time TDL optimally set through the ECG-prep scan as described above. The echo signals collected in each scan of the second half group scan S2 are arranged for each region by multiplying the low frequency region RL and the remaining high frequency region RH2 in the scan order.

  That is, the echo signal collected by the first scan in the second group scan S2 is in the first divided region of the low frequency region RL, and the echo signal collected by the second scan is in the remaining divided region of the low frequency region RL. The echo signals collected by the third scan are arranged in the first divided region of the high frequency region RH2, and the echo signals collected by the fourth scan are arranged in the next divided region of the high frequency region RH2. Moreover, the echo signal collected by the first cardiac cycle scan in the latter half group scan S2 is arranged at the position of the first phase encoding amount in each divided region. Are arranged at the position of the phase encoding amount.

  As described above, the electrocardiographic synchronization timing matched with the optimally set delay time TDL is the timing at which the blood flow signal is rendered highest at the imaging region Rima. Therefore, by collecting as described above, the echo signals collected by the initial scan (particularly the first and second scans) in the scan S2 of the latter half group are always preferentially arranged in the low frequency region RL. .

  The above seg. The execution of the scan based on the FFE method and the application of other MT pulses are repeated for each cardiac cycle, for example, “16 cycles (n = 16) × slice encoding amount”. As a result, the arrangement of the echo data in the three-dimensional k space is completed (FIG. 7, step S24-14).

  Thereafter, the sequencer 5 notifies the host computer 6 of the scan completion notification (step S24-15).

  Upon receiving the scan completion notification from the sequencer 5 (step S25), the host computer 6 outputs a breath holding release command to the voice generator 16 (step S26). Therefore, the voice generator 16 issues a voice message such as “It is fine to breathe” to the patient, and the breath holding period ends.

  Thereafter, the host computer 6 issues an image reconstruction and display command to the arithmetic unit 10 (step S27). In response to this, the arithmetic unit 10 performs, for example, three-dimensional Fourier transform on the k-space data to generate real-space three-dimensional image data, and stores this data in the storage unit 11. The arithmetic unit 10 further generates two-dimensional image data by performing, for example, MIP (maximum value projection) processing on the three-dimensional image data, and displays it on the display 12.

  According to the above image generation method, the synchronization timing of the electrocardiographic synchronization method is specified by the optimum delay time TDL obtained in advance through the ECG-prep scan, and is collected in the vicinity of the synchronization timing. An echo signal having a high signal intensity is preferentially arranged in the central portion in the phase encoding direction of the k space, that is, in the low frequency region. This data collection / arrangement method is achieved by the above-mentioned second group scan S2, and considering this group scan S2 as a center, it becomes a centric order data collection / arrangement method. ing.

  Therefore, the blood flow can be imaged without using an MR contrast agent, and even in the case of pulsating blood flow, the contrast between the substantial part and the blood flow in the reconstructed image is very high and stable. . That is, it is possible to perform echo data collection and k-space arrangement by making maximum use of such an inflow effect of blood flow, and to provide an image generation method excellent in entity (blood flow) rendering ability. Can do.

  In addition, according to a comparative experiment on the MRA image of the head conducted by the present inventor, 3Dseg. In the case of an FFE imaging scan, the blood flow signal intensity of the obtained image is improved by about 1.5 times compared to an imaging scan based on a normal 3DTOF method for the head, and the rendering performance is reliably improved. Has been confirmed.

  In this embodiment, a three-dimensional seg. When an imaging scan is executed according to the FFE method, echo data to be mapped to a high-frequency region in the k space is collected by a scan S1 of the first half group in a vacant time before reaching the ECG synchronization timing for each cardiac cycle. By collecting data without wasting idle time in this way, the entire imaging time can be shortened as much as possible. This also keeps the breath holding period during the imaging scan as short as possible and reduces the burden on the patient.

  Furthermore, since the ECG synchronization timing is measured by the “single slice / multiphase” method as described above, images of a plurality of time phases can be obtained at one time by one ECG-prep scan. That is, it is not necessary to perform a plurality of ECG-prep scans, the entire imaging time can be shortened, and patient throughput is improved.

  Various modifications can be further made to the MRI apparatus of the present embodiment. Hereinafter, this modification will be described.

(First variant)
The scan S1 of the first half group using the above-mentioned idle time is not necessarily executed. After applying the MT pulse, CHESS pulse, and spoiler pulse for each cardiac cycle, a series of echo data collections starting at the optimal synchronization timing is performed. Of course, the scanning may be continued.

(Second variant)
In addition, as shown in FIG. 11, an imaging scan using the above-described electrocardiographic synchronization method is a two-dimensional seg. You may carry out based on FFE method. In this imaging method, application of the slice encoding gradient magnetic field is omitted from the pulse train shown in FIG. 9, and only the phase encoding gradient magnetic field GE is applied as encoding. Also in this two-dimensional imaging scan, it is possible to suitably use the above-described method of presetting the optimum synchronization timing and collecting and arranging centric order data.

  In this case, the slice of the ECG-prep scan and the slice (imaging region) of the imaging scan may be different.

(Third variant)
This modification is characterized in that the collection and arrangement of echo data is performed by the scroll method. Depending on the imaging conditions such as the synchronization timing (delay time TDL) of the ECG synchronization method, the number of phase encodings in the k space, another scan is simply performed between the start timing of the scan S2 of the latter half group and the R wave. It is possible that there is no space. In such a case, scanning is started immediately at the set synchronization timing (corresponding to the scan S2 of the latter half group in the above-described embodiment), and the echo signals are collected and arranged based on the centric order method. In this case, the echo signal due to the later scan in the series of started scans is scrolled and arranged in the high frequency region of the k space as shown by the arrows in FIG. An encoding gradient magnetic field pulse is set. This can also shorten the imaging time.

