Matthew R Orton¹, David J Collins¹, Christina Messiou¹, Jean Tessier², and Martin O Leach^1
1CR-UK and EPSRC Cancer Imaging Centre, Institute of Cancer Research, Sutton, Surrey, United Kingdom, ²Formerly with Early Clinical Development, AstraZeneca, Alderley Park, Macclesfield, United Kingdom

Diffusion Weighted Imaging with multiple b-values is being used more widely in many application areas, including clinical trials for assessment of novel cancer therapeutics. Complex models are being increasingly used to extract more information from the data, and the suitability and interpretation of these is a matter of current debate. In this abstract we present results obtained from data acquired in a phase I clinical trial setting which demonstrate the effect of the diffusion model complexity, fitting methodology and tumour volume on the repeatability of functional parameter estimates.

He Wang¹, Yong Zhang¹, and Guang Cao^1
1Global Applied Science Laboratory, GE Healthcare, Shanghai, Shanghai, China, People's Republic of

A novel continuously distributed exponential model was explored to quantify diffusion in normal human brain. Diffusion-weighted images were acquired with twenty-one b-values ranging from 0 to 4000 s/mm2 to calculate the continuous distribution of diffusion coefficients in white matter, gray matter and cerebrospinal fluid. White matter showed different diffusion rates among three orthogonal orientations as contrast to CSF and gray matter with similar diffusion rates in all directions. More widely dispersed diffusion distribution in white matter benefits from the application of the continuously distributed exponential model. This technique may be used to characterize diffusion heterogeneity in different types of tumors.

Cheng Guan Koay^1,2, Andrew L. Alexander¹, and M. Elizabeth Meyerand^1
1Department of Medical Physics, University of Wisconsin-Madison, Madison, WI, United States, ²STBB, National Institutes of Health, Bethesda, MD, United States

Registration of DW and non-DW images is a critical data-analytics step. In this study, we proposed a novel DW and non-DW image registration strategy that provides the much needed information on the goodness-of-registration of DWI images to non-DWI (T2-weighted image or template) in terms of tensor estimation metric within the registration process. This strategy requires very minimal modification to the existing acquisition of DTI except two distinct b-values and is built upon weighted linear least squares estimation for computational efficiency consideration. Reduction in registration error as computed from the metric was about 48.6% for the experimental data set we tested.

Jin Kyu Gahm^1,2, Nicholas Wisniewski³, William S Klug⁴, Alan Garfinkel^3,5, and Daniel B Ennis^1,6
1Department of Radiological Sciences, University of California, Los Angeles, CA, United States, ²Department of Computer Science, University of California, Los Angeles, CA, United States, ³Department of Medicine, University of California, Los Angeles, CA, United States, ⁴Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, CA, ⁵Department of Physiological Science, University of California, Los Angeles, CA, United States, ⁶Biomedical Engineering Interdepartmental Program, University of California, Los Angeles, CA, United States

DT-MRI interpolation is the process of estimating diffusion tensors at arbitrary points in space from regularly sampled tensor data. Tensor interpolation is important for tensor-based fiber tractography, registration, volume rendering, and computational model building. In this work, we use bootstrap statistical methods to compare four different DT-MRI interpolation methods’ accuracies for recovering the tensor shape (invariants) and orientation of unknown tensors from known tensor data. By using a cardiac DT-MRI dataset, we show the statistical bootstrapping results for the paired comparisons, and present recommendations for the selection of the DT-MRI interpolation method.

Catherine Lebel¹, Thomas Benner², and Christian Beaulieu^3
1Biomedical Engineering, University of Alberta, Edmonton, AB, Canada, ²Athinoula Martinos Center for Functional and Structural Biomedical Imaging, Harvard University, Boston, MA, United States, ³Biomedical Engineering, University of Alberta, Edmonton, Alberta, Canada

Simulations suggest that using more than the minimum 6 diffusion-encoding gradient directions provides more robust DTI parameter estimates. Deterministic tractography is often criticized for using only 6 directions despite the lack of experimental evaluation. Diffusion measurements were compared between 6 versus 30 directions in healthy volunteers for 12 major white matter tracts. FA, MD and volume differences were very minimal (1/12 tracts had FA or MD differences, 2/12 had different volumes). These results suggest that 6-direction DTI data with adequate SNR is sufficient for FA, MD, and volume measurements and thus should not be considered inferior for population DTI studies.

