As TE increases, what happens to the signal to noise ratio?

As the echo time (TE) increases in MRI, the signal-to-noise ratio (SNR) generally decreases. This is primarily due to two factors: the relationship between signal decay and increasing time and the influence of noise.

With a longer TE, the time allowed for T2 relaxation increases, which means that the signal from the tissue will have decayed more by the time the echo is read. Tissues that lose their signal more quickly will exhibit significantly reduced SNR at longer TE values, as the available echo becomes weaker. Additionally, during the longer TE period, the system may also accumulate more thermal noise, which contributes to a lower SNR.

Consequently, as TE increases, the overall SNR diminishes, leading to less clear images. This is an essential consideration in MRI protocols, as optimizing TE is crucial for obtaining the best possible image quality while balancing other technical parameters.