JP4678926B2 - MRI equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a multi-slice ultra high speed MR (Magnetic Resonance Imaging) imaging method and an MRI (Magnetic Resonance Imaging) apparatus. More specifically, the present invention relates to a multi-slice ultrahigh-speed MR imaging method and an MRI apparatus that can obtain an MR image in which an artifact called N / 2 ghost is suppressed in any slice.
[0002]
[Prior art]
When the MR image is reconstructed using the image creation data obtained by the imaging pulse sequence of the single shot EPI (Echo Plannar Imaging) method as it is, it is called an N / 2 ghost on the MR image as shown in FIG. Artifact A is produced. Therefore, reference data is collected by reference scanning using a reference pulse sequence that does not add only the phase gradient of the pulse sequence of the single shot EPI method, and the image creation data is phase-corrected based on the phase information of the reference data. An MR image is reconstructed from phase-corrected image creation data.
[0003]
In the multi-slice ultrahigh-speed MR imaging method that obtains MR images for a plurality of slices in a short time, the image creation data for one slice is repeatedly collected while changing the slice position by the pulse sequence of the single shot EPI method.
In the case of this multi-slice ultra-high speed MR imaging method, the reference scan is performed on the central slice to collect reference data, and the phase correction is performed on the image creation data of all slices based on the phase information of the reference data. An MR image of each slice is reconstructed from the corrected image creation data.
[0004]
[Problems to be solved by the invention]
The cause of artifacts called N / 2 ghosts is the misalignment between the echo center and sampling center point caused by eddy currents. On the other hand, when the slice position is different, the influence of eddy current or the like is different. Therefore, in order to suppress an artifact called N / 2 ghost in the multi-slice ultra-high speed MR imaging method, it is necessary to perform different phase correction for slices at different positions.
However, in the conventional multi-slice ultrahigh-speed MR imaging method, the reference scan is performed only for one central slice, and therefore an artifact called N / 2 ghost can be suppressed for the slice and its neighboring slices. However, there is a problem that an artifact called an N / 2 ghost is generated on an image in a slice away from the image.
Accordingly, an object of the present invention is to provide a multi-slice ultrahigh-speed MR imaging method and an MRI apparatus capable of obtaining an MR image in which an artifact called N / 2 ghost is suppressed in any slice.
[0005]
[Means for Solving the Problems]
In a first aspect, the present invention is a multi-slice ultra-high-speed MR imaging method in which acquisition of image creation data for one slice is repeated while changing the slice position by a pulse sequence of the single shot EPI method. Collecting reference data for one slice by a reference pulse sequence without adding only the phase gradient of the pulse sequence of the shot EPI method is repeated at two or more slice positions while changing the slice position, and obtained reference data Provides a multi-slice ultra-high-speed MR imaging method characterized in that phase correction is performed on image data for a corresponding slice based on the phase information of the image, and MR images of each slice are reconstructed from the phase-corrected image generation data To do.
In the multi-slice ultrahigh-speed MR imaging method according to the first aspect, the reference scan is not performed for only one central slice but the reference scan is performed for each slice at two or more slice positions to collect reference data. To do. Then, the phase of the image creation data of each slice is corrected with the phase information of the reference data of the same slice or the nearest slice. Therefore, an MR image in which an artifact called N / 2 ghost is suppressed can be obtained in any slice.
[0006]
In a second aspect, the present invention provides a transmission coil for transmitting an RF pulse, a gradient coil for applying a gradient magnetic field, a reception coil for receiving an NMR signal, the transmission coil and the gradient coil, A main scanning unit that drives the reception coil and collects data for image creation for one slice by a pulse sequence of the single shot EPI method while changing the slice position; the transmission coil, the gradient coil, and the reception coil; A reference scan that repeats at two or more slice positions while changing the slice position by collecting the reference data for one slice by the reference pulse sequence without adding only the phase gradient of the pulse sequence of the single shot EPI method by driving And the corresponding slurry based on the phase information of the obtained reference data. Providing a phase correction means for image generation data phase correction of scan, an MRI apparatus wherein by comprising a reconstructing means for reconstructing an MR image of each slice from the image creation data phase correction.
In the MRI apparatus according to the second aspect, the multi-slice ultrahigh-speed MR imaging method according to the first aspect can be suitably implemented.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings. Note that the present invention is not limited thereby.
[0008]
FIG. 1 is a block diagram showing an MRI apparatus according to an embodiment of the present invention.
In the MRI apparatus 100, the magnet assembly 1 has a space portion (bore) for inserting the subject therein, and a static magnetic field that applies a constant static magnetic field to the subject so as to surround the space portion. Gradient for generating the coil 1p and gradient magnetic fields of the X, Y, and Z axes (a slice gradient axis, a lead gradient axis, and a phase encode gradient axis are formed by a combination of the X, Y, and Z axes) A magnetic field coil 1g, a transmission coil 1t that applies an RF pulse for exciting spins of nuclei in the subject, and a reception coil 1r that detects an NMR signal from the subject are arranged. The static magnetic field coil 1p, gradient magnetic field coil 1g, transmission coil 1t and reception coil 1r are connected to a static magnetic field power source 2, a gradient magnetic field drive circuit 3, an RF power amplifier 4 and a preamplifier 5, respectively.
[0009]
