JP2008272185A - Nuclear magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce uneven suppression of unnecessary signals due to spatial non-uniformity of a static magnetic field or a high-frequency magnetic field pulse without raising SAR (Specific Absorption Rate)in an MRI apparatus. <P>SOLUTION: The nuclear magnetic resonance imaging apparatus determines a pulse strength group whose signal strength has an extreme value of 0 or a value closest to 0 in a residual signal curve formed using signals obtained by changing pulse strength of a plurality of signal suppression pulses, and defines a determined pulse strength as the strength of the signal suppression pulse. As for the extreme value, the apparatus changes flip angles while retaining the pulse strength rate to select the minimum value, and then changes one pulse strength alone, and adjusts the pulse strength so that the extreme value is 0 or a value closest to 0. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a nuclear magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) and method, and more particularly to a technique for adjusting the intensity of a signal suppression pulse for suppressing a signal from an unnecessary substance that is not a measurement target.

  In the nuclear magnetic resonance signal measured by the MRI apparatus, a chemical shift phenomenon having a slightly different resonance frequency occurs due to a difference in molecular structure. Using this phenomenon, MRS (Magnetic Resonance Spectroscopy) that separates nuclear magnetic resonance signals for each molecule (metabolite) and obtains a spectrum, and MRSI (MRI) that images a spatial signal intensity distribution for each metabolite. Magnetic Resonance Spectroscopic Imaging (Nuclear Magnetic Resonance Spectroscopic Imaging) is known.

  Major metabolites of the human body that can be detected by MRS and MRSI include choline, creatine, N-acetylaspartic acid, lactic acid, and the like. From the amount of these metabolites, it is possible to determine the degree of progression and early diagnosis of metabolic disorders such as cancer. In addition, it becomes possible to perform non-invasive diagnosis of tumor malignancy.

  Since metabolites in the human body have only a signal intensity that is about 1/1000 of the water molecules in the body, they are buried in the water signal and cannot usually be detected. Therefore, in order to measure metabolites, water and fat signals unnecessary for measurement are suppressed. For example, there is a method in which unnecessary signals are suppressed in advance with a pulse having a frequency band similar to the frequency band of signals unnecessary for measurement, and a metabolite signal at the edge is detected (see, for example, Patent Document 1). . The method of suppressing the signal by pseudo-saturating the vicinity of the resonance frequency band of the unnecessary signal in this way is called a CHESS (CHEmical Shift Select) method.

  In the CHESS method, one pulse (CHESS pulse or signal suppression pulse) may be used for pseudo saturation. However, typically three CHESS pulses are used in combination with a triaxial spoiler gradient. In the CHESS method, in order to sufficiently suppress unnecessary water signals, it is important to irradiate the CHESS pulse intensity (flip angle) with an optimum value.

  The following conventional method 1 and conventional method 2 are representative methods for determining the optimum CHESS pulse intensity. In the conventional method 1, the intensity ratio of the three CHESS pulses is 1: 1: 1, and the water signal is acquired while finely changing the intensity (flip angle). The pulse intensity (flip angle) at which the water signal becomes 0 or a value close to 0 is set as the optimum pulse intensity. In the conventional method 2, the intensity (flip angle) of three CHESS pulses is set to 90 degrees, 90 degrees, and 180 degrees, respectively (for example, see Non-Patent Document 1).

Japanese Patent Laid-Open No. 60-168041 Journal of Magnetic Resonance, Vol. 106, pages 181-186, published in 1995

  In the conventional method 1, the spatial suppression unevenness of the unnecessary signal occurs due to the spatial nonuniformity of the static magnetic field and the high frequency magnetic field pulse. Further, the conventional method 2 reduces the unevenness in suppression of unnecessary signals due to the spatial nonuniformity of the static magnetic field and the high frequency magnetic field pulse. However, since the irradiation power of the high-frequency magnetic field pulse is increased, the power specific absorption rate (SAR) for the human body is increased.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to reduce unevenness in suppression of unnecessary signals due to spatial non-uniformity of a static magnetic field or a high-frequency magnetic field pulse without increasing the SAR.

  According to the present invention, a set of pulse intensities in which a signal intensity is 0 or an extremum closest to 0 in a residual signal curve created using a signal obtained by changing the pulse intensities of a plurality of signal suppression pulses is the signal suppression. The pulse intensity. For the extreme value, change the flip angle while maintaining the pulse intensity ratio, select the smallest one, and then change only one pulse intensity so that the extreme value becomes 0 or the value closest to 0. Adjust.

  Specifically, a gradient magnetic field applying means for applying a gradient magnetic field to a subject placed in a static magnetic field space, a high-frequency magnetic field pulse irradiating means for irradiating the subject with a high-frequency magnetic field pulse, and nuclear magnetic resonance generated from the subject A nuclear magnetic resonance imaging apparatus having imaging means including receiving means for receiving a signal, wherein the imaging means controls a plurality of high-frequency pulses for suppressing signals from predetermined nuclei, wherein at least one pulse intensity is Sequence control means for executing a signal suppression pulse sequence including irradiation of a high-frequency magnetic field pulse (signal suppression pulse) different from the pulse intensity of other signal suppression pulses, and pulse intensities of a plurality of signal suppression pulses used in the signal suppression pulse sequence Pulse intensity adjusting means for adjusting the intensity of the plurality of signal suppression pulses. A pulse intensity ratio setting means for setting the signal intensity, and changing the pulse intensity of the plurality of signal suppression pulses while maintaining the pulse intensity ratio set by the pulse intensity ratio setting means, and executing the signal suppression pulse sequence A first measurement control means for acquiring a residual signal curve calculation means for calculating a first residual signal curve in which the intensities of a plurality of signals acquired by the first measurement means are plotted, and the first residual signal Using a curve, a set of pulse intensities having the smallest sum of pulse intensities of the plurality of signal suppression pulses and having an extreme value is selected, and a first pulse intensity determination is made as the intensities of the plurality of signal suppression pulses And the signal suppression by changing the pulse intensity of one signal suppression pulse among the plurality of signal suppression pulses having the pulse intensity selected by the first pulse intensity determination unit Second measurement control means for acquiring a signal by executing a pulse sequence; residual signal curve calculation means for calculating a second residual signal curve in which the intensities of a plurality of signals acquired by the second measurement means are plotted; , Using the second residual signal curve, select a pulse intensity having a signal intensity of 0 or a value closest to 0, and determine the intensity of one signal suppression pulse changed by the second measurement control means And a second pulse intensity determination means for providing a nuclear magnetic resonance imaging apparatus.

