10a or c. The “noise” in Fig. 10b is primarily an artifact arising from the partial sampling of k-space used here. This artifact is eliminated by reconstruction with CS, as in Fig. 10c. There is some evidence of blurring in the UTE images shown in Fig. 10, especially where the beads touch the walls. This is likely due to slight Ivacaftor concentration errors in the k-space trajectory measurement . However, overall the resolution of all three images
is essentially equivalent, demonstrating the potential for UTE to obtain high-resolution images of complex samples. The UTE images shown in Fig. 10b and c were acquired using 64 center-out, radial spokes. Thus, these images were already obtained from only one quarter of the radial spokes required for a complete sampling of k-space at a resolution of 128 × 128 pixels. To further demonstrate the strength of the CS algorithm when reconstructing under sampled images, an image of the bead pack is shown in Fig. 10d obtained with only 32 center-out,
radial spokes. The acquisition time of this image is 1 min, half of that used for the images in Fig. 10b and c and an eighth of the time that would be required for a fully sampled center-out radial image. The intensity of the reconstructed image exhibits slightly more of the classic “stair-case” artifact , however, the structure of the bead pack is recovered accurately, with a clear demarcation between the solid beads (no signal) and the water. Cell Cycle inhibitor Indeed the image is very similar in quality to the UTE image acquired using all 64 radial spokes shown in Fig. 10c. To demonstrate the
strength of the UTE sequence for imaging short T2 material, we compare UTE and spin echo images of cork. A schematic of the sample is shown in Fig. 11a. The T2 of cork is much less than the minimum TE of the spin echo sequence, therefore there is no signal from the sample in the spin echo image shown in Fig. 11b. In contrast, the UTE image, in Fig. 11c, clearly shows the existence of a sample of cork. According to theory, the optimal bandwidth for the acquisition is defined by: equation(7) 1T=NπT2∗where T is the dwell time and N is the number of points in one image dimension . Considering the sample of cork, the optimal dwell time for a 128 × 128 Chloroambucil image would be 0.05 μs. This is not achievable with the present hardware, thus the image resolution is linewidth limited when using the minimum achievable dwell time of 1 μs per complex point. In a linewidth limited system with exponential decay, the resolution is defined by: equation(8) Δx=1πT2∗2πγGwhere γ is the gyromagnetic ratio of the nucleus and G is the acquisition gradient strength . However, as the gradient must ramp up to reach the constant value in UTE, the true resolution will be less than this. The ramp is on for 50 μs, with a 10 μs initial delay. The ramp up can be used to estimate the actual signal decay at each point in k-space.