MRI Frequency encoding and Phase encoding

投稿者: | 2017年7月5日
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MRI Frequency encoding and Phase encoding

 

This time, it is frequency encoding and phase encoding.

MRI data can not be imaged directly as you know.

So, put the MRI signal in k-space.

 

The vertical and horizontal axes of k-space are frequency encoding and phase encoding, respectively.

Which is not fixed vertically or horizontally.

 

The MRI signal is filled with k-space by frequency difference and phase difference.

Each of frequency encoding and phase encoding has its own characteristics.

 

Frequency encoding

It makes use of frequency difference.

The chemical shift artifact occurs.

 

Phase encoding

It makes use of phase difference.

The wraparound artifact occurs.

The motion artifact occurs phase direction.

It is related to scan time.

 

Phase encoding is more problematic.

The wraparound artifact appears when the FOV is smaller than the subject.

This artifact is because the phase difference outside the FOV can not be distinguished from the phase difference in the FOV.

To prevent it,

you have to increase the FOV in the phase encode direction above the subject.

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The scan time depends on the phase encoding number.

Therefore, in order to shorten the scan time, the phase encoding has to be reduced.

Normally, set the short direction of the subject to the phase encoding direction.

 

In case of head, it is RL direction.

In case of abdomen, it is AP direction.

 

In addition, we use parallel imaging to shorten imaging time.

That is a technique to reduce phase encoding.

 

To improve the artifact of the motion in the phase encoding direction, exchange the phase direction and the frequency direction.

In addition, it converts k-space collection to radial collection such as PROPELLAR method.

 

Radial collection is a method of filling k-space radially.

As a result, the phase direction becomes all directions and the artifact of movement is dispersed.

Furthermore, filling the center of k-space many times makes it possible to correct motion.

 

In frequency encoding and phase encoding, it is characterized by the phase encoding direction.

By understanding the features of the phase encoding direction, you can better control MRI.

 

Since I am a beginner in English, please correct the mistake grammar.

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