JPH03207343A - Mri apparatus - Google Patents

Abstract

PURPOSE:To perform photographing within a short time by comparing the longitudinal and lateral sizes of an ROI by an encode direction determining means and setting the shorter one to a phase encode direction while setting the longer one to a frequency encode direction. CONSTITUTION:The optimum photographing region (FOV), that is, FOVe and the photographing region in a frequency encode direction are determined by an optimum photographing region (FOV) determining means 22 so that no fold- back artifact L' enters the regions of RUIs set on the basis of the regions m1-m2 in a phase encode direction and the regions m3-m4 in the frequency encode direction. An encode direction determining means 23 compares the longitudinal and lateral sizes of an ROI to set the shorter one to the phase encode direction and sets the longer one to the frequency encode direction. By this method, the number of phase encodes is reduced and short-time photographing becomes possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an "rMRI apparatus").

(Prior art) Conventional MRI equipment uses two-dimensional Fourier transform method (2DFT).
law) is the mainstream. In this 2DFT method, MR
Data (hereinafter also referred to as "echo data") needs to be collected in two dimensions: a frequency encoding direction and a phase encoding direction. Data sampling in the frequency encoding direction can be performed within one RF pulse period TR, so it hardly affects the imaging time. However, data sampling in the phase encoding direction involves repeating irradiation of RF pulses by the number of phase encodes, which affects the imaging time. Therefore, in order to reduce the imaging time, it is important to reduce the number of phase encodes, especially in the long spin echo method.

There are a variable matrix method and an encode reduction method as methods for reducing the number of phase encodes. In the variable matrix method, the pixel size ΔX in the phase encoding direction
While keeping e constant, the imaging area in the phase encoding direction (
(hereinafter simply referred to as rFOVeJ) is varied according to the size of the subject or the region to be observed. In addition, the encode reduction method keeps FOVe constant,
The pixel size ΔXe in the phase encoding direction is varied according to the spatial frequency band of the subject or the part to be observed. Hereinafter, both the variable matrix method and the encode reduction method will be referred to as variable encoding methods.

In other words, the variable encoding method creates a narrow imaging area (hereinafter rFO
For example, by setting the image size to about 1.5 times the subject size (VJ), the number of phase encodes is reduced by trying to avoid photographing unnecessary areas and frequency areas as much as possible.

Furthermore, as a method of shortening the imaging time, the number of frequency encodes has not been particularly varied because the number of phase encodes mainly contributes to the imaging time.

On the other hand, in Japanese Patent Application Laid-Open No. 63-283632, in order to eliminate unnecessary data collection in the frequency domain, increase the imaging speed, and reduce aliasing artifacts, the FOV was changed to 9 frequencies in the phase encoding direction of the subject. A magnetic resonance imaging method and apparatus are disclosed that vary the encoding pitch according to the maximum dimension in the encoding direction.

(Problems to be Solved by the Invention) In the variable encoding method described above, since the number of phase encodes and the number of frequency encodes are not controlled, wasteful data in a band higher than the frequency band of the subject may be sampled, and the MR image There was a problem of lowering the S/N ratio.

Furthermore, since the invention according to JP-A-63-283632 does not control the pixel size, frequency components of the object may be sampled wastefully or in small quantities, resulting in optimal spatial resolution and S/N ratio. There was a problem with it not being set.

Moreover, only the lesion site as the observation target site falls into the FOV, and if only this FOV is photographed, the photographing can be completed in a short time. However, when the encoding pitch is determined based on the size of the FOV that matches the lesion site, the variable encoding method has the problem that aliasing occurs from locations other than the set FOV and the image deteriorates. Specifically, from the synchronization of the discrete Fourier transform, as shown in Fig. 9, FOVe
When set smaller than the object P size Le, FOVe
The part Pa that protrudes further to the left side enters the right side of the FOVe as a virtual image Pa', and the part Pb that protrudes to the right side of the FOVe enters the left side of the FOVe as a virtual image Pb', which impedes diagnosis.

