JP2001008918A - Mr imaging method and mri instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To satably control blood artifact even if T1 time of bloodstream is shorten by administering contrast medicine to a subject. SOLUTION: A saturation pulse to mitigate MR signal from bloodstream which is flown in a region of interest is applied then the region of interest is excited selectively and a scan for imaging which corrects MR signal, i.e., by exciting the region of interest selectively is carried out. In such a case, a flip angle for the saturation pulse is set as the same degree of the flip angle of the scan of imaging. For example, these flip angles are set to a smaller value than 90 deg. respectively. When the scan for imaging is carried out based on a selective exciting method to the region of interest, a moving direction of the part which is excited selectively is set toward an opposite direction to a flowing direction of bloodstream.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to MR (magnetic resonance) imaging for reducing blood flow artifacts (ghosts) by using RF pulses called saturation pulses (saturation pulses). The present invention relates to MR imaging that reliably and stably suppresses artifacts from arteries that become prominent when imaging is performed by administering to a subject.

[0002]

2. Description of the Related Art In MR (magnetic resonance) imaging, a nuclear spin of a subject placed in a static magnetic field is magnetically excited by a high frequency signal of its Larmor frequency, and an MR signal generated by this excitation is generated. This is a method of collecting and reconstructing an image from the collected data.

[0003] There are various forms of this MR imaging, and there are cases where an MR signal from a blood flow becomes an artifact. Conventionally, when performing such MR imaging, a presaturation pulse is applied immediately before the execution of a pulse sequence for main imaging (for imaging) by, for example, the FE method, and the presaturation pulse is applied to a region outside the region of interest (saturation region). ) To reduce the blood flow artifact by saturating the MR signal of the flowing blood flow and making the signal as silent as possible.

[0004] The blood flow artifact suppression method of applying a saturation pulse is used not only for general abdominal T2 images, but also for so-called "whole liver dynamic imaging".

[0005]

However, in the above-mentioned blood flow artifact suppression method in which a saturation pulse is applied, an MR contrast agent (hereinafter, simply referred to as a contrast agent) is administered to a subject to perform MR imaging. In this case, there is a problem that the effect of suppressing the blood flow artifact is extremely low.

This is because, when a contrast agent is administered, the T1 relaxation time of the blood flow is usually shortened, so that the MR signal (blood flow signal) from the blood flow flowing into the imaging region of interest from outside is once saturated. However, it is because T1 is recovered immediately, and it is almost difficult to maintain a state of no signal (saturated state) even after the inflow.

[0007] For this reason, in the conventional method of simply applying a saturation pulse to the outside of the blood flow in the region of interest, the saturation effect fluctuates along with the heartbeat, and thus the blood flow signal suppression effect fluctuates. As a result, blood flow artifacts could not be reliably and sufficiently suppressed.

[0008] Since this blood flow artifact spreads outside the encoding direction of the original blood vessel image, it greatly impairs the interpretation.

The present invention has been made in view of the situation facing such conventional MR imaging. For example, when a contrast medium is administered to a subject, the T1 time of blood flow is apparently reduced. An object of the present invention is to provide a highly reliable, high-quality, and high-definition MR image that can stably suppress blood flow artifacts even when it becomes short.

[0010]

The starting point of the idea of the present invention is that, when a contrast agent is administered, the T1 relaxation of the blood flow becomes very short, so that the blood flow signal in the imaging region of interest is turned off. Is in recognition of the fact that it is virtually impossible. That is, no matter how many presaturation pulses are applied, it is impossible to obtain a stable and effective saturation effect.

Therefore, from a different viewpoint, the signal intensity of the blood flow flowing into the region of interest is made substantially the same as the saturation effect due to the excitation in the main imaging, and the saturation effect is continuously and temporally and spatially. It is one gist of the present invention to achieve this.

Further, even if the blood flow moves due to the pulsation (it is pushed out vigorously), the moving direction of the imaging slice is opposite to the moving direction of the blood flow so that the uniformity of the signal intensity does not deteriorate as much as possible. It is another gist of the invention to achieve

An outline of the configuration of the present invention based on the above-described gist will be described in comparison with a conventional presaturation method.

