MRI Strain Imaging: Fundamentals & Applications for
Diagnosis and Therapy

Strain refers to changes in volume and/or shape that result from external forces acting upon an object.  The strain on a given bit of tissue can be described by a 3x3 matrix in which each entry is the deformation in one dimension divided by the original length in another dimension.  This 3x3 matrix can be transformed into a diagonal matrix containing 3 eigenvectors, which convey information about the strain in a form that is more easily interpreted.  Setting aside the mathematics involved, strain imaging can be thought of as a way of measuring how different parts of a tissue sample respond to external forces.

One type of strain imaging is referred to as Displacement Encoding Stimulated Echo (DENSE).  This technique uses MRI technology to obtain information about the strains within a tissue sample.  It has been used identify areas of ischemia in the hearts of patients who have recently suffered heart attacks.  Areas of the heart that are not functioning properly can be distinguished from health heart muscle based upon patterns of strain that emerge when the heart contracts.  DENSE imaging can also be used to visualize ablation injury that occurs during some trans-catheter coronary procedures.  The technique is not yet fast enough to provide real-time feedback for surgeons, but improvements seem possible. 

Another type of strain imaging is Magnetic Resonance Acoustic Radiation Force Imaging (MR-ARFI).  This technique involves pushing a tissue sample with ultrasonic waves in synchrony with MRI pulses.  It provides higher resolution than other MRI imaging techniques, but it still needs to be tested in vivo.

MRI strain imaging seems to be a promising technique, but significant room for improvement remains.  MRI imaging offers higher spatial resolution than ultrasonic imaging, but its temporal resolution is lower.  In the future, researchers hope to improve the clinical reliability of the technique, to optimize the technology involved, and to improve the temporal resolution. 

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