JPH0245449B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of the Invention] The present invention relates to nuclear magnetic resonance (nuclear magnetic resonance).
resonance) (hereinafter abbreviated as "NMR")
The present invention relates to a nuclear magnetic resonance examination method and apparatus that utilizes development to determine the distribution of specific atomic nuclei within a subject from outside the subject, and in particular relates to improvements in NMR imaging equipment suitable for medical equipment. .

[Prior Art] First, an outline of the principle of NMR will be explained.

The atomic nucleus consists of protons and neutrons, which are considered to be rotating as a whole with nuclear spin angular momentum →.

Figure 1 shows the hydrogen nucleus ^1H .
As shown in A, it consists of one proton p and rotates as represented by the spin quantum number 1/2. Here, since the proton p has a positive charge e ⁺ as shown in (b), a magnetic moment μ→ is generated as the atomic nucleus rotates. In other words, each hydrogen nucleus can be thought of as a small magnet.

Figure 2 is an explanatory diagram schematically showing this point.
In a ferromagnetic material such as iron, the directions of these micromagnets are aligned as shown in A, and magnetization is observed as a whole. On the other hand, in the case of hydrogen, etc., the direction of the micromagnets (the direction of the magnetic moment) is random as shown in (b), and no magnetization is observed as a whole.

Here, such a substance is subjected to a static magnetic field H ₀ in the z direction.
When applied, each atomic nucleus aligns in the direction of H ₀ . That is, the energy level of the nucleus is quantized in the z direction.

Figure 3A shows this situation for a hydrogen nucleus. The spin quantum number of hydrogen nucleus is 1/2
Therefore, as shown in Figure 3B, −1/2 and +1/
It is divided into two energy levels of 2. The energy difference ΔE between two energy levels is expressed by equation (1).

ΔE=γ〓H ₀ …(1) where γ is the gyromagnetic ratio 〓=h/2π h is Planck's constant Here, each atomic nucleus has a force of μ→×H _{0 → due to the static magnetic field H 0 →} is added, so the atomic nucleus precesses around the z-axis at an angular velocity ω as shown in equation (2).

ω = γH ₀ (Larmor angular velocity) ...(2) When an electromagnetic wave (usually a radio wave) with a frequency corresponding to the angular velocity ω is applied to a system in this state, resonance occurs, and the atomic nucleus has an energy difference ΔE shown by equation (1). absorbs energy equivalent to , and transitions to a higher energy level. Even if several types of atomic nuclei with nuclear spin angular momentum coexist, each nucleus has a different gyromagnetic ratio γ, so the resonant frequencies differ, and therefore only the resonance of a specific atomic nucleus can be extracted. Furthermore, by measuring the strength of the resonance, it is possible to determine the amount of nuclei present. Also,
After resonance, the atomic nucleus excited to a higher level returns to a lower level after a time determined by a time constant called relaxation time.

This relaxation time is classified into spin-lattice relaxation time (longitudinal relaxation time) T ₁ and spin-spin relaxation time (transverse relaxation time) T _2. By observing this relaxation time, data on material distribution can be obtained. be able to. In general, in solids, spins are almost fixed at fixed positions on the crystal lattice, so interactions between spins are likely to occur. Therefore, the relaxation time
T ₂ is short, and the energy obtained by nuclear magnetic resonance is first transferred to the spin system and then to the lattice system. Therefore, time T ₁ is significantly larger than T ₂ .
On the other hand, in a liquid, molecules move freely, so the spins interact with each other, and the spins and the molecular system (lattice)
The likelihood of energy exchange with is about the same.
Therefore, times T ₁ and T ₂ have approximately equal values. In particular, the time _T1 is a time constant that depends on the way each compound binds, and in normal tissues and malignant tumors,
It is known that the values differ greatly. Here, we have explained hydrogen nuclei ^1H , but it is possible to perform similar measurements with other nuclei that have nuclear spin angular momentum.In addition to hydrogen nuclei, phosphorus nuclei ^31P , carbon nuclei ^13C , sodium nucleus
Applicable to ²³ Na, fluorine nucleus ¹⁹ F, oxygen nucleus ¹⁷ O, etc.

In this way, NMR can measure the abundance of specific atomic nuclei and their relaxation times, so by obtaining various chemical information about specific atomic nuclei within a substance, it is possible to carry out various tests within the subject. It can be carried out.

