JP3197643B2 - MRI equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI apparatus for photographing a cross section of an object by utilizing a nuclear magnetic resonance phenomenon. Specifically, the RARE (Fast SE) method is used in which scanning can be completed in a short time by giving a plurality of inversion RF pulses and a plurality of phase encodings to one excitation to obtain a plurality of echoes (multi-echoes). MRI apparatus.

[0002]

2. Description of the Related Art Conventional MRI using the Fast SE method
The device will be described with reference to FIG. In the figure, reference numeral 201 denotes a magnetic moment in a desired slice oriented in the direction of the z-axis (xyz indicates a rotating coordinate system in which the direction of the static magnetic field is the z-axis and the direction of the static magnetic field is the axis and is rotated at the Larmor frequency). Shows an excitation pulse that rotates A by α (eg, π / 2) about the x-axis. Reference numerals 211 to 214 denote inversion pulses for rotating the magnetic moment rotated by α by π around an axis orthogonal to the z-axis. Reference numerals 221 to 224 denote a plurality (four in this case) of spin echo signals obtained by an inversion pulse and a gradient magnetic field (not shown) and phase-encoded according to the position in the slice. In this Fast SE, the phases of the inversion pulses 211 to 214 are changed in order to prevent the incompleteness of the inversion pulses 211 to 214 (deviation of the rotation angle from π) from being accumulated and the second and subsequent echoes to be reduced. CPMG (Carr-Purcell) determined to have a fixed phase shifted by π / 2 from the phase of the excitation pulse 201
-Meiboom-Gill) or the like.

[0005] Further, TR indicates a cycle of a pulse sequence of one cycle (for example, 600 msec), and a section A for data collection and a section B for waiting for relaxation of a magnetic moment.
Consists of Then, the TR cycle is repeated to obtain another phase-encoded signal corresponding to the position in the slice. Furthermore, the same measurement is repeated a plurality of times, and the averaging is taken to increase the SN ratio.

In general, a section B (for example, 500 msec) for waiting for relaxation of the magnetic moment is relatively longer than a section A for data collection (for example, 100 msec). Therefore, in order to increase the scanning throughput, a multi-slice in which a plurality of slices are scanned in parallel while shifting the timing by inserting a data acquisition section A in another slice into a relaxation waiting section B in the TR. A scan is taking place.

[0005]

SUMMARY OF THE INVENTION However, Fas
In t SE, although the scan time is short, a plurality of echo signals are used, so that the section A required for data collection needs to be longer than that for scanning with one echo.
Therefore, many slices cannot be scanned in the section B waiting for relaxation, and there is a problem that the number of slices that can be scanned within the time of TR is small, and the scan throughput is not good. High speed could not be fully utilized.

On the other hand, in the case of measuring one echo signal other than the Fast SE, N slices are simultaneously excited by synthesizing signal components having different frequencies according to the slice position, and the N slices included in the excitation signal are excited. By giving a predetermined phase to the signal component, an encode phase corresponding to the slice position is given to each slice, and a series of scans is performed using a series of excitation waveforms having different phase amounts to obtain data at each slice position.
Offset Multi-Planar) or H
There is adamard encoding. This requires N scans, so POMP
Alternatively, an image having the same SN ratio as an image obtained by performing averaging N times in a normal scan not using Hadamard encoding can be obtained in the same scan time, and the number of slices becomes N times.
(However, in the case of Hadamard encoding, N is limited to a power of 2)

However, this POMP or HaMP
In the damard encoding, different predetermined encoding phases are given to the respective slices by the excitation pulse. Therefore, it is necessary to satisfy conditions such as CPMG (conditions that do not integrate imperfect inversion pulses) F
Not applicable to ast SE.

The present invention has been made in view of the above, and an object of the present invention is to realize an MRI apparatus capable of increasing the number of slices that can be simultaneously scanned in the Fast SE, and more specifically, to provide a Fast SE. POMP while achieving conditions such as CPMG
Another object of the present invention is to realize an MRI apparatus capable of applying Hadamard encoding.

