JPH0453528A - Data collection method in mri apparatus - Google Patents

Abstract

PURPOSE:To improve the S/N ratio to obtain a proper image by varying the data collection time and the read inclined magnetic field according to the length of echo time in collecting signals after the second echo signal and on. CONSTITUTION:A selective excitation pulse H1 and a slice inclined magnetic field are applied to a tested body disposed in a static magnetic field. Subsequently, a non-selective excitation pulse H2 and slice inclined magnetic field are applied thereto. Read inclined magnetic field GR is applied to collect the first echo signal S1, and an image by the first echo signal S, is made into a PD intensified image as the echo time TE is short. Subsequently, a non-selective excitation pulse H3' and slice inclined magnetic field are applied. A read inclined magnetic field GR' is applied to collect the second echo signal S2' and an image by the second echo signal S2 is made into T2 intensified image as the echo time TE2 is long. These data groups are subjected to reconfiguration processing to obtain a tomographic image. Accordingly, PD intensified image by short echo time TE2 and T2 intensified image by long echo time are obtained.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to magnetic resonance (MR)
The present invention relates to a data collection method in an MRI apparatus (magnetic resonance imaging apparatus) for obtaining morphological information such as slice images of a subject (living body) and functional information such as spectroscopy using the phenomenon (sonance).

(Prior art) Magnetic resonance is a phenomenon in which an atomic nucleus with non-zero spin and magnetic moment placed in a static magnetic field resonantly absorbs and emits only electromagnetic waves of a specific frequency. The angular frequency ω shown in (ωo−2πshi0..;
It resonates at Larmor frequency) ν.

ω0−γHO Here, γ is the gyromagnetic ratio specific to the type of atomic nucleus,
Moreover, %HO is the static magnetic field strength.

This type of MRI apparatus, which performs biological diagnosis using the above-mentioned principle, processes electromagnetic waves of the same frequency as above, which are induced after the above-mentioned resonance absorption, to determine nuclear density, longitudinal relaxation time T.
1. Transverse relaxation time T2. flow.

Diagnostic information that reflects information such as chemical shifts, such as slice images of a subject, is obtained non-invasively.

Collecting diagnostic information by magnetic resonance is able to excite all parts of a subject (not shown) placed in a static magnetic field and collect signals, but there are limitations in the equipment configuration and clinical aspects of imaging images. Due to these demands, actual devices excite a specific region and collect its signals.

In this case, the specific region to be imaged is generally a sliced region with a certain thickness;
Magnetic resonance signals (MR multiplied signals) of echo signals and FID signals from this slice site are collected by performing a data encoding process many times, and these data groups are subjected to image reconstruction processing using, for example, a two-dimensional Fourier transform method. By doing so, an image of the specific slice region is generated.

Here, the hydrogen nuclear density PD, the longitudinal relaxation time T1, and the transverse relaxation time T2 are typically the nuclear densities mentioned above.
This is extremely important in image evaluation.

A multi-echo method has conventionally been used as a data collection method to obtain an image in which these three elements PD, T, , T2 are reflected.

FIG. 2 is a pulse sequence diagram showing a data acquisition method using a pulse echo method (also referred to as a spin echo method or SE method).

That is, a selective excitation pulse H+ (generally with a flip angle of 9
A high frequency pulse around 0') and a slicing gradient magnetic field (not shown) are applied. Next, after applying a phase encoding gradient magnetic field (not shown), a non-selective excitation pulse H2 (generally a high frequency pulse with a flip angle of about 1000) and a slicing gradient magnetic field (not shown) are applied. Then, by applying the read gradient magnetic field GR, the first echo signal S1 can be collected. Next, a non-selective excitation pulse H3 (generally a high frequency pulse with a flip angle of about 1000) and a slicing gradient magnetic field (not shown) are applied.

Then, by applying the read gradient magnetic field GR,
A second echo signal S2 can be collected.

The above is the data collection method in one encoding process,
By repeating this a predetermined number of times, the first echo signal S
A data group based on I and a data group based on the second echo signal S2 are obtained, and by performing reconstruction processing on these data groups, respective tomographic images are obtained.

(Problem to be Solved by the Invention) Here, the image based on the first echo signal S has an echo time T
The EI is short and the PD-enhanced image is created.
The image based on the second echo signal S2 becomes a T2-weighted image with echo time TF and □ being long<shi (long TE). but,
Since the data collection time (Ta) is the same for collecting the first echo signal S1 and collecting the second echo signal S2, the following problem occurs.

That is, the following relationship exists between the S/N ratio of the image, the signal band BW, and the data acquisition time Ta.

S/N oc 1/J Ding W, -(1) 1
/J-cho W Sen J7-cho, ... (2) That is, S/N oc 17-cho, ...
(3) Also, T a = Ns / (2・POV・G
R)...(4). Here, Ns is the number of sampling points, POV is the imaging area, and GR is the strength of the read gradient magnetic field.

