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11-04 MR Elastography

agnetic resonance elasticity imaging (MR elastography, MRE) is a conversion of palpation of tissues and organs by a physician into a two- or three-di­men­si­o­nal color-coded depiction of tissue stiffness. It was first described in 1995  [⇒ Muthupillai 1995].

The technique is non-invasive and per­mits the evaluation of the shear elasticity of tis­sues by using a mechanical excitation with a vibration source and special MR pul­se sequences with synchronized motion encoding (Figure 11-16). Stiffer tissues pos­sess longer wave lengths, softer shorter ones. The wave images are processed to create scaled, quantitative images representing the rela­tive stiffness (elastograms), com­mon­ly depicting shear stiffness on a scale from 0 to 8 kPa. The spatial re­so­lu­tion is one-third to one-fifth of the MRI resolution (Figure 11-17).

Figure 11-16:
A vibration source (green) is attached to the body of a patient to mechanically gener­ate waves through the organ of inter­est, in this case the liver. Then, a 2D or 3D gradi­ent echo-based MR elastography se­quen­ce is applied.

Figure 11-17:
Top: Wave image. Bottom: Processed elastogram.
Both are overlay images (cf. Chapter 15); they are superimposed on the respective high-resolution MR image in the liver/stomach,/spleen level.

At present, the main focus of MRE are hepatic diseases such as fibrosis and cir­rho­­sis. The stiffness of the diseased liver tissue is significantly higher than that of nor­mal tis­sue  [⇒ Venkatesh 2013]. Nearly everybody in MRE research is aiming for the liver; applications beyond the liver in­clude possible indications in the brain, breast, and the musculoskeletal system.

spaceholder redCritical remarks. MR elastography im­ages are colorful and nice to look at. How­­ever, there are no comparison and outcome studies. In many instances elastograms seem not to add any information of use to the clini­cal case.

Ascites, iron deposition, and obesity can cause failure of MRE studies. The technical failure rate increases substantially at ultrahigh fields  [⇒ Wagner 2017].