JP4067207B2 - Nuclear magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nuclear magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) that obtains a tomographic image of a target region of a subject (human body) by using a magnetic resonance phenomenon, and in particular, a signal from a main substance constituting an image signal. The present invention relates to an MRI apparatus that performs imaging for obtaining an image obtained by separating signals from signals from other substances.
[0002]
[Prior art]
Conventional MRI usually targets ¹ H. Since about 60% of a living body is composed of water, an image is composed of signals from ¹ H of water molecules (hereinafter referred to as biological water) in the living body. However, the living body includes a number of fat, it contains many of the ¹ H in fat. For this reason, the signal from fat becomes a high signal and may interfere with diagnosis. And by ¹ H contained to ¹ H and adipose tissue in a biological water, derived from the environment in which H is present are different, different resonant frequencies slightly. It is said that there is a frequency difference of about 3.5 ppm between the signal from the water in the living body and the signal from the fat tissue regardless of the static magnetic field strength. FIG. 6 schematically shows this.
[0003]
Various methods for suppressing a signal from fat using the difference in resonance frequency (chemical shift) have been developed. One of them is a method of obtaining an image in which signals from adipose tissue are suppressed by pre-saturating ¹ H of fat that hinders imaging before measurement. In this method, ¹ H of adipose tissue is excited using a prepulse having a frequency different from that of a radio frequency (RF) pulse to be irradiated in order to detect a signal from biological water. Since the pre-pulse is a 90 ° pulse, there is no longitudinal magnetization of the adipose tissue during signal measurement, and thus no image is formed.
[0004]
[Problems to be solved by the invention]
By the way, when the prepulse irradiation and the RF pulse irradiation for imaging are repeated, if the effective repetition time TR of the RF pulse is sufficiently longer than the longitudinal relaxation time T1 of the adipose tissue, the next prepulse irradiation is performed. At this point, the longitudinal magnetization of the adipose tissue is in the state before excitation by the previous prepulse, and the same fat suppression effect is always obtained. However, when the effective TR is shorter than the longitudinal relaxation time T1 of fat, the fat longitudinal magnetization has not returned to the state before being excited by the prepulse, and therefore, if the prepulse is irradiated under the same conditions, the fat suppression effect cannot be obtained.
[0005]
Accordingly, an object of the present invention is to provide an MRI apparatus that can always obtain an image having a fat suppression effect regardless of the effective TR.
[0006]
[Means for Solving the Problems]
The MRI apparatus of the present invention that achieves such an object detects a magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field in a space in which a subject is placed, and a nuclear magnetic resonance signal generated from the subject. Means for controlling the magnetic field generation means and the detection means to repeat the irradiation of the high-frequency magnetic field and the detection of the nuclear magnetic resonance signal at a predetermined repetition time, and the subject based on the detected nuclear magnetic resonance signal. Means for reconstructing and displaying the image, and the control means controls an application area of a suppression pulse for suppressing a signal from an unnecessary component in the subject corresponding to an effective TR.
[0007]
Here, the effective TR is a substantial repetition time of the RF pulse to be irradiated to excite the component to be imaged. In the case of imaging one slice, the 90 ° pulse is followed by the next 90 °. This is the time TR until the pulse is irradiated. In the case of multi-slice, it is a value obtained by dividing the repetition time TR for one slice by the number of slices.
[0008]
As described above, components (unnecessary components) other than the component to be imaged (unnecessary components), for example, the relaxation state of longitudinal magnetization of adipose tissue varies depending on the effective TR. By changing the integrated value), signals from unnecessary components can be effectively suppressed. The area of the prepulse is determined by the irradiation time and amplitude, and the area can be changed by changing both or one of them .
[0009]
The optimum amplitude value of the pre-pulse for the effective TR can be obtained in advance by experiment or simulation, and by storing it as a function or approximate expression, the control means can calculate the optimum amplitude for each effective TR. it can. When irradiating the repulse for exciting the unnecessary tissue, the control means calculates and sets the prepulse area according to the effective TR, and optimizes it according to the effective TR.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below in detail with reference to the drawings.
FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus to which the present invention is applied. This MRI apparatus uses a nuclear magnetic resonance phenomenon to obtain a tomographic image of a subject. As shown in the figure, a static magnetic field generating magnetic circuit 1, a gradient magnetic field generating system 2, a transmitting system 3, The receiving system 4 includes a signal processing system 5, a sequencer 6, a central processing unit (CPU) 7, and an operation unit 8.
[0011]
The static magnetic field generating magnetic circuit 1 generates a uniform static magnetic field around the subject 9 in the direction of the body axis or in a direction perpendicular to the body axis. The static magnetic field generating magnetic circuit 1 is permanent in a space having a certain extent around the subject 9. Magnetic, normal conducting, or superconducting magnetic field generating means are arranged. The gradient magnetic field generation system 2 includes a gradient magnetic field coil 10 wound in three axis directions of X, Y, and Z, and a gradient magnetic field power source 11 that drives the respective coils. By driving the gradient magnetic field power supply 11 of the coil, gradient magnetic fields Gs, Gp, and Gf in the three-axis directions of X, Y, and Z are applied to the subject 9. By applying this gradient magnetic field, the slice plane for the subject 9 can be set. Further, position information can be given to a nuclear magnetic resonance (NMR) signal generated from the subject.
[0012]
The transmission system 3 irradiates a high-frequency signal to cause nuclear magnetic resonance to occur in the atomic nuclei constituting the living tissue of the subject 9 by a high-frequency magnetic field pulse transmitted from the sequencer 6, and includes a high-frequency oscillator 12 and a modulator. 13, a high-frequency amplifier 15, and a high-frequency coil 15 on the transmission side. The high-frequency pulse output from the high-frequency oscillator 12 is amplified by the high-frequency amplifier 14, and then placed on the reception-side high-frequency coil 16. By supplying, the subject 9 is irradiated with electromagnetic waves.
[0013]
The receiving system 4 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nucleus of the biological tissue of the subject 9, and receives a high frequency coil 16, an amplifier 17, a quadrature detector 18 and A on the receiving side. The electromagnetic wave (NMR signal) of the response of the subject 9 due to the electromagnetic wave irradiated from the transmission-side high-frequency coil 15 is received by the reception-side high-frequency coil 16 arranged close to the subject 9. The two sequences detected and input to the A / D converter 19 via the amplifier 17 and the quadrature detector 18 and converted into a digital quantity, and further sampled by the quadrature detector 18 at the timing according to the command from the sequencer 6 The collected data is sent to the signal processing system 5.
[0014]
The signal processing system 5 performs an image reconstruction operation and displays an image using the echo signal detected by the receiving system 4, and performs Fourier transform, correction coefficient calculation, image processing on the echo signal detected by the receiving system 4. A CPU 7 that performs processing such as reconstruction and control of the sequencer 6, a ROM (read-only memory) 20 that stores a program for performing image analysis processing and measurement over time, an invariant parameter used in the execution, and a pre-measurement A RAM (anytime reading / reading memory) 21 for temporarily storing obtained measurement parameters, echo signals detected by the receiving system 4, and an image used for setting the region of interest, and a parameter for setting the region of interest, and the CPU 7 A magneto-optical disk 22 and a magnetic disk 23 serving as a data storage unit for recording the reconstructed image data, and The image data read from the al magneto-optical disk 22 or magnetic disk 23 to visualize consisting become the display 24. Display unit for displaying a tomographic image.
[0015]
The sequencer 6 serves as a control means for repeatedly applying a high-frequency magnetic field pulse for causing nuclear magnetic resonance to atomic nuclei constituting the living tissue of the subject 9 in a predetermined pulse sequence, and operates under the control of the CPU 7. Various commands required for collecting tomographic image data of the subject 9 are sent to the transmission system 3, the gradient magnetic field generation system 2, and the reception system 4.
