JP2006087778A - Mr scanning method and mri apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To precisely determine an RF pulse width in an MR (Magnetic Resonance) scanning method and MRI (Magnetic Resonance Imaging) apparatus. <P>SOLUTION: This MR scanning method is provided with a step Q1 of performing a localizer prescan for determining an RF pulse amplitude corresponding to an RF magnetic field intensity for the localizer scan, a step Q2 of performing the localizer san using the RF pulse of the RF pulse amplitude corresponding to the RF magnetic field intensity for the localizer scan, steps Q3 and Q4 of determining the RF pulse width of a full-scale scan based on data practically measured by the localizer prescan, a step Q5 of performing the prescan for the full-scale scan for determining the RF pulse amplitude corresponding to the RF magnetic field intensity of the full-scale scan, and a step Q6 of performing the full-scale scan using the RF pulse of the RF pulse amplitude corresponding to the RF magnetic intensity for the full-scale scan. This method can fully use the capacity of an RF power amplifier and avoid the occurrence of a situation of non-obtaining an intended RF magnetic field intensity. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an MR (Magnetic Resonance) scanning method and an MRI (Magnetic Resonance Imaging) apparatus, and more particularly to an MR scanning method and an MRI apparatus capable of accurately determining an RF pulse width.

Conventionally, various methods for obtaining the amplitude of an RF pulse for obtaining a desired RF magnetic field strength have been proposed (see, for example, Patent Document 1).
Also, an RF pulse having a reference amplitude is transmitted, the formed RF magnetic field strength is measured using a pickup coil, and the RF pulse amplitude for obtaining the RF magnetic field strength for the main scan is determined from the relationship between the reference amplitude and the RF magnetic field strength. A technique is known in which the original RF pulse width is extended when the calculated and amplitude cannot be output by the RF power amplifier (see, for example, Patent Document 2).
In addition, a technique for performing localized scan before the main scan is known (see Patent Document 3).
JP-A-11-290288 JP 2003-190118 A JP 2003-339622 A

Conventionally, when the amplitude of an RF pulse for obtaining a desired RF magnetic field intensity cannot be output by an RF power amplifier, the original RF pulse width has been extended. In Patent Document 2, a pickup coil is used. It has been proposed to extend the RF pulse width based on actually measured data. However, since actual measurement using a pickup coil is complicated, in practice, the RF pulse width is often extended by calculation based on parameters such as the body weight of the subject and the characteristics of the transmission coil and an empirical formula. ing.
However, the extension of the calculated RF pulse width depends on the subject and has a large error. The extended RF pulse width is excessive (cannot make full use of the power of the RF power amplifier), or the extended RF pulse width is insufficient. (The desired RF magnetic field strength could not be obtained).
Accordingly, an object of the present invention is to provide an MR scan method and an MRI apparatus that can accurately determine an RF pulse width.

In a first aspect, the present invention provides a localizer prescan step for performing a localizer prescan to determine an RF pulse amplitude corresponding to a localizer scan RF magnetic field strength, and the localizer scan RF magnetic field strength. A localizer scan step for performing a localizer scan using an RF pulse having an RF pulse amplitude corresponding to the above, and a main scan RF pulse for determining an RF pulse width of the main scan based on data measured by the localizer prescan A width determination step, a main scan pre-scan step for performing a main scan / pre-scan to determine an RF pulse amplitude corresponding to the RF magnetic field strength for main scanning, and an RF pulse corresponding to the RF magnetic field strength for main scanning Perform main scan using RF pulse of amplitude To provide an MR scan method characterized by having a main scanning step.
In the MR scan method according to the first aspect, it is not necessary to carry out complicated actual measurement using the pickup coil, but the RF pulse width of the main scan is determined based on the data actually measured by the localizer / prescan. Compared with the extension of the RF pulse width, the RF pulse width can be determined more accurately. Note that localizer prescan and localizer scan are performed irrespective of the present invention for slice positioning, and the number of processes increased for the present invention is small.