  According to the present invention, in the MRI apparatus, it is possible to reduce unnecessary signal suppression unevenness due to spatial nonuniformity of a static magnetic field or a high-frequency magnetic field pulse without increasing the SAR.

  Hereinafter, a first embodiment to which the present invention is applied will be described with reference to the drawings. FIGS. 1A to 1C illustrate an overall configuration and an external view of the nuclear magnetic resonance imaging apparatus of the present embodiment. FIG. 1A shows a horizontal magnetic field type MRI apparatus using a tunnel magnet that generates a static magnetic field with a solenoid coil. FIG. 1B shows a hamburger type (open type) vertical magnetic field type MRI apparatus in which magnets are separated into upper and lower sides in order to enhance the feeling of opening. FIG. 1C shows the same tunnel-type MRI apparatus as that in FIG. 1A, in which a feeling of openness is enhanced by shortening the depth of the magnet and tilting it obliquely. As the MRI apparatus of this embodiment, an MRI apparatus having a known structure including the MRI apparatus of these forms can be used. Moreover, it is not limited to this, It can use irrespective of the form and type of an MRI apparatus.

  Details of the MRI apparatus of this embodiment will be described. FIG. 2 is a functional block diagram of the MRI apparatus of this embodiment. As shown in the figure, the MRI apparatus according to the present embodiment is provided with a gradient for applying a static magnetic field coil 2 for generating a static magnetic field and a gradient magnetic field in three directions orthogonal to the static magnetic field to a space in which the subject 1 is placed. A magnetic field coil 3, a measurement high-frequency coil 5 (hereinafter simply referred to as a transmission coil) that irradiates a measurement region of the subject 1 with a high-frequency magnetic field (pulse), and reception that receives a nuclear magnetic resonance signal generated from the subject 1 And a high frequency coil 6 (hereinafter simply referred to as a receiving coil). Moreover, you may provide the shim coil 4 which adjusts a static magnetic field uniformity.

  Various types of static magnetic field coils 2 are adopted depending on the structure of the apparatus shown in FIG. The gradient magnetic field coil 3 and the shim coil 4 are driven by a gradient magnetic field power supply unit 12 and a shim power supply unit 13, respectively. In FIG. 2, the transmission coil 5 and the reception coil 6 are shown independently, but a configuration in which only one high-frequency coil is used for both transmission and reception may be used. A high frequency magnetic field (pulse) irradiated by the transmission coil 5 is generated by the transmitter 7. The signal suppression pulse irradiated in this embodiment is also one of the high-frequency magnetic field pulses generated by the transmitter 7 and irradiated by the transmission coil 5. The nuclear magnetic resonance signal detected by the receiving coil 6 is sent to the computer 9 through the receiver 8.

  The sequence control device 14 controls the drive power supply unit (gradient magnetic field power supply unit) 12 for the gradient magnetic field generating coil 3, the drive power supply unit (shim power supply unit) 13 for the shim coil 4, the transmitter 7, and the receiver 8. . A control time chart (pulse sequence) is set in advance by an imaging method and is stored in a storage device 11 to be described later.

  The computer 9 performs various arithmetic processes on the nuclear magnetic resonance signal to generate spectrum information, image information, and the like. The calculation process includes a nuclear magnetic resonance signal correction process. In addition, the computer 9 according to the present embodiment includes a pulse intensity adjustment processing unit 90 that performs an operation for determining the optimal signal suppression pulse intensity generated by the transmitter 7. A method for determining the optimum signal suppression pulse intensity by the pulse intensity adjustment processing unit 90 will be described later. Note that the calculation by the pulse intensity adjustment processing unit 90 may be performed by another computer outside the MRI apparatus.

  The computer 9 is connected to a display 10, a storage device 11, an input device 15, etc., and the above-described spectrum information and image information are displayed on the display 10 or recorded in the storage device 11. The input device 15 receives inputs such as measurement conditions by the MRI apparatus and conditions necessary for arithmetic processing, and the received various conditions are recorded in the storage device 11 as necessary.

  Here, the principle that the spatial suppression unevenness of the unnecessary signal occurs due to the spatial nonuniformity of the static magnetic field and the high frequency magnetic field pulse will be described with reference to FIG. Here, in order to simplify the explanation, it is assumed that one signal suppression pulse is emitted in a pulse sequence for signal suppression (signal suppression pulse sequence). Also, the relaxation time of nuclear magnetization is ignored.

  FIG. 3A shows the time change of the pulse intensity of the Gaussian signal suppression pulse used in the computer simulation. Further, the distribution of flip angles in the frequency direction is shown in FIG. FIG. 3C is an enlarged view of the vicinity of the center (0 Hz) of the transmission frequency in FIG. Here, the irradiation time of the signal suppression pulse is 41.7 ms. This corresponds to an excitation band of 1 ppm (about 64 Hz) in an MRI apparatus having a static magnetic field strength of 1.5 Tesla. The half width of the excitation profile excited by the signal suppression pulse is used as a reference for the excitation band.

  When the relaxation time is ignored and the flip angle of the signal suppression pulse is θ, the residual magnetization of unnecessary nuclei after applying the signal suppression pulse and the gradient magnetic field is a function of cos θ. Therefore, when the flip angle θ of the signal suppression pulse is 90 degrees, the unnecessary signal suppression effect is the highest. When the resonance frequency of the unnecessary signal deviates from the transmission frequency of the signal suppression pulse due to spatial static magnetic field inhomogeneity, the flip angle θ of the signal suppression pulse received by the unnecessary signal does not become 90 degrees. In addition, the desired flip angle θ cannot be obtained even when spatial nonuniformity of the high-frequency magnetic field coil that irradiates the signal suppression pulse occurs.

  As described above, if the static magnetic field and the transmission magnetic field are non-uniform, a desired flip angle cannot be achieved due to the non-uniformity, and the suppression effect of unnecessary signals by the signal suppression pulse is reduced. In the present embodiment, the optimum combination of three signal suppression pulse intensities in which the signal suppression effect is less likely to be reduced even if a deviation occurs in the flip angle is determined.

  Next, a pulse sequence executed by the MRI apparatus of this embodiment will be described. In the present embodiment, a case where a water signal is suppressed will be described as an example. The MRI apparatus of the present embodiment includes a CHESS method pulse sequence (hereinafter referred to as a CHESS sequence) that suppresses unnecessary signals, and an MRSI pulse sequence (MRSI sequence) that images metabolites while suppressing unnecessary signals. Are executed in this order. Details of both sequences will be described below.

  First, FIG. 4 shows an example of the MRSI sequence performed as the main measurement. In FIG. 4, RF indicates the application timing of the high-frequency magnetic field pulse, Gx, Gy, and Gz indicate the application timing of the gradient magnetic field pulse in the x, y, and z directions, respectively, and A / D indicates the signal measurement period. The pulse sequence shown in this figure is the same as a known MRSI pulse sequence. In this pulse sequence, one high-frequency magnetic field pulse (excitation pulse) RF1 and two high-frequency magnetic field pulses (inversion pulses) RF2 and RF3 are used to selectively excite a predetermined region of interest, and an FID signal ( Free induction decay) FID1 is obtained. The region excited by this pulse sequence is shown in FIG. Hereinafter, the operation of the MRSI sequence of the present embodiment shown in FIG. 4 will be briefly described with reference to FIG.

  First, a high frequency magnetic field pulse RF1 and gradient magnetic field pulses Gs1 and Gs1 'in the z direction are applied to excite the cross section 801 in the z direction. After TE / 4 (where TE is an echo time), a high-frequency magnetic field pulse RF2 and a gradient magnetic field Gs2 in the y direction are applied. As a result, only the phase of the nuclear magnetization in the region where the z-direction cross section 801 and the y-direction cross section 802 intersect is returned. Subsequently, the high frequency magnetic field pulse RF3 and the gradient magnetic field Gs3 in the x direction are applied after TE / 2 after the application of the high frequency magnetic field pulse RF2. In this way, only the phase of the nuclear magnetization in the region of interest 804 where the z-direction cross section 801, the y-direction cross section 802, and the x-direction cross section 803 intersect is returned, and the free induction decay signal FID1 from that area is measured.

  In FIG. 4, Gd1 to Gd4 and Gd1 ′ to Gd4 ′ do not disturb the phase of the nuclear magnetization excited by the high frequency magnetic field pulse RF1, but phase the phase of the nuclear magnetization excited by the high frequency magnetic field pulses RF2 and RF3. It is a gradient magnetic field to do. In addition, phase encode gradient magnetic fields Gp1 and Gp2 are applied between the high frequency magnetic field pulse RF1 and the high frequency magnetic field pulse RF2 in the x direction and the y direction, respectively. The intensity of these phase encoding gradient magnetic fields is changed for each excitation, and position information is given to the nuclear magnetic resonance signal generated from the region of interest 804. In this way, the nuclear magnetic resonance signal FID1 to which position information is added is measured and subjected to Fourier transform to obtain a distribution image of each metabolite included in the region of interest 804.

  Next, the CHESS sequence will be described. The CHESS sequence is executed prior to the above-described MRSI sequence, which is the main measurement, in order to reduce unnecessary water signals during the main measurement. FIG. 6 shows an example of a CHESS sequence that is normally used when this measurement is an MRSI sequence.

  In the CHESS sequence, Gaussian high-frequency magnetic field pulses RFC1, RFC2, and RFC3 with a water resonance frequency as a center frequency and a narrow excitation band (about 1.0 ppm) are irradiated as signal suppression pulses, and only nuclear magnetization contained in water is irradiated. Excited. After each high frequency magnetic field pulse RFC1, RFC2, RFC3 irradiation, a gradient magnetic field is sequentially applied in any one of the x-axis, y-axis, and z-axis directions as spoiler gradient magnetic fields Gsp1, Gsp2, and Gsp3. By executing such a pulse sequence, only the nuclear magnetization contained in water is pseudo-saturated and suppressed.

  The residual longitudinal magnetization (residual signal intensity) of the water spin after the last signal suppression pulse RFC3 is applied, that is, immediately before the excitation pulse of this measurement is applied, is the intensity of each signal suppression pulse RFC1, RFC2, RFC3 (flip angles θ1, θ2, This is a function with θ3) as a variable. Therefore, in order to reduce unnecessary water signals and obtain metabolite distribution images in the MRSI sequence, it is important to appropriately adjust the intensity of the signal suppression pulses RFC1, RFC2, and RFC3 used in this CHESS sequence. is there. In the MRI apparatus of this embodiment, the intensity of the signal suppression pulse used for the CHESS sequence is adjusted prior to the main measurement, as will be described in detail below.

  In this embodiment, the intensity of the signal suppression pulse is adjusted by the pulse intensity adjustment processing unit 90 of the computer 9. As shown in FIG. 2, the pulse intensity adjustment processing unit 90 includes a pulse intensity ratio setting processing unit 91, a measurement control processing unit 92, a residual signal curve calculation processing unit 93, and a pulse intensity determination processing unit 94. .