Conventionally, inspection equipment using NMR excites protons in a virtual cross-section of the subject using the same principle as X-ray CT, and generates NMR resonance signals corresponding to each protrusion of the subject. There is a method that calculates the NMR resonance signal intensity at each position of the object using a reconstruction method.

FIG. 4 is an operational waveform diagram for explaining an example of an inspection method in such a conventional device.

First, a z gradient magnetic field Gz ⁺ as shown in FIG. 4B and an RF pulse (90° pulse) with a narrow frequency spectrum as shown in FIG. 4A are applied to the subject. In this case, protons only on the plane with Larmor angular velocity ω = γ (H ₀ + ΔGz) are excited, and if the magnetization M is expressed on a rotating coordinate system rotating at an angular velocity ω as shown in Figure 5A, then the y′ axis The direction is changed by 90 degrees. Subsequently, as shown in FIG. 4C and D, an x gradient magnetic field Gx and a y gradient magnetic field Gy are applied, thereby creating a two-dimensional gradient magnetic field, and an NMR resonance signal as shown in E is detected. Here, as shown in Figure 5B, the magnetization M is x', -
As it gradually disperses in the direction of the arrow in the y' plane, the NMR resonance signal eventually decreases and disappears after the Ts time, as shown in Fig. 4(e). When the NMR resonance signal obtained in this manner is subjected to Fourier transformation, it becomes a projection in the direction perpendicular to the gradient magnetic field synthesized by the x gradient magnetic field Gx and the y gradient magnetic field Gy.

Thereafter, after waiting for a predetermined time Td, the next sequence is repeated in the same manner as described above. In each sequence, Gx and Gy are changed little by little. This allows each projection to
NMR resonance signals can be determined for numerous directions of the object.

By the way, in a conventional device that operates in this way, as shown in Figure 4, the time Ts until the NMR resonance signal disappears is 10 to 20 mS, but the predetermined time Td until moving to the next sequence is , approximately 1 sec is required due to the relaxation time T ₁ . Therefore, if a cross-section of a single object is to be reconstructed using, for example, 128 projections, the measurement requires a long time of at least 2 minutes, which is one of the major obstacles to achieving high speed. It's becoming one.

[Object of the Invention] In view of the above, the object of the present invention is to forcibly return the magnetization to a thermal equilibrium state after observing a large number of echo signals due to magnetic field reversal, and to obtain data at high speed by shortening the waiting time. can record
The purpose of the present invention is to provide an NMR testing method and testing device.

[Summary of the Invention] In order to achieve such an object, the present invention includes a step of applying a first 90° pulse to generate nuclear magnetic resonance in the nuclei of atoms constituting the tissue of a subject; a step of reversing the gradient magnetic field applied to the atomic nucleus multiple times after applying the 90° pulse and measuring the nuclear magnetic resonance signal generated at that time; and a second step after the step of measuring the nuclear magnetic resonance signal.
While repeating the sequence including the process of applying a 90° pulse, and the waiting time of waiting for Td time to elapse after applying this second 90° pulse, proceeding to the next process, the nuclear magnetic resonance The method is characterized by comprising the step of reconstructing an image related to the tissue of the subject based on the signal.

[Example] The present invention will be explained in detail below using the drawings. FIG. 7 is a block diagram showing the configuration of an embodiment of a device for implementing the method of the present invention. In the figure, 1 is a static magnetic field coil for generating a uniform static magnetic field H ₀ (the direction in this case is the z direction);
Reference numeral 2 denotes a control circuit for the static magnetic field coil 1, which includes, for example, a DC stabilized power source. Static magnetic field coil 1
It is desirable that the density H ₀ of the magnetic flux generated by this is about 0.1 T, and that the uniformity is 10 ⁻⁴ or more.

3 shows a general overview of gradient magnetic field coils;
4 is a control circuit for this gradient magnetic field coil 3.

FIG. 6A is a configuration diagram showing an example of the gradient magnetic field coil 3, in which the z gradient magnetic field coil 31, the y gradient magnetic field coils 32, 33, although not shown, are the same as the y gradient magnetic field coils 32, 33. It includes an x-gradient field coil which is rotated by 90°.

