Robison, Ryan K.

AbstractPurposeTo develop a robust 3D ultrashort‐TE (UTE) protocol that can reproducibly provide high‐quality images, assessed by the ability to yield clinically diagnostic images, and is suitable for clinical translation.Theory and MethodsBuilding on previous work, a UTE sampled with Fermat looped orthogonally encoded trajectories (FLORET) was chosen as a starting point due to its shorter, clinically reasonable scan times. Modifications to previous FLORET implementations included gradient waveform frequency limitations, a new trajectory ordering scheme, a balanced SSFP implementation, fast gradient spoiling, and full inline reconstruction. FLORET images were collected in phantoms and humans on multiple scanners and sites to demonstrate these improvements.ResultsThe updates to FLORET provided high‐quality images in phantom, musculoskeletal, and pulmonary applications. The gradient waveform modifications and new trajectory ordering scheme significantly reduced visible artifacts. Fast spoiling reduced acquisition time by 20%–28%. Across the various scanners and sites, the inline image quality was consistent and of diagnostic quality. Total image acquisition plus reconstruction time was less than 4 min for musculoskeletal and pulmonary applications with reconstructions taking less than 1 min.ConclusionRecently developed improvements for the FLORET sequence have enabled robust, high‐quality UTE acquisitions with short acquisition and reconstruction times. This enables clinical UTE imaging as demonstrated by the implementation of the sequence and acquisition on five MRI scanners, at three different sites, without the need for any additional system characterization or measurements.

PurposeThe need to detect and quantify brain lactate accurately by MRS has stimulated the development of editing sequences based on J coupling effects. In J‐difference editing of lactate, threonine can be co‐edited and it contaminates lactate estimates due to the spectral proximity of the coupling partners of their methyl protons. We therefore implemented narrow‐band editing 180° pulses (E180) in MEGA‐PRESS acquisitions to resolve separately the 1.3‐ppm resonances of lactate and threonine.MethodsTwo 45.3‐ms rectangular E180 pulses, which had negligible effects 0.15‐ppm away from the carrier frequency, were implemented in a MEGA‐PRESS sequence with TE 139 ms. Three acquisitions were designed to selectively edit lactate and threonine, in which the E180 pulses were tuned to 4.1 ppm, 4.25 ppm, and a frequency far off resonance. Editing performance was validated with numerical analyses and acquisitions from phantoms. The narrow‐band E180 MEGA and another MEGA‐PRESS sequence with broad‐band E180 pulses were evaluated in six healthy subjects.ResultsThe 45.3‐ms E180 MEGA offered a difference‐edited lactate signal with lower intensity and reduced contamination from threonine compared to the broad‐band E180 MEGA. The 45.3 ms E180 pulse had MEGA editing effects over a frequency range larger than seen in the singlet‐resonance inversion profile. Lactate and threonine in healthy brain were both estimated to be 0.4 ± 0.1 mM, with reference to N‐acetylaspartate at 12 mM.ConclusionNarrow‐band E180 MEGA editing minimizes threonine contamination of lactate spectra and may improve the ability to detect modest changes in lactate levels.

PurposeThe goal of this work is to present the implementation of 3D spiral high‐resolution MPRAGE and to demonstrate that SNR and scan efficiency increase with the increment of readout time.TheorySimplified signal equations for MPRAGE indicate that the T1 contrast can be kept approximately the same by a simple relationship between the flip angle and the TR. Furthermore, if T1 contrast remains the same, image SNR depends on the square root of the product of the total scan time and the readout time.MethodsMPRAGE spiral sequences were implemented with distributed spirals and spiral staircase on 3 Tesla scanners. Brain images of three volunteers were acquired with different readout times. Spiral images were processed with a joint water–fat separation and deblurring algorithm and compared to Cartesian images. Pure noise data sets were also acquired for SNR evaluation.ResultsConsistent T1 weighting can be achieved with various spiral readout lengths, and between spiral MPRAGE imaging and the traditional Cartesian MPRAGE imaging. Noise performance analysis demonstrates higher SNR efficiency of spiral MPRAGE imaging with matched T1 contrast compared to the Cartesian reference imaging.ConclusionFast, high SNR MPRAGE imaging is feasible with long readout spiral trajectories.

