JPH04141143A - Magnetic resonance angiography apparatus - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention (Industrial Application Field) The present invention relates to a magnetic resonance angiography apparatus that obtains an angiography image close to an angiography image by subtraction.

(Prior Art) Magnetic resonance angiography is a general term for a method of obtaining an image similar to an angiogram using magnetic resonance (MR) signals. Some of these methods include obtaining MR angiography images using only images obtained under imaging conditions that emphasize blood flow, and others using two sets of two-dimensional or three-dimensional MR signals obtained under different imaging conditions. There are two types of MR angiography images obtained by subtraction between. The different imaging conditions in the latter are such that the MR multiplied signals obtained in both cases are the same for those corresponding to the stationary part of the subject, and that there are some differences for those corresponding to moving parts such as blood flow. This is the condition to do so. Examples of the two imaging conditions include imaging in two different cardiac phases determined from electrocardiogram (ECG) waveforms, a rephasing sequence that performs flow-rephasing, and flow dephasing (f).
There are data collection using a dephasing sequence (low-dephasing), and imaging using the effect of phase shift due to flow. These conditions may also be used in combination.

In general, subtraction of two sets of MR multiplied signals collected by a rephasing sequence and a dephasing sequence is often performed. The phase ψ of the nuclear spin is O, but for the moving blood flow in the blood vessel, the phase ψ of the nuclear spin does not become 0 even at the echo center, causing a phase shift (day phase). be. Therefore, signals from the blood flow are extracted as low signals. A rephase sequence is a sequence that compensates for this phase shift so that the phase ψ of the nuclear spin at the echo center becomes O even for a moving object. Therefore, signals from the blood flow are extracted as high signals. In this way, there is no difference in the signals of both sequences for the stationary part, but there is a difference in the signals of the moving part, so by subtraction between the signals obtained in both sequences,
Signals from the blood flow area are obtained.

In order to increase the signal intensity from the blood flow in subtraction images and improve the ability to depict blood vessels, the rephase sequence generates as many signals as possible from the blood flow area, while the dephase sequence generates as many signals as possible from the blood flow area. It is necessary to prevent this from occurring. In other words, it is necessary to increase the re-phase effect and the day-phase effect respectively. In order to improve the rephasing effect, it is necessary to minimize the phase dispersion among many spins within a voxel of flow-based imaging. For this purpose, it is known that shortening the echo time and reducing the voxel size are effective in addition to performing the reaging itself with high precision. Note that the reduction in voxel size is also effective in meeting the most basic requirement of improving the resolution of three-dimensional magnetic resonance angiography.

Conventionally, in order to completely reduce the signal in the stationary part to O by subtraction, the echo time, which is a parameter unrelated to flow, has been set to the same value for both the rephase sequence and the dephase sequence. Additionally, to improve resolution, the voxel size needed to be as small as possible, and this was also set to the same size for both sequences. In this way, the rephase effect could be increased, but on the contrary, the dayphase effect was reduced. The reason for this is that the amount of gradient magnetic field for the day phase that can be applied within a short echo time is limited, and when the same gradient magnetic field is applied, the smaller the voxel size, the more the phase spread within it. This is because it is small. As a result, the difference between the signals acquired by the rephase sequence and the signals acquired by the dephase sequence becomes small, and blood vessels may not be visualized with sufficient contrast.

(Problems to be Solved by the Invention) The present invention has been made to address the above-mentioned circumstances, and its purpose is to greatly increase the difference between the signals collected by the rephase sequence and the signals collected by the dephase sequence. An object of the present invention is to provide a magnetic resonance angiography apparatus capable of obtaining high-contrast subtraction images.

[Structure of the Invention] (Means for Solving the Problems) In the magnetic resonance angiography apparatus according to the present invention, the echo time in the dephasing sequence is longer than the echo time in the rephasing sequence. In another magnetic resonance angiography apparatus according to the present invention, a pulse for presaturating areas other than the region of interest is added to the dephasing sequence. moreover,
Another magnetic resonance angiography apparatus of the present invention implements a two-echo mode in which the first echo is collected with a rephasing sequence and the second echo is collected with a dephasing sequence.

