Wang, Dinghui

AbstractPurposeTo develop a method that achieves simultaneous brain and neck time‐of‐flight (ToF) magnetic resonance angiography (MRA) within feasible scan timeframes.MethodsLocalized quadratic (LQ) encoding is efficient for both signal‐to‐noise ratio (SNR) and in‐flow enhancement. We proposed a spiral multiband LQ method to enable simultaneous intracranial and carotid ToF‐MRA within a single scan. To address the venous signal contamination that becomes a challenge with multiband (MB) ToF, tilt‐optimized non‐saturated excitation (TONE) and partial‐Fourier slice selection (PFSS) were further introduced in the LQ framework to mitigate the venous signal and improve artery contrast. A sequential spiral MB and LQ reconstruction pipeline was employed to obtain the brain‐and‐neck image volumes.ResultsThe proposed MB method was able to achieve simultaneous brain and neck ToF‐MRA within a 2:50‐min scan. The complementarily boosted SNR‐efficiency by MB and LQ acquisitions allows for the increased spatial coverage without increase in scan time or noticeable compromise in SNR. The incorporation of both TONE and PFSS effectively alleviated the venous contamination with improved small vessel sensitivity. Selection of scan parameters such as the LQ factor and flip angle reflected the trade‐off among SNR, blood contrast, and venous suppression.ConclusionsA novel MB spiral LQ approach was proposed to enable fast intracranial and carotid ToF‐MRA with minimized venous corruption. The method has shown promise in MRA applications where large spatial coverage is necessary.

AbstractPurposePerfusion MRI reveals important tumor physiological and pathophysiologic information, making it a critical component in managing brain tumor patients. This study aimed to develop a dual‐echo 3D spiral technique with a single‐bolus scheme to simultaneously acquire both dynamic susceptibility contrast (DSC) and dynamic contrast‐enhanced (DCE) data and overcome the limitations of current EPI‐based techniques.MethodsA 3D spiral‐based technique with dual‐echo acquisition was implemented and optimized on a 3T MRI scanner with a spiral staircase trajectory and through‐plane SENSE acceleration for improved speed and image quality, in‐plane variable‐density undersampling combined with a sliding‐window acquisition and reconstruction approach for increased speed, and an advanced iterative deblurring algorithm. Four volunteers were scanned and compared with the standard of care (SOC) single‐echo EPI and a dual‐echo EPI technique. Two patients were scanned with the spiral technique during a preload bolus and compared with the SOC single‐echo EPI collected during the second bolus injection.ResultsVolunteer data demonstrated that the spiral technique achieved high image quality, reduced geometric artifacts, and high temporal SNR compared with both single‐echo and dual‐echo EPI. Patient perfusion data showed that the spiral acquisition achieved accurate DSC quantification comparable to SOC single‐echo dual‐dose EPI, with the additional DCE information.ConclusionA 3D dual‐echo spiral technique was developed to simultaneously acquire both DSC and DCE data in a single‐bolus injection with reduced contrast use. Preliminary volunteer and patient data demonstrated increased temporal SNR, reduced geometric artifacts, and accurate perfusion quantification, suggesting a competitive alternative to SOC‐EPI techniques for brain perfusion MRI.

AbstractPurposeRobust implementation of spiral imaging requires efficient deblurring. A deblurring method was previously proposed to separate and deblur water and fat simultaneously, based on image‐space kernel operations. The goal of this work is to improve the performance of the previous deblurring method using kernels with better properties.MethodsFour types of kernels were formed using different models for the region outside the collected k‐space as well as low‐pass preconditioning (LP). The performances of the kernels were tested and compared with both phantom and volunteer data. Data were also synthesized to evaluate the SNR.ResultsThe proposed “square” kernels are much more compact than the previously used circular kernels. Square kernels have better properties in terms of normalized RMS error, structural similarity index measure, and SNR. The square kernels created by LP demonstrated the best performance of artifact mitigation on phantom data.ConclusionsThe sizes of the blurring kernels and thus the computational cost can be reduced by the proposed square kernels instead of the previous circular ones. Using LP may further enhance the performance.

Abstract Purpose To demonstrate T 2 ‐weighted (single‐echo) spin‐echo (SE) imaging with near‐optimal acquisition efficiency by applying SNR‐efficient RF slice encoding and spiral readout. Methods A quadratic‐phase (frequency swept) excitation RF pulse replaced the conventional excitation in T 2 ‐weighted SE sequence to excite a thick slab that is internally spatially encoded by a variable phase along the slice direction. Highly overlapping slabs centered at every desired slice location were acquired in multiple passes, such that the entire imaging volume was excited by contiguous slabs in any given pass. Following 90° excitation, each slab was refocused with a conventional 180° RF to produce a SE signal, followed by a spiral in‐out readout. A noise‐insensitive reconstruction removed the quadratic phase in the spatial frequency domain, yielding desired slice resolution and improved SNR. Results Increasing the RF frequency sweep (hence, excitation width) allowed more frequent encoding of each slice over the multiple passes, improving final image SNR, until crosstalk ensued at excessive slab widths compared to their center‐to‐center spacing. With an optimized slab width, the proposed technique used all passes to acquire every prescribed slice, with substantially improved SNR over conventional SE or 2D‐turbo‐spin‐echo (TSE) scans. Quantitative SNR measurements indicated similar SNR as 3D‐TSE, but radiologist scoring favored 3D‐TSE, mainly because of spiral‐related artifacts and possibly because of regularized reconstructions in 3D‐TSE. Conclusion Using SNR‐efficient slice excitation scheme and spiral readout helped eliminate SNR and temporal inefficiencies in conventional T 2 ‐weighted imaging, yielding SNR independent of TR or number of passes.

