Magnetic resonance imaging apparatus and control device of a magnetic resonance imaging apparatus
A control device of a magnetic resonance (MRI) imaging apparatus includes a condition setting unit and a judging unit. The condition setting unit sets an imaging sequence to be performed by the magnetic resonance imaging apparatus based on set conditions of the set imaging sequence. The judging unit then (a) calculates a value of electric current supplied to a gradient magnetic field coil of the MRI apparatus to perform that set imaging sequence based on the set conditions of the set imaging sequence, (b) calculates a value of voltage that would need to be applied to the gradient magnetic field coil based on a mutual inductance of the gradient magnetic field to cause electric current flowing to the gradient magnetic field coil to become equal to the value of the calculated electric current, and (c) judges whether the set imaging sequence is practicable or not based on the calculated value of voltage.
1. A magnetic resonance imaging apparatus that applies a gradient magnetic field to an imaging region, generates k-space data including a plurality of matrix elements by sampling a nuclear magnetic resonance signal acquired from the imaging region and reconstructs image data based on the k-space data, comprising:
a gradient magnetic field power supply configured to apply the gradient magnetic field to the imaging region by supplying gradient magnetic field current to a gradient magnetic field coil under an imaging sequence;
a gradient magnetic field calculating unit configured to calculate a waveform of the gradient magnetic field current based on conditions of the imaging sequence, and calculate a waveform of a gradient magnetic field in a readout direction based on a mutual inductance by which the gradient magnetic field coil causes mutual induction and the waveform of the gradient magnetic field current; and
a regridding processing unit configured to generate or rearrange the k-space data by sampling at unequally-spaced intervals, so that a part of the nuclear magnetic resonance signal acquired during a time span in which a time integral value of intensity of the gradient magnetic field in a readout direction is non-linear and each time integral value up to a sampling period corresponding to each of the matrix elements becomes equally-spaced.
2. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the regridding processing unit is configured to set unequally-spaced sampling periods so that each time integral value whose back end of integral interval is a representative time of each sampling period for the nuclear magnetic resonance signal is equally-spaced, and generate the k-space data by sampling the nuclear magnetic resonance signal based on the unequally-spaced sampling periods.
3. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the regridding processing unit is configured to generate the k-space data by sampling the nuclear magnetic resonance signal at equally-spaced intervals, and rearrange the k-space data so that each time integral value whose back end of integral interval is a representative time of each sampling period corresponding to each matrix element of the k-space data is placed at equal intervals.
4. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the gradient magnetic field calculating unit is configured to preliminarily store circuit constants of an equivalent circuit of a gradient magnetic field generation system that includes the gradient magnetic field coil, another coil aside from the gradient magnetic field coil, and the gradient magnetic field power supply, including a mutual inductance between said another coil and the gradient magnetic field coil, a self-inductance of the gradient magnetic field coil, a resistance value of the gradient magnetic field coil, a self-inductance of said another coil and a resistance value of said another coil as the circuit constants, and to calculate the waveform of the gradient magnetic field in a readout direction based on the waveform of the gradient magnetic field current, the equivalent circuit and the circuit constants.
5. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the gradient magnetic field calculating unit is configured to preliminarily store circuit constants of an equivalent circuit of a gradient magnetic field generation system that includes the gradient magnetic field coil, a plurality of coils aside from the gradient magnetic field coil, and the gradient magnetic field power supply, including mutual inductances between the gradient magnetic field coil and each of the plurality of coils, a self-inductance of the gradient magnetic field coil, a resistance value of the gradient magnetic field coil, respective self-inductances of the plurality of coils and each resistance value of each of the plurality of coils as the circuit constants, and to calculate the waveform of the gradient magnetic field in a readout direction based on the waveform of the gradient magnetic field current, the equivalent circuit and the circuit constants.
6. The magnetic resonance imaging apparatus according to claim 5 ,
wherein the circuit constants are constants preliminarily determined based on an actual measured value of frequency characteristics of impedance of the gradient magnetic fields.
7. The magnetic resonance imaging apparatus according to claim 4 , further comprising a constant correcting unit configured to correct the circuit constants in a prescan before a main scan;
wherein the regridding processing unit is configured to generate or rearrange k-space data of the prescan, so that each time integral value is equally-spaced for the nuclear magnetic resonance signal acquired in the prescan; and
the constant correcting unit is configured to calculate a numerical evaluation index reflecting a degree of an artifact for an evaluation image reconstructed based on the k-space data of the prescan, and correct the circuit constants based on the numerical evaluation index.
8. The magnetic resonance imaging apparatus according to claim 7 ,
wherein the imaging sequence is an imaging sequence of echo planar imaging, and the artifact is an N/2 artifact.
9. The magnetic resonance imaging apparatus according to claim 1 ,
further comprising a waveform correcting unit configured to correct a condition of the imaging sequence before performance of the imaging sequence, if the waveform of the gradient magnetic field in a readout direction calculated by the gradient magnetic field calculating unit is different from a target waveform for the gradient magnetic field in a readout direction.
10. The magnetic resonance imaging apparatus according to claim 9 ,
wherein the imaging sequence is an imaging sequence of echo planar imaging;
the gradient magnetic field calculating unit is configured to further calculate a waveform of a gradient magnetic field in a phase encode direction based on the mutual inductance and the waveform of the gradient magnetic field current; and
the waveform correcting unit is configured to shorten a pulse width of the gradient magnetic field in a phase encode direction and increase intensity of the gradient magnetic field in a phase encode direction so that that a posterior end part of the gradient magnetic field in a phase encode direction and a anterior end part of the gradient magnetic field in a readout direction do not overlap with each other, if the posterior end part of the gradient magnetic field in a phase encode direction calculated by the gradient magnetic field calculating unit and the anterior end part temporally overlap with each other.
11. The magnetic resonance imaging apparatus according to claim 1 ,
wherein the imaging sequence is an imaging sequence of echo planar imaging.