Recently, one of our members submitted a question to the Ask the Experts page. The query was whether CT or ultrasound was currently considered to be the best to use - or at least to use first - in an adult presenting with an acute abdomen. Addressing this issue led me to the academician's tongue-in-cheek reply, "I haven't got time for a short answer; you'll just have to put up with a long one." (My residents, incidentally, stopped thinking this was funny a long time ago.)

There is no single answer to whether ultrasound or CT is better to use in a patient with an acute abdomen. The literature addressing the sensitivities, specificities, diagnostic yields, costs, and benefits of each is vast. The decision cannot be made in isolation; at least some of the particular clinical features of the patient must be considered. What follows is an approach for adult patients and is confined to the relative merits of ultrasound and CT only. It is largely based on the extensive (and excellent) considerations presented in the ACR's Appropriateness Criteria publication (1).

If the patient has generalized abdominal pain and tenderness with guarding, fever, and an elevated white count - that is, has a "surgical" abdomen without signs or symptoms which might make a particular diagnosis likely; CT is the better first exam. CT can demonstrate a large variety of pertinent findings better than ultrasound including ischemic, inflammatory, and obstructive bowel disease, hemorrhage (at least in certain sites), intraperitoneal air, pancreatitis, abscesses (again, at least in certain sites), and urinary stones. It is also much more likely than ultrasound to demonstrate pathology in patients who are very obese. A number of diseases may progress to an intraabdominal abscess, and abscesses may appear at appropriate intervals after abdominal surgery. For most of these circumstances, CT is likely to be more accurate in depicting abscesses, especially if they are in the mid-abdomen and not amenable to ultrasound demonstration using solid organs as windows.

If a patient has right upper quadrant pain, so that acute cholecystitis is the most likely clinical diagnosis, ultrasound is almost always more accurate than CT in searching for cholelithiasis or choledocholithiasis. Furthermore, ultrasound can elicit or exclude a sonographic Murphy's sign. Abdominal scintigraphy using a biliary agent is competitive with ultrasound for acute gallbladder disease, but has the disadvantage of being useless for most other abdominal diseases which might masquerade as cholecystitis.

Patients who have clinical or laboratory evidence of pancreatitis, whether or not they have had CT, should also have ultrasound to search for biliary stone disease. The necessity for CT depends upon the severity of the clinical picture; if the condition is mild and resolves quickly, CT may not always be necessary. If it is moderate to severe, or persistent, CT should definitely be employed.

If the patient appears clinically to have bowel obstruction, CT is the better choice. Some authors claim that ultrasound is relatively accurate in diagnosing abdominal pathology in obstructed patients, but CT is still more likely to lead to the correct diagnosis in any department other than those that have exquisitely trained ultrasound personnel available around the clock. As a practical matter, most patients with questionable bowel obstruction have supine and upright abdomen films early in the course of their evaluations. Whether these exams add anything of value to the information available from a subsequent CT is doubtful, but they may, if normal, preclude the need for further imaging in some patients.

A patient who has pelvic pain may also be best managed by using ultrasound first. If the clinical picture suggests pelvic inflammatory disease in a woman, or other acute gynecologic events like torsion or rupture of an ectopic pregnancy, ultrasound is often the only imaging test that is needed. Right lower quadrant pain, which could be either pelvic inflammatory disease or appendicitis, can initially be investigated with ultrasound. If the signs and symptoms strongly suggest appendicitis, the choice between ultrasound and CT becomes a little less certain. In pediatric patients, thin adults, and pregnant women, graded compression transabdominal ultrasound is reasonably effective in diagnosing and excluding appendicitis. However, if the patient is older or obese, if the abdomen is rigid, if an abscess is suspected, or if an experienced sonologist is not available, CT is more likely to be effective.

To investigate left lower quadrant disease, the choice is a little easier. If the patient is felt most likely to have diverticulitis, CT is better; it is more likely than ultrasound to visualize the abnormality and is probably a better roadmap for planning any percutaneos therapy. For situations in which fluoroscopy is more readily available than CT, a barium enema is competitive with CT in terms of accuracy and safety, but still is much less likely than CT to demonstrate other acute abdominal conditions. If the likely condition is acute gynecologic disease, ultrasound is preferable.

Patients whose clinical pictures are most compatible with acute urinary system diseases, such as severe pyelonephritis or acutely obstructing ureteral stones, are best investigated with CT. CT is probably more sensitive than ultrasound in demonstrating renal inflammatory disease (not that history, physical examination and urinalysis don't usually suffice). CT is certainly more sensitive than ultrasound (and radiography) for detecting urinary tract calculi and by now has become nearly standard as a replacement for urography to investigate renal colic. The most commonly used CT protocol for ureteral stone disease is a spiral exam with neither oral nor intravenous contrast. It should be kept in mind that withholding contrast may obscure or make ambiguous certain findings in patients who turn out to have other abdominal diseases, and that excretory urography is still occasionally useful to demonstrate ureteral obstruction when CT findings are confusing.

Patients with abdominal trauma may appear with findings (broadly speaking) of an acute abdomen; for them, CT is almost always the better exam. Although major lacerations of solid organs and intraperitoneal hematomas may be detected equally well by both CT and ultrasound, mid-abdominal, retroperitoneal, intestinal, and mural bleeding can be better visualized by CT. This is also true for musculoskeletal trauma. CT can also more quickly evaluate traumatic vascular occlusive disease, and is a better single exam for determining whether a patient needs surgery, angiographic therapy, observation, or no treatment. CT may need to be augmented with CT cystography after transurethral instillation of contrast if bladder laceration is a consideration. Ultrasound may have a role for patients who cannot be moved to a CT scanner, however. If modern imaging is available, I am convinced that peritoneal lavage should be abandoned, despite the persistence of this procedure in certain corners of the literature.

A summary of the approach might be as follows:

• each patient should be assessed clinically
• if the likeliest diagnosis is acute biliary or gynecologic disease, ultrasound should probably be used first
• if appendicitis is the most likely diagnosis, either can be chosen depending upon the patient's age and habitus
• for most other acute abdominal disease, CT is likely to be the best first choice.

References