Certain arbitrary but generally accepted conventions are utilized for creating and displaying ultrasound images. The skin-transducer interface is placed at the top of the image. Each transducer has a mark of some sort, which is used for left-right orientation. The marker always points to the left side of the screen, as viewed from in front. When scanning in the transverse plane, the physician points the marker groove to the patient's right side, and the image is displayed as a cross section from the patient's feet (Fig. 295-1). For longitudinal or sagittal views, the marker points to the patient's head. The image then appears with the most cephalad portion on the left
FIG. 295-1. Transverse ultrasound image. This is obtained by directing the marker dot on the probe toward the patient's right side. (From Heller M, Jehle D, eds: Ultrasound in Emergency Medicine. Philadelphia, Saunders, 1995, with permission.)
FIG. 295-2. Longitudinal ultrasound image. The marker dot on the probe is directed toward the patient's head. (From Heller M, Jehle D, eds: Ultrasound in Emergency Medicine. Philadelphia, Saunders, 1995, with permission.)
The transmission of ultrasound waves is blocked by highly echogenic structures (e.g., gallstones), resulting in a relatively anechoic area distal to the echogenic structure. This effect is known as acoustic shadowing. Acoustic enhancement occurs distal to an anechoic, fluid-filled structure, such as the gallbladder. The area distal to the anechoic structure has increased echogenicity due to the greater number of ultrasound waves reaching this area through the anechoic structure. Examples of these phenomena are present in almost every ultrasound image. However, the many other technical factors that influence the creation of ultrasound images and artifacts are beyond the scope of this discussion. Two of the most common and useful are illustrated in Fig^QS-S.
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