JPH01166751A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE:To obtain an angiogrm and a blood flow velocity image having no artifact by a small inclined magnetic field power supply without elongating an echo time to a necessary degree or more, by arranging the flow encoding pulses of an inclined magnetic field having timewise allowance so that homospoit effect becomes max. CONSTITUTION:An examinee 102 is placed in a magnet 101 and a high frequency wave is generated by a transmission system 106 to irradiate the examinee 102 from a transmission coil 104. A nuclear magnetic resonance signal is received by a receiving coil 105 to be converted to audible frequency by a transmitting- receiving system 106 while said frequency is converted to a digital signal by an A/D converter 107 to be processed by a computer 109 to be displayed on a display device 110. An inclined magnetic field applying position data necessary for imaging is applied by an inclined magnetic field power supply 108 and an inclined magnetic field coil 103. The flow encoding pulse of a slice axis having timewise allowance is arranged so as to maximize homospoil effect. With respect to slice axes on and after a 180 deg.-pulse, the effect of an inclined magnetic field of positive polarity becomes large and the phase of dummy FID comes apart and no artifact is generated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magnetic resonance imaging apparatus, and particularly to a magnetic resonance imaging apparatus suitable for removing artifacts during imaging of blood vessels and blood flow velocity images.

[Conventional technology]

The known examples simply explain the method of drawing blood vessels.
The pulse sequence is devised so that pseudo FID artifacts from 180'' pulses do not occur.

Details will be explained in the examples, but the above-mentioned known example uses a flow insensitive sequence (phase insensitive sequence, Flow Insensitive Se
guence, Flow Rephased Sefue
flow sensitive sequence (phase sensitive sequence, also called Flow 5
uence.

Two images are taken in two sequences (also called Flow Dephased 5effence) and subtracted from each other to create an image of only the blood flow area. In the sequence diagram of the above-mentioned known example, the readout gradient magnetic field (Gr) is insensitive to blood flow. In other words.

The pair of the positive pulse before the 1806 pulse and the first half of the readout pulse up to the center of the echo is one flow encode pulse (in the above-mentioned known example, bipolar
(Although it is called gradient pulse, it is synonymous.) Also, there is another negative pulse before and after the 180° pulse.
two flow encode pulses. Since these two flow encode pulses have opposite phase shifting effects on the same fluid, they cancel each other out, resulting in no one phase shift and a large signal is obtained from the blood flow section.

At this time, by devising the arrangement so that the magnitude of the negative pulse (the product of intensity and time) is at least twice the magnitude of the positive pulse, the pseudo FID from the 1806 pulses is generated in the signal sample. I am trying to prevent it from appearing as an echo.

[Problem that the invention seeks to solve]

In order to realize the sequence of the above-mentioned known example, it is necessary to generate a negative gradient magnetic field that is larger than the gradient magnetic field strength at the time of readout, and a very large gradient magnetic field power source is required. In addition, if you try to realize the sequence of the above-mentioned known example with a small gradient magnetic field power supply, you will have to increase the application time of the negative gradient magnetic field, which will increase the echo time, and when dealing with moving objects such as blood flow, It will be very disadvantageous.

The purpose of the present invention is to solve these problems and to provide a magnetic resonance imaging apparatus that can obtain artifact-free blood vessel images, blood flow velocity images, etc. without unnecessarily lengthening the echo time using a small gradient magnetic field power source. It is about providing.

[Means for solving problems]

This objective is achieved by arranging the flow-encoding pulses of the gradient magnetic field, which are well-timed, such that the homospoiling effect is maximized.

[Effect]

By the means described above, an artifact-free blood vessel image and blood flow velocity image can be obtained using a small gradient magnetic field power supply without making the echo time longer than necessary.

〔Example〕

Embodiments of the present invention will be described with reference to FIGS. 1 to 3.