According to a second aspect, the present invention provides the MR scan method according to the first aspect, wherein the RF pulse amplitude is divided by the RF magnetic field strength for the localizer scan and output in the main scan RF pulse width determination step. A coefficient Coeff is calculated, and a value obtained by multiplying the target RF magnetic field strength B1_scan for main scan by the output coefficient Coeff is divided by the target amplitude Out_target for main scan to obtain the expansion / contraction ratio S_ratio, and the extension coefficient S_ratio is used for the original main scan. There is provided an MR scan method characterized by multiplying an RF pulse width PW_scan to determine a new main scan RF pulse width PW_scan.
In the MR scan method according to the second aspect described above, the RF pulse width for the main scan is determined based on the data measured by the localizer pre-scan. Therefore, the RF pulse width can be determined more accurately than the extension of the RF pulse width by calculation. Can be done.

According to a third aspect, the present invention provides the MR scan method according to the first or second aspect, wherein the RF pulse amplitude does not become insufficient even when the subject's weight is large as the RF magnetic field strength for localizer scanning. An MR scan method characterized by using a small magnetic field strength is provided.
In the MR scan method according to the third aspect, the RF magnetic field strength for localizer scan is limited, but the RF pulse width used in the localizer scan can be easily determined.

In a fourth aspect, the present invention is characterized in that, in the MR scan method according to the first or second aspect, the localizer scan RF pulse width is determined based on at least a parameter including the weight of the subject. An MR scan method is provided.
In the MR scan method according to the fourth aspect, it is necessary to determine the RF pulse width used in the localizer scan. However, the localizer has a small magnetic field intensity so that the RF pulse amplitude is not insufficient even when the subject's weight is large. -RF magnetic field strength for scanning is not limited.

In a fifth aspect, the present invention relates to a localizer prescan means for performing a localizer prescan to determine an RF pulse amplitude corresponding to the localizer scan RF magnetic field strength, and the localizer scan RF magnetic field strength. Localizer scan means for performing a localizer scan using an RF pulse having a corresponding RF pulse amplitude, and an RF pulse width determination for the main scan that determines the RF pulse width of the main scan based on the data measured by the localizer prescan Means, a main scan / pre-scan means for performing a main scan / pre-scan to determine an RF pulse amplitude corresponding to the main scan RF magnetic field strength, and an RF pulse amplitude RF corresponding to the main scan RF magnetic field strength. A main scanning means for performing a main scan using a pulse. Provides an MRI apparatus wherein Rukoto.
In the MRI apparatus according to the fifth aspect, the MR scan method according to the first aspect can be suitably implemented.

In a sixth aspect, the present invention provides the MRI apparatus according to the fifth aspect, wherein the main scanning RF pulse width determining means divides the RF pulse amplitude by the localizer scanning RF magnetic field intensity to output power coefficients. Coeff is calculated, and a value obtained by multiplying the target RF magnetic field strength B1_scan for main scanning by the output coefficient Coeff is divided by the target amplitude Out_target for main scanning to obtain the expansion / contraction ratio S_ratio, and the extension coefficient S_ratio is obtained from the original RF for main scanning. Provided is an MRI apparatus characterized by multiplying a pulse width PW_scan to determine a new main scan RF pulse width PW_scan.
In the MRI apparatus according to the sixth aspect, the MR scan method according to the second aspect can be suitably implemented.

In a seventh aspect, the present invention provides the MRI apparatus according to the fifth or sixth aspect, wherein the RF pulse amplitude does not become insufficient even when the subject's weight is large as the RF magnetic field strength for localizer scanning. An MRI apparatus characterized by using a small magnetic field strength is provided.
In the MRI apparatus according to the seventh aspect, the MR scan method according to the third aspect can be suitably implemented.