[0014] The conventional presaturation method is a method of saturating a signal from an inflowing blood flow prior to main imaging for image data collection. The word "pre" in the presaturation method expresses this. Therefore,
In the conventional case, the flip angle of the saturation pulse is 9
The angle is set to 0 degree or an angle deeper than 90 degrees (e.g., 110 degrees) in consideration of some T1 relaxation occurring before the main imaging after saturation.

However, when a so-called “full liver imaging dynamic imaging” is performed by administering a contrast agent and repeating, for example, a 2D-FE multi-slice scan under a breath-hold, the recovery of T1 becomes faster, and the inflow blood flow is reduced. It is practically impossible to perform actual shooting with no signal from the camera.

A calculation example of the T1 relaxation time indicating this will be described. The T1 value of blood is about 930 ms, and the relaxation rate R1 of the contrast agent Gd-DTPA is 4 [1 / mmol / l ·
sec], the concentration of the contrast agent is x [mmol /
l], the T1 value observed when

From (1 / T1) = (1 / 0.93) + 4x,

T1 = 1 / (1.075 + 4x) The maximum concentration of the contrast agent in the aorta is 6 [mmol /
l], the T1 time at this time is 4
It is about 0 ms. In other words, by administering a contrast agent,
For example, the T1 time is reduced by 1/20 or more.

Therefore, instead of the non-signaling based on the presaturation method, according to the present invention, as described above,
The signal intensity of the inflowing blood flow is set to the same level as the saturation effect due to the excitation in the main imaging, and the data is arranged so that the saturation effect is temporally and spatially continuous. However, the gist of the present invention is to set the moving direction of the imaging selection region (slice, slab) to be opposite to the moving direction of the blood flow so that the uniformity of the signal intensity is not broken as much as possible.

Therefore, as the main components that embody these points, (1). The flip angle of the saturation pulse is set to be substantially the same as the flip angle obtained by imaging in the main imaging (conventionally, in general, the flip angle of the presaturation pulse is set to 90 degrees or more in order to eliminate the signal of the inflow blood flow. (2). The order of selective excitation of slices and sliced slabs during the main imaging is set to the direction from the downstream of the main blood flow inflow direction to the upstream, that is, the direction opposite to the blood flow direction. Depends on the other parameters of), and,
(3). We propose to sharpen the slice characteristics of the saturation pulse, particularly the edge characteristics at the edges (conventionally, priority is given to shortening the scan time over the slice characteristics). These main components may be executed independently, or two or more of them may be executed in an appropriate combination.

In the present invention, since the meaning of the RF pulse for saturation is different from that of the prior art, the term "pre" is not used, and is simply referred to as "saturation pulse". Whereas conventional "pre-saturation" has the meaning of "saturate in advance", the "saturation" of the present invention "does not saturate in advance, but provides the saturation effect at the appropriate timing. It means that.

In order to achieve the above object, the present invention specifically adopts the following constitution.

According to the first aspect, after applying a saturation pulse for saturating an MR signal from an object flowing into a region of interest set to the object to the object,
In the MR imaging method and the MRI apparatus for performing an imaging scan on the region of interest, a flip angle of the saturation pulse is set to an angle lower than 90 degrees.

According to the MR imaging method and the MRI apparatus of the second invention, the flip angle of the saturation pulse is set to be substantially equal to the flip angle of the pulse train of the imaging scan. At this time, for example, both the flip angle of the saturation pulse and the flip angle of the pulse train of the imaging scan are set to values smaller than 90 degrees. The object is a blood flow flowing through a blood vessel.

According to the third aspect, after applying a saturation pulse to the subject to saturate the MR signal from the subject flowing into the region of interest set in the subject,
In an MR imaging method and an MRI apparatus for performing an imaging scan on the region of interest based on a selective excitation method, a direction in which a site selectively excited based on the selective excitation method moves is set in a predetermined direction with respect to a flow direction of the object. Is set. Preferably, the predetermined direction is a direction from a downstream to an upstream in a direction in which the site to be selectively excited moves. For example, the object is a blood flow flowing through a blood vessel, and the site to be selectively excited is a plurality of slices selected and sliced in the region of interest.
More preferably, the saturation pulse is applied only once every repetition time TR of a pulse sequence used for the imaging scan.

