JPH08154919A - Imaging method with magnetic resonance imaging method - Google Patents

Abstract

PURPOSE: To provide an imaging method which achieves a measurement in a short time without the lowering of the quality of picture in substance in an MRI apparatus used for IVR. CONSTITUTION: An operation device such as catheter 200 used for IVR is provided with an index 210 identifiable on an image to operate a sequence of measuring an object to be inspected over a preset area 10 and a sequence of measuring an area 20 containing the index as target and images obtained by both the sequences are combined to make and display one image. Here, in the sequence of measuring the area 20, an image pickup parameter is so set that the space resolving power (dimensions of area L1 /number of matrixes) thereof is higher than that in the measurement for the area 10 and a measurement is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging method in a magnetic resonance imaging apparatus (hereinafter abbreviated as MRI apparatus), and in particular, it is intended to make an operator easily view an image taken in an IVR using a catheter or the like. It relates to an imaging method.

[0002]

2. Description of the Related Art An MRI apparatus displays a density distribution, relaxation time distribution, etc. of nuclear spins in a subject as a tomographic image by processing a signal measured by utilizing an NMR (nuclear magnetic resonance) phenomenon. To do. For this reason, the subject is inserted into the space inside the magnet that generates a uniform static magnetic field, and the high frequency magnetic field is placed in the space to apply the high frequency magnetic field in a pulsed manner and the gradient magnetic fields in the three-axis directions as appropriate. The high-frequency coil for reception receives the NMR signal generated from the sample. Then, the tomographic image of the region of interest of the subject is obtained by processing the signal to which the position information is added by the gradient magnetic field. In this case, the cross section is selected by the gradient magnetic field in the one axis direction, the signal is frequency-encoded for the other one axis, and the signal is measured while changing the gradient magnetic field strength for the other one axis (referred to as a phase encode axis). , The required signal for one image for the selected cross section can be obtained.

By the way, in recent years, IVR (interventional ra
There is a dilogy). The IVR is for performing treatments such as treatment and surgery while imaging and monitoring a specific region of the observer. For example, as shown in FIGS. 5 and 6, the catheter 200 is inserted into the blood vessel from the base of the foot of the subject 100. When inserting into a blood vessel and performing procedures such as embolization and expansion of the blood vessel lumen, injection of drugs, etc.,
A treatment portion such as a catheter or a blood vessel is monitored by an X-ray fluoroscope or an endoscope. As such an IVR monitor device, research that uses an MRI device in place of the above-mentioned conventional device is becoming active due to advantages such as no exposure to X-rays. When the MRI apparatus is used for IVR, the region of interest 10 including the tip of the catheter is measured in the examples shown in FIGS.
Images need to be displayed in substantially real time.

[0004]

As described above, when the MRI apparatus is used as an IVR monitor apparatus and an image is provided in real time, the time taken to form one image is required to be as short as possible. As described above, in the MRI apparatus, it is necessary to phase-encode the signal in order to obtain one image, and one measurement requires a relatively long time.

In general, the time required for measurement in an MRI apparatus depends on the number of phase encodes and the number of signal samplings, that is, the number of measurement matrices (M ₁ × M ₂ ). The larger the number of matrices, the longer the measurement time. . On the other hand, the spatial resolution of an image is the size of the imaging region divided by the number of matrices, and when the size of the imaging region is constant, the larger the matrix number, the higher the spatial resolution. Therefore, the measurement time can be shortened by reducing the number of matrices, but this lowers the spatial resolution and deteriorates the image quality. In order to shorten the measurement time without deteriorating the spatial resolution, it is necessary to reduce the number of matrices and narrow the imaging area accordingly. But,
When the imaging area becomes narrow, it becomes difficult to grasp the overall positional relationship, which causes a problem. As described above, in the conventional technique, it is difficult to shorten the measurement time without adversely affecting the image quality.

The present invention solves the above-mentioned problems and provides MRI.
An object of the present invention is to provide an imaging method capable of performing measurement in a short time without deteriorating substantially the image quality in the apparatus. Another object of the present invention is to provide an imaging method suitable as a monitor for IVR, and particularly to provide an imaging method capable of real-time image display while maintaining high spatial resolution in a necessary region. .

[0007]

In order to achieve the above object, the imaging method of the present invention provides a first sequence for measuring a relatively wide region and a high space for a region within the first sequence and a region smaller than the first sequence. It is executed in combination with the second sequence measured at the resolution and displayed as one image. On this occasion,
The area measured in the second sequence is an area including the area indicated by the index, which is previously given to the inspection object by indicating the specific position of the inspection object in the image.

Further, the imaging method of the present invention repeats the measurement by the first and second sequences to sequentially display a plurality of images in real time. At this time, in a preferred aspect, the center position of the measurement region of the second sequence is the direction of movement of the index when the index moves.
Position is controlled according to speed. Further, the number of times the second sequence is executed is larger than the number of times the first sequence is executed.

[0009]

After the signal is acquired and image processed in the first sequence in a relatively wide area, a narrow area including a portion indicated by the index in the image is measured with a high spatial resolution in the second sequence, and both images are measured. By integrating
The positional relationship in the entire narrow area is shown, and an image with high resolution in that area can be displayed in a short time. Therefore, it is possible to display an image suitable for IVR in which the resolution of the region of interest is high without substantially lowering the image quality.

