Patent Description:
In an MRI apparatus, nuclear magnetic resonance signals generated from an examination subject caused by nuclear magnetic resonance are collected to create an image of the examination subject. The nuclear magnetic resonance signals are collected by operating a pulse sequence. In the pulse sequence, there are determined application intensity and the order of application to apply pulses, such as RF pulses for exciting nuclear spins (normal protons) of atoms in the tissue of the examination subject and gradient magnetic field pulses for adding position information to the nuclear magnetic resonance signals. Even when the nuclear magnetic resonance signals are generated from the same nuclear spin, the signal intensity and phase vary depending on the tissue where the nuclei are present. Thus, imaging is performed with settings of the pulse sequence and imaging conditions that allow enhancement of ability for visualizing the tissue, in response to properties of the tissue as a target of the imaging.

A typical one of pulse sequences used in the MRI apparatus is a spin echo (SE) type pulse sequence (hereinafter, also referred to as an "SE pulse sequence") that uses an excitation pulse (<NUM>° pulse) and a <NUM>° pulse to measure a nuclear magnetic resonance signal generated in the form of a spin echo (echo signal). For example, this pulse sequence is utilized for taking a T1 enhanced image reflecting a time constant of a longitudinal relaxation time of tissue.

In the imaging using the SE pulse sequence, since nuclear spins in blood that flows through the imaging plane depend on the velocity of the blood flow, there occurs a phase change different from that of the nuclear spins from stationary tissue in the imaging plane. Thus this causes a flow artifact along the phase-encoding direction in a reconstructed image. In general, there are widely known methods for reducing this kind of flow artifact, including following methods; a method of adding a flow compensation pulse for canceling the phase change of the spins of fluid at the time of collecting echo signals, and a method of applying a pre-saturation pulse for previously saturating spins in a nearby region outside the imaging target region to control the signals.

Chinese Patent <CIT> (hereinafter, referred to as Patent Literature <NUM>) and <NPL>, having the features in the pre-characterizing portion of Claim <NUM> in common with the present invention suggest that in the imaging using the SE pulse sequence, an excitation thickness is increased in either of a <NUM>° pulse and a <NUM>° pulse. In this technique, the center frequency positions of the <NUM>° pulse and the <NUM>° pulse are not changed, and one excitation width is extended to be wider than the other, thereby preventing the fluid, e.g., blood, excited in one cross section, from flowing into the next cross section and interfering with image formation of the next cross section, resulting in that this reduces the flow artifact.

In the technique disclosed in Patent Literature <NUM>, the regions on both sides adjacent to the imaging plane are excited by the <NUM>° pulse or the <NUM>° pulse, so that magnetization of the excited regions does not contribute to forming an image of the imaging plane, and the magnetization is reduced in the next cross section. However, when focusing on the imaging plane, the blood located on the upstream side of the cross section and excited by the <NUM>° pulse or the <NUM>° pulse, that is, the blood having extra magnetization, flows into the imaging plane, resulting in that this increases artifacts.

An object of the present invention is to prevent the increase of the blood flow artifacts in the imaging plane, so that the blood flow artifacts can be reduced in the cross section to be excited next.

In order to solve the above-described problem, in the imaging using an SE pulse sequence, the excitation width of a <NUM>° pulse is extended to one side from the imaging plane. The one side is assumed as the downstream side of a blood flow with respect to a vessel of interest. When imaging of multiple cross sections is performed, the order of measuring the imaging planes is set along the direction in which the width is extended.

The MRI apparatus and the control method of the MRI apparatus of the present invention are defined in the independent claims. Further advantageous features are set out in the dependent claims.

According to the present invention, it is possible to reduce artifacts caused by the fluid flowing into the imaging plane, and also to reduce artifacts caused by the fluid flowing into another cross section on the downstream side of the imaging plane. As a result, an image can be obtained with reduced flow artifacts in the entire volume of the examination subject.

