JPH0919415A - Magnetic resonance imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To photograph an image off-center without artifact in the sequence that EPI (echo planner) is continuously inverse-read out, gradient magnetic field is applied, and echo signal is measured. SOLUTION: RF pulses having the magnetic resonance frequency (f) are irradiated to a testee, slice selection gradient magnetic field pulses are applied, the readout gradient magnetic field off-set quantity is applied, then continuously inverted readout gradient magnetic field pulses are applied, and the echo signals generated in cycles of the inverted readout gradient magnetic field are detected in time series. The reference frequency of (resonance frequency (f) + off-set frequency Δf) is used to detect the signal (101), the time series data are phase- corrected by the frequency (2Δf) twice the off-set frequency Δf (104) only when the polarity of the inverted readout gradient magnetic field pulse is negative, and the signal arrangement is inverted in the time series direction (103). The signals when the polarity of the inverted readout gradient magnetic field pulse thus processed is negative and the signals when the polarity is positive are used to reconstitute an image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures nuclear magnetic resonance (hereinafter referred to as "NMR") signals from hydrogen, phosphorus, etc. in an object to visualize nuclear density distribution and relaxation time distribution. A magnetic resonance imaging (MRI) method.

[0002]

2. Description of the Related Art MRI targets a specific nuclide (proton, ¹³ C, ³¹ P), which is a constituent material of an object, for radiography at a frequency equal to its Larmor frequency in these atomic nuclei placed in a static magnetic field. A nuclear spin is excited by irradiating a magnetic field, and the NMR signal generated after that is measured. Proton is the most common object to be imaged, and the spatial distribution of its density and the spatial distribution of the relaxation phenomenon of the excited state are obtained, and by imaging this, the form or function of the human head, abdomen, extremities, etc. 2D or 3D image.

In the MRI imaging method, in order to give positional information to the NMR signal, it is superimposed on the static magnetic field in the measurement space,
Gradient magnetic field pulses in three orthogonal directions are given. That is, different phase encodes are given by the gradient magnetic field, and the echo signals obtained by each phase encode are detected. The number of phase encodes is usually 12 per image.
Values such as 8, 256, 512 etc. are chosen. Each echo signal is usually obtained as a time series signal composed of 128, 256, 512, 1024 sampling data. Two-dimensional Fourier transform is performed on these data to create one MR image.

By the way, conventionally, as a typical imaging method of MRI, methods such as a spin echo method and a gradient echo method have been known. In recent years, a high-speed imaging method for measuring a plurality of echo signals by one excitation is widely used. It is supposed to be done. FIG. 2 shows a sequence of an echo planar (EPI) method which is a typical sequence of such a high speed imaging method. In this sequence, first, a high-frequency pulse 201 is applied to a subject including magnetization to be detected, and at the same time, a gradient magnetic field (Gs) pulse 202 for selecting a slice is applied to select and excite a slice to be imaged. Next, a pulse 203 that gives an offset for phase encoding (Ge)
And a pulse 205 which gives an offset of the read gradient magnetic field (Gr) is applied. After that, the read gradient magnetic field pulse 206 whose polarity is reversed is continuously applied. The inverted read gradient magnetic field pulse 206 is trapezoidal in the illustrated example, but may be a sine wave. In synchronization with the read gradient magnetic field pulse 206, the phase encoding gradient magnetic field pulse 20
4 is applied discretely. In each cycle of the gradient magnetic field 206, when the integrated value of the inverted read gradient magnetic field 206 becomes 0, the echo signals 207 of each phase encoding are generated in time series. Therefore, the echo signals 207 are sampled for a predetermined time range 208. Get time series data. Each time range 208 is typically on the order of 1 ms. A series of operations 209 collects all echoes required for image reconstruction. That is, if the number of phase encodes is 128, 128 sets of time series data are collected.

