FIGURE 3.

Chronograms of basic spin‐echo (SE) gradient‐echo (GRE), ultrashort echo time (UTE), and zero echo time (ZTE) pulse sequences. Note the difference in echo time (TE) and repetition time (TR) between the sequences. Typical values of TE in the UTE sequence are in the range of 100 μs and of δ in the ZTE sequence in the range of 10 μs. In particular, in the UTE sequence, there is a fast gradient switching between TR's and the acquisition starts during the gradient ramp up. In the ZTE sequence, gradient switching is smooth and the gradient is on before the excitation but there is a delay between the excitation and the acquisition. In this basic UTE sequence, a free induction echo is acquired but more complex sequences can acquire gradient‐recalled echoes. RF = radiofrequency; SS = slice selection; PE = phase encoding; RO = readout; DAQ = data acquisition.