JPH0439857B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention utilizes the nuclear magnetic resonance (hereinafter abbreviated as NMR) phenomenon to detect the distribution of specific atomic nuclei within a specimen from outside the specimen. Regarding a nuclear magnetic resonance imaging device that reconstructs and outputs a cross-sectional image of a desired examination site of a subject in a destructive manner,
In particular, it relates to automatic or manual adjustment of high frequency pulses (also referred to as RF (radio frequency) pulses).

(Prior Art) This type of MNR imaging device has been well known. In order for this device to obtain images with the theoretical signal strength without artifacts, the RF pulses used to apply the high-frequency magnetic field and excite the magnetization must be accurate.

Normally, the characteristics of the transmission system change due to changes in the resonance sharpness Q of the coil that generates the RF pulse, so-called RF coil, due to differences in the object or region to be imaged. For this reason, it is necessary to readjust the RF pulse, but the conventional adjustment method is to set the RF pulse with the maximum received signal strength to a 90° pulse, which has the following problems.

(1) The signal strength S when shifted from 90° to α° is S=So cosα, with So as a constant, so α=0
The differential coefficient is zero when , the sensitivity to the pulse error (α) when determining the maximum point of the received signal strength is very poor, and the adjustment accuracy is poor.

(2) In the case of a selective pulse, the angle at which the magnetization falls varies depending on the location due to the selection of the slice plane, and is maximum at the center of the slice plane.

Accurate 90° pulses ensure that the magnetization at the center of the slice plane
Although the pulse is tilted at 90°, the maximum point of the received signal strength is when the magnetization at the center of the slice exceeds the condition where the magnetization is tilted by 90° (accurate 90° pulse), that is, when the condition is tilted by 90° or more.

On the other hand, in order to solve the above problem, the RF pulse with the minimum received signal strength is set as a 180° pulse.
The product of RF pulse length and RF pulse height is 1/ of a 180° pulse.
One possible method is to use a 90° pulse as the second pulse, but if the object to be imaged is larger than the imaging area, such as a human body, there is a problem that this method cannot be simply applied because signals from outside the imaging area will be mixed in. .

In view of these points, an object of the present invention is to provide an NMR imaging device that can adjust RF pulses such as 90° pulses and 180° pulses with high precision.

(Means for solving the problem) In order to achieve such an object, in the present invention, a 180° pulse is applied together with a gradient magnetic field, and a gradient magnetic field in the same direction as the gradient magnetic field is applied to receive an NMR signal. The RF pulse length or RF
The present invention is characterized in that it is configured to allow highly accurate RF pulse adjustment by adjusting the pulse height or both at the same time.

(Example) The present invention will be explained in detail below using the drawings. FIG. 1 is a diagram showing the configuration of essential parts of an embodiment of an NMR imaging device according to the present invention. In the figure, reference numeral 1 denotes a magnet assembly, which has a space (hole) inside for inserting an object. a magnetic field coil 2, and a gradient magnetic field coil 3 for generating a gradient magnetic field (consisting of an x gradient magnetic field coil, a y gradient magnetic field coil, and a Z gradient magnetic field coil configured to be able to individually generate gradient magnetic fields) ), an RF transmitter coil 4 that provides high-frequency pulses to excite the spins of atomic nuclei within the object, and a receiver coil 5 that detects NMR signals from the object.

The main magnetic field coil is connected to the static magnetic field control circuit 15, Gx,
Gy, Gz each gradient magnetic field coil is gradient magnetic field control circuit 1
4, the RF transmitting coil is connected to the power amplifier 18, and the NMR signal receiving coil is connected to the preamplifier 19.
are connected to each.

13 is a controller that controls the sequence of generation of gradient magnetic fields and high-frequency magnetic fields.
Performs necessary control to capture the NMR signal into the waveform memory 21.

17 is a gate modulation circuit, and 16 is a high frequency oscillator that generates a high frequency signal. Gate modulation circuit 17
appropriately modulates the high frequency signal output from the high frequency oscillator 16 using a control signal from the controller 13 to generate a high frequency pulse with a predetermined phase. This high frequency pulse is applied to the RF transmitting coil 4 through the RF power amplifier 18.

19 is a preamplifier that amplifies the NMR signal obtained from the detection coil 5; 20 is a phase detection circuit that phase-detects the NMR signal by referring to the output signal of the high-frequency oscillator; and 21 stores the phase-detected waveform signal from the preamplifier. In the waveform memory, here is A/
Contains a D converter.

11 is a computer which receives the signal from the waveform memory 21 and performs predetermined signal processing to convert it into an NMR signal.
A function to obtain the center frequency component of the RF pulse, a function to set the RF pulse height in the high frequency oscillator 16, and the above-mentioned
A function of adjusting the RF pulse height so that the center frequency component of the RF pulse of the NMR signal is minimized;
It also has the ability to reconstruct tomographic images from NMR signals. Incidentally, an input means (not shown) for inputting various information necessary for this apparatus is connected to the computer 11, so that the operator can input appropriate information.

12 is a display layer like a television monitor that displays the obtained tomographic image.

In such a configuration, an RF pulse and a magnetic field are applied to an object placed in the magnet assembly 1 to generate an NMR signal, which is phase-detected, and this signal is processed by the compisutator 11 to detect the object. The general operation of the imaging device of reconstructing an image related to tissue and displaying it on the display is the same as that of a conventional NMR imaging device, and furthermore, this operation has little direct relation to the present invention. A detailed explanation of its operation will be omitted here.

