JPH0467860B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention relates to magnetic resonance (MR)
Regarding a magnetic resonance diagnostic apparatus that utilizes the resonance phenomenon to obtain diagnostic information that can be used for medical diagnosis, such as slice images of a subject, the present invention aims to shorten scan time and obtain optimal slice characteristics. It relates to a magnetic resonance diagnostic device.

(Prior art) Magnetic resonance is a phenomenon in which atomic nuclei with non-zero spin and magnetic moment placed in a static magnetic field resonantly absorb and emit only electromagnetic waves of a specific frequency. If H ₀ is the static magnetic field strength, then this nucleus resonates at the angular frequency ω ₀ shown in the following equation.

The method of performing biological diagnosis using the principle of ω ₀ = γH ₀ or more is as follows:
The electromagnetic waves of the same frequency as those induced after the above-mentioned resonance absorption are subjected to signal processing to obtain, for example, a tomographic image of the subject.

In this case, the collection of diagnostic information by magnetic resonance is
Although it is possible to excite and collect signals from all parts of a subject placed in a static magnetic field, due to limitations in the device configuration and clinical requirements for imaging images, in reality it is only possible to excite and collect signals on specific slice planes. I am trying to collect signals.

Here, the desired conditions for the imaging method are that the TE (echo time) is short at the same gradient magnetic field strength in order to shorten the scan time, and furthermore, for example, gear press multi-slice imaging can be realized. In order to achieve this, it must have a rectangular wave-like slice characteristic. Generally, a ±4π Sinc function, a ±2π Sinc function, or the like is used as the selective excitation pulse wave (SEP wave).

(Problem to be solved by the invention) In the above, according to the selective excitation pulse wave using the Sinc function of ±4π, the waveform is close to a rectangular wave, so the slicing characteristics are good and high-quality gear press multi-slice can be performed. However, the problem is that the TE becomes longer at the same gradient magnetic field strength. On the other hand, ±
If a selective excitation pulse wave using a 2π Sinc function is used, a short TE (echo time) can be achieved with the same gradient magnetic field strength, but the slice characteristics are not good.
There was a problem that gear press multi-slice could not be performed.

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic resonance diagnostic apparatus that can improve slice characteristics and shorten scan time.

[Structure of the Invention] (Means for Solving the Problems) The present invention is characterized by taking the following measures in order to solve the above problems and achieve the object. That is, the present invention applies a gradient magnetic field together with a selective excitation pulse wave to a subject placed in a static magnetic field to generate excitation by magnetic resonance in a specific slice plane part of the subject, and to excite the subject from the part. In a magnetic resonance diagnostic apparatus that collects induced magnetic resonance signals to obtain diagnostic information, the waveform characteristics are determined by parameters that are expressed by a Sinc function of ±2π and define the amplitude and frequency of the function. a holding means for holding a selective excitation pulse wave; a means for executing a slice characteristic measurement sequence using the selective excitation pulse wave of the holding means; means for calculating an error from an ideal value; means for changing parameters for determining the amplitude and frequency of the selective excitation pulse wave so that the error value determined by this means becomes a minimum value; and a means for calculating a selective excitation pulse based on the change parameter obtained by changing the parameter and holding it in the holding means, and performing a series of processing by each of the means at least once.

(Function) According to this configuration, the selective excitation pulse wave can be made into an ideal waveform (rectangular wave) by transforming the ±2π Sinc function, and this selective excitation pulse wave can be used to By executing the image acquisition sequence using TE, it becomes possible to obtain high-quality gear-press multislice images while shortening the scan time using short TE.

(Embodiment) An embodiment of the magnetic resonance diagnostic apparatus according to the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of the magnetic resonance diagnostic apparatus of this embodiment. As shown in FIG. 1, the magnet assembly MA that accommodates the subject P therein is
A static magnetic field coil 1 of, for example, a superconducting type or a normal conducting type that generates a high-intensity static magnetic field to act on the subject P, and three gradient magnetic field coils 2 that generate gradient magnetic fields along the X, Y, and Z axis directions. and a transmitting/receiving coil 3, which serves both as transmitting and receiving functions, for excitation and signal collection.

It also has a static magnetic field control system 4 that performs excitation control of the static magnetic field coil 1 and coolant supply control. X, Y,
The Z-axis gradient magnetic field coil 2 is excitation-controlled by X, Y, and Z-axis gradient magnetic field power supplies 5, 6, and 7. The transmitter/receiver coil 3 is driven by a transmitter 8 during transmission (excitation), and is driven by a transmitter 8 during reception (signal collection).
In this case, it is driven by the receiver 9.

In addition, a sequencer 10 controls the generation sequence of gradient magnetic fields and transmission/reception signals by the X, Y, and Z-axis gradient magnetic field power supplies 5, 6, and 7, the transmitter 8, and the receiver 9.
A computer system 11 that performs overall control and signal processing of the entire system including the sequencer 10 and accessory equipment such as a bed, and a display system 12 that displays generated images.

It also includes an optimal SEP calculation unit 13 that calculates an ideal selective excitation pulse wave by transforming the ±2π Sinc function to approximate an ideal rectangular wave. This optimum SEP calculation unit 13 holds a selective excitation pulse wave, executes a slice characteristic measurement sequence using the held selective excitation pulse wave, and compares it with the actual measured value of the slice characteristic obtained by this execution. Calculate the error from the ideal value, change the parameter for determining the selective excitation pulse so that the error value obtained by this becomes the minimum value, and use the changed parameter to create a new selective excitation pulse. By finding a pulse and performing this series of processing at least once, an ideal rectangular wave is obtained.