This gradient magnetic field coil generates a magnetic field having linear gradients in the x-, y-, and z-axis directions in the same direction as the uniform static magnetic field H ₀ . The control circuit 4 is controlled by a controller 20 (details will be described later).

Reference numeral 5 denotes an excitation coil that provides an RF pulse with a narrow frequency spectrum as an electromagnetic wave to the subject, and its configuration is shown in FIG. 8B.

6 is the frequency corresponding to the NMR resonance condition of the atomic nucleus to be measured (for example, 42.6M for protons)
Hz/T), the output of which is applied to the excitation coil 5 via a gate circuit 30 (details will be described later) whose opening and closing are controlled by signals from the controller 20. has been done. Reference numeral 8 denotes a detection coil for detecting the NMR resonance signal in the subject, and its configuration is the same as the excitation coil shown in FIG. Although it is desirable that this detection coil be installed as close to the subject as possible, it may also be used as an excitation coil if necessary.

9 is an amplifier that amplifies the NMR resonance signal obtained from the detection coil 8; 10 is a phase detection circuit; and 11 is a signal from a wave memory circuit 11 that stores the phase-detected waveform signal from the amplifier 9, which is composed of, for example, an optical fiber. A computer inputs the tomographic image through a transmission line 12 and performs predetermined signal processing to obtain a tomographic image, and 14 is a display device such as a television monitor that displays the obtained tomographic image.

The controller 20 can output signals (analog signals) necessary for controlling the gradient magnetic fields Gz, Gx, Gy and modulation signals, and control signals (digital signals) necessary for transmitting RF pulses and receiving NMR signals. It is configured so that it can be done.

FIG. 8 is a configuration diagram showing an example of such a controller, which is particularly capable of high-speed control and allows easy changes in control sequences and analog waveforms. In the figure, 221 is a write control circuit that writes data sent from the console 210 or the computer 13 via the console to each memory;
222, 225, 228, 231 are waveform storage memories in which waveform data of the x, y, z gradient signal and modulation signal given from this write control circuit are respectively written; 223, 226, 229, 232 are waveform storage memories 222; ,225,228,23
Latch circuits 224, 227, 230, and 233 temporarily hold the waveform data output from 1, respectively.
This is a DA conversion circuit that converts the outputs from the DA converter into DA converters. x ₂ , y ₂ , z ₂ , M ₂ are the DA conversion circuit 22
4, 227, 230, and 233 are x, y, and z gradient signal outputs and modulation signal outputs, respectively. 234, 236, 238, and 240 are transmitting/receiving circuit control signals, that is, an AD conversion control signal, a transmitting gate control signal, a receiving gate control signal, and a phase selection signal (any one of four types of RF pulses having mutually different phases). waveform storage memories 235 and 237 into which data of RF pulses, which are signals for selecting pulses (however, only two types of RF pulses are selected and used here), are respectively written from the write control circuit 221; , 239, 241 are the waveform storage memories 234, 236, 238, 2
A latch circuit that temporarily holds the data output from 40, T ₂ , S ₂ , R ₂ , PS is this latch circuit 235,
These are an AD conversion control signal, a transmission gate control signal output, a reception gate control signal output, and a phase selection signal output from 237, 239, and 241. 243 is each waveform storage memory 222, 225, 228,
a read control circuit 24 for reading out the contents of 231, 234, 236, 238, 240 to the latch circuit;
2 sets the value of the write/read start address from the console or computer (hereinafter simply referred to as "from the computer"), and sequentially adds the value +1 given from the write/read control circuit to the value of that address; 244 is an output count register in which the number of output steps given by the computer is set and notifies the read control circuit 243 of the end of output; 245 is a memory address register which outputs 1 given from the computer as a write/read address;
This is a 1-step length pulse generation circuit in which the time length of a step (1 step length) is set and a 1-step length pulse is generated.

The operation of the controller having such a configuration is as follows.

(b) Write operation In the write operation, the waveform data sent from the computer is written to the specified address of the waveform storage memory specified by the computer. That is, first, a write start address is set in the memory address register 242. The data sent from the computer along with the write command is sent to the write control circuit 221.
The signal is written to the address specified by the memory address register 242 in the waveform storage memory (for example, the waveform storage memory 222) selected by the . After that, the write control circuit 221 automatically adds 1 to the memory address register 242 to make it the memory address for the next write. Data is sequentially written to other waveform storage memories by the same operation as described above.

