JPS6349148A - Method for measuring number of atomic nuclei - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention "Industrial Application Field" This invention relates to a method for obtaining nuclear magnetic resonance information from a subject, for example, in a diagnostic device for the human body, and in particular for measuring the number of excited atomic nuclei. It concerns the method of doing so.

"Prior Art" Conventionally, a technique for obtaining the spatial distribution of nuclear magnetic resonance information of a specific atomic nucleus from a subject has been established. The spatial distribution represents the number of specific atomic nuclei per unit volume, that is, the relative comparison of concentrations. Furthermore, although a lot of chemical shift information can be obtained by spectroscopy technology, the spectral IP turn only shows a relative comparison of the concentrations of nuclei with different chemical shifts.

In this way, in order to measure the absolute number of nuclei rather than the relative number of nuclei, a reference material, a so-called phantom, is placed on the detection area of the specimen at the same time, and the nuclear magnetic resonance signal from the reference material is measured under the same conditions as the detection area. A method has been proposed in which the concentration in the specimen, that is, the number of nuclei, is determined by detecting the nuclear magnetic resonance signal and comparing this with the nuclear magnetic resonance signal intensity from the detected part. This conventional method has a complicated calibration method and is not expected to have sufficient accuracy.
Particularly in measurements using surface coils, it has not been widely used because the magnetic field cannot be set to the same conditions for the detected part and the reference material.

An object of the present invention is to provide a method for measuring the number of atomic nuclei from nuclear magnetic resonance signals.

"Means for Solving the Problem" According to the present invention, a magnetic field is applied to an object and a high frequency pulse is applied to excite the nuclear spins of the object to be detected, and the nuclear magnetic resonance signal generated by the excitation is transmitted over a period of time. TsO
The nuclear magnetic resonance signal intensity S of the selected portion of the subject is calculated from the received signal. Its signal strength S
Using equation (1), the number n of the target nuclei in the selected portion is obtained.

K is a constant 6 when the power W of the high-frequency pulse is an envelope power, and is a constant 6 x K when it is an average power, and Δtl
dThe pulse width of the above-mentioned high-frequency Noculus, k is the Boltzmann constant, T is the absolute temperature of the sample, ■ is the number of nuclear spin quantum numbers to be detected, W is the value obtained by dividing the blank constant by 2π, ω0 is the nuclear magnetic resonance signal angular frequency, (2) is the volume of the part selected above, and N is the noise.

First, the reason why formula (1) is obtained will be explained below.

It is known that when a main magnetic field is applied to an object, the relationship between the detected atomic nucleus deviation/quantum number and magnetization M is given by equation (2) (for example, A, AB Hiro GAM, 'THE
.

PRINCIPLES OF NTJCLEARMAG
NETISM, 0XFORD UNIV.

RRESS, P. 2 (1961)).

M = n r2Fr2I(I+1)B /3kT
(2) n: Number of nuclei to be detected per unit volume γ: Value obtained by dividing the gyromagnetic bhikuni blank constant by 2π B: Main magnetic field strength k: Boltzmann constant T: Absolute temperature of the object.

On the other hand, the voltage S induced across the coil receiving the nuclear magnetic resonance signal is S=ω. MBlv (3
). Here, ω0 is the nuclear magnetic resonance angular frequency, B.

is the magnetic flux density generated at the position of the subject when the current of the coil IA is applied, and ■ is the volume of the selected part of the subject.

In order to generate a nuclear magnetic resonance signal of a detected atomic nucleus from a subject, a main magnetic field is applied to the subject and a radio frequency pulse is applied, and the main magnetic field strength B0 at that time and the carrier wave angular frequency ω of the radio frequency pulse are set. And that high frequency, Zorus no? By selecting the pulse width Δt, the direction of the detected nuclear spin is turned 90 degrees from the steady state to cause free precession of the nuclear spin, that is, the nuclear spin is excited, and the free precession causes The generated free induction decay signal, or nuclear magnetic resonance signal, is received by the coil. The high frequency that excites this nuclear spin! Luz is 90°RF A? It's called Luz.

The magnetic flux density B, which is generated when this 90°RF/#rus is passed through the coil (if the receiving coil is the same). 0 is (
4) Equation becomes.

B. o(t)=B1×I(t)×this magnetic flux density B. o(t) is the magnetic flux density of a 90° RF pulse in a rotating coordinate system rotating at the same frequency as the nuclear spin, and lXT is the value of the current flowing through the coil at time t. % on the right side of equation (4) is the current flowing through the coil 1. This means that of the envelope of the 0° RF pulse, the rotational component that is the same as the nuclear spin accounts for % of the total.

