MR ELASTOGRAPHY

Magnetic Resonance Elastography (MRE) is a technique that images propagating mechanical waves using MRI. This is performed by synchronizing motion-sensitive MR imaging sequences with the application of acoustic waves in the 100 to 1000 Hz range. Work in this area involves development of the basic sequences, study of the appropriate means for applying the waves, and the interpretation of the data, including conversion into estimates of elastic moduli.

To illustrate how materials may be characterized by imaging propagating shear waves, we constructed an agarose gelatin phantom. This is the standard MR modulus image of the tissue-simulating phantom showing a wedge-shaped middle layer (made with a lower percentage of gelatin).

The waves visualized when a transducer moves back and forth across the top of the gel are seen here. The wavelength (distance between black lines) is much shorter in the middle layer, indicating that the middle layer has a lower shear modulus. The lower shear modulus means that the middle layer is more easily deformed, or softer. The wave can be observed at various points in time by changing the timing of the acquisition. Compiling these images together in time order makes a "movie" of the propagating wave.

These wave images can be processed to estimate the wavelength everywhere, and this wavelength estimate converts directly to a shear modulus value, producing a shear modulus map.

The clinical interest in elastography arises from the importance of palpation in diagnosis. Palpation assesses the stiffness of a region with respect to surrounding tissue, and is the basis for clinical presentation of many breast, thyroid, prostate and abdominal pathologies. MRE provides quantitative stiffness information, which may prove even more useful in diagnosing certain types of diseases. Our current research efforts include tissue characterization, breast, prostate, muscle and brain projects. The tissue characterization effort is designed to determine the range of possible applications of MRE by determining the stiffness of healthy tissues. The breast project is an effort to explore the potential of MRE to detect breast cancer. The prostate project is attempting to validate the potential contribution of MRE by using ex vivo samples The muscle project has been able to demonstrate that MRE can provide more information than standard MRI, because stiffness images can differentiate between an activated muscle and a relaxed one.

We can easily produce and image shear waves in ex vivo and in vivo muscle. Click here for more details about MRE of muscle.

The brain project has demonstrated that MRE can go where no modality has gone before, to non-invasively measure the stiffness of human brains in vivo.