JPH01250234A - Method of collecting mr data - Google Patents

Abstract

PURPOSE:To reconcile the collection of data not affected by the movement of an object to be examined and the shortening of the time required in the completion of the collection of all of data, by changing a data collecting period corresponding to the phase encoding quantity in each pulse sequence. CONSTITUTION:In a pulse sequence due to a spin echo process, a threshold value is linearily changed from T1 to T2 corresponding to phase encoding quantity (phase encoding number K). A period t1 short in a stable state is regarded as a respiration stop period in the low threshold value T1 while a longer period t2 is regarded as the respiration stop period in the higher threshold value T2 to collect data. Therefore, when the absolute value of the phase encoding number K is small, data is collected only within the period t1 and the threshold value is made large as the absolute value becomes large and data is collected during a long period like the period t2. As a result, the collection of data receives the effect of movement slightly but the effect of the movement of an image is substantially little. Contrarily, since data to be discarded is reduced by making a data collecting time long like the period t2, the data collecting time can be prevented from becoming long.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method of collecting MRI (imaging using nuclear magnetic resonance) data, and particularly to a method of collecting data in synchronization with periodic motion of a subject.

[Conventional technology]

MHI uses excitation radio frequency signals (RF) and three orthogonal directions (
A series of procedures (pulse sequence ) is repeated at regular intervals to collect data for image reconstruction, but if the subject moves due to breathing movements or other causes during this data collection period, artifacts called motion artifacts will occur in the reconstructed image. . Therefore, conventionally, when the movement is periodic such as breathing movement, the movement is captured by a respiratory sensor, etc., and the data obtained by repeating it at a certain period is obtained at the point of intense movement. A data collection method is used in which data is discarded and only data obtained when movement has stopped is used. For example, when trying to avoid the effects of respiratory motion,
The state of abdominal movement is determined by a respiratory sensor. The temporal fluctuations in the position of the abdomen detected in this way are usually as shown in Figure 7. When the abdomen expands when you breathe in and the position of the abdomen rises, the movement of the abdomen is rapid, and when you breathe out, the abdomen moves rapidly. When the belly is depressed and the position of the belly is low, it is a stable period with little movement. Therefore, an appropriate threshold value is set for the signal from the respiratory sensor, and this threshold value determines the period ta during which the abdomen is depressed within the period tc of one cycle of respiratory movement.
and a period tb in which the abdomen is inflated, and this period ta is a breathing rest period, and the period tb is a breathing movement period, and a period t is defined.
Collect data only at point a.

[Problem to be solved by the invention]

However, with this method of setting a certain threshold and distinguishing between the data collection period and the data discarding period, depending on how the threshold is set, the data discarding period may become longer and it may not be possible to complete all data collection. The problem is that the time it takes to complete the process becomes longer, or conversely, the period during which data is discarded is shortened, but data that is affected by movement is also collected. An object of the present invention is to provide an MR data acquisition method that can both acquire data that is not affected by the movement of a subject and shorten the time required to complete all data collection. .

[Means to solve the problem]

In order to achieve the above object, according to the present invention, in a method of collecting data during a period in which it is determined that periodic motion of a subject is not intense, the period for data collection is changed according to the amount of phase encoding in each pulse sequence. It is characterized by causing

[For production]