Abstract Purpose This work proposes a 2D/3D hybrid inflow MRA technique for fast scanning and high SNR and contrast‐to‐noise (CNR) efficiencies. Methods Localized quadratic (LQ) encoding was combined with a sliding‐slice spiral acquisition. Inflow MRAs around the circle of Willis and the carotid bifurcations were collected on four healthy volunteers. Spiral images were deblurred without or with water–fat separation for sliding‐slice LQ (ssLQ) out‐of‐phase (OP) and Dixon inflow MRAs, respectively. Results were compared to multiple overlapping thin slab acquisitions (MOTSA) and 2D OP inflow MRAs. Noise data were also acquired with RF and gradients turned off to compute maps of SNR and SNR efficiency. Quantitative assessment of relative contrast, CNR, and CNR efficiency for flow were performed in regions of interest. Results The sliding‐slice spiral technique alone reduces scan time by 10% to 40% compared with a standard spiral acquisition scheme. The proposed spiral ssLQ OP achieves 50% higher scan speed than the spiral MOTSA with comparable SNR and CNR efficiencies, which are ∼100% higher than the Cartesian MOTSA for intracranial inflow MRAs. Spiral ssLQ Dixon inflow MRA provides better visibility for vessels around the fat compared to spiral ssLQ OP inflow MRA, with a trade‐off of scan speed. Spiral ssLQ MRA with thinner slice thickness is two to five times faster than the 2D Cartesian inflow neck MRA around the carotid bifurcations, while also achieving higher SNR efficiency. Conclusion The proposed spiral ssLQ is a fast and flexible MRA method with improved SNR and CNR efficiencies over traditional Cartesian inflow MRAs.

Purpose The combination of SENSE and spiral imaging with fat/water separation enables high temporal efficiency. However, the corresponding computation increases due to the blurring/deblurring operation across the multi‐channel data. This study presents two alternative models to simplify computational complexity in the original full model (model 1). The performances of the models are evaluated in terms of the computation time and reconstruction error. Methods Two approximated spiral MRI reconstruction models were proposed: the comprehensive blurring before coil operation (model 2) and the regional blurring before coil operation (model 3), respectively, by altering the order of coil‐sensitivity encoding process to distribute signals among the multi‐channel coils. Four subjects were recruited for scanning both fully sampled T 1 ‐ and T 2 ‐weighted brain image data with simulated undersampling for testing the computational efficiency and accuracy on the approximation models. Results Based on the examples, the computation time can be reduced to 31%–47% using model 2, and to 39%–56% using model 3. The quality of the water image remains unchanged among the three models, whereas the primary difference in image quality is in the fat channel. The fat images from model 3 are consistent with those from model 1, but those from model 2 have higher normalized error, differing by up to 4.8%. Conclusion Model 2 provides the fastest computation but exhibits higher error in the fat channel, particularly in the high field and with long acquisition window. Model 3, an abridged alternative, is also faster than the full model and can maintain high accuracy in reconstruction.

PurposeAn accurate field map is essential to separate fat and water signals in a dual‐echo chemical shift encoded spiral MRI scan. A rapid low‐resolution B0 map prescan is usually performed before each exam. Occasional inaccuracy in these field map estimates can lead to misclassification of the water and fat signals as well as blurring artifacts in the reconstruction. The present work proposes a self‐consistent model to evaluate residual field offsets according to the image data to improve the reconstruction quality and facilitate the scan efficiency.Theory and MethodsThe proposed method compares the phase differences of the two‐echo data after correcting for fat frequency offsets. A more accurate field map is approximated according to the phase discrepancies and improved image quality. Experiments were conducted with simulated off‐resonance on a numerical phantom, five volunteer head scans, and four volunteer abdominal scans for validation.ResultsThe initial reconstruction of the demonstrated examples exhibit blurring artifacts and misregistration of fat and water because of the inaccuracy of the field map. The proposed method updates the field map to amend the fat and water estimation and improve image quality.ConclusionsThis work presents a model to improve the quality of fat‐water imaging of the spiral MRI by estimating a better field map from the acquired data. It allows reducing the field map pre‐scans before each spiral scan under normal circumstances to increase scan efficiency.

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.

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 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.