Ultrasound in gastroenterology

Medical imaging has a strong impact on clinical decision making and its significance and utility seems ever increasing. In the field of gastroenterology, new imaging methods such as double-balloon and capsula enteroscopy have recently added to traditional endoscopic methods. In contrast to x-ray, computed tomography, MRI, single photon emission computed tomography and PET scanning, ultrasonography is a clinical method that can be easily applied, even at the bedside, also using mobile, hand-carried scanners [1]. Dedicated methods, such as Doppler ultrasound, 3D ultrasound and endosonography, have expanded the options for diagnostic ultrasound in the field of digestive diseases [2,3]. In addition to its favorable cost, high availability, great flexibility and user friendliness, ultrasonography exhibits very high temporal and spatial resolution. Ultrasonography can provide physiological, pathophysiological and biomechanical information to the clinician and constitutes an important tool in the diagnosis and follow-up of large populations of patients in gastroenterology.

Ultrasonography in digestive diseases can, in principle, be applied in two different ways to patient management. It can be used as one imaging modality among others, for which the clinician would refer the patient on to a radiologist and later receive a report. However, ultrasound in gastroenterology can also be used as a clinical tool, similar to the stethoscope, expanding on the clinical findings and paving the way for efficient work-up of the patients. In clinical practice, rapid intervention, often based on beside judgment only, is necessary to avoid serious complications of a gastrointestinal (GI) disorder. Under these circumstances ultrasound imaging by a clinician saves critical time and may guide necessary intervention. It is my opinion that ultrasonography in most countries is markedly under-used in a clinical setting. In this editorial I will demonstrate how ultrasonography can be applied in gastroenterology, particularly to evaluate motility and functional disorders.

Functional ultrasonography

A considerable portion of patients with symptoms from the digestive system have no organic lesions but rather a functional disorder. Accordingly, diagnostic methods to support the differentiation between structural and functional abnormalities of the gut are highly warranted. Functional ultrasonography (f-US) is ultrasound imaging of organ function, in contrast to conventional imaging of anatomic structures. Using f-US, clinical information about motility, gastric and gallbladder emptying, biomechanics, flow, perfusion, organ filling, and emptying can be obtained noninvasively. It is well recognized that ultrasonography can provide both qualitative and quantitative data about motility, in a fasting state and after meal ingestion [4–6]. High-resolution ultrasound using frequencies in the range 7–15 MHz permits detailed studies of gastric wall layer involvement during peristalsis. Ultrasound is more sensitive than manometry in detecting antral contractions, particularly nonocclusive contractions [7]. Ultrasonography has been widely used to assess gastric emptying rates [8–11], and good correlation to radionuclide estimates of emptying rates have been detected [12–14].

Tissue Doppler imaging enables mapping of local tissue velocities, thus increasing the physiological information regarding moving walls [15]. A Doppler method based on strain rate imaging (SRI) and estimation of relative strain was developed to enable visualization of tissue deformation [16,17]. Doppler SRI was evaluated in vitro using a silicone strip phantom mimicking slowly moving tissue [18] and in measuring strain in the porcine antral wall in vitro[19]. These studies showed that SRI gave accurate measurement of radial strain of the antral wall with reasonable interindividual variation. Estimation of relative strain of the muscle layer of the gastric wall by Doppler ultrasonography is feasible and enabled detailed mapping of local strain distribution [20]. SRI is capable of distinguishing contractile activity of the longitudinal and circular muscle layers, even though the two layers cannot be separated visually in the 2D images.

A sonographic method for assessment of proximal gastric size was developed to estimate accommodation of meals [21]. By this technique the existence of impaired proximal gastric accommodation to a meal in patients with functional dyspepsia was observed [22]. Nitric oxide is a key neurotransmitter in the reflex regulating gastric adaptive relaxation. Therefore, a double blind placebo-controlled cross-over study was performed showing that glyceryl trinitrate caused a concomitant improvement of proximal gastric accommodation and epigastric pain in response to a soup meal [23]. This 2D method has also been used to study gastric accommodation in diabetes mellitus, in patients with reflux esophagitis, in liver cirrhosis and in children with recurrent abdominal pain [24–27]. This ultrasound method can also be applied to study pharmacological intervention and combined with a barostat bag in the stomach [28,29].