By controlling the measurement center of a narrow area based on the information on the moving direction and speed of the index, it is possible to prevent the image from becoming difficult to see by unnecessarily moving the center position, or to follow the movement of the index. It is possible to move the center position. In this case, the center position can shift the image center from the position of the index depending on which of the indices has more important information.

For a wide area with little change, the number of times the first sequence is executed is reduced to shorten the measurement time, and for an area with many changes, the number of times the second sequence is executed is increased. , The time resolution for that part can be increased.

[0012]

Embodiments of the present invention will be specifically described below with reference to the drawings. Also in this embodiment, as shown in FIGS. 5 and 6, the catheter 20 is inserted from the base of the foot of the subject 100.
An example will be described of imaging in IVR in which 0 is inserted into a blood vessel, and procedures such as embolization and expansion of the blood vessel lumen and drug injection are performed. The subject 100 is placed in a static magnetic field space of an MRI apparatus (not shown), a high frequency magnetic field pulse and a gradient magnetic field are applied in a predetermined pulse sequence controlled by a sequencer of the MRI apparatus, and acquisition of an NMR signal and image processing are performed. Then, the image is displayed in real time on a monitor (not shown).

In the illustrated embodiment, an index 210 is provided to identify the tip of the catheter. Index 21
0 may be any as long as it can be identified as an index in the image when the region including it is imaged, for example, one that generates an NMR signal having a higher density than biological tissue,
On the contrary, a substance that does not generate an NMR signal, a magnetic substance, or the like is used. If the catheter 210 itself can be sufficiently discriminated from the living tissue, it functions as an index by itself, and thus it is not necessary to provide another index.

In the imaging method of this embodiment, as shown in FIG.
6, a first sequence for measuring an NMR signal from a relatively wide region 10 and a second sequence for measuring an NMR signal in a narrow region 20 including the index 210 in the region 10 as shown in FIG. To execute. At this time, a sequence with high spatial resolution is executed in the narrow region 20 including the distal end portion 210 of the catheter 200. Generally, in the IVR, the actual work for performing the treatment is limited to the periphery of the catheter, and information on the relative positional relationship may be obtained in the peripheral portion. In other words, the spatial resolution in the peripheral area is somewhat
There is no problem even if it is low. Here, the spatial resolution is given by spatial resolution d = area diameter / matrix number (L / M), where L is the field of view (area) diameter of the image and M is the number of matrices. The imaging parameter may be set so that the value of "M" becomes small. As the first and second sequences, an echo planar method (EPI) capable of forming one image in a short time by repeating a few or few pulse sequences is used.
Method) or a high speed spin echo method (FSE method).

In the imaging method of this embodiment, first, the number of matrices (the number of phases and frequency encodes) is set so that a normal spatial resolution can be obtained for the region 10, for example, and it is necessary for one image by the EPI method. Measure the signal. The signal acquired by this first sequence is subjected to image processing to obtain, for example, an NM having a higher density than that of living tissue.
In the case of an index that generates an R signal, the brightest part is recognized as the index, and then the area 20 centered on this index is used.
For, the second sequence is executed. At this time, matrix number first measurement smaller, for example as a matrix number M ₁ of _{1/2 '(= M 1/2} ), measures the signals necessary for one image for the region 20. in this case,
The spatial resolution of the region 10 obtained by the first measurement is d (≡L ₁ / M ₁ ) when considering the vertical direction of FIG.
And the spatial resolution d '(≡L ₁ ' / M ₁ ') in the narrow region 20 near the catheter is M ₁ ' = M _1/2 , where L ₁ 'is 1/5 of L _1. Therefore, d ′ = 5 × d /
2, which is 2.5 times d. Moreover, the measurement time for the region 20 is 1/2 compared with the measurement for the region 10 because the number of matrices is 1/2, and the total measurement time of the measurement of both sequences is only 1.5 times. Do not extend.

Next, one image is composed of the image data of the area 10 and the image data of the area 20 obtained by such two sequences. At this time, the image data obtained in the first sequence is used for the portion (outer peripheral portion) of the region 10 excluding the region 20, and the image data obtained in the second sequence is used for the region 20. As a result, the spatial resolution of the central region 20 is 2.
It is possible to obtain an image which is increased by a factor of 5 and the spatial resolution of the outer peripheral portion is the same as the conventional value. By repeating such measurement, a plurality of images are sequentially obtained and these images are displayed in real time.

