What is the effect on signal intensity as TE increases?

As the echo time (TE) increases in magnetic resonance imaging (MRI), the signal intensity typically decreases due to the effects of T2 relaxation. T2 relaxation time refers to the time it takes for the transverse magnetization (the component of the magnetization that is perpendicular to the magnetic field) to decay after the initial excitation pulse due to interactions among neighboring spins in the tissue.

Initially, when TE is short, the signal is strong because the spins have not yet decayed significantly. However, as TE increases, more time has passed for T2 decay to occur, leading to a lower signal intensity from the tissues being imaged. Different tissues have varying T2 relaxation times; for instance, fluids have longer T2 times, while fat and muscle typically have shorter T2 times. As a result, the signal intensity from these different tissues will diminish over longer TEs, further contributing to the overall decrease in signal intensity.

Consequently, indicating that as TE increases, the signal intensity decreases is accurate because it reflects the underlying physical principles governing MRI signal generation and behavior. This principle is crucial for understanding imaging sequences and optimizing techniques for specific clinical scenarios.