(b) Read operation In read operation, each memory is read in parallel. 9th
The figure shows an example of a time chart of read signal waveforms. The computer first sets the reading start address of the waveform storage memory in the memory address register 242.
Set to . Next, the number of read steps is set in the output count register 244. In addition, 1 step length (time per 1 step during readout) is set to 1.
The step length pulse generation circuit 245 is set.
Next, the waveform storage memories 222, 225, 228, 231, 2 at the address indicated by the memory address register 242 are given a reading start command from the computer.
34, 236, 238, and 240 at the same time, and when the data is complete, the read control circuit 243 sends the latch circuits 223, 226, 22
A latch pulse is output to 9, 232, 235, 237, 239, and 241 to latch the data. Next, 1 is added to the value of the memory address register 242. If the output count register 244 indicates completion, the latch circuit 2 is output from the read control circuit 243.
23, 226, 229, 232, 235, 23
A clear pulse is output to 7, 239, and 241 to end the read operation. Output count register 244
is not completed, output count register 2
44 and waits for one step time based on the output from the one step length pulse generation circuit 245, and then moves to the next read step. Thereafter, the same process can be repeated to read out a waveform as shown in FIG. 9, for example. x, y, z gradient signals x ₂ , y ₂ , z ₂
and modulation signal M ₂ is outputted from the latch circuit by further converting it into DA converters 224, 227, 230, 233.
This is an analog signal obtained by DA conversion, and is a modulated signal.
M ₂ is guided to a gate circuit 30, and the x, y, z gradient signals are respectively guided to a control circuit 4 for the gradient magnetic field.

Since such a controller is equipped with dedicated hardware such as a waveform storage memory, it is possible to read and output a large amount of data at high speed. Further, since the contents of the waveform storage memory can be rewritten as necessary, any analog/digital signal waveform can be output. Furthermore, it is easy to use part of the signal waveform (which is often actually used) by appropriately providing the read start address and the number of read steps.

The gate circuit 30 receives the signal from the oscillator 6, generates four types of signals with different phases by 90 degrees, selects one of the four types of signals based on instructions from the controller 20, and selects one of the four types of signals based on instructions from the controller 20. is further modulated with an RF modulation signal to obtain a drive signal for the excitation coil 5, and its detailed configuration is shown in FIG. 10 (only two of these types are used in the present invention). In the figure, 311 is a 90° phase shifter that can obtain two signals with a phase shift of 0° and 90° relative to the input RF signal, and 312 and 323 are 90° phase shifters that can obtain two signals with a phase shift of 0° and 90° relative to the input signal. Two 180° signals can be obtained at the same time.
It is a 180° phase shifter. 90° phaser 31 as shown
By applying each output of 1 to each of the 180° phase shifters, signals having phase differences of 0°, 180°, and 270° with respect to the RF signal are obtained. These signals are each connected to a high frequency switch (for example, a double balanced mixer: DBM can be used) 314 to
317 to a combiner 321, where the four signals are summed. In this case, the high frequency switches are individually driven by the output of the decoder driver 320, and the high frequency switches are driven individually by the output of the decoder driver 320.
The four outputs (X,
Y, -X, -Y), one of them becomes active. As a result, only the corresponding switch becomes conductive (the other three switches are non-conductive).
Therefore, only one signal is input to the combiner 321.

The output of the coupler 321 is input to the modulator 323 after passing through the amplifier 322, where the RF modulation signal (a pulse signal whose pulse width and peak value determine the rotation of the magnetization M) is provided by the controller 20. For example, the signal is modulated into a Gaussian waveform shown in FIG. 4A and output.

According to the gate circuit with such a configuration, one
Based on the RF signal, obtain RF signals exhibiting phase differences of 0°, 90°, 180°, and 270°, selectively select a desired one from these signals, and further output the desired signal at an appropriate timing. It has the advantage that it is extremely easy to modulate the signal with a waveform of .

The operation of the apparatus of the present invention constructed in this way will be explained step by step with reference to FIG.

1 Selectively excite the subject with the first 90° pulse and Gz (Fig. 11 A and B). This generates an FID signal.

Magnetic fields Gx and Gy of magnitude g _x1 g _y1 are applied for 2 t _n1 hours (Fig. 11 C and D).

