JPH0429377B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention is applicable to NMR imaging devices, in particular when imaging information derived from relaxation time, by simulating changes in excitation pulse conditions. The present invention relates to a method for easily obtaining an image of.

[Background of the invention]

In an NMR imaging device, the atomic nuclei of a sample placed in a magnetic field precess, and this angular velocity is
Image reconstruction is performed using two factors: being proportional to the strength of the magnetic field, and obtaining positional information by applying a gradient magnetic field and gradually shifting the angular frequency at each point in the spatial coordinates. I'm going. Now, static magnetic field
The atomic nucleus of the sample placed in H ₀ precesses around the z-axis in the direction of H ₀ as shown by the solid line in Figure 1. The angular frequency ω ₀ of the motion at this time is the static magnetic field H ₀
is proportional to the strength of ω ₀ = γH ₀ ……(1). Here, γ is called the gyromagnetic ratio,
This value is unique to the nucleus. At this time, when a high frequency magnetic field H ₁ equal to ω ₀ is applied in the x direction, this energy is received, and the precession suddenly becomes intense, causing a nuclear magnetic resonance phenomenon. As a result, the magnetization M that was rotating around the z-axis is tilted in the y-axis direction. This tilt angle θ is proportional to the received energy. The excited nucleus returns to its original state of thermal equilibrium. The energy release process at this time includes spin-spin and spin-lattice relaxation, and the time constants of their relaxation are called the transverse relaxation time T ₂ and the longitudinal relaxation time T ₁ , respectively.
Next, an example of the principle of measuring longitudinal relaxation T ₁ will be explained using FIG. 2. As mentioned above, a 180° pulse is applied by _H1 , and the magnetization that was directed in the z direction is turned to the -z direction. Thereafter, it relaxes in the original z direction due to the spin-lattice longitudinal relaxation process. The state of magnetization at this time is shown in FIG. The magnetization M after an arbitrary time τ ₁ after the 180° pulse is expressed by the following formula: M=M ₀ (1−2e ⁻ 〓 ^1/T1 ) ……(2) (2). As a result, T ₁ measures M ₁ and M ₂ from two measurement points τ ₁ and τ ₂ , T ₁ = −τ ₂ /n (1 + M ₂ /M ₁ ) − n2……(3) (3) It can be calculated using the formula. In order to improve the measurement accuracy of this T ₁ , it is also possible to use a larger number of measurement points. However, the process of longitudinal relaxation shown in Figure 3 cannot be directly detected as information, and the free induction signal is detected by tilting it to the Y and Z planes with a 90° pulse at an arbitrary time τ after applying the 180° pulse. There is a need. After one measurement, it is necessary to wait for sufficient relaxation before starting the next measurement, and for this reason, it takes a considerable amount of measurement time to obtain a large number of measurement points with different τ.

Now, in NMR imaging, there are mainly density images that show density distribution, density images emphasized by relaxation time, and pure relaxation time images, each of which has clinical significance. Here, the image emphasized by the relaxation time is an image in which the density image is emphasized by the effect of relaxation by changing the time τ from the 180° pulse to the 90° pulse. For example, the T ₁ values of white matter and gray matter in the human head are 400 ms and 600 ms, respectively, and it is difficult to distinguish between the two in density images.
It is also possible to distinguish between the two with good contrast in the enhanced image. For this reason, enhanced images often have more clinical significance than density images. When τ ₁ , it is necessary to appropriately select 180°−τ−90° depending on the relaxation time. That is, by changing this τ on the reconstructed image, the same region can be displayed brightly or darkly. This means that when depicting a tissue with a relaxation time that is difficult to predict in advance or a mixture of tissues with various relaxation times, it is necessary to perform trial and error by varying the value of τ. On the other hand, NMR imaging has the disadvantage that measurement takes a long time. After one excitation, it is necessary to wait at least three times the relaxation time before the next excitation.
It takes about 1 to 1.5 seconds for each measurement.
It takes at least 100 excitations using normal means to construct one screen. Therefore, a long measurement time of 2 minutes or more is required. At this time, if the above-mentioned τ is changed and another condition is set, the time will be many times longer. In this way, in the conventional method, in the 180°−τ−90° pulse sequence,
Reproducing an image with optimal contrast has the disadvantage that measurement time is too long, making it impractical.

[Purpose of the invention]

In view of the above drawbacks, the purpose of the present invention is to arbitrarily change the contrast of an image containing various relaxation times by simulating excitation pulse conditions, thereby obtaining pulse conditions for obtaining an optimal image, and at the same time making it possible to diagnose The object of the present invention is to provide a method for easily obtaining a suitable contrast depending on the purpose.

[Summary of the invention]

Normally, in relaxation time imaging, the time constant is calculated from two images with different excitation pulse times, and the image is reproduced using this as the relaxation time. However, the present invention does not calculate the relaxation time itself. The method is characterized in that the value of τ on the relaxation time curve is obtained by interpolation or extrapolation, and a density distribution emphasized by the relaxation time is obtained.

[Embodiments of the Invention] The present invention will be described in detail below with reference to the embodiments shown in FIG. The transmitter 2 is driven in accordance with instructions from a sequencer 1 that generates a sequence of high-frequency pulses, and applies high-frequency pulses to the transmitting coil of the NMR main body 3. The free induction signal that can be observed thereby is detected by the receiver coil of the NMR main body 3 and sent to the data preprocessor 4. The data preprocessor 4 reconstructs a two-dimensional image from these signals. This reconstruction method includes a back projection method and a 2-D Fourier transform method used in NMR imaging, but the details are already well known and will therefore be omitted. The image created here is sent to the two-dimensional memory 5. The two-dimensional memory 5 has two sides 5-1 and 5-2. The screen synthesizer 6 can perform a synthesis process, which will be explained in detail later, on the signals sent from the two screens of the memory 5-1 and the memory 5-2, and send the signals to the display 7 for observation. The controller 8 is in charge of overall control and controls the flow.