  The pulse intensity ratio setting processing unit 91 sets the intensity of each of the plurality of signal suppression pulses. The measurement control processing unit 92 changes the intensity (flip angle) of the signal suppression pulse while maintaining the intensity (flip angle) ratio between the plurality of signal suppression pulses set by the pulse intensity ratio setting processing unit 91, and performs predetermined processing. The pulse sequence (here, CHESS sequence) is executed, and a nuclear magnetic resonance signal (hereinafter simply referred to as a signal) is measured. The residual signal curve calculation processing unit 93 calculates a residual signal (described later) from the signal measured by the measurement control processing unit 92, and calculates a curve (residual signal curve) corresponding to the intensity (flip angle). The pulse intensity determination processing unit 94 determines the pulse intensity from the residual signal curve calculated by the residual signal curve calculation processing unit 93 according to a predetermined rule.

  Hereinafter, in the MRI apparatus of this embodiment, a procedure (adjustment procedure) for adjusting and setting the intensity of the signal suppression pulse by each of these processing units will be described. Here, a case where there are three signal suppression pulses of the CHESS sequence will be described as an example. As described above, the number of signal suppression pulses in the CHESS sequence is not limited to this. FIG. 7 is a processing flow of the procedure for adjusting the intensity of the signal suppression pulse of this embodiment. FIG. 8 is a graph of a residual signal curve in which the residual signal intensity of this embodiment is plotted.

  Prior to the description, an outline of the adjustment procedure of the present embodiment will be described. In the present embodiment, first, the intensity of the signal suppression pulse is changed under the first condition, and there is some error in the flip angle θ among the combinations of the intensity (flip angle) of the three signal suppression pulses. Also, a signal with little change in the signal suppression effect is determined. Specifically, the CHESS sequence is executed by changing the flip angle (pulse intensity) θ while maintaining the intensity (flip angle) ratio (θ: tθ: uθ) of the three signal suppression pulses, A residual signal curve having an extreme value is obtained from the signal. Then, the minimum flip angle θ1 is selected from among the flip angles θ that take extreme values, and three signal suppression pulses are determined as θ1, tθ1, and uθ1 using the selected flip angle θ1.

  The reason for selecting the minimum flip angle θ1 is to select the smallest flip angle among the flip angles having extreme values. Since the intensity ratio of the three signal suppression pulses is constant, this selection selects the flip angle that takes the extreme value that minimizes the sum of the intensity (flip angle) of the three signal suppression pulses. become. As a result, the overall signal suppression pulse intensity can be reduced as compared with the case where other extreme values are used, and SAR can be suppressed.

  Next, the signal suppression pulse intensity set obtained under the first condition is changed under the second condition to determine the signal suppression pulse intensity set that makes the residual signal intensity as small as possible. Specifically, the intensity (flip angle θ) of any two of the three signal suppression pulses is fixed, and the intensity (flip angle θ) of the remaining one signal suppression pulse is changed to change the CHESS sequence. To obtain a residual signal curve from the plurality of acquired signals. Then, the intensity (flip angle θ2) at which the extreme value of the residual signal curve becomes 0 or the value closest to 0 is selected. A combination of the intensity of the selected one signal suppression pulse and the intensity of the two fixed signal suppression pulses is determined as an optimal combination of pulse intensity. Hereinafter, the adjustment procedure will be described in detail.

First, the pulse intensity ratio setting processing unit 91 determines the flip angle ratios t and u of the second and third signal suppression pulses RFC2 and RFC3 with respect to the first signal suppression pulse RFC1 (step 901). If the flip angles of the three signal suppression pulses RFC1, RFC2, and RFC3 are α, β, and γ, respectively, a: β: γ = θ: tθ: uθ. Hereinafter, t and u are referred to as flip angle coefficients. At this time, in order to calculate the extreme value and the zero cross point within a reasonable range, the following restrictions are set on the flip angle coefficients t and u.
1 ≦ t <u <2 or 1 ≦ u <t <2 (Formula 1)
Here, a case where the first signal suppression pulse RFC1 is used as a reference will be described as an example, but any of the three signal suppression pulses RFC1, RFC2, and RFC3 may be used as a reference. Regardless of which case is used as a reference, the flip angle coefficients of the other two signal suppression pulses are limited in the same way as t and u.

  Next, the measurement control processing unit 92 changes the flip angle θ itself while maintaining the flip angle ratio of the three signal suppression pulses RFC1, RFC2, and RFC3 determined in Step 901, and executes the CHESS sequence to the sequence control device 14. And measure the signal. And the residual signal curve calculation process part 93 calculates the residual signal strength of the water signal for every measurement, and plots it as a residual signal curve (step 902). The plot result at this time is shown in FIG. Reference numeral 1001 denotes a residual signal curve in which the intensity of the residual signal of the water signal is plotted at each flip angle θ.

  In addition, what is necessary is just to change flip angle (theta) in the range in which the pulse intensity (flip angle (theta)) which takes the extreme value of a residual signal curve is included. Specifically, one end (initial value) of the residual signal curve is set to a flip angle smaller than a reference value that is a 90-degree pulse intensity adjusted in advance. The reference value may be a value that is adjusted and held in advance for each coil or part. On the other hand, the other end (terminating value) estimates the intensity of the signal suppression pulse taking an extreme value from experience and the like, and is set to a flip angle larger than the pulse intensity and smaller than the 180-degree pulse intensity.

  The number of times of measurement (the number of times the flip angle θ is changed), that is, the number of signals to be acquired is not particularly limited as long as the pulse intensity (flip angle θ) that takes an extreme value is included in the plotted range. For example, several points such as 10 points may be used. The residual signal curve 1001 may be calculated by using various fitting methods such as polynomial fitting of the acquired signal and fitting by changing the parameter of the model function. In the example shown in FIG. 8A, the flip angle θ is changed from 50 degrees to 110 degrees and measured 12 times. In the example of this figure, the residual signal curve 1001 has two extreme values, 1002 and 1003.

