Magnetic Resonance
Elastography
Richard L. Ehman, MD, Professor of Radiology, Mayo
Clinic, Rochester, MN, ehman.richard@mayo.edu
Moderator:
Ferenc Jolesz, MD, CIMIT Co-Program Leader,
Image Guided Therapy; Director, Division of MRI and Director, Image Guided
Therapy Program, Brigham and Women's Hospital; Director, National Center
for Image Guided Therapy; Professor of Radiology, Harvard Medical School,
fjolesz@partners.org
Many disease processes cause profound
changes in the mechanical properties of tissues. This accounts for the
efficacy of palpation for detecting abnormalities and provides motivation
for developing practical methods to quantitatively image tissue elasticity.
Magnetic Resonance Elastography (MRE) is an emerging imaging technique
that uses a modified phase-contrast MRI technique to visualize propagating
acoustic waves generated by surface drivers, inertial effects, acoustic
radiation pressure, or endogenous mechanisms. MRE acquisition sequences
are capable of visualizing waves of less a micron in amplitude in vivo.
Inversion algorithms are used to process the wave data to generate maps
of properties such as stiffness, viscosity, attenuation, and anisotropic
behavior, providing access to a new range of previously unexplored tissue
imaging biomarkers.
Human studies have demonstrated
that it is feasible to quantitatively image the mechanical properties
of skeletal muscles, gray and white matter in the brain, thyroid, myocardium,
kidney, liver, and skin. The first established clinical application
of the technology is for detection of hepatic fibrosis. Emerging evidence
suggests that in addition to being safer, more comfortable, and less
expensive, MRE is at least as accurate as liver biopsy for this diagnosis.
Other clinical and scientific applications await the development of
specialized acoustic driver systems
MRI Strain Imaging:
Fundamentals & Applications for Diagnosis and Therapy
Ehud Schmidt, PhD, Director, Engineering Physics,
Radiology, Brigham and Women's Hospital, eschmidt3@partners.org
Moderator: Van Wedeen, MD, Assistant in Neuroscience,
Massachusetts General Hospital and Associate Professor in Radiology,
Harvard Medical School, van@nmr.mgh.harvard.edu
Strain constitutes a change in
the dimensions (compression, dilation) and/or shape (shear, torsion)
of a sample tissue element due to external forces, such as pressure.
Elastic deformation constitutes a sub-set of the above changes which
is reversible in nature. Information on elastic deformation can be obtained
in MRI with multiple imaging techniques. Cardiac MRI strain has been
obtained via analysis of wall-motion cine, myocardial tagging, HARP
and DENSE. Focusing on the Displacement Encoding Stimulated Echo Technique
(DENSE) technique, it is possible to obtain multidirectional strain
components in practical acquisition times (<20 seconds). Since changes
in tissue elastic constants are associated with multiple physiological
conditions and changes, Strain imaging can be used in the diagnosis
and therapy of many diseases. We will demonstrate applications of DENSE
imaging to the diagnosis of acute ischemia in the heart, to monitoring
therapy for mitral regurgitation and heart failure, to visualizing ablation
injury created by Electro-physiology RF ablation in the left atrium.
Finally, MR Radiation Force Imaging (MR-ARFI), a variant of Strain imaging
that includes a synchronized force-displacement field, can visualize
micrometer-scale displacement fields produced by low-power focused ultrasound
beams, fields that produce only minute (0.2 Celsius) Temperature changes.
MR-based strain techniques will be compared to Ultrasound-based imaging
in flexibility, spatial and temporal resolution. Images contributed
by researchers at MGH, NAMC, NIH and BWH are gratefully acknowledged.
For the CIMIT webpage please visit cimit.org here

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