First, sequencer 1 calculates the aforementioned 180°−τ−90°.
An image is reconstructed for τ=0 using the pulse sequence 5-1 and stored in 5-1. Note that 180°−τ− where τ=0
A 90° pulse sequence is difficult to understand literally, but it is sufficient to excite magnetization without giving time for longitudinal relaxation and measure nuclear magnetic resonance signals. In other words, a pulse sequence is used in which a -90° pulse is applied, equivalent to applying a 180° pulse and a 90° pulse simultaneously, and the resulting nuclear magnetic resonance signal is measured. The data measured multiple times is reconstructed into two-dimensional data according to the back projection method or the 2-D Fourier transform method, and is stored in the two-dimensional memory 5-1. If the content of the two-dimensional memory at the point ij on any picture element at this time is M _1ij , its value will be the value where τ = 0 in equation (2), that is, M _1ij
= −M _0ij .

Next, the pulse sequence is similarly measured with τ=τ ₁ and stored in 5-2. In other words, a 180° pulse is applied to change the magnetization of the object in the direction of the demagnetizing field (-z in Figure 2).
direction), wait for longitudinal relaxation of each magnetization for a time of τ ₁ , and then apply a 90° pulse (strictly speaking,
A plurality of measurements are performed using a pulse sequence in which the time difference between the pulse centers of the 180° pulse and the 90° pulse is τ ₁ ), and the data is reconstructed into two-dimensional data and stored in the memory 5-2. Contents of 2D memory in 5-2 at this time
M _2ij is (hereinafter, ij is omitted as it becomes complicated) M ₂ = −M ₁ (1−2e ⁻ 〓 ^1/T1 ) ……(4) Equation (4).

That is, now that we have found the values of the magnetization at each pixel position at the point τ = 0 and the point τ = τ ₁ of the longitudinal relaxation process as shown in Figure 3, we can calculate the value at these arbitrary parameters τ. Understand what you can do. Now let any τ be τ _x and τ _x =
If α = ₁ , the value of M at this time M _x is It can be obtained using equation (5). This is calculated for all picture elements ij. By displaying this result on the display 7, it is possible to easily know the result for a desired τ without actually changing the parameter τ and performing measurements. In other words, measurements are only made using the two types of pulse sequences described above, and then the screen synthesizer 6 performs calculations by varying the value of α in equation (5), resulting in a synthesis that best emphasizes the desired part. Just get the image. In addition, the screen synthesizer 6 can also be calculated by interpolation or extrapolation using an appropriate approximation function, instead of using the calculations described above. Furthermore, it is also possible to determine a time constant curve from the ratio of M ₁ and M ₂ and use it by indexing a table.

In the above explanation, the enhanced image of the longitudinal relaxation time T ₁ has been described, but it is clear that the same effect can be obtained using the same means for the transverse relaxation time T _z as well. The enhanced image of transverse relaxation time T _z is 90°−τ−180°
This can be obtained by selecting an appropriate value for the pulse interval τ as a parameter using a pulse sequence that measures resonance echoes using a pulse sequence of Instead of experimenting, measurements are taken using only two types of τ, and from the two images obtained in the same way as described above, a calculation formula based on the relaxation process formula is used to synthesize a composition that corresponds to the measurement of any value of τ. Get the image.

〔Effect of the invention〕

As explained in detail above, according to the present invention, images corresponding to various times emphasized by relaxation time can be easily obtained from a density image without performing measurements using a desired pulse sequence.
It is now possible to observe tissues with various relaxation times with the best contrast. As a result, the diagnosis of the lesion is facilitated, which is of great clinical benefit. In addition, the conventional method requires one measurement for one τ, which takes 2 minutes to more than 10 minutes, and as a result, it is almost impossible to obtain multiple images by changing the τ conditions. However, this method has the effect of easily obtaining the best screen without causing unnecessary pain to the subject.

[Brief explanation of drawings]

1 and 2 are explanatory diagrams for explaining the nuclear magnetic resonance phenomenon, FIG. 3 is a curve showing longitudinal relaxation, and FIG. 4 is an embodiment of the present invention.

Claims (1)

[Claims] 1. An NMR imaging method that sequentially measures nuclear magnetic resonance signals by exciting nuclear spins of a target using a pulse sequence including a plurality of high-frequency magnetic field pulses, and obtains an image of the target from these nuclear magnetic resonance signals, The pulse sequence imparts a nuclear spin relaxation effect to the nuclear magnetic resonance signal depending on the time interval between the high-frequency magnetic field pulses.
In the NMR imaging method, a plurality of pulse sequences are performed, each of which is determined by selecting a plurality of values as the time interval between the high-frequency magnetic field pulses that determines the effect of nuclear spin relaxation, and the nuclei obtained from each pulse sequence are Images of the object are reconstructed from the data of the magnetic resonance signals, and an enhanced image that should be obtained when the time interval between the high-frequency magnetic field pulses is an arbitrary value is synthesized by calculation from the data of the plurality of images. An NMR imaging method characterized by: