# US 11,079,451 B2 · MRI apparatus

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- Number: US 11,079,451 B2
- Document ID: US11079451B2
- Status: Granted
- Kind: B2
- Application number: 16/031,383
- Filed: 2018-07-10
- Published: 2019-01-17
- Granted: 2021-08-03
- Earliest claimed: 2017-07-12
- Assignees: Canon Medical Systems Corporation
- Applicants: Canon Medical Systems Corporation
- Inventors: Masashi Ookawa; Kazuyuki Soejima
- Web page: https://patentgrep.com/patent/US11079451B2

## CPC

- G01R 33/385: using gradient magnetic field coils
- G01R 33/36: Electrical details, e.g. matching or coupling of the coil to the receiver
- G01R 33/3875: using correction coil assemblies, e.g. active shimming
- G01R 33/4818: {MR characterised by data acquisition along a specific k-space trajectory or by the temporal order of k-space coverage, e.g. centric or segmented coverage of k-space}
- G01R 33/561: by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
- G01R 33/56518: {due to eddy currents, e.g. caused by switching of the gradient magnetic field}
- G01R 33/56572: {caused by a distortion of a gradient magnetic field, e.g. non-linearity of a gradient magnetic field (G01R33/56509, G01R33/56518, G01R33/56536 take precedence)}
- G01R 33/3621: {NMR receivers or demodulators, e.g. preamplifiers, means for frequency modulation of the MR signal using a digital down converter, means for analog to digital conversion [ADC] or for filtering or processing of the MR signal such as bandpass filtering, resampling, decimation or interpolation}
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)

## Abstract

In one embodiment, an MRI apparatus includes a gradient coil, a receiving circuit, and processing circuitry. The gradient coil is configured to superimpose a gradient magnetic field on a static magnetic field. The receiving circuit is configured to receive an MR (magnetic resonance) signal from an object placed in the gradient magnetic field. The processing circuitry is configured to estimate time variation of an MR (magnetic resonance) frequency during a sampling period of the MR signal based on waveform data of a gradient current applied to the gradient coil, perform correction on a frequency or phase of the MR signal received by the receiving circuit based on the estimated time variation of the MR frequency during the sampling period, and reconstruct an image by using the MR signal subjected to the correction.

## Claims

### Claim 1

An MRI apparatus comprising:
a gradient coil configured to superimpose a gradient magnetic field on a static magnetic field;
a receiving circuit configured to receive an MR (magnetic resonance) signal from an object placed in the gradient magnetic field; and
processing circuitry configured to
estimate time variation of an MR (magnetic resonance) frequency during a sampling period in which a readout gradient is applied for sampling the received MR signal based on waveform data of a gradient current applied to the gradient coil,
perform correction on a frequency or phase of the received MR signal during the sampling period in which the readout gradient is applied, based on the estimated time variation of the MR frequency, and
reconstruct an image by using the MR signal subjected to the correction,
wherein,
the receiving circuit is configured to digitize the MR signal and then output the digitized MR signal to the processing circuitry; and
the processing circuitry is configured to generate k-space data by filling k-space with the digitized MR signal outputted from the receiving circuit, perform correction on a frequency or phase of the k-space data based on the estimated time variation of the MR frequency during the sampling period, and reconstruct an image by using the k-space data after correction.

### Claim 2 (depends on 1)

The MRI apparatus according to claim 1, wherein the processing circuitry is configured to estimate the time variation of the MR frequency caused by time variation of an eddy current magnetic field due to an eddy current.

### Claim 3 (depends on 2)

The MRI apparatus according to claim 2, wherein the processing circuitry is configured to sequentially estimate the eddy current magnetic field at a certain timing during the sampling period, by using waveform data of a plurality of gradient currents that are applied during a predetermined period prior to the certain timing.

### Claim 4 (depends on 2)

The MRI apparatus according to claim 2, wherein the processing circuitry is configured to estimate variation in frequency or phase of the MR signal during a sampling period of the MR signal by using time variation of an estimated zero-order component of the eddy current magnetic field.

### Claim 5 (depends on 1)

The MRI apparatus according to claim 1, wherein the processing circuitry is configured to sequentially estimate variation in frequency or phase of the MR signal, at a same interval as a sampling interval of the MR signal or at an interval shorter than the sampling interval, and sequentially correct the variation in frequency or phase of the MR signal in the sampling period, at a same interval as the sampling interval or at an interval shorter than the sampling interval.

### Claim 6 (depends on 1)

The MRI apparatus according to claim 1, wherein the processing circuitry is configured to perform the correction on the frequency or phase of the MR signal by applying phase rotation to the k-space data of the MR signal, in such a manner that the phase rotation cancels a frequency or phase corresponding to the estimated time variation of the MR frequency.

### Claim 7 (depends on 6)

The MRI apparatus according to claim 6, wherein the processing circuitry is configured to perform filtering for removing a signal unnecessary for image reconstruction out of the sampled MR signal, and then perform the correction on the frequency or phase of the MR signal.

### Claim 8

An MRI apparatus comprising:
a gradient coil configured to superimpose a gradient magnetic field on a static magnetic field;
a receiving circuit configured to receive an MR (magnetic resonance) signal from an object placed in the gradient magnetic field; and
processing circuitry configured to
estimate time variation of an MR (magnetic resonance) frequency during a sampling period in which a readout gradient is applied for sampling the received MR signal based on waveform data of a gradient current applied to the gradient coil,
perform correction on a frequency or phase of the received MR signal received by the receiving circuit during the sampling period in which the readout gradient is applied, based on the estimated time variation of the MR frequency during the sampling period, and
reconstruct an image by using the MR signal subjected to the correction, wherein
the receiving circuit includes a detector configured to detect the MR signal;
the processing circuitry is configured to control the receiving circuit in such a manner that a frequency or phase of the MR signal outputted from the detector is corrected according to the estimated time variation of the MR frequency during the sampling period, and reconstruct an image by using the MR signal after correction that are outputted from the detector;
the receiving circuit further includes a synthesizer configured to output a reference signal corresponding to the estimated time variation of the MR frequency, the reference signal being used in the detector for detecting the received MR signal, and
the detector is configured to correct variation in frequency or phase of the MR signal by using the reference signal.

### Claim 9 (depends on 8)

The MRI apparatus according to claim 8, wherein the processing circuitry is configured to set the reference signal to the receiving circuit in such a manner that a frequency or phase of the reference signal continuously varies, following the estimated variation in frequency or phase of the MR signal, at least during the sampling period.