Further, as a high-speed imaging method for measuring a plurality of echo signals with one excitation, in addition to the above EPI, GE & SE (gradient echo.
And spin echo method) and a split type EPI method are also proposed. For example, GE & SE irradiates a 180 ° RF pulse that further inverts spin during operation 209 in FIG. 20
4. The application of the reverse readout gradient magnetic field 206 is continuously repeated to measure the echo signal and collect all the echoes necessary for image reconstruction. In addition, the split type EPI method uses the operation 20 of FIG.
In 9, the phase encode data of only a part is acquired, and then the operation 209 is repeated while changing the pulse 203 that gives the offset of the phase encode, and the remaining echo signal 207 is acquired. In the split-type EPI, since the operation 209 is repeated, the data acquisition time for acquiring one image becomes long, but the number of echoes 207 acquired in the operation 209 decreases, and the operation 209 itself becomes short. As a result,
A reduction in echo signal is reduced, and high-quality pictures can be acquired.

Next, FIG. 7 shows a method of image reconstruction and signal correction in the above-mentioned EPI and a method similar thereto. First, the reference frequency is effectively set to the resonance frequency f [Hz], and data is acquired (701). Here, the resonance frequency f is used as the reference frequency because the signal is
This is because the signal is converted into a frequency and only the frequency shift corresponding to the position information is extracted as a signal. In this case usually 2
It is realized by signal frequency conversion of three stages or three stages. That is, the reference frequencies of the respective stages are f1, f2, and f3, and f = f1 + f2 + f3. The final stage is quadrature detection (analog or digital). The term "effective" means that such multistage conversion is also included.

Next, the polarity of the inversion gradient magnetic field pulse 206 is confirmed (702). For example, in FIG. 2, the gradient of the gradient magnetic field pulse is negative in the first echo, and the polarity of the gradient magnetic field pulse is positive in the second echo. When the polarity is negative, the signal array is inverted in the time direction (703). An image is reconstructed by a known method, that is, a two-dimensional Fourier transform or the like, using the signal (time series data) subjected to such inversion processing and the signal when the polarity is positive (704).

FIG. 8 shows a typical signal sequence acquired by the division type EPI in x-ky space display in the case of reference frequency = resonance frequency f, and shows the state after reversal in the time direction. The display of the x-ky space means the measurement space k
It is a display in which the signals arranged on x-ky are Fourier transformed only in the kx direction and the kx axis is transformed to the x axis. In this figure, the signals are divided into four groups in the kx direction,
The signal groups 81 and 83 are signals when the polarity of the gradient magnetic field is negative, and the signal groups 82 and 84 are signals when the polarity of the gradient magnetic field is positive. After the time direction inversion, the signals of the signal groups 81 to 84 are arranged in the kx direction without displacement. K these
When the Fourier transform is performed in the y direction, the image of FIG. 9 is obtained.
The image center 91 coincides with the center of the gradient magnetic field, that is, the cross section where the gradient magnetic field strength is always zero. Therefore, when the center of the subject is set to coincide with the center of the gradient magnetic field, the subject 92 appears at the image center 91 as shown in the figure. In addition,
93 is the background.

At this time, the influence of unavoidable misalignment of the device at the time of signal acquisition, such as residual offset component of the gradient magnetic field and non-uniformity of the static magnetic field, on the signal is removed by known signal correction. Particularly in EPI imaging, when there is a frequency difference between the RF center frequency and the resonance frequency, the signal groups measured at the different polarities of the read gradient magnetic field are displaced due to the frequency difference, which causes artifacts. As a phase correction method for preventing this, for example, Japanese Patent Laid-Open No. 5-31095 discloses that sampled data is multiplied by a function of the frequency difference between the RF center frequency and the Larmor frequency.

[0010]

As described above, in the conventional EPI sequence, the reference frequency is the magnetic resonance frequency f.
In this case, the center of the field of view becomes the center of the gradient magnetic field. On the other hand, if the center of the imaging field of view can be changed arbitrarily, only the important parts of the subject are arbitrarily extracted,
There are advantages such as magnified shooting. It is known that in order to shift the center of the imaging field of view by Δx [m] in the reading direction, the effective reference frequency may be set to f + γGxΔx in the normal imaging sequence such as the SE method described above. Here, γ [Hz / T] is the gyromagnetic ratio, Gx [T /
m] is the read gradient magnetic field strength. However, in the imaging method in which a plurality of signals are measured by continuously applying a reading gradient magnetic field that inverts, such as EPI, when such an offset (γGxΔx) is applied to the reference frequency, a strong artifact corresponding to Δf = γGxΔx occurs. Will end up.

