The UTE sequence is developed using a sample of doped water and the potential of UTE is demonstrated using samples of cork and rubber that have short T2* and T2. UTE uses a soft excitation pulse, typically of a half Gaussian shape, to minimize the IWR1 echo time (TE) [23]. Slice selection is achieved by applying a gradient at the same time as the soft pulse. When using a full Gaussian pulse, a second gradient is used to refocus the spins that have dephased during the second half

of the radiofrequency (r.f.) pulse. This gradient must have the same area, but opposite sign, as that used during the second half of the r.f. pulse. Therefore, the refocusing gradient is typically of half the duration of the r.f. pulse. The duration of the refocusing gradient limits the minimum TE for slice selective excitations.

The minimum TE for the sequence would occur if the acquisition were to begin immediately after the negative gradient lobe typically corresponding to around 0.5 ms or more. UTE overcomes this limitation by using the half shape which is formed by truncating the full shape at the zero phase point [24]. As the excitation ends at the zero phase point, the refocusing gradient is not needed and the acquisition can begin as soon as the r.f. pulse ends. However, as the excitation is truncated it gives a dispersion excitation, that is an excitation selleck chemicals llc with both real and imaginary terms. To eliminate Y-27632 2HCl the imaginary component of the excitation the sequence needs to be executed twice. The two acquisitions are identical except that the slice select gradient has

opposite sign. The sum of these two acquisitions produces an identical slice to that produced by a full Gaussian and refocusing gradient as the imaginary signals, i.e. the dispersion peaks, cancel and the real signals, i.e. the absorption peaks, add [24]. A half Gaussian excitation requires the slice gradient to be switched off at the same time as the r.f. pulse ends. In practice it is impossible to switch off a gradient immediately owing to limitations in the slew rate that can be achieved by the gradient hardware. It is therefore necessary to switch the gradient off relatively slowly using a ramp. However, as the gradient strength decreases the instantaneous, apparent slice thickness of the r.f. pulse increases. Variable Rate Selective Excitation (VERSE) [25] and [26] is used to reshape the r.f. pulse to account for the time varying strength of the slice gradient. The VERSE pulse is designed such that the real-space bandwidth of the pulse remains constant as the gradient is decreased. A constant bandwidth is achieved by decreasing the power of the r.f. pulse, whilst increasing its duration and keeping the total applied power constant. This allows for the r.f. and gradient pulses to be switched off simultaneously.