Therefore, the present invention has been made in view of the above circumstances, and
To provide an MRI apparatus which can perform imaging of a necessary part in a short time with the necessary spatial resolution, and can obtain an MR image with an optimal S/N ratio and spatial resolution/power without aliasing artifacts. The purpose is to

[Structure of the Invention] (Means for Solving the Problem) In order to achieve the above object, the invention according to claim 1 provides an M
At the MRIR, which performs R imaging and obtains diagnostic MR images,
It has encoding direction determining means for comparing the vertical and horizontal sizes of the ROI and setting the shorter one as the phase encoding direction and the longer one as the frequency encoding direction, and the encoding direction determining means determines the length and width of the ROI. The present invention is characterized in that MR imaging is performed based on both encoding directions to obtain diagnostic MR images.

In the invention according to claim 2, in the invention according to claim 1, the ROI is set in a rectangular shape on the positioning MR image inward in the phase encoding direction from the outline of the subject in accordance with the observation target region. The diagnostic MR
It has an optimal FOV determination means that automatically sets the FOV so that aliasing artifacts do not appear in the image, and this optimal FOV
MR imaging is performed with respect to the FOV automatically set by the V determining means to obtain a diagnostic MR image.

Furthermore, the invention as claimed in claim 3 provides an MR multiplied signal obtained at each application of a phase encoding gradient magnetic field to which a predetermined phase encoding gradient magnetic field increment is added stepwise based on the control of the sequencer, and the MR multiplied signal is collected at a predetermined sampling pitch. MRIR
a threshold storage means for storing in advance a threshold value of a signal value of the collected MR data, and a comparison between the signal value of the MR data and the threshold value stored in the threshold storage means for each collected MR data; and sequencer control means for sending a stop signal to the sequencer to stop adding the gradient magnetic field increment for phase encoding when the signal value of the MR data and the threshold value become substantially equal. It is.

(Function) In the invention according to claim 1, which features the following functions of the apparatus configured as described above, the encoding direction determining means compares the vertical and horizontal sizes of the ROI and selects the shorter one as the phase encoding direction. , and set the longer one in the frequency encoding direction. Because the number of phase encodes is reduced, it is possible to shoot for a short time.

In the invention as set forth in claim 2, the optimum FOV determining means is configured to prevent aliasing artifacts from entering the diagnostic MR image when the ROI is set in a rectangular shape according to the observation target region inside the outline of the subject in the phase encoding direction. Automatically set the FOV so that This allows you to shoot in a short time,
Moreover, an MR image with a good S/N ratio can be obtained.

In the invention according to claim 3, the sequencer control means compares the signal value of the MR data with the threshold value stored in the threshold value storage means, and when the signal value of the MR data and the threshold value are almost equal, the sequencer control means controls the sequencer. A stop signal is sent to stop adding the gradient magnetic field increment for phase encoding. This makes it possible to collect MR data in the optimal frequency band of the subject.
An MR image with optimal S/N ratio and spatial resolution can be obtained.

(Example) Examples of the present invention will be described in detail below.

FIG. 1 is a block diagram showing the configuration of this device.

This device includes a static magnetic field coil 1 that generates a static magnetic field, and a gradient magnetic field Gs for slicing, a gradient magnetic field Ge for encoding, and a gradient magnetic field Gr for readout that are orthogonal to each other for providing position information of the MR multiplied signal excitation site. Obtaining gradient magnetic field coil 2
, an RF coil 3 that irradiates RF pulses toward the subject P and detects an MR multiplied signal from the subject P, a static magnetic field control system 4 that controls the static magnetic field of the static magnetic field coil 1, and an X-axis gradient magnetic field power supply 5. , a Y-axis gradient magnetic field power supply 6, a two-axis gradient magnetic field power supply 7, a transmitter 8 that transmits an RF pulse signal to the RF coil 3, and a receiver 9 that receives the MR multiplied signal from the RF coil 3.
and a sequencer 1 that executes a pulse sequence to be described later.
0 and an image creation unit 11 that performs 2DFT processing based on the sent MR data to reconstruct an MR image of the subject P.
, a display section 12 that visualizes the MR image reconstructed by the image creation section 11, selection of an imaging mode such as MR imaging for positioning, R imaging for cutting, and setting of ROI.

Operation unit 1 for manually setting pixel size ΔX, etc.
3, and a system controller 20 that controls the operation of this device.
It has

The system controller 20 includes an outer rectangular area determining means 21, an optimum FOV determining means 22, an encoding direction determining means 23, a parameter determining means 24 comprising an FOv parameter determining means 24a and a pixel size parameter determining means 24b, It has a threshold storage means 25 and a sequencer control means 26.