Hu Cheng¹, Dae-Jin Kim¹, Olaf Sporns¹, Yang Wang², Jinhua Sheng², and Andrew Saykin^2
1Indiana University, Bloomington, IN, United States, ²Indiana University, Indianapolis, IN, United States

DTI has been used for mapping the structural network of human brain. The network is constructed by choosing various brain regions as nodes and fiber tracts connecting the regions as links. To investigate the effect of DTI SNR on the constructed network, a set of DTI data with different SNR was created from the 48 direction dataset. The result shows that SNR affects the accuracy of network while has less effect on the variation.

Rafael Luis O'Halloran¹, Samantha Holdsworth¹, and Roland Bammer^1
1Radiology, Stanford University, Palo Alto, CA, United States

One of the fundamental problems with multi-shot diffusion weighted MRI is phase mismatch between shots. The ultimate source of these errors is motion, however, one component of it is reproducible and correlated with the cardiac cycle. In this work the reproducibility of cardiac induced phase is investigated with a rapid time-resolved single-shot gradient echo DW-EPI. Phase maps showed that, although there is a large component of the phase that is highly correlated with the cardiac cycle, there are still residual variations in the phase that do not correlate.

Lin-Ching Chang¹, Lindsay Walker², Babak Behseta³, and Carlo Pierpaoli^2
1Department of Electrical Engineering and Computer Science, The Catholic University of America, Washington, DC, United States, ²STBB, NICHD, National Institutes of Health, Bethesda, Maryland, United States, ³Pediatric & Developmental Neuroscience Branch, NIMH, National Institutes of Health, Bethesda, Maryland, United States

Artifacts are common in diffusion weighted images (DWIs) especially those originating from cardiac pulsation in ungated acquisitions and from subject motion. Neglecting to account for them can result in erroneous diffusion tensor values. The Robust Estimation of Tensors by Outlier Rejection (RESTORE) is an effective method for improving tensor estimation presence of artifactual data points. However, RESTORE relies heavily on data redundancy (large DWIs datasets), which may not be acquired in some clinical settings. This paper proposes a method called informed RESTORE (iRESTORE) that incorporates the notion that physiological noise artifacts are more likely to results in signal drops rather than signal increases to achieve an accurate rejection of artifactual data points in low redundancy DWIs datasets.

Kim Mouridsen¹, Leif Østergaard¹, Søren Christensen², and Sune Nørhøj Jespersen^1
1Center for Functionally Integrative Neuroscience, Aarhus University, Aarhus University Hospital, Aarhus, Denmark, ²Department of Neurology, Royal Melbourne Hospital, Melbourne, Australia

DSC-MRI has proven very useful for characterizing tissue perfusion indices such as CBF, CBV and MTT. However, whereas CBF and MTT only represent average capillary flow and transit time, important physiological parameters such as flow heterogeneity and oxygen extraction capacity depend on the shape characteristics of the complete distribution of transit times. We demonstrate in a simulation study, that by using a Bayesian estimation approach and a parameterization of the residue function based on a vascular model, shape characteristics of the residue function can be reliably estimated for a range of microvascular flow patterns.

Nicolas Pannetier^1,2, Clément Debacker^1,2, Franck Mauconduit^1,2, Thomas Christen^1,3, and Emmanuel Luc Barbier^1,2
1U836, INSERM, Grenoble, France, ²Grenoble Institut des Neurosciences, Université Joseph Fourier, Grenoble, France, ³Department of Radiology, Stanford University, Stanford, California, United States

Quantification of Dynamic Susceptibility Contrast MRI remains a challenge when contrast agent (CA) extravasates into the interstitium. Indeed, CA extravasation increases tissue R1 relaxation(signal enhancement) and competes with R2* increase (signal destruction) used to compute perfusion estimates. However, the consequences of CA extravasation on R2* are often overlooked. CA extravasation reduces the magnetic susceptibility difference at the intra-/extravascular interface and CA extravasation yields to the emergence of magnetic susceptibility differences at the interfaces between cells and interstitium.In this study, we evaluated these two competing R2* effects in case of blood-brain barrier permeability using numerical simulations for various porosities and cell sizes.

