Eric T. Han

Q-ball reconstruction of multimodal fiber orientations using the spherical harmonic basis.

Diffusion tensor imaging (DTI) accurately delineates white matter pathways when the Gaussian model of diffusion is valid. However, DTI yields erroneous results when diffusion takes on a more complex distribution, as is the case in the brain when fiber tracts cross. High angular resolution diffusion imaging (HARDI) overcomes this limitation of DTI by more fully characterizing the angular dependence of intravoxel diffusion. Among the various HARDI methods that have been proposed, QBI offers advantages such as linearity, model independence, and relatively easy implementation. In this work, reconstruction of the q‐ball orientation distribution function (ODF) is reformulated in terms of spherical harmonic basis functions, yielding an analytic solution with useful properties of a frequency domain representation. The harmonic basis is parsimonious for typical b‐values, which enables the ODF to be synthesized from a relatively small number of noisy measurements and thus brings the technique closer to clinical feasibility from the standpoint of total imaging time. The proposed method is assessed using Monte Carlo computer simulations and compared with conventional q‐ball reconstruction using spherical RBFs. In vivo results from 3T whole‐brain HARDI of adult volunteers are also provided to verify the underlying mathematical theory. Magn Reson Med, 2006.

PROMO: Real-time prospective motion correction in MRI using image-based tracking.

Artifacts caused by patient motion during scanning remain a serious problem in most MRI applications. The prospective motion correction technique attempts to address this problem at its source by keeping the measurement coordinate system fixed with respect to the patient throughout the entire scan process. In this study, a new image‐based approach for prospective motion correction is described, which utilizes three orthogonal two‐dimensional spiral navigator acquisitions, along with a flexible image‐based tracking method based on the extended Kalman filter algorithm for online motion measurement. The spiral navigator/extended Kalman filter framework offers the advantages of image‐domain tracking within patient‐specific regions‐of‐interest and reduced sensitivity to off‐resonance‐induced corruption of rigid‐body motion estimates. The performance of the method was tested using offline computer simulations and online in vivo head motion experiments. In vivo validation results covering a broad range of staged head motions indicate a steady‐state error of less than 10% of the motion magnitude, even for large compound motions that included rotations over 15 deg. A preliminary in vivo application in three‐dimensional inversion recovery spoiled gradient echo (IR‐SPGR) and three‐dimensional fast spin echo (FSE) sequences demonstrates the effectiveness of the spiral navigator/extended Kalman filter framework for correcting three‐dimensional rigid‐body head motion artifacts prospectively in high‐resolution three‐dimensional MRI scans. Magn Reson Med, 2010.

Isotropic MRI of the knee with 3D fast spin-echo extended echo-train acquisition (XETA) : Initial experience

OBJECTIVE The purpose of our study was to prospectively compare a recently developed method of isotropic 3D fast spin-echo (FSE) with extended echo-train acquisition (XETA) with 2D FSE and 2D fast recovery FSE (FRFSE) for MRI of the knee. SUBJECTS AND METHODS Institutional review board approval, Health Insurance Portability and Accounting Act (HIPAA) compliance, and informed consent were obtained. We studied 10 healthy volunteers and one volunteer with knee pain using 3D FSE XETA, 2D FSE, and 2D FRFSE. Images were obtained both with and without fat suppression. Cartilage and muscle signal-to-noise ratio (SNR) and cartilage-fluid contrast-to-noise ratio (CNR) were compared using a Students t test. We also compared reformations of 3D FSE XETA with 2D FSE images directly acquired in the axial plane. RESULTS Cartilage SNR was higher with 3D FSE XETA (56.8 +/- 9 [SD]) compared with the 2D FSE (45.8 +/- 8, p < 0.01) and 2D FRFSE (32.5 +/- 5.3, p < 0.01). Muscle SNR was significantly higher with 3D FSE XETA (52.1 +/- 4.3) than 2D FSE (45.2 +/- 9, p < 0.01) and 2D FRFSE (23.6 +/- 6.2, p < 0.01). Fluid SNR was significantly higher for 2D FSE (144.9 +/- 33) than 3D FSE XETA (104.7 +/- 18, p < 0.01). Compared with 2D FSE and 2D FRFSE, 3D FSE XETA had lower cartilage-fluid CNR due to higher cartilage SNR (p < 0.01). Three-dimensional FSE XETA acquired volumetric data sets with isotropic resolution. Reformatted images in the axial plane were similar to axial 2D FSE acquisitions but with thinner slices. CONCLUSION Three-dimensional FSE XETA acquires high-resolution (approximately 0.7 mm) isotropic data with intermediate and T2-weighting that may be reformatted in arbitrary planes. Three-dimensional FSE XETA is a promising technique for MRI of the knee.