The following is a second explanation of the characteristic operation of the present invention.
This will be explained in detail with reference to the figures.

(1) With the static magnetic field control circuit 15 driving the static magnetic field coil 2 to apply a static magnetic field to the subject, the controller 13 controls the gradient magnetic field control circuit 14 and the gate circuit 17, as shown in FIG. With the gradient magnetic field b for the period t ₁ to _{t 2} as
A 180° pulse a is applied to selectively excite the subject.

At this time, the relationship between the angle at which the magnetization falls and the position is as shown in FIG. 3C. In other words, when the RF pulse height is too large (curve Vo ₂ ), the magnetization at the center of the slice plane tilts by more than 180°, and conversely, when the RF pulse height is too small (curve Vo ₃ ), the magnetization at the center of the slice plane tilts more than 180°. It can only fall at a small angle. Appropriate RF pulse height (curve Vo ₁ :
(which is the case for an exact 180° pulse), then
The magnetization at the center of the slice plane is tilted exactly 180°.

(2) Subsequently (after time t ₂ ), a gradient magnetic field C of opposite polarity is applied in the same direction as gradient magnetic field b and generated.
The NMR signal d is received via a preamplifier 19 and a phase detection circuit 20, and the data is stored in a waveform memory 21.

The signal strength S when the angle of magnetization is α° is:
Since S=So sin α, where So is a constant, the relationship between the signal intensity and frequency of the NMR signal obtained via the phase detection circuit 20 is as shown in E of FIG.

Therefore, the center frequency (fo) component of the RF pulse of the NMR signal becomes the minimum (curve ``e'' in the same figure).
V ₁₁ ) RF pulse height becomes accurate 180° pulse.

(3) Based on the above theory, adjust the RF pulse height so that the center frequency component of the RF pulse of the NMR signal is minimized.

(4) For pulses with angles (α°) other than 180°, the product of RF pulse height and RF pulse length is α/180 of 180° pulse.
The pulse height and pulse length are set so that the pulse height and pulse length are doubled.

In this way, the RF pulse can be precisely adjusted.

In addition, in the above embodiment, FID using 180° pulse
Although the signal was used, it is also possible to adjust the RF pulse in the same way using an echo signal as shown in FIG.
In FIG. 4, the RF pulse a is a 180° pulse to be adjusted, b is a 180° pulse for echo, and c and d are gradient magnetic fields in the same direction and with the same polarity.

Further, in the embodiment, the computer 11 calculates the center frequency component of the RF pulse of the NMR signal, but it is also possible to use a dedicated circuit to calculate the center frequency component of the RF pulse of the NMR signal.

Further, the adjustment of the RF pulse ((3) above) is not limited to adjusting the pulse height, but may also adjust the pulse length or both the pulse height and the pulse length.

(Effects of the Invention) As explained above, the present invention has the following effects.

(1) NMR when the 180° pulse deviates from 180° by α°
Since the center frequency component S of the RF pulse of the signal is S=So sinα, where So is a constant, (ds/dα) ₌₀ =So, and the sensitivity to pulse errors is good.

(2) When the 180° pulse is accurate, the center frequency component of the RF pulse of the NMR signal becomes zero, allowing accurate adjustment.

(3) All pulses can be adjusted regardless of whether they are selected or not, and regardless of the pulse angle.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the main part of an embodiment of the NMR imaging device according to the present invention, Figs. 2 and 4 are time charts, and Fig. 3 is for explaining the relationship between the angle of magnetization and signal intensity. This is a diagram. DESCRIPTION OF SYMBOLS 1... Magnet assembly, 2... Main magnetic field coil, 3... Gradient magnetic field coil, 4... RF transmitting coil, 5... Receiving coil, 11... Computer, 12... Display, 13... Controller ,1
4... Gradient magnetic field control circuit, 15... Static magnetic field control circuit, 16... High frequency oscillator, 17... Gate circuit, 18... Power amplifier, 19... Preamplifier,
20... Phase detector, 21... Waveform memory.

Claims (1)

[Scope of Claims] 1. Means for applying a magnetic field to an object, means for applying a high frequency pulse to an object, means for receiving a nuclear magnetic resonance signal, and a means for applying a magnetic field to a target object, a means for receiving a nuclear magnetic resonance signal, and a means for applying a magnetic field to a target object, comprising means for obtaining an image related to the image, and a display device for displaying the obtained image,
In a nuclear magnetic resonance imaging apparatus capable of applying a high frequency pulse and a magnetic field to a target object to generate a nuclear magnetic resonance signal, using the signal to obtain an image regarding the tissue of the target object and displaying the image on a display device, the above-mentioned The means for obtaining an image regarding the tissue of the target object includes means for obtaining a value of a predetermined frequency component of the NMR signal, and the means for applying the high frequency pulse includes a means for obtaining an image of a predetermined frequency component of the NMR signal. , comprising means capable of controlling the radio frequency pulse length and the radio frequency pulse height, applying a 180° radio frequency pulse together with a gradient magnetic field,
Subsequently, a gradient magnetic field in the same direction as the gradient magnetic field is applied to receive an NMR signal, and the high-frequency pulse length is adjusted so that the value obtained by the means for determining the value of a predetermined frequency component of the NMR signal is the minimum. A nuclear magnetic resonance imaging apparatus characterized in that the high-frequency pulse can be adjusted to be accurate by adjusting one or both of the high-frequency pulse heights.