JP3112866B2 - MR imaging method and MRI apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an MR (Magnetic)
(Resonance) imaging method and MRI (Magneti)
c Resonance Imaging)
The present invention relates to an MR imaging method and an MRI apparatus capable of achieving a short repetition time TR without deteriorating an MTC (Magnetization Transfer Contrast).

[0002]

2. Description of the Related Art As a conventional example related to the present invention, there is, for example, an MR imaging method and an MRI apparatus described in Japanese Patent Application Laid-Open No. 9-108196. In this MR imaging method and MRI apparatus, NMR (Nuclear Magnetic Resonanc) of a nuclide (for example, proton of bound water) other than a target nuclide (for example, free water proton) for which data is to be collected.
e) To suppress the signal, apply an MT (Magnetization Transfer) pulse to RF-saturate the nuclides other than the target,
After that, an imaging sequence for collecting data from a target nuclide is executed.

[0003] The amplitude, pulse width, flip angle, and offset frequency of the MT pulse are determined in advance, and when an operator inputs a repetition time TR, the predetermined MT pulse is generated even at the repetition time TR input by the operator. It was checked whether or not hardware constraints such as heat generation of the coil were satisfied. If satisfied, the repetition time TR was allowed, and if not satisfied, the repetition time was extended to satisfy the hardware constraints. . FIG. 5 is a flowchart of the repetition time TR check process.
In step J1, the operator inputs the repetition time TR. In step J2, a minimum repetition time min_TR that satisfies hardware constraints such as electromagnetic wave absorption (Specific Absorption Rate) of the subject and heat generation of the transmission coil and the reception coil is calculated using a predetermined MT pulse. In step J3, the repetition time TR input by the operator is compared with the minimum repetition time min_TR. If the former is equal to or longer than the latter, the process is terminated.
In step J4, the minimum repetition time min_TR is forcibly set as the repetition time TR, and the process ends.

That is, as shown in FIG. 6A, the repetition time TR inputted by the operator is equal to the minimum repetition time min_T.
If not less than R, the repetition time TR input by the operator is employed as it is, as shown in FIG. On the other hand, the repetition time TR input by the operator as shown in FIG.
Is shorter than the minimum repetition time min_TR, the minimum repetition time min_TR is forcibly set as the repetition time TR as shown in FIG. 7B.

[0005]

In the above prior art, a predetermined MT pulse having a predetermined amplitude, pulse width, flip angle and offset frequency is applied so as to obtain a required MTC. However, there is a problem that it is not possible to set a repetition time TR shorter than the minimum repetition time min_TR that satisfies the hardware constraint with a predetermined MT pulse. If the MT pulse is changed so as to reduce the product of the MT pulse amplitude and pulse width, the minimum repetition time mi
Since n_TR can be shortened, a short repetition time TR can be set, but another problem arises in that a required MTC cannot be obtained. Therefore, an object of the present invention is to provide an MR imaging method and an MRI apparatus that can achieve a short repetition time without impairing MTC.

[0006]

According to a first aspect of the present invention, an MT pulse is applied to selectively remove non-target nuclides by RF.
There is provided an MR imaging method for saturating, wherein the flip angle and the offset frequency of an MT pulse are optimized according to a repetition time input by an operator. The minimum repetition time min_TR that satisfies the hardware constraint with a certain MT pulse and the effect of the flip angle of the MT pulse on the free water parasitic oscillation and MTC are examined. When the flip angle of the MT pulse is reduced, the minimum repetition time min_TR is reduced And free water parasitic oscillation can be reduced. However, MT
C decreases. Next, the minimum repetition time min_TR
When the offset frequency of the MT pulse is reduced, the minimum repetition time min_TR
And free water parasitic oscillations increase. However, the MTC can be improved. Therefore, in the MR imaging method according to the first aspect, the minimum repetition time mi shorter than the repetition time TR input by the operator.
The flip angle of the MT pulse is optimized so as to be n_TR. Thus, the repetition time TR input by the operator
Can be directly used as the repetition time of the pulse sequence.
At this time, the MTC may decrease. Therefore, the offset frequency is optimized to improve the MTC so as to compensate for the decrease in the MTC. As described above, MT
A short repetition time can be obtained without impairing C.
The effect on the free water parasitic oscillation is small because the effect of optimizing the flip angle and the effect of optimizing the offset frequency are offset.

