Magnetic resonance imaging apparatus and method for setting shim values

The invention is intended to obtain optimal shim values even if slice planes are slanted with respect to the system-inherent coordinate system. The section for setting a plane for calculating shim values sets planes perpendicular to and a plane parallel with a slice plane in a system of coordinates x′, y′, and z′ perpendicular to the slice plane. The section for calculating shim values obtains shim values with regard to this slice plane in this coordinate system, based on data acquired by the data acquiring section. The coordinates converting section converts the thus obtained shim values to shim values in the system-inherent x-y-z coordinate system of the MRI apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Application No. 2006-156426 filed Jun. 5, 2006.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic resonance imaging system that calculates shim values and corrects inhomogeneity of static magnetic field based on these values and a method for setting shim values.

An MRI apparatus applies a magnetic field to a subject and produces a tomographic image of the subject, based on detected echo signals from the subject. If the static magnetic field is not homogeneous, phase rotation occurs due to this and causes ghosting and positional distortion (artifacts); it is thus required to keep the static magnetic field homogeneous in order to accurately reflect the internal structure of a probed portion of the subject.

For the MRI, shimming (correcting inhomogeneity of static magnetic field) is important to reduce such artifacts.

When shimming is carried out, conventionally, shim values which are parameters for shimming are calculated based on data acquired by scanning the planes (e.g., x, y, and z planes) based on a system-inherent coordinate system (e.g., a system of x, y, and z coordinates). However, this method has a drawback in which, when slice planes (including tomographic imaging data) are slanted with respect to the system-inherent coordinate system, acquired shim values are non-optimal and it is unable to well correct inhomogeneity.

SUMMARY OF THE INVENTION

It is desirable that the problem described previously is solved.

A first aspect of the invention resides in a magnetic resonance imaging apparatus, which carries out shimming, comprising a section for setting a slice plane, which sets at least one slice plane for obtaining desired tomographic image data; and a section for setting shim values, which calculates and sets shim values for shimming with regard to the slice plane set by the section for setting a slice plane, wherein the section for setting shim values calculates shim values in a system of coordinates perpendicular to the slice plane and, based on these shim values, calculates shim values in a system-inherent coordinate system for the whole of the magnetic resonance imaging apparatus.

A second aspect of the invention resides in a method for setting shim values in a magnetic resonance imaging apparatus, which carries out shimming, the method comprising a first step of setting at least one slice plane for obtaining desired tomographic image data; and a second step of calculating shim values in a system of coordinates perpendicular to the slice plane set in the first step and, based on these shim values, calculating shim values in a system-inherent coordinate system for the whole of the magnetic resonance imaging apparatus.

According to the invention, it is possible to provide a magnetic resonance imaging apparatus and a method for setting shim values, wherein optimal shim values can be obtained even if slice planes are slanted with respect to the system-inherent coordinate system.

DETAILED DESCRIPTION OF THE INVENTION

In the following, an embodiment of the invention will be described.

MRI apparatus (magnetic resonance imaging apparatus)1of the invention has a feature in which it applies a unique manner of determining target regions from where shim values are acquired for shimming to correct inhomogeneity of static magnetic field applied to a subject.

As is shown inFIG. 1, the MRI apparatus1comprises a scanning unit2and an operating console3.

A system of x, y, and z coordinates, which is shown inFIG. 1, is the coordinate system for the whole of the MRI apparatus1(system-inherent coordinate system).

The scanning unit2, as shown inFIG. 1, comprises static magnetic field generating magnet assemblies21, gradient coil assemblies22, an RF coil assembly23, and a table24, and, within an imaging space in which a static magnetic field is formed, performs scanning which involves applying electromagnetic waves to a subject SU to excite an imaging region of the subject SU and acquiring magnetic resonance signals generated in the imaging region of the subject SU.

The components of the scanning unit2are described one by one.

The static magnetic field generating magnet assemblies21are configured using, for example, a pair of permanent magnet assemblies and form the static magnetic field within the imaging space where the subject SU lies. Herein, the static magnetic field generating magnet assemblies21form the static magnetic field so that the direction of the static magnetic field aligns with a direction z vertical to the axial direction of the body of the subject SU. The static magnetic field generating magnet assemblies21may be configured using superconductive magnet assemblies.

