Ha-Kyu Jeong

High-resolution human diffusion tensor imaging using 2-D navigated multishot SENSE EPI at 7 T.

The combination of parallel imaging with partial Fourier acquisition has greatly improved the performance of diffusion‐weighted single‐shot EPI and is the preferred method for acquisitions at low to medium magnetic field strength such as 1.5 or 3 T. Increased off‐resonance effects and reduced transverse relaxation times at 7 T, however, generate more significant artifacts than at lower magnetic field strength and limit data acquisition. Additional acceleration of k‐space traversal using a multishot approach, which acquires a subset of k‐space data after each excitation, reduces these artifacts relative to conventional single‐shot acquisitions. However, corrections for motion‐induced phase errors are not straightforward in accelerated, diffusion‐weighted multishot EPI because of phase aliasing. In this study, we introduce a simple acquisition and corresponding reconstruction method for diffusion‐weighted multishot EPI with parallel imaging suitable for use at high field. The reconstruction uses a simple modification of the standard sensitivity‐encoding (SENSE) algorithm to account for shot‐to‐shot phase errors; the method is called image reconstruction using image‐space sampling function (IRIS). Using this approach, reconstruction from highly aliased in vivo image data using 2‐D navigator phase information is demonstrated for human diffusion‐weighted imaging studies at 7 T. The final reconstructed images show submillimeter in‐plane resolution with no ghosts and much reduced blurring and off‐resonance artifacts. Magn Reson Med, 2013.

Characterizing fiber directional uncertainty in diffusion tensor MRI

Image noise in diffusion tensor MRI (DT‐MRI) causes errors in the measured tensor and hence variance in the estimated fiber orientation. Uncertainty in fiber orientation has been described using a circular “cone of uncertainty” (CU) around the principal eigenvector of the DT. The CU has proved to be a useful construct for quantifying and visualizing the variability of DT‐MRI parameters and fiber tractography. The assumption of circularity of the CU has not been tested directly, however. In this work, bootstrap analysis and simple theoretical arguments were used to show that the CU is elliptical and multivariate normal in the vast majority of white matter (WM) voxels for typical measurement conditions. The dependence of the cone angle on the signal‐to‐noise ratio (SNR) and eigenvalue contrast was established. The major and minor cone axes are shown to be coincident with the second and third eigenvectors of the tensor, respectively, in the limit of many uniformly spaced diffusion‐encoding directions. The deviation between the major cone axis and the second eigenvector was quantified for typical sets of diffusion‐weighting (DW) directions. The elliptical CU provides more realistic error information for fiber‐tracking algorithms and a quantitative basis for selecting DT imaging acquisition protocols. Magn Reson Med 60:1408–1421, 2008.

Effects of MR Parameter Changes on the Quantification of Diffusion Anisotropy and Apparent Diffusion Coefficient in Diffusion Tensor Imaging: Evaluation Using a Diffusional Anisotropic Phantom

Objective To validate the usefulness of a diffusional anisotropic capillary array phantom and to investigate the effects of diffusion tensor imaging (DTI) parameter changes on diffusion fractional anisotropy (FA) and apparent diffusion coefficient (ADC) using the phantom. Materials and Methods Diffusion tensor imaging of a capillary array phantom was performed with imaging parameter changes, including voxel size, number of sensitivity encoding (SENSE) factor, echo time (TE), number of signal acquisitions, b-value, and number of diffusion gradient directions (NDGD), one-at-a-time in a stepwise-incremental fashion. We repeated the entire series of DTI scans thrice. The coefficients of variation (CoV) were evaluated for FA and ADC, and the correlation between each MR imaging parameter and the corresponding FA and ADC was evaluated using Spearmans correlation analysis. Results The capillary array phantom CoVs of FA and ADC were 7.1% and 2.4%, respectively. There were significant correlations between FA and SENSE factor, TE, b-value, and NDGD, as well as significant correlations between ADC and SENSE factor, TE, and b-value. Conclusion A capillary array phantom enables repeated measurements of FA and ADC. Both FA and ADC can vary when certain parameters are changed during diffusion experiments. We suggest that the capillary array phantom can be used for quality control in longitudinal or multicenter clinical studies.

Improved diffusion tensor imaging of the optic nerve using multishot two‐dimensional navigated acquisitions

A diffusion‐weighted multishot echo‐planar imaging approach combined with SENSE and a two‐dimensional (2D) navigated motion correction was investigated as an alternative to conventional single‐shot counterpart to obtain optic nerve images at higher spatial resolution with reduced artifacts.