For pulse sequences with a large amount of phase encoding, NMR
The amplitude of the signal (data) becomes small, and the amplitude of the NMR signal becomes large in a pulse sequence with a small amount of phase encoding. This is because when the phase encoding amount is increased, the spin phase is disturbed and the NMR signal becomes smaller. Therefore, if even the slightest movement of the subject is included in the data obtained with a pulse sequence with a small amount of phase encoding, this will appear as a major influence on the reconstructed image.
Conversely, even if some movement is included in the data obtained from a pulse sequence with a large amount of phase encoding, it will not appear as a significant effect on the reconstructed image. Therefore, the period during which data is collected is not fixed, but is varied according to the amount of phase encoding in each pulse sequence.
When the amount of phase encoding is large, by expanding the data collection period and collecting data even when the subject is moving slightly, it is possible to collect data that is virtually unaffected by the movement of the subject. This can prevent the period for collecting all the data necessary for image reconstruction from becoming long.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. First, in this embodiment, a pulse sequence based on the spin echo method as shown in FIG. 5, for example, is adopted. In FIG. 5, Gx is a gradient magnetic field for slice selection, Gy is a gradient magnetic field for phase encoding, and Gz is a gradient magnetic field for frequency encoding. This FIG. 5 represents one pulse sequence, and this pulse sequence is
Repeat 2N times by changing the phase encode amount. As shown in Figure 6A, the Gy waveform is based on the basic Gy waveform by /N (K=-N, -N+1..., -1°0,
+1. ..., N-1). Then, for each pulse sequence, the phase encode number K is increased one by one from -N to N-1, and the pulse sequence is repeated while changing the phase encode amount.
An echo signal as shown in Figure B is obtained. Then, the threshold value is changed as shown in FIG. 1 according to the phase encode amount (phase encode number K). In this embodiment, the movement state of the abdomen is grasped by a respiratory sensor, and the influence of respiratory movement is avoided.The threshold value of the signal from the respiratory sensor is determined by the phase encode number K as shown in FIG. It changes linearly from T1 to T2 according to the change in temperature. When a low threshold value T1 is set, data is collected by regarding a short period t1 in which the position of the antrum is lower and stable as shown in FIG. 2 as a respiratory pause period. In the case of a higher threshold value T2, data is collected over a longer period of time, including a period in which the position of the antinode is somewhat higher and moving to some extent, and 2 is regarded as a respiratory pause period. Therefore, the absolute value of the phase encoding number is small (
(near O), data is collected only during periods t1 with less movement. At this time, the echo signal has become large as shown in FIG. This can be avoided by collecting data. On the other hand, as the absolute value of the 11 phase encode number becomes larger, the threshold value is increased like T2 and data is collected over a longer period like Sit2. As a result, data that has been somewhat affected by movement will also be collected, but at this time the echo signal will be small as shown in Figure 6B, and the data that has been somewhat affected by movement will be used to create an image. Even if you reconfigure it,
The effect of motion on the image is substantially less; on the other hand, increasing the data collection period to t2 means that less data is discarded, so
It is possible to prevent the time required to collect all the data necessary for image reconstruction from increasing. In this embodiment, basically, the phase encode number K is increased in order and the pulse sequence is repeated at a constant period, and the pulse sequence is maintained during the data collection period t.
If it does not enter l(t2), the data obtained there is discarded, and the pulse sequence for the same phase encode number is repeated until the period enters. In addition, in the above embodiment, the threshold value is changed linearly according to the amount of phase encoding, but it may be changed to a function as shown in FIG. 3 or as a step-like function as shown in FIG. You can also do things like

【Effect of the invention】

According to the MR data acquisition method of the present invention, it is possible to acquire data that is practically unaffected by the movement of the subject and to shorten the time required to complete all data acquisition.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the amount of phase encoding and the threshold value according to an embodiment of the present invention, FIG. 3 and 4 are graphs showing the relationship between the phase encode amount and the threshold according to the modified example, FIG. 5 is a time chart showing an example of a pulse sequence, and FIGS. 6A and B
7 is a graph showing the phase encode amount and the echo signal, and FIG. 7 is a graph showing the relationship between the antinode position fluctuation and the data acquisition period in the conventional example. Tl, T2...Threshold value, tl, t2...Data collection period, RF...High frequency signal for excitation, Gx...Gradient magnetic field for slice selection, G, y...Gradient magnetic field for phase encoding , Gz...Gradient magnetic field for frequency encoding, ta
... Respiratory pause period, tb... Breathing movement period, tc.
...1 breathing cycle.

Claims (1)

[Claims] (1) An MR data acquisition method characterized by changing the data acquisition period according to the amount of phase encoding in each pulse sequence, in a method of acquiring data during a period in which it is determined that periodic motion of the subject is not intense. .