  Next, the pulse intensity determination processing unit 94 determines whether or not the residual signal curve 1001 calculated in step 902 has an extreme value, and notifies the pulse intensity ratio setting processing unit 91 of the result (step 903). When it is determined that there is no extreme value, the pulse intensity ratio setting processing unit 91 returns to step 901, and the flip angle coefficient t of the second signal suppression pulse in the range of the above equation 1 or / and the third The flip angle coefficient u of the first signal suppression pulse is changed so as to increase the difference between t and u, and is reset. That is, when t <u, t is decreased or / and u is increased, and when t> u, t is increased or / and u is decreased.

  On the other hand, if it is determined in step 903 that there is an extreme value, the pulse intensity ratio setting processing unit 91 extracts all the flip angles θ that take the extreme values (1002 and 1003 in FIG. 8) on the residual signal curve 1001. Among the extracted flip angles θ, the minimum flip angle θ1 (1004 in FIG. 8) is selected (step 904). Using this θ1, the flip angles α, β, γ of the three signal suppression pulses RFC1, RFC2, RFC3 are set to θ1, tθ1, uθ1.

  Next, the pulse intensity ratio setting processing unit 91 changes the flip angle α, β, γ (= θ1, tθ1, uθ1) of the three signal suppression pulses RFC1, RFC2, and RFC3 by changing only one flip angle. I do. Here, a case where only the flip angle α (= θ1) of RFC1 is changed will be described as an example. α = φ, β and γ are fixed to tθ1 and uθ1, respectively, and the flip angle φ is changed to cause the sequence controller 14 to execute the CHESS sequence and measure the signal. And the residual signal curve calculation process part 93 calculates the residual signal strength of the water signal for every measurement, and plots it as a residual signal curve (step 905). The plot result at this time is shown in FIG. Reference numeral 1005 denotes a residual signal curve in which the strength of the residual signal of the water signal is plotted at each flip angle φ.

  Note that the flip angle φ may be changed within a range including the flip angle φ (optimum pulse intensity) at which the residual signal curve is 0 or a value closest to 0. Specifically, one end (initial value) of the residual signal curve is set to a flip angle smaller than a reference value that is a 90-degree pulse intensity adjusted in advance. The reference value may be a value that is adjusted and held in advance for each coil or part. On the other hand, the other end (end value) is set to a flip angle larger than the optimum pulse intensity estimated from experience or the like and smaller than the 180-degree pulse intensity.

  Again, the number of times of measurement (number of times φ is changed), that is, the number of signals to be acquired, is not particularly limited as long as the pulse intensity taking the optimum pulse intensity is included in the plotted range. For example, it may be several points such as five points. Further, the residual signal curve 1005 may be calculated using various fitting methods such as polynomial fitting of the acquired signal and fitting by changing the parameter of the model function. In the example shown in FIG. 8B, the flip angle φ is changed from 35 degrees to 125 degrees and measured ten times. In the example of this figure, the residual signal curve 1005 has a zero cross point 1006.

  Next, the pulse intensity determination processing unit 94 determines whether or not the residual signal curve 1005 calculated in Step 905 has a zero cross point (Step 906). If it is determined that there is no zero cross point, the pulse intensity ratio setting processing unit 91 is notified to that effect, and the pulse intensity ratio setting processing unit 91 that has received the notification returns to step 901 to determine the second signal suppression pulse. For example, the flip angle coefficient t is decreased or the flip angle coefficient u of the third signal suppression pulse is increased.

  On the other hand, if it is determined in step 906 that the residual signal curve 1005 has a zero cross point, the fact is notified to the pulse intensity ratio setting processing unit 91. Upon receiving the notification, the pulse intensity ratio setting processing unit 91 selects a flip angle φ1 (1007 in FIG. 8) that becomes 0 or a value closest to 0 (1006 in FIG. 8) on the residual signal curve 1005 (step 907). .

  Then, the pulse intensity ratio setting processing unit 91 determines the set of flip angles α, β, and γ of the three signal suppression pulses RFC1, RFC2, and RFC3 as φ1, tθ1, and uθ1 (step 908).

  In the present embodiment, a sequence for suppressing a specific signal such as a CHESS sequence is executed with the flip angles α, β, and γ adjusted in the above procedure, and then the MRSI sequence shown in FIG. 4 is executed as the main measurement. In the above description, the case where the water signal is suppressed in the CHESS sequence and the MRSI sequence is performed as the main measurement has been described as an example. However, the signal to be suppressed may be any substance including not only a water signal but also a fat signal. Further, the sequence performed as the main measurement is not limited to MRSI, but may be MRS or MRI.

  Next, an example of MRI measurement performed by suppressing the fat signal using the signal suppression pulse after adjustment according to the adjustment procedure of the present embodiment will be described. FIG. 9 is an example of a spin echo pulse sequence that combines a sequence for performing fat suppression and the MRI sequence of the main measurement. In FIG. 9, RF indicates the application timing of the high-frequency magnetic field pulse, Gx, Gy, and Gz indicate the application timing of the gradient magnetic field pulse in the x, y, and z directions, respectively, and A / D indicates the signal measurement period. The pulse sequence shown in this figure is the same as a known fat suppression spin echo pulse sequence. In this pulse sequence, the spin echo signal Sig is obtained from a cross section perpendicular to the z-axis. This cross section is a cross section excited by the excitation pulse RF1 and the inversion pulse RF2.

  The image acquisition procedure when performing measurement while suppressing fat signals will be described briefly with reference to FIG. 9. The following processing is realized by the sequence controller 14 operating each unit according to a predetermined program or an instruction from the user and the computer 9 performing the processing.

  This pulse sequence includes a fat suppression unit 1301 that suppresses a signal due to fat, and a main measurement unit 1302 that measures a desired signal. First, the fat suppression unit 1301 irradiates broadband high-frequency magnetic field pulses RF01, RF02, and RF03 for fat suppression with the optimum pulse intensity adjusted in the above procedure, and excites only unnecessary fat signals. Thereafter, gradient magnetic field pulses Gsp1 to Gsp3 are applied, the phase of the excited fat signal is disturbed, and the fat signal disappears.

