Could an internal hernia, in the mid-abdomen, be missed on CT scan? Is MRI more accurate? Thank you.

No. An internal hernia can only be suspected post surgery when signs of partial bowel obstruction develops due to a loop entering a defect in mesentery or equivalent and bowel dilatation develops which can be defined by both CAT and MRI.
Surgery. The only way to definitively diagnose (and treat if it exists) an internal hernia is an operation, typically done laparoscopically. While CT and MRI can diagnose it, a "negative" study does not rule it out. Symptoms are typically intermittent pain, occasionally progressing to bowel obstruction. Intussusception can also have the same symptoms. Hope this helps!