Magnetic resonance imaging apparatus and method of controlling the same
Generation of artifacts caused by the FID signal is suppressed even when the parallel imaging is applied to the imaging using a spin echo type pulse sequence. In performing a pulse sequence of a spin echo type using an excitation RF pulse for exciting nuclear spin and an inversion RF pulse for inverting excited nuclear spin as a high-frequency magnetic field pulse, a high-frequency transmitter of a MRI apparatus changes the phase of the inversion RF pulse according to the phase encoding and the phase encoding number imparted for each echo signal. Specifically, the phase of the inversion RF pulse is controlled to be a quadratic function of the phase encode of the echo signal.
1. A magnetic resonance imaging apparatus comprising:
a static magnetic field generating magnet for generating a static magnetic field;
a high-frequency transmitting unit for irradiating an RF pulse to a subject placed in a static magnetic field space;
a receiving unit for receiving an echo signal generated from the subject;
a gradient magnetic field generating unit for generating a gradient magnetic field for encoding the echo signal;
a control unit for controlling the high-frequency transmitting unit, the receiving unit and the gradient magnetic field generating unit according to a predetermined pulse sequence;
and an arithmetic unit for image reconstruction using the echo signal,
wherein in irradiating while using a pulse sequence of a spin echo system including irradiation of an excited RF pulse for exciting nuclear spin and irradiation of inversion RF pulses for inverting the excited nuclear spin as the pulse sequence, the high-frequency transmission unit changes an irradiation phase of each inversion RF pulse of the pulse sequence of the spin echo system that is to be irradiated after a predetermined repetition time TR from spin excitation, according to phase encode imparted for the echo signal and a phase encoding number, the irradiation phase of said each inversion RF pulse to be irradiated after the predetermined repetition time TR from the spin excitation being represented by a quadratic function of the phase encode.
2. The magnetic resonance imaging apparatus of claim 1 , wherein the irradiation phase θ of said each inversion RF pulse is expressed by equation (1),
[
Equation
1
]
θ
(
k
)
=
π
(
2
l
+
1
)
N
k
2
(
1
)
where l is an integer of 0 or more, k is the phase encode and N is the phase encoding number or a substantial phase encoding number obtained by dividing the phase encoding number by a reduction factor.
3. The magnetic resonance imaging apparatus of claim 2 , wherein
the control unit performs control for under-sampling k-space data at a predetermined reduction factor R in a phase encoding direction,
the high-frequency transmission unit, as N represented by the equation (1), uses the substantial phase encoding number to control the irradiation phase of said each inversion RF pulse.
4. The magnetic resonance imaging apparatus of claim 3 , wherein
the control unit performs a control for not measuring a portion of a high-frequency region of the phase encoding direction of the k-space data, and
the arithmetic unit, for the unmeasured high-frequency region, uses an absolute value of a signal of a measured high-frequency region and the irradiation phase of said each inversion RF pulse calculated for the unmeasured high-frequency region to create data of the unmeasured high-frequency region.
5. The magnetic resonance imaging apparatus of claim 4 , wherein
the control unit performs control for under-sampling the k-space data that is a measurement target at a predetermined reduction factor R in the phase encoding direction, and
the high-frequency transmitting unit, as N represented by the equation (1), uses the substantial phase encoding number to control the irradiation phase of said each inversion RF pulse.
6. The magnetic resonance imaging apparatus of claim 1 , wherein the irradiation phase θ of said each inversion RF pulse is expressed by equation (5),
[
Equation
5
]
θ
(
k
)
=
φ
(
2
l
+
1
)
N
k
2
(
5
)
where φ≠nπ, n is an integer greater than or equal to 1, l is an integer of 0 or more, k is the phase encode and N is the phase encoding number or a substantial phase encoding number obtained by dividing the phase encoding number by a reduction factor.
7. The magnetic resonance imaging apparatus of claim 6 ,
wherein the control unit performs control for under-sampling k-space data that is a measurement target at a predetermined reduction factor R in a phase encoding direction, and
the high-frequency transmission unit uses the substantial phase encoding number obtained by dividing a phase encoding number of an image by a reduction factor R as N represented by equation (5), to control the irradiation phase of said each inversion RF pulse.
8. The magnetic resonance imaging apparatus of claim 6 , which further includes
an acceptance unit for accepting designation of a region of interest,
wherein the high-frequency transmission unit calculates the irradiation phase θ of said each inversion pulse which makes a pixel value generated by a FID signal substantially zero or constant in the region of interest received by the receiving unit, and controls the irradiation phase of said each inversion RF pulse to be the calculated phase.
9. A method of controlling a magnetic resonance imaging apparatus, comprising:
performing a spin echo type pulse sequence which includes irradiation of RF pulses exciting nuclear spin and irradiation of inversion RF pulses inverting excited nuclear spin; and
changing an irradiation phase of each inversion RF pulse of the inversion RF pulses after a predetermined repetition time TR from spin excitation, according to a phase encode imparted for each echo signal and a phase encoding number, the irradiation phase of said each inversion RF pulse being represented by a quadratic function of the phase encode.
10. The method according to claim 9 , wherein
the spin echo type pulse sequence is executed at a predetermined reduction factor, and
the irradiation phase of said each inversion RF pulse is made to a phase θ represented by-equation (1),
[
Equation
1
]
θ
(
k
)
=
π
(
2
l
+
1
)
N
k
2
(
1
)
where l is an integer of 0 or more, k is the phase encode and N is a substantial phase encoding number obtained by dividing the phase encoding number by a reduction factor.