The outer rectangular area determining means 21 determines the contour of the subject P by threshold processing based on the positioning MR image shown in FIG. 2 obtained by positioning MR imaging using a known technique. The region in the phase encoding direction that is in contact with L2 is determined.

The optimum FOV determining means 22 determines the regions m to m2 in the phase encoding direction, the region m in the frequency encoding direction,
The optimum FOV, that is, the FOVe shown in FIG. 2 and the imaging region in the frequency encoding direction (hereinafter referred to as rFOVrJ) are determined so that the aliasing artifact L' does not enter the region of the ROIs set from m4 to m4.

It is in FOVe that the aliasing artifact L' becomes a problem in relation to the imaging time. In order to prevent the aliasing artifact L' from entering the ROI s, the second
Area X shown in the figure may be determined as FOVe. That is, it is sufficient to find X such that L2-X<ml L1+X>m2. Therefore, X=min (L2-ml, m2-LL).

Further, FOVr may be obtained in the same way as the encoding direction, but here, the sampling pitch ΔTr of the echo data
is usually constant, and since the imaging time does not change even if sampling is performed at the minimum value in the system, the value Lr corresponding to ΔTr is set.

The encoding direction determining means 23 compares the vertical and horizontal sizes of the ROI, and as a result of the comparison, sets the shorter one of both sides of the ROI as the phase encoding direction, and sets the longer one as the frequency encoding direction. .

The FOV parameter determining means 24a is configured to determine the optimum FOV.
With the FOV determined by the V determining means 22, the parameters of the following equation (1), (2+) are determined.

The imaging area FOVr in the frequency encoding direction and the imaging area FOVe in the phase encoding direction are expressed as 0Vr = 17 (2πγ・Gr・ΔTr) − (1) 0Ve = 1/(2πy − ΔGe − Te) − (2) be done.

However, γ: nuclear gyromagnetic ratio Gr: reading gradient magnetic field strength ΔTr: sampling pitch ΔGe: phase encoding gradient magnetic field intensifier e: phase encoding gradient magnetic field application time Figure 3 shows an example of a pulse sequence. .

A gradient magnetic field G for reading out an MR multiplied signal obtained each time a plurality of phase encoding gradient magnetic field increments ΔGe are obtained by irradiating a π/2 RF pulse and a πRF pulse and applying a slicing gradient magnetic field Gs.
r is given and collected. Each of the gradient magnetic field power supplies 5 to 7 is appropriately selected as a slicing gradient magnetic field Gs, an encoding gradient magnetic field Ge, and a reading gradient magnetic field Gr according to the direction of the slice plane under the control of the sequencer 10. will be dealt with accordingly.

Since each parameter is configured in this way, the FOV parameter determining means 24a calculates the formula (
From 1), since the reading gradient magnetic field strength Gr is known based on the capability of the apparatus, etc., the sampling pitch ΔTr is determined from this. Furthermore, when FOVe is determined in the same manner, the FOV parameter determining means 24a calculates the gradient magnetic field application time Te for phase encoding from equation (2).
Since it is known based on the capability of the device, etc., the phase encoding gradient magnetic field increment ΔGe is determined from this.

The pixel size parameter determining means 24b determines the pixel size Δ
This is to find the parameters of equations (3) and (4), which will be described later, regarding X. Pixel size Δx in frequency encoding direction
r and the pixel size Δxe in the phase encoding direction are Δxr = 1/ (2π7−Gr−Tr) (3) Δx
It is expressed as e = 1/C2yr7 ・Gem-Te)-(4). However, Gem: Maximum gradient magnetic field strength for phase encoding Tr: Total sampling time of one echo data However, in the above equations (3) and (4), Tr.

Since Gem is not known, it is difficult to immediately obtain Δxr and Δxe from equations (3) and (4).

Therefore, the maximum gradient magnetic field strength Gem for phase encoding and the total sampling time Tr are determined by the method described later.
The pixel dimensions Δxr and Δxe are calculated.

Furthermore, with reference to FIGS. 4 to 6, the function of the pixel size parameter determining means 24b will be explained.