There will now be described embodiments of an MRI apparatus according to the present invention. First, an outline of the MRI apparatus to which the present invention is applied will be described. As shown in <FIG>, MRI apparatus <NUM> comprises, as a measurement unit <NUM>, a static magnetic field generation magnet <NUM>, a gradient coil <NUM> configured to apply a magnetic field gradient to a static magnetic field, an RF transmission coil <NUM> configured to irradiate a subject <NUM> in a static magnetic field space with an RF magnetic field, an RF receive coil <NUM> configured to receive a nuclear magnetic resonance signal generated from a subject <NUM>, an RF reception unit <NUM>, an RF transmission unit <NUM> configured to supply RF pulse current to the RF transmission coil <NUM>, a signal processing unit <NUM> configured to detect an NMR (nuclear magnetic resonance) signal received by the RF receive coil <NUM> to perform signal processing, and a gradient magnetic field power supply <NUM> configured to supply a current to the gradient coil <NUM>. The subject <NUM> is laid on a table <NUM> and disposed in the static magnetic field space generated by the static magnetic field generation magnet <NUM>.

Further, the MRI apparatus comprises, as a control/arithmetic system, a measurement control unit <NUM> configured to control operations of the RF transmission unit <NUM>, the RF reception unit <NUM>, the signal processing unit <NUM>, and the gradient magnetic field power supply <NUM>, a computer <NUM> including a CPU <NUM>, a memory <NUM>, and an internal storage device (not shown), an external storage device <NUM> connected to the computer <NUM>, and a user interface (UI) unit <NUM> including a display device, an input device, and others. The computer <NUM> may be connected to an external network such as the Internet and an intranet, via a network IF (not shown).

The static magnetic field generation magnet <NUM> comprises a static magnetic field generating source of a permanent magnet type, a normal conducting type, or a superconducting type, and depending on the direction of the static magnetic field being generated, the magnet may be a vertical magnetic field type, a horizontal magnetic field type, and so on. The present invention can be applied to any type of the static magnetic field generation magnets.

The gradient coils <NUM> are wound respectively in three axial directions of X, Y, and Z, which are real space coordinate systems (stationary coordinate systems) of the MRI apparatus. Pulse-like current is supplied from the gradient magnetic field power supply <NUM> to each of the gradient coils, so that gradient magnetic field pulses can be applied in desired directions. By applying the gradient magnetic field, position information is added to the NMR signals. Specifically, for example, in the case of imaging a two-dimensional cross section (slice), a gradient magnetic field is applied in the slice direction, thereby determining the slice position. In the state that this slice position is excited, one of the two directions orthogonal to the slice plane is set as the phase encoding direction, and the other direction is set as the readout direction, and the gradient magnetic field pulses are applied respectively to the directions, thereby encoding the generated NMR signals in the respective directions. Usually, the NMR signals are collected as echo signals of the RF pulse or of the gradient magnetic field pulses, and thus they are also referred to as echo signals.

The RF transmission coil <NUM> generates a pulse-like induced magnetic field (RF pulse) by the RF pulse current supplied from the RF transmission unit <NUM>. The RF pulse excites nuclear spins of the atoms constituting the tissue of the subject <NUM>, and generates NMR signals. The RF receive coil <NUM> detects the echo signals from the subject <NUM> and sends the echo signals to the signal processing unit <NUM>. The signal processing unit <NUM> performs quadrature detection and AD conversion on the echo signals to obtain time-series digital data, and then performs various processing required for image reconstruction.

Settings such as the intensity and timing of the RF pulse and the gradient magnetic field pulses of respective axes, and the timing of signal collection (sampling) are predetermined in the form of a pulse sequence for each imaging, and such settings are provided in the measurement control unit <NUM>. The measurement control unit <NUM> uses thus provided pulse sequence, and imaging conditions and imaging parameters set by a user via the UI unit <NUM> so as to calculate the pulse sequence used for the imaging (imaging pulse sequence). Then, according to the calculated imaging pulse sequence, the measurement control unit <NUM> controls the RF transmission unit <NUM>, the gradient magnetic field power supply <NUM>, and the signal processing unit <NUM>. This allows collection of echo data necessary for reconstructing the image of the subject <NUM>.

