Junmin Liu

Intervention-based multidimensional phase unwrapping using recursive orthogonal referring

We present a new intervention‐based phase unwrapping algorithm, which solves the inherent integration‐path‐dependent problem (typically resulting in streaks), by using a 2D recursive orthogonal referring (PUROR) approach. The streaks were removed by three consecutive procedures: intra‐image phase unwrapping, inter‐image cross‐referring a “good‐strip,” and cross‐referring line segments. The application of these procedures results in streak‐free 2D phase images. The phase inconsistencies across slices in a 3D image were removed using a hybrid 3D PUROR algorithm: the two step approach involves stacking the individual slices, by using the mean phase values of each slice, then applying the 2D PUROR algorithm to reformatted 2D images that include the slice direction. The described approach was tested with in vivo multislice phase images acquired in the axial, sagittal, and coronal orientation. The results of the unwrapped phase volume recovered using the PUROR algorithm have equivalent quality to that achieved by using established methods, but the PUROR algorithm is about two orders of magnitude faster (between 1 and 5 s per 256 × 256 slice; independent of slice orientation and echo time). Magn Reson Med, 2012.

Phase-unwrapping algorithm for translation extraction from spherical navigator echoes

Spherical navigator echoes have been shown to determine rigid‐body rotation and translation simultaneously. Following the determination of rotation, translations are determined from the phase change between the baseline and transformed spherical navigator echoes. Because the measured phase change is limited in the interval (−π, π), a phase‐unwrapping algorithm is required to recover the true phase change in absolute values. The unwrapping algorithm presented in this article is based on a priori information about the true translation‐induced phase‐change function. The algorithm is verified using simulation and in vivo experiments, and the accuracy and precision of translation determination are evaluated. Specifically, the effects of background and off‐resonance‐induced phase noise are explored. When the proposed phase‐unwrapping algorithm was used, translations up to 15 mm were measured, with accuracy better than 5%; for translations up to 40 mm, an error of approximately 10% was observed. Magn Reson Med, 2010.

Inter-echo variance as a weighting factor for multi-channel combination in multi-echo acquisition for local frequency shift mapping

To develop and evaluate a local frequency shift (LFS) mapping method specifically designed for multi‐echo acquisitions and multi‐channel receive coils.

Rapid six-degree-of-freedom motion detection using prerotated baseline spherical navigator echoes.

A new spherical navigator echo (SNAV) registration technique is presented. This technique starts by collecting a set of SNAV templates at a reference position. These templates are acquired by rotating the gradient system to result in rotation angles that uniformly cover a predefined range of rotation. The rotation angles between an unknown physically transformed position and the reference position are subsequently determined by finding the template with the lowest sum of squared differences with SNAV at the transformed position. Translations are calculated from the phase differences between the best‐match SNAV template and the SNAV acquired at the transformed position. In comparison with the conventional SNAV registration technique, the proposed technique is noniterative, robust, and can detect 3‐dimensional rigid body motion in less than 50 msec. The technique was verified with phantom and in vivo experiments, which demonstrated subdegree rotational and submillimeter translational accuracy over a range of simultaneous ±20° and ±10° mm of motion. Magn Reson Med, 2011.

Combination of multidimensional navigator echoes data from multielement RF coil

Until now, only one‐dimensional navigator‐echo techniques have been implemented with multielement RF coils. For the multidimensional navigator echoes, which extract six‐degree of freedom motion information from the raw k‐space data, an efficient raw data combination approach is needed. In this work, three combination approaches, including summation of the complex raw data, summation following phase alignment, and summation of the squares of the k‐space magnitude profiles, were evaluated with the spherical navigator echoes (SNAV) technique. In vivo brain imaging experiments were used to quantify accuracy and precision and demonstrated that SNAVs acquired with an eight‐channel head coil can determine the rotation and translation in range up to 10° and 20 mm with subdegree and submillimeter accuracy, respectively. Results from a 3D brain volume realignment experiment showed excellent agreement between baseline images and SNAV‐aligned follow‐up volumes. Magn Reson Med, 2010.

Susceptibility-weighted imaging using inter-echo-variance channel combination for improved contrast at 7 tesla.

To implement and optimize a new approach for susceptibility‐weighted image (SWI) generation from multi‐echo multi‐channel image data and compare its performance against optimized traditional SWI pipelines.

Design and evaluation of an MRI-compatible linear motion stage

PURPOSE To develop and evaluate a tool for accurate, reproducible, and programmable motion control of imaging phantoms for use in motion sensitive magnetic resonance imaging (MRI) appli cations. METHODS In this paper, the authors introduce a compact linear motion stage that is made of nonmagnetic material and is actuated with an ultrasonic motor. The stage can be positioned at arbitrary positions and orientations inside the scanner bore to move, push, or pull arbitrary phantoms. Using optical trackers, measuring microscopes, and navigators, the accuracy of the stage in motion control was evaluated. Also, the effect of the stage on image signal-to-noise ratio (SNR), artifacts, and B0 field homogeneity was evaluated. RESULTS The error of the stage in reaching fixed positions was 0.025 ± 0.021 mm. In execution of dynamic motion profiles, the worst-case normalized root mean squared error was below 7% (for frequencies below 0.33 Hz). Experiments demonstrated that the stage did not introduce artifacts nor did it degrade the image SNR. The effect of the stage on the B0 field was less than 2 ppm. CONCLUSIONS The results of the experiments indicate that the proposed system is MRI-compatible and can create reliable and reproducible motion that may be used for validation and assessment of motion related MRI applications.