Lin-Wei Hsu¹, Yeng-Peng Liao¹, and Ho-Ling Liu^1,2
1Institute of Medical Physics and Imaging Science, Chang Gung University, Taoyuan, Taiwan, ²Department of Medical Imaging and Intervention, Chang Gung Memorial Hospital, Taoyuan, Taiwan

Mathematical models have been previously developed to correct for T1 and T2 effects originated from contrast agent extravasations in dynamic susceptibility contrast MRI, and showed additional potentials for assessing the vessel permeability. This study aimed to assess variability of the blood volume (BV) correction and permeability estimation using computer simulations based on varied SNR, permeability and whether baseline T1 is available. The results showed that quality of BV correction and permeability estimation depended on both SNR and severity of contrast agent leakage. Accurate measurement of baseline T1 is especially helpful for the situation of high permeability and low SNR acquisition.

Natenael B Semmineh¹, Junzhong Xu¹, and Christopher Chad Quarles^1
1Radiology and Radiological Sciences, Vanderbilt University, Nashville, TN, United States

Given the heterogeneous nature of blood vessels within tissue and the dependence of susceptibility field gradients on vascular geometry the assumption that a linear relationship, with a spatially uniform rate constant termed the vascular susceptibility calibration factor (kp), exists between the CA concentration and the measured transverse relaxation rate change, could significantly impact the reliability of DSC-MRI hemodynamic measurements. We propose the use of an efficient computational method to estimate the extent of kp heterogeneity across normal and tumor tissue and to assess the reliability of DSC-MRI measures of blood volume and blood flow.

Abby Ying Ding^1,2, and Ed X Wu^1,2
1Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Hong Kong, Hong Kong SAR, China, People's Republic of, ²Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, Hong Kong SAR, China, People's Republic of

The accuracy of DTI quantitation can directly affect the interpretation of underlying biological microstructures. DTI measurements can be confounded by the presence of vasculature and blood perfusion. So far, little is known about the extent of such effects in quantitative DTI. In this study, we quantitatively examined the effect of hypercapnia on quantitative DTI indices in vivo in rodent brains. DTI indices were found to alter during hypercapnia, including various diffusivities and FA. These findings indicated that alterations in physiologic condition, vascular characteristics and hemodynamic regulations can affect the in vivo quantitation of various DTI indices. Therefore, caution must be taken in designing experiments and interpreting DTI indices.

Debbie Anaby¹, Ian D Duncan², and Yoram Cohen^1
1School of Chemistry, Tel Aviv University, Tel Aviv, Israel, ²School of Veterinary Medicine, University of Wisconsin-Madison, Madison, Wisconsin, United States

The contribution of myelin on the observed diffusion anisotropy of water in neuronal tissue is not clear. Here we used high-b-value q-space DWI to probe the effects of myelin on diffusion indices in Long Evans shaker (les) and control rat brains. We compared the QSI indices with the FA which was extracted from the low-b-value regime of the QSI data. We found that the QSI indices are significantly different between the groups while the FA blurs the differences between them. Therefore, high b-value QSI should be considered as a means for achieving better distinction between les and control brain tissues.

Shin-Lei Peng¹, Chih-Kuang Yeh¹, Chung-Hsin Wang¹, Hsu-Hsia Peng¹, and Fu-Nien Wang^1
1Department of Biomedical Engineering and Environme, National Tsing Hua University, Hsin-Chu, Taiwan

Recently, microbubbles were used as a MR susceptibility contrast agent because of the susceptibility differences by the gas-liquid interface. However, the relationship between rCBV deriving from Gd-DTPA and that from microbubbles has not been investigated. In this study, we aim to investigate correlation between rCBV from both contrast agents and find the key factor influencing the correlation. The results showed that the time-to-peak for microbubbles increases, the correlation between microbubbles and Gd-DTPA increases. When time-to-peak of microbubbles increases, it may mean that more microbubbles aggregate and trap in local tissue vasculature. As a result, the stronger susceptibility effect will be produced and lead to high correlate with Gd-DTPA.