FIG. 1 shows a configuration diagram of an embodiment of the present invention. A subject 102 is placed in a magnet 101 that generates a uniform static magnetic field, and a transmission system 106 generates a high frequency necessary to generate magnetic resonance for zH in the subject, and the high frequency is transmitted to a transmitting coil. 104 irradiates the subject 102. After irradiation for a certain period of time, the receiving coil 105 detects a nuclear magnetic resonance signal emitted by the IH in the subject, and the detected resonance signal is converted to an audio frequency by the transmitting/receiving system 106, and further converted to an audio frequency by the A/D converter 107. It becomes a digital signal. The signal is subjected to necessary processing by the computer 109, and the resulting image is displayed on the display device 110. Further, a gradient magnetic field for adding position information necessary for imaging to a signal is generated by a gradient magnetic field power supply 108 controlled by a computer 109 so as to satisfy predetermined necessary conditions, and a gradient magnetic field connected to the power supply. Applied by coil 103. Further, the transmission/reception system 106 is similarly controlled by a computer.

FIG. 2 shows a basic spin echo pulse sequence and explains the principle of imaging.

The magnetization of a subject placed in a static magnetic field points in the direction of the static magnetic field. When a 90' pulse 201 is irradiated thereon, the magnetization of the subject is oriented in a direction perpendicular to the static magnetic field, and an NMR signal having a frequency proportional to the static magnetic field strength is generated. This signal attenuates rapidly due to the inhomogeneity of the static magnetic field in addition to the effects of transverse relaxation, but when the 180" pulse 202 is applied after τ time, a strong signal 203 is generated again after another τ time. This signal Sample this measurement for a certain period of time TR
Repeat each time. In order to provide position information to this signal, three mutually orthogonal gradient magnetic fields are used. Measurements are repeated by changing the gradient magnetic field application method little by little.

First, in order to cut out a plane, slicing gradient magnetic fields 204 to 206 with the same output value are applied for each measurement when applying 90'' and 180° pulses 201 and 202.Then, the frequencies of 90@pulse and 180@pulse are applied. Only the magnetization with the magnetic field strength corresponding to is excited. This is called slicing.

Regarding the magnetization within the plane, frequency encoding gradient magnetic fields 208 to 209 and phase encoding gradient magnetic field 207 are applied to provide position information within the plane. Frequency encoding is a method of applying a gradient magnetic field with the same output value for each measurement during signal sampling 210, and making the frequency of the NMR signal correspond to the signal generation position.

Moreover, phase encoding is a method of applying a gradient magnetic field that changes for each measurement before signal sampling, and making the change in phase of each measurement correspond to the position.

An NMR signal S when there is a density of stationary hydrogen atoms in one pixel in a plane cut out by slicing, and the frequency is encoded by a gradient magnetic field for frequency encoding into a frequency wk, and the gradient magnetic field for phase encoding is encoded into a phase change θ.
o Dt n) is 5o(t, n)=ζexp (jw
kt) exp (j 13 an) − (2). This is first Fourier transformed in the time axis direction to obtain So(w, n) = ζδ(W - Wk) exp(jθ, n
>-(3) Next, Fourier transform is performed in the phase axis direction, that is, in the measurement number axis direction, and 5o (W, θ) = ζδ (W Wk) δ (e - a
t>...C4), and (Wk, 0
It is converted into an image signal with intensity ζ at the point of

Next, the flow encode pulse will be explained.

Regarding the principle of flow encoding pulses, see Miyamoto et al.
Please refer to "Blood flow measurement by MRI".

By using a plurality of these flow encode pulses, a blood flow insensitive sequence (phase insensitive sequence, Flow In5en) that does not give any phase change to the NMR signal from the constant velocity fluid
sitive 5equence, Flow Rap
hased sequence) and a blood flow sensitive sequence (phase sensitive sequence, Flow 5eus') that causes a large phase change in the NMR signal from a constant velocity fluid.
1tive 5equence, Flow Dep
(also called hased 5aquence) can be created.

