JP2002253527A - Magnetic resonance imaging system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To acquire an MRA image by appreciable 3D-TOF in a short time. SOLUTION: A sequence is operated for obtaining three-dimensional data on each of a plurality of adjacent slabs substantially perpendicular to a bloodstream direction in a subject. A signal generating means provided is necessary for every intravenous infusion of a given amount of a contrast medium at timing a given time earlier than a start of measurement of each slab. The timing is conformed to timing when the contrast medium reaches each related slab as center data on a three-dimensional k space of the slab are imaged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging (MRI) apparatus for obtaining a tomographic image of a desired part of a subject by utilizing a nuclear magnetic resonance (hereinafter abbreviated as NMR) phenomenon. The present invention relates to a magnetic resonance imaging apparatus capable of improving the visualization ability when visualizing the travel of a vascular system.

[0002]

2. Description of the Related Art An MRI apparatus measures a nuclear spin (hereinafter, simply referred to as spin) density distribution, a line sum time distribution, and the like at a desired inspection site in a subject by utilizing an NMR phenomenon.
An arbitrary cross section of the subject is displayed as an image based on the measurement data. As one imaging function of the MRI apparatus, there is MR angiography (hereinafter abbreviated as MRA) for drawing a blood flow without using a contrast agent or the like.

[0003] As a blood flow drawing technique of MRA, a time-of-flight (Time-of-fl
ight: TOF method, phase-sensitive (Phase-sensitive: P)
Phase contrast (PC), in which the polarity of phase diffusion due to blood flow is inverted and the difference is calculated
Method is mainly used.

Further, the TOF method includes a two-dimensional method in which thin slices are continuously imaged and a three-dimensional method in which a plurality of volume data are imaged with a predetermined overlap. The two-dimensional method is called 2D-TOF, which can draw relatively slow flows and the venous system, and is applied to imaging of the neck and lower limbs because of the excellent contrast of plane flow, but it is difficult to draw the flow in the plane. There is a problem that the resolution and continuity in the slice direction are low.
On the other hand, the three-dimensional method is called 3D-TOF, which is used for depiction of the vascular system of the head by high resolution in the slice direction, high depiction of in-plane flow, and high S / N by volume measurement. Have been.

However, in the 3D-TOF, due to the thickness of the slab, not only the stationary tissue but also the blood flow signal is likely to be saturated by continuous short time interval (TR) excitation. Various ideas have been proposed to avoid this problem (for example,
TONE: “Improved MR Angiography: Magnetization Tra
nsfer Suppression with Variable Flip Angle Excitat
ion and Increased Resolution. Atkinson D et al. Ra
diology 190; pp890-894, 1994 ”, MTC:“ Magnetizat
ion transfer time of flight magnetic resonance an
giography. Pike GB et a1. , Magnetic Resonance in
Medicine 25; pp372-379, 1992 ”, multislab measurement:“ MR angiography by multiple thin slab 3D acqu
isition. Parker DL et a1. , Magnetic Resonance in
Medicine 17; pp. 434-451, 1991 ”). Among them, the method using multislab is only suitable for uniformly rendering a wide area, and is also highly effective for delineating a vascular system with a low flow rate or a blood vessel on the peripheral side.

On the other hand, in an imaging method using a contrast agent, a bolus-shaped contrast agent is injected, blood is signaled by the contrast agent, and an image is taken by a short TR gradient echo method or the like. Generally, a method of obtaining a difference from a case where the image is taken in a state where no image is taken is generally used.

[0007]

By the way, in the above-mentioned conventional multi-slab 3D-TOF measurement, a signal drop of a peripheral artery or an artery running in a plane is suppressed as compared with a single slab, but the blood flow velocity is extremely slow. In some cases, a signal drop has occurred. This tendency is large in the blood flow downstream in the slab as shown in FIG. 7, and there is a possibility that information important for diagnosis may not be obtained. The projection image obtained is
The image quality deteriorated, such as a difference in density at the boundary between slabs.

Further, since the optimum value of the relationship between the excitation repetition time TR and the slab thickness is determined by the blood flow velocity of the target blood vessel, the TR cannot be simply reduced to reduce the imaging time, and the number of slice encodes cannot be reduced. However, the time cannot be reduced by means other than limiting the area such as the number of phase encodes and the number of slabs or sacrificing the spatial resolution.