In an eighth aspect, the present invention is characterized in that, in the MRI apparatus according to the fifth or sixth aspect, the localizer scan RF pulse width is determined based on at least a parameter including the weight of the subject. An MRI apparatus is provided.
In the MRI apparatus according to the eighth aspect, the MR scan method according to the fourth aspect can be suitably implemented.

  According to the MR scanning method and the MRI apparatus of the present invention, accurate RF pulse width determination can be performed. For this reason, it is possible to make full use of the capability of the RF power amplifier or to avoid a situation in which a desired RF magnetic field strength cannot be obtained.

  Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings. Note that the present invention is not limited thereby.

FIG. 1 is a block diagram illustrating an MRI apparatus 100 according to the first embodiment.
In the MRI apparatus 100, the magnet assembly 1 has a space portion (bore) for inserting a subject therein, and an X-axis gradient coil 1X that forms an X-axis gradient magnetic field around the space portion. A Y-axis gradient coil 1Y that forms a Y-axis gradient magnetic field, a Z-axis gradient coil 1Z that forms a Z-axis gradient magnetic field, and a transmission coil 1T that provides an RF pulse for exciting spins of nuclei in the subject. A receiving coil 1R for detecting an NMR signal from the subject and a permanent magnet pair 1M for forming a static magnetic field are provided. A superconducting magnet may be used instead of the permanent magnet pair 1M.

The X-axis gradient coil 1X, the Y-axis gradient coil 1Y, and the Z-axis gradient coil 1Z are connected to the X-axis gradient coil drive circuit 3X, the Y-axis gradient coil drive circuit 3Y, and the Z-axis gradient coil drive circuit 3Z, respectively.
The transmission coil 1T is connected to the RF power amplifier 4.
The receiving coil 1R is connected to the preamplifier 5.

  The sequence storage circuit 8 operates the gradient coil drive circuits 3X, 3Y, and 3Z based on the stored pulse sequence in accordance with a command from the computer 7 to generate gradient magnetic fields from the gradient coils 1X, 1Y, and 1Z. The gate modulation circuit 9 is operated to modulate the carrier wave output signal of the RF oscillation circuit 10 into a pulse signal having a predetermined timing, a predetermined envelope shape, and a predetermined phase, which is added to the RF power amplifier 4 as an RF pulse, and the RF power amplifier After power amplification at 4, the voltage is applied to the transmission coil 1T.

  The preamplifier 5 amplifies the NMR signal from the subject received by the receiving coil 1 </ b> R and inputs it to the phase detector 12. The phase detector 12 uses the carrier wave output signal of the RF oscillation circuit 10 as a reference signal, phase-detects the NMR signal from the preamplifier 5, and provides it to the AD converter 11. The AD converter 11 converts the analog signal after phase detection into digital data and inputs it to the computer 7.

The computer 7 is responsible for overall control such as receiving information input from the console 13. The computer 7 reads digital data from the AD converter 11 and performs arithmetic processing to generate an image.
The display device 6 displays images and messages.

FIG. 2 is a flowchart of the MR scan process according to the first embodiment.
In step Q1, localizer prescan for determining the localizer scan RF pulse amplitude Out_loc is performed. For example, the localizer prescan sequence is a spin echo sequence, the 90 ° RF pulse width PW — 90.loc is 3.2 [ms], and the 90 ° RF field intensity B1 — 90.loc.pre is 0.09 [ Gauss], RF pulse width PW_180.loc of 180 ° pulse is 6.4 [ms], and RF magnetic field intensity B1_180.loc.pre of 180 ° pulse is 0.09 [Gauss], as shown in FIG. The 90 ° pulse R1 and the 180 ° pulse R2 are applied while changing the RF pulse amplitude A, and the RF pulse amplitude Amax at which the intensity of the spin echo signal is maximized is obtained. Here, it is assumed that the RF pulse amplitude Amax is 60 [%] (the maximum RF pulse amplitude possible with the RF power amplifier 4 is 100 [%]).
Note that the RF magnetic field strengths B1_90.loc.pre and B1_180.loc.pre are small magnetic field strengths so that the RF pulse amplitudes PW_90.loc and PW_180.loc are not insufficient even when the weight of the subject is large. As a result, the RF pulse widths PW_90.loc and PW_180.loc can be easily determined.

