What is the effect of using a 180° pulse in spin echo imaging?

Using a 180° pulse in spin echo imaging primarily serves to refocus spins that have dephased due to magnetic field inhomogeneities, which helps to produce clearer and more homogeneous images. The introduction of the 180° refocusing pulse compensates for any phase shifts that may have occurred among the spins, particularly those caused by variations in the magnetic field. By refocusing these spins, the technique effectively reduces inhomogeneity artifacts that can degrade image quality. This ensures that the signals from the tissue being imaged are more coherent, enhancing the overall resolution of the resulting images.

The other options represent effects that are not accurately attributed to the use of a 180° pulse. For instance, while scan time can be influenced by various parameters, the use of a 180° pulse does not inherently decrease scan time; rather, it often adds to the total time because an additional radiofrequency pulse is introduced. Susceptibility artifacts are typically exacerbated by field inhomogeneities and are not increased by the refocusing pulse; hence, the choice that suggests it increases susceptibility artifacts is misleading. Lastly, the repetition time (TR) is determined by the imaging sequence design rather than directly impacted by the introduction of a 180° pulse,