3 After that, the sign of the magnetic field is reversed so that the following equation is satisfied for t' _n1 time. This generates an echo signal.

g _x1 ×g' _x1 <0 g _y1 ×g' _y1 <0 4 At the end of the time t' _n1 , the echo signal reaches its maximum. However, g _x1 ×t _n1 =−g′ _x1 ×t′ _n1 g _y1 ×t _n1 = −g′ _y1 ×t′ _n1 5. Repeat steps 2 to 4 above n−1 times (n>1).
However, g _xp ×t _np = −g′ _xp ×t′ _np g _yp ×t _np = −g′ _yp ×t′ _np where p = 1, 2, ..., n 6 Where the echo signal is maximum, 2nd 90° _-x
Add a pulse and Gz to turn the magnetization upward.

7. Next, homogeneity spoil pulses a, b, and c are applied to Gx, Gy, and Gz as shown in B, C, and D of FIG. 11. This makes it possible to completely return the magnetization vector to the direction of the static magnetic field and eliminate the correlation with the next sequence.

8 Wait for Td time and move on to the next sequence.
In the next sequence, steps 1 to 7 above are repeated.

In this way, a large number of NMR signals are obtained as echo signals, and at the same time, the following sequence is repeated with a short waiting time Td, making it possible to obtain projections in many directions in a short time. Additionally, averaging processing can be performed to improve image quality. The NMR signal thus obtained is sent to the computer 13 via the wave memory 11, where a reconstructed image is obtained by image reconstruction calculation. The obtained image is displayed on the display 14.

Note that the present invention is not limited to the above-mentioned embodiments, but can be applied to various types of systems such as those described below.

B In the embodiment shown in Fig. 11, n=2, g _x1 >>g' _x1 , g _y1 >>g' _y1 , g _x2 <<g' _x2 , g _y2
<<g' _y2 g' _x1 = g _x2 and g' _y1 = g _y2 as shown in Fig. 12.

Using the echo signals occurring during periods t′ _n1 and t _n2 ,
The signals generated during the t _n1 and t′ _n2 periods are RF pulses and
It is not used as measurement data because it is affected by switching noise, etc. from Gx, Gy, and Gz.

According to the above conditions, t _n1 <<t′ _n1 and t _n2 >>
t′ _n2 , the signal acquisition time is long and the S/N is
It is possible to record a good signal.

It can be applied to the loss spin warp method (Figure 13). In this case, t _kp (p=1, 2,...
n) is constant, and g _xp (p=1, 2,...n) is changed.

It can be applied to the Fourier method (Figure 13). In this case, t _knp (p=1, 2,...n)
By changing g _xp (p=1, 2,...n)
is constant.

2) It can be applied to the echo planar method (Fig. 14).

It can be applied to the selective excitation line method (Fig. 15).

It can be applied to the inversion recovery method (Figure 16).

G. A system that uses inter-image calculations to obtain T ₁ images, T ₂ images, spin density images, and images that are a combination of these.

Chi Using the waiting time of Td to excite other planes,
It can be applied to a multi-slice method that obtains information from this.

[Effects of the Invention] As explained above, according to the present invention, since the magnetization is forcibly directed upward after a series of scans,
It is possible to move on to the next scan with a short waiting time Td. In addition, by reversing the magnetic field, a large number of echo signals can be obtained in a short period of time. Therefore, the overall scan time can be shortened.

Furthermore, since a large amount of data can be obtained in a short time,
Averaging of signals or averaging of multiple images can be performed, resulting in high quality images.

Furthermore, the homogeneity spoil pulse breaks the correlation between the previous and subsequent sequences, allowing the magnetization to move correctly.

In the case of the method shown in Fig. 12, echo signals are observed, so RF pulses, Gx, Gy, Gz
The negative effects of this will be extremely small. At the same time, useless
T _s1 and T′ _s2 are short and the signal is large.

Further, the inter-image calculation method as described in the above G has the advantage that an image suitable for the purpose can be easily obtained.