However, in the conventional method, the echo time T82 of the second echo signal S2 is the echo time TE1 of the first echo signal S1.
Since the data collection time Ta is the same even though it is longer than , both were supposed to have the same S/N ratio.

Therefore, an object of the present invention is to
An object of the present invention is to provide a data acquisition method in an MRI apparatus that improves the /N ratio and makes it possible to obtain appropriate images.

[Structure of the Invention (Means for Solving the Problems) The present invention has a structure in which the following means are taken to solve the above problems. That is, the present invention provides a data collection method in which a specific region of a subject is excited by magnetic resonance and a plurality of echo signals are collected from the region, and the signal collection after the second echo signal is performed according to the length of the echo time. Features (effects) characterized in that the data collection time and the read gradient magnetic field are varied. According to such a configuration, the second echo signal (subsequent) with a longer echo time is collected in order to obtain a T2-weighted image. In this case, by increasing the data collection time Ta and lowering the strength GR of the read gradient magnetic field, the S/N ratio can be improved with the same imaging area and number of sampling points while satisfying the relationship of equation (4) above. An improved image is obtained.

(Example) A data acquisition method in an MRI apparatus according to the present invention will be described below with reference to FIG.

FIG. 1 is a pulse sequence diagram showing the data acquisition method of this embodiment using the pulse echo method.

That is, a selective excitation pulse H+ (generally with a flip angle of 9
A high-frequency pulse of around 0°) and a slicing gradient magnetic field (not shown) are applied. Next, after applying a phase encoding gradient magnetic field (not shown), a non-selective excitation pulse H2 (generally a high frequency pulse with a flip angle of about 180°) and a slicing gradient magnetic field (not shown) are applied. Then, by applying the read gradient magnetic field GR, the first echo signal S1 can be collected, and the image based on the first echo signal S1 has a short echo time TE (short TE) and becomes a PD-enhanced image. This is the same as before.

Next, a non-selective excitation pulse H3 (generally a high frequency pulse with a flip angle of about 180') and a slicing gradient magnetic field (not shown) are applied. Then, the read gradient magnetic field GR
By applying -, the second echo signal 82' can be collected, and the image based on the second echo signal S2 has a long echo time TE2 (long TE) and becomes a T2-weighted image.

The above is the data collection method in one encoding process,
By repeating this a predetermined number of times, the first echo signal S
A data group based on the second echo signal 82' and a data group based on the second echo signal 82' are obtained, and tomographic images are obtained by respectively reconstructing these data groups.

Note that the data collection time (Ta=) is different from Ta in the case of collecting the first echo signal, and Ta>Ta, and the pulse repetition time TR is the same in both the conventional and this embodiment.

Here, Ta- and GR- satisfy the relationship of formula (4), and generally a combination of the smallest GR- and the largest Ta- is preferred. because,
According to equation (3), if the data collection time is increased, (
This is because according to equation 3), the S/N ratio is improved.

As described above, according to this embodiment, in the second echo signal S2- having the echo time Tgz longer than the echo time TE1 in the first echo signal S1, the first echo signal S
Since the second echo signal S2- was collected in the data collection time Ta- which is longer than that in case 1, it is possible to obtain an image with a higher S/N ratio than in the case of the first echo signal S1.

Therefore, a PD-weighted image with a short echo time T61 (Shaw Tg) and a T2-weighted image with a long echo time T82 can be obtained, which are useful in clinical practice.

In addition, although the case where two echo signals are obtained by the multi-echo method was illustrated above, the multi-echo method for obtaining three or more echo signals can also be applied. In that case as well, the data collection time Ta and the strength GR of the read gradient magnetic field satisfy the relationship of equation (4), and the S/N
In order to improve the ratio, the strength GR of the gradient magnetic field for read must be made the smallest, and the data collection time Ta be made the largest.
In combination with

In addition, although the data acquisition method described above is based on the pulse echo method, the magnetic field for refocusing the spin phase is applied to the above-mentioned non-selective excitation pulse H3 (generally a high-frequency pulse with a flip angle of around 180"). ), the field echo method (also called gradient-end field echo method, FE method) uses an inverted gradient magnetic field for reading.

) can also be applied.

In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] As described above, according to the present invention, when collecting the second echo signal (subsequent) with a longer echo time, the data collection time is lengthened and the strength of the REET gradient magnetic field is lowered. By doing so, the transverse relaxation time is emphasized in the same imaging area and the same number of sampling points, and an image with an improved S/N ratio can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram of a pulse sequence showing a data collection method according to an embodiment of the present invention, and FIG. 2 is a diagram of a pulse sequence showing a data collection method according to a conventional example. Applicant's agent Patent attorney Takehiko Suzue

Claims (1)

[Claims] In a data collection method in which a specific region of a subject is excited by magnetic resonance and a plurality of echo signals are collected from the region,
A data acquisition method in an MRI apparatus, characterized in that signal acquisition after the second echo signal varies data acquisition time and read gradient magnetic field according to the length of the echo time.