JP2018011751A - Magnetic resonance imaging apparatus and slab signal correction method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resolve blood vessel discontinuity at a slab end caused by a gap between a function (a weighting function) to be used for connection and a signal profile in a slab when creating a blood vessel image of an area of interest by connecting a plurality of slag signals obtained by a non-contrast blood vessel contrast method that performs division into a plurality of selection areas (slabs) and performs imaging (multi-slab imaging).SOLUTION: A coefficient table holding each order term and value of an intercept of a polynomial expressing a weighting function is stored every imaging condition, each order term and value of an intercept corresponding to the imaging condition is read from a coefficient table 601 stored in the storage means, a polynomial is composed on the basis of each order term and value of an intercept that are read to provide a weighting function, and by using a value obtained by the weighting function, a slab profile changing as an RF pulse waveform for excitation defined in accordance with the imaging condition is corrected.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and more particularly, to a non-contrast blood vessel imaging method for performing imaging by dividing a region of interest into a plurality of selected regions (slabs).

  The MRI apparatus uses the so-called NMR phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter referred to as “spins”) at a desired examination site in the subject, and examines the subject from the measured data. The part is displayed as an image.

  The 3D Time of Fright method (3D TOF method), which is one of the non-contrast blood vessel imaging methods in MRI equipment, is obtained by dividing the region of interest into multiple selected regions (slabs) and performing imaging (multi-slab imaging). A plurality of slab signals are processed and connected to reconstruct a blood vessel image of the region of interest.

  In such a three-dimensional imaging method used for multi-slab imaging, the signals at both ends of the slab are lower than the center due to imperfectness of the excitation profile of the slab. Therefore, when slabs are joined together, if both ends of the slab where the signal is lowered are used, discontinuity of the blood flow signal occurs at the connection part, resulting in boundary artifacts that may hinder diagnosis.

  As a technique for solving the discontinuity of the blood flow signal, Patent Document 1 discloses a technique using a weighting function.

JP 2000-189395 A

  However, the technique disclosed in Patent Document 1 cannot cope with all imaging conditions that can be selected by the operator, and discontinuity of the blood flow signal may still occur.

  Therefore, the present invention has been made in view of the above problems, and in any imaging condition that can be selected by the operator, a slab edge caused by a divergence between a function used for splicing (weighting function) and a signal profile in the slab. It is an object to provide an MRI apparatus and a slab signal correction method capable of eliminating the blood vessel discontinuity.

  In order to achieve the above object, the present invention is configured as follows. That is, when imaging the region of interest of a subject into a plurality of selected regions (slabs), a slab profile that changes according to the excitation RF pulse waveform determined according to the imaging conditions is used using a weighting function. To correct.

  Preferably, a coefficient table holding each order term and intercept value of a polynomial representing a weighting function is stored in the storage unit for each imaging condition, and each coefficient corresponding to the imaging condition is stored from the coefficient table stored in the storage unit. Read out the order terms and intercept values, construct a polynomial based on the read out order terms and intercept values, and use the weighting function to determine the excitation RF determined according to the imaging conditions. The slab profile that changes according to the pulse waveform is corrected.

  According to the MRI apparatus and the slab signal correction method of the present invention, the discontinuity of blood vessels between slab signals is eliminated by selecting and using a suitable weighting function under any imaging condition that can be selected by the operator. It becomes possible. As a result, it is possible to acquire a good three-dimensional non-contrast blood vessel image.

1 is a block diagram showing an entire MRI apparatus to which the present invention is applied. Pulse sequence diagram of 3D TOF sequence adopted by MRI system of the present invention Symmetrical slab profile Asymmetric slab profile Example of using a symmetric weighting function for an asymmetric slab profile Control diagram of the present invention Diagram showing an example of a slab profile approximated by a fourth-order polynomial Figure showing the change of the fourth-order polynomial obtained from the slab profile of Fig. 7 Diagram showing an example of coefficient table

  Hereinafter, an embodiment of the MRI apparatus of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram of an overall configuration showing an MRI apparatus to which the present invention is applied. This MRI apparatus mainly includes a central processing unit (CPU) 1, a sequencer 2, a transmission system 3, a static magnetic field generating magnet 4, a receiving system 5, a gradient magnetic field generating system 6, and a signal processing system 18. ing.