  In the measurement unit 1302, the high-frequency magnetic field pulse RF1 and the gradient magnetic field pulses Gs1 and Gs1 'are applied to excite a cross section perpendicular to the z direction. After the TE / 2 hours, a high frequency magnetic field pulse RF2 and a gradient magnetic field Gs2 are applied. As a result, only the phase of the nuclear magnetization in the same cross section as described above is returned, and the spin echo signal Sig is measured. Note that the gradient magnetic fields Gd1, Gd2, and Gd1 ′, Gd2 ′ are gradient magnetic fields for dephasing only the phase of the nuclear magnetization excited by the RF2, without disturbing the phase of the nuclear magnetization excited by the high-frequency magnetic field pulse RF1. .

  Further, the phase encode gradient magnetic field Gp1 in the x-axis direction is applied before obtaining the spin echo signal Sig. The intensity of the phase encoding gradient magnetic field Gp1 is changed for each excitation, and position information in the x direction is given to the nuclear magnetic resonance signal generated from the excitation cross section. Further, the phase gradient magnetic field Gr1 and the frequency encoding gradient magnetic field Gr2 are applied in the y direction, and position information in the y direction is given to the nuclear magnetic resonance signal generated from the excitation cross section by one excitation. The spin echo signal Sig to which the position information obtained by the above procedure is added is subjected to Fourier transform to obtain an MRI image in which the fat signal is suppressed.

Here, the case where the adjustment procedure of the present embodiment is combined with the spin echo sequence has been described as an example, but the present invention is not limited to this. It can be combined with various known sequences such as a gradient echo sequence. For example, the present invention can also be applied to the pulse sequences shown in Non-Patent Document 1 and Non-Patent Document 2 shown below. By using the signal suppression pulse intensity adjusted by the procedure of the present embodiment, Non-Patent Document 1 can effectively remove subcutaneous fat signals existing in the whole body and acquire a diffusion-weighted image with high diagnostic ability.
Radiation Medicine Vol.22, 275-282 Published in 2004

  As described above, according to the present embodiment, the extreme value is calculated from the residual signal curve when the three signal suppression pulses are irradiated, and the extreme value having a small sum of the intensities of the three signal suppression pulses is calculated. , 0 or the value closest to 0 is determined by adjusting the combination of the intensity of the three signal suppression pulses. As a result, the residual longitudinal magnetization after the signal suppression sequence becomes 0 or minimal, and even if there is a spatial nonuniformity of the static magnetic field and the transmission magnetic field, signals unnecessary for detection of metabolites can be sufficiently suppressed. For this reason, diagnostic ability improves. In addition, since the sum of the intensity of the three signal suppression pulses is smaller than when other extreme values are selected, SAR is reduced. In particular, it is useful in MRSI and MRS that measure metabolites that are weaker than water signals.

<Example 1>
In order to compare the effect of the present invention with the conventional method 1, a computer simulation was performed using a water signal ignoring the T1 relaxation time as 2000 ms and the T2 relaxation time as a model.

  FIG. 12 is a diagram for explaining a method of determining the optimum signal suppression pulse intensity according to the conventional method 1. FIG. 12A shows a water signal when θ is changed with α: β: γ = θ: θ: θ when the three signal suppression pulse intensities (flip angles) are α, β, and γ, respectively. It is a graph of the curve (residual signal rate curve) which shows the residual signal rate. The combination of the flip angles θ at the point 301 at which the residual signal rate is 0 is α: β: γ = θ: θ: θ = 106.6 °: 106.6 °: 106.6 °.

  FIG. 12B is an enlarged view of the vicinity of the point 301 of the residual signal rate curve of the water signal. As shown in the figure, the gradient of the residual signal rate curve according to the conventional method 1 in the vicinity of the intersection 301 with the straight line with the residual signal rate of 0 is steep. For this reason, the change in the residual signal rate due to the deviation of the pulse intensity (flip angle θ) is large. Specifically, as shown in FIG. 12B, the θ range 302 where the residual water signal rate is ± 0.5% or less is approximately 5 degrees.

  Thus, in the conventional method 1, the rate at which the spatial deviation of the flip angle reduces the suppression effect is high. Therefore, when the spatial deviation occurs in the flip angle received by the water signal due to the spatial non-uniformity between the static magnetic field and the transmission magnetic field, in the conventional method 1, the difference in the suppression effect increases depending on the position.

  FIG. 13 is a diagram for explaining a method for determining the optimum signal suppression pulse intensity in the adjustment procedure of the present invention. FIG. 13A is a graph of a residual signal rate curve as a result of performing a computer simulation using the adjustment procedure of the present invention. The combination of the flip angles of the point 401 having a residual signal rate having an extreme value and being closest to 0 or 0 is α: β: γ = 72.2 °: 80.5 °: 108.3 °.

  FIG. 13B is an enlarged view of the vicinity of the residual signal rate curve 401 of the water signal. As shown in the figure, the gradient of the intersection 401 of the residual signal rate curve according to the adjustment procedure of the present invention and the straight line with the residual signal rate of 0 is gentle. For this reason, even if the pulse intensity (flip angle θ) shifts, the change in the residual signal rate is small. Specifically, as shown in FIG. 13B, the θ range 402 where the residual water signal rate is ± 0.5% or less is approximately 15 degrees.

  Thus, according to the adjustment procedure of the present invention, the range of θ in which the residual signal rate after the sequence execution by the signal suppression pulse is ± 0.5% or less is wider than that of the conventional method 1. That is, according to the adjustment method of the present invention, there is little deterioration in the residual signal rate due to the deviation of the pulse intensity (flip angle θ). Therefore, even if there is a spatial nonuniformity between the static magnetic field and the transmission magnetic field, spatial signal suppression unevenness is reduced.

According to the adjustment procedure of the present invention, a signal unnecessary for the detection of a metabolite can be sufficiently suppressed as compared with the conventional method 1, so that the detectability of a trace amount of metabolite is improved and the diagnostic ability is improved.
Further, as described above, since the sum of the strengths (flip angles) of the three signal suppression pulses is smaller than that in the conventional method 1, the SAR can be reduced.