The computer <NUM> functions as an overall controller to control the entire apparatus including the measurement unit <NUM> and the measurement control unit <NUM>, and also functions as an arithmetic unit that performs various calculations using data such as the processing results from the signal processing unit <NUM>. Data items such as the data required for the processing by the computer <NUM> and the data in the middle of processing are stored in the memory <NUM> or in the internal storage unit. Images as the processing results, calculation results, and others, can be displayed on the display of the UI unit <NUM>, stored in the external storage device <NUM>, and further transferred via a network. The user is allowed to set imaging parameters via the UI unit <NUM>, and in addition, the user is also allowed to send a command or an instruction required for the processing in the computer <NUM>.

In the MRI apparatus <NUM> of the present embodiment, the measurement unit <NUM> performs imaging using the SE pulse sequence under the control of the measurement control unit <NUM>. In this situation, the measurement control unit <NUM> sets the excitation width of a <NUM>° pulse to extend to be wider than the thickness (slice width) of the imaging plane provided as an imaging condition. Hereinafter, embodiments of a specific pulse sequence and control of the excitation width will be described.

<FIG> illustrates an example of the SE pulse sequence executed by the MRI apparatus of the present embodiment. There are some types of imaging including three-dimensional imaging and two-dimensional imaging. In the three-dimensional imaging, a certain volume is excited at a time, and position information is given to the signals generated from within the volume after encoded by gradient magnetic field pulses. In the two-dimensional imaging, multiple cross sections (multi-slices) are sequentially excited and signals are collected for respective cross sections. <FIG> illustrates an example of the multi-slice two-dimensional imaging.

As illustrated, the SE pulse sequence applies an RF pulse (<NUM>° pulse) <NUM> for excitation and applies a <NUM>° pulse <NUM> that reverses the magnetization after a lapse of a predetermined time (TE/<NUM>) to generate a spin echo <NUM> that peaks at the echo time (TE). When the <NUM>° pulse <NUM> and the <NUM>° pulse <NUM> are applied, slice gradient magnetic field pulses <NUM> and <NUM> are respectively applied for selecting a cross section to be simultaneously excited. Thereafter, the gradient magnetic field pulse <NUM> in the phase encoding direction is applied to give position information in the phase encoding direction to the echo signal, then a gradient magnetic field pulse <NUM> in the readout direction is applied, and the echo signals <NUM> are collected for a predetermined time. The sequence <NUM> is repeated for a predetermined repetition period TR while varying the application intensity of the gradient magnetic field pulse <NUM> in the phase encoding direction, and echo signals of the number required for image reconstruction are collected. Within the repetition time TR in the pulse sequence for the present cross section, the above sequence is performed for another cross section and the echo signals are collected as well. In <FIG>, though the gradient magnetic field pulses in the slice direction (Gs), the phase encoding direction (Gp), and the readout direction (Gr) are not shown for other cross sections, they are the same as those for the slice #<NUM>, and the sequence <NUM> is repeated every TR.

In the multi-slice imaging, assuming that the cross sections are arranged in the spatial order of #<NUM>, #<NUM>, #<NUM>. , the spatially continuous cross sections are usually not selected consecutive in time, but as shown in <FIG>, for instance, the cross sections are selected every other one, such as #<NUM>, #<NUM>, #<NUM>,. and sequentially imaged.

Here, in a typical SE pulse sequence, the <NUM>° pulse and the <NUM>° pulse are applied with selecting the same cross section. As shown in <FIG>, the MRI apparatus of the present embodiment controls the <NUM>° pulse and the gradient magnetic field pulses to be applied simultaneously in such a manner that the width of the region to be excited by the <NUM>° pulse (excitation width) is extended to one side with respect to the imaging plane. That is, it is assumed that the excitation width corresponds to a width; including the imaging plane and a region that is adjacent to the imaging plane and located on the outflow side (downstream side) with respect to the inflow direction of the fluid (cerebrospinal fluid, blood, etc., hereinafter referred to as "blood" as a representative example), which flows into the imaging plane.

Which side is the downstream side is determined according to the information as the following; for example, regarding a blood vessel included in the imaging slice, when it is already known that the blood having a flow artifact as a problem flows through either an artery or a vein, the directions of the artery flow and the vein flow is substantially determined by the examination site. Thus, the user can designate the downstream side of the slice based on this information. Alternatively, when a positioning image (e.g., a scout image) for determining the slice upon imaging is displayed on the UI unit <NUM>, a GUI for receiving a designation of the downstream side as to the examination site may be displayed, and the received information can be set in the measurement control unit <NUM>.

