JP7152146B2 - Magnetic resonance imaging device - Google Patents

Description

Embodiments of the present invention relate to magnetic resonance imaging apparatus.

Conventionally, in a magnetic resonance imaging apparatus, as a technique for suppressing image quality deterioration due to eddy currents generated in the gradient magnetic field coil, the gradient magnetic field waveform input to the gradient magnetic field power supply is changed so as to cancel the eddy magnetic field generated by the eddy currents. Methods for pre-correction are known. In this method, for example, the gradient magnetic field waveform input to the gradient magnetic field power supply is corrected to emphasize the amplitude in the rising portion of the waveform (hereinafter referred to as eddy correction).

When such eddy correction is performed, the gradient magnetic field waveform before correction is usually set so that the amplitude of the gradient magnetic field waveform after correction does not exceed the maximum value of amplitude allowed by the gradient magnetic field power supply. Amplitude limitation is applied at that time. As a result, when the gradient magnetic field power supply outputs a current to the gradient magnetic field coil according to the corrected gradient magnetic field waveform, the current is output with a limited amplitude in the period other than the rising portion of the waveform. The capacity of the gradient magnetic field power supply will not be utilized to the maximum.

U.S. Patent Application Publication No. 2015/0035531 U.S. Pat. No. 9,157,973 U.S. Pat. No. 8,890,530

"Pre-emphasis Questions and Answers in MRI", [searched November 16, 2017], Internet <URL: http://mrequestions.com/what-is-pre-emphasis.html>

The problem to be solved by the present invention is to make more effective use of the power of the gradient magnetic field power supply.

A magnetic resonance imaging apparatus according to an embodiment includes a sequence control section and an eddy correction section. A sequence control unit calculates a gradient magnetic field waveform based on imaging conditions. The eddy correction section performs eddy magnetic field correction processing on the gradient magnetic field waveform output from the sequence control section, and outputs the corrected gradient magnetic field waveform to the gradient magnetic field power supply. The sequence control unit controls that, in the corrected gradient magnetic field waveform, the magnitude of the amplitude in at least a part of the period is close to the maximum value within a range that does not exceed the maximum value of the amplitude allowed by the gradient magnetic field power supply. The waveform of the gradient magnetic field calculated based on the imaging condition is changed so as to be continuously maintained, and output to the eddy correction unit.

FIG. 1 is a block diagram showing the configuration of a magnetic resonance imaging (MRI) apparatus according to this embodiment. FIG. 2 is a diagram showing an example of eddy correction according to a comparative example of this embodiment. FIG. 3 is a diagram showing an example of vortex correction according to a comparative example of this embodiment. FIG. 4 is a flow chart showing the procedure of processing performed by the MRI apparatus according to this embodiment. FIG. 5 is a diagram showing an example of eddy correction according to this embodiment. FIG. 6 is a diagram showing an example of eddy correction according to this embodiment. FIG. 7 is a diagram showing another example of eddy correction according to this embodiment.

An MRI apparatus according to this embodiment will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of the MRI apparatus according to this embodiment.

As shown in FIG. 1, the MRI apparatus 100 according to the present embodiment includes a static magnetic field magnet 1, a static magnetic field power supply 2, a gradient magnetic field coil 3, a processing circuit 4, a processing circuit 5, a processing circuit 6, a processing circuit 10, an RF ( radio frequency) coil 7, transmitter 8T, receiver 8R, memory circuit 11, display 12, interface 13, shim coil 14, shim coil power supply 15, voice generator 16, ECG (Electrocardiogram) unit 18, gradient magnetic field power supply 19, etc. .

The static magnetic field magnet 1 is, for example, a superconducting magnet or the like, receives current supply from a static magnetic field power supply ₂ , and generates a static magnetic field B0 in an imaging space where the subject is placed. The gradient magnetic field coil 3 is supplied with current from the gradient magnetic field power source 19 and applies a gradient magnetic field to the imaging space. The RF coil 7 applies RF pulses to the imaging space and receives MR signals (echo signals) generated from the subject under the influence of the RF pulses. The transmitter 8T supplies the RF coil 7 with RF pulses corresponding to the Larmor frequency. Receiver 8R sends the MR signals received by RF coil 7 to processing circuitry 10 . The shim coil 14 is supplied with current from the shim coil power supply 15 to homogenize the static magnetic field B ₀ .

The processing circuit 4 has an eddy correction function 4a. The processing circuit 5 also has a sequence control function 5a. The processing circuit 6 also has a main control function 6a. The processing circuit 10 also has an arithmetic function 10a. The eddy correction function 4a is an example of means for implementing the eddy correction section, and the sequence control function 5a is an example of means for implementation of the sequence control section.

For example, the processing circuit 4, the processing circuit 5, the processing circuit 6, and the processing circuit 10 are each implemented by a processor. In this case, the processing functions of each processing circuit are stored in the storage circuit 11 in the form of a computer-executable program. Each processing circuit reads out each program from the storage circuit 11 and executes it, thereby realizing a function corresponding to each program. In other words, each processing circuit with each program read has each function shown in each processing circuit in FIG. In the example shown in FIG. 1, it is assumed that each processing function is realized by a plurality of processors. I don't mind if it's a thing. Also, the processing functions of each processing circuit may be appropriately distributed or integrated in a single or a plurality of processing circuits. In the example shown in FIG. 1, the single memory circuit 11 stores programs corresponding to each processing function. A configuration in which the corresponding program is read out from the circuit may be used.