Yi-Ling Wu¹, Chien-Chung Chen¹, Yi-Chun Wu¹, Chia-Hao Chang¹, and Fu-Nien Wang^1
1Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, Taiwan

In this work, we take a closer look at the different integrated parts of CT, such as the anterior and posterior part of CT peak, direct summation of first pass part, the fitted gamma-variate curve (conventional CBV map), recirculation part, and finally the whole curve. Since the extravasation supposed to be minor in the short duration of DSC imaging, we confirmed the feasibility of integrating the total area under the CT curve including both the first pass and recirculation parts could further enrich the data points and to enhance the reliability of measured CBV.

Marco Castellaro¹, Denis Peruzzo¹, Massimiliano Calabrese², Francesca Rinaldi², Valentina Bernardi², Alice Favaretto², Irene Mattisi², Paolo Gallo², and Alessandra Bertoldo^1
1Department of Information Engineering, University of Padova, Padova, Italy, ²Multiple Sclerosis Centre, Department of Neuroscience, University of Padova, Padova, Italy

In this Dynamic Susceptibility Contrast (DSC-MRI) study we investigated the hemodynamic changes that may occur in Cortical Lesions (CLs) in 20 patients with Relapsing Remitting multiple sclerosis. 91% of the CLs show a statistical significant decreased CBF (-53±21%, p<0.001) when compared to the Normal Appearing Grey Matter CBF values. The remaining 9% of the CLs show a statistical significant higher CBF (+39±30% p < 0.05). This confirms neuropathological observations showing a proportion of active CLs characterized by a high degree of inflammated cells. On the other hand, the ipoperfused CLs may suggests a neuronal damage and loss.

E. M. Haacke¹, Meng Li¹, and Flavia Juvvigunta^1
1Department of Radiology, Wayne State University, Detroit, Michigan, United States

Multiple Sclerosis is a chronic inflammatory demyelinating disease of the central nervous system. The cerebral perfusion in MS is reduced with the function of severity of the disease. In this study we use a new approach to processing PWI data that we call Tissue Similarity Maps. It can identify those lesions linked by the same vascular response to the contract agent. Our results showed that the TSM of nulling the MS lesions is successful in enhancing specifically MS lesions. It is a very useful means to reveal information about tissues otherwise difficult to see with conventional PWI processing approaches.

Egbert J.W. Bleeker¹, Andrew G. Webb¹, Marianne A.A. van Walderveen², Mark A. van Buchem^1,2, and Matthias J.P. van Osch^1
1Radiology, C.J. Gorter Center for high field MRI, Leiden University Medical Center, Leiden, Netherlands, ²Radiology, Leiden University Medical Center, Leiden, Netherlands

Local or regional arterial input function (AIF) measurements aim for voxel specific AIFs from small arteries. These local AIFs are assumed to reflect the true input of the microvasculature much better than global AIFs. However, do the measured local AIFs reflect the true concentration-time curve of small arteries? The current study investigated this question with numerical modeling that simulated partial volume effects in local AIF measurements. In addition, local AIF candidates selected using angiograms were evaluated in vivo. The findings suggest that local AIF measurements do not reflect the true concentration-time curve in small arteries.

Adam Martin Winchell^1,2, Ralf B Loeffler¹, Ruitian Song¹, Himanshu Bhat³, Michael Hamm³, Alberto Broniscer⁴, and Claudia M Hillenbrand^1
1Radiological Sciences, St. Jude Children's Research Hospital, Memphis, TN, United States, ²Biomedical Engineering, University of Memphis, Memphis, TN, United States,³Siemens Healthcare, Charlestown, MA, United States, ⁴Oncology, St. Jude Children's Research Hospital, Memphis, TN, United States

The aim of our study was to compare two DSC methods, a manual global and an automatic local AIF approach, and examine their calculated regional perfusion differences as a function of vascular territory.