  Hereinafter, the results of actual measurement using the flip angle determined by the method of the present invention will be described.

<Example 2>
Water suppression measurement using an N-acetylalanine phantom was performed using the conventional method 1 and the method of the present invention. FIGS. 10A and 10C are a spectrum and a water signal image measured after water suppression using the water suppression pulse intensity determined by the conventional method 1. FIG. FIGS. 10B and 10D are a spectrum and a water signal image measured after water suppression using the water suppression pulse intensity determined by the adjustment procedure of the present invention. Here, in the figure, 1101, 1103 are N-acetylalanine signals, and 1102, 1104 are water signals.

  As shown by 1102 and 1104 in the figure, according to the water suppression pulse intensity determined by the adjustment procedure of the present invention, the water signal is suppressed by about 3 times compared to the case where the water suppression pulse intensity determined by the conventional method 1 is used. Is done. That is, it is shown that the combination of water suppression pulse intensities determined by the adjustment procedure of the present invention is more optimal.

  Further, as shown in FIGS. 10B and 10D, according to the adjustment procedure of the present invention, compared with the conventional method 1, there is no unevenness in water suppression, and the water is suppressed uniformly. That is, it is shown that the combination of pulse intensities determined by the adjustment procedure of the present invention is sufficiently suppressed without being affected by the nonuniformity of the static magnetic field and the transmission magnetic field.

<Example 3>
An example of water suppression measurement using the human head was performed using the conventional method 1 and the method of the present invention. In general, the human head has a larger static magnetic field non-uniformity than the above-described N-acetylalanine phantom. 11A and 11C are a spectrum and a water signal image measured after water suppression using the water suppression pulse intensity determined by the conventional method 1. FIG. 11B and 11D are a spectrum and a water signal image measured after water suppression using the water suppression pulse intensity determined by the adjustment procedure of the present invention. Here, 1201 and 1205 are choline signals, 1202 and 1206 are creatine signals, 1203 and 1207 are N-acetylaspartic acid signals, and 1204 and 1208 are water signals.

  As shown by 1204 and 1208 in this figure, according to the water suppression pulse intensity determined by the adjustment procedure of the present invention, the water signal is suppressed by about 3 times compared to the case of using the water suppression pulse intensity determined by the conventional method 1. Is done. That is, it is shown that the combination of water suppression pulse intensities determined by the adjustment procedure of the present invention is more optimal. Further, according to the adjustment procedure of the present invention, it is shown that even a human head that is greatly affected by non-uniform static magnetic fields is sufficiently suppressed.

1 is an overall configuration and external view of a nuclear magnetic resonance imaging apparatus according to an embodiment of the present invention. It is a functional block diagram of the MRI apparatus of embodiment of this invention. It is a figure for demonstrating the principle which the spatial suppression nonuniformity of an unnecessary signal produces by the spatial nonuniformity of a static magnetic field and a high frequency magnetic field pulse. It is a figure which shows the MRSI sequence of embodiment of this invention. It is a figure which shows the area | region excited by the MRSI sequence of embodiment of this invention. It is a figure which shows the CHESS sequence of embodiment of this invention. It is a processing flow of the adjustment procedure of the intensity | strength of the signal suppression pulse of embodiment of this invention. It is a graph of the residual signal curve of embodiment of this invention. It is a figure which shows the fat suppression spin echo pulse sequence of embodiment of this invention. It is a figure which shows an example of the spectrum and water signal image which were measured after water suppression. It is a figure which shows an example of the spectrum and water signal image which were measured after water suppression. It is a figure for demonstrating the determination method of the optimal signal suppression pulse intensity | strength by the conventional method. It is a figure for demonstrating the determination method of the optimal signal suppression pulse intensity | strength in the adjustment procedure of this invention.

Explanation of symbols

2: static coil, 3: gradient coil, 4: shim coil, 5: transmitter coil, 6: receiver coil, 7: transmitter, 8: receiver, 9: computer, 10: display, 11: storage device, 12 : Power supply unit for gradient magnetic field, 13: power supply unit for shim, 14: sequence control device, 15: input device, 90: pulse intensity adjustment processing unit, 91: pulse intensity ratio setting processing unit, 92: measurement control processing unit, 93 : Residual signal curve calculation processing unit, 94: Pulse intensity determination processing unit

Claims (11)

