Which of the following investigation is used in diagnosing intestinal obstruction?

Obtain plain radiographs first for patients in whom small-bowel obstruction (SBO) is suspected. At least 2 views, supine or flat and upright, are required. Plain radiographs are diagnostically more accurate in cases of simple obstruction. 2 findings predictive of a higher grade or complete SBO: (1) the presence of an air-fluid differential height in the same small-bowel loop and (2) the presence of a mean level width greater than 25 mm. Dilated small-bowel loops with air-fluid levels indicate SBO, as does absent or minimal colonic gas.