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Chapter 10 (page 42)
B-mode and real-time ultrasonography is a well-established and valuable diagnostic
procedure in the examination of various intra-abdominal conditions. However, factors
such as obesity, alimentary gas and ribs may limit the resolution and field of view in
conventional surface echography. Hisanaga et al (l980) developed a sonographic scanner
which could be introduced into the lumen of the stomach; as the gastric walls are quite
thin, adjacent organs such as the pancreas could be observed from within the gastric
lumen.
Strohm et al. (l980) and DiMagno et al. (l980) were the first to incorporate an ultrasonic
probe into the tip of an endoscope, thereby eliminating most of the barriers of
conventional surface sonography and allowing endoscopic ultrasonic scanning (EUS) of
internal organs in the vicinity of the oesophagus, stomach and duodenum. Initially
Strohm et al. (l980) reported their experiences with a prototype radial scanning ultrasonic
endoscope, and DiMagno et al. (l980) introduced a prototype of a linear arrayed
ultrasonic endoscope. Later Strohm (l984) pointed out that better resolution was
achieved with high frequency transducers which allowed successful visualization of the
pancreas, liver, gall bladder and spleen in many cases. Using a high frequency transducer
of 7.5 MHz, Nakazawa et al. (l984) called endoscopic ultrasonography a diagnostic tool
of great value in gastroenterology.
Caletti et al. (l984 a) showed that the wall structure of the stomach could be studied by
endoscopic ultrasonography and that the different layers could be identified. This was
not possible with either conventional abdominal sonography or endoscopy alone. The
gastric walls were best explored when the stomach was distended and filled with 300 to
500ml de-aired water. To facilitate distension and avoid peristaltic contractions, 2.0 ml
propantheline bromide or 1.0 mg glucagon was administered intravenously. Initially 4
layers of different echogenicity were discerned. From within outwards these were:
firstly, an echogenic layer considered to correspond to the mucosa and submucosa;
secondly, a hypoechoic layer probably corresponding to the muscularis externa; thirdly,
an echogenic layer indicating the serosa, and fourthly a hypoechoic layer due to the
periserosal fat. The "antral" wall was thicker than that of the body of the stomach. A
peristaltic contraction also caused wall thickening; this could mimic infiltration of the
wall, e.g. by carcinoma, and was the reason for distending the lumen and paralyzing the
walls by appropriate premedication.
At present there is substantial agreement that 4 and often 5 different layers can be
identified in the gastric wall by means of endoscopic ultrasonography (Caletti et al. l984
b, l986; Aibe et al. l986; Bolondi et al. l986). From within outwards these are:
- a thin hyperechoic layer, corresponding to the mucosa
- a thin hypoechoic layer, the muscularis mucosae
- a wide hyperechoic layer, the submucosa
- a thin hypoechoic layer, the muscularis externa
- a thin hyperechoic layer, the serosa
Aibe et al. (l986) compared endoscopic ultrasonic images of the wall with histological
sections of resected specimens. It was found that the first and second layers sometimes
merged in the antral wall, together representing the mucosal epithelium, the lamina
propria and the muscularis mucosae. Occasionally an extra thin layer was discernible in
both the second and fourth layers. In the pyloric region of the stomach the fourth layer,
i.e. the muscularis externa, was relatively thicker and the third layer, the submucosa,
relatively thinner than in other parts of the stomach.
Seeing that the muscularis mucosae and the serosa are only a few micrometers thick,
Bolondi et al. (l986) thought it was unlikely that they could produce such echographically
evident layers; the acoustic reflection caused by interfaces created by different
anatomical layers should also be taken into account. It was thought that any echogenic
band should correspond not only to a distinct anatomical layer, but also to an interface
between tissues of differing acoustic impedance. Accordingly Bolondi et al. (l986)
postulated that the first layer was partially generated by ultrasound reflection at the
interface between intraluminal fluid and the mucosa, while the fifth layer indicated the
serosa/fluid reflection.
In pathological conditions the extension of lesions into the various layers of the gastric
wall may be precisely determined by means of endoscopic ultrasonography. In patients
with gastric ulceration of the "antrum", Strohm and Classen (l986) demonstrated
considerable thickening of the wall and especially of the submucosa and muscularis
externa. Benign gastric submucosal tumors could readily be localized between the
different layers and could be distinguished from malignant processes. In cases of gastric
carcinoma Aibe et al. (l986) showed that it was possible to determine whether the lesion
was limited to the mucosa (as in early gastric carcinoma) or whether it had spread into the
other layers of the wall. In many cases the depth of tumor invasion could be determined
precisely (Ohashi et al. l989).
According to direct viewing by endoscopic ultrasonography, the "antral" wall is thicker
than that of the body of the stomach; a peristaltic contraction also causes wall thickening.
The method may perhaps be utilized to investigate contractions of the pyloric sphincteric
cylinder in subjects in whom the walls have not been paralyzed. As far as we are aware
such an investigation has not yet been reported.
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