EUS sonographic layers of the wall of the GI tract have been correlated to histopathological layers in several studies. 4 The standard five layer EUS image of the GI tract wall and its correlation to histological layers is shown in Fig. 13.1a.16 However, additional histological layers can be seen when different factors change the sonographic resolution of the intestinal wall. When a pathological process changes sonographic interfaces, seven layers (Fig. 13. 1 b) can be seen (Snady H, unpublished observations). UP to nine layers have been observed when different frequencies are used.27,28 Optimal imaging of the GI wall is essential to determine the wall layer(s) affected by disease. With the sonographic plane oriented as closests possible to perpendicular to the intestinal wall, optimal imaging and limitation of artifacts can be achieved. Substances injected into the GI wall can be located with EUS (Snady H, unpublished observations) to demonstrate which layers of the GI tract wall are altered. (Fig. 13. 1c). EUS images of organs, large vessels and lymph nodes contiguous to the GI tract each have their own characteristic appearance from seven standard position 7 used for orientation and finding anatomical landmarks. The normal anatomy of the oesophagus, stomach, pancreas, retroperitoneurn and hepatobiliary tract have been described previously. 4,7,8,10,11,15-19

Direct correlation of EUS images to anatomy, particularly for the pancreatobiliary systern, have been difficult to perform. Consequently, precise correlation of various EUS findings to actual histopathology has been possible only through indirect methods, which rely on the analysis of pathology from operative or autopsy specimens. Although the pancreatobiliary system can be imaged in various animal models, the variations in anatomy from the human anatomy make EUS animal studies of limited value (H Snady, unpublished observations). Intraoperative EUS has also not been helpful; when the peritoneal cavity is opened, air distorts the EUS image (H Snady, unpublished observations). However, intraoperative EUS has been useful for analysis of lymph nodes. 29 An understanding of lymph nodes associated with each organ is crucial to understanding where to scan for spread of disease to regional lymph nodes.

The role of EUS in determining patient outcome, both in terms of prognosis and quality of life, depends upon co-ordination of EUS with meaningful pathological staging and effective treatment. As more effective treatments for GI neoplasms involving combined chernotherapy/ radiotherapy plus surgery have evolved, increased response rates and improved survival have been reported.4,14 Even when not curative, therapy may alter the course and life history of GI cancer. Studies show that accurate preoperative staging is essential in determining the timing and dosage of specific treatments for greatest efficacy and impact.4

For most cancer sites, the staging recommendations that are accepted and used worldwide are concerned only with anatomical extent of disease. An untreated primary cancer or tumour (T) progressively increases in depth of invasion, spreads to regional lymph nodes (N), and finally, as a result of continued extension of disease, metastasizes (M) to distant lymph nodes or organs. TNM classification and stage grouping is thus a method of designating the extent of a particular type of cancer as it is related to the natural course. Manuals 30,31 with recommendations for stage groupings of cancer at all anatomical sites have been published by The American joint Committee of Cancer (AJCC) and The International Union Against Cancer (UICC). These evolving recommendations of the TNM system, developed by Denoix, 32 are based on contributions from 400 expert participants over 40 years and replace other non-uniform staging methods.33.34 The TNM tumour staging method has proven to be very accurate in determining prognosis. For GI tumours, EUS is the most accurate single technique to determine stage according to the TNM system.

Progress in predicting prognosis of a tumour will continue with further refinement of both the TNM system as well as the ability of EUS to predict the extent of tumour spread. Limitations of the TNM system for GI neoplastic disease have been reviewed.35 Limitations of prediction of tumour stage with EUS are related to the overlap of current criteria used to differentiate the echo patterns and features of disease processes. Part of this overlap is inherent to the ultrasound Pulses; however, part is a result of limited experience with the subtle distinctions required for the most refined EUS interpretation that is possible. As criteria continue to be developed, clarified and conjoined, image interpretation will continue to improve.36-38 In addition, EUS will have a major role in defining abnormal areas as small as 5-10 mm for directed deep biopsies from the GI lumen through the GI tract wall. 20,39-41 Combining histology of a tissue sample with EUS imaging of the entire tumour will improve the process of differentiating neoplastic tissue from inflammation.

The display of the interaction of sound waves with tissues on a monitor is the basis of clinical US. Understanding this interaction is the primary factor in using artifacts and in achieving optimal images. Accurate clinical US image interpretation depends on understanding the properties of sound waves. the mechanics of the equipment, the characteristics of the tissue or suspension media. as well as the proper technique.3-6,42-46 Errors in EUS can occur at any phase of image generation and interpretation.5,6,43-47 They can be divided into categories according to origin (Table 13.1). If maximum effort is not made to produce an optimal image, artifacts inherent to ultrasonography are magnified and even created, so that accurate interpretation is virtually impossible. Each of these features of EUS will be discussed to highlight aspects required to produce consistent, quality imaging.

The term ultrasound refers to sound wave frequencies >20,000 Hz that are beyond human hearing. Ultrasound frequencies from 1 to 50 MHz have been investigated for clinical use. When performing US, factors that must be considered include: certain basic and necessary reductive assumptions that must be made to build an image-producing machine, the physical nature of the sound beam itself, and the acoustic properties of the objects being imaged.43-46

Imaging artifacts are misrepresentations of the true nature of the structure or tissue being displayed on a monitor. Artifacts can cause misreading of EUS images because of optical illusions, errors of interpretation, and interobserver variability related to perception and/or incorrect or incomplete definitions and criteria of various terms and findings.36-38,42-47

There are certain artifacts which relate to equipment malfunction. These occur when air, water or dust gain access to the oil-immersed-transducer housing, the transducer, or the electrical connections. These types of artifacts are generally easy to detect as they will be present on the screen even when the instrument is not being used for scanning.

Because of attenuation, the optimal focal range of an instrument will vary with frequency. For higher frequencies, the focal length is smaller and closer to the transducer; for lower frequencies, the focal length is further away from the transducer. The optimal focal range for 7.25 MHz is 1-4 cm and for 12 MHz, 2-20 mm. Objects that are not in the focal range of the Sound beam cannot be consistently and accurately deciphered (Fig. 13.2a and 13.2b). Adjustment of image intensity with amplification (gain) or using adjustments in contrast, focal zone and depth can compensate for some of these factors in producing an accurate image. However, improper machine settings as well as improper adjustments of equipment controls during, scanning can cause artifacts.