[0009] On the other hand, contrast-enhanced MRA has the feature that a wide range of blood vessels can be visualized in a short time, but it is necessary to complete basic data collection while the contrast agent is stored in the blood vessel. A sufficient phase encode step and slice encode step to reduce the time could not be secured. For this reason, only images with low spatial resolution were created compared to 3D-TOF, and they were not used for vascular morphology diagnosis of the head region.

Accordingly, the present invention provides a multi-slab 3D-TOF imaging system which displays a target blood vessel with a high-intensity pixel value relative to a stationary tissue without signal deterioration and image deterioration irrespective of blood flow velocity. It is an object of the present invention to provide an MRI apparatus capable of performing the operation. Another object of the present invention is to provide an MRI apparatus in which saturation is unlikely to occur even with a short TR, and as a result, the imaging time can be reduced by shortening the TR.

[0011]

In order to achieve the above object, the present invention provides multi-slab 3D-TOF measurement in an imaging region including a target blood vessel of a subject and synchronizes with measurement of each slab. A magnetic resonance imaging method for injecting a small amount of a contrast agent is provided. An MRI apparatus according to the present invention that can realize such a magnetic resonance imaging method includes a static magnetic field generating unit that applies a static magnetic field to a subject, a gradient magnetic field generating unit that applies a gradient magnetic field to the subject, and the subject. A transmission system that irradiates a high-frequency magnetic field to cause nuclear magnetic resonance in an atomic nucleus of a living tissue, a reception system that detects an echo signal emitted by the nuclear magnetic resonance, and the gradient magnetic field generation unit,
Control means for controlling a transmission system and a reception system based on a predetermined pulse sequence, a signal processing system for performing image reconstruction calculation using an echo signal detected by the reception system, and means for displaying an obtained image. In the magnetic resonance imaging apparatus provided, the control unit includes a multi-slab sequence for measuring a plurality of adjacent slabs as the pulse sequence, and determines a timing for injection of a contrast agent in synchronization with a measurement timing of each slab. It is characterized by comprising means for transmitting a timing signal.

According to the MRI apparatus configured as described above, M
When executing the RA measurement method, since the contrast agent can be distributed in the target blood vessel when measuring each slab,
Even when a TR shorter than a normal 3D-TOF or a thicker slab is employed, saturation due to multiple excitation of intravascular blood flow is suppressed, and a relatively high-intensity pixel value can be obtained. As a result, it is possible to draw a wide range in a shorter time, and it is also possible to obtain a good blood vessel image even in a portion where a signal drop is likely to occur, such as an in-plane flow.

In the MRI apparatus of the present invention, the control means preferably sends the timing signal a predetermined time before the start of each slab measurement. In particular, the predetermined time is
A point in time when the center data of the three-dimensional k-space of each slab is imaged,
The timing is determined so that the timing at which the contrast agent reaches the slab matches.

By determining the timing signal as described above and injecting the contrast agent at each slab measurement at this timing, the contrast of the image is affected during the time when the contrast agent concentration reaches the optimum concentration in the slab being photographed. Exert
Since the central region of the three-dimensional k-space can be measured, a high-contrast, high-spatial-resolution image unique to 3D-TOF can be obtained.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus according to the present invention. This MRI apparatus obtains a tomographic image of a subject using an NMR phenomenon, and as shown in FIG.
Static magnetic field generation magnet 2, gradient magnetic field generation system 3, and transmission system 5
, A receiving system 6, a signal processing system 7, a sequencer 4, and a central processing unit (CPU) 8.

The static magnetic field generating magnet 2 generates a uniform static magnetic field around the subject 1 in the direction of its body axis or in a direction perpendicular to the body axis. A permanent magnet type, a normal conduction type, or a superconducting type magnetic field generating means is disposed in the apparatus.

The gradient magnetic field generating system 3 comprises a gradient magnetic field coil 9 wound in three directions of X, Y and Z, and a gradient magnetic field power supply 10 for driving each gradient magnetic field coil. By driving the gradient magnetic field power supplies 10 of the respective coils in accordance with the above-mentioned command, gradient magnetic fields Gx, Gy, Gz in three axes of X, Y, Z directions are applied to the subject 1. The slice plane for the subject 1 can be set by the method of applying the gradient magnetic field, and the NMR signal generated from the subject can be phase-encoded.

