MRI apparatus and method using sample filtering based on time shifts caused by temporal variations in magnetic field strength

1. An MRI apparatus comprising:

an application unit that applies a readout gradient magnetic field in which magnetic field strength thereof varies with time and RF pulses to a subject placed in a static magnetic field in accordance with a predetermined pulse sequence;

a receiving unit that receives an MR signal obtained from the subject to which the readout gradient magnetic field and the RF pulses are applied;

an AD converter that converts the received MR signal from analog to digital, and samples discrete MR-signal values;

a filtering processing unit that performs filtering processing on the sampled discrete MR-signal values, including

a filter-coefficient setting unit that sets a plurality of filter coefficients to be used for the filtering processing with respect to each point of sampling time at which the filtering processing is to be performed, based on time-shift amounts corresponding to variations in an MR-signal frequency arising from temporal variations in the magnetic field strength, and

a product-sum operating unit that performs the filtering processing on the MR-signal values by performing a product-sum operation between the filter coefficients and the MR-signal values; and

an image-data creating unit that creates image data based on the MR-signal values on which the filtering processing is performed.

2. The MRI apparatus according to claim 1 , wherein the filtering processing is performed for correcting the variations in the MR-signal frequency arising from the temporal variations in the magnetic field strength with the product-sum operating unit.

3. The MRI apparatus according to claim 1 , wherein the filtering processing unit performs the filtering processing by setting a larger sampling cycle than a sampling cycle of analog-digital conversion.

4. The MRI apparatus according to claim 3 , wherein the filtering processing unit sets the larger sampling cycle based on a signal band of the MR signal.

5. The MRI apparatus, according to claim 1 , wherein

the filter-coefficient setting unit includes a filter-coefficient interpolation calculating unit, and

the filter-coefficient interpolation calculating unit calculates the filter coefficients by performing interpolation processing based on each of the time-shift amounts on standard filter-coefficients of a filter function with a center on each of the sampling time at which the filtering processing is to be performed, the standard filter-coefficients being predetermined with respect to respective points of the sampling time.

6. The MRI apparatus according to claim 5 , further comprising:

a shift-amount storage unit that prestores the time-shift amounts corresponding to respective readout gradient magnetic fields; and

a filter-coefficient storage unit that prestores standard filter-coefficients corresponding to respective filter characteristics, wherein the filter-coefficient interpolation calculating unit calculates the filter coefficients by performing interpolation processing on the standard filter-coefficients read from the filter-coefficient storage unit based on the filter characteristic of the filtering processing by using the time-shift amounts read from the shift-amount storage unit based on a magnetic-field characteristic of the readout gradient magnetic field.

7. The MRI apparatus according to claim 6 , wherein the readout gradient magnetic field has a magnetic field strength of which temporal variations are sinusoidal in shape.

8. The MRI apparatus according to claim 6 , wherein the standard filter-coefficients are set as those of a sinc function as a filter function.

9. An MRI method comprising:

applying a readout gradient magnetic field in which magnetic field strength thereof varies with time and RF pulses to a subject placed in a static magnetic field in accordance with a predetermined pulse sequence;

receiving an MR signal obtained from the subject to which the readout gradient magnetic field and the RF pulse are applied;

converting the received MR signal from analog to digital, and sampling discrete MR-signal values;

performing filtering processing on the sampled discrete MR-signal values, by

setting a plurality of filter coefficients to be used for the filtering processing with respect to each point of sampling time at which the filtering processing is to be performed, based on time-shift amounts corresponding to variations in an MR-signal frequency arising from temporal variations in the magnetic field strength, and

performing the filtering processing on the MR-signal values by performing a product-sum operation between the filter coefficients and the MR-signal values; and

creating image data based on the MR-signal values on which the filtering processing is performed.