JPH0318453B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a nuclear magnetic resonance imaging apparatus (hereinafter referred to as NMR), and particularly relates to a nuclear magnetic resonance imaging apparatus (hereinafter referred to as NMR), and in _particular , to
This invention relates to improvements in means for obtaining calculation images of T ₂ and proton density ρ.

(Prior art) Longitudinal relaxation time T ₁ , which is considered to be medically useful, has traditionally been measured using NMR imaging equipment from images measured.
There is a technique to obtain an image related to the value (T ₁ image) and an image related to the transverse relaxation time T ₂ value (T ₂ image).

(Problems to be Solved by the Invention) However, the T ₁ image and the T ₂ image have been obtained by different methods as follows.

For example, the T ₁ image is calculated as follows. The IRSE method is a combination of the Inversion Recovery method (hereinafter referred to as IR method) and the Spin Echo method (hereinafter referred to as SE method) as shown in Figure 7. The saturation recovery method (Saturation
One original image is obtained using the SRSE method, which is a combination of Recovery (hereinafter abbreviated as SR method) and SE method, and calculations are performed using these two images and an approximation formula for signal strength.

As shown in Figure 8, the SRSE method is a pulse sequence in which an echo signal is obtained by applying a 90° pulse and then a 180° pulse, and the time from the 90° pulse to the center of the echo signal is Ts, 90°. The time from pulse application to application of a 90° pulse in the next view is defined as Tr.

In addition, as shown in Figure 7, the IRSE method is a pulse sequence in which a 180° pulse for inversion recovery is applied before each 90° pulse of the SRSE method shown in Figure 8. The time from the application of the 180° pulse to the application of the 90° pulse is Td, the time from the 90° pulse to the center of the echo signal is Ts, and the time from the inversion pulse to the center of the echo signal is Ts.
The time from application of a 180° pulse for recovery to application of a 180° pulse for the next view is defined as Tr.

Theoretical formula for signal strength in _the SRSE method I _SR is I _SR = I ₀・exp(−Ts/T ₂ )
s/2T ₁ ) + exp (-Tr/T ₁ )} In addition, the theoretical formula for signal strength in the IRSE method I _IR is I _IR = I ₀・exp (−Ts/T ₂ ) {1−2・exp (− Td/ _T1 )
+2・exp(−Tr/T ₁ +Ts/2T ₁ )−exp(−Tr/
T ₁ )} For this theoretical formula, here, as Tr≫T ₁
If exp(−Tr/T ₁ )=0, I _SR ≒I ₀・exp(−Ts/T ₂ ) I _IR ≒I ₀・exp(−Ts/T ₂ ){1−2・exp(−
Td/T ₁ )} Therefore, I _IR / I _SR = 1-2・exp(-Td/T ₁ ) T ₁ = Td/ln {2I _SR / (I _SR − I _IR )} From this formula, T ₁ value seek.

If you want to obtain the _T2 image, please refer to the publication "Eizo Information (M)" June 1984 issue (Vol.16 No.11), for example.
CPMG described on pages 570 to 576 of
T ₁ and ρ are removed from a plurality of echo data by the method, and the T ₂ value is determined by the least squares method.

A CP that allows you to continuously extract multiple pieces of echo data and calculate the _T2 value in one data collection.
This method had the drawback that if the length of the applied pulse was imperfect, the error would accumulate as echoes were obtained, resulting in an error in the _T2 value.
The CPMG method solves this problem, and is a pulse sequence in which a 90° pulse is followed by a 180° pulse n times to generate n echoes, as shown in FIG.

Conventional methods using such techniques have the following drawbacks.

(1) The T ₁ image and the T ₂ image are obtained separately, and the calculation images of T ₁ , T ₂ , and ρ cannot be obtained at the same time.

(2) Exact values cannot be determined because approximate formulas are used.

(3) Due to the condition of Tr≫T ₁ , Tr must be made long, and the total scan time is long.

In view of these points, an object of the present invention is to use an NMR imaging device to obtain T ₁ , T ₂ ,
NMR that accurately and simultaneously obtains ρ calculation images
The purpose of the present invention is to provide an imaging device.

Another object of the present invention is that even when using an original image obtained from an arbitrary pulse sequence, T ₁ , T ₂ , ρ
Made it possible to obtain computational images accurately and simultaneously.
Our objective is to provide an NMR imaging device.

(Means for Solving the Problem) In order to achieve such an object, the present invention provides the following methods (a) to (b) for obtaining a calculated image based on relaxation time and proton density in a nuclear magnetic resonance imaging apparatus. ) It is characterized by comprising a control means having a function.

Note (a) Execute the determined pulse sequence to obtain at least three original images with different pulse parameters.

(b) Using the at least three original images and a theoretical formula for the signal intensity of each image according to the scan conditions of each image, simultaneously determine the values of relaxation times T ₁ , T ₂ and proton density ρ, and calculate these values. Each value is imaged to obtain each calculation image.

(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 for carrying out the method of 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.

Next, the procedure for creating a calculated image in such a configuration will be explained.

First, determine the desired pulse sequence and scan parameters. Here, we will take as an example a case where three images obtained by the SRSE method are used as original images. The scan parameters of the three images are selected to minimize the variance or standard deviation of the calculated images.

These conditions such as the pulse sequence and scan parameters determined above are set in the controller 13. Under the control of the controller 13, a 90° pulse as shown in FIG. 2A is obtained through the gate modulation circuit 17 and applied to the RF transmitting coil 4 via the power amplifier 18 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 the gradient magnetic field Gy, and at the same time, a gradient magnetic field Gx is applied (d) to prepare for observing 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.

