Which sequence is best described as a 180° pulse followed by a 90°/180° combination?

The sequence described as a 180° pulse followed by a 90°/180° combination is best identified as an Inversion Recovery (IR) sequence. In an IR sequence, the initial 180° pulse is responsible for inverting the longitudinal magnetization of the tissue. This inversion creates a delay until a subsequent 90° pulse is applied, allowing for the recovery of the longitudinal magnetization before the imaging is performed.

Following the 180° pulse, the timing is critical, as it dictates how long the inversion occurs before the 90° pulse is applied. This timing can be adjusted to optimize contrast between different tissues based on their T1 relaxation times. The IR sequence is particularly effective in enhancing contrast, especially when trying to differentiate between various tissue types, such as fat and water, or in identifying lesions.

The combinations of 90° and 180° pulses are key in defining the behavior of the IR sequence as they establish both the initial inversion and the subsequent imaging signal.

This context highlights the function of IR sequences in MRI, which is critical for distinguishing between different tissues based on their relaxation properties.