The present invention eliminates this artifact,
Even in the EPI and similar imaging methods, the center of the imaging field of view can be shifted.

[0012]

In order to solve such a problem, in the present invention, the R of the magnetic resonance frequency f is applied to the subject.
A step of applying a slice selection gradient magnetic field pulse at the same time as irradiating an F pulse and performing slice selection, and a read gradient magnetic field pulse whose polarity is inverted are continuously applied, and a phase encode gradient is synchronized with the inversion of the read gradient magnetic field pulse. In a MRI method including a step of applying a magnetic field pulse and a step of detecting time-series data by time-sequentially detecting echo signals generated within each cycle of a reversing read gradient magnetic field, a reference frequency for detecting an echo signal As a predetermined offset frequency Δf to the magnetic resonance frequency f
The signal is detected by using the reference frequency (f + Δf) added with, and the phase correction process of double the offset frequency Δf is performed only on the time-series data acquired when the read gradient magnetic field pulse has the first polarity (negative). At the same time, the signal array is inverted in the time series direction, and image reconstruction is performed using the processed data and the data acquired when the inverted read gradient magnetic field pulse has the second polarity (positive), and differs from the gradient magnetic field center. An image having a photographing center at a position is obtained.

When the reference frequency is f + Δf, the time-series data acquired when the read gradient magnetic field pulse has the first polarity (negative) is data having an offset component of −Δf, while the read gradient magnetic field pulse is the first. The time-series data acquired when the polarity is 2 (positive) is data having an offset component of + Δf. Therefore, by performing the phase correction processing of 2Δf only on the time-series data acquired when the first polarity (negative), both data become data having the offset component of Δf with the reference frequency f as the center. This offset component corresponds to a deviation ΔX (= Δf / γGx) from the center of the gradient magnetic field in the reading direction. Therefore, the time-series data acquired when the first polarity (negative) is subjected to the phase correction process and the inversion process in the time direction, and then the time-series data after the process and the second polarity are acquired. By reconstructing the image using the time-series data, it is possible to obtain an image with the imaging center at a position deviated from the center of the gradient magnetic field by ΔX.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The MRI method of the present invention will be described below with reference to the drawings. FIG. 4 shows a configuration of a typical MRI apparatus to which the present invention is applied. This MRI apparatus includes a magnet 402 that generates a static magnetic field around a subject 401 and a gradient magnetic field that generates a gradient magnetic field in the space. A coil 403, an RF coil 404 that generates a high-frequency magnetic field in this region, and an RF probe 405 that detects an MR signal generated by the subject 401 are provided. The gradient magnetic field coil 403 is composed of gradient magnetic field coils in the directions of three axes (X, Y, Z) orthogonal to each other, and generates a gradient magnetic field according to a signal from the gradient magnetic field power source 409. The RF coil 404 generates a high frequency magnetic field according to the signal from the RF transmitter 410. The signal of the RF probe 405 is detected by the signal detection unit 406,
The signal processing unit 407 performs signal processing, and converts it into an image signal by calculation. The image is displayed on the display unit 408.
Gradient magnetic field power source 409, RF transmitter 410, signal detector 4
The control unit 411 controls 06 to activate a pulse sequence for a predetermined imaging method. The bed 412 is for the subject to lie on. The present invention is directed to the signal detecting section 406 and the signal processing section 4 in such an MRI apparatus.
It mainly relates to 07.

Next, the MRI method of the present invention will be described by taking a divided EPI as an example. In the pulse sequence by the divided EPI, similarly to the EPI sequence shown in FIG. 2, first, the subject including the magnetization to be detected is irradiated with the high-frequency pulse 201, and at the same time, the gradient magnetic field pulse 202 for selecting the slice is applied to select the slice for imaging. And excite. Next, a pulse 203 that gives an offset for phase encoding and a pulse 205 that gives an offset for a read gradient magnetic field are applied. After that, the read gradient magnetic field pulse 206 whose polarity is reversed is continuously applied. The phase encoding gradient magnetic field pulse 204 is discretely applied in synchronization with the gradient magnetic field pulse 206. When the integrated value of the inverted read gradient magnetic field 206 becomes 0, that is, in each period of the gradient magnetic field 206, the echo signals 207 of each phase encoding are generated in time series. Get series data. In this example, a series of operations 20
9 collects 1/4 of all echoes needed for image reconstruction. That is, if the number of phase encodes is 256, time series data consisting of 64 echo signals is collected. Next, a gradient magnetic field pulse 203 that gives an offset for phase encoding
The operation 209 is repeated three more times while changing the above, and the remaining echo signal 207 is acquired.