The gradient coil assemblies22form gradient magnetic fields in the imaging space where the static magnetic field is formed to add spatial position information to magnetic resonance signals which the RF coil assembly23receives. Herein, the gradient coil assemblies22consist of three systems of x, y, and z directions and form gradient magnetic fields in a frequency encoding direction, in a phase encoding direction, and in a slice selection direction, respectively, according to imaging conditions. In particular, the gradient coil assemblies22apply a gradient magnetic field in a slice selection direction of the subject SU and the RF coil assembly23sends RF pulses; thereby, a slice of the subject SU, which is excited, is selected. Also, the gradient coil assemblies22apply a gradient magnetic field in a phase encoding direction of the subject SU to phase encode the magnetic resonance signal from the slice excited by the RF pulses. Besides, the gradient coil assemblies22apply a gradient magnetic field in a frequency encoding direction of the subject SU to frequency encode the magnetic resonance signal from the slice excited by the RF pulses.

This MRI apparatus1adopts a method in which magnetic field inhomogeneity correction in the imaging region within the static magnetic field is performed by primary shimming and is arranged so that the primary shimming is performed by adjusting the gradient coils in the x, y, and z axis directions.

However, the invention is not so limited and may be configured such that shim coils exclusively used for shimming are provided in the static magnetic field generating magnet assemblies21and magnetic field inhomogeneity correction is performed by these shim coils.

The RF coil assembly23is placed to enclose the imaging region of the subject SU, as shown inFIG. 1. The RF coil assembly23sends RF pulses of electromagnetic waves to the subject SU and forms a radio-frequency magnetic field within the imaging space where the static magnetic field is formed by the static magnetic field generating magnet assemblies21and thus excites spins of protons in the imaging region of the subject SU. Then, the RF coil assembly23receives electromagnetic waves generated from the excited protons inside the subject SU as magnetic resonance signals (MR signals).

The table24serves as a platform on which the subject SU is rested. The table24moves in the imaging space and moves out therefrom, according to a control signal from the operating console3.

The operating console3, as shown inFIG. 1, comprises a section for setting a slice plane31, a section for setting shim values32, an image producing section33, an operating section34, a display section35, and a storage section36.

The components of the operating console3are described one by one.

The section for setting a slice plane31sets a slice plan including tomographic image data acquired with regard to the subject SU, based on instructions or the like given via the operating section34which will be described later. Here, the section for setting a slice plane31may set a plurality of slice planes.

The section for setting shim values32sets parameters (shim values) for shimming appropriate for the set slice plane. Here, the shimming is a processing of inhomogeneity correction in the imaging region where the static magnetic field has been disordered due to the entrance of the subject SU into the static magnetic field that was homogeneous.

The section for setting shim values32comprises a section for setting a plane for calculating shim values321, which sets a plane on which data is acquired in a data acquiring section322for calculating shim values appropriate for the slice plane set by the section for setting a slice plane31, a section for calculating shim values323, which calculates shim values with regard to the plane set by the section for setting a plane for calculating shim values321, and a coordinates converting section324.

The section for setting a plane for calculating shim values321sets a system of coordinates x′, y′, and z′ perpendicular to the slice plane, as is shown inFIG. 2.

This will be further explained below, taking an instance where the section for setting a plane for calculating shim values321sets a coordinate system such that the slice plane becomes parallel with the z′-x′ plane of the x′-y′-z′ coordinate system. This is only illustrative and the preset invention is not so limited; the only requirement of a coordinate system that is set by the section for setting a plane for calculating shim values321is that its coordinates are perpendicular to the slice plane. For example, a coordinate system such that the slice plane becomes parallel with its x′-y′ plane may be set.

FIG. 2is a diagram showing a relationship between a slice plane P0and an x′-y′-z′ coordinate system set by the section for setting a plane for calculating shim values321.

Next, the section for setting a plane for calculating shim values321sets planes P1and P2perpendicular to the slice plane P0, as is shown inFIG. 3.