Further, if the multi-slice method is used, it is possible to realize even higher speed in appearance.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the spin of a hydrogen atom, Figure 2 is a diagram explaining the spin of a hydrogen atom.
The figure is a schematic diagram of the magnetic moment of a hydrogen atom,
Figure 3 is a diagram explaining the state in which the nuclei of hydrogen atoms are aligned in the direction of the magnetic field, Figure 4 is a diagram showing an example of a conventional NMR test pulse waveform, and Figure 5 is a diagram showing magnetization M in rotational coordinates. , FIG. 6 and FIG. 7 are configuration diagrams of an embodiment of the present invention, FIG. 8 is a configuration diagram showing an example of a controller, FIG. 9 is an operation waveform diagram for explaining the operation according to the present invention, and FIG. FIG. 10 is a diagram showing an example of a gate circuit, FIG. 11 is an operational waveform diagram for explaining the sequence according to the present invention, and FIGS. 12 to 16 are operational waveform diagrams showing other embodiments according to the present invention. It is. 1... Static magnetic field coil, 2, 4... Magnetic field control circuit, 3... Gradient magnetic field coil, 5... Excitation coil, 6... Oscillator, 8... Detection coil, 10...
Phase detection circuit, 11...Wave memory circuit, 1
3...Computer, 20...Controller, 3
0...Gate circuit.

Claims (1)

[Scope of Claims] 1. A step of applying a first 90° pulse for causing nuclear magnetic resonance to the nucleus of an atom constituting the tissue of a subject, and applying a first 90° pulse to the atomic nucleus after applying the 90° pulse. a step of reversing a gradient magnetic field multiple times and measuring a nuclear magnetic resonance signal generated at the time; a step of applying a second 90° pulse after the step of measuring the nuclear magnetic resonance signal;
Waiting for Td time to elapse after applying the 90° pulse, and then proceeding to the next step.The sequence is repeated, and an image related to the tissue of the subject is regenerated based on the nuclear magnetic resonance signal. 1. An inspection method using nuclear magnetic resonance, comprising the steps of configuring. 2. In the step of reversing the gradient magnetic field applied to the atomic nucleus multiple times after applying the 90° pulse and measuring the nuclear magnetic resonance signal generated at that time, one of the reversing magnetic fields is made stronger than the other, 2. The nuclear magnetic resonance testing method according to claim 1, wherein only the echo signals of the nuclear magnetic resonance signals generated thereby are observed. 3. In the step of reversing the gradient magnetic field applied to the atomic nucleus at least once after the application of the 90° pulse and measuring the nuclear magnetic resonance signal generated at that time, the gradient magnetic field to be applied is appropriately determined, and two-dimensional PR is performed. By nuclear magnetic resonance according to claim 1, the image can be reconstructed by any one of the method, the spin warp method, the echo planar method, or the selective excitation line method. Inspection method. 4. Claim 1, characterized in that in the step of applying the first 90° pulse, a 180° pulse for inversion recovery is applied prior to the first 90° pulse. Inspection method using nuclear magnetic resonance as described in section. 5. The method according to claim 1, wherein in the step of applying the second 90° pulse, a homogeneity spoil pulse is applied using a gradient magnetic field after the second 90° pulse. Inspection method using nuclear magnetic resonance. 6. In the step of reconstructing an image related to the tissue of the subject based on the nuclear magnetic resonance signal, a T ₁ image or a T ₂ image or a spin density is determined by inter-image calculation from a plurality of images obtained by changing time parameters. 2. An examination method using nuclear magnetic resonance according to claim 1, characterized in that an image or a combination image thereof is obtained. 7 Selectively excite another surface during the waiting time,
Claim 1, characterized in that multi-slicing is performed by obtaining surface information.
Inspection method using nuclear magnetic resonance as described in Section 1. 8. An inspection device using nuclear magnetic resonance signals, comprising means for applying a high-frequency pulse to cause nuclear magnetic resonance to the nuclei of atoms constituting the tissue of a subject, and means for measuring the nuclear magnetic resonance signals generated in the nuclei. In the method, the means for applying the high-frequency pulse is capable of applying a first 90° pulse and reversing the gradient magnetic field applied to the nucleus multiple times after application of the 90° pulse, and applying the high-frequency pulse. The means for applying the second 90° pulse is configured to wait a predetermined period of time before proceeding to the next sequence, and the means for measuring the nuclear magnetic resonance signal generated in the nucleus is configured to invert the pulse. 1. An inspection device using nuclear magnetic resonance, characterized in that it is configured to measure nuclear magnetic resonance signals generated during a period when a magnetic field is applied.