Magnetic resonance imaging device and control method thereof
Distortion generated in an image is effectively corrected in imaging using an EPI sequence such as DWI without extending an imaging time. After one excitation RF pulse of EPI is applied, a navigator scan in which the polarity of the phase encoding is opposite to that of the main scan is performed continuously to the main scan, and the distortion of the image by using the navigator scan data obtained by the navigator scan is corrected. In a case of multi-shot, phase information obtained from the navigator scan data for each shot is used to perform phase correction and multi-shot reconstruction on the main scan data of each shot.
1. A magnetic resonance imaging device comprising:
a measurement unit configured to collect a nuclear magnetic resonance signal from a subject, the measurement unit executing a pulse sequence including performing, after applying one excitation RF pulse (1 shot), a main scan for measuring an echo signal for image reconstruction, and a navigator scan for measuring an echo signal for distortion correction, an applied polarity of phase encoding in the navigator scan being opposite to an applied polarity of phase encoding in the main scan; and
an image calculation unit configured to generate an image by using the nuclear magnetic resonance signal collected by the measurement unit,
the image calculation unit including
a displacement amount calculation unit configured to calculate a displacement amount of a main scan image, by using the echo signal obtained by the navigator scan, and
a distortion correction unit configured to correct distortion of the main scan image by using the displacement amount calculated by the displacement amount calculation unit, to cause a corrected image to be output.
2. A control method of a magnetic resonance imaging device comprising: a measurement unit configured to collect a nuclear magnetic resonance signal from a subject using a pulse sequence; and an image calculation unit configured to generate an image by using the nuclear magnetic resonance signal collected by the measurement unit, the control method comprising:
a measuring step of causing the measurement unit to execute a pulse sequence including performing, after applying one excitation RF pulse, a main scan for measuring an image reconstruction echo signal, and a navigator scan for measuring a distortion correction echo signal, an applied polarity of phase encoding in the navigational scan being opposite to an applied polarity of phase encoding in the main scan; and
an image processing step of calculating a displacement amount of a main scan image, by using an echo signal obtained by the navigator scan, and correcting distortion of the main scan image by using the displacement amount, to cause a corrected image to be output.
3. The magnetic resonance imaging device according to claim 1 , wherein
the pulse sequence is a single shot sequence that collects all echoes for image reconstruction with one excitation RF pulse, and
the distortion correction unit uses the distortion correction by the navigator scan to correct distortion of the main scan image corresponding to the echo signal measured by the main scan, by using the displacement amount calculated by the displacement amount calculation unit.
4. The magnetic resonance imaging device according to claim 1 , wherein
the pulse sequence is a multi-shot sequence that collects echoes for image reconstruction by using two or more excitation RF pulses, and
the image calculation unit further includes a multi-shot reconstruction unit configured to synthesize the echo signal obtained in the main scan of each shot, and the multi-shot reconstruction unit uses phase information on a navigator scan image obtained by the navigator scan of each shot to perform phase correction and synthesize an image corresponding to the echo signal obtained by the main scan of each shot.
5. The magnetic resonance imaging device according to claim 4 , wherein the distortion correction unit uses the navigator scan image obtained by the navigator scan to correct distortion of the main scan image synthesized by the multi-shot reconstruction unit by using the calculated displacement amount.
6. The magnetic resonance imaging device according to claim 1 , wherein
the navigator scan includes a first navigator scan in which an applied polarity of phase encoding is the same as that of the main scan, and a second navigator scan in which the applied polarity of the phase encoding is opposite to that of the main scan, and
the displacement amount calculation unit calculates the displacement amount by using a navigator scan image obtained by the first navigator scan and a navigator scan image obtained by the second navigator scan.
7. The magnetic resonance imaging device according to claim 6 , wherein
the pulse sequence is a multi-shot echo planar sequence that collects the echoes for image reconstruction by using the two or more excitation RF pulses,
the image calculation unit further includes a multi-shot reconstruction unit configured to synthesize the echo signal obtained in the main scan of each shot, and
the distortion correction unit uses the displacement amount to correct distortion of the main scan image synthesized by the multi-shot reconstruction unit.
8. The magnetic resonance imaging device according to claim 7 , wherein
the multi-shot reconstruction unit uses the phase information on the navigator scan image obtained in the first navigator scan to perform phase correction and synthesize the image obtained in the main scan of each shot.
9. The magnetic resonance imaging device according to claim 1 , wherein
the pulse sequence is a multi-shot echo planar sequence that collects echoes for image reconstruction by using two or more excitation RF pulses, the navigator scan includes a first navigator scan in which an applied polarity of phase encoding is the same as that of the main scan, and a second navigator scan in which the applied polarity of the phase encoding is opposite to that of the main scan,
the image calculation unit further includes a multi-shot reconstruction unit configured to synthesize the echo signal obtained in the main scan of each shot, and a navi-distortion correction unit configured to generate a distortion-corrected navigator scan image by using a navigator scan image obtained by the first navigator scan and a navigator scan image obtained by the second navigator scan, and
the multi-shot reconstruction unit uses phase information on a distortion-corrected navigator scan image generated by the navi-distortion correction unit to perform phase correction and synthesize the image obtained in the main scan of each shot.
10. The magnetic resonance imaging device according to claim 9 , wherein
the displacement amount calculation unit calculates the displacement amount by using the navigator scan image obtained by the first navigator scan and the navigator scan image obtained by the second navigator scan, and
the distortion correction unit uses the displacement amount to correct the main scan image synthesized by the multi-shot reconstruction unit.
11. The magnetic resonance imaging device according to claim 1 , wherein
the pulse sequence includes an MPG pulse, and
the image calculation unit generates a DWI image as the main scan image.
12. The control method of the magnetic resonance imaging device according to claim 2 , wherein
the navigator scan includes a first navigator scan in which an applied polarity of phase encoding is the same as an applied polarity of the main scan, and a second navigator scan in which an applied polarity of the phase encoding is opposite to that of the main scan, and
in the image processing step, the displacement amount used for the distortion correction of the main scan image is calculated by using an image obtained by the first navigator scan and an image obtained by the second navigator scan.
13. The control method of the magnetic resonance imaging device according to claim 2 , wherein
the navigator scan includes a first navigator scan in which an applied polarity of phase encoding is the same as an applied polarity of the main scan, and a second navigator scan in which applied polarity of the phase encoding is opposite to that of the main scan, and
the image processing step further includes a step of correcting a phase of the main scan image by using phase information on the navigator scan image obtained in the first navigator scan.
14. The control method of the magnetic resonance imaging device according to claim 2 , wherein
the navigator scan includes a first navigator scan in which an applied polarity of phase encoding is the same as an applied polarity of the main scan, and a second navigator scan in which applied polarity of the phase encoding is opposite to that of the main scan, and
the image processing step further includes a step of generating a distortion-corrected navigator scan image by using an image obtained by the first navigator scan and an image obtained by the second navigator scan and a step of correcting a phase of a main scan image by using the distortion-corrected navigator scan image.
15. The control method of the magnetic resonance imaging device according to claim 2 , wherein
the pulse sequence is a multi-shot echo planar sequence that collects echoes for image reconstruction by using two or more excitation RF pulses, and includes application of an MPG pulse for each shot.