The sequencer 4 repeatedly applies a high-frequency magnetic field pulse for causing nuclear magnetic resonance to the nuclei of the atoms constituting the living tissue of the subject 1 in a predetermined pulse sequence. Various commands necessary for data collection of one tomographic image are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

The transmission system 5 irradiates a high-frequency magnetic field to cause a nuclear magnetic resonance in the nuclei of the atoms constituting the living tissue of the subject 1 by the high-frequency pulse sent from the sequencer 4. A high-frequency pulse output from the high-frequency oscillator 11 is composed of a device 12, a high-frequency amplifier 13, and a high-frequency coil 14a on the transmission side.
The amplitude is modulated by the modulator 12 in accordance with the instruction, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then supplied to the high-frequency coil 14a arranged close to the subject 1, whereby the electromagnetic wave is 1 is to be irradiated.

The receiving system 6 receives an echo signal (NMR signal) emitted by nuclear magnetic resonance of an atomic nucleus of a living tissue of the subject 1.
The high-frequency coil 14b on the receiving side and the amplifier 1
5, a quadrature detector 16 and an A / D converter 17, and an electromagnetic wave (NMR signal) of the response of the subject 1 due to the electromagnetic wave radiated from the high-frequency coil 14 a on the transmitting side is arranged close to the subject 1. Detected by the high-frequency coil 14b, input to an A / D converter 17 via an amplifier 15 and a quadrature phase detector 16, converted into a digital quantity, and further converted by a quadrature phase detector 16 at a timing according to a command from the sequencer 4. The two sets of sampled collected data are sent to the signal processing system 7.

The signal processing system 7 includes a CPU 8, a recording device such as a magnetic disk 18 and a magnetic tape 19, a display 20 such as a CRT, and an input device 25 such as a keyboard and a mouse. The CPU 8 performs processing such as Fourier transform, correction coefficient calculation, image reconstruction, etc., and forms an image of a signal intensity distribution of an arbitrary section or a distribution obtained by performing an appropriate operation on a plurality of signals, and displays the image on the display 20 as a tomographic image. It is supposed to.

In FIG. 1, the high-frequency coils 14a and 14b and the gradient coil 9 on the transmitting and receiving sides are connected to the subject 1
Is installed in the magnetic field space of the static magnetic field generating magnet 2 arranged in the space around the. In addition to the MRI apparatus, the subject 1 is provided with an automatic injector 30 for injection of a contrast agent to enable imaging of the contrast agent.
8 and operates.

Next, in the above configuration, the multi-slab 3D-T
A method for performing OF imaging will be described. In multi-slab imaging,
For example, as shown in FIG. 2, a slab surface is usually set in a cross section so as to be substantially orthogonal to a main blood vessel in an imaging region including a target blood vessel of a subject.

A pulse sequence for multi-slab photographing is incorporated in the CPU 8 in advance as a program, and a pulse sequence for multi-slab photographing is selected by an input device 25 such as a keyboard and a mouse provided in the CPU 8. At this time, the slab thickness, the number of slabs, the degree of slab overlap, and the like are set. In multi-slab 3D-TOF imaging, the sequencer 4 controls the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6 according to a pulse sequence as shown in FIG.

That is, first, a high-frequency pulse 301 is applied together with a gradient magnetic field (Gs) 302 for selecting the entire first slab to selectively excite the first slab. Next, gradient magnetic fields (Ge1, Ge2) 303 and 304 for encoding in the slice direction and the phase encode direction are applied, and a gradient magnetic field (Gr) 305 in the readout direction is applied, and an echo signal 3 from the slab is applied.
Measure 06. For example, by fixing the intensity of the encoding gradient magnetic field Ge1 in the phase encoding direction and changing the intensity of the encoding gradient magnetic field Ge2 in the slice direction, for example, the number of steps in FIG.
Is repeated, and then the intensity of the gradient magnetic field Ge1 in the phase encoding direction is changed (the number of steps is increased). Similarly, the sequence is repeated while changing the intensity of the encoding gradient magnetic field Ge2 in the slice direction.

When all the steps in the phase encoding direction have been completed in this way, measurement is performed on the second slab in the same manner as described above. The echo signals thus measured for each slab become three-dimensional data arranged in the k-space having the respective encoding amounts of the phase encoding gradient magnetic field and the slice encoding gradient magnetic field as coordinates.