Further, the range of the excitation width to be extended to one side of the imaging plane can be appropriately set considering other information such as the speed of the blood flow into the imaging plane. For example, it is preferable to set the range that can cover the region between the target imaging plane and the cross section to be imaged subsequent thereto. That is, it is preferable to set the excitation width such that the end of the region excited by the <NUM>° pulse is as close as possible to the end of the next imaging plane.

The <NUM>° pulse is applied without changing the position and width of the imaging plane which are provided as the imaging conditions.

The width and position of the region excited by the excitation pulse are determined by the center frequency and the bandwidth of the RF pulse, the envelope of the RF pulse, and the intensity of the slice gradient magnetic field pulse. Thus, adjustment of these information items allows variation of the position, width, and excitation profile of the cross section excited by the <NUM>° pulse. Specific techniques for changing the width of the excited region will be described later.

In the MRI apparatus, when it is set to perform imaging, the computer (control unit) <NUM> provides via the UI unit <NUM>, the pulse sequence and the imaging conditions (such as TE, TR, the slice number, the slice thickness, and the order of slice measurement) for executing the pulse sequence. Then, the computer controls the RF transmission unit <NUM> and the gradient magnetic field power supply <NUM> via the measurement control unit <NUM>, to widen the region excited by the <NUM>° pulse to the lower side of the imaging plane as shown in <FIG>.

Other imaging processing steps are similar to those of conventional SE pulse sequence imaging. For example, when the imaging is the same as the multi-slice imaging as shown in <FIG>, echo signals are sequentially collected for multiple cross sections, and images of the respective cross sections are reconstructed.

As described above, in the MRI apparatus of the present embodiment, in executing the SE pulse sequence, the excitation width of the <NUM>° pulse is extended to the downstream side with respect to the imaging plane (the downstream side with respect to the blood flowing into the cross section). Thus, magnetization is reduced in the blood that is within the excitation width and to which the <NUM>° pulse is applied, and when the blood flows into the imaging of the cross section on the downstream side, it is possible to reduce generation of signals that may cause artifacts in the cross section on the downstream side.

The blood in the upstream side of the imaging plane, flowing into the imaging plane, is not subjected to the application of the <NUM>° pulse at the time of applying the <NUM>° pulse (TE/<NUM>), and only the <NUM>° pulse is applied. Therefore, no spin echo is generated in the echo time (TE). In addition, the blood to which both the <NUM>° pulse and the <NUM>° pulse are applied partially flows out from the imaging plane. Thus, this allows reduction of the spin echoes that cause the flow artifacts.

<FIG> illustrate a difference in effects between the case where the width excited by the <NUM>° pulse is extended only to the downstream side (downstream side in the blood flow direction) of the imaging plane (<FIG>) and the case where the width excited by the <NUM>° pulse is extended to both sides of the imaging plane (<FIG>) (technique disclosed by Patent Literature <NUM>). In <FIG> and subsequent figures, the slice and the blood flow are shown in a manner rotated by <NUM> degrees relative to the illustration of <FIG>.

In both cases where the excitation width of the <NUM>° pulse is extended to both sides (<FIG>) and where it is extended only to the downstream side (<FIG>), the blood located in the region on the downstream side of the imaging plane is reduced by the application of the <NUM>° pulse, and therefore, both cases can contribute to reduction of the flow artifacts in imaging the subsequent cross section.

In the meantime, in <FIG>, at the time point (TE/<NUM>) when the <NUM>° pulse is applied after the <NUM>° pulse is applied, the blood <NUM> present in the imaging plane is applied with both the <NUM>° pulse and the <NUM>° pulse, resulting in that spin echoes are generated and flow artifacts are increased. On the other hand, in the case of <FIG>, the proportion of blood <NUM> that causes flow artifacts decreases by the amount of the blood <NUM> that has flowed into the imaging plane from the upstream side. In addition, the blood <NUM> is not subjected to application of the <NUM>° pulse and only the <NUM>° pulse is applied. Thus, the spin echo that causes flow artifacts is not generated in this portion, and the flow artifacts are not increased.