In step Q2, the RF pulse amplitude Out_loc of the localizer scan is determined based on the data actually measured by the localizer prescan, and the localizer scan is performed using the RF pulse of the determined RF pulse amplitude Out_loc. For example, if the localizer scan sequence is a gradient echo sequence, the flip angle is 30 °, the RF pulse width PW_loc is 3.2 [ms], and the RF magnetic field strength B1_loc is 0.03 [Gauss], the localizer pre- If the RF pulse amplitude Amax is 60% with respect to the RF magnetic field intensity B1_90.loc.pre = B1_180.loc.pre = 0.09 [Gauss] in the scan, the RF pulse amplitude Out_loc of the localizer scan is ,
Out_loc = 0.03 [Gauss] × 60 [%] / 0.09 [Gauss] = 20 [%]
And decide. Then, an RF pulse R as shown in FIG. 4 is applied to collect data for localization.

In step Q3, the expansion / contraction ratio S_ratio is calculated based on the data actually measured by the localizer / prescan. For example, first, the output coefficient Coeff is calculated by dividing the RF pulse amplitude Out_loc by the localizer scan RF magnetic field intensity B1_loc.
Coeff = Out_loc / B1_loc = 20 [%] / 0.03 [Gauss] ≈667 [%] / [Gauss]
Alternatively, the RF pre-scan RF magnetic field intensity B1 — 90.loc.pre = B1 — 180.loc.pre = 0.09 [Gauss] is divided by the RF pulse amplitude Amax to calculate the output coefficient Coeff.
Coeff = 60 [%] / 0.09 [Gauss] ≈667 [% / Gauss]
Next, the expansion / contraction ratio S_ratio is obtained by the following equation.
S_ratio = B1_scan.target x Coeff / Out_scan.target
That is, the expansion / contraction ratio S_ratio can be obtained by dividing the main scan target RF magnetic field intensity B1_scan.target by the output coefficient Coeff by the main scan target amplitude Out_scan.target. For example, the main scan sequence is a spin echo sequence, the 90 ° RF field strength B1_scan.target is 0.09 [Gauss], and the 180 ° pulse RF field strength B1_scan.target is 0.18 [Gauss]. When the target amplitude for main scan Out_scan.target is 80 [%],
S1_ratio = 0.09 [Gauss] × 667 [% / Gauss] / 80 [%] = 0.75
For the 180 ° pulse,
S2_ratio = 0.18 [Gauss] × 667 [% / Gauss] / 80 [%] = 1.5
It becomes.
In order to effectively use the RF power amplifier 4 with a margin, it is preferable to set the target amplitude for main scan Out_scan.target to 60 [%] to 90 [%].

In step Q4, the RF pulse width for main scanning is determined by multiplying the provisional RF pulse width by the expansion / contraction ratio S_ratio. For example, in the above-mentioned example, as shown in FIG. 5, when the 90 ° pulse RF pulse width PW_90.scan is set to the temporary RF pulse width of 90 ° pulse is 3.2 [ms],
PW_90.scan = 3.2 [ms] × 0.75 = 2.4 [ms]
The RF pulse width PW_180.scan of the 180 ° pulse is set to 3.2 [ms] when the temporary RF pulse width of the 180 ° pulse is 3.2 [ms]
PW — 180.scan = 3.2 [ms] × 1.5 = 4.8 [ms]
Extend to.