The sequence storage circuit 8 operates the gradient magnetic field drive circuit 3 based on the stored pulse sequence in accordance with a command from the computer 7, generates a gradient magnetic field from the gradient coil 1 g of the magnet assembly 1, and performs gate modulation. The circuit 9 is operated, and the carrier wave output signal of the RF oscillation circuit 10 is modulated into a pulse signal having a predetermined timing and a predetermined envelope shape, which is added as an RF pulse to the RF power amplifier 4 and power amplified by the RF power amplifier 4 Thereafter, it is applied to the transmission coil 1t of the magnet assembly 1 to selectively excite a desired imaging surface.
[0010]
The preamplifier 5 amplifies the NMR signal from the subject detected by the receiving coil 1 r of the magnet assembly 1 and inputs it to the phase detector 12. The phase detector 12 uses the carrier wave output signal of the RF oscillation circuit 10 as a reference signal, phase-detects the NMR signal from the preamplifier 5, and provides it to the A / D converter 11. The A / D converter 11 converts the analog signal after phase detection into digital data and inputs the digital data to the computer 7.
[0011]
The computer 7 is responsible for overall control such as receiving information input from the console 13. Further, the computer 7 reads digital data from the A / D converter 11 and performs an image reconstruction operation to generate an MR image.
The display device 6 displays the MR image.
[0012]
FIG. 2 is a flowchart of multi-slice ultrahigh-speed MR imaging processing by the MRI apparatus 100.
In step P1, the slice number counter i is initialized to “1”.
[0013]
In Step P2, reference data for the i-th slice is collected by a pulse sequence of the single shot EPI method without adding a phase gradient.
FIG. 3 illustrates the first slice to the Ith slice.
FIG. 4 illustrates a pulse sequence of the single shot EPI method without adding a phase gradient.
In this pulse sequence, an excitation pulse R1 and a slice gradient S1 are applied. Next, the inversion pulse R2 and the slice gradient RS are applied. Next, positive and negative lead gradients RO and RE are alternately applied m (for example, m = 256) times in succession, and a plurality of echoes e1 to em are sampled in time and sampled, and each echo e1 to e1 is sampled. Data d1 to dm corresponding to em are collected. No phase encoding gradient is applied.
[0014]
In Steps P3 and P4, Step P2 is repeated for the second slice to the Ith slice.
[0015]
In step P11, the slice number counter i is initialized to “1”.
[0016]
In Step P12, image creation data for the i-th slice is collected by a pulse sequence of the single shot EPI method.
FIG. 5 illustrates a pulse sequence of the single shot EPI method.
In this pulse sequence, an excitation pulse R1 and a slice gradient S1 are applied. Next, the inversion pulse R2 and the slice gradient RS are applied. Next, positive and negative lead gradients RO and RE are alternately and continuously applied m (for example, m = 256) times, and phase encode gradients V and W are applied, and a plurality of echoes e1 to em are sequentially imaged. And the data d1 to dm corresponding to the echoes e1 to em are collected.
[0017]
In Steps P13 and P14, Step P12 is repeated for the second slice to the Ith slice.
[0018]
In step P21, the slice number counter i is initialized to “1”.
[0019]
In step P22, phase correction is performed on the image creation data of the i-th slice with the phase information of the reference data for the i-th slice.
In step P23, the MR image of the i-th slice is reconstructed from the phase-corrected image creation data for the i-th slice.
[0020]
In Steps P24 and P25, Steps P22 and P23 are repeated for the second slice to the Ith slice.
[0021]
As described above, MR images of the first slice to the Ith slice can be obtained in a short time. Since these MR images are created from image creation data that has been phase-corrected with reference data at the same slice position, an artifact called N / 2 ghost is sufficiently suppressed.
[0022]
The first slice to the I-th slice are divided into groups of, for example, three adjacent slices, reference data is collected only for the middle slice of each group, and image creation data for the same group is used for reference of the group. The phase may be corrected based on the phase information of the data. In this case, the effect of suppressing an artifact called N / 2 ghost is slightly lower in the slices at both ends of each group than in the middle slice, but there is an advantage that the reference scan time can be shortened to 1/3.
[0023]
【The invention's effect】
According to the multi-slice ultra-high-speed MR imaging method and the MRI apparatus of the present invention, it is possible to shorten the MR image for a plurality of slices by repeating the acquisition of image creation data by the pulse sequence of the single shot EPI method while changing the slice position. When obtained in time, an MR image in which an artifact called N / 2 ghost is suppressed can be obtained in any slice.
[Brief description of the drawings]
FIG. 1 is a block diagram of an MRI apparatus according to an embodiment of the present invention.
2 is a flowchart of multi-slice ultrahigh-speed MR imaging processing in the MRI apparatus of FIG. 1;
FIG. 3 is a diagram illustrating a plurality of slice positions.
FIG. 4 is an exemplary diagram of a pulse sequence of a reference scan.
FIG. 5 is an exemplary diagram of a pulse sequence of the main scan.
FIG. 6 is an exemplary view of an MR image in which an artifact called an N / 2 ghost has occurred.
[Explanation of symbols]
100 MRI apparatus 1 Magnet assembly 3 Gradient magnetic field drive circuit 7 Computer 8 Sequence storage circuit V, W Phase encoding gradient

Claims (1)

A transmission coil for transmitting RF pulses;
A gradient coil for applying a gradient magnetic field;
A receiving coil for receiving NMR signals;
Collecting image creation data for one slice by a single shot EPI pulse sequence by driving the transmission coil, gradient coil, and reception coil, and repeating the image for a plurality of slices arranged while changing the slice position A main scanning means for collecting data for creation ;
The three slices in the order in which the plurality of slices are arranged are grouped into one group, and the transmitting coil, the gradient coil, and the receiving coil are driven for the slice at the same position as the central slice of each group. Reference scanning means for collecting reference data by a reference pulse sequence that does not add only the phase gradient of the pulse sequence of the single shot EPI method;
Phase correction means for correcting the phase of the image creation data of the slices in each group based on the obtained phase information of the reference data in each group divided into groups .
An MRI apparatus comprising: reconstruction means for reconstructing an MR image of each slice from the phase-corrected image creation data.

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