Ultrashort Echo Time Acquisition Research Articles

Multicomponent T2* mapping of knee cartilage: Technical feasibility ex vivo

Disorganization of collagen fibers is a sign of early-stage cartilage degeneration in osteoarthritic knees. Water molecules trapped within well-organized collagen fibrils would be sensitive to collagen alterations. Multicomponent effective transverse relaxation (T2*) mapping with ultrashort echo time acquisitions is here proposed to probe short T(2) relaxations in those trapped water molecules. Six human tibial plateau explants were scanned on a 3T MRI scanner using a home-developed ultrashort echo time sequence with echo times optimized via Monte Carlo simulations. Time constants and component intensities of T2* decays were calculated at individual pixels, using the nonnegative least squares algorithm. Four T2*-decay types were found: 99% of cartilage pixels having mono-, bi-, or nonexponential decay, and 1% showing triexponential decay. Short T2* was mainly in 1-6 ms, while long T2* was ∼ 22 ms. A map of decay types presented spatial distribution of these T2* decays. These results showed the technical feasibility of multicomponent T2* mapping on human knee cartilage explants.

Highly localized positive contrast of small paramagnetic objects using 3D center‐out radial sampling with off‐resonance reception

In this article, we present a 3D imaging technique, applying center-out RAdial Sampling with Off-Resonance reception, to accurately depict and localize small paramagnetic objects with high positive contrast while suppressing long T(2) (*) components. The center-out RAdial Sampling with Off-Resonance reception imaging technique is a fully frequency-encoded 3D ultrashort echo time acquisition method, which uses a large excitation bandwidth and off-resonance reception. By manually introducing an offset, Δf(0), to the central reception frequency (f(0)), the typical radial signal pileup observed in 3D center-out sampling caused by a dipolar magnetic field disturbance can be shifted toward the source of the field disturbance, resulting in a hyperintense signal at the magnetic center of the small paramagnetic object. This was demonstrated both theoretically and using 1D time domain simulations. Experimental verification was done in a gel phantom and in inhomogeneous porcine tissue containing various objects with very different geometry and susceptibility, namely, subvoxel stainless steel spheres, a puncture needle, and paramagnetic brachytherapy seeds. In all cases, center-out RAdial Sampling with Off-Resonance reception was shown to generate high positive contrast exactly at the location of the paramagnetic object, as was confirmed by X-ray computed tomography.

High-resolution spiral imaging on a whole-body 7T scanner with minimized image blurring.

High-resolution (approximately 0.22 mm) images are preferably acquired on whole-body 7T scanners to visualize minianatomic structures in human brain. They usually need long acquisition time ( approximately 12 min) in three-dimensional scans, even with both parallel imaging and partial Fourier samplings. The combined use of both fast imaging techniques, however, leads to occasionally visible undersampling artifacts. Spiral imaging has an advantage in acquisition efficiency over rectangular sampling, but its implementations are limited due to image blurring caused by a strong off-resonance effect at 7T. This study proposes a solution for minimizing image blurring while keeping spiral efficient. Image blurring at 7T was, first, quantitatively investigated using computer simulations and point-spread functions. A combined use of multishot spirals and ultrashort echo time acquisitions was then employed to minimize off-resonance-induced image blurring. Experiments on phantoms and healthy subjects were performed on a whole-body 7T scanner to show the performance of the proposed method. The three-dimensional brain images of human subjects were obtained at echo time = 1.18 ms, resolution = 0.22 mm (field of view = 220 mm, matrix size = 1024), and in-plane spiral shots = 128, using a home-developed ultrashort echo time sequence (acquisition-weighted stack of spirals). The total acquisition time for 60 partitions at pulse repetition time = 100 ms was 12.8 min without use of parallel imaging and partial Fourier sampling. The blurring in these spiral images was minimized to a level comparable to that in gradient-echo images with rectangular acquisitions, while the spiral acquisition efficiency was maintained at eight. These images showed that spiral imaging at 7T was feasible.