(Function) According to the magnetic resonance angiography apparatus according to the present invention,
By increasing the echo time, the amount of phase shift increases only in the dephasing sequence, and the dephasing effect that reduces the signal in the flow area can be increased without reducing the rephasing effect, which improves the visualization of blood flow. It is possible to improve the performance and obtain high-contrast subtraction images. Further, according to another magnetic resonance angiography apparatus according to the present invention, it is possible to suppress blood flow signals flowing into the region of interest, and it is possible to collect signals that do not include any signals in the flow portion by a dephasing sequence. It is possible to improve the ability to depict blood flow and obtain high-contrast subtraction images. Furthermore, according to another magnetic resonance angiography apparatus of the present invention, the rephasing sequence has a short echo time and increases the rephasing effect, and the dephasing sequence has a long echo time and increases the dayphasing effect, thereby increasing the contrast of the subtraction image. By simultaneously executing both sequences in two echo modes, the imaging time can be shortened.

(Example) An example of a magnetic resonance angiography apparatus according to the present invention will be described below with reference to the drawings. FIG. 2 is a block diagram showing a schematic configuration of the first embodiment.

Inside the gantry 20, a static magnetic field magnet 1, X-axis, Y-axis, and Z-axis gradient magnetic field coils 2, and a transmitting/receiving coil 3 are provided.

The static magnetic field magnet 1 as a static magnetic field generator is configured using, for example, a superconducting coil or a normal conducting coil. The X-axis, Y-axis, and Z-axis gradient magnetic field coils 2 include an X-axis gradient magnetic field Gx,
This is a coil for generating a Y-axis gradient magnetic field Gy and a Z-axis gradient magnetic field Gz.

The transmitter/receiver coil 3 is used to generate high frequency pulses and detect magnetic resonance (MR) signals generated by magnetic resonance. The subject P on the bed 13 is inserted into an imaging possible area (a spherical area in which an imaging magnetic field is formed, and diagnosis is possible only within this area) in the gantry 20.

The static magnetic field magnet l is driven by the static magnetic field control device 4.

The transmitter/receiver coil 3 is driven by a transmitter 5 during magnetic resonance excitation, and is coupled to a receiver 6 during MR multiplication signal detection. The X-axis, Y-axis, and Z-axis gradient magnetic field coils 2 are driven by an X-axis gradient magnetic field power supply 7, a Y-axis gradient magnetic field power supply 8, and a Z-axis gradient magnetic field power supply 9.

The X-axis gradient magnetic field power supply 7, the Y-axis gradient magnetic field power supply 8, the Z-axis gradient magnetic field power supply 9, and the transmitter 5 are driven by a sequencer 10 according to a predetermined sequence, and the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient A magnetic field G2 and radio frequency (RF) pulses are generated in a predetermined pulse sequence described later. In this case, the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field Gz are mainly, for example, a readout gradient magnetic field Gr,
They are used as a gradient magnetic field Ge for encoding and a gradient magnetic field Gs for slicing, respectively. The computer system 11 drives and controls the sequencer IO, captures the spin echo signal as an MR multiplied signal received by the receiver 6, and performs predetermined signal processing to obtain a tomographic image of a predetermined slice site of the subject. An image is generated and displayed on the display unit 12.

A pulse sequence for MR excitation/MR data acquisition in an embodiment with such a configuration will be explained with reference to FIG. Here, a gradient field echo method will be described as an example. The first sequence in Figure 1 shows a rephasing sequence, and the second sequence shows a dephasing sequence.Although not shown, during the period when the sequence shown in Figure 1 is performed, the static magnetic field A static magnetic field is statically applied to the subject P by a static magnetic field magnet l driven by the control device 4 .

By applying an RF pulse consisting of a selective excitation pulse while applying a slicing gradient magnetic field Gs to the subject P, magnetic resonance is excited in the nuclear spins of a specific region (region of interest) of the subject P, and the primary readout is performed. By applying a gradient magnetic field Or, the nuclear magnetization of nuclear spins excited by magnetic resonance is rephased, and by applying an encoding gradient magnetic field Ge with an amplitude corresponding to the encoding step, phase encoding is performed, and then Collect echo signals. RF pulses are generated at regular intervals TR. After the rephasing sequence, a dephasing sequence is performed. Also in the dephasing sequence, by applying a readout gradient magnetic field Gr, the nuclear magnetization of nuclear spins excited by magnetic resonance is dephased, and the encoding step is Applying an encoding gradient magnetic field Ge with an amplitude corresponding to
The above-described sequence for performing phase encoding is repeated while changing the amplitude of the encoding gradient magnetic field Ge by a predetermined value at each encoding step.