(Fourth modification)
Further, in the above-described embodiment, an excitation pulse (empty pulse) that does not collect echo signals may be applied instead of the scan S1 of the first half group. In this way, since the idle pulse is always applied in the idle time from the appearance of the R wave to the scan S2 by the centric order method, the signal value from the substantial part of the imaging region is suppressed and slice encoding is performed. It is possible to reduce signal variations between slices or slices.

(5th modification)
This variant relates to the application of MT pulses. As shown in FIG. 14, a plurality of divided MT pulses are continuously applied as MT pulse: Pmt. Each of the divided MT pulses has a small flip angle, but the pulse waveform area is set so that a predetermined flip angle can be secured as a whole of the plurality of divided MT pulses. The signal value of the substantial part of the imaging region can also be suppressed by the MT effect even with the plurality of divided MT pulses, and at the same time, by adjusting the number of application of the plurality of MT pulses, between slice encoding or between slices The variation of the signal can be adjusted.

(Sixth variation)
This modification relates to the start control of the imaging scan. The outline of this control is shown in FIG. 15 (the processing in FIG. 15 is performed jointly by a sequencer, a host computer, and an arithmetic unit). When this imaging scan is carried out, as the previous scan, electrocardiogram synchronization is not used and no encoding gradient magnetic field is applied. Echo signals are collected from the entire imaging region (voxel) in real time by the pulse sequence of the FFE method (FIG. 15, step S41). When the intensity of the echo signal exceeds a certain threshold, 3Dseg. An imaging scan based on the FFE method is executed in the same manner as described above (steps S42 to S44). Thereby, the start timing of the dynamic scan can be automatically set to the optimum value.

(Seventh variation)
This modification is characterized in that the above-described pre-optimal setting of the synchronization timing of the electrocardiographic synchronization method and the data collection / placement method based on the centric order method are implemented in the normal TOF method, that is, the FE method. In this case, for example, the ECG-prep scan is executed by the 2D-FE method (normal 2D-TOF method), and the imaging scan is executed by the 3D-FE method (normal 3D-TOF method). Of course, you may perform an imaging scan by 2D-FE method (normal 2D-TOF method). Data collection and arrangement by the imaging scan is performed by the centric order method as described above.

  In this case, in particular, when the ECG-prep scan is the 2D-FE method, the scan time becomes long. Therefore, in order to avoid this, it is desirable to use a so-called keyhole imaging method in combination. That is, in this ECG-prep scan, only the central portion (low frequency region) in the phase encoding direction of the two-dimensional k-space is collected by the 2D-FE method, and the data once captured is copied to the remaining high frequency region. Then, each data collection is completed. Thereby, the time of the ECG-prep scan based on 2D-FE method can be shortened.

(Eighth modification)
This modification is an example in which an ECG-prep scan is performed with a two-dimensional phase image (2D-PS image). As another method for measuring the synchronization timing (delay time TDL) of the electrocardiographic synchronization method described above, a two-dimensional phase image is taken based on the single slice multiphase method, and a flow image in each time phase for each cardiac cycle is created. To do. When this flow image is comparatively observed, for example, it is possible to determine an image in which the flow peaks when time elapses from the R wave as a trigger. That is, the time phase at which this peak flow image is collected can be set as the optimum synchronization timing, and this synchronization timing can be reflected in the imaging scan as described above.

  The description of the embodiment is as described above, but the present invention is not limited to the configuration described in the embodiment, and those skilled in the art can appropriately change the configuration without departing from the gist described in the claims. These can be modified, and their configurations are also included in the present invention.

  For example, in the above-described embodiment and modifications thereof, the description has been made assuming that the synchronization timing (delay time) of the electrocardiographic synchronization method is synonymous with the start timing of the second-half group scan S2, but this synchronization timing is , It may be defined as a timing to collect data arranged at the phase encoding amount of k space = 0 through the ECG-prep scan. That is, it may be determined by calculation in consideration of conditions such as the optimum delay time obtained by ECG-prep scan, the number of phase encodings, and the range of the low frequency region.

DESCRIPTION OF SYMBOLS 1 Magnet 2 Static magnetic field power supply 3 Gradient magnetic field coil unit 4 Gradient magnetic field power supply 5 Sequencer 6 Host computer 7 RF coil 8T Transmitter 8R Receiver 10 Arithmetic unit 11 Storage unit 12 Display 13 Input device 17 ECG sensor 18 ECG unit

Claims (1)

In an MRI apparatus configured to perform 3D imaging by an electrocardiographic synchronization method on a desired region of a subject,
Preparatory scanning means for performing a preparatory two-dimensional scan on a part of the desired region and acquiring in advance a plurality of preparatory images having different delay times from a reference wave;
Analyzing the signal intensity distribution of the blood flow portion of the plurality of preparation images, and setting an optimum delay time setting means for setting a delay time used for the three-dimensional imaging scan by the electrocardiogram synchronization method;
An imaging scanning means for collecting an echo signal by executing a three-dimensional imaging scan by the electrocardiographic synchronization method with a delay time set by the optimum delay time setting means;
An image generating means for arranging the collected echo signals in k-space and performing reconstruction processing on the echo signals arranged in k-space to generate an MRA image without using an MR contrast agent;
With
The plurality of preparation images are two-dimensional phase images (2D-PS).
An MRI apparatus characterized by that.

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