According to the fourth aspect, after applying a saturation pulse for saturating the MR signal from the target flowing into the region of interest set in the subject to the subject,
In the MR imaging method and the MRI apparatus for performing an imaging scan on the region of interest, a thickness of a saturation region to which the saturation pulse is applied is increased. For example, the object is a blood flow flowing through a blood vessel. As an example, the thickness of the saturation region is 10 in the direction of movement of the object.
cm or more. Further, the saturation pulse may be an RF pulse having improved edge characteristics when selectively exciting the saturation region. The RF pulse may have a Sinc function waveform, and the π length of at least one of the positive and negative π lengths may be 5π or more.

According to a fifth aspect of the present invention, at least two of the three types of MR imaging methods comprising the MR imaging method of the second aspect, the MR imaging method of the fifth aspect, and the MR imaging method of the tenth aspect. A combination of different types of MR imaging methods may be executed.

[0026]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

First, the MR used in the following embodiment will be described.
FIG. 1 shows a schematic configuration of an I (magnetic resonance imaging) apparatus.

This MRI apparatus uses a patient P as a subject.
Bed, a static magnetic field generator for generating a static magnetic field, a gradient magnetic field generator for adding positional information to the static magnetic field, a transmitter / receiver for transmitting and receiving high-frequency signals, control of the entire system and image reconstruction A control / arithmetic unit responsible for
An electrocardiographic measurement unit that measures an ECG signal as a signal representing the cardiac phase of the subject P. Note that a pulse wave measurement unit that measures a pulse wave of the subject may be provided instead of the electrocardiogram measurement unit.

The static magnetic field generating section includes, for example, a superconducting magnet 1 and a static magnetic field power supply 2 for supplying a current to the magnet 1, and has a cylindrical opening (diagnostic space) into which the subject P is loosely inserted. the axial direction (Z axis direction) to generate a static magnetic field H _0.
Note that a shim coil 14 is provided in this magnet portion. The shim coil 14 is supplied with a current for homogenizing a static magnetic field from a shim coil power supply 15 under the control of a host computer described later. The couch part can retreatably insert the top plate on which the subject P is placed into the opening of the magnet 1.

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

The x, y, z coils 3x from the gradient magnetic field power supply 4
By controlling the pulse current supplied to ~ 3z,
The gradient magnetic fields in the three axes X, Y, and Z directions, which are physical axes, are synthesized, and the respective logics of a slice-direction gradient magnetic field Gs, a phase encoding direction gradient magnetic field Ge, and a readout direction (frequency encoding direction) gradient magnetic field Gr that are orthogonal to each other. The axial direction can be set and changed arbitrarily. Slice direction, phase encoding direction, and gradient magnetic fields in the readout direction are superimposed on the static magnetic field H _0.

The transmission / reception unit includes an RF coil 7 disposed near the subject P in the imaging space in the magnet 1,
And a transmitter 8T and a receiver 8R, which are connected to each other. The transmitter 8T and the receiver 8R are connected to a sequencer 5 described later.
It operates under the control of. The transmitter 8T supplies the RF coil 7 with an RF current pulse having a Larmor frequency for exciting nuclear magnetic resonance (NMR). The receiver 8R has R
The F coil 7 receives the received MR signal (high-frequency signal), performs various signal processing such as pre-amplification, intermediate frequency conversion, phase detection, low-frequency amplification, and filtering, and then performs A / D conversion. The digital quantity data (original data) of the MR signal is generated.

The control / arithmetic unit includes a sequencer (also called a sequence controller) 5, a host computer 6, an arithmetic unit 10, a storage unit 11, and a display unit 1.
2 and an input device 13. Among them, the host computer 6 has a function of commanding pulse sequence information to the sequencer 5 and controlling the operation of the entire apparatus by a pre-stored software procedure based on various aspects as described later.

