What structures are visible on a plain abdominal X ray?
Structures are visible on a plain radiograph as a result of differential attenuation of the x-ray beam by air, fat, soft tissue, calcium, and various metals. The edge of an organ composed of soft tissue is visible only where it interfaces with fat of the retroperitoneum or mesenteries, gas within the lumen of the adjacent bowel, or gas within the intraperitoneal or retroperitoneal spaces. Interfaces are best shown when the x-ray beam is tangential to the interface. Calcium density is detected in bones or abnormal calcifications, such as gallstones or kidney stones, or within vessel lumens or vascular walls.
The inferior edge of the liver may be visible. The soft tissue of the spleen may be outlined by air in the stomach and gas in the splenic flexure of the colon. The kidneys and lateral border of the psoas muscles are outlined by retroperitoneal fat. A moderately distended urinary bladder may be outlined by pelvic fat. Portions of the uterus may also be outlined by pelvic fat.
The stomach is defined by its rugal folds. Air and fluid in a nondistended stomach is a normal finding. The duodenal bulb may be shown by air. Small bowel is identified by its thin, transverse folds (valvulae conniventes). The large bowel is defined by its haustral sacculations ( Figure 23-2 ). A small amount of gas may be present in nondistended small bowel. Gas may outline the nondependent portions of the large bowel, in particular, the transverse colon. Although the lung bases and posterior costophrenic angles are usually underexposed during chest radiography, they are well shown on plain abdominal radiographs.