The echo time TEd of the dephasing sequence is set longer than the echo time TEr of the rephasing sequence. Dephasing is for day-phasing (i.e. for flow encoding, or bipolar type) during the echo time.
is caused by applying a gradient magnetic field. By lengthening the echo time TE, the application time of this flow encoding gradient magnetic field can be lengthened, and therefore, the phase shift can be made thicker (then, the amount of phase shift can be increased).
The day phase effect becomes large and signals from moving parts are reduced.

However, if the echo time is changed depending on the sequence, the signal strength of the stationary part may change between the two.For example, if T E = 6m5ec (rephase sequence) and 12m5ec (dephase sequence), the brain background The signal strength of the white matter (WM) and gray matter (GM) during the day phase is approximately 92 to 93% of that during the rephase.

In order to extract a blood vessel image, it is not necessarily necessary to make the background part completely zero, but depending on the case, it may be possible to perform weighting and subtraction.As explained above, according to the first embodiment, For example, since the echo time of the dephasing sequence is made longer than that of the rephasing sequence, the dephasing effect can be increased without reducing the rephasing effect, which improves the subtraction result.
It is possible to obtain high-contrast images of moving parts.

Next, another embodiment of the invention will be described. The structure of the other embodiments is the same as that of the first embodiment shown in FIG. 2, so the explanation of the structure will be omitted. Rephasing Sequence of Second Embodiment The pulse sequences of the dephasing sequence are shown in FIGS. 3 and 4, respectively. The rephasing sequence shown in FIG. 3 is the same as the rephasing sequence of the first embodiment. The dephase sequence shown in FIG. 4 is obtained by adding a pre-saturation pulse to the first embodiment. In the second embodiment, the echo time may be the same in both sequences or may be varied as in the first embodiment.

Assuming that the region of interest to be selectively excited by the rephase sequence is the region shown in FIG. Figure 5(b)
Both sides of the region of interest can be saturated as shown by the diagonal lines. After this, when the same sequence as in the first embodiment is executed, the blood flow signal flowing into the region of interest is suppressed, and the dephase sequence collects data in which no blood flow portion appears on the image.

However, in such a presaturation method, it is not preferable to alternately repeat the rephasing sequence and dephasing sequence once each time, because the influence of the saturation effect due to bripulse may be affected when data is collected in the next rephasing sequence. This is because the signal from the blood flow portion cannot be made sufficiently large. Therefore, in the second embodiment, data collection using the rephase sequence R and data collection using the dephase sequence D are basically performed separately, as shown in FIG.

However, since an actual device undergoes some kind of change over time, if images are taken separately, some deviation may occur between the two data, which may cause a positional deviation between the two images. Therefore, as shown in Figure 7,
It is best to collect data for rephase sequence R to some extent all at once, then collect data for dephase sequence D in the same way, and repeat this process.For example, in the case of data collection using the 3DFT method, each slice Each sequence may be performed together for each direction encoding, ie, for 256×256×32 data acquisition, the rephase sequence is performed in 256 encoding steps, and the dephasing sequence is similarly performed in 256 encoding steps.

This is done 3 times while changing the amount of phase encoding in the slice direction.
Repeat twice.

Next, estimate the time required for the region of interest to become saturated after applying the Bripulse. 1. Assuming that the flow velocity of the major intracranial blood vessels is 50 cm/sec, the blood flow winds through the region of interest with a thickness of 5 cm. The time required to pass through the area is estimated to be 1 sec. Then, T R==
When loOmsec is assumed, about 10 encoding steps are required from application of the pre-pulse to saturation.

Therefore, it is considered that about 10 encoding steps are required to change the saturated/unsaturated state of the blood flow spins flowing into the region of interest depending on the presence or absence of presaturation. When executing both sequences alternately with 256 encoding steps as in the example above, there is no substantial effect even if the blood flow spins flowing into the region of interest are unsaturated for the first 10 encoding steps. This situation is shown in Figure 8.

To deal with this delay in changing the saturation/unsaturation state, the timing of applying the pre-pulse may be advanced and the pre-pulse may be applied from the last few sequences of the rephase sequence group, as shown in Figure 9. Then, the saturation/desaturation state in the blood vessel of interest can be adjusted to the data collection of the dephase sequence and rephase sequence, and the saturation effect in the region of interest is saturated when necessary, and desaturated when it is not necessary. be able to.