With such a configuration, in the MRI apparatus 100 according to the present embodiment, the processing circuit 6, the processing circuit 10, the storage circuit 11, the display 12, and the interface 13 can detect the type of pulse sequence to be imaged, the size of the imaging region, Imaging conditions such as slice thickness, spatial resolution, repetition time (TR), and echo time (TE) are set. Further, the main control function 6a of the processing circuit 6 creates a timing chart (pulse sequence) of control signals to each unit that meet the imaging conditions.

Here, the processing circuit 5 is connected to three main system units, a gradient magnetic field system, a shim coil system, and a transmission/reception system, as well as other units such as an ECG unit 18 and a voice generator 16 . The ECG unit 18 receives electrocardiographic signals from the ECG sensor 17 attached to the subject and detects a predetermined electrocardiographic waveform. The gradient magnetic field system controls and supplies gradient magnetic field coils 3 (three channels of x, y, and z; X channel coil 3x, Y channel coil 3y, and Z channel coil 3z, respectively) and the current applied to the gradient magnetic field coils 3. It consists of a processing circuit 4 and a gradient magnetic field power supply 19 . The shim coil system consists of a shim coil 14 and a shim coil power supply 15 that supplies current to it. The transmitting/receiving system includes an RF coil 7 for radiating high frequency waves to an imaging target 200, which is a subject, and receiving magnetic resonance signals generated thereby, a transmitter 8T, and a receiver 8R.

Then, when imaging is started, the sequence control function 5a of the processing circuit 5 controls each connected unit at the necessary timing based on the pulse sequence sent from the main control function 6a of the processing circuit 6. give instructions. Specifically, the sequence control function 5a sends to the gradient magnetic field system a gradient magnetic field waveform 20 indicating the strength of the current flowing through the gradient magnetic field coil 3 in addition to the control signal 21 of the gradient magnetic field power supply 19. FIG. The sequence control function 5a also sends information such as the waveform of the RF pulse to be transmitted, reception start timing and sampling pitch to the transmission/reception system. Based on these signals, units of each system connected to the processing circuit 5 operate to generate echo signals in the space 23 to be imaged. This echo signal is received by the RF coil 7, sent to the processing circuit 10 via the receiver 8R and the sequencer, and reconstructed into an image by the arithmetic function 10a of the processing circuit 10. FIG. The reconstructed image is stored in the memory circuit 11 and displayed on the display 12 as required.

Next, the gradient magnetic field system according to this embodiment will be described in more detail.

When the current flowing through the gradient magnetic field coil 3 changes abruptly, eddy currents are generated in the surrounding conductors due to the change in magnetic flux, thereby generating an eddy magnetic field that tries to cancel the gradient magnetic field. This eddy magnetic field disturbs the desired gradient magnetic field, and thus can be a cause of image quality deterioration of MRI images.

Therefore, in the MRI apparatus 100 according to the present embodiment, in order to correct this eddy magnetic field, the gradient magnetic field waveform 20 generated by the sequence control function 5a is not passed through the gradient magnetic field coil 3 as it is. The correction function 4a corrects (eddy correction) the input of the gradient magnetic field power supply 19 so as to cancel the eddy magnetic field generated by the eddy current. Generally, this correction is also called pre-emphasis because the input waveform of the gradient magnetic field power supply 19 is emphasized in advance by the amount of the eddy magnetic field (for example, http://mrequestions.com/what- is-pre-emphasis.html).

Here, the contents of the time constant and strength of the eddy magnetic field and the correction process will be described assuming that the eddy magnetic field is represented by a simple one-component model. In the following, the magnitude of the amplitude of the gradient magnetic field waveform is represented by the gradient magnetic field strength, and the gradient magnetic field power supply 19 allows the maximum value of the gradient magnetic field strength that can be output by the gradient magnetic field amplifier included in the gradient magnetic field power supply 19. An example in which the amplitude is set to the maximum value will be described.

For example, if the gradient magnetic field waveform output by the sequence control function 5a is G _seq(t) , the eddy magnetic field intensity is A _eddy , and the time constant is Tc _eddy , the sequence A magnetic field waveform G _gcoil (t) generated in the gradient magnetic field coil 3 when G _seq(t) generated by the control function 5a is passed through the gradient magnetic field coil 3 as it is can be expressed by the following equation (1). .

このように、本来は、シーケンス制御機能５ａの出力波形を忠実に傾斜磁場コイル３内で磁場波形として発生させたいが、その場合には、（１）式の第２項に示すような渦磁場による誤差が入ってしまう。

As described above, originally, it is desirable to faithfully generate the output waveform of the sequence control function 5a as a magnetic field waveform in the gradient magnetic field coil 3. There is an error due to

Therefore, in the present embodiment, the eddy correction function 4a performs the following (2) on the input G _seq(t) , where A _ecc is the strength of the corrected eddy magnetic field and Tc _ecc is the time constant. The corrected gradient magnetic field waveform G _ecc(t) is calculated by performing the correction represented by the formula.