PurposeThe purpose of this study was to develop a spiral‐based combined spin‐ and gradient‐echo (spiral‐SAGE) method for simultaneous dynamic contrast‐enhanced (DCE‐MRI) and dynamic susceptibility contrast MRI (DSC‐MRI).MethodsUsing this sequence, we obtained gradient‐echo TEs of 1.69 and 26 ms, a SE TE of 87.72 ms, with a TR of 1663 ms. Using an iterative SENSE reconstruction followed by deblurring, spiral‐induced image artifacts were minimized. Healthy volunteer images are shown to demonstrate image quality using the optimized reconstruction, as well as for comparison with EPI‐based SAGE. A bioreactor phantom was used to compare dynamic‐contrast time courses with both spiral‐SAGE and EPI‐SAGE. A proof‐of‐concept cohort of patients with brain tumors shows the range of hemodynamic maps available using spiral‐SAGE.ResultsComparison of spiral‐SAGE images with conventional EPI‐SAGE images illustrates substantial reductions of image distortion and artifactual image intensity variations. Bioreactor phantom data show similar dynamic contrast time courses between standard EPI‐SAGE and spiral‐SAGE for the second and third echoes, whereas first‐echo data show improvements in quantifying T1 changes with shorter echo times. In a cohort of patients with brain tumors, spiral‐SAGE–based perfusion and permeability maps are shown with comparison with the standard single‐echo EPI perfusion map.ConclusionSpiral‐SAGE provides a substantial improvement for the assessment of perfusion and permeability by mitigating artifacts typically encountered with EPI and by providing a shorter echo time for improved characterization of permeability. Spiral‐SAGE enables quantification of perfusion, permeability, and vessel architectural parameters, as demonstrated in brain tumors.

Purpose Hyperpolarized 129 Xe MRI characterizes regional lung ventilation in a variety of disease populations, with high sensitivity to airway obstruction in early disease. However, ventilation images are usually limited to a single breath‐hold and most‐often acquired using gradient‐recalled echo sequences with thick slices (~10‐15 mm), which increases partial‐volume effects, limits ability to observe small defects, and suffers from imperfect slice selection. We demonstrate higher‐resolution ventilation images, in shorter breath‐holds, using FLORET (Fermat Looped ORthogonally Encoded Trajectories), a center‐out 3D‐spiral UTE sequence. Methods In vivo human adult ( N = 4; 2 healthy, 2 with cystic fibrosis) 129 Xe images were acquired using 2D gradient‐recalled echo, 3D radial, and FLORET. Each sequence was acquired at its highest possible resolution within a 16‐second breath‐hold with a minimum voxel dimension of 3 mm. Images were compared using 129 Xe ventilation defect percentage, SNR, similarity coefficients, and vasculature cross‐sections. Results The FLORET sequence obtained relative normalized SNR, 40% greater than 2D gradient‐recalled echo ( P = .012) and 26% greater than 3D radial ( P = .067). Moreover, the FLORET images were acquired with 3‐fold‐higher nominal resolution in a 15% shorter breath‐hold. Finally, vasculature was less prominent in FLORET, likely due to diminished susceptibility‐induced dephasing at shorter TEs afforded by UTE sequences. Conclusion The FLORET sequence yields higher SNR for a given resolution with a shorter breath‐hold than traditional ventilation imaging techniques. This sequence more accurately measures ventilation abnormalities and enables reduced scan times in patients with poor compliance and severe lung disease.

Purpose B0 eddy currents are a subtle but important source of artifacts in spiral MRI. This study illustrates the importance of addressing these artifacts and presents a system response‐based eddy current correction strategy using B0 eddy current phase measurements on a phantom. Methods B0 and linear eddy current system response measurements were estimated from phantom‐based measurement and used to predict residual eddy current effects in spiral acquisitions. The measurements were evaluated across multiple systems and gradient sets. The corresponding eddy current corrections were studied in both axial spiral‐in/out TSE and sagittal spiral‐out MPRAGE volunteer data. Results Correction of B0 eddy currents using the proposed method mitigated blurriness in the axial spiral‐in/out images and artifacts in the sagittal spiral‐out images. The system response measurement was found to yield repeatable results over time with some variation in the B0 eddy current responses measured between different systems. Conclusions The proposed eddy current correction framework was effective in mitigating the effects of residual B0 and linear eddy currents. Any spiral acquisition should take residual eddy currents into account. This is particularly important in spiral‐in/out acquisitions.