Why must TE be long for a T2 weighted image?

In T2-weighted imaging, the echo time (TE) is crucial for capturing the differences in signal decay related to T2 relaxation times of tissues. A long TE is necessary because it allows enough time for the transverse magnetization to decay, which is where T2-weighted contrast is generated.

As tissues relax, they lose their transverse magnetization at different rates, which reflects the inherent properties of different tissues. For example, fluids like cerebrospinal fluid (CSF) will have a longer T2 time and thus produce a stronger signal at longer TE compared to more solid tissues. By using a longer TE, it's possible to effectively differentiate between tissues with varying T2 decay rates, enhancing the contrast of the image and facilitating better visualization of pathology, particularly in lesions or conditions where T2 contrast is critical, such as edema or tumors.

This understanding of TE is fundamental for optimizing MRI sequences and ensuring high-quality T2-weighted images that highlight the differences in tissue characteristics based on their relaxation properties.