In the imaging method executed by the MRI apparatus of the present invention, the imaging is performed by injecting a contrast agent at the timing of each measurement of one slab in such multi-slab 3D-TOF imaging. Therefore, the CPU 8 sends a timing signal to the automatic injector 30.

The timing for injecting the contrast agent is set so that imaging is performed at the time when the concentration of the contrast agent in the blood vessel to be imaged becomes maximum after the injection of the contrast agent. In particular, it is preferable that the timing of measuring data in the low-frequency region of the k space coincides with the timing at which the contrast agent concentration becomes maximum.
Therefore, prior to the main imaging, first, for example, a time point t1 at which a contrast agent is to be injected is determined by using a technique called test injection. Then, as shown in FIG. 4, for example, in the sequential order in which the phase encode is measured from the high frequency region to the low frequency region of the k space and further measured in the high frequency region, the contrast agent concentration is approximately at the intermediate point of the first slab measurement. The contrast agent arrival time (time from injection to the maximum contrast agent concentration) Δt1 is determined so as to be the maximum. In the centric order in which measurement is first performed from the low frequency region, Δt1 is determined so that the contrast agent concentration becomes maximum at the start of measurement. From the arrival time Δt1 and the measurement time Δt2 of one slab, a contrast agent injection timing Δt (difference between the injection start and the start of the slab meter) is obtained.

The time difference Δt between the start of the injection and the start of the measurement may be obtained by measuring the arrival time of the contrast agent as described above, or may be set to an empirical time. It is further desirable to determine Δt for each slab.

The contrast agent injection timing determined in this way is automatically or manually set in the CPU 8 via the input means. Next, the CPU 8 sends a control signal indicating the start of multi-slab imaging to the sequencer 4 and creates a synchronization signal s1 synchronized with the timing of slab measurement.
The synchronization signal s1 is generated at a timing Δt earlier than the slab measurement start time as described above. For example, when the automatic injector 30 is used for injection of the contrast agent as shown in FIG. 1, the synchronization signal s1 is directly sent to the automatic injector 30, and the automatic injector 30 uses the signal s1 as a trigger. A predetermined amount of a contrast agent is injected. Alternatively, the synchronization signal s1 may be used as notification means for notifying the operator of the injection timing, for example, an audio notification means, or a drive signal for turning on a buzzer or a lamp. In this case, the operator intravenously administers the contrast agent to the subject at each time taught by the notification means.

The injection amount of the contrast agent varies depending on the thickness of the slab. For example, about 1 to 2 cc is injected at an injection rate of 0.1 to 3 cc / s, or a solution diluted about 10 times with physiological saline is injected. The injection may be performed at a speed of 1 cc / s or less and an injection duration of 10 seconds or more. Thus, the 3D-TOF measurement of the first slab can be performed during the time when the contrast agent stays in the target blood vessel.

On the other hand, the sequencer 4 starts measuring the first slab after a lapse of a predetermined timing after receiving the control signal from the CPU 8. This measurement is repeated while sequentially changing the phase encoding step and the slice encoding step in the pulse sequence shown in FIG. 3 as described above. By this repetition, the slab to be measured is repeatedly excited, and the signal is reduced if no contrast agent is present. In particular, the signal is significantly reduced on the downstream side of the blood flow. Since the concentration of the contrast agent in the blood vessels is maximized at the time of the measurement, the signal does not decrease and the blood flow can be drawn with a high signal as shown in FIG.

During the measurement of the first slab, the injection of the contrast agent for the measurement of the second slab is carried out in exactly the same procedure as described above, and the measurement of the second slab is subsequently performed.
This is repeated for the number of slabs to collect the respective slab data.

These slab data (k-space data) are
The image data is reconstructed into three-dimensional image data by performing a three-dimensional Fourier transform, and the image data of all the slabs is synthesized to generate a three-dimensional image of all the slabs. In this case, if there is an overlap between the slabs, the overlapping data is subjected to processing such as weighted averaging.