[0009]

An MRI apparatus according to the present invention comprises:
By applying an excitation pulse having a frequency that selectively excites three or more slices and including three or more signal components whose phases are shifted by a predetermined angle, N slices are simultaneously excited and the encoding phase is increased. And three or more signal components having a frequency for selectively inverting the three or more slices, wherein the phase of each signal component of the excitation pulse corresponding to each slice is π /
Inverting means for applying a plurality of inversion pulses composed of signal components having two phase offsets after excitation by the excitation means, and a plurality of slices respectively generated after application of each of the inversion pulses by the inversion means. And a reconstructing means for reconstructing an image by processing the detected echo signal based on the encode phase.

[0010]

An RF pulse for selectively inverting each slice can be inverted by an RF pulse having a phase component shifted by π / 2 with respect to the excitation pulse for each slice. Can be satisfied in each of three or more slices.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram showing an example of hardware of an MRI apparatus according to the present invention. The magnet assembly 11 has a space portion for inserting the subject therein, and surrounds the space portion to generate a static magnetic field coil for applying a constant static magnetic field to the subject and a gradient magnetic field. , A transmission coil for applying a high-frequency RF pulse, a reception coil for detecting an NMR echo signal from the subject, and the like. The sequence storage circuit 16 has a means for generating a sequence signal for acquiring scan data based on the Fourier method by a pulse sequence described below in accordance with a command from the computer 21,
Gradient magnetic field drive circuit 13, RF oscillation circuit 18, transmission unit 14
And the receiving unit 20 is operated. The gradient magnetic field drive circuit 13 applies a gradient magnetic field having a desired waveform from the gradient magnetic field coil of the magnet assembly 11 according to a timing signal from the sequence storage circuit 16. The transmission unit 14 modulates and amplifies the high-frequency signal from the RF oscillation circuit 18 with the timing signal from the sequence storage circuit 16, and transmits an excitation pulse or an inverted pulse from the transmission coil. The receiving unit 20 receives the reference signal from the RF oscillation circuit 18 based on the timing signal from the sequence storage circuit 16, performs phase detection on the echo signal detected by the receiving coil, performs digital conversion, and inputs the digital signal to the computer 21. Calculator 2
Reference numeral 1 denotes switching of operation of the sequence storage circuit 16, rewriting of memory, and reception of information in order to exchange information with the operation console 22 and realize various scan sequences.
An image reconstruction operation is performed using data from the R> unit 20.

Here, the excitation means and the inversion means include a sequence storage circuit 16 and sequence information stored therein, an RF oscillation circuit 18, a transmission section 14, a gradient magnetic field drive circuit 13, and the like. Then, not only the exciting means but also the inverting means are configured to be able to transmit RF pulses of a composite waveform of a plurality of signal components having a desired phase and a frequency for selectively exciting or inverting a plurality of slice positions. .

A case where a plurality of slices are simultaneously excited and inverted by the inversion means and the excitation means will be described with reference to FIG. Waveforms 32 and 33 whose frequencies are shifted by + w0 and −w0 with respect to a waveform 31 having a center frequency modulated by a sinc function (which excites or inverts a slice area indicated by 41 with the application of a gradient magnetic field) obtain. Each of these waveforms is generated by applying a gradient magnetic field.
Excitation or inversion of the slice area indicated by 2, 43. An RF pulse 34 obtained by combining these two waveforms 32 and 33 with a phase difference of 0 excites or inverts two separated slices simultaneously in the same direction as indicated by 44. An RF pulse 35 obtained by combining the two waveforms 32 and 33 with a phase difference π is
Two separate slices are simultaneously excited or inverted in the opposite directions as shown at 45. Therefore, since not only the excitation but also the inversion can be selectively changed for each slice, even if a different predetermined encoding phase is given to each slice by the excitation pulse, the desired phase signal component is inverted for each slice. A pulse can be applied. Therefore, the inversion pulse can be applied while satisfying the conditions such as the CPMG in each slice (the condition not to integrate the incompleteness of the inversion pulse).