FIG. 3is a diagram showing an example of how to set planes P1and P2.

The plane P1is, for example, a plane that is parallel with the x′-y′ plane of the x′-y′-z′ coordinate system and the plane P2is a plane that is parallel with the y′-z′ plane of the x′-y′-z′ coordinate system.

Although a plurality of coordinate systems are possible as those in which the coordinates are perpendicular to one slice plane, any of them may be set by the section for setting a plane for calculating shim values321. Although a plurality of planes are possible as those perpendicular to the slice plane, any of them may be set as P1and P2and, additionally, multiple P1and P2planes may be set. In the present embodiment, an instance where P1and P2are set as shown inFIG. 3is discussed by way of illustration.

Next, the section for setting a plane for calculating shim values321sets a plane P3that is parallel with the slice plane P0, as is shown inFIG. 4. Although a plurality of planes are possible as those parallel with the slice plane, any of them may be set as P3and, additionally, a plurality of P3planes may be set. In the present embodiment, an instance where P3is set as shown inFIG. 4is discussed by way of illustration.

FIG. 4is a diagram showing an example of how to set a plane P3.

The data acquiring section322sends the scanning unit2a control signal for acquiring data for calculating shim values with regard to the planes set by the section for setting a plane for calculating shim values321and acquires the data. Shimming is performed with regard to the planes P1to P3set by the section for setting a plane for calculating shim values321.

The section for calculating shim values323calculates shim values based on the data acquired from the planes P1to P3by the data acquiring section322.

Here, the section for calculating shim values323performs a weighting operation to weight the shim values calculated based on the data acquired from the planes P1to P3by the data acquiring section322so that more precise shimming can be performed.

Referring toFIG. 5, the weighting operation that is performed by the section for calculating shim values323will be explained below in detail.

FIG. 5is a set of diagrams to explain the weighting operation that is performed by the section for calculating shim values323.

FIG. 5(a) shows an original image produced based on the data acquired by based on the data acquiring section322with regard to the slice plane set by the section for setting a slice plane31.

FIG. 5(b) shows a masked image resulting from filtering the original image shown inFIG. 5(a) using predetermined threshold values. The predetermined threshold values are not specified here. These values can be varied optionally according to an intended image and a probed portion of the subject.

FIG. 5(c) shows a weighted image obtained by logically multiplying the original image shown inFIG. 5(a) with the masked image shown inFIG. 5(b).

Then, the section for calculating shim values323further produces another weighted image which is obtained by logically multiplying another original image obtained at a slight time shift from the time at which the original image shown inFIG. 5(a) was produced with another masked image resulting from filtering the original image obtained at the time shift.

Finally, the section for calculating shim values323obtains a phase difference between the two weighted images obtained at different time instants and produces a phase image as shown inFIG. 5(d).

Here, as shown inFIG. 5, when a region where data for shimming can be taken (hereinafter termed a signal region; a white portion shown inFIG. 5(a)) is separated into a plurality of regions (regions a and b shown inFIG. 5(d) inFIG. 5), weighting in proportion to area ratio is performed for the respective signal regions.

In particular, if, in the phase image obtained inFIG. 5(d), the signal region a occupies an area of x % within the entire phase image shown inFIG. 5(d) and the region b occupies an area of y % within the entire phase image shown inFIG. 5(d), shim values are calculated as follow:
(shim values obtained in region a)×x/100+(shim values obtained in region b)×y/100.

The resulting values are taken as the shim values with regard to the image region shown inFIG. 5(a).

In this manner, the section for calculating shim values323automatically detects signal regions for calculating shim values and thereby can calculate optimal shim values.

Although a method of setting target regions for calculating shim values has been described here, the invention does not specify a method in which the section for calculating shim values323calculates shim values and, therefore, a description of how shim values are calculated from the set regions is omitted. The section for calculating shim values323calculates shim values from the set regions, using a known technique.