In the obtained three-dimensional data set, it is difficult to grasp the running and shape of the blood vessel as it is. Therefore, three-dimensional image data is converted to the maximum intensity projection method (Maximum Intensity).
(projection processing) using a known technique such as volume rendering.
For example, in the projection processing, projection is performed in a predetermined direction such as a coronal section, a sagittal section, or a transverse axis. Alternatively, as shown in FIG. 6, it is also possible to create projection images every 5 ° to 10 ° from a projection having an angle of about ± 45 ° around a certain axis C and display them as moving images. It is possible. Thereby, depth perception such as the context of the blood vessels A and B can be obtained, and the structure of the blood vessels can be easily recognized.

The embodiment of the present invention has been described above.
The present invention can be variously modified without being limited to the above embodiment. For example, in the above description, in the multi-slab photographing, the case where the phase encoding step is set to the outer loop and the slice encoding step is set to the inner loop has been described. Spatial data may be measured in any order.

As described above, in the MRA measurement method, the Tl value of blood in a blood vessel is shortened at the time of measurement of each slab, saturation is hardly caused even by excitation by a short TR, and an echo signal from a stationary tissue is extremely low. Therefore, the target blood vessel shows a relatively high-intensity pixel value. In addition, the injection timing of the contrast agent can be set to a timing close to the measurement timing of each slab, whereby high contrast can be achieved and fairness between slabs can be maintained.

In the 3D-TOF measurement, it is possible to obtain a good MRA image in a short time by enabling measurement using an extremely short TR.

[0040]

According to the MRI apparatus of the present invention, a contrast agent can be injected at an appropriate timing in multi-slab 3D-TOF imaging, so that a shorter TR can be adopted as compared with the conventional art, and the imaging time can be reduced. In addition to this, it is possible to improve the suppression of the stationary part and increase the signal intensity of the arteries in the region including the bent blood vessel, and to render the vascular system with a higher contrast than that of the conventional MRI apparatus. Also, since a sufficient phase encode step and slice encode step can be secured, it is possible to obtain a high spatial resolution image utilizing the features of 3D-TOF while being a contrast MRA.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a position embodiment of an MRI apparatus according to the present invention.

FIG. 2 is a multi-slab 3D-TO adopted by the MRI apparatus of the present invention.
Diagram explaining F

FIG. 3 is a diagram showing a pulse sequence of 3D-TOF adopted by the MRI apparatus of the present invention.

FIG. 4 is an explanatory diagram showing the concept of an MR angiography measurement method executed by the MRI apparatus of the present invention.

FIG. 5 is a diagram showing an image and a signal intensity obtained by the MR angiography measurement method according to the present invention.

FIG. 6 is a view for explaining one method of projection.

FIG. 7 is a diagram illustrating a problem of conventional multi-slab measurement.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Subject, 2 ... Static magnetic field generation magnet, 3 ... Gradient magnetic field generation system, 4 ... Sequencer, 5 ... Transmission system, 6 ... Reception system, 7 ... Signal processing system, 8 ... CPU, 9 ... Gradient magnetic field coil, 14a ... High-frequency coil on the transmitting side, 14b ... High-frequency coil on the receiving side

Claims (3)

[Claims] 1. A static magnetic field generating means for generating a static magnetic field in a space where a subject is placed, a gradient magnetic field generating means for applying a gradient magnetic field to the subject, and a nuclear magnetic resonance to an atomic nucleus of a biological tissue of the subject. A transmission system that irradiates a high-frequency magnetic field to cause a magnetic field, a reception system that detects an echo signal emitted by the nuclear magnetic resonance, and controls the gradient magnetic field generation unit, a transmission system, and a reception system based on a predetermined pulse sequence. Control means, a signal processing system for performing an image reconstruction operation using an echo signal detected by the receiving system, and a magnetic resonance imaging apparatus comprising means for displaying the obtained image, the control means, A multi-slab sequence for measuring a plurality of adjacent slabs is provided as the pulse sequence, and the timing of contrast agent injection is determined in synchronization with the measurement timing of each slab. A magnetic resonance imaging apparatus comprising means for transmitting a timing signal. 2. The control means according to claim 1, wherein
2. The magnetic resonance imaging apparatus according to claim 1, wherein the transmission is performed a predetermined time before the start of each slab measurement. 3. The control device according to claim 1, wherein the predetermined time is determined such that a point in time at which the center data of the three-dimensional k-space of each slab is imaged coincides with a timing at which the contrast agent reaches the slab. The magnetic resonance imaging apparatus according to claim 2, wherein