In a spin echo pulse sequence, what increases if the time from the 90 degree RF pulse is increased?

In a spin echo pulse sequence, the time between the application of the 90-degree radiofrequency (RF) pulse and the subsequent echo is known as the echo time (TE). This interval is crucial because it defines how long the spins in the tissue experience the influence of magnetic field inhomogeneities and the longitudinal relaxation process. When the time from the initial 90-degree RF pulse is increased, it directly results in a longer TE.

A longer TE allows for more signal decay, which means that tissues with longer T2 relaxation times will provide stronger echoes. This can lead to improved contrast between different types of tissues in imaging. Therefore, the increase in the time from the 90-degree RF pulse directly correlates to an increase in the echo time (TE), making it the correct answer.

Other factors, such as repetition time (TR), scan time, and phase encoding, do not directly relate to the time increased from the 90-degree RF pulse in the context of spin echo sequences, which makes TE the most relevant and correct consideration in this scenario. In summary, increasing the time from the 90-degree RF pulse specifically enhances the echo time (TE), thereby contributing to image contrast and quality.