At this time, the eddy correction function 4a corrects the intensity A _eddy and the time constant Tc _eddy of the observed eddy magnetic field according to the relationships shown in the following formulas (3) and (4). The strength A _ecc and the time constant Tc _ecc of the eddy magnetic field are calculated.

As a result, the deformation of the gradient magnetic field waveform due to the eddy magnetic field is canceled, and as shown in the following equation (5), the gradient magnetic field waveform G _seq(t) output from the sequence control function 5a and the gradient magnetic field coil 3 are The generated magnetic field waveform G _{gcoil,corrected} (t) almost matches.

2 and 3 are diagrams showing an example of eddy correction according to a comparative example of this embodiment.

Here, the graphs shown in (a) to (f) of FIG. 2 respectively show gradient magnetic field waveforms, with the vertical axis showing gradient magnetic field intensity and the horizontal axis showing time.

Specifically, FIGS. 2A to 2C show examples of gradient magnetic field waveforms when eddy correction is not performed, and FIGS. An example of a gradient magnetic field waveform in such eddy correction is shown. 2(a) and (d) show the input waveform (20) of the eddy correction function 4a, and (b) and (e) show the output waveform (22) of the eddy correction function 4a. , and (c) and (f) show the magnetic field waveform (23) in the gradient magnetic field coil 3. FIG.

In the example shown in FIG. 2, the maximum value of the gradient magnetic field strength that can be output by the gradient magnetic field amplifier is 100 mT / m, the Slew rate (time change rate of the gradient magnetic field) is 200 mT / m / ms, and the strength of the eddy magnetic field is 10 %, a time constant of 4 ms was assumed.

Also, FIG. 3 is obtained by superimposing each waveform shown in (d) to (f) of FIG. 2 on the same graph. In FIG. 3, line 40 shows the input waveform (20) of the eddy correction function 4a, and line 41 shows the output waveform (22) of the eddy correction function 4a (the output current of the gradient magnetic field power supply 19 is proportional to this waveform). ) and line 42 indicates the magnetic field waveform ( 23 ) in the gradient coil 3 . Also, G _max is the maximum value of the gradient magnetic field strength that can be output by the gradient magnetic field amplifier.

In the example shown in FIG. 3, the input _waveform (20 ), the maximum value G _max ' of the gradient magnetic field strength in the flat top portion was set. In this case, the input waveform (20) of the eddy correction function 4a indicated by line 40 and the magnetic field waveform (23) in the gradient magnetic field coil 3 indicated by line 42 match, indicating that the eddy correction has been performed as intended. I understand.

However, in the example shown in FIG. 3, if the gradient magnetic field strength in the flat top portion of the input waveform (20) of the eddy correction function 4a indicated by line 40 exceeds G _max ', then the eddy correction indicated by line 41 In the output waveform (22) of function 4a, there is a period during which the gradient magnetic field intensity exceeds _Gmax . In this case, a gradient magnetic field having a magnitude exceeding G _max is not output during that period. will not match the magnetic field waveform (23) of , and correct eddy correction will not be possible.

On the other hand, for example, in the sequence control function 5a, a maximum value equal to or less than G _max ' is set so that the gradient magnetic field strength of the input waveform (20) of the eddy correction function 4a indicated by the line 40 does not exceed G _max '. It is conceivable to limit the output range as follows. In this case, if the time constant of the eddy magnetic field is sufficiently long with respect to the rise time of the gradient magnetic field, the magnitude of G _max ' is approximately (1−A _eddy )·G _max , _and the vortex It becomes lower by the strength of the magnetic field. For example, if the eddy field strength is 0.1 (=10%), then G _max ' will be less than G _max by approximately the same 10%.

Thus, for eddy correction, when limiting the amplitude (gradient magnetic field strength) of the input waveform (20) of the eddy correction function 4a indicated by the line 40, which is the output of the sequence control function 5a, the expected The limit range is determined by the strength of the eddy magnetic field, the time constant, etc., but basically this limit range is assumed to be set at a fixed value regardless of the shape of the waveform in order to simplify control. be.

In this case, when the gradient magnetic field power supply 19 outputs a current to the gradient magnetic field coil 3 according to the output waveform (22) of the eddy correction function 4a (=output current of the gradient magnetic field power supply 19), a period other than the rising moment , the current is output with a gradient magnetic field intensity smaller than the maximum value G _max , and the capacity of the gradient magnetic field power supply 19 is not utilized to the maximum. In particular, when the output waveform (22) of the eddy correction function 4a is a waveform that rises and falls at relatively long intervals with respect to the time constant of the eddy magnetic field, the ability of the gradient magnetic field power supply 19 is utilized. degree is also lower.

Regarding specific imaging conditions of the pulse sequence, for example, in the case of diffusion-weighted imaging using MPG (Motion Probing Gradient) pulses, originally only MPG pulses weaker than the power of the gradient magnetic field power supply 19 can be output. is less emphasized, resulting in lower image quality. In addition, in the case of an imaging method for measuring the flow velocity of an imaging target called the phase contrast method, the intensity of the gradient magnetic field pulse (flow-encoding gradient magnetic field pulse) for adjusting the sensitivity for detecting slow flow velocity is originally determined by the gradient magnetic field power supply 19. Since the intensity is weaker than the ability of , the ability to detect the flow velocity is reduced.

