5 ms. In UTE, the TE is defined as the time between the end of the r.f. pulse and the beginning
of the data acquisition, a center-out trajectory is used hence the TE can be short. Typically a TE of 100–250 μs is used, however times as short as 8 μs have been reported . The implementation of the sequence can be separated into two key areas: (i) the implementation of slice selection and (ii) the image reconstruction. Each of these aspects of the UTE sequence will be affected by the particular hardware used. In the following we discuss both aspects of UTE and present a simple technique http://www.selleckchem.com/products/VX-765.html to visualize the slice excitation profile. The r.f. and gradient shape must be well matched to ensure accurate slice selection. Here, the slice select gradient is AZD2014 ramped down from constant strength to zero over a few microseconds. The r.f. pulse for UTE excitation was reshaped to match the gradient using VERSE
. To apply VERSE, the center point of the gradient ramp is placed at the original end point of the half Gaussian r.f. pulse. The VERSE principle is then used to reshape the r.f. pulse to match the ramped switch off of the slice gradient. The r.f. pulse is scaled such that the area of the new pulse shape is equivalent to that of the original half Gaussian r.f. pulse. The use of VERSE compensates for the limited slew rate achievable by the gradient amplifiers and helps to ensure accurate slice selection. For the experiments shown here, the gradient pulse was defined with a 50 μs linear ramp from the constant value to zero and the r.f. ramp down time was therefore set to match this. The oscilloscope can be used to measure the output gradient shape from the amplifiers; however, there
is still some variation between the amplifiers and the gradient input to the sample. It is therefore desirable to measure the applied gradient directly. Here, the applied gradient is measured using the technique of Duyn et al. . The sequence measures the phase change across a thin slice in a homogeneous sample. This phase change corresponds to a direct measurement of position in k-space. The derivative of the measured phase change however provides the strength of the gradient that is produced by the gradient coil as a function of time. Using measurements of the gradient, the shape of the gradient is corrected using a method known as gradient pre-equalization . The method is outlined in Fig. 2. Initially, the input gradient is defined as a step function, u(t), that switches instantaneously from zero to a constant value. The resulting output, y(t), is measured using the gradient measurement technique of Duyn et al. . The measured gradient shape is then used to approximate the impulse response, h(t), of the gradient amplifier and coils.