JPH0318452B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a nuclear magnetic resonance imaging device (hereinafter referred to as NMR), and in particular, the present invention relates to a nuclear magnetic resonance imaging system (hereinafter referred to as NMR), and in particular, to find an optimal scan parameter and use the scan parameter to generate an original image. The present invention relates to a nuclear magnetic resonance imaging apparatus that can capture images and obtain calculated images of relaxation times T ₁ and T ₂ and proton density ρ.

(Prior art) Longitudinal relaxation time T ₁ , which is considered to be medically useful from measured images, has conventionally been used in NMR imaging devices.
There is a technique to determine the transverse relaxation time T ₂ proton density ρ. This technique is usually called IR (Inversion Recovery)
This method is calculated using two or more images with different pulse application conditions using the SR (Saturation Recovery) method or SR (Saturation Recovery) method.

(Problems to be Solved by the Invention) However, since the evaluation method for the pulse sequence and scan parameters when obtaining the original image of the calculation image of T ₁ , T ₂ , and ρ is not clear, the conditions are not necessarily optimal. There was a problem that T ₁ , T ₂ , and ρ were not always calculated.

In view of these points, the purpose of the present invention is to calculate T ₁ , T ₂ , and
An object of the present invention is to provide an NMR imaging device capable of determining the optimum value of a scan parameter for the value of ρ.

(Means for solving the problem) In order to achieve such an objective, the present invention uses a theoretical formula for signal strength, a scan parameter, and a relaxation time.
From T ₁ , T ₂ , proton density ρ, and the variance of the original image, use the law of error propagation to find the variance or standard deviation of the calculated image, and then find the value of the scan parameter that minimizes this variance or standard deviation. It is characterized by the following.

(Example) The present invention will be explained in detail below using the drawings. FIG. 1 is a block diagram of essential parts showing an embodiment of an NMR imaging apparatus 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 (composed 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 other.

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 that receives the signal from the waveform memory 21 and performs predetermined signal processing to obtain a tomographic image;
12 is a display device such as a television monitor that displays the obtained tomographic image.

Note that the control means is a portion consisting of the controller 13 and a predetermined functional portion of the computer 11.

In such a configuration, the saturation recovery method (SR method)
and NMR using the spin echo method (SE method) (hereinafter such a method is simply referred to as the SRSE method).
The pulse sequence when observing the signal is
The following description will be made with reference to a waveform diagram showing an example of the pulse sequence shown in the figure.

Under the control of the control means, that is, the controller 13, a 90° pulse as shown in FIG.
It is applied to the RF transmitting coil 4 to excite the object. At this time, a gradient magnetic field Gz is also applied (FIG. 2B) to selectively excite only the spins within a specific slice plane.

Next, phase encoding is performed using a gradient magnetic field Gy,
At the same time, a gradient magnetic field Gx is applied (d in the same figure).
Prepare to observe echoes.

Next, the application of the gradient magnetic field is stopped, and a 180° pulse is applied to reverse the spin. Thereafter, as shown in Figure D, an echo signal (Figure E) generated while applying Gx is detected by the receiving coil 5 and observed.
The spin echo signal detected by the receiving coil is stored in a waveform memory 21 via a preamplifier 19 and a phase detection circuit 20.

The echo signal thus obtained corresponds to one line of the two-dimensional Fourier transform of the spin density distribution within the slice plane. Therefore, if we collect a series of data while changing the size of Gy, that is, the size of phase encoding, for each view, we can obtain a reconstructed image by performing a two-dimensional inverse Fourier transform on these data. can.

Furthermore, the computer can obtain calculated images of T ₁ , T ₂ , and ρ from the original image obtained in this way, if necessary. In order to obtain a high-quality calculated image, the scan parameter (time Ts in Figure 2)
and Tr) need to be optimal values.