The sequencer 5 includes a CPU and a memory, stores pulse sequence information sent from the host computer 6, and controls the operation of the gradient magnetic field power supply 4, the transmitter 8T, and the receiver 8R according to the information. At the same time, the digital data of the MR signal output from the receiver 8R is input once and transferred to the arithmetic unit 10. Here, the pulse sequence information is
Gradient power supply 4, according to a series of pulse sequences,
All the information necessary to operate the transmitter 8T and the receiver 8R, for example, x, y, z coils 3x to 3z
And information on the intensity of the pulse current to be applied to the device, application time, application timing, and the like.

The MR imaging performed in this embodiment is the same as that shown in FIG.
As shown in the figure, this is a so-called whole liver angiography dynamic imaging. In other words, this is a technique in which a contrast agent is administered to a subject, and an imaging region covering the entire region of the liver is dynamically photographed by a multi-slice method. The pulse sequence used for this is an FE-based pulse train, such as the FE (gradient field echo) method or the FE-based EPI (echo planar imaging) method. FIG.
Shows a partial FE-based pulse sequence.

The arithmetic unit 10 inputs the digital data (original data or raw data) output from the receiver 8R through the sequencer 5, and stores the digital data in a Fourier space (also called k-space or frequency space) on its internal memory. The data is arranged, and the original data is subjected to two-dimensional or three-dimensional Fourier transform for each set to reconstruct image data in real space. The arithmetic unit is also capable of performing data combining processing, difference calculation processing, and the like.

The storage unit 11 has a memory, and can store not only reconstructed image data but also image data on which the above-described combining processing and difference processing have been performed. Further, the storage unit includes, as a recording medium, a ROM (not shown) which records a pulse sequence in MR imaging in the form of program data and is readable by a computer. In this ROM, information of a pulse sequence (here, a pulse train of the multi-slice method by the FE method) is recorded.

The display 12 displays an image. In addition, the operator can input, to the host computer 6, imaging conditions desired by the operator, pulse sequences, and information regarding image synthesis and difference calculation.

Further, the electrocardiogram measuring section comprises an ECG sensor 17 which is attached to the body surface of the subject and detects an ECG signal as an electric signal. 6 and an ECG unit 18 for outputting to the sequencer 5. The measurement signal from the electrocardiograph is used by the sequencer 5 when performing an imaging scan. This gives E
The synchronization timing by the CG gate method (electrocardiographic synchronization method) can be appropriately set, and data can be collected by performing an imaging scan of the ECG gate method based on the synchronization timing.

A voice generator 16 for instructing the subject P to hold his / her breath by voice is connected to the host computer 6 and the sequencer 5.

Next, the operation and effect of this embodiment will be described.

In the MRI apparatus of this embodiment, as shown in FIG. 2, dynamic liver imaging of the liver LV of the subject P is performed. As shown in FIG. 3, the scanning for this imaging is performed by a pulse sequence in which a pulse train of the FE method is applied according to the multi-slice method.

By appropriately setting the slice selection parameters in this pulse sequence, as shown in FIG. 2, imaging consisting of a plurality of slices S1 to Sn (for example, about 14 to 18) over the entire region of the liver LV. An area RG _ima is set. Each slice S is, for example, 8 mm thick, and the gap between slices is, for example, 2 mm. Each slice is sequentially excited by the multi-slice method, and the excitation order forms one of the features of the present invention.
That is, here, the parameters of the pulse sequence (for example, the intensity of the slice gradient magnetic field) are determined so as to sequentially excite the slices in the direction opposite to the inflow direction of the blood flow of the descending aorta AR as the blood flow.

[0044] Also, by selecting a parameter related to the saturation pulse is applied in a pulse sequence appropriately, almost adjacent state, saturation region RG _{s at} covering the heart HT and descending aorta AR is set to an imaging region RG _ima You. One feature of the saturation region RG _sat is to make it as thick as possible, and the heart HT and the descending aorta AR
Is set to a value of, for example, 10 cm or more that sufficiently covers the ejection side.

A pulse sequence according to the multi-slice method will be described.