According to a second aspect, the present invention is an MRI apparatus provided with an MT pulse applying means for applying an MT pulse to selectively RF-saturate a non-target nuclide, wherein the MRI apparatus includes a repetition input by an operator. MT that optimizes the flip angle and offset frequency of the MT pulse in accordance with time TR
An MRI apparatus comprising a pulse optimizing means is provided. In the MRI apparatus according to the second aspect,
The MR imaging method according to the first aspect can be suitably performed.

According to a third aspect, the present invention provides an M
In the RI apparatus, the MT pulse optimizing means obtains a maximum flip angle that can satisfy hardware constraints such as heat generation of the coil even at the repetition time input by the operator,
An MRI apparatus characterized in that the maximum flip angle is determined as the flip angle of the MT pulse, an appropriate offset frequency is determined from the flip angle, and the appropriate offset frequency is determined as the offset frequency of the MT pulse. provide. In the MRI apparatus according to the third aspect, the MT
The flip angle of the pulse is set to the maximum flip angle such that the minimum repetition time min_TR is shorter than the repetition time TR input by the operator. Thus, the repetition time TR input by the operator can be used as it is as the repetition time of the pulse sequence. At this time, the MTC decreases. Therefore, an offset frequency suitable for the flip angle is selected in order to improve the MTC so as to compensate for the decrease in the MTC. As described above, a short repetition time can be achieved without deteriorating MTC. Note that the influence on the free water parasitic vibration is small because the influence of the flip angle and the influence of the offset frequency cancel each other.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the embodiments shown in the drawings. Note that the present invention is not limited by this.

First Embodiment FIG. 1 is a block diagram showing an MRI apparatus according to a first embodiment of the present invention. In the MRI apparatus 100, the magnet assembly 1 has a space (hole) for inserting a subject therein, and a static magnetic field for applying a constant static magnetic field to the subject so as to surround the space. Coil 1
p and a gradient magnetic field coil 1g for generating a gradient magnetic field
(The gradient magnetic field coil includes coils for each of the X, Y, and Z axes), a transmission coil 1t for applying an MT pulse or an excitation pulse to the subject, and an NMR signal from the subject. The receiving coil 1r is arranged. The static magnetic field coil 1p is connected to the static magnetic field power supply 2, and the gradient magnetic field coil 1
g is connected to the gradient magnetic field drive circuit 3, the transmission coil 1t is connected to the RF power amplifier 4, and the reception coil 1r is connected to the preamplifier 5.

The sequence storage circuit 6 is operated by a gradient magnetic field driving circuit 3 based on a pulse sequence such as a gradient echo method or a spin echo method in accordance with a command from a computer 7.
To generate a gradient magnetic field from the gradient magnetic field coil 1g of the magnet assembly 1, and operate the gate modulation circuit 8 to convert the high-frequency output signal from the RF oscillation circuit 9 into a pulse signal having a predetermined timing and a predetermined envelope. Modulate,
This is applied to the RF power amplifier 4 as an MT pulse or an excitation pulse, and after the power is amplified by the RF power amplifier 4, it is applied to the transmission coil 1t of the magnet assembly 1 and R
Transmit the F pulse.

The preamplifier 5 includes a magnet assembly 1
, And amplifies the NMR signal from the subject detected by the receiving coil 1 r and inputs the amplified signal to the phase detector 10. Phase detector 10
Uses the output of the RF oscillation circuit 9 as a reference signal, performs phase detection on the NMR signal from the preamplifier 5, and outputs the A / D converter 11
Give to. The A / D converter 11 converts the analog signal after the phase detection into a digital signal and inputs the digital signal to the computer 7.

The computer 7 performs an image reconstruction operation on the digital signal from the A / D converter 11 to generate an image (proton density image) of a target excitation region. This image is displayed on the display device 13. Computer 7
Is responsible for overall control such as receiving information input from the console 12. Further, the computer 7 optimizes the flip angle and offset frequency of the MT pulse based on the repetition time TR input by the operator (optimal MT pulse flip angle / offset frequency determination processing), and executes a pulse sequence for applying the MT pulse. It is designed and passed to the sequence storage circuit 6.