FIG. 6is a conceptual diagram of coordinates conversion that is performed by the coordinates converting section324. Here, the shim values calculated by the section for calculating shim values323in the manner described above are the values in the system of coordinates perpendicular to the slice plane, as set by the section for setting a plane for calculating shim values321, and represented as follows: (shim_x′, shim_y′, shim_z′). Hence, the shim values calculated by the section for calculating shim values323have to be converted to those values in the system-inherent coordinate system, because the MRI apparatus1must use the shim values in the system-inherent coordinate system of the MRI apparatus1when actually performing shimming.

For the above-noted reason, the coordinates converting section324converts the shim values (shim_x′, shim_y′, shim_z′) in the x′-Y′-z′ coordinate system, calculated by the section for calculating shim values323, to shim values (shim_x, shim_y, shim_z) in the x-y-z coordinate system that is the system-inherent coordinate system of the MRI apparatus1.

The conversion method may be a conversion method using tensor R for converting the x′-Y′-z′ coordinate system to the x-y-z coordinate system. The invention does not specify this conversion method.

The section for setting shim values32performs shimming based on the shim values (shim_x, shim_y, shim_z) in the system-inherent coordinate system, output by the coordinates converting section324. The invention does not specify a method in which shimming is performed.

The image producing section33sends the scanning unit2a control signal to execute a scan after shimming is performed by the section for setting shim values32with regard to the slice plane set by the section for setting a slice plane31and produces an MR image based on MR signals from the RF coil assembly23.

The operating section34comprises operating devices such as a keyboard and a pointing device. The operating section34takes instruction data input by an operator and outputs the instruction data to each section.

The display section35comprises a display device such as CRT and displays an image on a screen. For example, the display section35displays on the screen an image including a plurality of entry items for which instruction data is input to the operating section34by the operator. Also, the display section35receives data corresponding to a slice image of the subject SU, which is generated based on magnetic resonance signals from the subject SU, and displays the slice image on the screen.

The storage section36comprises a memory and stores a variety of data. The data stored in the storage section36is accessed as required.

Then, how the MRI apparatus1of the present embodiment operates when calculating shim values is described.

FIG. 7is a flowchart to illustrate an example of the operation of the MRI apparatus1when calculating shim values.

The section for setting a slice plane31sets a slice plane according instructions or the like specified via the operating section34.

The section for setting a plane for calculating shim values321sets a system of coordinates x′, y′, and z′ perpendicular to the slice plane set by the section for setting a slice plane31in step ST1.

The section for setting a plane for calculating shim values321sets planes P1and P2which are perpendicular to the slice plane and a plane P3which is parallel with the slice plane in the x′-y′-z′ coordinate system set in step ST2.

The data acquiring section322acquires data with regard to the planes P1to P3set in step ST3.

The section for calculating shim values323obtains shim values (shim_x′, shim_y′, shim_z′) with regard to the slice plane in the x′-y′-z′ coordinate system, based on the data acquired in step ST4.

The section for calculating shim values323performs a weighting operation to weight the shim values calculated in step ST5, based on the area ratios of signal regions.

The coordinates converting section324converts the shim values (shim_x′, shim_y′, shim_z′) in the x′-Y′-z′ coordinate system to shim values (shim_x, shim_y, shim_z) in the system-inherent x-y-z coordinate system of the MRI apparatus1.

As described above, according to the MRI apparatus1of the present embodiment, because data is acquired and the section for calculating shim values323calculates shim values in a system of coordinates perpendicular to a slice plane that was set, independent of the system-inherent coordinate system, the shim values optimal for the slice plane can be calculated.

Since the section for calculating shim values323calculates shim values based on data acquired with regard to two planes perpendicular to the slice plane and one plane parallel with the slice plane in the system of coordinates perpendicular to the slice plane, more precise shim values can be calculated.

Since the section for calculating shim values323calculates shim values weighted in proportion to the area ratios of signal regions, more precise shim values can be calculated.

In the foregoing embodiment, data is acquired and shim values are calculated, based on planes P1, P2perpendicular to and a plane P3parallel with one slice plane set by the section for setting a plane for calculating shim values321; however, the invention is not so limited. It may be possible to acquire data and calculate shim values based on only one of the planes P1to P3or to acquire data and calculate shim values from all of a plurality of planes perpendicular to and planes parallel with a plurality of slice planes.