As described above, the example shown in FIG. 3 is simple because it is restricted by a fixed value, but it is inefficient because the ability of the gradient magnetic field power supply 19 is not maximized at all times. Further, in the example shown in FIG. 3, imaging can be performed only under imaging conditions lower than the original capability of the gradient magnetic field power supply 19, which may deteriorate image quality or extend the imaging time. .

For this reason, in the MRI apparatus 100 according to the present embodiment, the sequence control function 5a causes the output waveform (22) of the eddy correction function 4a indicated by the line 41 in FIG. The input waveform (20) of the eddy correction function 4a, indicated by line 40, is determined to have a maximum value for a longer period of time.

Specifically, the sequence control function 5a keeps the input waveform of the gradient magnetic field power supply 19 including the waveform for eddy correction (the output waveform (22) of the eddy correction function 4a indicated by the line 41 in FIG. 3) always The input waveform (20) of the eddy correction function 4a indicated by the line 40 is determined so as not to exceed but to be as close to the upper limit as possible. For this reason, the sequence control function 5a estimates the input waveform of the gradient magnetic field power supply 19 using a predetermined eddy correction model at the stage of generating the pulse sequence.

With such a configuration, a waveform close to the upper limit of the gradient magnetic field power supply 19 can be output for a longer period of time within the range in which eddy correction can be performed correctly. As a result, for example, in diffusion weighted imaging, an MPG pulse with a larger area can be output with the same echo time TE, so that imaging with a higher b value can be performed, and the degree of emphasizing diffusion is greater. A high-quality image can be obtained. In addition, even in many other imaging methods, imaging can be performed under imaging conditions that require higher gradient magnetic field capability, and images with higher image quality can be obtained.

FIG. 4 is a flow chart showing the procedure of processing performed by the MRI apparatus 100 according to this embodiment.

As shown in FIG. 4, in the MRI apparatus 100 according to this embodiment, first, the main control function 6a sets initial conditions (step S101). Specifically, the main control function 6a controls the maximum value G _max (unit: mT/m) of the gradient magnetic field strength that can be output by the gradient magnetic field amplifier, the maximum Slew rate SR _max (unit: mT/m/ms), the vortex Current intensity A _eddy , time constant Tc _eddy (unit: s), etc. are set. These values are values unique to the MRI apparatus 100, and are actually measured maximum values and measured values.

The main control function 6a also determines the strength A _ecc and the time constant Tc _ecc (unit: s) of the corrected eddy magnetic field based on the initial conditions (step S102). Specifically, the main control function 6a calculates A _ecc from A _eddy using equation (3) described above. Also, the main control function 6a calculates Tc _ecc from Tc _eddy and A _eddy using the formula (4) described above.

The main control function 6a also sets imaging conditions (step S103). Specifically, the main control function 6a sets the imaging conditions based on input values of various imaging parameters received from the operator via the interface 13, SR _max set as initial conditions, and the like.

Then, when the operator instructs to start imaging (step S104, Yes), the main control function 6a calculates the input waveform G _seq(t) (unit: mT/m) is calculated (step S105). Specifically, the main control function 6a has an eddy correction function corresponding to the gradient magnetic field waveform 20 generated by the sequence control function 5a based on the set imaging conditions, the SR _max set as the initial condition, and the like. Calculate the input waveform G _seq(t) of 4a.

After that, the sequence control function 5a performs waveform calculation for each time when the period of the gradient magnetic field waveform is divided by a constant time interval t_div, as follows, so that the input waveform G _{seq( t)} .

First, the sequence control function 5a sets the time T to an initial value (step S106), and then calculates the gradient magnetic field waveform G _ecc(T) after correction of the eddy magnetic field (step S107). Specifically, the sequence control function 5a calculates G _ecc(T) from G _seq(T) , A _ecc , and Tc _ecc using equation (2) described above.

Subsequently, the sequence control function 5a compares G _ecc(T) and G _max (step S108). Then, when G _{ecc(T) exceeds} G _max (step _S108 , G _max <G _ecc(T) ), the sequence control function 5a After reducing G _seq(T) by (step S109a), the corrected gradient magnetic field waveform G _ecc(T) is calculated again (step S107), and G _ecc(T) and G _max are compared ( step S108). Similarly, when G _ecc(T) is less than -G _max (Step S108, G _ecc(T) <-G _max ), the sequence control function 5a controls G _ecc(T) to make G _max negative. After increasing G _seq(T) by the excess (step S109b), the corrected gradient magnetic field waveform G _ecc(T) is calculated again (step S107), and G _ecc(T) and G _max are compared (step S108).

On the other hand, when G _ecc(T) satisfies the condition of -G max ≤ G ecc(T ₎ ≤ G _max (step S108, -G _max ≤ G _ecc(T) ≤ G _max ), the sequence control function 5a does not modify G _seq(T) and only determines whether T has reached the final value (step S110). Then, when T has not reached the final value (step S110, No), the sequence control function 5a increases T by t_div (step S111), and then repeats the corrected gradient magnetic field waveform G _{ecc(T )} is calculated (step S107), and G _ecc(T) and G _max are compared (step S108).