Next, the detection, signal processing and image reconstruction of the echo signal thus measured according to the present invention will be described with reference to FIG.
This will be described with reference to the flowchart of FIG. Incidentally, these processes can be performed digitally. First, the echo signal is
Reference frequency f + Δ in the signal detector (406 in FIG. 4)
It is detected using f (101). Here, f is a resonance frequency (RF frequency), Δf is an offset frequency for shifting the image center, and is determined by Δx = Δf / γGx corresponding to the shift amount Δx from the image center. In particular,
Static magnetic field strength is 1.5T, resonance frequency f is 63.8MHz
(The magnetic resonance ratio γ is 42.5 × 10 ⁶ Hz / T) and the gradient magnetic field strength Gx is 0.01 T / m, the off-center amount (shift amount from the gradient magnetic field center) Δx is 0.0.
In order to obtain 2 m, the frequency offset amount Δf is 8500 Hz from Δx = Δf / γGx. When detecting with the reference frequency f + Δf, frequency conversion may be performed in multiple stages so that the total of the reference frequencies of the respective stages becomes f + Δf.

Next, the polarity of the gradient magnetic field pulse 206 when each echo is generated is confirmed (102), and the offset frequency Δ is measured for the echo signal measured when the polarity is negative.
The phase is corrected twice as much as f (103). Further, the echo signal measured when the polarity is negative is inverted in the time direction (104). Generally, the sampled echo signals S are arranged in the measurement space in the reading direction (kx direction) as shown in FIG. 3, but the data measured when the polarity is negative and the data measured when the polarity is positive. Are different in the arrangement direction in the kx direction (note that the number of scans is simplified in FIG. 3). Therefore, the reversal in the time direction is a process for making the arrays in the reading directions of both data the same. Either the phase correction (103) or the time direction inversion (104) may be performed first.

The phase correction is performed by changing the sampling time of the signal by Δ
When t (for example, 4 ms), 2Δf · 360 · Δt
The phase correction amount can be obtained from kx. That is,
After inverting the signal S (kx, ky) in the measurement space in the time direction, its phase θ (kx, ky) is changed to θ (kx, ky) + Δ.
The signal S'after conversion to f.360.Δt · kx and correction
(Kx, ky) can be obtained. Note that kx is a data number.

This phase correction is performed by converting the time series data into x-ky.
Further description will be given with reference to FIG. 5 displayed in space. FIGS. 5A and 5B show time-series data measured by the split-type EPI in x-ky space, and FIG. 5A shows a case where no phase correction is performed and FIG. () Shows the state after the time direction reversal in the case where the phase is corrected. In this embodiment, since the number of phase encodes is 256, the signal groups 51 to 54 are 64 (= 256/4) phase encodes each, and the signal group 5
Reference numerals 1 and 53 represent signal groups when the polarity of the gradient magnetic field is negative, and signal groups 52 and 54 represent signal groups when the polarity of the gradient magnetic field is positive.

As described above, in the MRI method of the present invention,
Since f + Δf is used as the reference frequency for signal detection, a signal when the polarity of the gradient magnetic field is negative and a signal when the polarity of the gradient magnetic field is positive are different from each other in that there is a deviation in the opposite direction corresponding to Δf about the center of the gradient magnetic field. Become. That is, FIG.
As shown in (a), the signal center 56 of the signal groups 52 and 54 when the polarity of the gradient magnetic field is positive is shifted to the right by Δx = Δf / γGx with respect to the gradient magnetic field center 55, and the polarity of the gradient magnetic field is The signal center 57 of the signal groups 51 and 53 when is negative
Is shifted to the left by Δx = Δf / γGx with respect to the gradient magnetic field center 55. Such signal groups 51 to 54 are
If the Fourier transform is performed in the ky direction as it is, the subject 62 is out of focus and becomes a blurred image as shown in FIG. 6A.