The number of matrices and the size of the regions in both of these measurements are the characteristics of the tissue to be measured and the IV.
It can be appropriately selected depending on the size of the region to be subjected to R treatment.
For example, as described above, when it is sufficient to display the area 10 to such an extent that the positional relationship of the central area 20 can be understood, the measurement time can be shortened by reducing the number of matrices in the outer peripheral portion and decreasing the spatial resolution. You can also

The size L of the region in the frequency encoding direction is

[0019]

[Equation 1] (Wherein G is the gradient magnetic field strength at the time of reading, Δt is the sampling interval, N is the number of samples, and T is the sampling time). Therefore, if the measurement conditions such as the sampling interval are the same, The range can be selected by changing the sampling number N of the signal. Further, the size Lp of the region in the phase encode direction is

[0020]

[Equation 2] (Gp is the intensity of the phase encode gradient magnetic field, Δτ is the application time of the phase encode gradient magnetic field, and M is the number of samplings)
Is represented by Sampling number M and gradient magnetic field strength Gp
By changing the, the size of the imaging area and the spatial resolution can be selected.

In the embodiment shown in FIG. 1, the position of the index 210, that is, the tip of the catheter is aligned with the central position of the central region 20, but the central position of the region 20 is aligned with the position of the index. It is not necessary, and the region 20 may be deviated in that direction depending on which direction has more important information in relation to the catheter. 2 and 3 show such an example. Here, the center of the imaging region is biased toward the front (FIG. 2) or the rear (FIG. 3) of the catheter, rather than measuring symmetrically with respect to the tip of the catheter. This is because when the catheter is being moved, the previous information in the moving direction (indicated by the arrow in the figure) is often more important. Also in this case, it is the same as in the embodiment of FIG. 1 that the imaging area around the tip portion of the catheter is measured with high spatial resolution.

In actual measurement, it is effective to detect the moving direction of the catheter and automatically bias the center of the imaging region 20 having a high spatial resolution in accordance with the moving direction. To detect the moving direction, a means such as temporally differentiating the measured raw data or image can be used. The center position of the imaging region can be easily set by rewriting parameters in the CPU that controls the MRI apparatus, such as changing the center frequency of the high-frequency magnetic field.

However, if the center of the image changes following an extremely small movement during the operation, it may be rather difficult to see. Therefore, in order to make the image easier to see, it is effective to detect the moving speed or acceleration in addition to the moving direction of the index (catheter tip) 210, and control according to the magnitude. For example, a certain lower limit may be set for the moving speed or the acceleration, and the imaging center may be moved only when the lower limit is exceeded. Note that the moving speed and acceleration of the index can be obtained by temporally differentiating the images as described above.

In the above embodiment, the case where two sequences are sequentially repeated to obtain a plurality of images has been described, but it is not necessary to repeat the same number of the first and second sequences. In other words, in the IVR, in a region away from the catheter, it often causes no problem if not only the spatial resolution but also the temporal resolution is low. Therefore, as shown in FIG. 4, the sequence 50 for imaging the vicinity of the catheter tip is performed more frequently than the sequence 60 for imaging the peripheral portion. In the figure, for one execution of the first sequence 60, the second sequence 50
Is continuously executed four times. By doing this,
The vicinity of the catheter of interest is always displayed in real time, and the peripheral portion where information is not required is updated at a certain time interval. The ratio of the number of execution times of both sequences may be appropriately specified according to the situation, and there is no particular limitation.

The case of using a catheter as an example of IVR has been described above. However, the present invention is not limited to this, and even when a surgical instrument other than a catheter is used, the surgical instrument can be used as an index. It is possible to apply the imaging method of the present invention with or with a surgical instrument as an index.

[0026]

As is apparent from the above description, according to the imaging method of the present invention, the measurement of a wide area and the measurement of a narrower area than that indicated by the index are combined to form one image. As a result, the measurement can be performed in a short time without substantially lowering the image quality, and an image suitable for IVR having a high resolution of the region of interest can be displayed. Further, by making the number of times of the two measurements different, it is possible to display an image of a region of more interest in real time with high time resolution.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an imaging region according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing an imaging region according to another embodiment of the present invention.

FIG. 3 is a diagram schematically showing an imaging region according to another embodiment of the present invention.

FIG. 4 is a diagram schematically showing an embodiment of an imaging sequence according to the present invention.

FIG. 5 is a side view showing a situation of an IVR to which the present invention is applied.

6 is a front view of FIG.

[Explanation of symbols]

 10 imaging region 20 high-resolution imaging region 50 first sequence 60 second sequence 100 subject 200 catheter 210 index

Claims (4)

[Claims] 1. A high-frequency magnetic field and a gradient magnetic field are applied to an inspection target placed in a magnetic field in a predetermined sequence, and a magnetic resonance signal generated from the inspection target is detected by a nuclear magnetic resonance phenomenon. An imaging method by a magnetic resonance imaging apparatus for forming a tomographic image of the inspection target based on, an index indicating a specific position of the inspection target in the image is given to the inspection target, and the inspection target is measured over a preset region. 1 sequence and a second sequence for measuring a region including the index in the measurement region of the first sequence and narrower than that with a higher spatial resolution than the first sequence, and performing both sequences. The magnetic resonance imaging apparatus is characterized in that the images obtained in step 1 are combined and displayed as one image. Imaging method. 2. The imaging method according to claim 1, wherein a plurality of images are sequentially displayed by repeating the measurement by the first and second sequences. 3. The center position of the measurement region of the second sequence is controlled according to the direction and speed of movement of the index when the index moves. Imaging method. 4. The second sequence is executed more times than the first sequence.
The described imaging method.