Retrospective 3D motion correction using spherical navigator echoes

PURPOSE To develop and evaluate a rapid spherical navigator echo (SNAV) motion correction technique, then apply it for retrospective correction of brain images. METHODS The pre-rotated, template matching SNAV method (preRot-SNAV) was developed in combination with a novel hybrid baseline strategy, which includes acquired and interpolated templates. Specifically, the SNAV templates are only rotated around X- and Y-axis; for each rotated SNAV, simulated baseline templates that mimic object rotation about the Z-axis were interpolated. The new method was first evaluated with phantom experiments. Then, a customized SNAV-interleaved gradient echo sequence was used to image three volunteers performing directed head motion. The SNAV motion measurements were used to retrospectively correct the brain images. Experiments were performed using a 3.0T whole-body MRI scanner and both single and 8-channel head coils. RESULTS Phantom rotations and translations measured using the hybrid baselines agreed to within 0.9° and 1mm compared to those measured with the original preRot-SNAV method. Retrospective motion correction of in vivo images using the hybrid preRot-SNAV effectively corrected for head rotation up to 4° and 4mm. CONCLUSIONS The presented hybrid approach enables the acquisition of pre-rotated baseline templates in as little as 2.5s, and results in accurate measurement of rotations and translations. Retrospective 3D motion correction successfully reduced motion artifacts in vivo.

Single multi-echo GRE acquisition with short and long echo spacing for simultaneous quantitative mapping of fat fraction, B0 inhomogeneity, and susceptibility

&NA; Multi‐echo gradient echo (mGRE) sequences have been widely adapted in clinical and scientific practice for different purposes to their capability of performing Dixon MRI, generating multi‐contrast images and extracting multi‐parametric maps. This work aims to extend mGRE‐based techniques for imaging whole head, where further technical developments are required due to the co‐existence of fat and large B0 inhomogeneity in regions such as the skull base and neck. Specifically, bipolar mGRE data were acquired with a single sequence that contains both a short echo‐spacing (&Dgr;TE) echo train to capture water‐fat and B0 phase shifts (for proton density fat‐fraction (FF) and B0 mapping) and a longer &Dgr;TE echo train (and long echo times) to capture subtle susceptibility variations and R2* information. The mGRE images covering the whole head (spatial resolution 1.0 × 1.0 × 2.0 mm3) were acquired in 5 min. An automated processing pipeline was implemented to use the FF and B0 maps determined from the short‐TE train to compensate for the effects of fat, remove the background phase for whole‐head quantitative susceptibility mapping, and reduce the difficulty of spatial phase unwrapping of the long echo‐time data. Data from healthy volunteers imaged on a 3 T scanner along with phantom validation are presented. Co‐registered quantitative multi‐parametric maps (FF, B0 inhomogeneity, R2*, local frequency shift and quantitative susceptibility) and multi‐contrast images covering the whole head were successfully generated in processing times of several minutes. HighlightsA dual echo train mGRE sequence to capture water‐fat and B0 phase shifts as well as susceptibility and R2* is proposed.Multi‐parametric maps automatically generated from a whole head (and neck) bipolar mGRE acquisition.Accurate fat fraction and susceptibility estimation achieved simultaneously.Presented acquisition/processing method has potential to replace set of currently separated acquisitions that measure FF, R2*, SWI and QSM in neuroradiology.

Morphology-Specific Discrimination between MS White Matter Lesions and Benign White Matter Hyperintensities Using Ultra-High-Field MRI

BACKGROUND AND PURPOSE: Recently published North American Imaging in Multiple Sclerosis guidelines call for derivation of a specific radiologic definition of MS WM lesions and mimics. The purpose of this study was to use SWI and magnetization-prepared FLAIR images for sensitive differentiation of MS from benign WM lesions using the morphologic characteristics of WM lesions. MATERIALS AND METHODS: Seventeen patients with relapsing-remitting MS and 18 healthy control subjects were enrolled retrospectively. For each subject, FLAIR and multiecho gradient-echo images were acquired using 7T MR imaging. Optimized postprocessing was used to generate single-slice SWI of cerebral veins. SWI/FLAIR images were registered, and 3 trained readers performed lesion assessment. Morphology, location of lesions, and the time required for assessment were recorded. Analyses were performed on 3 different pools: 1) lesions of >3 mm, 2) nonconfluent lesions of >3 mm, and 3) nonconfluent lesions of >3 mm with no or a single central vein. RESULTS: The SWI/FLAIR acquisition and processing protocol enabled effective assessment of central veins and hypointense rims in WM lesions. Assessment of nonconfluent lesions with ≥1 central vein enabled the most specific and sensitive differentiation of patients with MS from controls. A threshold of 67% perivenous WM lesions separated patients with MS from controls with a sensitivity of 94% and specificity of 100%. Lesion assessment took an average of 12 minutes 10 seconds and 4 minutes 33 seconds for patients with MS and control subjects, respectively. CONCLUSIONS: Nonconfluent lesions of >3 mm with ≥1 central vein were the most sensitive and specific differentiators between patients with MS and control subjects.