Changes in Integrity of Normal-Appearing White Matter in Patients with Moyamoya Disease: A Diffusion Tensor Imaging Study

BACKGROUND AND PURPOSE: DTI is widely used for the evaluation of white matter integrity in various neurologic diseases. The purpose of this study was to investigate changes in white matter integrity by using DTI in NAWM of patients with MMD and to evaluate the correlation between diffusion and perfusion characteristics through an interhemispheric comparison. MATERIALS AND METHODS: We retrospectively reviewed 20 primary MMD patients with asymmetric disease stage and 20 age-matched healthy controls. FACS and ADCCS values of bilateral centrum semiovale were measured by using region of interest analysis. Mean FACS and ADCCS were compared between patient and control groups by unpaired t test. Interhemispheric differences in FACS and ADCCS were assessed and compared between the H-TTPdelayed and the H-TTPshorter by using paired t test. AIs also were assessed to verify interhemispheric differences. RESULTS: The patient group showed a significantly lower mean FACS and a higher mean ADCCS value than the control group. In the patient group, the H-TTPdelayed had a significantly lower FACS and higher ADCCS value than the H-TTPshorter. Both AIFA and AIADC were significantly higher in the patient compared with the control group. CONCLUSIONS: DTI can describe subtle changes in white matter integrity in NAWM of patients with primary MMD that are not detected by conventional MR imaging. In addition, diffusion characteristics are well correlated with perfusion characteristics. We believe that DTI is a useful ancillary tool to evaluate patients with MMD.

A comparison of readout segmented EPI and interleaved EPI in high-resolution diffusion weighted imaging

PURPOSE To provide a comprehensive understanding of multi-shot EPI diffusion imaging methods by comparing Readout segmented EPI (RS-EPI) and interleaved EPI (iEPI). MATERIALS AND METHODS RS-EPI and iEPI were compared on the same 3T scanner. A 2D navigator was used for both RS-EPI and iEPI for phase correction. Signal to noise ratio (SNR), fractional anisotropy (FA) and distortion level were compared using phantom data. Distortion reduction capability and scan efficiency were compared with different protocols with simulations. In addition, distortion reduction capability and diffusion tensor imaging performance were compared using in vivo data. RESULTS Our phantom data showed that the mean SNRs were 50.5, 86.6 and 45.4 for RS-EPI using GRAPPA=3, fully sampled iEPI and iEPI using GRAPPA=2 respectively. The mean FA values were 0.08, 0.05 and 0.09 for RS-EPI using GRAPPA=3, fully sampled iEPI and iEPI using GRAPPA=2 respectively. The distortion levels were 1.34mm, 1.29mm and 0.61mm for RS-EPI using GRAPPA=3, fully sampled iEPI and iEPI using GRAPPA=2 respectively. The effective echo spacing could be reduced by increasing the number of shots for both methods but more prominent for iEPI. The scan time was approximately proportional to the number of shots for both methods and RS-EPI showed a shorter scan time. Our in vivo data for distortion comparison showed consistent results with the effective echo spacing study. The mean difference of the FA and MD values between the high resolution sequences and SS-EPI was all within 7%. CONCLUSION For high resolution diffusion imaging, iEPI has more potential in distortion reduction than RS-EPI when increasing the number of shots. RS-EPI can achieve a reasonable SNR with a shorter scan time than iEPI. RS-EPI and iEPI have similar performance in FA and MD quantifications as well as showing structure details when using eleven shots for in vivo diffusion tensor imaging.

Integrating Medical Imaging Analyses through a High-throughput Bundled Resource Imaging System.

Exploitation of advanced, PACS-centric image analysis and interpretation pipelines provides well-developed storage, retrieval, and archival capabilities along with state-of-the-art data providence, visualization, and clinical collaboration technologies. However, pursuit of integrated medical imaging analysis through a PACS environment can be limiting in terms of the overhead required to validate, evaluate and integrate emerging research technologies. Herein, we address this challenge through presentation of a high-throughput bundled resource imaging system (HUBRIS) as an extension to the Philips Research Imaging Development Environment (PRIDE). HUBRIS enables PACS-connected medical imaging equipment to invoke tools provided by the Java Imaging Science Toolkit (JIST) so that a medical imaging platform (e.g., a magnetic resonance imaging scanner) can pass images and parameters to a server, which communicates with a grid computing facility to invoke the selected algorithms. Generated images are passed back to the server and subsequently to the imaging platform from which the images can be sent to a PACS. JIST makes use of an open application program interface layer so that research technologies can be implemented in any language capable of communicating through a system shell environment (e.g., Matlab, Java, C/C++, Perl, LISP, etc.). As demonstrated in this proof-of-concept approach, HUBRIS enables evaluation and analysis of emerging technologies within well-developed PACS systems with minimal adaptation of research software, which simplifies evaluation of new technologies in clinical research and provides a more convenient use of PACS technology by imaging scientists.