As is obvious from the comparison between the two cases above, according to the present embodiment, by extending the excitation width of the <NUM>° pulse only to the downstream side to increase the thickness, it is possible to prevent the increase in artifacts of the imaging plane, in addition to the effect of reducing artifacts on the cross section on the downstream side.

Next, there will now be described examples of specific techniques for extending the excitation width.

In the present embodiment, as shown in <FIG>, the center frequency (excitation frequency) of the <NUM>° pulse is shifted to the downstream side and the excitation range is extended to the downstream side. Which side is the downstream side is determined as provided in advance by the user designation, for example. Assuming that the excitation width is {original excitation width/<NUM> + [shift amount of excitation position]} × <NUM>, it is possible to make one end of the original excitation width coincide with one end after the excitation width is extended. That is, the excitation range includes both the region of the target slice and the region on the downstream side of the slice.

The position (z) of the slice to be excited is determined by the center frequency and the gradient magnetic field intensity, and the excitation width is determined by the bandwidth or the gradient magnetic field intensity of the <NUM>° pulse. Thus, controlling and adjusting the values set in RF transmission unit <NUM> and the gradient magnetic field power supply <NUM> allow the excitation range to extend to the downstream side as shown in <FIG>. These conditions for applying the <NUM>° pulse may be set in the measurement control unit <NUM> at the time when the slice thickness and the order of slice measurement are provided. Alternatively, only adjustment amounts may be calculated in advance respectively for multiple slice thicknesses to select and apply an appropriate adjustment amount in accordance with the imaging conditions.

In the example illustrated in <FIG>, Slice #<NUM> and Slice #<NUM> to be scanned next are not adjacent to each other, and the excitation range is extended in the region between the two slices. This technique can also be applied to the case where the slice #<NUM> and the slice #<NUM> are located within close vicinity of each other, or to the case where the region between them is narrow. In addition, <FIG> shows an example that the excitation position (center position), which corresponds to the excitation frequency after the shift, is located outside the slice #<NUM>. The excitation position, however, may be inside the slice #<NUM> if the excitation range includes the downstream side of the slice #<NUM>.

According to the present technique, the excitation width can be easily adjusted by switching the conditions for applying the <NUM>° pulse with respect to each slice.

This technique changes the excitation width using a <NUM>° pulse that is referred to as a multi-band RF pulse.

As shown in <FIG>, the multi-band RF pulse is represented as the RF pulse <NUM> obtained by combining two types of RF pulses <NUM> and <NUM> having different center frequencies. One RF pulse <NUM> indicates the <NUM>° pulse that causes the excitation position and the excitation range to be included in the scan targeted slice (here, slice #<NUM>), and the other RF pulse <NUM> is the <NUM>° pulse that causes the excitation position and the excitation range to be included in a region on further downstream of the fluid flowing into the slice, than the slice to be scanned. The two <NUM>° pulses <NUM> and <NUM> differ only in excitation frequency and have the same excitation width. By applying the combined multi-band RF pulses <NUM>, the slice #<NUM> and the neighboring downstream-side region are simultaneously excited, as shown in <FIG>.

In the present technique as well, this multi-band RF pulse is prepared in advance, and the frequency is adjusted and applied for each slice position, so that excitation can be easily performed by the <NUM>° pulse, allowing the excitation range to extend to the downstream side. In addition, in the present technique, unlike Technique <NUM>, it is not necessary to adjust the bandwidth or the excitation frequency to be shifted, and thus the control can be performed more easily. In addition, since the excitation thicknesses of the two <NUM>° pulses are kept thin, the individual excitation profiles can be kept clean. Accordingly, this may prevent the influence of extra excited magnetization at the edges of the profiles and inadvertent interference to the adjacent slice.

In Technique <NUM>, the center frequency of the <NUM>° pulse is shifted. In the present technique, the excitation profile is deformed to an asymmetric shape about the position of the center frequency without changing the center frequency, thereby extending the excitation width to the downstream region adjacent to the slice. <FIG> illustrate examples of this change of the excitation profile.