An embodiment of the phase of the inversion pulse of the inversion means corresponding to the phase of the excitation pulse of the present invention will be described in detail with reference to FIG. In the figure, reference numeral 101 denotes an excitation pulse that selectively and simultaneously excites the magnetic moments in the slices of slice 1, slice 2,... Oriented in the z-axis direction, and rotates by α (for example, π / 2) about the x-axis. Is shown. The selection of the slice is made based on the gradient magnetic field and the frequency of the signal component included in the excitation pulse. The rotation direction is determined by the phase of the signal component that excites each slice, and each slice is given an encode phase.

Numerals 111 to 114 denote inversion pulses which simultaneously act on the magnetic moment in each phase-encoded slice and rotate the magnetic moment by π about an axis orthogonal to the Z axis. The selection of the slice by the inversion pulses 111 to 114 is made by the frequency of the signal component and the gradient magnetic field, similarly to the excitation pulse. Then, like the excitation pulse, the rotation direction by the inversion pulse is determined by the phase of the signal component that inverts each slice, and as shown in FIG. Done.

Reference numerals 121 to 124 denote a plurality (four in this case) of spin echo signals obtained by inversion pulses and a gradient magnetic field (not shown) and phase-encoded according to the position in the slice. However, this echo signal includes signals from a plurality of slices that have been phase-encoded.
In the Fast SE of the present invention, the phase of the inversion pulse is set so that each slice satisfies the condition such as CPMG (the condition for not integrating the incompleteness of the inversion pulse). The effects of perfection are not cumulative.

In FIG. 3, the excitation pulse 10 of α
The value shown in the column of each slice below the inversion pulses 111 and 114 of 1, π indicates the phase of each signal component of the RF pulse applied at the time of excitation and inversion in each scan (that is, the encoding phase of each slice). , 0 indicates rotation about the x axis, π is the reverse rotation about the x axis, and π /
Reference numeral 2 denotes rotation around the direction of π / 2 from the x axis, that is, around the y axis, and 2π / 3 denotes rotation around the direction of 2π / 3 from the x axis.

Inversion pulses 111 to 114 shown in FIG.
Will be described. This selectively acts on three slices simultaneously. The phases of the excitation pulses having three frequency components are shifted by 2π / 3 from each other in the first scan, and the phases of the three signal components are aligned in the second scan.
In the third scan, it is shifted by 2π / 3 in the direction opposite to the first scan. That is, in scans 1 and 3, the excitation pulse 101 changes the magnetic moment of three slices by x.
This is to rotate around three axes shifted by 2π / 3 in the y-plane. The phase of each signal component of an inverted pulse for selectively inverting three slices at the same time is shifted by π / 2 with respect to the phase of each signal component of the excitation pulse. 111
To 114 are all equal. In this way, the imperfections of the inversion pulses 111 to 114 are not integrated in all three slices, and the multi-echo can be accurately measured. In this case, in this embodiment, the direction of the phase shift of π / 2 is the positive direction.
If the phases of 1 to 114 are equal, they may be shifted by π / 2 in the opposite direction. Also, for slice 1, π /
2. For slice 2, the direction of displacement may be different depending on the slice, such as -π / 2.

In POMP, Fourier transform (F) is applied to three sets of data obtained by three scans.
Slice 1 and Slice 2 by applying FT or DFT)
Of each data. When the number of slices to be simultaneously excited is a power of 2, Hadamard
The matrix calculation using the encoding method is solved, and addition or subtraction is performed to obtain data of each slice.