3D ultrasonography

A method for volume estimation of the stomach based on mechanical tilting acquisition of 3D ultrasonography was developed [30]. This 3D ultrasound system demonstrated excellent accuracy in vitro both on phantoms, on animal organs and when validated in vivo against MRI this 3D system was in good agreement and presented high precision [30–32]. This system has also been used to study diseases of the liver [33], and to evaluate patients with functional dyspepsia [34–36]. However, this 3D system could only acquire a 90° fan-like data set from a predetermined, fixed position of the transducer. To enable scanning of larger organs such as the stomach, a commercially available magnetometer-based position and orientation measurement device was interfaced to the scanner. This system for magnetic scanhead tracking (Bird, Ascencion Technology Inc., VT, USA) was validated both with respect to its precision in locating specific points in space [37] and to its accuracy in volume estimation [18,18,38]. For the first time, total gastric volumes and intragastric distribution of meals could be studied by ultrasonography [39]. The 3D ultrasonographic method was validated in vivo in healthy controls [40]. This 3D ultrasound system correlated very well to infused volumes and showed very good agreement with true volumes, as well as low interobserver variation. Regarding gastric emptying the 3D system has been validated against scintigraphy yielding comparable results [14].

In functional dyspepsia, poor accommodation of the proximal stomach to a meal has been found in many studies [22,41,42]. Drinking capacity is often reduced in these patients and drink tests may therefore have a diagnostic potential. A simple drink test in combination with 3D ultrasonography was applied in a test using Toro^® meat soup, Nutridrink^® and water [43]. Interestingly, optimal discrimination between patients and controls was obtained by the combination of symptoms and intragastric volume using meat soup as the test meal. In another study, an analytical method was developed to describe the 3D geometry of the gastric antrum; gastric fundus and the whole stomach-based 3D ultrasound acquistitions [44]. The 3D ultrasound system currently used in our laboratory is outlined in more depth in a previous review [45].

Endosonography

Endosonography performed with intraluminal ultrasound transducers (EUS) enables detailed imaging of the mucosal surface of the GI tract as well as the individual wall layers, adjacent organs and tissue [2]. The normal GI wall is imaged by EUS as a layered structure consisting of several layers (five–nine layers), but mostly a five-layer structure is seen [46]. EUS has become not only an important diagnostic tool for gastroenterologists, but also a very flexible and versatile instrument in guiding biopsy-taking and various interventions. For thorax surgeons, EUS examination with lymph node biopsy is now a standard preoperative evaluation regarding staging of pulmonary cancer. 3D EUS images can be obtained using images acquired by both echoendoscopes and miniprobes. 3D-EUS may be applied for improved recognition of the GI anatomy and pathological lesions. Mostly, a pullback device has been used to acquire parallel 2D images which are reformatted before 3D reconstruction and volume estimation is performed [47]. However, miniature position sensors are being employed to facilitate more flexible acquisition with echoendoscopes to promote clinical applications. The great advantage using 3D-EUS is better standardization of data, shortened acquisition time, easier interpretation of ultrasonographic images and improved application in telemedicine.

It is my opinion that the diagnostic potential of ultrasonographic imaging in the field of gastroenterology has not been fully exploited. In many clinics worldwide, the management of patients with digestive diseases can be significantly improved by using ultrasound more systematically in daily routine. However, ultrasonography is a rather observer-dependent method where the diagnostic and therapeutic confidence relies on the operator’s competence and experience. Therefore, the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) has published [101] proposals for standardization of education in ultrasonography in Europe. In this document, the importance of supervision, courses and continuous education is emphasized, and EFSUMB is suggesting three different competence levels for the post-graduate ultrasound education.

When a clinical imaging modality like ultrasonography is brought closer to the patient and integrated in the work-up of patients, rapid diagnosis and tailored treatment are enabled.

Financial & competing interests disclosure

The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.