  The central processing unit (CPU) 1 controls each of the sequencer 2, the transmission system 3, the reception system 5, and the signal processing system 6 in accordance with a program based on the present invention. The sequencer 2 operates based on a control command from the central processing unit 1, sends various commands necessary for data collection from the region of interest of the subject 7, a transmission system 3, and a gradient magnetic field generation system 6 of the static magnetic field generation magnet 4. And sent to the receiving system 18. The CPU 1 and the sequencer 2 constitute control means in the present invention.

  The transmission system 3 has a high-frequency oscillator 8, a modulator 9, and an irradiation coil 11 as a high-frequency coil, and an amplitude of a high-frequency pulse from the high-frequency oscillator 8 (hereinafter referred to as an RF pulse) is modulated by the modulator 9 according to a command from the sequencer 2. The subject 7 is irradiated with a predetermined pulse-shaped electromagnetic wave by modulating and amplifying the amplitude-modulated RF pulse by the high-frequency amplifier 10 and supplying it to the irradiation coil 11. The application phase of this RF pulse is controlled by the central processing unit 1.

  The static magnetic field generating magnet 4 is for generating a uniform static magnetic field around the subject 7 in an arbitrary direction. Inside the static magnetic field generating magnet 4, in addition to the irradiation coil 11, a gradient magnetic field coil 13 for generating a gradient magnetic field and a reception coil 14 of the reception system 5 are installed.

  The gradient magnetic field generation system 6 includes a gradient magnetic field coil 13 that applies a gradient magnetic field independently in Cartesian coordinate axis directions orthogonal to each other, and a gradient magnetic field power supply 12 that supplies current to the gradient magnetic field coil 13, and the gradient magnetic field power supply 12 Is controlled by the sequencer 2 as described above.

  The receiving system 5 includes a receiving coil 14 as a high frequency coil, an amplifier 15 connected to the receiving coil 14, a quadrature phase detector 16, and an A / D converter 17, and receives an NMR signal from the subject 7 as a receiving coil. When the signal 14 is detected, the signal is converted into a digital quantity via the amplifier 15, the quadrature detector 16, and the A / D converter 17. At that time, it is converted into two series of collected data sampled by the quadrature detector 16 at a timing according to a command from the sequencer 2 and sent to the CPU 1. The measurement phase of the NMR signal is controlled by the CPU 1 in the same manner as the RF pulse application phase.

  The signal processing system 18 has an external storage device (storage means) such as a magnetic disk 21 and a magneto-optical disk 19, and a display 20 composed of a CRT or the like, and when data from the reception system 5 is input to the CPU 1, The CPU 1 executes processing such as signal processing and image reconstruction, and displays the image of the region of interest of the subject 7 as a result on the display 20 and records it on the magnetic disk 21 or the like of the external storage device.

  Next, a pulse sequence of the 3D-TOF method will be described with reference to FIG. First, an excitation RF pulse 202 is applied together with a slice selection gradient magnetic field 201 for selecting a slice of the subject 7, and a phase encoding gradient magnetic field 203 and a read gradient magnetic field 205 are applied.

  The flip angle (excitation angle) of the RF pulse 202 is arbitrary, but is preferably 0 to 90 degrees. Further, a slice encode gradient magnetic field 204 is applied to perform three-dimensional measurement. Slice encoding is applied by changing the application amount by the number of slices in the slab. The echo signal 206 is measured while applying the read gradient magnetic field 205 after the echo signal time TE from the application of the RF pulse 202. In multi-slab imaging, these processes are repeated for the number of slabs selected by the operator.

  Next, FIG. 3 shows a profile (hereinafter referred to as slab profile) showing signal values of each slice in the slab obtained when the pulse sequence of the 3D-TOF method shown in FIG. 2 is used. The horizontal axis indicates the slice number in the slab, and the vertical axis indicates the signal value (value obtained by normalizing the maximum value to 1). When a general irradiation waveform is used for the excitation RF pulse 202, a slab profile having a bilateral symmetry (that is, a slab direction or a slice direction) as shown in FIG. 3 is obtained. 301 is an actual signal value, and 302 is a signal value obtained by approximating 301 with a quadratic polynomial. In Patent Document 1, a weighting function is created based on this symmetric slab profile, and the weighting function is used to carry out the weighting process so that the slab profile becomes uniform.