FIG. 2 is a flowchart showing an optimum MT pulse flip angle / offset frequency determination process according to an embodiment of the present invention. In step S1, the operator inputs a repetition time TR. In step S2, an MT pulse having a flip angle x = xo and an offset frequency f = fo that gives a minimum repetition time min_TR smaller than the minimum value of the repetition time TR that may be input by the operator is assumed (the waveform is fixed). And). In step S3, a minimum repetition time min_TR that satisfies hardware constraints such as electromagnetic wave absorption of the subject and heat generation of the transmission coil and the reception coil is calculated using the assumed MT pulse. In step S4, the repetition time TR input by the operator is compared with the minimum repetition time min_TR. If TR is larger than min_TR as shown in FIG. 3A, the process proceeds to step S5. x
Since the value of o has been reduced, first of all, step S5
Proceed to. In step S5, the flip angle x is increased by a predetermined angle θ as shown in FIG. Then, the process returns to step S3. The min_TR calculated in the returning step S3 becomes longer by the time corresponding to the predetermined angle θ. Then, steps S3 to S5 are repeated, and when min_TR exceeds TR as shown in FIG. 3C, the process proceeds to step S6. In step S6, the flip angle x is reduced by a predetermined angle θ as shown in FIG. The flip angle after the decrease is the maximum flip angle that gives the minimum repetition time min_TR shorter than the repetition time TR input by the operator.

In step S7, the optimum offset frequency corresponding to the flip angle x is calculated or calculated. Then, the process ends. Note that in order to find the optimum offset frequency corresponding to the flip angle x, it is necessary to create an arithmetic expression and a table in advance by experiments and simulations. For example, T1, T2 close to real
And the minimum offset frequencies at which the signal attenuation due to the MT pulses having different flip angles are less than or equal to a certain value are calculated based on the block equation in consideration of the above, and the relationship between the flip angles and the offset frequencies is tabulated. Keep it. Alternatively, an arithmetic expression is obtained from the table by, for example, least squares approximation.

According to the above-described MRI apparatus 100, MR imaging can be performed with a pulse sequence of a short repetition time TR input by the operator as shown in FIG. And MTC will not be spoiled.

In the above embodiment, the minimum repetition time min_TR shorter than the repetition time TR input by the operator.
Although the maximum flip angle that gives is obtained by loop calculation, it may be obtained by an arithmetic expression. In the above embodiment, the pulse sequence of the gradient echo method has been described as an example, but the present invention may be applied to a pulse sequence of the spin echo method.

[0018]

The MR imaging method and M according to the present invention.
According to the RI device, since the flip angle and the offset frequency of the MT pulse are optimized in accordance with the repetition time input by the operator, a short repetition time can be achieved without impairing the MTC.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an MRI apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart showing an optimal MT pulse flip angle / offset frequency determination process according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram of a process of optimizing a flip angle.

FIG. 4 is an explanatory diagram of an imaging sequence of the gradient echo method according to the present invention.

FIG. 5 is a flowchart showing a conventional TR check process.

FIG. 6 is an explanatory diagram of an imaging sequence of the gradient echo method when the repetition time TR input by the operator can be adopted as it is.

FIG. 7 is an explanatory diagram of an imaging sequence of the gradient echo method when the repetition time TR input by the operator is extended.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 MRI apparatus 1 Magnet assembly 2 Static magnetic field power supply 3 Gradient magnetic field drive circuit 4 RF power amplifier 5 Preamplifier 6 Sequence storage circuit 7 Computer 8 Gate modulation circuit 9 RF oscillation circuit 10 Phase detector 11 A / D converter 12 Console 13 Display apparatus

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-10-33498 (JP, A) JP-A-9-108196 (JP, A) JP-A-4-170937 (JP, A) Ayumu Okumura et al. Brain edema research by Magnetizion Transfer Controller (1st report) ", Report of Brain Edema Study Group (1996), Vol. 19th, p89-p97 (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. An MRI apparatus comprising an MT pulse applying means for applying an MT pulse for selectively RF-saturating an undesired nuclide, wherein the MRI apparatus includes an MT pulse applying means for applying an MT pulse in accordance with a repetition time TR inputted by an operator. M to optimize flip angle and offset frequency
MRI having T pulse optimizing means
apparatus. 2. The MRI apparatus according to claim 1, wherein
The MT pulse optimizing means obtains a maximum flip angle that can satisfy hardware constraints such as heat generation of the coil even at the repetition time input by the operator, and determines the maximum flip angle as a flip angle of the MT pulse; Next, an MRI apparatus characterized in that an appropriate offset frequency is obtained from the flip angle and the appropriate offset frequency is determined as an offset frequency of the MT pulse. 3. The MRI apparatus according to claim 2, wherein
The MRI apparatus, wherein the MT pulse optimizing means has an arithmetic expression or a table for finding an optimum offset frequency corresponding to the determined MT pulse flip angle.