Bryce Wilkins¹, Namgyun Lee¹, and Manbir Singh^1
1Radiology and Biomedical Engineering, University of Southern California, Los Angeles, California, United States

The effect of truncated-sphere sampling commonly used in DSI to estimate single fiber orientation errors is simulated using fully-sampled (7x7x7) q-space data (343 samples), truncated-sphere (203 samples), zero-padding to (21x21x21), noise-free and noisy (SNR = 30) scenarios. We show that the error of resolving a single fiber direction depends on the fiber orientation, and the “corners of q-space”, which are not sampled when truncated sampling schemes are used, help reduce error (mean error 3.5deg versus 3.9deg for the noise case and zero-padding used). As truncated-sphere sampling requires 40% less time, its use could justify the small increase in error.

Silvia De Santis^1,2, Yaniv Assaf³, Christopher John Evans¹, and Derek K Jones^1
1CUBRIC, School of psychology, CARDIFF University, United Kingdom, ²Physics department, Sapienza University, Rome, Italy, ³Tel Aviv University, Israel

We have developed a comprehensive optimised CHARMED pipeline, comprising an optimised data acquisition scheme, based on the electrostatic repulsion algorithm, combined with optimised ordering and extended to different b-value shells, and an analysis approach that incorporates a model parsimony. Marked improvements in data quality are demonstrated using a bootstrap approach. Our proposed pipeline clearly improves the data quality and results in better confidence in the estimated parameters such as fibre orientation.

Alessandro Daducci¹, Jason McEwen², Dimitri Van De Ville^3,4, Jean-Philippe Thiran¹, and Yves Wiaux^2,4
1Signal Processing Laboratory (LTS5), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland, ²Institute of Electrical Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland, ³Institute of Bioengineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland, ⁴Department of Radiology and Medical Informatics, University of Geneva (UniGE), Geneva, Switzerland

Diffusion MRI has become a powerful tool to non-invasively study white-matter integrity in the brain. Recently multi-shell spherical acquisitions have been advocated for mapping the diffusion signal, notably through its ODF, with a lower number of q-space samples, hence providing acceleration. In this context the spherical harmonic (SH) transform has gained a great deal of popularity. This study presents a theoretical framework and numerical simulations aiming to provide initial guidance in designing efficient multi-shell spherical sampling strategies in a model independent approach. It is based on the use of equiangular grids on the sphere, for which exact sampling theorems exist.

Daniel Güllmar¹, Christian Ros¹, and Jürgen R Reichenbach^1
1Medical Physics Group, Jena University Hospital, Jena, Thuringia, Germany

2-D multislice Diffusion-MRI typically suffers from lower resolution in the slice direction compared to in-plane resolution. Thus, we applied super-resolution technique in slice direction by acquiring overlapping slices (negative slice gap). Based on this dataset we evaluated the effect on DTI fiber tractography. We observe an overall improvement in diffusion property maps (FA, ADC, etc.) as well as in fiber tractography.

Michael Zeineh¹, Samantha Holdsworth¹, Stefan Skare¹, Scott Atlas¹, and Roland Bammer^1
1Stanford Univeristy, Stanford, CA, United States

For clinical entities such as Alzheimer’s disease and epilepsy, it would be beneficial to perform diffusion imaging of the medial temporal lobes with higher resolution than typically acquired for a more detailed analysis of hippocampal microstructure. In this study, we have compared 8-channel and 32-channel coil acquisitions at 3T while altering the direction of phase encoding and use of GRAPPA-accelerated echo-planar imaging. We demonstrate that an optimal balance of SNR, distortion, and symmetry is achieved using the 32-channel coil utilizing parallel imaging with phase-encoding in the supero-inferior direction.

Samantha J Holdsworth¹, Stefan Skare², Murat Aksoy¹, Rafael O'Halloran¹, and Roland Bammer^1
1Department of Radiology, Stanford University, Palo Alto, CA, United States, ²Clinical Neuroscience, Karolinska Institute, Stockholm, Sweden

Two approaches for Readout-Segmented (RS)-EPI have purported to increase the SNR for DWI. One fills k-space with full readout segments (RS-EPI-X), and the other which minimizes the TE with partial readout segments (RS-EPI-Y). Here we explore the scan efficiency of RS-EPI-Y and RS-EPI-X. Human brain data show that SNR efficiency is about equivalent, except for the case of a higher matrix size combined with twice-refocused DW preparation – whereby RS-EPI-Y has 1.6 times better scan efficiency than RS-EPI-X.