  In step Q5, a main scan / pre-scan is performed to determine an RF pulse amplitude Out_scan corresponding to the RF magnetic field strength for main scan using the RF pulse having the determined RF pulse width. For example, a 90 ° pulse R1 (RF pulse width 2.4 [ms]) and a 180 ° pulse R2 (RF pulse width 4.8 [ms]) as shown in FIG. Then, the RF pulse amplitude Hmax that maximizes the intensity of the spin echo signal is obtained.

In step Q6, the RF pulse amplitude Hmax measured in the main scan / pre-scan is determined as the main scan RF pulse amplitude Out_scan.
Out_scan = Hmax
Then, the main scan is performed using the RF pulse having the determined RF pulse amplitude Out_scan. For example, RF pulses R1 (RF pulse width PW_90.scan = 2.4 [ms]) and R2 (RF pulse width PW_180.scan = 4.8 [ms]) as shown in FIG. 6 are applied for imaging. Collect data.

According to the MRI apparatus 100 of the first embodiment, it is not necessary to perform complicated measurement using the pickup coil, but the RF pulse width of the main scan is determined based on the data measured by the localizer / pre-scan. Compared with the extension of the RF pulse width by, the RF pulse width can be determined more accurately. Further, the RF power amplifier 4 can be used effectively.
Note that localizer prescan and localizer scan are performed irrespective of the present invention for slice positioning, and the number of processes increased for the present invention is small.

In order to simplify the processing, the RF pulse width PW may not be shortened but only extended.
In this case, for example, a 90 ° pulse R1 (RF pulse width PW — 90.scan = PW — 90.scan.target = 3.2 [ms]) and a 180 ° pulse R2 (RF pulse width PW — 180.scan = 4) as shown in FIG. .8 [ms]) is applied while changing the RF pulse amplitude H, and the RF pulse amplitude Hmax that maximizes the intensity of the spin echo signal is obtained.
Then, the RF pulse amplitude Hmax is determined as the main scan RF pulse amplitude Out_scan, and the main scan is performed using the RF pulse of the determined RF pulse amplitude Out_scan. For example, RF pulses R1 (RF pulse width PW — 90.scan = 3.2 [ms]) and R2 (RF pulse width PW — 180.scan = 4.8 [ms]) as shown in FIG. 8 are applied for imaging. Collect data.

  Parameters and empirical formulas including the weight of the subject and the characteristics of the coil may be used supplementarily.

FIG. 9 is a flowchart of the MR scan process according to the third embodiment.
In step P1, the localizer scan RF pulse width PW_loc is determined based on parameters and empirical formulas including the weight of the subject and the coil characteristics.

In Step P2, the same localizer prescan as in Step Q1 is performed using the RF pulse having the determined localizer scan RF pulse width PW_loc.
In step P3, the localizer scan is performed in the same manner as in step Q2.

  In step P4, the temporary RF pulse width PW_scan.target for main scanning is determined based on parameters and empirical formulas including the weight of the subject and the characteristics of the coil.

In step P5, the expansion / contraction ratio S_ratio is calculated in the same manner as in step Q3.
In step P6, the main scan RF pulse width PW_scan is determined in the same manner as in step Q4.
In step P7, the main scan / pre-scan is performed as in step Q5.
In step P8, the main scan is performed in the same manner as in step Q6.

  According to the MRI apparatus of the third embodiment, the parameters including the weight of the subject and the characteristics of the coil and the empirical formula are used supplementarily so that the RF pulse amplitude is not insufficient even when the subject weight is large at the time of localization. There is no limitation as in the first embodiment in which the RF field strength is small.

  The MR scanning method and MRI apparatus of the present invention can be used to obtain a tomographic image of a subject.

1 is a block diagram illustrating a configuration of an MRI apparatus according to a first embodiment. FIG. 6 is a flowchart illustrating MR scan processing according to the first embodiment. It is a conceptual diagram which shows the RF pulse at the time of a localizer prescan. It is a conceptual diagram which shows the RF pulse at the time of a localizer scan. FIG. 3 is a conceptual diagram illustrating RF pulses at the time of main scanning and pre-scanning according to the first embodiment. FIG. 3 is a conceptual diagram illustrating an RF pulse during a main scan according to the first embodiment. FIG. 10 is a conceptual diagram illustrating RF pulses during main scan and pre-scan according to the second embodiment. FIG. 6 is a conceptual diagram showing an RF pulse at the time of a main scan according to Example 2. FIG. 10 is a flowchart illustrating MR scan processing according to the third embodiment.