Using the method described above, echoes in a large number of views with different amounts of phase encoding are observed, and the computer 11 obtains an original image.

Calculation images of T ₁ , T ₂ , and ρ are obtained using the iteratively corrected least squares method from the three original images obtained above and the theoretical formula.

Here, a calculation example using the iterative corrected least squares method will be explained.

Regarding the scan parameters Tr and Ts of the three original images, the first image is Tr ₁ and Ts ₁ , the second image is Tr ₂ and Ts ₂ , and the third image is Tr ₃ and Ts ₃ . , and if the theoretical formula for signal strength in the SRSE method is F(Tr, Ts, T ₁ , T ₂ , ρ), it is 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 effect of slicing 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 effect of this slice is 90° except at the center of the slice. This is due to the fact that the magnetization does not tilt by 90° due to the pulse.

Next, the calculation procedure will be described with reference to the flow of calculation processing shown in FIG. Initial approximation value〓(T^ ₁ , T^ ₂ ,
By performing a Taylor expansion of the theoretical formula around ρ^) and taking its first order, the signal strength I ₁ in each image can be obtained.
I ₂ and I ₃ are as follows.

I ₁ = F (Tr ₁ , Ts ₁ , 〓) + (∂F／∂T ₁ ) _Tr1Ts1 〓・ΔT
₁ + (∂F/∂T ₂ ) _Tr1Ts1 〓・ΔT ₂ + (∂F/∂T ₁ ) _Tr1Ts
1 〓・Δρ I ₂ = F (Tr ₂ , Ts ₂ , 〓) + (∂F／∂T ₁ ) _Tr2Ts2 〓・ΔT
₁ + (∂F/∂T ₂ ) _Tr2Ts2 〓・ΔT ₂ + (∂F/∂ρ) _Tr2Ts
2 〓・Δρ I ₃ = F (Tr ₃ , Ts ₃ , 〓) + (∂F／∂T ₁ ) _Tr3Ts3 〓・ΔT
₁ + (∂F/∂T ₂ ) _Tr3Ts3 〓・ΔT ₂ + (∂F/∂ρ) _Tr3Ts
3 〓・Δρ Next, ΔT ₁ , ΔT ₂ , and Δρ are determined by the least squares method from the values of the three images and the above equation.

Next, set new approximations for T^ ₁ , T^ ₂ , and ρ^ as T^ ₁ +ΔT ₁ T^ ₂ +ΔT ₂ ρ^+Δρ and iterate until T^ ₁ , T^ ₂ , and ρ^ converge. . If convergence is not possible, change the initial value and repeat the calculation from the beginning. When it is determined that it has converged, T^ ₁ at that time,
T^ ₂ and ρ^ are the required T ₁ , T ₂ , and ρ. This slice effect is due to the fact that the magnetization does not tilt by 90° due to the 90° pulse except at the center of the slice.

By the above procedure, calculation images of T ₁ , T ₂ , and ρ can be obtained simultaneously.

Note that in the present invention, the number of original images is not limited to three, and may be more than three. Alternatively, the image may be a mixture of three images in different pulse sequences, and the combination of pulse sequences is free.

The theoretical formula for each pulse sequence and the signal strength at that time is as follows.

FIGS. 4 to 6 are diagrams showing other pulse sequences. However, the manner in which the gradient magnetic field is applied is omitted. Figure 4 shows the case of the SR2SE method in which a 180° pulse is applied twice for each view in the SRSE method, and the theoretical formula in this case is:
Assuming Mo as the following formula, Mo=C _SR2SE {1-2・exp(-Tr/T ₁ +Ts ₁ /T ₁ +Ts ₂ /2
T ₁ ) +2・exp(-Tr/T ₁ +Ts ₁ /2T ₁ )-exp(-
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 5 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. 6 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. 5.

The theoretical formula for signal strength in this case is Mo=1-exp(-Td/ _T1 )/1-exp( _-Ts1 / _T2 - _Ts2 /
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 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 no approximation formula is used, accurate values can be obtained.

T ₁ , T ₂ , ρ can be calculated without eliminating them by calculating the theoretical formulas, so T ₁ , T ₂ , ρ
calculation images can be obtained at the same time.

Since the characteristics of the theoretical formula are not used, the pulse sequence and scan parameters can be freely selected. Therefore, the total scan time can be shortened.

By increasing the number of images used, the dispersion can be reduced over a wide range of T ₁ and T ₂ .

[Brief explanation of drawings]

FIG. 1 is a diagram showing the main part configuration of an embodiment of the NMR imaging device according to the present invention, FIG. 2 is a diagram showing an embodiment of the pulse sequence, FIG. 3 is a diagram showing the flow of calculation processing, and FIG. 9 through 9 are diagrams showing various pulse sequences. 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 signal is generated by applying a high frequency pulse and a gradient magnetic field to an object, this signal is detected, and the detected signal is used to obtain an image regarding the tissue of the object. 1. A nuclear magnetic resonance imaging apparatus comprising: a control means having the following functions (a) to (b) for obtaining a calculated image based on relaxation time and proton density. Note (a) Execute the determined pulse sequence to obtain at least three original images with different pulse parameters. (b) Based on the at least three original images and a theoretical formula for the signal intensity of each image according to the scan conditions of each image, the relaxation times T ₁ , T ₂ and protons are calculated using the iteratively corrected least squares method. The value of density ρ is determined at the same time, and each of these values is converted into an image to obtain each calculated image. 2. The theoretical formula for the signal strength is such that when the image is affected by the slice, the theoretical formula for the signal strength that includes the influence of the slice is used to obtain the calculated image. The nuclear magnetic resonance imaging device described in .