DWI of the spinal cord with reduced FOV single-shot EPI

Single‐shot echo‐planar imaging (ss‐EPI) has not been used widely for diffusion‐weighted imaging (DWI) of the spinal cord, because of the magnetic field inhomogeneities around the spine, the small cross‐sectional size of the spinal cord, and the increased motion in that area due to breathing, swallowing, and cerebrospinal fluid (CSF) pulsation. These result in artifacts with the usually long readout duration of the ss‐EPI method. Reduced field‐of‐view (FOV) methods decrease the required readout duration for ss‐EPI, thereby enabling its practical application to imaging of the spine. In this work, a reduced FOV single‐shot diffusion‐weighted echo‐planar imaging (ss‐DWEPI) method is proposed, in which a 2D spatially selective echo‐planar RF excitation pulse and a 180° refocusing pulse reduce the FOV in the phase‐encode (PE) direction, while suppressing the signal from fat simultaneously. With this method, multi slice images with higher in‐plane resolutions (0.94 × 0.94 mm2 for sagittal and 0.62 × 0.62 mm2 for axial images) are achieved at 1.5 T, without the need for a longer readout. Magn Reson Med 60:468–473, 2008.

In vivo 3T spiral imaging based multi‐slice T1ρ mapping of knee cartilage in osteoarthritis

T1ρ describes the spin‐lattice relaxation in the rotating frame and has been proposed for detecting damage to the cartilage collagen‐proteoglycan matrix in osteoarthritis. In this study, a multi‐slice T1ρ imaging method for knee cartilage was developed using spin‐lock techniques and a spiral imaging sequence. The adverse effect of T1 regrowth during the multi‐slice acquisition was eliminated by RF cycling. Agarose phantoms with different concentrations, 10 healthy volunteers, and 9 osteoarthritis patients were scanned at 3T. T1ρ values decreased as agarose concentration increased. T1ρ values obtained with imaging methods were compared with those obtained with spectroscopic methods. T1ρ values obtained during multi‐slice acquisition were validated with those obtained in a single slice acquisition. Reproducibility was assessed using the average coefficient of variation of median T1ρ, which was 0.68% in phantoms and 4.8% in healthy volunteers. There was a significant difference (P = 0.002) in the average T1ρ within patellar and femoral cartilage between controls (45.04 ± 2.59 ms) and osteoarthritis patients (53.06 ± 4.60 ms). A significant correlation was found between T1ρ and T2; however, the difference of T2 was not significant between controls and osteoarthritis patients. The results suggest that T1ρ relaxation times may be a promising clinical tool for osteoarthritis detection and treatment monitoring. Magn Reson Med, 2005.

Probabilistic streamline q-ball tractography using the residual bootstrap.

Q-ball imaging has the ability to discriminate multiple intravoxel fiber populations within regions of complex white matter architecture. This information can be used for fiber tracking; however, diffusion MR is susceptible to noise and multiple other sources of uncertainty affecting the measured orientation of fiber bundles. The proposed residual bootstrap method utilizes a spherical harmonic representation for high angular resolution diffusion imaging (HARDI) data in order to estimate the uncertainty in multimodal q-ball reconstructions. The accuracy of the q-ball residual bootstrap technique was examined through simulation. The residual bootstrap method was then used in combination with q-ball imaging to construct a probabilistic streamline fiber tracking algorithm. The residual bootstrap q-ball fiber tracking algorithm is capable of following the corticospinal tract and corpus callosum through regions of crossing white matter tracts in the centrum semiovale. This fiber tracking algorithm is an improvement upon prior diffusion tensor methods and the q-ball data can be acquired in a clinically feasible time frame.