As schematically shown in FIG. 3, in this pulse sequence, one phase encode of echo data is collected for a plurality of slices (for example, about 14 to 18) during each repetition time TR. . The echo data collection for each repetition time TR is performed by phase encoding 1 to 12
8 to collect all echo data of a plurality of slices.

In the first time slot for each repetition time TR, the saturation pulse train PT
_sat is applied. The pulse train PT _sat is composed of a saturation pulse PS _sat, a slice gradient magnetic field pulse Gs applied in parallel with the saturation pulse PS _sat, and a gradient magnetic field spoiler G _spoil for dephase applied subsequently.

The saturation pulse PS _sat is formed by, for example, a Sinc function, and its waveform area is set so that the flip angle SatFA = 70 °. Flip angle Sat of this saturation pulse PS _sat
In the present embodiment, the value of FA is set to a value lower than 90 degrees. Preferably, the flip angle Sa
tFA is set to be substantially the same as the flip angle associated with RF excitation by an imaging pulse train applied to the imaging region RGima. Saturation pulse PS
RF frequency of _sat and slice gradient pulse Gs
Is, as described above, the saturation region RG _sat
Is set to select a slice.

At each repetition time TR, the above-described
Curation pulse PS_satWhen the application of
Of all slices S1 to Sn for one phase encode amount
Echo data is collected. For each echo data collection
Imaging pulse train PT _imaIs the excitation RF pulse
PS_extAnd slice gradient applied in parallel
Field Gs, phase encoding gradient magnetic field Ge (not shown), and
And the readout gradient magnetic field Gr. Excitation RF pulse PS
_extIs set to 70 ° here.
I have. This flip angle FA is desirably 90 ° or less.
Best, saturation pulse PS_satFlip of
It is determined by comparison with the value of the angle SatFA.

The slice excitation order under each echo data acquisition for one encoding amount is set to a direction reverse to the descending aorta AR, for example, by controlling the setting order of RF frequencies.

In the execution of the above pulse sequence,
The repetition time TR is, for example, about 160 ms, and the entire scanning time is, for example, about 20 sec.

As shown schematically in FIG. 2, whole liver angiography dynamic imaging is liable to generate blood flow artifacts under the influence of the descending aorta AR. In particular, the influence of the pulsation from the heart HT is large, and the T1 of the blood flow is apparently shortened by the contrast agent, so that the T1 is rapidly relaxed (saturation is ineffective). Periodic inflow and outflow of the MR signal due to periodic inflow into the region _RGima . As a result, FIG.
As shown in (a), when the signal strength is taken in the time (encode number) axis direction, the signal strength periodically fluctuates. Therefore, when the fluctuating signal values in FIG. 7A are subjected to Fourier transform (image formation), a ghost (blood flow artifact) occurs as shown in FIG.

The present invention has been proposed to prevent or suppress this blood flow artifact. In order to confirm the effectiveness of the above-described embodiment method to which the present invention is applied, the inventor performed a comparison simulation of the signal intensity between the conventional method and the embodiment method. 5 to 9 for representative examples
This will be described with reference to FIG.

As the photographing conditions according to the conventional method, those assumed to be the most typical conditions were assumed. That is, RR interval = 800 ms, TR = 160 ms, application of saturation pulse once per TR, flip angle SatFA of saturation pulse = 90 °, flip angle FA of imaging excitation pulse FA = 70 °, and slice excitation order = The forward direction follows the flow of the descending aorta. Considering the model shown in FIG. 2, a saturation region was taken upstream of the blood flow, a gap of 2 cm was opened, and an imaging region of 14 to 18 slices was taken downstream.

T1 = 200 ms (contrast agent 1 mmol / l)
FIG. 5 shows a simulation result of the signal strength SI when (corresponding to). The vertical axis represents the signal strength SI, the horizontal axis represents the encode number, and the longitudinal direction represents the slice number.
The signal strength is sequentially calculated for each slice in order from the first encoding, and the first to fifteenth of the slices are calculated.
The signal intensity up to the encoding point (for 3R-R) is graphically shown in FIG. This indicates the behavior of the signal strength up to the 15th echo of each slice.