Also, this magnetic flux density B. 0 has the following relationship.

However, Δt is 90' RF/J? Luz's A? It is the width of the loop.

Equation (6) is obtained from equations (4) and (5).

On the other hand, when the transmission power W of the 90° RF pulse is expressed by envelope power, it becomes equation (7).

Here, R is the resistance value of the coil.

In addition, the magnitude of noise N induced in the coil is N=7
It is known that 5T(8). Here, 73 is the length of the reception period of the nuclear magnetic resonance signal.

Therefore, formula (9) is obtained from formula (3) and formulas (6) to (8), and formula 01 is defined.

Once the corrugation (t) of 90°RF A is determined, this α
is determined.

From equations (2), (9), and 60, the number n of nuclei per unit volume is expressed as ql). S, W, and N on the right side of the αη formula are measured, and the others are determined from the measurement conditions. Therefore, by calculating the α formula, the number n of nuclei can be quantitatively determined. Note that when using the average power as the power W of the 90° RF pulse, the right side of equation (11) may be multiplied by 4.

If the received signal strength S is small, it is sufficient to measure it repeatedly and use the average value of the signal strength. In this case, if the measurement repetition period is tr, and the longitudinal relaxation time of the nuclear stripe is T0n, then the number of repetitions is N, then this addition results in a signal of Nr.
However, the noise is multiplied by t:. Also, in order to make a correction based on starting the measurement again before the nuclear spin completely returns to the steady state, multiply equation (6) by equation <11). .

Here, the transverse relaxation time T2 of the nuclear spin was set to be sufficiently shorter than the excitation.

Chopsticks 11 in nuclear magnetic resonance diagnostic equipment (NMR-CT)
For example, a main magnetic field in biaxial directions is applied to the subject, a magnetic field in the Z-axis direction whose magnetic field strength gradually changes along z4Nfi, a so-called gradient magnetic field G2, is applied, and in this state, 90°p, F/
Apply J rus to the subject and take a cross section perpendicular to this on the Z axis (
After exciting the spin of a specific atomic nucleus, such as a proton, in the specimen at the slice plane), and then applying a gradient magnetic field Gy in the Z-axis direction whose intensity gradually changes along y + (x), Applying a gradient magnetic field Gx in the Z-axis direction whose intensity changes along the axis, the nuclear magnetic resonance signal generated from the subject is sampled at a constant period for a constant time T3, and the above sequence is repeated by changing the gradient magnetic field Gy. The detection time series of the nuclear magnetic resonance signals thus obtained is subjected to two-dimensional Fourier transform to determine the relative distribution of the density of the specific atomic nuclei within the cross section of the object.

In order to obtain the distribution of the number of atomic nuclei instead of the relative distribution of the nuclear density using such a method, the signal intensity of each pixel (voxel) after Fourier transform is used as the signal intensity S in the equation (1υ), and the volume V is the volume of the pixel (voxel).If the 90° RF /F pulse used for excitation of nuclear spins is of the Ussian t2 waveform (envelope), then I(tl = e area α
is determined from the waveform of the 90° RF pulse, and when a rectangular wave is used as the 90° RF pulse, α=1.

In addition, to calculate the number of nuclei as the two-dimensional image information mentioned above,
For example, in the case of protons, the sampling period T3 is about 16 milliseconds, which is smaller than the horizontal and vertical sum time T2, and generally T
, < 72, the influence of the horizontal and vertical sum can be ignored, and the signal strength of each pixel can be used as S as is.

Further, in each pixel, signals are added and averaged the number of times the gradient magnetic field G is changed in the above example, that is, generally the number of lines of the image, so this number becomes the number of repetitions Nr.

For example, the noise N can be determined as follows. Impedance matching is performed between the coil and the receiving system, and a pseudo impedance element is connected to the coil, for example, when the impedance is matched with the receiving system with 50Ω, a 50Ω resistance element is connected, and the same as signal strength measurement is performed. Measurement is performed and the measured data is subjected to two-dimensional Fourier transform to obtain the noise level of each pixel. By measuring in this way, the influence of external noise received by the coil can be avoided. The noise level obtained at this time is increased by the noise figure of the receiving system, so the noise figure must be measured in advance and corrected. Furthermore, the noise level after the correction is, for example, W, A.