In vivo T1ρ mapping in cartilage using 3D magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (3D MAPSS)

For T1ρ quantification, a three‐dimensional (3D) acquisition is desired to obtain high‐resolution images. Current 3D methods that use steady‐state spoiled gradient‐echo (SPGR) imaging suffer from high SAR, low signal‐to‐noise ratio (SNR), and the need for retrospective correction of contaminating T1 effects. In this study, a novel 3D acquisition scheme—magnetization‐prepared angle‐modulated partitioned‐k‐space SPGR snapshots (3D MAPSS)—was developed and used to obtain in vivo T1ρ maps. Transient signal evolving towards the steady‐state were acquired in an interleaved segmented elliptical centric phase encoding order immediately after a T1ρ magnetization preparation sequence. RF cycling was applied to eliminate the adverse impact of longitudinal relaxation on quantitative accuracy. A variable flip angle train was designed to provide a flat signal response to eliminate the filtering effect in k‐space caused by transient signal evolution. Experiments in phantoms agreed well with results from simulation. The T1ρ values were 42.4 ± 5.2 ms in overall cartilage of healthy volunteers. The average coefficient‐of‐variation (CV) of mean T1ρ values (N = 4) for overall cartilage was 1.6%, with regional CV ranging from 1.7% to 8.7%. The fitting errors using MAPSS were significantly lower (P < 0.05) than those using sequences without RF cycling and variable flip angles. Magn Reson Med 59:298–307, 2008.

Ankle: Isotropic MR Imaging with 3D-FSE-Cube—Initial Experience in Healthy Volunteers

The purpose of this prospective study was to compare a new isotropic three-dimensional (3D) fast spin-echo (FSE) pulse sequence with parallel imaging and extended echo train acquisition (3D-FSE-Cube) with a conventional two-dimensional (2D) FSE sequence for magnetic resonance (MR) imaging of the ankle. After institutional review board approval and informed consent were obtained and in accordance with HIPAA privacy guidelines, MR imaging was performed in the ankles of 10 healthy volunteers (four men, six women; age range, 25-41 years). Imaging with the 3D-FSE-Cube sequence was performed at 3.0 T by using both one-dimensional- and 2D-accelerated autocalibrated parallel imaging to decrease imaging time. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) with 3D-FSE-Cube were compared with those of the standard 2D FSE sequence. Cartilage, muscle, and fluid SNRs were significantly higher with the 3D-FSE-Cube sequence (P < .01 for all). Fluid-cartilage CNR was similar for both techniques. The two sequences were also compared for overall image quality, blurring, and artifacts. No significant difference for overall image quality and artifacts was demonstrated between the 2D FSE and 3D-FSE-Cube sequences, although the section thickness in 3D-FSE-Cube imaging was much thinner (0.6 mm). However, blurring was significantly greater on the 3D-FSE-Cube images (P < .04). The 3D-FSE-Cube sequence with isotropic resolution is a promising new MR imaging sequence for viewing complex joint anatomy.

Quantitative apparent diffusion coefficients and T2 relaxation times in characterizing contrast enhancing brain tumors and regions of peritumoral edema

To investigate the potential value and relationship of in vivo quantification of apparent diffusion coefficients (ADCs) and T2 relaxation times for characterizing brain tumor cellularity and tumor‐related edema.

Prospective motion correction of high-resolution magnetic resonance imaging data in children

Motion artifacts pose significant problems for the acquisition and analysis of high-resolution magnetic resonance imaging data. These artifacts can be particularly severe when studying pediatric populations, where greater patient movement reduces the ability to clearly view and reliably measure anatomy. In this study, we tested the effectiveness of a new prospective motion correction technique, called PROMO, as applied to making neuroanatomical measures in typically developing school-age children. This method attempts to address the problem of motion at its source by keeping the measurement coordinate system fixed with respect to the subject throughout image acquisition. The technique also performs automatic rescanning of images that were acquired during intervals of particularly severe motion. Unlike many previous techniques, this approach adjusts for both in-plane and through-plane movement, greatly reducing image artifacts without the need for additional equipment. Results show that the use of PROMO notably enhances subjective image quality, reduces errors in Freesurfer cortical surface reconstructions, and significantly improves the subcortical volumetric segmentation of brain structures. Further applications of PROMO for clinical and cognitive neuroscience are discussed.