  However, in the 3D-TOF method in which multi-slab imaging is performed, a blood flow signal is reduced at the blood flow outflow portion in the slab due to multiple excitation by continuously imaging a plurality of slabs. In order to solve this decrease in the blood flow signal, the blood flow outflow in the slab can be obtained by using the excitation RF pulse 202 having an irradiation waveform (SSP waveform) having an excitation profile inclined in the slab direction or slice direction. Suppresses blood flow signal drop in the area. Since this SSP waveform has an inclination in the slab direction or slice direction, the obtained slab profile has an asymmetric inclination in the slab direction or slice direction as shown in FIG. Here, 401 is an actual signal value of each slice, and 402 is a signal value approximated by a fourth-order polynomial. The horizontal axis in FIG. 4 indicates the slice number in the slab, and the vertical axis indicates the signal value (value obtained by normalizing the maximum value to 1).

  Therefore, if a slab profile having an asymmetric slope in the slab direction or slice direction as shown in FIG. 4 is used with a weighting function created from a slab profile symmetric in the slab direction or slice direction as shown in FIG. There will be a discrepancy between the obtained slab profile and the weighting function. An example is shown in FIG. The solid line in FIG. 5 is a slab profile obtained when using the SSP waveform, and the dotted line is a symmetric slab profile used for weighting. For example, a slice surrounded by 501 in the figure is a slice that has a large signal difference between a solid line and a dotted line and is difficult to perform appropriate weighting. Such a slice cannot be properly weighted, resulting in a discontinuity in the blood flow signal.

  This slab profile having an asymmetric slope can be approximated by a third-order or fourth-order polynomial, as shown in Expression (1) or Expression (2). The order of the polynomial approximation may be 3rd order or higher, but the 3rd order is particularly effective for a waveform having a gentle slope, and the 4th order approximation is effective for a waveform having a steep slope. In equations (1) and (2), a, b, c, and d are coefficients for each order term, e is the intercept, X is the slice position (slice number) in the slab, and X can be operated from 1. Is the number of slices in the slab selected by the person.

F (x) = bx ^ 3 + cx ^ 2 + dx + e (1)
F (x) = ax ^ 4 + bx ^ 3 + cx ^ 2 + dx + e (2)
In addition, when a general irradiation waveform that is not an SSP waveform is selected, since the slab profile shown in FIG. 3 is provided, approximation can be performed using a second-order polynomial shown in Equation (3). In Equation (3), c, d, and e are the coefficients of each term, e is the intercept, X is the slice position (slice number) in the slab, and the possible range of X is 1 Is a number.

F (x) = cx ^ 2 + dx + e (3)
Patent Document 1 also describes the use of polynomials, but the use of weighting functions using polynomial approximation requires that the coefficients indicated by a, b, c and d in the polynomial equation (1) and the equation (2) It is necessary to clarify the section indicated by e.

  As the types of SSP waveforms, waveforms having different slopes can be selected by the operator, and when the slopes are different, these coefficients and intercepts are also different. Therefore, it is difficult to uniformly obtain appropriate values for all the SSP waveforms. That is, it is difficult to correct the slope of the slab profile that changes according to the SSP waveform with the same single weighting function.

  Therefore, the present invention acquires a slab profile of all SSP waveforms that can be selected by the operator using a test instrument at the time of device development and performs polynomial approximation, and the order of the polynomial as a weighting function, its coefficient and intercept Is set for each SSP waveform, and the degree, coefficient, and intercept defining a polynomial as a weighting function for each set SSP waveform are recorded as a coefficient table in an external storage device such as the magnetic disk 20 or a reconstruction program. Thus, the slab profile can be corrected using a weighting function configured using an appropriate polynomial, its coefficient, and intercept according to the SSP waveform selected by the operator.

  In addition to the SSP waveform, the operator can change the imaging conditions. In particular, since the number of slices in the slab is frequently changed by the subject or the like, it is not possible to deal with all imaging conditions simply by storing the slab profile.

  Therefore, if the number of slices of the slab profile acquired at the time of device development is M and the number of slices selected by the operator is N, correction using the equation shown in equation (4) becomes possible. Thereby, it becomes possible to cope with all the imaging conditions.

