Magnetic resonance imaging apparatus, image processing apparatus, and phase correcting method
To provide a technique in which, in imaging using an EPI method, an occurrence of an artifact when phase correction is performed for each channel is avoided and the phase correction is accurately performed. A common phase correction value to be applied to data of all channels is calculated using pre-scan data of each channel. The common phase correction value is obtained by combining a difference phase obtained for each of the channels. The difference phase is obtained by complex integration, while an absolute value of each channel is maintained as it is. The combination is performed by complex average, and averaging processing according to a weight of the absolute value is performed. The occurrence of an artifact can be prevented by using the common phase correction value, and robust phase correction can be performed by including the weight of the absolute value.
1. A magnetic resonance imaging apparatus, comprising:
an imaging unit configured to apply a high-frequency magnetic field pulse and a gradient magnetic field pulse according to a predetermined pulse sequence and collect a magnetic resonance signal generated from an inspection object; and
a calculation unit configured to generate an image of the inspection object using the magnetic resonance signal, wherein
the imaging unit includes a receiving coil provided with a plurality of channels and is configured to execute, as the pulse sequence, a pulse sequence of an EPI method of obtaining a plurality of magnetic resonance signals after a high-frequency magnetic field pulse for excitation is applied one time, the calculation unit includes a preprocessing unit configured to correct a phase of data of magnetic resonance signals respectively obtained in the plurality of channels of the receiving coil by executing the pulse sequence of the EPI method, and
the preprocessing unit includes a correction value calculation unit configured to calculate a phase correction value by using data for correction received in each of the plurality of channels and calculate a common phase correction value by combining the phase correction value of each channel, and a phase correction unit configured to correct a phase of data for image-forming, which is received in each channel, by using the common phase correction value.
2. The magnetic resonance imaging apparatus according to claim 1 , wherein
the correction value calculation unit is configured to calculate the phase correction value of each channel as a difference phase obtained by complex integration between a set of line data, and calculate the common phase correction value by performing complex average on the difference phases.
3. The magnetic resonance imaging apparatus according to claim 2 , wherein
the correction value calculation unit is configured to calculate, for a plurality of slices, the difference phases calculated for each channel, and combine the difference phases of the plurality of slices.
4. The magnetic resonance imaging apparatus according to claim 3 , wherein
the correction value calculation unit is configured to use a Gaussian kernel function to combine the difference phases of the plurality of slices.
5. The magnetic resonance imaging apparatus according to claim 1 , wherein
the preprocessing unit is configured to convert the data for correction into x-ky space data and perform phase correction value calculation and phase correction using the x-ky space data.
6. The magnetic resonance imaging apparatus according to claim 5 , wherein
the preprocessing unit includes an x-direction correction unit configured to perform phase correction in the x-direction and an ky-direction correction unit configured to perform phase correction in the ky-direction, and the preprocessing unit is configured to calculate the common phase correction value for each of the x-direction correction unit and the ky-direction correction unit.
7. The magnetic resonance imaging apparatus according to claim 6 , wherein
the x-direction correction unit is configured to calculate, for each channel, a difference phase by complex integration between a set of line data in the x-ky space, calculate an inter-channel average difference phase by performing complex average on the difference phase of each channel, calculate a gradient in the x-direction and an intercept in the x-direction based on the inter-channel average difference phase, and set the gradient in the x-direction and the intercept in the x-direction as the common phase correction value.
8. The magnetic resonance imaging apparatus according to claim 6 , wherein
the ky-direction correction unit includes a data dividing unit configured to divide data for correction of each channel into a plurality of patterns according to the number of shots of EPI and whether the data is in an even line or an odd line, and the ky-direction correction unit is configured to calculate a phase correction value for each pattern divided by the data dividing unit, then combine the phase correction value of each pattern, and set the combined phase correction value as the common phase correction value in the ky-direction.
9. The magnetic resonance imaging apparatus according to claim 1 , wherein
the data for correction is data obtained by pre-scanning performed by the imaging unit.
10. The magnetic resonance imaging apparatus according to claim 9 , wherein
the imaging unit is configured to obtain the data for correction and the data for image-forming in the same EPI sequence.
11. The magnetic resonance imaging apparatus according to claim 10 , wherein
the imaging unit is configured to execute two EPI sequences in which scan directions of a k-space are opposite to each other, and the preprocessing unit is configured to calculate a phase correction value using data obtained in the two EPI sequences as one set of data.
12. The magnetic resonance imaging apparatus according to claim 1 , wherein
the calculation unit includes an image reconstruction unit configured to reconstruct an image by parallel imaging calculation using under-sampled k-space data obtained by the imaging unit.
13. An image processing apparatus configured to reconstruct an image by processing data, which is received by a receiving coil provided with a plurality of channels in a magnetic resonance imaging apparatus, for each channel, the image processing apparatus comprising:
a correction value calculation unit configured to calculate a phase correction value by using data for correction received in each of the plurality of channels and calculate a common phase correction value by combining the phase correction value of each channel;
a phase correction unit configured to correct a phase of data for image-forming, which is received in each channel, by using the common phase correction value; and
an image reconstruction unit configured to reconstruct an image by using the phase-corrected image-forming data.
14. A phase correction method of correcting a phase of data for image-forming, which is obtained using EPI sequence in a magnetic resonance imaging apparatus having a receiving coil with a plurality of channels, by each of the plurality of channels, the phase correction method comprising:
a step of calculating a phase correction value by using data for correction received in each of the plurality of channels;
a step of calculating a common phase correction value by combining the phase correction value of each channel; and
a step of correcting a phase of the data for image-forming, which is received in each channel, using the common phase correction value, wherein
the step of calculating the phase correction value includes a step of calculating an inter-line difference phase in x-ky space data of the data for correction by complex integration, and the phase correction value is calculated based on the difference phase, and
the step of calculating the common phase correction value includes a step of integrating and averaging the phase correction value of each channel.
15. The phase correction method according to claim 14 , wherein
the step of calculating the phase correction value includes a step of calculating a phase correction value in the x-direction of the x-ky space and a step of calculating a phase correction value in the ky-direction of the x-ky space, and
the step of calculating the phase correction value in the x-direction and the step of calculating the phase correction value in the ky-direction are executed independently.