Denis Kokorin^1,2, Martin Haas¹, Frederik Testud¹, Jürgen Hennig¹, and Maxim Zaitsev^1
1Medical Physics, University Medical Center Freiburg, Freiburg, Germany, ²International Tomography Center, Novosibirsk, Russian Federation

Parallel transmission in combination with spatially selective excitation allows a reduction of field of view in the phase encoding direction. In this study this novel principle is examined for multi slice inner volume imaging and the advantages for artifact suppression in EPI are presented. The method was successfully implemented for DWI applications on Siemens MAGNETOM TRIO human system.

HA-KYU JEONG^1,2, JOHN C GORE^1,2, and ADAM W ANDERSON^1,2
1Vanderbilt University Institute of Imaging Science, Nashville, TN, United States, ²Radiology and Radiological Sciences, Vanderbilt University, Nashville, TN, United States

High spatial-resolution DTI at 7 T has not been commonly implemented using single-shot methods due to technical difficulties in reducing the effects of geometric distortions and/or blurring artifacts and B1-inhomogeneities that cause SNR decreases in some regions. We have previously suggested methods for diffusion-weighted multi-shot SENSE acquisitions with 2-D navigators and image-domain column-by-column reconstructions using a point spread function. With a 32 channel head coil, the B1-inhomogeneity induced regional SNR decrease can be ameliorated, and diffusion-weighted single-shot and multi-shot images show higher SNR compared with 16 channel coil data. Moreover, the suggested approach can delineate tiny white matter fiber structures at sub-millimeter resolution even in regions with relatively high B1-inhomogeneity, where conventional single-shot methods do not perform adequately.

Marco Palombo¹, Andrea Gabrielli², Silvia De Santis¹, and Silvia Capuani^1,3
1Physics Department, Sapienza University of Rome, Rome, Italy, ²ISC, CNR, Rome, Italy, ³IPCF UOS Roma, CNR, Rome, Italy

This work introduces a novel method to give more insides on anomalous diffusion processes of water in heterogeneous systems, by means of PGSTE techniques. When a competition between subdiffusive and superdiffusive processes occurs, it is possible to introduce a pseudo-RMSt^/ for which / grater or smaller than 1 defines superdiffusion and subdiffusion processes respectively. Experimental data obtained in micro-beads water suspension samples, demonstrate that  and  reflect some additional microstructural information which cannot be obtained using conventional diffusion procedures based on Gaussian diffusion. Specifically,  can provide information on the degree of order and/or disorder within the investigated systems.

Martin A. Koch¹, and Jürgen Finsterbusch^1
1Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Hamburg, Germany

Double wave vector diffusion weighting may help to assess pore size and shape in tissue. It employs two successive diffusion weighting periods with independent gradient direction. With a short delay between the periods, the signal difference between parallel and antiparallel gradient orientation for restricted diffusion depends on the mean pore size. It is often very small when whole body gradients are employed, hampering routine application. Recently, a "concatenated" experiment was proposed where the double diffusion weighting is applied multiple times. It should provide a larger antiparallel-parallel difference. The predicted increase is investigated experimentally in excised pig spinal cord.

Jun-Cheng Weng^1,2
1School of Medical Imaging and Radiological Sciences, Chung Shan Medical University, Taichung, Taiwan, ²Department of Medical Imaging, Chung Shan Medical University Hospital, Taichung, Taiwan

Fundamental relationships between diffusion tensor imaging (DTI) and q-space imaging can be derived which establish conditions when these two complementary MR methods are equivalent. When the 3D displacement distribution is measured by q-space imaging with large displacement and small q vector, the result is similar to 3D Gaussian assumed in DTI. Combing displacement information from q-space imaging and fiber direction from DTI, distribution of axonal diameters and directions could be derived at the same time. Based on the assumption, the study proposed a novel technique, q-space diffusion tensor imaging (qDTI), to map orientationally invariant axonal diameter distribution of human brain. The goal could be achieved with any of two image reconstruction methods described below. One was tensor-based method. The 3D Gaussian displacement distribution could be obtained directly from the displacement tensor. The other was displacement projection method. The fiber directions were first calculated from conventional DTI, and the mean displacement as well as maximum diffusivity of water molecules along specific direction were then obtained with q-space imaging. The effective axonal diameter was defined as the average of several displacements projected to the direction of the fiber cross section. Our results demonstrated that two qDTI methods both produced reasonable distribution of orientationally invariant axonal diameters in human brain.