F (x) =
a * (x * M / N) ^ 4 + b * (x * M / N) ^ 3 + c * (x * M / N) ^ 2 + d * (x * M / N) ^ 1 + e ・..Formula (4)
Equation (4) shows a correction example using the fourth-order polynomial shown in Equation (2), but when the third-order polynomial shown in Equation (1) is used, the coefficient a in Equation (4) Correction can be made by setting the value to 0. Similarly, when the quadratic polynomial shown in Expression (3) is used, correction can be performed by setting the values of the coefficients a and b in Expression (4) to 0.

FIG. 9 shows an example of the coefficient table. For example, when the slab profile shown in FIG. 7 is approximated by a fourth-order polynomial,
-9.8e-6 * x ^ 4 + 0.00065 * x ^ 3-
0.01514 * x ^ 2 + 0.1516 * x ^ 1 + 0.3270 (5)
It becomes. Here, when the number of slices of the slab profile acquired at the time of device development is 32 and the number of slices selected by the operator is 24, each weight of the 24 slices is a calculation result like the coefficient table of FIG. The slice correction value in the coefficient table indicates M / N (herein 32/24) in equation (4), the coefficient indicates the coefficient in equation (5), and the intercept indicates the intercept in equation (5). . When this weighting value is graphed, a profile as shown in FIG. 8 is drawn, which is very close to FIG. Such a coefficient table is prepared in advance for each imaging condition in association with the imaging condition.

  Next, the processing flow of the present invention will be described based on the flowchart shown in FIG. The coefficient table is represented as a coefficient table 601 stored in the magnetic disk 21 in the flowchart of FIG.

  In step S602, the CPU 1 reads out the coefficient corresponding to the imaging condition selected by the operator from the coefficient table 601 stored in the magnetic disk 20, and changes the slab that changes according to the excitation RF pulse waveform determined according to the imaging condition. A polynomial expressed by equation (4) is set as a weighting function for correcting the profile.

In step S603, the CPU 1 corrects the image data of each slice in the slab based on the polynomial set in step S602, and synthesizes the corrected slice images.
The above is the outline of the processing flow of the present invention.

  As described above, according to the present invention, in multi-slab imaging, the slab profile that changes according to the excitation RF pulse waveform determined according to the imaging condition selected by the operator is corrected using the weighting function. At that time, the weighting function is expressed as a polynomial, and the coefficients and intercepts of the respective order terms of the polynomial are stored in advance in the MRI apparatus as a coefficient table.

  Then, a coefficient corresponding to the imaging condition selected by the operator is read from the coefficient table, and a polynomial expressed by the equation (4) is set. Based on the value obtained by this polynomial, the image data of each slice in the slab is corrected. This makes it possible to eliminate slab end vascular discontinuity caused by the divergence between the weighting function and the signal profile in the slab.

  1 central processing unit (CPU), 2 sequencer, 3 transmission system, 4 static magnetic field generation magnet, 5 reception system, 6 gradient magnetic field generation system, 7 subject, 18 signal processing system (image processing means)

Claims (3)

A static magnetic field generating means for generating a static magnetic field;
Transmitting means for applying a high-frequency electromagnetic wave to the subject placed in the static magnetic field;
A gradient magnetic field generating means for applying a magnetic field gradient to the static magnetic field;
Receiving means for measuring a nuclear magnetic resonance signal generated from the subject;
Control means for controlling the transmitting means, the gradient magnetic field generating means and the receiving means, and performing image processing based on the measured nuclear magnetic resonance co-signal;
In a magnetic resonance imaging apparatus comprising:
The control means, when dividing the region of interest of the subject into a plurality of selected regions (slabs), weights a slab profile that changes according to an excitation high-frequency pulse waveform determined according to an imaging condition. A magnetic resonance imaging apparatus, wherein correction is performed using The magnetic resonance imaging apparatus according to claim 1.
Storage means for storing a coefficient table for holding each degree term and intercept value of a polynomial representing the weighting function for each imaging condition;
The control means reads out each order term and intercept value corresponding to the imaging condition from the coefficient table stored in the storage means, sets a polynomial representing the weighting function, and corrects the slab profile. A magnetic resonance imaging apparatus. A slab signal correction method for correcting a slab signal imaged by a magnetic resonance imaging apparatus having a storage means,
Reading each order term and intercept value corresponding to the imaging condition from the coefficient table stored in the storage means;
Configuring a polynomial based on each read order term and intercept value to be a weighting function;
Using the value obtained by the weighting function, correcting the slab profile that changes according to the excitation high-frequency pulse waveform determined according to the imaging conditions;
A slab signal correction method comprising:

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