Explanation of symbols

1T Transmitting coil 1X X-axis gradient coil 1Y Y-axis gradient coil 1Z Z-axis gradient coil 4 RF power amplifier 100 MRI apparatus

Claims (8)

  A localizer prescan step for performing a localizer prescan to determine an RF pulse amplitude corresponding to the RF magnetic field strength for localizer scan, and an RF pulse having an RF pulse amplitude corresponding to the RF magnetic field strength for localizer scan A localizer scan step for performing a localizer scan, a main scan RF pulse width determination step for determining a main scan RF pulse width based on data measured by the localizer prescan, and a main scan RF magnetic field A main scan pre-scan step for performing a main scan / pre-scan to determine an RF pulse amplitude corresponding to the intensity, and an RF pulse having an RF pulse amplitude corresponding to the RF magnetic field intensity for the main scan is performed. To perform this scan step MR scanning method which is characterized in that.   2. The MR scan method according to claim 1, wherein in the RF pulse width determination step for the main scan, an output coefficient Coeff is calculated by dividing the RF pulse amplitude by the RF magnetic field intensity for the localizer scan, and the target for the main scan is calculated. The value obtained by multiplying the RF magnetic field strength B1_scan by the output coefficient Coeff is divided by the target amplitude for main scanning Out_target to obtain the expansion / contraction ratio S_ratio, and the original main scanning RF pulse width PW_scan is multiplied by the extension coefficient S_ratio to obtain a new one. An MR scan method characterized by determining a main scan RF pulse width PW_scan.   3. The MR scanning method according to claim 1, wherein the localizing / scanning RF magnetic field strength is a small magnetic field strength that does not cause a short RF pulse amplitude even if the subject's weight is large. MR scanning method.   3. The MR scanning method according to claim 1, wherein the localizer scanning RF pulse width is determined based on a parameter including at least the weight of the subject.   Using localizer prescan means for performing localizer prescan to determine the RF pulse amplitude corresponding to the localizer scan RF magnetic field intensity, and an RF pulse having an RF pulse amplitude corresponding to the localizer scan RF magnetic field intensity Localizer scan means for performing localizer scan, RF pulse width determination means for main scan for determining the RF pulse width of the main scan based on data measured by the localizer prescan, and RF magnetic field intensity for main scan A main scan / pre-scan means for performing a main scan / pre-scan to determine a corresponding RF pulse amplitude, and a main scan for performing a main scan using an RF pulse having an RF pulse amplitude corresponding to the RF magnetic field intensity for the main scan. MRI characterized by comprising: Location.   6. The MRI apparatus according to claim 5, wherein the main scan RF pulse width determining means calculates an output coefficient Coeff by dividing the RF pulse amplitude by the localizer scan RF magnetic field intensity, and calculates the target scan target RF. The value obtained by multiplying the magnetic field intensity B1_scan by the output coefficient Coeff is divided by the main scan target amplitude Out_target to obtain the expansion / contraction ratio S_ratio, and the extension coefficient S_ratio is multiplied by the original main scan RF pulse width PW_scan to obtain a new book. An MRI apparatus characterized by determining a scanning RF pulse width PW_scan.   7. The MRI apparatus according to claim 5, wherein the localizing / scanning RF magnetic field strength is a small magnetic field strength that does not cause a short RF pulse amplitude even if the subject's weight is large. MRI equipment.   7. The MRI apparatus according to claim 5, wherein the localizer scan RF pulse width is determined based on a parameter including at least the weight of the subject.

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