Gradient magnetic field applying means for applying a gradient magnetic field to a subject placed in a static magnetic field space, high-frequency magnetic field pulse irradiating means for irradiating the subject with a high-frequency magnetic field pulse, and reception for receiving a nuclear magnetic resonance signal generated from the subject A nuclear magnetic resonance imaging apparatus having imaging means comprising means,
Irradiation with a high-frequency magnetic field pulse (signal suppression pulse) that controls a plurality of high-frequency pulses for controlling the imaging means and suppresses a signal from a predetermined nucleus, wherein at least one pulse intensity is different from that of other signal suppression pulses. Sequence control means for executing a signal suppression pulse sequence including:
Pulse intensity adjusting means for adjusting the pulse intensity of a plurality of signal suppression pulses used in the signal suppression pulse sequence,
The pulse strength adjusting means executes the signal suppression pulse sequence by changing a set of pulse strengths of the plurality of signal suppression pulses, and the signal strength becomes 0 or 0 in a signal strength curve obtained by plotting the strength of the obtained signal. A nuclear magnetic resonance imaging apparatus characterized by selecting a set of pulse intensities that take the extreme values that are closest to each other and setting the pulse intensities of the plurality of signal suppression pulses. The nuclear magnetic resonance imaging apparatus according to claim 1,
The pulse intensity adjusting means includes
First pulse intensity determination means for changing a set of pulse intensities of the plurality of signal suppression pulses while satisfying a first condition, and selecting a set of pulse intensity taking the extreme value;
A second set of pulse intensities determined by the first pulse intensity determining means is further changed while satisfying the second condition, and a second set of pulse intensities that is the value closest to 0 or 0 is selected. A nuclear magnetic resonance imaging apparatus comprising: a pulse intensity selection determining unit; The nuclear magnetic resonance imaging apparatus according to claim 2,
The first condition is to fix a pulse intensity ratio of the plurality of signal suppression pulses,
The nuclear magnetic resonance imaging apparatus characterized in that the second condition is to change only one of the plurality of signal suppression pulse intensities. The magnetic resonance imaging apparatus according to claim 2 or 3,
The first pulse intensity determination means selects a set of pulse intensities having the smallest sum of the pulse intensities among the sets of pulse intensities that take extreme values in the signal intensity curve. Shooting device. Gradient magnetic field applying means for applying a gradient magnetic field to a subject placed in a static magnetic field space, high-frequency magnetic field pulse irradiating means for irradiating the subject with a high-frequency magnetic field pulse, and reception for receiving a nuclear magnetic resonance signal generated from the subject A nuclear magnetic resonance imaging apparatus having imaging means comprising means,
Irradiation with a high-frequency magnetic field pulse (signal suppression pulse) that controls a plurality of high-frequency pulses for controlling the imaging means and suppresses a signal from a predetermined nucleus, wherein at least one pulse intensity is different from that of other signal suppression pulses. Sequence control means for executing a signal suppression pulse sequence including:
Pulse intensity adjusting means for adjusting the pulse intensity of a plurality of signal suppression pulses used in the signal suppression pulse sequence,
The pulse intensity adjusting means includes
Pulse intensity ratio setting means for setting the intensity ratio of the plurality of signal suppression pulses;
First measurement control means for acquiring a signal by executing the signal suppression pulse sequence by changing the pulse intensity of the plurality of signal suppression pulses while maintaining the pulse intensity ratio set by the pulse intensity ratio setting means; ,
A residual signal curve calculating means for calculating a first residual signal curve plotting the intensities of the plurality of signals acquired by the first measuring means;
Using the first residual signal curve, a set of pulse intensities having the smallest sum of pulse intensities of the plurality of signal suppression pulses and an extreme value is selected and set as the intensities of the plurality of signal suppression pulses. A first pulse intensity determining means;
A signal is obtained by executing the signal suppression pulse sequence by changing the pulse intensity of one signal suppression pulse among the plurality of signal suppression pulses having the pulse intensity selected by the first pulse intensity determination means. Two measurement control means;
A residual signal curve calculating means for calculating a second residual signal curve in which the intensities of the plurality of signals acquired by the second measuring means are plotted;
Using the second residual signal curve, a pulse strength having a signal strength of 0 or a value closest to 0 is selected and determined as the strength of one signal suppression pulse changed by the second measurement control means. And a second pulse intensity determining means. A nuclear magnetic resonance imaging apparatus. The nuclear magnetic resonance imaging apparatus according to any one of claims 3 to 5,
The signal suppression pulse is three,
The pulse intensity ratio is set such that, when the intensity of one of the three signal suppression pulses is 1, the intensity of the other two pulses is set in a range of 1 or more and less than 2, respectively. Magnetic resonance imaging device. The nuclear magnetic resonance imaging apparatus according to any one of claims 1 to 6,
The nuclear magnetic resonance imaging apparatus, wherein the sequence control means includes a main measurement pulse sequence including irradiation of at least one high-frequency magnetic field pulse as a pulse sequence subsequent to the signal suppression pulse sequence. The nuclear magnetic resonance imaging apparatus according to claim 7,
The signal suppression pulse is a high-frequency magnetic field pulse that suppresses a signal from water protons,
The nuclear magnetic resonance imaging apparatus, wherein the main measurement pulse sequence is an MRS pulse sequence including acquisition of a free decay signal of at least one excitation high-frequency magnetic field pulse. The nuclear magnetic resonance imaging apparatus according to claim 7,
The signal suppression pulse is a high-frequency magnetic field pulse that suppresses a signal from water protons,
The main measurement pulse sequence is an MRSI pulse sequence including at least one excitation high-frequency magnetic field pulse, application of a phase encoding gradient magnetic field, and acquisition of a free decay signal after irradiation of the inverted high-frequency magnetic field pulse. Nuclear magnetic resonance imaging device. The nuclear magnetic resonance imaging apparatus according to claim 7,
The signal suppression pulse is a high-frequency magnetic field pulse that suppresses a signal from a fat proton,
The measurement pulse sequence is an MRI pulse sequence including irradiation of at least one excitation high-frequency magnetic field pulse, application of a phase encoding gradient magnetic field and a read gradient magnetic field, and acquisition of the echo signal. Magnetic resonance imaging device. A gradient magnetic field applying means for applying a gradient magnetic field to a subject placed in a static magnetic field space, a high frequency magnetic field pulse irradiating means for irradiating the subject with a high frequency magnetic field pulse, and a nuclear magnetic resonance signal generated from the subject A high-frequency magnetic field pulse (signal) that controls a plurality of high-frequency pulses that control the imaging means and suppress signals from predetermined nuclei, wherein at least one pulse intensity is different from the pulse intensity of other signal suppression pulses A pulse intensity adjusting method for adjusting the pulse intensities of the plurality of signal suppression pulses in a nuclear magnetic resonance imaging apparatus having imaging means comprising sequence control means for executing signal suppression pulse sequences including irradiation of suppression pulses) ,
A signal strength curve is an extreme value in a signal strength curve obtained by executing the signal suppression pulse sequence by changing a set of pulse strengths of the plurality of signal suppression pulses while satisfying the first condition, and plotting the signal strength obtained. A first pulse strength determination step for selecting a set of pulse strengths to take
The set of pulse intensities determined in the first pulse intensity determining step is further changed while satisfying the second condition, the signal suppression pulse sequence is executed, and the signal intensity in the signal intensity curve is 0 or And a second pulse intensity selection determining step for selecting a set of pulse intensities that are values closest to 0. A pulse intensity adjustment method comprising:

Images