World J Gastroenterol. 2009 March 21; 15(11): 1319-1330.
Published online 2009 March 21. doi: 10.3748/wjg.15.1319.
Sonography of the small intestine
Kim Nylund, Svein Ødegaard, Trygve Hausken, Geir Folvik, Gülen Arslan Lied, Ivan Viola, Helwig Hauser and Odd-Helge Gilja.
Kim Nylund, Trygve Hausken, Svein Ødegaard, Odd-Helge Gilja, National Centre for Ultrasound in Gastroenterology, Department of Medicine, Haukeland University Hospital, 5021 Bergen, Norway
Geir Folvik, Gülen Arslan Lied, Department of Medicine, Haukeland University Hospital, 5021 Bergen, Norway
Ivan Viola, Helwig Hauser, Institute of Informatics, University of Bergen, 5008 Bergen, Norway
Kim Nylund, Trygve Hausken, Svein Ødegaard, Odd-Helge Gilja, Institute of Medicine, University of Bergen, 5021 Bergen, Norway
Author contributions: All authors have contributed to the manuscript.
Correspondence to: Kim Nylund, Institute of Medicine, University of Bergen, Haukeland University Hospital, Jonas Lies vei 65, 5021 Bergen, Norway. kim.nylund@med.uib.no
Telephone: +47-55-973075
Fax: +47-55-972950
Received December 17, 2008; Revised January 26, 2009; Accepted February 2, 2009;
Abstract
In the last two decades, there has been substantial development in the diagnostic possibilities for examining the small intestine. Compared with computerized tomography, magnetic resonance imaging, capsule endoscopy and double-balloon endoscopy, ultrasonography has the advantage of being cheap, portable, flexible and user- and patient-friendly, while at the same time providing the clinician with image data of high temporal and spatial resolution. The method has limitations with penetration in obesity and with intestinal air impairing image quality. The flexibility ultrasonography offers the examiner also implies that a systematic approach during scanning is needed. This paper reviews the basic scanning techniques and new modalities such as contrast-enhanced ultrasound, elastography, strain rate imaging, hydrosonography, allergosonography, endoscopic sonography and nutritional imaging, and the literature on disease-specific findings in the small intestine. Some of these methods have shown clinical benefit, while others are under research and development to establish their role in the diagnostic repertoire. However, along with improved overall image quality of new ultrasound scanners, these methods have enabled more anatomical and physiological changes in the small intestine to be observed. Accordingly, ultrasound of the small intestine is an attractive clinical tool to study patients with a range of diseases.
Keywords: Contrast-enhanced ultrasound, Crohn’s disease, Endoscopic sonography, Endosonography, Enteroclysis, Hydrosonography, Magnetic resonance imaging, Ultrasonography, Virtual endoscopy, Visualization
INTRODUCTION
The small intestine is the most difficult part to examine of the gastrointestinal (GI) tract because of its length and tortuous course. The traditional investigations with small bowel enteroclysis and small bowel follow-through reveal information sparingly, and unfortunately involve radiation exposure of the patient. Although it is an organ that is spared from frequent disease, more precise and patient-friendly methods are needed. In the last three decades, new imaging techniques have been developed that have proven useful. Computerized tomography (CT), magnetic resonance imaging (MRI), wireless capsule endoscopy and double-balloon endoscopy are all relatively new additions to the diagnostic armamentarium.
Compared with these methods, transabdominal bowel sonography (TABS), has the advantage of being cheap, portable, flexible and user- and patient-friendly. There are challenges with depth penetration and intestinal air precluding optimal image quality, and the flexibility of ultrasonography (US) warrants a systematic approach by the examiner. However, the development of improved scanner technology and high-resolution transducers has provided the clinician with image data of high temporal and spatial resolution, thus making it a useful tool in the diagnosis of small intestinal diseases[13]. This paper presents an overview of established US techniques for examining the small bowel and a description of new, promising scanning techniques.
When using US frequencies in the range of 7.5 to 14 MHz, the wall of the small intestine usually exhibits five different layers that correspond well to the histological layers. When looking at the mesenteric side of the intestine transabdominally, the first layer that is observed is the serosa/subserosa. It can be difficult to define exactly, as it is an interface echo between the surrounding structures (peritoneal wall/intestine/fat) and the serosa. The interface echo is thicker than the actual serosa and extends into the muscularis propria. Thus, the first layer that is clearly defined is echo-poor and corresponds to the muscularis propria. Next, is an echo-rich layer that corresponds to the submucosa, and an interface echo between the submucosa and the mucosa. Subsequently, there is an echo-poor layer followed by an echo-rich layer that corresponds to the mucosa and the interface echo between the mucosa and the luminal content. The wall layers as seen from the luminal side are described in the section on endoscopic sonography. The GI wall has a normal stratification if five US layers are visible, and there is loss of stratification if one or more US layers are missing[45].
BASIC SCANNING TECHNIQUES
Preparations
The patient should be examined in the supine position and usually the examination requires no special preparation. Fasting for > 6 h, however, leads to smaller amounts of fluid and air in the intestine and reduces motility. The intake of fluids orally or through a feeding tube reduces the air content in the intestine and makes it easier to separate the lumen from the wall and different bowel loops from each other. Furthermore, the mesenteric wall of the intestine, which is often hidden behind pockets of air, can be more easily examined with these preparations[6]. Another technique for dealing with the problem of air is graded compression, for which the examiner uses the US transducer to squeeze the air away from the region of interest[7].
High frequency B-mode
It is recommended that the examiner starts performing a regular scan of the abdomen with a 3.5-5 MHz transducer. This might give additional information, a better overview, and larger lesions in the intestine can be imaged completely. For a detailed transabdominal examination of the small intestine, a curved or linear transducer with frequencies in the range 7.5-14 MHz should be used. If pathology is detected, wall thickness, stratification, luminal patency, degree of stenosis or dilatation, and motility pattern should be determined.
Doppler modalities
Duplex scanning of the blood velocity in the superior mesenteric artery (SMA) provides several quantifiable parameters. The peak systolic (PV) and end diastolic velocity (EDV) can be used to calculate the resistive index (RI) = (PV-EDV)/PV, and the estimated mean velocity together with the inner diameter of the SMA is used to calculate the mean blood flow (MBF). The SMA is examined in the long axis. The best place to position the sample area is 2 cm after the SMA branches off from the aorta, and as it runs parallel to the aorta. The examiner should tilt the probe towards the epigastrum to obtain an angle < 60°. Intra- and interobserver agreements are good when