Pediatric Abdominal and Pelvic Imaging
Pediatric abdominal imaging is dominated by age-specific diagnoses and by knowing which study is urgent. Bilious vomiting in a neonate is malrotation with volvulus until excluded; obtain an upper gastrointestinal series, not a barium enema, and remember that a normal plain film does not exclude malrotation. Duodenal atresia produces a double bubble without distal gas, while jejunoileal atresia shows multiple dilated loops. Hypertrophic pyloric stenosis occurs at 3-6 weeks with nonbilious projectile vomiting; ultrasound shows a thickened pyloric muscle (at least 3 mm) and elongated canal (at least 15 mm). Intussusception in a toddler presents with colicky pain and currant-jelly stool; ultrasound shows a target or pseudokidney sign, and air or contrast enema is both diagnostic and therapeutic unless perforation or peritonitis is present.
Know neonatal bowel disease: necrotizing enterocolitis shows pneumatosis, portal venous gas, and pneumoperitoneum; never give a contrast enema when NEC is suspected. Meconium ileus occurs in cystic fibrosis with a microcolon and inspissated meconium; meconium plug syndrome is usually transient. In older children, appendicitis is diagnosed by ultrasound or MRI, with CT reserved for equivocal cases. Renal and pelvic topics include hydronephrosis and UPJ obstruction, vesicoureteral reflux, multicystic dysplastic kidney, ovarian torsion, and cryptorchidism. Distinguish Wilms tumor (intrarenal, displaces the collecting system) from neuroblastoma (adrenal, calcifies, crosses midline).
Worked rule: A neonate with bilious emesis and a normal abdominal radiograph still needs an urgent upper GI series; do not wait for acidosis or a gasless abdomen.