On the other hand, according to the present invention, the phase correction of 2Δf is given in advance to the signal groups 51 and 53 when the polarity of the gradient magnetic field is negative. Therefore, as shown in FIG.
1 and 53 are 2x (= 2Δ from the position without phase correction).
f / γGx) is shifted to the right by a signal group 5 such that the signal center 57 ′ overlaps the signal center 56 of the signal groups 52 and 54.
Converted to 1 ', 53'.

Then, the signals of the signals 51 ', 52, 53' and 54 are subjected to Fourier transform in the ky direction to reconstruct an image (FIG. 1, 105), and an object as shown in FIG. 6 (b) is obtained. Acquire 63 images. This image is formed without blurring as shown in FIG. 6A, and the image center 61 is Δ with respect to the subject center 64 (= gradient magnetic field center).
It will be x-shifted.

As described above, by performing the phase correction in the measurement space, it is possible to perform off-center imaging with a desired shift amount. The off-center shooting according to the present invention
Unlike the conventional method in which data is shifted in real space, shifts of one picture element or less can be corrected, so that artifacts do not occur, which is desirable. The MRI method of the present invention has been described above with the split-type EPI. However, the present invention is a one-shot EP that measures all echoes necessary for image formation with one excitation.
I, the GE & SE method, the spiral scanning method of scanning the k space in a spiral shape instead of the raster scanning as shown in FIG. All can be applied and the same effect can be obtained.

[0024]

As is apparent from the above description, according to the MRI method of the present invention, an echo generated in each cycle of the reversal magnetic field is applied by applying a read gradient magnetic field pulse that is continuously reversed like an EPI sequence. An off-center imaging of an image is possible without artifacts in a nuclear magnetic resonance imaging method having a step of detecting signals in time series.

[Brief description of the drawings]

FIG. 1 is a flowchart showing processing in an MRI method of the present invention.

FIG. 2 is a diagram showing an example of an MRI imaging sequence to which the present invention is applied.

FIG. 3 is a diagram showing an array of echo signals in a measurement space.

FIG. 4 is an MRI apparatus to which the present invention is applied.

5A and 5B are diagrams illustrating processing contents of signal processing by MRI, FIG. 5A shows processing contents before phase correction, and FIG. 5B shows a result thereof.

6A and 6B are diagrams illustrating the processing contents of signal processing by MRI, in which FIG. 6A shows the processing contents after phase correction, and FIG. 6B shows the result.

FIG. 7 is a flowchart showing a process in a conventional MRI method.

FIG. 8 is a diagram illustrating processing contents in a conventional MRI method.

9 is a diagram showing a result of the processing contents of FIG.

[Explanation of symbols]

 201-High frequency pulse 202-Slice selection gradient magnetic field pulse 204-Phase encoding gradient magnetic field pulse 206-Reading gradient magnetic field pulse

Claims (1)

[Claims] 1. A step of irradiating a subject with an RF pulse having a magnetic resonance frequency f and at the same time applying a slice selection gradient magnetic field pulse to perform slice selection, and continuously applying a readout gradient magnetic field pulse whose polarity is reversed, and Applying a phase-encoding gradient magnetic field pulse in synchronization with the inversion of the readout gradient magnetic field pulse, and obtaining time-series data by time-sequentially detecting echo signals generated in each cycle of the inversion readout gradient magnetic field. A magnetic resonance imaging method including: a signal using a reference frequency (f + Δf) obtained by adding a predetermined offset frequency Δf to the magnetic resonance frequency f as a reference frequency for detecting the echo signal, and the read gradient magnetic field pulse; Offset frequency Δf only in the time series data acquired when is the first polarity (negative) Image processing using the data acquired when the signal arrangement is inverted in the time series direction and the processed data and the inverted read gradient magnetic field pulse have the second polarity (positive) A magnetic resonance imaging method comprising reconstructing and obtaining an image having an imaging center at a position different from the gradient magnetic field center.