Magnetic resonance imaging apparatus and method for calculating correction value as application amount of refocusing pulse for UTE sequence

Patent metadata

Field	Value
Publication number	US-9594140-B2
Application number	US-201214232470-A
Country	US
Kind code	B2
Filing date	Aug 21, 2012
Priority date	Aug 23, 2011
Publication date	Mar 14, 2017
Grant date	Mar 14, 2017

Abstract

In order to obtain a high-quality image even in multi-slice imaging in a UTE sequence that uses a half RF pulse, a refocusing pulse of the slice gradient magnetic field is adjusted and applied so that the excitation profiles of positive polarity data and negative polarity data have phase distributions that are 180 [deg] inverted with respect to each other in side lobe portions. In addition, the irradiation frequency of the half RF pulse is adjusted so as to eliminate a position shift between the intensity distributions of the positive polarity data and the negative polarity data.

First claim

The invention claimed is:

1. A magnetic resonance imaging apparatus, comprising:
a static magnetic field generation system;
a measurement system including a gradient magnetic field generation system, a high frequency magnetic field generation system, and a high frequency magnetic field detection system; and
an arithmetic system that controls an operation of the measurement system according to a pulse sequence to measure a nuclear magnetic resonance signal and performs calculation using data obtained from the nuclear magnetic resonance signal,
wherein the pulse sequence is an ultra-short echo time sequence to obtain each echo signal by performing two slice selection excitations by inverting a polarity of a slice gradient magnetic field, which is applied together with a half RF pulse, between a positive polarity and a negative polarity,
the arithmetic system includes a pre-processing unit that calculates a correction value used in the measurement and the calculation and a main measurement unit that sets the correction value calculated by the pre-processing unit in the pulse sequence, performs main measurement by controlling the measurement system according to the pulse sequence after the setting, and reconstructs an image,
the pre-processing unit includes a refocusing pulse application amount calculation section that calculates an application amount of a refocusing pulse of each slice gradient magnetic field as the correction value,
the main measurement unit includes an image reconstruction section that adds positive polarity data, which is an echo signal obtained when the slice gradient magnetic field having a positive polarity is applied, and negative polarity data, which is an echo signal obtained when the slice gradient magnetic field having a negative polarity is applied, in the main measurement and reconstructs the image using polarity-added data after the addition, and
the refocusing pulse application amount calculation section calculates the application amount of each refocusing pulse so as to reduce a side lobe signal of an excitation profile obtained by adding the positive polarity data and the negative polarity data.

2. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the pre-processing unit further includes an irradiation frequency calculation section that calculates each irradiation frequency of the half RF pulse as the correction value, and
the irradiation frequency calculation section calculates each irradiation frequency so as to eliminate a position shift between excitation positions by the two slice selection excitations.

3. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the pre-processing unit further includes a zero-order phase difference calculation section that calculates a zero-order phase difference, which is a zero-order term of a phase difference between the positive polarity data and the negative polarity data, as the correction value, and
the image reconstruction section corrects the phase difference between the positive polarity data and the negative polarity data using the zero-order phase difference before the addition.

4. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the pre-processing unit further includes an irradiation frequency calculation section that calculates an irradiation frequency of each half RF pulse as the correction value and a zero-order phase difference calculation section that calculates a zero-order phase difference, which is a zero-order term of a phase difference between the positive polarity data and the negative polarity data, as the correction value,
the irradiation frequency calculation section calculates each irradiation frequency so as to eliminate a position shift between excitation positions by the two slice selection excitations, and
the image reconstruction section corrects the phase difference between the positive polarity data and the negative polarity data using the zero-order phase difference before the addition.

5. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the pre-processing unit further includes a slice gradient magnetic field waveform measurement section that measures a slice gradient magnetic field waveform of the pulse sequence, and
the refocusing pulse application amount calculation section calculates the application amount of each refocusing pulse using the measured slice gradient magnetic field waveform.

6. The magnetic resonance imaging apparatus according to claim 2 ,
wherein the pre-processing unit further includes a slice gradient magnetic field waveform measurement section that measures a slice gradient magnetic field waveform of the pulse sequence,
the refocusing pulse application amount calculation section calculates the application amount of each refocusing pulse using the measured slice gradient magnetic field waveform, and
the irradiation frequency calculation section calculates the position shift using a strength of a slice gradient magnetic field obtained from the measured slice gradient magnetic field waveform.

7. The magnetic resonance imaging apparatus according to claim 3 ,
wherein the pre-processing unit further includes a slice gradient magnetic field waveform measurement section that measures a slice gradient magnetic field waveform of the pulse sequence,
the refocusing pulse application amount calculation section calculates the application amount of each refocusing pulse using the measured slice gradient magnetic field waveform, and
the zero-order phase difference calculation section calculates the zero-order phase difference using a difference between a slice gradient magnetic field waveform in which the polarity is positive and a slice gradient magnetic field waveform in which the polarity is negative.

8. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the pre-processing unit further includes an excitation profile measurement section that measures excitation profiles of the positive polarity data and the negative polarity data and a first-order phase difference calculation section that calculates a coefficient of a first-order term of the phase difference as a first-order phase difference using the excitation profiles, and
the refocusing pulse application amount calculation section determines each refocusing pulse application amount using the first-order phase difference.

9. The magnetic resonance imaging apparatus according to claim 2 ,
wherein the pre-processing unit further includes an excitation profile measurement section that measures excitation profiles of the positive polarity data and the negative polarity data, a first-order phase difference calculation section that calculates a coefficient of a first-order term of the phase difference as a first-order phase difference using the excitation profiles, and a shift amount calculation section that calculates an excitation position shift amount between the excitation profiles,
the refocusing pulse application amount calculation section determines each refocusing pulse application amount using the first-order phase difference, and
the irradiation frequency calculation section determines the position shift using the shift amount.

10. The magnetic resonance imaging apparatus according to claim 3 ,
wherein the pre-processing unit further includes an excitation profile measurement section that measures excitation profiles of the positive polarity data and the negative polarity data and a first-order phase difference calculation section that calculates a coefficient of a first-order term of the phase difference as a first-order phase difference using the excitation profiles,
the refocusing pulse application amount c

Assignees

• Hanada Hikaru
• Oka Kuniharu
• Ono Masaharu
• Takizawa Masahiro
• Hitachi Ltd

Inventors

• Hanada Hikaru
• Oka Kuniharu
• Ono Masaharu
• Takizawa Masahiro

Classifications

• G01R33/561 Primary

  by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences · CPC title

• G01R33/4816

  NMR imaging of samples with ultrashort relaxation times such as solid samples, e.g. MRI using ultrashort TE [UTE], single point imaging, constant time imaging · CPC title

• A61B5/055 Primary

  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging · CPC title