Elastography and strain rate imaging of the gastrointestinal tract
Introduction
Ultrasonography of the wall of the GI tract shows a layered structure of 3–9 layers, depending on the frequency of the transducer used [1]. Usually 5 layers are observed representing from inside-out, the interface between mucosa and lumen, the mucosa, the submucosa and two layers of muscularis propria [1]. When examining the intestines, it is sufficient to use frequencies of around 5 MHz, but preferably between 10 and 15 MHz, to enable optimal visualisation of wall layers, thickened bowel walls and target lesions. This also applies for strain imaging and, fortunately, the linear high-frequency external probes or endoluminal probes are usually well suited for elasticity imaging.
Section snippets
Methods of strain imaging
Several methods for elasticity imaging have developed over the last years. Basically these methods can be divided into strain elastography [2] and shear-wave elastography [3], [4]. Strain elastography methods map the shape change of the tissue over a time interval by tracking the echogenic spots in the B-mode image as the tissue is being exposed to a stretching or contracting force. When this is done manually the method is referred to as quasi-static elastography [2]. Induction of the
Gastrointestinal wall pathology
Because the bowel wall is a long and thin organ largely surrounded by the serosa on the outside and with a luminal inside frequently containing gas and food undergoing digestion, it is not an ideal organ to visualise with US elastography with the current methods. Both the serosa and the luminal side allow substantial movement, which may appear in elastography images as very soft tissue or even no-signal areas. One of the advantages of B-mode ultrasonography is the detailed visualisation of wall
Gastrointestinal motility
The trans-abdominal application has been used for Strain rate imaging (SRI), which use tracking of tissue Doppler and preserving the sign of recorded strain in order to map contractions (negative strain) and relaxation/stretch (positive strain) [12]. Strain rate imaging is the temporal derivative of strain, i.e. it images the amount of strain per time unit. SRI is capable of differentiating between actively contracting muscle and passively following tissue. SRI enables separation of the
Limitations of methods
It is not possible to obtain a complete overview of all bowel segments using transabdominal ultrasound. Therefore, only selective segments of the intestines can be scanned. A major challenge to strain imaging of the bowel is peristaltic movements that introduce error in strain imaging. Also the thin structure of a normal bowel wall and the serosa and luminal surface that allows relatively large movements along these planes impairs at least strain based imaging. For endoscopic application, only
Summary
Imaging of the elastic properties in the GI tract may be technically demanding and not all of the current methods may be useful. Pathological lesions often represent areas of bowel wall thickening with reduced peristalsis, which enhance the applicability of elasticity imaging. Both neoplastic and inflammatory lesions may increase the bowel wall hardness. Weather ultrasound elastography may be used for differentiation between predominantly fibrotic or predominantly inflammatory lesions in IBD
Conflicts of interest
No conflicts of interests.
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