[0016]
In the present invention, a pulse sequence including a pre-pulse for suppressing a signal other than a component to be imaged as a pulse sequence, for example, a signal from fat is executed. Therefore, the CPU 7 calculates the optimum prepulse amplitude based on the effective repetition time TR set by the pulse sequence in addition to the above-described calculation for image reconstruction and control of the sequencer, and the prepulse is irradiated with the calculated amplitude. The transmission system 3 is controlled as described above.
[0017]
The operation unit 8 inputs control information for processing performed in the signal processing system 5 and includes a trackball 25 and a keyboard 26. The operation unit 8 also sets an imaging method (pulse sequence) involving pre-pulses.
[0018]
Next, a method of performing imaging using the fat suppression method in the MRI apparatus having the above configuration will be described. FIG. 2 is a diagram showing an example of a pulse sequence incorporated in the sequencer 6. Here, the fat suppression method based on the SE method is shown. In this pulse sequence, first, a pre-pulse 201 for exciting ¹ H of fat is irradiated, and then a crusher gradient magnetic field 202 is applied in each axial direction of the gradient magnetic field. In this state, a 90 ° pulse 203 for exciting ¹ H of biological water is applied together with the slice selective gradient magnetic field Gs204, a gradient magnetic field Gp205 in the phase encoding direction is applied, and then a 180 ° pulse 207 is applied together with the slice selective gradient magnetic field Gs208. To do. Gradient magnetic fields 206 and 209 in the reading direction are applied, and an echo signal is measured after the elapse of the echo time TE from the 90 ° pulse 203. After the signal measurement, the crusher gradient magnetic field 210 is applied again, and after the repetition time TR elapses, the next prepulse 201 is applied. Such a sequence is repeated while changing the intensity of the phase encoding gradient magnetic field 205, and an echo signal necessary for image reconstruction is measured.
[0019]
Although FIG. 2 shows the case of measuring one slice, it is also possible to excite a plurality of slices by changing the slice to be excited within the repetition time TR for one slice. In this case, the effective TR is a value obtained by dividing TR in FIG. 2 by the number of slices.
[0020]
In the above-described sequence, the pre-pulse 201 is a high-frequency pulse having a center frequency that excites ¹ H of fat, and its amplitude is determined to be an optimum value based on the effective TR. The optimum amplitude value for each effective TR is calculated by the CPU 7 from an approximate expression obtained in advance by experiment or simulation, and the amplitude of the prepulse is set to the obtained optimum amplitude value. Hereinafter, a calculation formula for calculating the amplitude by the CPU 7 will be exemplified.
[0021]
FIG. 3 is a plot of values obtained by experimentally determining the optimum amplitude of the prepulse when the effective TR is changed with the prepulse irradiation time constant. The figure shows the result in a 1.5T apparatus, and the unit of effective TR (horizontal axis) is ms. Further, the amplitude of the pre-pulse indicates a value normalized by assuming that the effective TR is sufficiently longer than the longitudinal relaxation time (here, 500 ms).
[0022]
From such experimental results, the optimum amplitude of the pre-pulse can be approximated by Expression (1) or Expression (2) as shown in FIG. 4 (a) or (b), for example.
f (x) = (a / x) + b (1)
In the equation, a and b are variables that determine the slope and intercept of the curve, respectively, and x is the effective TR.
f (x) = a · exp (b / x) + c (2)
[0023]
Thus, by approximating the optimal amplitude value with respect to the effective TR by a linear expression and always setting the optimal amplitude value, a stable fat suppression effect can be obtained regardless of the effective TR. In this case, the amplitude value is changed with respect to the effective TR. However, since the area of the pre-pulse only needs to be optimized with respect to the effective TR, the shape and the irradiation time can be maximized to obtain the fat suppression effect. It is also possible to optimize.
[0024]
In the above two examples, the approximation is performed by the first-order equation, but theoretically, the change (decrease) in the fat suppression effect becomes significant when the effective TR is shorter than the longitudinal relaxation time T1, and thus the longitudinal relaxation time. Only when it is shorter than T1, the area of the pre-pulse may be changed according to the effective TR. Alternatively, the approximate expression can be changed with the longitudinal relaxation time T1 as a boundary.
