Investigation of choice for pneumoperitoneum is:

Ref: Sabiston Textbook of Surgery 19th Edition & Radiology for Surgeons in Clinical Practice, First Edition, Pg: 42.Explanation:Erect abdominal X-ray is the first test ordered for suspected pneumoperitoneum.However CT abdomen is the most sensitive test for demonstrating pneumoperitoneum.Upright chest radiographs can detect as little as 1 ml of air.Lateral decubitus abdominal radiographs can also detect pneumoperitoneum effectively in patients who cannot stand: as little as 5 to 10 mL of gas may be detected with this technique.Approximately 75% of these patients will have a large enough pneumoperitoneum to be visible on X-rays.Where the plain radiographs are equivocal, then CT is the most sensitive imaging technique for diagnosis of free intraperitoneal air.Signs of free intraperitoneal air on a supine abdominal radiograph areAir overlying or outlining solid organs such as liver and spleen (Lucent liver sign)A triangle of air outlining Morrison's pouch (Doges cap sign)Air outlining intraabdominal structures such as falciform ligament, umbilical median, medial and lateral ligaments (Inverted V sign)Bowel wall outlined by air both inside and outside (Rigler's sign)Centrally placed intraperitoneal free air (football sign)Scrotal air in childrenCupola sign of air under the central diaphragmConditions mimicking pneumoperitoneumIt is very important to be able to recognize conditions that can simulate pneumoperitoneum because an observational error may lead to unnecessary surgery.Chilaiditi's syndrome in which the colon is interposed between the liver and the diaphragm can mimic pneumoperitoneum, but usually haustral folds can be seen that will allow- identification of the colon.A gas-containing subphrenic abscessBasal pulmonary atelectasis.On the left side, a gas-distended fundus of the stomach may cause confusion, and clarification by a decubitus film may be necessary.Postoperative air may sometimes cause confusion, but this should have resolved by 5 to 7 days and should decrease in size with time.