Thus, the sequence control function 5a ensures that G _{ecc(T) satisfies} the condition of -G _max ≤ G ecc(T) ≤ G _max (step S108, -G _max ≤ G _ecc(T) ≤ G _max ). And, until T reaches the final value (step S110, Yes), the calculation of the corrected gradient magnetic field waveform G _{ecc (T)} is repeated. is output to the eddy correction function 4a (step _S112 ).

After that, the eddy correction function 4a performs eddy magnetic field correction processing on G _seq(T) output from the sequence control function 5a (step S113), and converts the corrected gradient magnetic field waveform G _ecc(t) to the gradient magnetic field Output to the power supply 19 (step S114).

Then, the gradient magnetic field power supply 19 supplies a current to the gradient magnetic field coil 3 based on the corrected gradient magnetic field waveform G _ecc(t) , and imaging is performed by the gradient magnetic field system, the transmission/reception system, and the like (step S115 ).

Here, among the processing procedures shown in FIG. 4, the processing of steps S101 to S105 is realized, for example, by the processing circuit 6 reading out a predetermined program corresponding to the main control function 6a from the storage circuit 11 and executing it. be. Further, the processing of steps S106 to S113 is realized by, for example, the processing circuit 5 reading out a predetermined program corresponding to the sequence control function 5a from the storage circuit 11 and executing it. Further, the process of step S114 is realized, for example, by the processing circuit 4 reading out a predetermined program corresponding to the eddy correction function 4a from the storage circuit 11 and executing it.

5 and 6 are diagrams showing examples of eddy correction according to this embodiment.

Here, the graphs shown in (d) to (i) of FIG. 5 each show a gradient magnetic field waveform, similar to FIG. 2, the vertical axis shows the gradient magnetic field strength, and the horizontal axis shows time. .

Specifically, (d) to (f) in FIG. 5 show the same gradient magnetic field waveforms as (d) to (f) in FIG. The gradient magnetic field waveform in such eddy correction is shown. 5(d) and (g) show the input waveform (20) of the eddy correction function 4a, and (e) and (h) show the output waveform (22) of the eddy correction function 4a. , and (f) and (i) show the magnetic field waveform (23) in the gradient magnetic field coil 3. FIG.

In the example shown in FIG. 5, it is assumed that the maximum value of the gradient magnetic field strength that can be output by the gradient magnetic field amplifier is 100 mT/m, the Slew rate is 200 mT/m/ms, the strength of the eddy magnetic field is 10%, and the time constant is 4 ms. did.

In addition, (j) in FIG. 6 is obtained by superimposing each waveform shown in (d) to (f) in FIG. 5 on the same graph, and (k) is the are superimposed on the same graph. In FIG. 6, lines 40 and 70 indicate the input waveform (20) of the eddy correction function 4a, and lines 41 and 71 indicate the output waveform (22) of the eddy correction function 4a (the output of the gradient magnetic field power supply 19). The current is proportional to this waveform), and lines 42 and 72 show the magnetic field waveform (23) in the gradient coil 3. FIG. Further, G _max is the maximum value of the gradient magnetic field strength that can be output by the gradient magnetic field amplifier, and G _max ′ is the maximum gradient magnetic field strength in the flat top portion of the input waveform (20) of the eddy correction function 4a in the comparative example. value.

As can be seen by comparing (j) and (k) in FIG. 6, in this embodiment, the output waveform (22) of the eddy correction function 4a indicated by the line 71 is stuck to G _max immediately after the rise. As output, the input waveform (20) of the eddy correction function 4a, indicated by line 70, has changed. As a result, in the comparative example, the input waveform (20) of the eddy correction function 4a indicated by the line 40 does not exceed G _max ' at all _times . , and with the lapse of time, the intensity almost reaches G _max .

As described above, in this embodiment, the sequence control function 5a calculates the gradient magnetic field waveform based on the imaging conditions. Further, the eddy correction function 4 a performs eddy magnetic field correction processing on the gradient magnetic field waveform output from the sequence control function 5 a and outputs the corrected gradient magnetic field waveform to the gradient magnetic field power supply 19 . Then, the sequence control function 5a causes the amplitude of the corrected gradient magnetic field waveform output from the eddy correction function 4a to the gradient magnetic field power supply 19 to exceed the amplitude allowed by the gradient magnetic field power supply 19 in at least a part of the period. The gradient magnetic field waveform calculated based on the imaging conditions is changed and output to the eddy correction function 4a so that the magnitude close to the maximum value is continuously maintained within a range not exceeding the maximum value.

In addition, the sequence control function 5a ensures that the amplitude of the corrected gradient magnetic field waveform output from the eddy correction function 4a to the gradient magnetic field power supply 19 does not exceed the amplitude allowed by the gradient magnetic field power supply 19 in at least a part of the period. The gradient magnetic field waveform calculated based on the imaging conditions is changed so that the magnitude that matches the maximum value is continuously maintained.

In addition, the sequence control function 5a determines whether the amplitude of the gradient magnetic field waveform exceeds the maximum value for each time obtained by dividing the period of the gradient magnetic field waveform calculated based on the imaging conditions by a certain time interval. The gradient magnetic field waveform calculated based on the imaging condition is changed by changing the magnitude of the amplitude by changing the amplitude.

