Pipe, James G.

PurposeTo generate efficient gradient waveforms for spiral MRI which mitigate the high‐frequency attenuation inherent in gradient systems.Theory and MethodsSpiral MRI has many clinical advantages, including high temporal and SNR efficiency. One of the challenges for robust spiral MRI is a high sensitivity to imperfections in the gradient system, which requires some form of correction in order to map data correctly in k‐space. A previous numerical algorithm for generating spiral gradient waveforms was modified to reduce its high‐frequency content with minimal increase in waveform duration.ResultsExamples are shown of compact frequency gradient waveforms. Software implementing the algorithm is made available.ConclusionAn algorithm to produce gradient waveforms with a compact frequency spectrum is described. This algorithm results in greatly reduced overall error and better compatibility with gradient systems than the original algorithm from which it was derived.

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.

PurposeTo address the need for a method to acquire 3D data for MR elastography (MRE) of the whole brain with substantially improved spatial resolution, high SNR, and reduced acquisition time compared with conventional methods.MethodsWe combined a novel 3D spiral staircase data‐acquisition method with a spoiled gradient‐echo pulse sequence and MRE motion‐encoding gradients (MEGs). The spiral‐out acquisition permitted use of longer‐duration motion‐encoding gradients (ie, over two full oscillatory cycles) to enhance displacement SNR, while still maintaining a reasonably short TE for good phase‐SNR. Through‐plane parallel imaging with low noise penalties was implemented to accelerate acquisition along the slice direction. Shared anatomical information was exploited in the deblurring procedure to further boost SNR for stiffness inversion.ResultsIn vivo and phantom experiments demonstrated the feasibility of the proposed method in producing brain MRE results comparable to the spin‐echo–based approaches, both qualitatively and quantitatively. High‐resolution (2‐mm isotropic) brain MRE data were acquired in 5 minutes using our method with good SNR. Joint deblurring with shared anatomical information produced SNR‐enhanced images, leading to upward stiffness estimation.ConclusionA novel 3D gradient‐echo–based approach has been designed and implemented, and shown to have promising potential for fast and high‐resolution in vivo MRE of the whole brain.

PurposeTo introduce a modified 3D stack‐of‐spirals trajectory and efficient SENSE reconstruction for improved through‐plane undersampling, while maintaining SNR efficiency and other benefits of spiral acquisitions.MethodsA novel spiral staircase trajectory is introduced. This trajectory is a modified stack of spirals, in which spiral arms are distributed between partitions along kz. The trajectory maintains the efficient separable reconstruction with a Cartesian fast Fourier transform along the kz direction, followed by a 2D slice‐by‐slice gridding reconstruction. An additional intermediate step introduces a phase correction to collapse the spiral arms into the prescribed slice planes. For data undersampled through plane, this produces aliasing with reduced coherence, controlled by the arm‐ordering. Undersampled data can then be reconstructed with reduced g‐factor using a conjugate gradient–based iterative SENSE algorithm.ResultsThe trajectory significantly improves g‐factor for through‐plane accelerated acquisitions. Improvement manifests through both reduced overall g‐factor and reduced structure in the g‐factor maps. In the presented experiments, the mean g‐factor decreased from 1.26 to 0.93 and the maximum g‐factor decreased from 3.89 to 1.15 for R = 2 spiral staircase when compared with stack of spirals, and the mean g‐factor decreased from 2.51 to 0.94 and the maximum g‐factor decreased from 8.26 to 1.35 for R = 3 spiral staircase when compared with stack of spirals.ConclusionThe novel spiral staircase trajectory offers improved aliasing characteristics for through‐plane parallel imaging acceleration in 3D spiral acquisitions.

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.

PurposeFluid‐attenuated inversion recovery (FLAIR) nulls the CSF signal and is widely used in neuro MRI exams. A 3D scan can provide high SNR, contiguous coverage, and reduced sensitivity to through‐plane CSF flow. In this work, a 3D spiral FLAIR technique is proposed to improve the image quality of conventional 3D Cartesian FLAIR.MethodsThe 3D spiral FLAIR sequence incorporated a spiral‐in/out readout to preserve higher scan efficiency and eliminate off resonance‐induced artifacts observed with a commonly implemented spiral‐out readout, a compensation approach to minimize phase errors due to the concomitant fields accompanying the spiral gradient, and an adapted variable flip angle scheme to preserve scan efficiency and maintain a long and stable echo train. 3D Cartesian and spiral FLAIR (~6 min each) were acquired on a 3 Tesla scanner from 6 subjects (age range: 31‐64 years; mean: 39.5). Two neuroradiologists rated the images in a blinded fashion on a 5‐point scale. The noise performance was assessed quantitatively.ResultsCompared to 3D Cartesian FLAIR, 3D spiral FLAIR exhibits greater reduction of artifacts from CSF, especially anterior to the brain stem (rated better in 4 cases), artifacts attributed to blood/flow in the deep brain (better or much better in all 6 cases), and superior overall image quality (much better in 5 cases) despite residual susceptibility artifacts near the nasal cavity. Quantitative assessment demonstrates ~1.5× higher average SNR than Cartesian data.Conclusion3D spiral FLAIR achieves higher SNR, reduced CSF, and blood/flow artifacts, providing an alternative to 3D Cartesian FLAIR for neurological exams.

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.

There is increasing scrutiny from healthcare organizations towards the utility and associated costs of imaging. MRI has traditionally been used as a high‐end modality, and although shown extremely important for many types of clinical scenarios, it has been suggested as too expensive by some. This editorial will try and explain how value should be addressed and gives some insights and practical examples of how value of MRI can be increased. It requires a global effort to increase accessibility, value for money, and impact on patient management. We hope this editorial sheds some light and gives some indications of where the field may wish to address some of its research to proactively demonstrate the value of MRI.Level of Evidence: 5Technical Efficacy: Stage 5J. Magn. Reson. Imaging 2019;49:e14–e25.