Demo: MRI contrast - Machine Learning Satellite

In this demo post we will describe how the signal strength is determined in a MRI spin-echo sequence. You can find more information about MRI, echoes, image contrasts and how to use machine learning to manipulate them in future Kappa posts:

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In spin-echo sequences, the echoes are often repeated multiple times to increase the signal. The time difference between the radiofrequency (RF) pulses is called the repetition time ( $T_R$ ), while the time between turning off the RF pulse and achieving the echo is called the echo time ( $T_E$ ).

The selected $T_E$ and $T_R$ will affect the signal according to the $T_1$ and $T_2$ relaxation times of the protons. The signal equation of a spin echo sequence is the following:

$s = PD \cdot \left( 1 - e^{-\frac{TR}{T1}} \right) \cdot e^{-\frac{TE}{T2}}$

where $PD$ is proton density, another tissue-dependent property. The selection of $T_E$ and $T_R$ will define the contrast of the acquired image.

The below figure visualizes how changing the contrast settings ( $T_E$ and $T_R$ ) affects the signal strength.

Echo Time [TE]

Repetition Time [TR]

As an example, we can see how changing the $T_R$ has a larger effect than $T_E$ on the signal from water than from muscle. This is due to the large $T_1$ relaxation time of water, which is very dominant in the bladder.

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