Now, the variance or standard deviation of the calculated image of T ₁ , T ₂ , ρ can be calculated from the theoretical formula of signal strength, the scan parameter, T ₁ , T ₂ , ρ, and the variance of the original image. For example, the law of error propagation (this law is described in S.Brandt's "Statistical and
Computational Methods in Data Analysis:
North-Holland Publishing Company, 1976,
Or "Methods of Data Analysis" by S. Brandt (translated by Yoshiki and others), Misuzu Shobo, 1976, explained on pages 33 to 35. ) can be calculated.

Further, since this value represents the S/N of the calculated image (more precisely, the standard deviation of the calculated image is proportional to the S/N), the calculated image can be evaluated using this value. In this way, the variance or standard deviation of the calculated image can be used as the evaluation function. For example, a weighted function as shown below may be used as the evaluation function.

σ _T1 /T+σ _T2 /T2+σ _p /ρ where σ _T1 , σ _T2 , and σ _p are the respective standard deviations of the T ₁ , T ₂ , and ρ calculation images.

In short, in order to obtain a high-quality calculated image, it is sufficient to find the times Ts and Tr at which the evaluation function is the best (the variance or standard deviation of the calculated image is minimized), and use these as the scan parameters.

Next, from the three original images in the SRSE method, T ₁ ,
A method for determining the optimal scan parameter when determining T ₂ and ρ images will be explained. The principle adopted here is as follows.

Theoretical formula F(Tr,
Ts, T ₁ , T ₂ , ρ) are expressed as follows.

F(Tr, Ts, T ₁ , T ₂ , ρ) = C _SRSE・exp(−Ts/T ₂ ) {
1-2·exp(−Tr/T ₁ +Ts/2T ₁ )+exp(−Tr/T ₁ )}·ρ Here, C _SRSE is a coefficient representing the influence of the slice and is a function of T ₁ /Tr. For example, if a Gaussian 90° pulse is used, _CSRSE can be expressed by the following equation.

0.2<T ₁ /Tr<10.0 and C _SRSE = 8.1537 E-6 (T ₁ /Tr) ⁶ -2.95086 E-4 (T ₁ /Tr) ⁵ +4.27675 E-3 (T ₁ /Tr) ⁴ -3.17902 E-2 (T ₁ /Tr) ³ +1.29262 E-1 (T ₁ /Tr) ² -2.8554 E-1 (T ₁ /Tr) +1.0557 The scan parameters of the three original images are Tr ₁ ,
If Ts ₁ , Tr ₂ , Ts ₂ , Tr ₃ , Ts ₃ , then the covariance matrix V _T1T2 〓 of the values of T ₁ , T ₂ , ρ calculated from the original image by the least squares method is V _T1T2 〓 = (A ^T V ^-1 ₁₂₃ A) ^-1 However, V ₁₂₃ is the covariance matrix of the original image, and the variance σ ² of the original image is σ ² ∝n ^-1 Ta ^-1 , where the number of averages is n and the sampling time is Ta. becomes. Therefore, the variance of the values of T ₁ , T ₂ , and ρ is
It is found as the diagonal element of V _T1T2 〓.

Based on this principle, scan parameters are determined by the following procedure.

First, the pulse sequence to be used is determined, and a theoretical formula for signal strength is determined.

From the theoretical formula, the range of T ₁ , T ₂ and ρ to be measured, and the variance of the original image, the variance or standard deviation of the calculated image is determined as a function of the scan parameter.

In the above, the variance or standard deviation of the calculated image was found as a multivariable function of the scan parameter, so the scan parameter Ts ₁ that minimizes the variance or standard deviation is determined by a method of finding the extreme value of the multivariable function (such as the simplex method). , Ts ₂ ,
Ts ₃ , Tr ₁ , Tr ₂ , Tr ₃ , Ta ₁ , Ta ₂ , Ta ₃ , n ₁ ,
Find n ₂ and n ₃ .

Note that in the present invention, the number of original images is not limited to three, but may be any number greater than the number of variables to be determined (at least two). Furthermore, there are no restrictions on the pulse sequence.

