Magnetic resonance imaging apparatus and flip angle determination method

Patent metadata

Field	Value
Publication number	US-10073159-B2
Application number	US-201214116421-A
Country	US
Kind code	B2
Filing date	May 23, 2012
Priority date	Jun 9, 2011
Publication date	Sep 11, 2018
Grant date	Sep 11, 2018

Abstract

In order to maximize the SNR of an image in consideration of signal correction in a multi-echo sequence, flip angles of a plurality of refocus high frequency magnetic field pulses are determined in a multi-echo imaging sequence. Using an index that reflects the SNR of an image after signal correction of a plurality of acquired echo signals, a flip angle at which the SNR of the image becomes a maximum is determined by repeatedly calculating the index by changing information specifying the flip angle of each refocus RF pulse according to an optimization method set in advance.

First claim

The invention claimed is:

1. A magnetic resonance imaging apparatus, comprising:
a static magnetic field generation system that generates a static magnetic field;
a gradient magnetic field generation system that applies a gradient magnetic field to an object disposed in the static magnetic field;
a high frequency magnetic field transmission system that transmits a high frequency magnetic field pulse to excite magnetization of the object at a predetermined flip angle;
a signal receiving system that receives an echo signal generated by the object; and
a control system that reconstructs an image from the echo signal received by the signal receiving system and controls operations of the gradient magnetic field generation system, the high frequency magnetic field transmission system, and the signal receiving system according to an imaging sequence, the control system including:
a receiving unit that receives, from a user, imaging conditions to determine the imaging sequence; and
a sequence generation unit that generates the imaging sequence according to (a) the imaging conditions received by the receiving unit, and (b) a pulse sequence that includes applying a plurality of refocus high frequency magnetic field pulses after application of one excitation high frequency magnetic field pulse, in which flip angles of the refocus high frequency magnetic field pulses are not constant,

wherein the sequence generation unit determines the flip angles of the respective refocus high frequency magnetic field pulses to maximize an SNR (signal to noise ratio) in a maximum resolution of the reconstructed image by considering contribution of each echo signal to the SNR, said contribution of the echo signal being dependent on amplification of the echo signal caused by a signal correction to suppress image blur as post-processing, and
wherein for each refocus pulse amongst the plurality of refocus high-frequency magnetic field pulses, the flip angle of the refocus pulse is determined so as to minimize a noise amplification factor that reflects not only signal strength of the echo signals arranged in a center of k-space but also signal strength of the echo signals arranged in a region other than the center of k-space and the noise amplification factor is determined based on only the signal strength.

2. A flip angle determination method performed by a magnetic resonance imaging apparatus, the method comprising:
a flip angle determination performed by the magnetic resonance imaging apparatus to determine (i) a plurality of refocus high frequency magnetic field pulses to be applied after an excitation high frequency magnetic field pulse in a multi-echo imaging sequence in the magnetic resonance imaging apparatus, in which angles of the refocus high frequency magnetic field pulses are not constant, and (ii) flip angles of the respective refocus high frequency magnetic field pulses to maximize an SNR (signal to noise ratio) in a maximum resolution of a reconstructed image by considering contribution of each echo signal to the SNR, wherein the contribution is dependent on amplification of the echo signal caused by a signal correction to suppress image blur as post-processing,
wherein for each refocus pulse amongst the plurality of refocus high-frequency magnetic field pulses, the magnetic resonance imaging apparatus determines the flip angle of the refocus pulse so as to minimize a noise amplification factor that reflects not only signal strength of the echo signals arranged in a center of k-space but also signal strength of the echo signals arranged in a region other than the center of k-space and the noise amplification factor is determined by only the signal strength.

3. A method performed by a magnetic resonance imaging apparatus to improve SNR (signal to noise ratio) of a reconstructed image, the magnetic resonance imaging apparatus comprising a static magnetic field generation system to generate a static magnetic field, a gradient magnetic field generation system to apply a gradient magnetic field to an object disposed in the static magnetic field, a high frequency magnetic field transmission system to transmit a high frequency magnetic field pulse to excite magnetization of the object at a predetermined flip angle, and a signal receiving system to receive an echo signal generated by the object, the method comprising:
generating an imaging sequence according to a pulse sequence and imaging conditions set by a user, the pulse sequence including a plurality of refocus high frequency magnetic field pulses to be applied after application of one excitation high frequency magnetic field pulse, in which flip angles of the respective refocus high frequency magnetic field pulses are not constant and are configured to maximize an SNR in a maximum resolution of a reconstructed image by considering contribution of each echo signal to the SNR, said contribution of the echo signal being dependent on amplification of the echo signal caused by a signal correction to suppress image blur as post-processing;
controlling operations of the gradient magnetic field generation system, the high frequency magnetic field transmission system, and the signal receiving system according to the imaging sequence; and
reconstructing an image from the echo signal received by the signal receiving system
wherein for each refocus pulse amongst the plurality of refocus high-frequency magnetic field pulses, the magnetic resonance imaging apparatus determines the flip angle of the refocus pulse so as to minimize a noise amplification factor that reflects not only signal strength of the echo signals arranged in a center of k-space but also signal strength of the echo signals arranged in a region other than the center of k-space and the noise amplification factor is determined by only the signal strength.

4. The magnetic resonance imaging apparatus according to claim 1 , wherein
the sequence generation unit includes a flip angle determination section that determines the flip angle of each of the refocus high frequency magnetic field pulses from the imaging conditions, and
the flip angle determination section determines the flip angle by repeatedly calculating an index by changing information specifying the flip angle of each of the refocus high frequency magnetic field pulses according to an optimization method set in advance.

5. The magnetic resonance imaging apparatus according to claim 4 , wherein
the flip angle determination section includes a signal strength calculation section that calculates signal strength of each echo signal, which is obtained by application of a refocus high frequency magnetic field pulse corresponding to each information item, from the information specifying the flip angle of each of the refocus high frequency magnetic field pulses, and
using the noise amplification factor, which is obtained from a square root of a mean of a square of a reciprocal of the calculated signal strength of each echo signal, as the index, the flip angle of each of the refocus high frequency magnetic field pulses is determined so as to minimize the noise amplification factor.

6. The magnetic resonance imaging apparatus according to claim 4 , wherein the information specifying the flip angle of each of the refocus high frequency magnetic field pulses is a flip angle of each of the refocus high frequency magnetic field pulses.

7. The magnetic resonance imaging apparatus according to claim 4 , wherein the information specifying the flip angle of each of the refocus high frequency magnetic field pulses is constituted by a number of items, said number of items being smaller than the number of refocus high frequency magnetic field pulses.

8. The magnetic resonance imaging apparatus according to claim 4 , wherein
the i

Assignees

• Kurokawa Shinji
• Hitachi Ltd

Inventors

• Kurokawa Shinji

Classifications

• G01R33/5617 Primary

  using RF refocusing, e.g. RARE · CPC title

• G01R33/583

  Calibration of signal excitation or detection systems, e.g. for optimal RF excitation power or frequency (G01R33/246 takes precedence) · CPC title

• G01R33/543

  Control of the operation of the MR system, e.g. setting of acquisition parameters prior to or during MR data acquisition, dynamic shimming, use of one or more scout images for scan plane prescription (G01R33/546 takes precedence) · CPC title

• G01R33/56 Primary

  Image enhancement or correction, e.g. subtraction or averaging techniques {, e.g. improvement of signal-to-noise ratio and resolution} · CPC title