[0025]
FIGS. 5A and 5B show examples in which different approximate expressions are adopted with the longitudinal relaxation time T1 as a boundary. In FIG. 5A, as shown in the following equation, when the effective TRx is less than the longitudinal relaxation time, the same approximate expression as that in FIG. 4A is used, and the longitudinal relaxation time or more is approximated by a straight line.
x <T1 → f (x) = (a / x) + b
x ≧ T1 → f (x) = − cx + d
[0026]
In FIG. 5B, as shown in the following expression, when the effective TRx is less than the longitudinal relaxation time, the same approximate expression as in FIG. 4B is used to approximate the longitudinal relaxation time or more with a straight line.
x <T1 → f (x) = a · exp (b / x) + c
x ≧ T1 → f (x) = − dx + e
[0027]
The longitudinal relaxation time T1 value of the fat tissue in the 1.5T apparatus is about 150 ms.
Also in this case, by always calculating and irradiating the optimum fat suppression pulse area for the effective TR using these approximate equations, a stable fat suppression effect can be obtained regardless of the effective TR.
[0028]
In the above embodiment, the pulse sequence based on the spin echo method has been described. However, the pulse sequence is not limited to the spin echo method as long as it uses a signal suppression pre-pulse for a specific nuclear spin. Applicable to sequences. In the above embodiments, the case where the fat suppression method is applied to the imaging target of hydrogen protons has been described. All are applicable. In this case, by appropriately adjusting the frequency of the pre-pulse for exciting the nuclear spins whose signals should be suppressed and changing the amplitude based on the effective TR, the signal from one of the nuclear spins was always stably suppressed. A photographing method can be realized.
[0029]
【The invention's effect】
As described above, the MRI apparatus of the present invention can control the area of the fat suppression pulse in accordance with the effective TR, thereby creating a stable fat suppression image.
[Brief description of the drawings]
FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus to which the present invention is applied.
FIG. 2 is a timing chart showing an embodiment of a fat suppression sequence employed in the MRI apparatus of the present invention.
FIG. 3 is a diagram in which the optimum amplitude (experimental result) of a fat suppression pulse is plotted against the effective TR in the fat suppression sequence.
4A and 4B are diagrams each showing an approximate expression of the experimental result of FIG. 3;
FIGS. 5A and 5B are diagrams showing cases where the experimental results of FIG. 3 are approximated by different functions with the longitudinal relaxation time T1 of fat as a boundary.
FIG. 6 is a schematic diagram showing MR spectra of water and fat.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Static magnetic field generation magnetic circuit 2 Gradient magnetic field generation system 3 Transmission system 4 Reception system (detection means)
5 Signal processing system 6 Sequencer 7 CPU (control means)
8 Operation unit 9 Subject

Claims (4)

Magnetic field generating means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field in a space in which the subject is placed, means for detecting a nuclear magnetic resonance signal generated from the subject, and irradiation of the high-frequency magnetic field at a predetermined repetition time Nuclear magnetism comprising means for controlling the magnetic field generation means and the detection means so as to repeat detection of the nuclear magnetic resonance signal, and means for reconstructing and displaying the image of the subject based on the detected nuclear magnetic resonance signal In the resonance imaging apparatus,
The said control means controls the application area of the suppression pulse for suppressing the signal from the unnecessary component in the said test object corresponding to effective TR , The magnetic resonance imaging device characterized by the above-mentioned. The magnetic resonance imaging apparatus according to claim 1,
Wherein the control means is greater than said first applied area in the case of the first relative to the effective TR, the shorter than the first effective TR second effective TR for the suppression pulse first applied area A magnetic resonance imaging apparatus that applies a suppression pulse of a second application area. The magnetic resonance imaging apparatus according to claim 1 or 2,
The magnetic resonance imaging apparatus, wherein the control means controls the amplitude of the suppression pulse corresponding to the effective TR . The magnetic resonance imaging apparatus according to claim 1 or 2,
The magnetic resonance imaging apparatus, wherein the control means controls an application time of the suppression pulse corresponding to the effective TR .

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