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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]. MR elastography has become a popular tool in the diagnosis and staging of liver fibrosis. Publications have reported MRE techniques and results using many different parameters, acquisition techniques, processing methods, and varied nomenclature. The diversity of terminology can lead to confusion  [⇒ Manduca 2021].

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].