Hereinafter, this optimum SEP calculating section 13 will be explained in detail with reference to FIG. 2. That is, the optimum SEP wave calculation unit 13 has a RAM (random access memory) not shown, and a selective excitation pulse wave generated by a waveform generator not shown is stored in this RAM (step S1). ).

In step S2, a selective excitation pulse wave is extracted from the RAM to obtain a 90° SEP wave and a 180° SEP wave, and the slicing gradient magnetic field, encoding gradient magnetic field, and read gradient magnetic field are all directed in the same direction (in the illustration, the Z-axis direction). ) are combined to generate the slice characteristic measurement sequence shown on the right in the figure.
This is given in sequence 10 of FIG.

In step S3, the sequencer 10 executes a slice characteristic measurement sequence, and generates a slicing gradient magnetic field, an encoding gradient magnetic field, and a read gradient magnetic field in the Z-axis direction by the X, Y, and Z-axis gradient magnetic field coils 2, and further By transmitting/receiving coil 3
A 90° SEP wave and a 180° SEP wave are generated to cause a magnetic resonance phenomenon in a specific part of the subject P, and the resulting magnetic resonance signals are collected by the transmitting/receiving coil 3 and transmitted via the receiver 9. The measured values are taken into the computer system 11 and subjected to one-dimensional Fourier transform processing to obtain actual measured values, which are displayed on the display system 12. Here, in step S4, as shown on the right side of FIG. 2, the error E between the slice characteristic actual value obtained in step S3 and the slice characteristic ideal value when using a rectangular wave is calculated with a rise of 0 to 10% and a rise of 10 to 10%. 90% rise, 90
Calculations are performed for three types of rises: ~100%.

Next, in step S5, parameters for determining the selective excitation pulse (a modified version of the ±2π Sinc function) are changed so that the error E is minimized. Here, the selective excitation pulse is expressed by the following equation.

f(t)=sin t+al cos t+a2 sin 2t+a3 cos
2t+a4 sin 3t+a5 cos 3t/t One method for changing the parameter that minimizes the error E is as follows.

Initial value a1=a2=a3=a4=a5=0 Initial value step Δa1=Δa2=Δa3=Δa4=Δa5 If error E decreases with ai←ai+Δai, Δai←2Δai If error E increases with Δai←ai+Δai, Δai←−1 /
2Δai As step S6, find the parameters using the method described above, calculate a new SEP wave, and proceed to step S6.
Return to S1 and store the new SEP wave in RAM.

The above series of calculations in steps S1 to S6 is performed at least once and stored in the RAM in the optimum SEP wave calculating section 13 as an ideal selective excitation pulse wave.

Various image sequences such as a normal single-slice image acquisition sequence, a multi-slice image acquisition sequence, and a gear press multi-slice image acquisition sequence can be executed using the above-mentioned optimal SEP wave, and thereby the patient P Various slice images can be obtained and displayed on the display system 12.

As described above, according to this embodiment, it is possible to optimize the SEP waves, shorten the scan time by short-time TE, and obtain high-quality gearless multislice images. become able to. Furthermore, in the imaging sequence of the multi-echo method, it becomes possible to realize high image quality of the third and fourth echo images.

[Configuration of the Invention] As described above, according to the present invention, slice characteristics can be made into an ideal waveform (rectangular wave) by using an ideal selective excitation pulse wave obtained by modifying the ±2π Sinc function. By executing the image acquisition sequence using this selective excitation pulse wave, it is possible to obtain high-quality gear press multi-slice images while shortening the scan time due to short TE. Therefore, it is possible to provide a magnetic resonance diagnostic apparatus that can improve slice characteristics and shorten scan time.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an embodiment of the magnetic resonance diagnostic apparatus according to the present invention, and FIG. 2 is an optimal diagram of the embodiment.
3 is a flowchart showing detailed operations of a SEP wave calculating section. MA... Magnet assembly, 1... Static magnetic field magnet, 2... Gradient magnetic field coil, 3... Transmission/reception coil, 4... Static magnetic field control system, 5... X-axis gradient magnetic field power supply, 6... Y-axis gradient Magnetic field power supply, 7... Z-axis gradient magnetic field power supply, 8... Transmitter, 9... Receiver, 10...
Sequencer, 11... Computer system, 1
2... Display system, 13... Optimal SEP wave calculation unit.

Claims (1)

[Claims] 1 Applying a gradient magnetic field together with a selective excitation pulse wave to a subject placed in a static magnetic field to generate magnetic resonance excitation in a specific slice plane part of the subject, and generate a magnetic resonance signal induced from the part. In a magnetic resonance diagnostic apparatus that obtains diagnostic information by collecting A holding means for holding, a means for executing a slice characteristic measurement sequence using a selective excitation pulse wave of this holding means, and an error between the actual measured value of the slice characteristic obtained by executing this means and a predetermined ideal value. means for changing the parameters for determining the amplitude and frequency of the selected excitation pulse wave so that the error value obtained by this means becomes the minimum value, and the changed parameters obtained by this means. A magnetic resonance diagnostic apparatus comprising means for calculating a selective excitation pulse and holding it in the holding means, and performing a series of processing by each of the means at least once.