ZTE imaging with long‐T₂ suppression

Abstract

Three-dimensional radial zero echo time (ZTE) imaging enables efficient direct MRI of tissues with rapid transverse relaxation. Yet, the feature of capturing signals with a wide range of T2 and T2 * values is accompanied by a lack of contrast between the corresponding tissues. In particular, the targeted short-T2 tissues may not be easily identified, and various approaches have been proposed to generate T2 contrast by reducing the long-T2 signal of water and/or fat. The aim of this work was to provide efficient long-T2 suppression for selective direct MRI of short-T2 tissues using the ZTE technique. For magnetization preparation, suppression pulses for water and fat were designed to provide both good T2 selectivity and off-resonance performance. To obtain high efficiency at short TRs, the pulses were applied in a segmented sequence scheme with minimized timing overhead, thus leading to a quasi-steady state of magnetization. The sequence timing was adjusted for optimal tissue contrast in musculoskeletal applications by means of simulations and experiments, incorporating both T2 and T1 of the involved tissues. The developed technique was employed for imaging of a lamb joint sample at 4.7 T. ZTE images were obtained with effective suppression of signals from tissues with long-T2 water, such as muscle or articular spaces, and fat. Hence, primarily short-T2 tissues were visible, such as bone and tendon. The MR image intensity of bone showed strong similarity with bone density imaged with micro-computed tomography.

Towards quantification of myelin by solid-state MRI of the lipid matrix protons

“…IR-ZTE, therefore, cannot be directly compared to an optimized IR-UTE protocol, where the duration of the excitation pulse is more flexible, and the flip angle can be increased to the optimal 90°. This problem could be mitigated on our 3T clinical scanner by applying multiple readouts per inversion in an IR-rZTE-PETRA sequence (Li et al, 2016), and has also been reported on a small-bore imager (Weiger et al, 2015), but this sequence enhancement could not be implemented on the 9.4T system used in this study.…”

“…Some of these limitations in ZTE can be alleviated with a custom-designed multiple-readout pulse sequence as described in Section 2.2 above and references (Li et al, 2016) and (Weiger et al, 2015). No such pulse sequence was available for the experiments conducted on the 9.4T micro-imaging system.…”

Quantification of Human Cortical Bone Bound and Free Water in Vivo with Ultrashort Echo Time MR Imaging: A Model-based Approach

“…Therefore, they possess extremely short relaxation times (T2* ,400 µsec), and conventional MR imaging modalities with an approximate echo time (TE) on the order of a few milliseconds (5-7 msec) are not capable of enabling any considerable information to be acquired from these water molecules. To obviate this limitation, solidstate MR imaging has been introduced, employing short, ultrashort, and, most recently, zero TEs, and is acknowledged to be a prevalent tool for imaging short T2* components (11,12).…”

Magnetic Resonance Imaging of Hard Tissues and Hard Tissue Engineered Bio-substitutes

“…Further ZTE imaging optimizations, which consisted of short amplitude-and frequency-modulated pulses for high bandwidth RF excitation, and long T 2 suppression, led to a significant improvement of this technique for clinical use [40,41].…”