In addition, the sequence control function 5a determines whether the amplitude of the gradient magnetic field waveform exceeds the maximum value at each time when the period of the gradient magnetic field waveform is divided by a certain time interval. If the amplitude does not exceed the maximum value, the magnitude of the amplitude is reduced by the difference from the maximum value, and it is determined again whether the amplitude is within the range that does not exceed the maximum value. , determines whether or not the amplitude of the gradient magnetic field waveform exceeds the maximum value at the next time.

In the above-described embodiment, the sequence control function 5a determines whether G _{ecc (T)} at that time exceeds G _max at each time T obtained by dividing the period of the gradient magnetic field waveform by the time interval t_div. Although it was decided to confirm and recalculate, the embodiment is not limited to this. For example, the sequence control function 5a estimates the value of G _seq(T) at which G _ecc(T) itself becomes lower by the amount that G ecc(T ₎ exceeds G _max using the formula (2) described above. and that value may be used as the value of the next G _seq(T) . Alternatively, the sequence control function 5a may reduce G _seq(T) by multiplying the amount by which G _ecc(T) exceeds G _max by a positive proportionality constant of 1 or less.

In any case, the sequence control function 5a estimates G _ecc(T) by some means, instead of constraining G _seq(T) to within a fixed upper or lower limit, so that G _ecc(T) A feature of this embodiment is that G _seq(T) is changed by constantly confirming whether G _max is not exceeded (or whether it is within the range of −G _max to +G _max ). The process of changing G _seq(T) may be modified without departing from the spirit of the above.

Further, in the above-described embodiment, an example in which the component of the eddy magnetic field is one component has been shown, but the embodiment is not limited to this. For example, even if a plurality of components of the eddy magnetic field are used, G _seq(T) can be similarly changed within a range that does not deviate from the spirit described above. In this case, a model including a plurality of components is used as the model of the eddy magnetic field.

Further, in the above-described embodiment, the case where the gradient magnetic field power supply 19 has a fixed maximum current value has been described, but the embodiment is not limited to this. In the above-described embodiment, whether or not G _ecc(T) constantly exceeds G _max is checked at sufficiently small time intervals with respect to changes in the gradient magnetic field waveform. can be applied in real time, ie G _max varies as a function of time. In this way, it is also an advantage of the present embodiment that the ability of the gradient magnetic field system can be efficiently exhibited by flexibly responding to changes in circumstances.

In the above-described embodiment, after the gradient magnetic field waveform rises, the amplitude is adjusted so as to draw out the limit of the ability, so the waveform does not become flat. For this reason, for example, for MPG pulses used in diffusion-weighted imaging and flow-encoding gradient magnetic field pulses used in the phase method, it is desirable to calculate sensitivity in consideration of waveform deformation. In addition, when used as a readout gradient magnetic field for an echo signal, the waveform is not flat, so echo sampling is performed by changing the interval according to the change in the gradient magnetic field strength, that is, by performing so-called non-uniform sampling, or by sampling at equal intervals. , it is desirable to perform a process called regridding before reconstructing the image. It is desirable that the sequence control function 5a appropriately performs sequence generation and post-processing in consideration of these points.

Further, in the above-described embodiment, an example in which the waveform of the gradient magnetic field is a waveform having a flat positive polarity (hereinafter referred to as a RAMP waveform) has been described, but the embodiment is not limited to this. For example, the above-described embodiments can be similarly applied even when the waveform of the gradient magnetic field includes a gradient magnetic field pulse having a negative polarity and a gradient magnetic field pulse having a positive polarity.

FIG. 7 is a diagram showing another example of eddy correction according to this embodiment.

Here, in FIG. 7, the input waveform (20) of the eddy correction function 4a is a waveform including one gradient magnetic field pulse having a negative polarity and one gradient magnetic field pulse having a positive polarity (hereinafter referred to as a BI-POLAR waveform is called).

Specifically, (l) of FIG. 7 shows a case where the amplitude of the input waveform (20) of the eddy correction function 4a is limited in the same manner as in the comparative examples shown in (d) to (f) of FIGS. , and (m) in FIG. 7 shows an example in which the eddy correction according to this embodiment is performed. Similarly to FIGS. 3 and 6, (l) and (m) in FIG. 7 respectively show the input waveform (20) of the eddy correction function 4a, the output waveform (22) of the eddy correction function 4a, and the gradient magnetic field. The magnetic field waveform (23) in the coil 3 is superimposed on the same graph.

In FIG. 7, lines 80 and 90 indicate the input waveform (20) of the eddy correction function 4a, and lines 81 and 91 indicate the output waveform (22) of the eddy correction function 4a (the output of the gradient magnetic field power supply 19). The current is proportional to this waveform), and lines 82 and 92 show the magnetic field waveform (23) in the gradient coil 3. FIG. Further, G _max is the maximum positive value of the gradient magnetic field strength that can be output by the gradient magnetic field amplifier, and G _max ′ is the gradient magnetic field strength at the flat top portion of the input waveform (20) of the eddy correction function 4a in the comparative example. is the maximum positive value of . -G _max is the maximum negative value of the gradient magnetic field strength that can be output by the gradient magnetic field amplifier, and -G _max ' is the gradient in the flat top portion of the input waveform (20) of the eddy correction function 4a in the comparative example. is the maximum negative value of the magnetic field strength.