According to this, the influence of the upstream slice is large and the signal becomes unstable. More specifically, first, the signal strength of the first encoding, that is, the signal strength of each slice is sequentially calculated along the arrow (1) in FIG.
The signal strength is large because the first one is a spin that is hardly saturated. However, the signal intensity sharply decreases from the eighth slice (see the part (A) in the figure). This drop is assumed to be caused by the fact that the flow velocity becomes large due to the heart beat after about 100 ms from the R wave, so that the partially saturated spin used in one slice is repeatedly used in the downstream slice. Conceivable. This effect appears when the maximum blood flow velocity is higher than the excitation movement velocity of the slice.

Next, calculations for the second and subsequent encodings are sequentially performed (see arrows (2), (3),... In the figure). After the second encoding, the signal intensity is lower than that of the first encoding due to the influence of the blood flow flowing from the saturation area. Note that the signal strength is increased in the part (B) in the figure. In the high signal area (B), the blood flow spin which is not saturated in the gap between the saturation area and the imaging area (here, set to 2 cm) is caused by pulsation, and this slice NO. This is probably because they have reached around 16 and 17 (without being excited by other slices).

FIG. 6 shows the result of estimating the ghost by the conventional method. This estimate was obtained by performing a discrete Fourier transform on the calculation data. The Fourier transform data represents an original image, a first ghost, and a second ghost.
As shown in the figure, the ghost amount reaches about 20% of the original image. FIG. 7A shows the dependence of the magnitudes of the original image and the ghost (first and second ghosts) on the T1 value (contrast agent concentration) and the delay of the data acquisition start timing from the R wave at this time. ) And (b). In both dependency graphs, the ghost shows a considerably large value.

On the other hand, simulation results based on the method of the present embodiment implementing the present invention are as shown in FIGS.

FIG. 8 is a graph showing the calculation result of the signal value of each slice according to the method of the present embodiment, which is compared with FIG. The imaging conditions at this time are, among the most typical imaging conditions described with reference to FIG. 5, the flip angle of the saturation pulse SatFA = 70 °, the flip angle of the imaging excitation pulse FA = 70 °, and the slice excitation order =
The direction is opposite to the flow of the descending aorta, and the remaining parameters are the same as described above.

According to this, it is understood that the fluctuation of the signal intensity is considerably reduced due to the fact that the influence when the upstream slice is excited becomes small.

Specifically, 1) the slice excitation order is reversed with respect to the blood flow in consideration of the fact that the maximum speed of the blood flow and the moving speed of the slice excitation are the same, and 2) both of them. By setting the flip angle SatFA = FA and a shallow value, the effect of signal saturation from other excitation slices and the effect of signal saturation of blood flow flowing from the saturation region can be made the same, thereby reducing the saturation state. The echo data can be continuously provided from the downstream to the upstream, and the echo data for imaging of each slice is collected in a state where the fluctuation is small. Therefore, collected data also forms a more continuous data group with little temporal and spatial fluctuations, and ghosts, that is, blood flow artifacts can be reduced.

As a result of estimating the ghost from the signal intensity according to the method of the present embodiment in the same manner as described above, FIG.
As shown in (a) and (b), a graph was formed. It is obvious at a glance as compared with the graph of FIG. 7 described above, and the ghosts (first and second ghosts) are significantly reduced. Specifically, it has been found that the ghost size can be reduced to several percent or less, except for some slices. In particular, even when the T1 value (contrast agent concentration) or the delay time from the R wave is changed,
It was also found that the size (amount) of the ghost was stably reduced.

As discussed in the above simulation, a certain gap is set between the upstream saturation region RG _sat and the downstream imaging region RG _ima . The width of the gap is determined in accordance with the degree of the edge characteristic when the saturation pulse is applied so that the saturation pulse PS _sat does not affect the imaging area. The gap needs to be set so large that the edge portion of the slice characteristic at the time of application is distorted from the rectangular wave. Therefore, in the above-described embodiment, the saturation pulse PS _sat is configured by a Sinc function, and its π length is set longer. For example, FIG.
As shown in FIG. 0 (a), the positive and negative π lengths of the Sinc function are ± 5.
It may be set to π or the π length only on the negative side of the Sinc function may be set to −5π as shown in FIG. Conversely,
Only the positive side may be set to 5π. The positive and negative sides may be set to -5π and 2π.