Technical Note: Clustering-based Motion Compensation Scheme for Multishot Diffusion Tensor Imaging

PURPOSE To extend image reconstruction using image-space sampling function (IRIS) to address large-scale motion in multishot diffusion-weighted imaging (DWI). METHODS A clustered IRIS (CIRIS) algorithm that would extend IRIS was proposed to correct for large-scale motion. For DWI, CIRIS initially groups the shots into clusters without intracluster large-scale motion and reconstructs each cluster by using IRIS. Then, CIRIS registers these cluster images and combines the registered images by using a weighted average to correct for voxel mismatch caused by intercluster large-scale motion. For diffusion tensor imaging (DTI), CIRIS further reduces the effect of motion on diffusion directions by treating motion-induced direction changes as additional diffusion directions. CIRIS also introduces the detection and rejection of motion-corrupted data to avoid corresponding image degradation. The proposed method was evaluated by simulation and in vivo diffusion datasets. RESULTS Experiments demonstrated that CIRIS can reduce motion-induced blurring and artifacts in DWI and provide more accurate DTI estimations in the presence of large-scale motion, compared with IRIS. CONCLUSION The proposed method presents a novel approach to correct for large-scale in-plane motion for multishot DWI and is expected to benefit the practical application of high-resolution diffusion imaging.

Reduction of Oxygen-Induced CSF Hyperintensity on FLAIR MR Images in Sedated Children: Usefulness of Magnetization-Prepared FLAIR Imaging.

BACKGROUND AND PURPOSE: Oxygen-induced CSF hyperintensity on FLAIR MR imaging is often observed in sedated children. This phenomenon can mimic leptomeningeal pathology and lead to a misdiagnosis. The purpose of this study was to investigate whether magnetization-prepared FLAIR MR imaging can reduce oxygen-induced CSF hyperintensity and improve image quality compared with conventional (non-magnetization-prepared) FLAIR MR imaging. MATERIALS AND METHODS: Bloch simulation for magnetization-prepared and non-magnetization-prepared FLAIR sequences was performed for tissue contrast. We retrospectively reviewed 85 children with epilepsy who underwent MR imaging under general anesthesia with supplemental oxygen (41 with non-magnetization-prepared FLAIR and 44 with magnetization-prepared FLAIR). CSF hyperintensity was scored from 0 to 3 points according to the degree of CSF signal intensity and was compared between the 2 sequences. The contrast-to-noise ratios among GM, WM, and CSF were evaluated to assess general image quality from both sequences. To assess the diagnostic accuracy for hemorrhage, we reviewed an additional 25 patients with hemorrhage. RESULTS: Bloch simulation demonstrated that CSF hyperintensity can be reduced on magnetization-prepared FLAIR compared with non-magnetization-prepared FLAIR. CSF hyperintensity scores were significantly lower in magnetization-prepared FLAIR than in non-magnetization-prepared FLAIR (P < .01). The contrast-to-noise ratios for GM-WM, GM-CSF, and WM-CSF were significantly higher in magnetization-prepared FLAIR than in non-magnetization-prepared FLAIR (P < .05). Hemorrhage was clearly demarcated from CSF hyperintensity in the magnetization-prepared group (100%, 12/12) and non-magnetization-prepared group (38%, 5/13). CONCLUSIONS: Magnetization-prepared 3D-FLAIR MR imaging can significantly reduce oxygen-induced CSF artifacts and increase the tissue contrast-to-noise ratio beyond the levels achieved with conventional non-magnetization-prepared 3D-FLAIR MR imaging.

Improved DTI of the optic nerve using multishot 2-D navigated acquisitions

A diffusion‐weighted multishot echo‐planar imaging approach combined with SENSE and a two‐dimensional (2D) navigated motion correction was investigated as an alternative to conventional single‐shot counterpart to obtain optic nerve images at higher spatial resolution with reduced artifacts.