As can be seen by comparing (l) and (m) in FIG. 7, in this example as well, the output waveform (22) of the eddy correction function 4a indicated by line 91 is stronger for a longer period of time. is output in a state of sticking to G _max or -G _max . As a result, both the input waveform (20) of the eddy correction function 4a indicated by the line 90 and the magnetic field waveform in the gradient magnetic field coil 3 can be increased in strength (absolute value is increased).

Table 1 shown below shows the magnetic field waveform in the gradient magnetic field coil 3 in the comparative example indicated by line 42 in (j) and the line 72 in (k) in the example of the RAMP waveform shown in FIG. Regarding the magnetic field waveform in the gradient magnetic field coil 3 in this embodiment, the ratio of the area ratio from the rise to 10 ms, the area ratio from the rise to 30 ms, and the maximum amplitude (the maximum value of the gradient magnetic field strength in the flat top portion), Each improvement rate is shown.

As shown in Table 1, when comparing the time integral value (area) of the gradient magnetic field from the rise of the gradient magnetic field to 10 ms for the magnetic field waveform in the gradient magnetic field coil 3 shown in FIG. .41% larger. Further, when the time integral value (area) of the gradient magnetic field from the rise of the gradient magnetic field to 30 ms is compared, it can be seen that this embodiment is 8.9% larger. Also, when comparing the maximum amplitude, it can be seen that the present embodiment is 10.3% larger.

In addition, Table 2 shown below shows the magnetic field waveform in the gradient magnetic field coil 3 in the comparative example indicated by line 82 in (l) and the line in (m) in the example of the BI-POLAR waveform shown in FIG. Regarding the magnetic field waveform in the gradient magnetic field coil 3 in this embodiment indicated by 92, the ratio of the maximum amplitude (maximum value of the gradient magnetic field strength in the flat top portion) in the period of 5 ms which is the pulse width and the improvement rate are shown. It is a thing.

As shown in Table 2, when comparing the maximum amplitudes of the magnetic field waveforms in the gradient magnetic field coil 3 shown in FIG. 7, it can be seen that the present embodiment is 17.3% larger.

As described above, in the present embodiment, the gradient magnetic field power supply 19 having the same performance can output a gradient magnetic field pulse having a larger area, and imaging can be performed under stricter imaging conditions. In other words, the ability of the gradient magnetic field power supply 19 can be exhibited more effectively for a longer period of time. As a result, for example, in diffusion weighted imaging, an MPG pulse with a larger area can be output with the same echo time TE, so that imaging with a higher b value can be performed, and the degree of emphasizing diffusion is greater. A high-quality image can be obtained. In addition, even in many other imaging methods, imaging can be performed under imaging conditions that require higher gradient magnetic field capability, and images with higher image quality can be obtained.

Therefore, according to this embodiment, the ability of the gradient magnetic field power supply can be utilized more effectively.

In the above-described embodiment, the sequence control section in this specification is implemented by the sequence control function 5a of the processing circuit 5, and the eddy correction section in this specification is implemented by the eddy correction function 4a of the processing circuit 4. has been described, the embodiments are not limited to this. For example, the sequence control unit and the eddy correction unit in this specification may be realized by the sequence control function 5a and the eddy correction function 4a described in the embodiment, respectively, or may be realized only by software, only by hardware, or by combining hardware. The same function may be realized by mixing with software.

Further, the term "processor" used in the description of the above-described embodiments is, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), or an application specific integrated circuit (ASIC), Circuits such as programmable logic devices (e.g., Simple Programmable Logic Devices (SPLDs), Complex Programmable Logic Devices (CPLDs), and Field Programmable Gate Arrays (FPGAs)) means. Instead of storing the program in the memory circuit, the program may be directly incorporated in the circuit of the processor. In this case, the processor implements its functions by reading and executing the program embedded in the circuit. Further, each processor of the present embodiment is not limited to being configured as a single circuit for each processor, but may be configured as one processor by combining a plurality of independent circuits to realize its function. good. Furthermore, a plurality of components in FIG. 1 may be integrated into one processor to realize its functions.

Here, the program to be executed by the processor is provided by being pre-installed in, for example, a ROM (Read Only Memory), a storage circuit, or the like. This program is a file in a format that can be installed in these devices or a file in a format that can be installed on a computer such as CD (Compact Disk)-ROM, FD (Flexible Disk), CD-R (Recordable), DVD (Digital Versatile Disk), etc. may be recorded on a readable storage medium and provided. Also, this program may be provided or distributed by being stored on a computer connected to a network such as the Internet and downloaded via the network. For example, this program is composed of modules including each functional unit described above. As actual hardware, the CPU reads out a program from a storage medium such as a ROM and executes it, so that each module is loaded onto the main storage device and generated on the main storage device.

According to at least one embodiment described above, the capacity of the gradient magnetic field power supply can be used more effectively.

While several embodiments of the invention have been described, these embodiments have been presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and modifications can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, as well as the scope of the invention described in the claims and equivalents thereof.