Wenjin Zhou¹, and David Laidlaw^1
1Brown University, Providence, RI, United States

We present an analytical diffusion model for measuring axon properties in biological tissue of unknown orientation using low-q angular double-pulsed gradient spin echo (d-PGSE) NMR. The axon properties we aim to recover are axon radii distribution, axon orientation, and axon volume fraction. Our simulation results demonstrate that clinically feasible acquisition protocols are sufficient to accurately recover these axon properties in typical human brain tissue range (1-5µm).

Sinyeob Ahn¹, Ki Sueng Choi¹, and Xiaoping Hu^1
1Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA, United States

Distortion in diffusion tensor imaging (DTI) causes inter-subject variance and intensity modulation, which hinders an accurate analysis of diffusion characteristics. View angle tilting (VAT) technique has been used to correct distortion in the readout direction in spin echo imaging. This work describes a method for correcting image distortion along the phase-encode (PE) direction using the VAT technique in twice-refocused DTI. Phase generated by the VAT gradient concurrently applied with the PE gradient offsets an unwanted phase by field inhomogeneity, leading to the correction of image distortion. B0 images and tractography were used to evaluate the performance of the proposed method.

Myung-Ho In¹, and Oliver Speck^1
1Biomedical Magnetic Resonance, Otto-von-Guericke-University, Magdeburg, Germany

Although diffusion tensor imaging (DTI) allows the observation of molecular diffusion in tissues, it severely suffers from distortions due to B0 field inhomogeneity, susceptibility, chemical shift, as well as eddy current effects, especially at ultra high field such as 7T. These artifacts cause geometric and intensity distortions in MR image formation. Moreover, the distortions vary with the diffusion gradient direction. In this study, an acceleration point spread function (PSF) mapping based reconstruction is proposed in order to correct for direction-dependent distortions in diffusion tensor imaging. The results demonstrate that the proposed method can fully correct the direction-dependent distortions with high fidelity.

Wanyong Shin¹, Erik B Beall¹, Ken Sakaie¹, Mingyi Li¹, Dominic Holland², Anders M Dale³, and Mark Lowe^1
1Radiology, Imaging Institute, Cleveland Clinic, Cleveland, OH, United States, ²Neuroscience, University of California, San Diego, CA, United States, ³Radiology, University of California, San Diego, CA, United States

Recently, Holland et al. (Neuroimage, 2010) presented a fast, highly computationally efficient unwarping method using forward and reverse phasing encode (PE) directional EPI scans. In this study, we propose a single diffusion tensor imaging (DTI) scan in which a forward / reverse PE direction scan with b = 0 is implemented as a first acquisition. We test the reproducibility and robustness of the proposed unwarping technique from voxel-wise fractional anisotropy comparison using the proposed DTI sequence.

ALKATHAFI ALI Alhamud¹, Aaron Hess¹, Matthew Dylan Tisdall², Ernesta M. Meintjes¹, and Andre J. van der Kouwe^2
1University of Cape Town, Cape Town, South Africa, ²Department of Radiology, Harvard Medical School, MA, United States

This paper presents a novel technique to correct, in real time, spatial misalignment of diffusion volumes. A 3D low resolution navigator based on T1 weighted imaging is inserted between the diffusion volumes to compensate for rotation and translation in all 6 degrees of freedom using PACE. The DTI sequence was modified and ran three (Moco, NoMoco and Baseline) on a subject. The results show a substantial improvement in the diffusion tensor parameters with Moco, also the FA histogram shows a shift and a broadening in the FA distribution with NoMoco data, while a significant recovering with Moco was noted.