The example illustrated in <FIG> shows the excitation profile obtained by reducing the flip angle (FA) to the downstream side while maintaining the FA of <NUM>° in the slice #<NUM>, and thereby widening the excitation range to the downstream side. The example illustrated in <FIG> shows that the FA of the slice #<NUM> is <NUM>° and the FA becomes larger than <NUM>° toward the downstream side.

The excitation profile may have any of the shapes in <FIG>, but in the case of <FIG>, by increasing the intensity in the slice to be scanned, high signal intensity can be ensured with preventing an increase of flow artifacts in the slice. In the case of <FIG>, by increasing the signal intensity on the downstream side, the effect for reducing artifacts can be improved, and this leads to enhancement of the effect to reduce the flow artifacts in the slice to be scanned next.

The shape of the excitation profile is determined by the shape of the RF pulse (the shape of the envelope), and Fourier-transforming of one shape gives the shape of the other. Generally, in order to obtain a rectangular excitation profile, the RF pulse takes the shape of the Sinc function. In this technique, the predetermined profile is determined, assuming that the shape obtained by Fourier-transforming a predetermined excitation profile represents the shape of the RF pulse. Accordingly, without changing the center frequency, it is possible to excite the range asymmetric with respect to the center frequency, that is, the range extended to the region adjacent to the slice plane.

The excitation profile with such deformed shape can be easily achieved by calculating the RF pulse shape superimposed on the RF signal in advance, and setting thus calculated RF pulse shape in the measurement control unit <NUM>.

According to the present technique, it is not necessary to change the center frequency, and only the shape needs to be controlled. Therefore, the excitation width can be easily controlled by calculating the excitation profile in advance.

In the first embodiment, there has been described the case that when the excitation width is extended to a region located on the downstream side of inflowing fluid, a user designates which side is the downstream side with respect to the imaging plane according to an examination site. In the present embodiment, the system automatically makes this determination and controls the excitation width, based on the order of slice measurement (slice measurement order) set by the user. Other configurations are the same as those of the first embodiment.

When the user is aware of the blood flow (in an artery or in a vein) having a problem of flow artifacts, and the direction of the flow on the examination site, the user designates, in many cases, the measurement order to perform scanning from the upstream side of the flow, aiming at an effect of reduction in flow artifacts after applying the excitation pulses repeatedly.

The measurement control unit <NUM> extends the excitation width to the downstream side, automatically following the order based on the designated measurement order. In this case, for example, as illustrated in <FIG>, there may be provided a GUI displaying the information such as the slice positions <NUM> and the measurement order information <NUM> on the display screen <NUM> of the UI unit <NUM>, prompting the user to make an approval or correction.

According to the present embodiment, it is possible to reduce the burden on the user in setting the imaging conditions. Further, displaying on the UI unit <NUM> the automatically determined result regarding the region in which the excitation width is extended, can prompt the user to change the measurement order, or to change the region where the excitation width is extended. Therefore, this enhances the effectiveness in reducing the flow artifacts.

Claim 1:
A magnetic resonance imaging apparatus comprising,
a measurement unit (<NUM>) configured to apply an RF pulse (<NUM>) to excite a predetermined cross section of an examination subject (<NUM>) and gradient magnetic field pulses (<NUM>, <NUM>; <NUM>; <NUM>), and to collect nuclear magnetic resonance signals (<NUM>) generated from the predetermined cross section, and
a measurement control unit (<NUM>) configured to control the measurement unit so that the measurement unit collects the nuclear magnetic resonance signals according to a spin-echo pulse sequence comprising the RF pulse as an excitation pulse and the gradient magnetic field pulses,
characterized in that
the measurement control unit is configured to control at least either of the excitation pulse and a gradient magnetic field pulse of the gradient magnetic field pulses that is applied simultaneously with the excitation pulse in the spin-echo pulse sequence in such a manner that a width in a thickness direction of the predetermined cross section excited by the excitation pulse in the spin-echo pulse sequence is extended from an imaging plane of the predetermined cross section only to a downstream side of a liquid that flows into the predetermined cross section, so that the width includes the predetermined cross section and a region adjacent to the predetermined cross section on the downstream side.