For example, when a human body is photographed using a blood flow insensitive sequence, there is no phase change in the signal from the flow area, that is, the blood flow area.
Since the phases are aligned, a large signal can be obtained from the blood flow area. When imaging with the blood flow sensitive sequence, the signals from the blood flow part change in phase depending on the velocity, so the phases are not aligned and a large signal cannot be obtained. Since the sizes are the same, if two images are taken in a blood flow insensitive sequence and a blood flow sensitive sequence and subtracted from each other, an image of only the blood flow part can be obtained.

In this way, a sequence utilizing flow encoding pulses is used. In particular, the blood flow insensitive sequence is a very important sequence in the sense that it obtains signals from the blood flow section.

FIG. 3 shows a blood flow-insensitive spin echo sequence created by incorporating a plurality of flow encode pulses into the normal spin echo sequence of FIG. 2. 301 to 310 are the second
Same as 201 to 210 in the figure.

Paying attention to the slice axis, the second half from the center of the 90″ pulse 304 and the second half of the 90″ pulse 305 are one flow encode pulse (
One flow encode pulse (FE2) in the first half and the second half of the 306 (7) 180' pulse, 31
One flow encode pulse (FE3) at 1 and 312
, FEI and FE2 cause a phase shift in the same direction for the same fluid, and FE3 causes a phase shift in the opposite direction. In other words, the effects of FEI, FE2, and FE3 cancel out the phase shift, and as a result, no phase shift occurs.

Also, if you pay attention to the readout axis, the first half of 308 and 309 up to the echo center is one flow encode pulse (FE4), and 313 and 314 are one flow encode pulse (FE5), which are opposite to each other for the same fluid. directional phase shifts, and the phase shifts cancel each other out, resulting in no phase shift. Pseudo FID generated from 1806 pulses of this sequence
If you pay attention to , the first half of 314 and 309 have opposite polarities and the same size, so the spins in the readout direction align in phase at the center of sampling, and since 306 and 312 have opposite polarities, the spins in the slice direction align. Although the phases are not perfect, they are in the direction of aligning. Therefore, the pseudo FID becomes an echo at the center of sampling and enters the image as an artifact.

Therefore, as shown in FIG. 4, the flow encode pulses on the slice axis, which have sufficient time, are arranged so that the Homosboil effect is maximized. That is, the application interval of 411 and 412 is made as large as possible, and the flow encode pulses of 415 and 416, which have the same effect as the increment of phase shift, are incorporated. At this time, the application interval is made as short as possible. Then, on the slice axis after the 180° pulse, the effect of the positive polarity gradient magnetic field becomes large, and the phase of the pseudo FID becomes disjointed. Therefore, no artifacts occur.

〔Effect of the invention〕

The present invention enables artifact-free vascular images without making the echo time unnecessarily long with a small gradient magnetic field power source.
A blood flow velocity image can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a pulse sequence diagram of a normal spin echo, Fig. 3 is a pulse sequence diagram of a spin echo insensitive to blood flow, and Fig. 4 is a diagram of an embodiment of the present invention. FIG. 3 is a pulse sequence diagram of a spin echo that is insensitive to blood flow. 101... Magnet, 102... Subject, 103...
Gradient magnetic field coil, 104... Transmission coil, 105...
・Reception coil, 106... Transmission/reception system, 107・
...A/D converter, 108... Gradient magnetic field power supply, 109
...Calculator, 110...Display device.

Claims (1)

[Claims] 1. Apply a high-frequency magnetic field to the subject placed in a static magnetic field,
In a magnetic resonance imaging apparatus that performs image reconstruction processing on nuclear magnetic resonance signals emitted from a subject to obtain images, a gradient magnetic field pulse called a flow encode pulse that gives a constant phase change to an NMR signal from a constant velocity fluid is used. Magnetic resonance imaging is characterized in that, when controlling the phase of the NMR signal from the subject, flow encode pulses on the gradient magnetic field axis with sufficient time are arranged so as to maximize the homospoiling effect. Device.