By setting the π length to be long as described above, the edge portion of the slice characteristic when the saturation pulse PS _sat is applied has a waveform closer to a rectangular shape.
As a result, the gap between the saturation region RG _sat and the imaging region RG _ima can be set smaller. This is the saturation region RG _s
It is also suitable for setting the width of _at to be large, for example, 10 cm or more. Therefore, the number of unsaturated spins in this gap is reduced as much as possible, and it is possible to suppress a situation in which such spins flow into the imaging region and cause signal fluctuation. That is, it is possible to eliminate or suppress the high signal region in the portion (B) of FIG. 5, further reduce the overall signal fluctuation, and more reliably suppress the blood flow artifact.

Note that, in the above configuration, 1)
The flip angle SatFA of the saturation pulse is set equal to the flip angle FA of the imaging RF pulse and slightly suppressed (preferably, a value of 90 ° or less).
2) Selective excitation of a plurality of slices is performed in the order from the downstream side to the upstream side of the main blood vessel, and 3) Improving the edge characteristics of the slice characteristics of the saturation pulse. Only one configuration or only two configurations may be implemented together.

In the above-described embodiment, an example is shown in which the technique of the present invention is applied to imaging based on the 2D multi-slice method. The present invention may be applied between a plurality of thin slabs and a saturation region when imaging a large-sized imaging region. At this time, the slab is moved not only by collecting all necessary data in the first slab and then sequentially collecting data in the second and third slabs, but also by continuously moving the slab for data collection. (See US Pat. No. 5,631,560).

Further, scanning of the imaging area
This may be performed using a pulse sequence based on the FE-based EPI method.

The present invention is not limited to the embodiments described above, and those skilled in the art can further appropriately modify the present invention without departing from the scope of the invention described in the claims.

[0070]

As described above, according to the imaging method and the MRI apparatus according to the present invention, the slice or slice at the time of the main photographing is set so that the flip angle of the saturation pulse is substantially the same as the flip angle at the time of the main photographing. The selective excitation order of the slab is set to a direction from the downstream of the main blood flow inflow direction to the upstream direction (reverse direction), and the slice characteristics of the saturation pulse, particularly the edge characteristics at the end portions, are sharpened. It is carried out alone or in combination as appropriate. Therefore, even if the T1 value of the blood flow is apparently lowered by administering the contrast agent, the effect of spin transfer due to pulsation and the effect of unsaturated spin transfer existing between the saturation region and the imaging region. , The influence of the movement of the saturated spin accompanying the other slice excitation becomes less likely, and the state of the spin saturation is continuously and stably received from the downstream to the upstream. Therefore, blood flow artifacts can be reliably and stably suppressed, and therefore, highly reliable, high-quality and high-resolution MR imaging can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an outline of a configuration of a magnetic resonance imaging (MRI) apparatus according to an embodiment of the present invention.

FIG. 2 is a view for explaining a positional relationship between a saturation region and an imaging region and a slice excitation order.

FIG. 3 is a timing chart showing a scanning method together with a pulse sequence.

FIG. 4 is a diagram illustrating the occurrence of a blood flow artifact.

FIG. 5 is a graph of signal intensity as a simulation result based on a conventional method.

FIG. 6 is a graph showing the size of a ghost as a simulation result based on a conventional method.

FIG. 7 is a graph showing the T1 value dependence of the original image and the ghost signal and the delay time dependence from the R wave, respectively, as simulation results based on the conventional method.

FIG. 8 is a graph of signal strength as a simulation result of the method according to the present invention.

FIG. 9 is a graph showing the T1 value dependence of the original image and the ghost signal and the delay time dependence from the R wave, respectively, as simulation results of the method according to the present invention.