Figure 4 shows the signal value 5 of MR data collected in frequency space.
(fr, fe), the horizontal axis is the frequency f in the phase encoding direction
e, the vertical axis shows the frequency fr in the frequency encoding direction, and FIG. 5 shows the profile of the MR data in FIG. In the profile equivalent to the figure, the horizontal axis is the number of encodes n, and the vertical axis is the direct current (Ge) of one echo data.
The signal strength at m0) is expressed as S (0, n). In FIG. 6, the threshold value is indicated by Ss (0, n), and the number of encodes at that time is N0pt.

The threshold value storage means 25 stores the threshold value 5s (0°fe) of the signal value 5 (fr, fe) of the MR data shown in FIG.
It stores the threshold value Ss (0, n) shown in the figure.

The sequencer control means 26 controls the signal value 5 of the MR data.
When (fr, fe) and the threshold value Ss (0, fe) become approximately equal, a stop signal is sent to the sequencer 10 to control the addition of the encoding gradient magnetic field increment ΔGe. Therefore, the maximum gradient magnetic field Gem of the final encoding gradient magnetic field is determined by this.

In this way, the magnetic field G is controlled by the sequencer control means 26.
Once em is determined, the pixel size parameter determining means 24b calculates the equation (4) to determine the pixel dimension Δxe in the phase encoding direction, and similarly determines the pixel dimension Δxr in the frequency encoding direction.

On the other hand, the relationship between the frequency bandwidth F' e m and the pixel size Δxe in the phase encoding direction shown in FIG. 5 is Δxe=1/
It is also expressed as 2Fem = (5). Once the frequency bandwidth Fem is determined, the pixel size Δxe in the phase encoding direction can be determined in the same manner as the above equation (4), so the pixel size parameter determining means 24b may determine each parameter using the above equation (5). . Further, the frequency encoding direction may also be determined in the same manner.

The operation and effects of the apparatus having the above configuration will be explained with reference to FIGS. 7 and 8.

First, the operator sets an ROI smaller than the object size,
Section 7 regarding the case where the pixel size ΔX is set manually
The explanation will be given according to the flowchart shown in the figure.

When the operator selects the positioning MR imaging mode on the operation unit 13, the system controller 20
is controlled to perform positioning MR imaging, and an MR image of the head is obtained (STI) as shown in FIG. In addition, in the same figures (a) thru|or (e), the site|part shown with the diagonal line shows a lesion site.

Next, as shown in FIG. 4B, the operator operates the operation unit 13 to set the ROI on the lesion depicted inside the subject (Sr1). The outer rectangular area determining means 21 is configured as shown in the same figure (
As shown in C), the circumscribed rectangular area is determined (Sr3
). Next, the optimal FOV determining means 22 determines the optimal FOV (5T4), and after determining the optimal FOV, the encoding direction determining means 24 determines each encoding direction. Next, the FOv parameter determining means 24a determines the phase encoding gradient magnetic field increment ΔGe and the sampling pitch ΔTr based on the above equations (1) and (2) (Sr1). Then, the system controller 20 controls the sequencer 10 based on the imaging conditions for diagnostic MR imaging input from the operation unit 13 to perform diagnostic MR imaging (Sr1). Since MR data is not collected for the entire area of the subject, and imaging is performed for the minimum optimal FOV, this diagnostic MR imaging is completed in a short time. When the MR data is taken into the image creation section 11, the image is reconstructed in this image creation section 11 (
Sr7), as shown in FIG. 6(e), an MR image with a good S/N ratio is displayed on the display section 12 without the aliasing artifact L' (Sr8).

Next, a case where the optimum pixel size ΔX is automatically set by the apparatus without the operator setting the ROI will be explained according to the flowchart of FIG.

The sequencer 10 sets the maximum number of encodes Nmax shown in FIG. 6 under the control of the system controller 20 (STII). Next, initialize the number of encodings n (n
= O) (ST12).

Furthermore, initialize the encode gradient magnetic field strength (ST13)
. Next, the sequencer 1o adds phase encode gradient magnetic field increments Δ+Ge and Δ−Ge to separately collect two lines of MR data (ST14). Further, the sequencer 10 determines for each line whether the signal strength S (0, n) of the MR data is larger than the threshold value Ss (S (0, n)≧S+n) (ST15). Subsequently, the sequencer 1o adds the increment ΔGe to the previous phase encode gradient magnetic field Ge to set the next phase encode gradient magnetic field Ge (ST16).
). Then, the sequencer control means 26 controls Nmax/2>
Make a judgment about lnl (ST17), and if Yes,
Proceed to step 5T14, and if No, N.
=n, and the collection of MR data ends (ST18).

