What effect does increasing TE (echo time) have on SNR?

Increasing the echo time (TE) in MRI typically results in a decrease in signal-to-noise ratio (SNR). This is primarily because longer TE values allow for more signal decay due to T2 relaxation processes. When the echo time is extended, the transverse magnetization diminishes more significantly, leading to a weaker echo signal received by the MRI coil. Since SNR is defined as the ratio of the signal strength to the noise level, a reduction in the signal (due to increased TE) will directly contribute to a lower SNR.

As TE increases, the time allowed for dephasing of spins also increases, which further diminishes the signal that reaches the receiver coil. Therefore, understanding the relationship between TE and SNR is crucial for optimizing MRI protocols, as longer echo times can be beneficial for certain contrast mechanisms but typically come at the cost of reduced SNR. This knowledge helps radiologists and MRI technologists make informed decisions on imaging parameters to balance image quality with scan time.