Edelstein, etal, Med, Phy.
s, 11(2), 180 (1984).

As can be understood from the detailed description in , the magnitude of noise N used in the above-mentioned Oji formula is 1.25 times, so N is determined by taking this into consideration as well.

The excitation/gurus power W may be determined as follows. 90°RF i? The signal strength is measured by changing the signal strength, and the power of the 90'RF signal when the signal strength reaches the maximum is detected. At this time, the power supplied to the coil is 90° RF power W, so W may be the value obtained by subtracting the loss between the transmitter and the coil from the transmission power of the transmitter that gives the maximum signal strength. . Also, 90°RF'
Another method for measuring power is to place a small coil (search coil) near the nuclear spin excitation coil, and before measuring the object, use a phantom (base thread sample) to , 90°RF yF pulse is determined in advance, and the 90°RF
・Record the wave height of the glucose and use it as a reference when measuring the object.
The wave height value of 0°RF/P Luss was set to 90° during phantom measurement.
°RF・? It is also possible to obtain the 90' RF power by adjusting the RF power so that it is equal to the peak value of the pulse. However, in this case, it is necessary that the electrical coupling between the search coil and the excitation coil be sufficiently small. This means that
It can be confirmed whether the change in noise level due to search coil settings is within the measurement accuracy range.

For example, when obtaining two-dimensional image information, the phase of nuclear spins within a sliced plane becomes slightly irregular due to the selection of a slice (a cross section of a subject). This influence can be corrected nine times each time a slice of the phantom is selected and measured in advance.

Using the values measured as above, α0 formula.

Using equation I5a, the number of specific atomic nuclei in a selected portion of the object, that is, in each pixel, can be quantitatively determined.

When spectroscopy is used to calculate the number of specific nuclei in a selected part of an object, - generally 90° RF
/# A rectangular wave is used as the pulse, therefore, as mentioned above, α=1, and the volume V is, for example, JP-A-59-10
Publication No. 7245, W, P, AVE.

et al, J, Magn, Re5on,, 5
6.350 (1984), etc., may be used. In spectroscopy, T3>T2 must be established, and in this case, the influence of the horizontal and vertical sum time T2 cannot be ignored, and at this time,
The received and detected nuclear magnetic resonance signal may be Fourier transformed, and the signal strength S may be the integral value of the signal that is a set of multiple spectra corresponding to the specific atomic nuclear spin, that is, the value that is the sum of the intensities of each spectrum. .

"Effects of the Invention" As described above, according to the present invention, substances containing atomic nuclei measured by nuclear magnetic resonance imaging 1 and nuclear magnetic resonance spectroscopy, that is, substances containing 'H, such as water and fat, Those containing 3'p'k are adenosine trisonic acid and creatyrelic acid.

For substances containing Na, such as inorganic phosphorus, the number of nuclei, ie, the concentration, of Na + ions in the measured portion can be determined.

It should be noted that in the past, in nuclear magnetic resonance apparatuses, it was not possible to perform an absolute evaluation of the device, that is, how far the device could reach S, but by modifying equation (1), 0
The equation is KΔt(kT)2iv'i;i By calculating this a1 equation, the device can be evaluated. In other words, when looking at the reference sample, n is known, W is determined by the device, and other things are determined by the measurement conditions, so Qlk can be calculated and the S/N of the device can be determined.

Claims (1)

[Claims] (1) Apply a magnetic field to the object and a high-frequency pulse to excite the nuclear spins to be detected in the object, receive the nuclear magnetic resonance signal generated by the excitation for a period T_S, and use the received signal to derive the nuclear spin of the object to be detected. A nuclear magnetic resonance signal intensity S of a selected portion of a specimen is calculated, and the following equation is calculated using the signal intensity S to obtain the number of nuclei to be detected in the selected portion. {[KΔt(kT)3/2√α]/[π@n@＾2I(
I+1)ω_O^2V√T_S]}×(S√W/N)K
is 6 if the power W of the high-frequency pulse is the envelope power, 6×√2 if it is the average power, Δt is the pulse width of the high-frequency pulse, k is Boltzmann's constant, T is the absolute temperature of the object to be tested, ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ @n@ is the value obtained by dividing Planck's constant by 2π, I is the above-mentioned detected nuclear spin quantum number, ω_O is the above-mentioned nuclear magnetic resonance signal angular frequency, and V is the above-mentioned selected part. Volume, N is noise.