Subject: WashCAS news:  next talk will be Tuesday May 20th on Diffusion
Tensor Imaging


Dear WashCAS attendee:

After a brief break due to events in the Middle East, we are back on
schedule with the WashCAS talks.

We are pleased to announce that the May 20th WashCAS talk will be given
by Peter Basser, PhD, of the National Institute of Child Health and Human
Development. The topic will be "Diffusion Tensor Imaging". The abstract
is pasted below and is on the web site at www.washcas.org. We may also have
a special guest presentation at the beginning of the meeting - details
will be forthcoming.

The meeting will be held in the Lister Hill Center, Building 38A, behind
the National Library of Medicine on the NIH Campus in Bethesda. The
visitor's entrance is on South Drive off of Wisconsin Avenue, next to the metro stop. Drivers and passengers should be prepared to present photo IDs. Vehicle hoods and trunks may be opened for inspection.

For directions, see http://www.nlm.nih.gov/about/directions.html

The meeting schedule is:
        6:30 - 7:00 pm: refreshments and food in lobby
        7:00 - 8:00 pm: speaker and discussion

Please remember that absolutely no food is allowed in the meeting room.

At this point, we don't anticipate any problems with holding the meeting
at the Lister Hill Center. If the situtation changes, we will move the
meeting to a local Bethesda hotel and notify you.

If you are even considering attending, please RSVP by replying to this
email or let one of the society officers know. This is not required, but helps us in planning the food order as well as giving a list of potential
attendees to the security personnel.

We are now using our mailing list for these announcements. If you wish
to unsubscribe, please send email to cleary@georgetown.edu. For those who
did not receive this announcement and wish to subscribe to the mailing list, ask them to send email to join@washcas.org and they will be automatically added. WashCAS meetings are held every other month on the third Tuesday of the month. For WashCAS news, see www.washcas.org

Sincerely,

Kevin Cleary, PhD
Georgetown University
ISIS Center
(202) 687-8253
cleary@georgetown.edu

James Burgess, M.D.
Neurosurgery
Inova Fairfax Hospital
(703) 718-0314
burgessj@pol.net

Gerald Higgins, Ph.D.
Simquest
(301) 565-2033
Higgins@simquest.com

Terry Yoo, PhD
National Library of Medicine
(301) 435-3268
yoo@nlm.nih.gov

Diffusion-Tensor MRI: An Overview

Peter Basser, Ph.D.

Section on Tissue Biophysics & Biomimetics, NICHD

National Institutes of Health, Bethesda, MD, USA

pjbasser@helix.nih.gov

This talk deals with the measurement (or estimation) of the effective
diffusion tensor (1) of water in tissues. We review the history of
diffusion measurements using NMR and MRI methods, the definition and physical interpretation of the effective diffusion tensor, as well as
quantitative MR parameters derived from it, such as the Trace and measures of diffusion anisotropy (2). We will also provide an overview of several biological and clinical applications of DT-MRI.

The NMR measurement of the effective diffusion tensor of water, D,
within each voxel of an imaging volume, and its analysis and display is called Diffusion Tensor MRI (DT-MRI) or Diffusion Tensor Imaging (DTI) (3).
This measurement can be obtained in vivo, non-invasively, without requiring
exogenous contrast agents. The MR measurement of D in tissues can
provide unique biologically and clinically relevant information that is not
available from other imaging modalities, particularly quantitative
parameters that help characterize tissue composition, the physical
properties of tissue constituents, and tissue microstructure and its
architectural organization.

In tissues, such as brain gray matter, where the measured apparent
diffusivity is approximately independent of the orientation of the
tissue (i.e. isotropic) at a voxel length scale, it is usually sufficient to describe the diffusion characteristics using a single(scalar) apparent
diffusion coefficient (ADC). However, in anisotropic media, such as
skeletal and cardiac muscle (4-6), and in white matter (7-9) where the measured apparent diffusivity depends upon the orientation of the tissue, a
single ADC is not adequate to characterize the orientation-dependent water
mobility. For such a free anisotropic diffusion process, one can replace
the scalar diffusion coefficient appearing in Fick's law with a symmetric
effective or apparent diffusion tensor of water, D (e.g., see (10)).

In DT-MRI, D, is estimated from a series of DW images and their
corresponding b-matrices (that summarize the effects of all applied
gradients on the amount of diffusion weighting on each element of D)
(3). DT-MRI is inherently three-dimensional. To measure D, one must apply
diffusion gradients along at least six non-collinear, non-coplanar
directions (1).

Quantitative parameters can be extracted from diffusion-tensor MRI data.
Intrinsic quantities can be found to characterize distinct features
describing the size, shape, orientation, or pattern of rms diffusion
ellipsoids within an imaging volume. These quantities are rotationally
invariant, i.e., independent of the orientation of the tissue
structures, the patient's body within the MR magnet, the applied diffusion
sensitizing gradients, and the choice of the laboratory coordinate system in which the components of the diffusion tensor and magnet field gradients are measured (3; 11). Some examples are Trace(D), which is proportional to the orientationally-averaged intrinsic diffusivity or mean ADC; the
eigenvalues of D, which are the principal diffusivities along the local principal axes; the eigenvectors of D, which define the orientations of the local principal axes; as well as many parameters and constructs derived from these quantities. For instance, color maps that indicate the local fiber-tract orientation, are created by combining information contained in the eigenvector associated with the large principal diffusivity and a
measure of diffusion anisotropy (12-15). Fiber tract trajectories can be
constructed from D data by generating streamlines that follow the direction of maximum diffusivity (e.g., see (3; 16-28). Probabilistic schemes for
tractograph and connectivity analysis (29-33) use D data to construct fiber
orientation distributions.

The main artifacts in obtaining DT-MRI data are those associated with
acquiring the DWIs from which D is measured. These include subject
motion, eddy currents, magnetic susceptibility effects, and image noise.
Hardware issues, such as background gradients, and gradient miscalibration should also be considered. Some errors and artifacts that are peculiar to
tractography will be described.

The majority of applications of DT-MRI to date have been to diagnosing
and assessing various neurological disorders (e.g., chronic and acute
ischemia (stroke), Wallerian degeneration, ALS, Alzheimers disease) as well as behavior and cognitive disorders (e.g., schizophrenia, dyslexia, ADD,
ADHD). More recently a number of therapeutic applications have emerged. The
most notable and promising is in the assessment of tumor type and in the
planning of surgical procedures to remove cancerous tissue.

DT-MRI provides new information with which to probe tissue structure at
different levels of hierarchical organization. New structural parameters
provided by DT-MRI, such as maps of the principal diffusivities of D,
its Trace, measures of the degree of diffusion anisotropy and organization,
and estimates of fiber direction are all helping to advance our
understanding of nerve pathways in the CNS, as well as the anatomical organization of some soft fibrous tissues.

Biography

Peter J. Basser is Senior Investigator and Chief of the Section on
Tissue Biophysics and Biomimetics within the National Institute of Child Health and Human Development (NICHD). Peter received his A.B, S.M., and Ph.D. from Harvard University in '80,'82 and '86 respectively. In 1986 he received a Staff Fellow appointment in the Biomedical Engineering and
Instrumentation Program (BEIB) at the National Institutes of Health (NIH). In 1998, he joined NICHD in his current position.

Dr. Basser's research focuses primarily on transport processes in
tissues. He has contributed to the understanding of the mechanisms of action of electromagnetic fields on excitable tissues (magnetic stimulation), measuring and characterizing diffusive transport of water in tissues using magnetic resonance methods, and explaining the functional properties of extracellular matrix in terms of the osmotic properties of its constituents.

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