In this embodiment, scanning is performed three times, and data of each slice is measured three times.
Therefore, the result of the image reconstruction operation by Fourier transform or the like is based on the conventional Fast S which does not simultaneously excite a plurality of slices.
In E, the processing is the same as the one obtained by performing three measurements and averaging. In this embodiment of the present invention, the phases of the three signal components are respectively shifted by π /
Invert pulse 1
Since generation of 11 to 114 makes it possible to scan a plurality of slices at the same time, the same SN as that of the image obtained by the conventional three-time averaging in the same time as the imaging time of the conventional three-time averaging image is used. Three times the normal number of slices of the image with the ratio can be photographed. In the embodiment of the present invention, as in the related art, it is possible to excite and further scan a plurality of other sets of slices during the waiting time for relaxation.

In these embodiments, the number N of slices to be simultaneously excited is not limited to three, but may be more. If the number of sheets is large, it takes time until the scan is completed, but since the image is reconstructed using a correspondingly large amount of data, the SN ratio also increases. And the conventional Fas
.., N times as many slices can be photographed in the same photographing time as the images having the same SN ratio obtained at t SE.

In the conventional MRI apparatus, a multi-slice in Fast SE (scanning another slice during relaxation waiting time) and a multi-slice in Fast SE of the present invention (scanning another slice during relaxation waiting time). The actual effect of the example of the combination of the above and the POMP is shown below.

A case will be described in which the SN ratio is equivalent to three averages and an image having a matrix size of 256 × 256 is reconstructed in a sequence for obtaining four echo signals. F
In a scan that does not employ the ast SE (does not use the multi-echo), the TR cycle is 256 × 3 = 76.
It is necessary to repeat eight times, but when Fast SE is adopted, the TR cycle may be repeated 256/4 × 3 = 192 times in both the conventional example and the present invention, and TR =
In the case of 600 msec, a scan time of about 120 seconds is sufficient. Time required for data collection in TR is 100m
In the case of sec, six groups of scans can be performed in parallel using the relaxation waiting time. Therefore, when such a scan is performed, an image having an SN ratio equivalent to three times of averaging (that is, when N = 3) takes about 120 seconds of scan time,
Conventionally, six images can be taken, whereas in the present invention, eighteen images can be taken.

[0024]

The MRI apparatus of the present invention selectively excites three or more slices and applies a predetermined encoding phase to each slice, and sets a frequency for selectively inverting each slice. A plurality of inversion pulses comprising three or more signal components having a phase offset shifted by π / 2 with respect to the phase of each signal component of the excitation pulse corresponding to each slice; And an inversion means for applying an inversion pulse after the excitation by the above, and can satisfy the condition of CPMG in which incomplete inversion pulses are not integrated in three or more slices. Therefore, if the number of slices to be simultaneously excited is N, an image having the same SN ratio as the image obtained by the conventional N-times averaging in the same time as the imaging time of the conventional N-times averaging image, N times as many slices can be taken. In this way, the number of slices that can be simultaneously photographed is improved, and the high speed of multi-echo can be fully utilized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention.

FIG. 2 is a configuration diagram showing one embodiment of the present invention.

FIG. 3 is a configuration diagram showing one embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a conventional example.

[Explanation of symbols]

 101 Excitation pulse 111-114 Inversion pulse 121-124 Echo signal

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-6-197881 (JP, A) JP-A-4-15039 (JP, A) JP-A-59-105550 (JP, A) JP-A-62 54148 (JP, A) JP-A-61-77748 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. An N slice is simultaneously excited by applying an excitation pulse having a frequency for selectively exciting N slices and including N signal components whose phases are shifted by a predetermined angle. And excitation means for providing an encoding phase together with N signal components having a frequency for selectively inverting the N slices, wherein π is determined with respect to the phase of each signal component of the excitation pulse corresponding to each slice. Inverting means for applying a plurality of inversion pulses consisting of signal components having a phase offset shifted by / 2 after excitation by the excitation means; and a plurality of slices respectively generated after application of the plurality of inversion pulses by the inversion means. Detecting means for detecting an echo signal from the apparatus, and reconstructing means for processing the detected echo signal based on the encode phase to reconstruct an image. M with
RI equipment. However, N is a natural number of 3 or more.