According to the second embodiment as well, the signal intensity of the blood flow portion in the data collected by the dephasing sequence can be reduced, and a subtraction image with high contrast can be obtained. Note that in the second embodiment as well, the effect can be further improved by lengthening the echo time of the dephase sequence.

Next, a third example will be described. The two embodiments described above are 2
Since two sequences are performed and subtraction is performed, even if the echo times are different in both sequences,
Even if they are equal, the total imaging time T = TRXNA (number of additions) x NE (number of encodes) x NS (number of encodes in the slice direction) x 2, which is 2 times longer than normal imaging.
It takes twice as long. Therefore, in the third embodiment, two echo modes are used to collect data using both a rephasing sequence and a dephasing sequence in one MR excitation, which reduces the imaging time compared to the two embodiments described above. can be halved. Or, conversely, if the same imaging time is compared, the S/N ratio will be f2 times higher.

The pulse sequence of the third embodiment is shown in FIG.
After applying the pulse, first, a gradient magnetic field for a rephase sequence similar to that in the first embodiment is generated, and a first echo is collected. At this time, the echo time TER is short, for example, 5 to 8 m.
Set to 5ec. After collecting the first echo, continue to collect the first echo.
A gradient magnetic field for a dephasing sequence similar to that in the example is generated, and a second echo is collected. The echo time TEd of the dephasing sequence is set to be sufficiently long as in the first embodiment so that a sufficient gradient magnetic field for the dayphase is applied.

In this way, in the 2-echo mode, by collecting the first echo using the re-phase sequence and collecting the second echo using the de-phase sequence, the echo time of the de-phase sequence can be changed by the re-phase sequence as in the first embodiment. The contrast, S/
A subtraction image with a high N ratio can be obtained.

Note that the present invention is not limited to the embodiments described above, and can be implemented with various modifications without changing the gist of the invention. For example, although the above embodiments used a gradient field echo method, spin echo A method, a three-dimensional Fourier transform method, etc. may be used, and each embodiment may be combined as appropriate.

[Effects of the Invention] As explained above, according to the present invention, the rephase sequence can increase the signal of a moving part, the dephasing sequence can reduce the signal of a moving part, and subtraction can reduce the motion. A magnetic resonance angiography apparatus is provided that can obtain high contrast images of a portion.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a pulse sequence of a first embodiment of a magnetic resonance angiography apparatus according to the present invention, and FIG.
A block diagram showing the schematic configuration of the embodiment, FIGS. 3 and 4 are diagrams showing the rephase sequence and dephase sequence in the second embodiment, respectively, FIG. 5 is a diagram showing the state of selective excitation, and FIG. 6 , FIG. 7 is a diagram showing the execution order of the rephasing and dephasing sequences in the second embodiment, FIGS. It is a figure which shows the pulse sequence of 3rd Example. DESCRIPTION OF SYMBOLS 1... Static magnetic field magnet, 2... X-axis, Y-axis, Z-axis gradient magnetic field coil, 3... Transmission/reception coil, 4... Static magnetic field control device, 5... Transmitter, 6...・Receiver, 7...X-axis gradient magnetic field critical condition 8...Y-axis gradient magnetic field type temperature 9...Z
Axial gradient magnetic field critical condition 10-...Sequencer, 11...
Computer system, 12...display section. Applicant's agent Patent attorney Takehiko Suzue (a) (b) Figure RR-1-DD Figure 8

Claims (3)

[Claims] (1) In a magnetic resonance angiography device that performs subtraction between signals collected by a rephasing sequence and signals collected by a dephasing sequence, the echo time in the dephasing sequence is longer than the echo time in the rephasing sequence. Features of magnetic resonance angiography equipment. (2) In a magnetic resonance angiography apparatus that performs subtraction between signals collected by a rephasing sequence and signals collected by a dephasing sequence, adding a pulse to the dephasing sequence to presaturate areas other than the region of interest. A magnetic resonance angiography device featuring: (3) In a two-echo mode magnetic resonance angiography device that performs subtraction of signals collected by a rephasing sequence and signals collected by a dephasing sequence, the first echo is collected by the rephasing sequence, and the second echo is A magnetic resonance angiography device that collects images using a dephasing sequence.