3 to 5 are diagrams showing other pulse sequences. However, the manner in which the gradient magnetic field is applied is omitted. Figure 3 shows the case of the SR2SE method in which a 180° pulse is applied twice for each view in the SRSE method.The theoretical formula for signal strength in this case is Mo = C _SR2SE {1-2・exp(-Tr/T ₁ +Ts ₁ /T ₁ +Ts
₂ /2T ₁ ) +2・exp(−Tr/T ₁ +Ts ₁ /2T ₁ )−e
xp(−Tr/T ₁ )}・ρ Here, C _SR2SE is a coefficient representing the influence of slice,
It is a function of T ₁ /Tr.

The intensity of the first echo signal is expressed as Mo·exp(−Ts ₁ /T ₂ ), and the intensity of the second echo signal is expressed as Mo·exp(−Ts ₁ /T ₂ −Ts ₂ /T ₂ ).

However, Ts ₁ is the time from application of the 90° pulse to the peak time of the first echo signal, and Ts ₂ is the time from the peak time of the first echo signal to the peak time of the second echo signal.

Figure 4 shows a pulse sequence in which a 180° pulse and a 90° pulse are applied after an echo signal is generated to shorten the waiting time until the next view starts (FRSE method). The theoretical formula is I=exp(−Ts ₁ /T ₂ )・1−exp(−Td/T ₁ )/
1−exp(−Ts ₁ /T ₂ −Ts ₂ /T ₂ −Td/T ₁ )·ρ.

Furthermore, FIG. 5 shows a method (FR2SE method) in which an echo signal is generated twice for each view, and a 180° pulse is added to the pulse sequence of FIG. 4.

The theoretical formula for signal strength in this case is Mo = 1-exp(-Td/T ₁ )/1-exp(-Ts ₁
/T ₂ −Ts ₂ /T ₂ −Ts ₃ /T ₂ −Td/T ₁ )・ρ The intensity of the first echo signal is Mo・exp(−Ts ₁ /T ₂ ) The intensity of the second echo signal is It is expressed as Mo・exp(−Ts ₁ /T ₂ −Ts ₂ /T ₂ ).

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

Since it has become possible to evaluate calculated images through calculations, it has become possible to select the optimal scan parameters under given conditions (total scan time, etc.).

It is possible to estimate the image quality of a calculated image without actually capturing it.

Since the calculated images can be compared, the pulse sequence to be used can be evaluated.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of main parts showing an embodiment of an NMR imaging device according to the present invention, FIG. 2 is a diagram showing an embodiment of a pulse sequence, and FIGS. 3 to 5 are applicable to the present invention. It is a figure which shows another pulse sequence. 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 modulation circuit, 18... Power amplifier, 19... Preamplifier, 20... Phase detection circuit, 21... Waveform memory.

Claims (1)

[Claims] 1. A nuclear magnetic resonance imaging apparatus that measures nuclear magnetic resonance signals and uses the signals to reconstruct a tomographic image of an object, wherein at least relaxation times (T ₁ , T ₂ ) or A nuclear magnetic resonance imaging apparatus characterized by comprising a control means having the following functions (a) to (f) for obtaining a calculated image regarding any one of proton densities (ρ). Note (a) Determine the pulse sequence to be used and obtain the theoretical formula for the signal strength in that pulse sequence. (b) From the above theoretical formula, scan parameters, relaxation times (T ₁ , T ₂ ), proton density (ρ), and original image variance, calculate the variance or standard deviation of the calculated image, or their weighted sum. Using the law of error propagation, the defined evaluation function is
It is determined as a function of the scan parameter. (c) Newly find scan parameters that give the best evaluation function for the calculated image found in (b) above. (d) Imaging is performed using the new scan parameters determined in (c) above. (e) Obtain a new original image from the signal obtained in imaging in (d) above. (f) The theoretical formula for the determined signal strength and the above
From the original image obtained in (e), use the least squares method to calculate at least one of T ₁ , T ₂ , and ρ values, or at least one of T ₁ , T ₂ , and ρ calculation image. Ask for one. 2. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein variance or standard deviation of the calculated image is used as the evaluation function. 3. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the following value is used as the variance (σ ² ) of the original image. Note σ ² ∝n ^-1 Ta ^-1where , n: Average number of original images Ta: Sampling time