FIG. 10 is a real-time waveform diagram of a saturation pulse with improved slice characteristics, described as another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Static magnetic field magnet 3 Gradient magnetic field coil 4 Gradient magnetic field power supply 5 Sequencer 6 Host computer 8T transmitter 8R receiver 10 Operation unit 11 Storage unit 12 Display 13 Input device

Claims (18)

[Claims] 1. An MR imaging method comprising: applying a saturation pulse for saturating an MR signal from a target flowing into a region of interest set to a subject to the subject; and performing an imaging scan on the region of interest. 3. The MR imaging method according to claim 1, wherein a flip angle of the saturation pulse is set to an angle lower than 90 degrees. 2. After applying a saturation pulse for saturating an MR signal from a target flowing into a region of interest set in the subject to the subject, the region of interest is selectively excited to collect an MR signal. An MR imaging method for executing an imaging scan, wherein a flip angle of the saturation pulse is set to be substantially equal to a flip angle of a pulse train of the imaging scan. 3. The MR imaging method according to claim 2, wherein the flip angle of the saturation pulse and the flip angle of the pulse train of the imaging scan are both 90.
An MR imaging method, wherein the value is set to a value smaller than the degree. 4. The MR imaging method according to claim 1, wherein the object is a blood flow flowing through a blood vessel.
R imaging method. 5. After applying a saturation pulse for saturating an MR signal from an object flowing into a region of interest set in the subject to the subject, an imaging scan is performed on the region of interest by a selective excitation method. In the MR imaging method to be executed, a moving direction of a site selectively excited according to the selective excitation method is set to a predetermined direction with respect to a flow direction of the target. 6. The MR imaging method according to claim 5, wherein the predetermined direction is a direction from a downstream to an upstream in a moving direction of the site to be selectively excited. 7. The MR imaging method according to claim 5, wherein the object is a blood flow flowing through a blood vessel, and the site to be selectively excited is a plurality of slices selected and sliced in the region of interest. An MR imaging method, comprising: 8. The M according to claim 5, wherein
In the R imaging method, the saturation pulse is applied only once every repetition time TR of a pulse sequence used for the imaging scan. 9. An MR imaging method for applying a saturation pulse for saturating an MR signal from a target flowing into a region of interest set to the subject to the subject, and then performing an imaging scan on the region of interest. 3. The MR imaging method according to claim 1, wherein the thickness of the saturation region to which the saturation pulse is applied is increased. 10. The MR imaging method according to claim 9, wherein the object is a blood flow flowing through a blood vessel.
R imaging method. 11. The MR imaging method according to claim 10, wherein the thickness of the saturation region is 10 cm or more in the direction of movement of the object. 12. The MR imaging method according to claim 11, wherein the saturation pulse is an RF with improved edge characteristics when selectively exciting the saturation region.
An MR imaging method characterized by being a pulse. 13. The MR imaging method according to claim 12, wherein the RF pulse has a Sinc function waveform, and at least one of the positive and negative π lengths has a π length of 5π or more. MR imaging method. 14. Three types of MRs comprising the MR imaging method according to claim 2, the MR imaging method according to claim 5, and the MR imaging method according to claim 10.
M wherein at least two types of MR imaging methods are combined and executed.
R imaging method. 15. An imaging scan is performed on the region of interest after applying a saturation pulse for saturating an MR signal from a target flowing into the region of interest set on the object to the object of interest. The MRI apparatus according to claim 1, wherein a flip angle of said saturation pulse is set to an angle lower than 90 degrees. 16. After applying a saturation pulse for saturating an MR signal from a target flowing into a region of interest set in the subject to the subject, the region of interest is selectively excited to collect an MR signal. An MRI apparatus configured to execute an imaging scan, wherein a flip angle of the saturation pulse is set to be substantially equal to a flip angle of a pulse train of the imaging scan. 17. After applying a saturation pulse for saturating an MR signal from a target flowing into a region of interest set to the subject to the subject, imaging scan of the region of interest is performed by a selective excitation method. An MRI apparatus configured to be executed, wherein a direction in which a site selectively excited according to the selective excitation method moves is set to a predetermined direction with respect to a flow direction of the target. 18. An imaging scan is performed on the region of interest after applying a saturation pulse to the object to saturate an MR signal from an object flowing into the region of interest set on the object. The MRI apparatus according to claim 1, wherein a thickness of a saturation region to which the saturation pulse is applied is increased.