100 magnetic resonance imaging (MRI) device 5 processing circuit 5a sequence control function 4 processing circuit 4a eddy correction function 19 gradient magnetic field power supply

Claims (7)

a sequence control unit that calculates a gradient magnetic field waveform based on imaging conditions;
an eddy correction unit that performs eddy magnetic field correction processing on the gradient magnetic field waveform output from the sequence control unit and outputs the corrected gradient magnetic field waveform to a gradient magnetic field power supply,
The sequence control unit calculates a gradient magnetic field waveform after correction of the eddy magnetic field from the gradient magnetic field waveform calculated based on the imaging conditions. For each period of the imaging, the magnitude of the amplitude in the period is continuously maintained at a magnitude close to the maximum value within a range that does not exceed the maximum value of the amplitude allowed by the gradient magnetic field power supply. changing the gradient magnetic field waveform calculated based on the conditions, and outputting the changed gradient magnetic field waveform to the vortex correction unit;
Magnetic resonance imaging equipment. a sequence control unit that calculates a gradient magnetic field waveform based on imaging conditions;
an eddy correction unit that performs eddy magnetic field correction processing on the gradient magnetic field waveform output from the sequence control unit and outputs the corrected gradient magnetic field waveform to a gradient magnetic field power supply,
The sequence control unit corrects the eddy magnetic field from the gradient magnetic field waveform calculated based on the imaging conditions at each time obtained by dividing the period of the gradient magnetic field waveform calculated based on the imaging conditions by a constant time interval. By calculating the gradient magnetic field waveform of, determining whether the amplitude of the corrected gradient magnetic field waveform exceeds the maximum value of the amplitude allowed by the gradient magnetic field power supply, and changing the magnitude of the amplitude , in the corrected gradient magnetic field waveform, the magnitude of the amplitude in at least a part of the period is continuously maintained at a magnitude close to the maximum value within a range that does not exceed the maximum value, the imaging condition changing the gradient magnetic field waveform calculated based on and outputting the changed gradient magnetic field waveform to the eddy correction unit;
Magnetic resonance imaging equipment. In the corrected gradient magnetic field waveform, for each period from immediately after the rise to immediately before the fall of the waveform, the sequence control unit continues to have an amplitude that matches the maximum value in that period. changing the gradient magnetic field waveform calculated based on the imaging conditions so that it is maintained;
The magnetic resonance imaging apparatus according to claim 1. Based on the imaging condition, the sequence control unit continuously maintains the magnitude of the amplitude corresponding to the maximum value in the at least part of the period in the corrected gradient magnetic field waveform. change the gradient magnetic field waveform calculated by
3. The magnetic resonance imaging apparatus according to claim 2 . The sequence control unit corrects the eddy magnetic field from the gradient magnetic field waveform calculated based on the imaging conditions at each time obtained by dividing the period of the gradient magnetic field waveform calculated based on the imaging conditions by a constant time interval. Calculate the gradient magnetic field waveform of, determine whether the amplitude of the gradient magnetic field waveform after the correction exceeds the maximum value, and change the magnitude of the amplitude, thereby calculating based on the imaging conditions change the gradient field waveform,
A magnetic resonance imaging apparatus according to any one of claims 1 to 4 . The sequence control unit corrects the eddy magnetic field from the gradient magnetic field waveform calculated based on the imaging conditions at each time obtained by dividing the period of the gradient magnetic field waveform calculated based on the imaging conditions by a constant time interval. , and as a result of determining whether the amplitude of the corrected gradient magnetic field waveform exceeds the maximum value, if it exceeds the maximum value, the calculation is performed based on the imaging conditions. The amplitude of the corrected gradient magnetic field waveform is reduced by the difference between the amplitude of the corrected gradient magnetic field waveform and the maximum value, and the corrected gradient magnetic field waveform of the eddy magnetic field is calculated again, and the corrected gradient magnetic field waveform is calculated. Changing the gradient magnetic field waveform calculated based on the imaging conditions by determining whether the amplitude of the gradient magnetic field waveform exceeds the maximum value;
The magnetic resonance imaging apparatus according to any one of claims 2-5 . a sequence control unit that calculates a gradient magnetic field waveform based on imaging conditions;
an eddy correction unit that performs eddy magnetic field correction processing on the gradient magnetic field waveform output from the sequence control unit, and outputs the corrected gradient magnetic field waveform to a gradient magnetic field power supply;
with
The sequence control unit corrects the eddy magnetic field from the gradient magnetic field waveform calculated based on the imaging conditions at each time obtained by dividing the period of the gradient magnetic field waveform calculated based on the imaging conditions by a constant time interval. of the gradient magnetic field waveform is calculated, and as a result of determining whether the amplitude of the corrected gradient magnetic field waveform exceeds the maximum value of the amplitude allowed by the gradient magnetic field power supply, if it exceeds the maximum value reduces the magnitude of the amplitude of the gradient magnetic field waveform calculated based on the imaging conditions by the difference between the amplitude of the gradient magnetic field waveform after the correction and the maximum value, and again the gradient magnetic field after correction of the eddy magnetic field By calculating the waveform and determining whether the amplitude of the gradient magnetic field waveform after the correction exceeds the maximum value, the magnitude of the amplitude in at least a part of the period in the corrected gradient magnetic field waveform The gradient magnetic field waveform calculated based on the imaging conditions is changed so that the magnitude close to the maximum value is continuously maintained within the range not exceeding the maximum value, and the gradient magnetic field waveform after the change is changed to the output to the eddy correction unit,
Magnetic resonance imaging equipment.

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