Intestinal Obstruction — Imaging

On CT imaging of small bowel obstruction, which sign indicates the transition point between proximal dilated bowel and distal collapsed bowel, and is the most specific for mechanical obstruction?

On CT imaging, which finding is most specific for distinguishing simple small bowel obstruction from strangulated obstruction?

A 62-year-old man presents with acute abdominal distension, colicky pain, and vomiting. Plain abdominal radiograph shows dilated small bowel loops with air-fluid levels. Regarding the imaging features of small bowel obstruction, all of the following are characteristic findings EXCEPT:

A 4-year-old child presents with acute onset colicky abdominal pain, palpable abdominal mass, and currant jelly stools. Clinical suspicion for intussusception is high. What is the most appropriate next investigation?

A 62-year-old man with a history of abdominal surgery 15 years ago presents with acute onset colicky abdominal pain, abdominal distension, and bilious vomiting for 8 hours. On examination, he is dehydrated with visible peristaltic waves. Plain abdominal X-ray shows dilated small bowel loops (diameter 4 cm) with multiple air-fluid levels in a "staircase" pattern, and minimal colonic gas. CT abdomen with IV contrast reveals transition zone with abrupt change in caliber at the distal ileum, with no evidence of mass, perforation, or ischemia. What is the most likely diagnosis?