The image is then reconstructed by the image creation unit 11 in the same manner as described above (ST19), and an MR image with a good S/N ratio is displayed on the display unit 12 based on the MR data collected in the optimal frequency band of the subject. (ST20).

Although one embodiment has been described above, the present invention is not limited to this, and various modifications can be made without changing the gist thereof.

For example, step ST6 and subsequent steps shown in FIG. 7 may be performed after step 5T11 shown in FIG. 8.

By doing this, the optimum FOV and pixel size Δ
X is automatically determined.

Furthermore, although the case where the ROI is set in a rectangular shape has been described, even if the ROI is set in an arbitrary shape, it may be recognized as a rectangular line circumscribing the ROI.

[Effect of the Invention] According to the invention described in claim 1 detailed above, the encoding direction determining means compares the vertical and horizontal sizes of the ROI, sets the shorter one as the phase encoding direction, and sets the longer one as the phase encoding direction. is set in the direction of frequency encoding, so
It is an object of the present invention to provide an MRI apparatus that can perform imaging of a necessary part in a short time with the necessary spatial resolution, and can obtain an MR image with an optimal S/N ratio and spatial resolution without aliasing artifacts. can.

Further, according to the invention as claimed in claim 2, even when the Rot is set in a rectangular shape inward in the phase encoding direction from the contour of the subject and in accordance with the observation target region, the optimum FOV determining means prevents aliasing artifacts from occurring in the diagnostic MR image. Since the FOV is automatically set so as not to interfere, the same effect as claimed in claim 1 can be obtained.

Furthermore, according to the third aspect of the invention, the sequencer control means compares the signal value of the MR data with the threshold value stored in the threshold value storage means, and when the signal value of the MR data and the threshold value are approximately equal, the sequencer control means compares the signal value of the MR data with the threshold value stored in the threshold value storage means. Since a stop signal is sent to stop the addition of the gradient magnetic field increment for phase encoding, other parameters that determine the pixel size can be easily determined, and MR data can be collected in the optimal frequency band of the subject. , the same effect as claimed in claim 1 can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the operation of the optimum FOV determination means of this apparatus, and FIGS. 3 to 6 are operation of the parameter determination means and threshold storage means. FIG. 7 and FIG. 8 are flowcharts showing the operation of this device, and FIG. 9 is a diagram showing an example of an MR image in a conventional device. 10... Sequencer, 22... FOV determining means, 2
3...Encoding direction determining means, 25...Threshold value storage means, 26...Sequencer control means, Ss...Threshold value of signal value of MR data, ΔGe...Gradient magnetic field increment for phase encoding, LO・...Subject outline, P...Subject, ΔTr...Sampling pitch.

Claims (3)

[Claims] (1) ROI set in a rectangular shape on the positioning MR image
In an MRI apparatus that performs MR imaging to obtain a diagnostic MR image, the vertical and horizontal sizes of the ROI are compared, and the shorter one is set in the phase encoding direction, and the longer one is set in the frequency encoding direction. MR imaging is performed based on both encoding directions determined by the encoding direction determining means, and diagnostic MR
An MRI apparatus characterized by obtaining images. (2) The ROI is set in a rectangular shape on the positioning MR image inward in the phase encoding direction from the outline of the subject according to the observation target region, and
It has an optimal FOV determination means that automatically sets the FOV so that aliasing artifacts do not appear in the image, and this optimal FOV
2. The MRI apparatus according to claim 1, wherein MR imaging is performed with respect to the FOV automatically set by the V determining means to obtain a diagnostic MR image. (3) In an MRI apparatus that collects, at a predetermined sampling pitch, MR signals obtained each time a phase encoding gradient magnetic field is applied to which predetermined phase encoding gradient magnetic field increments are added stepwise based on the control of a sequencer. a threshold storage means that stores in advance a threshold value of a signal value of the MR data to be collected; and for each collected MR data, the signal value of this MR data is compared with the threshold value stored in the threshold storage means;
An MRI apparatus comprising: sequencer control means for sending a stop signal to the sequencer to stop adding a phase encoding gradient magnetic field increment when a signal value of the MR data and a threshold value become substantially equal.