Acute Abdominal Pain: CT or US?

January 2011

By Richard Hyer
Elsevier Global Medical News 

CHICAGO - The choice of computed tomography versus ultrasound for diagnosis of right lower quadrant pain is not only about radiation, but also about the operator's level of training, the patient's age and sex, and several other factors, according to Dr. Stephanie R. Wilson and Dr. Ajay Singh.

"I would choose ultrasound for everyone," Dr. Wilson said at the annual meeting of the Radiological Society of North America. But Dr. Singh praised CT "because of its high sensitivity, high specificity, accuracy, availability, lack of operator dependence, and finally the wide range of disease processes it can diagnose."

Acute abdominal pain accounts for 117 million emergency department visits in the United States each year, and CT is the diagnostic modality of choice, said Dr. Singh of the radiology department at Harvard Medical School, Boston. Exceptions are suspected appendicitis in children and in pregnancy, right upper quadrant pain, and pelvic pain in young women.

"CT is considered the most appropriate modality for adults with right lower quadrant pain. Ultrasound is the most appropriate imaging modality for assessing right lower quadrant pain in children and in pregnant patients," Dr. Singh said.

However, ultrasound is quite effective for diagnosing the scope and extent of disease in the GI tract, with excellent spatial and temporal resolution, said Dr. Wilson of the department of radiology at the University of Calgary (Alta.).

The modality is inexpensive, portable, and safe, and has no requirement for radiation or for functioning kidneys, she said.

Choosing ultrasound is especially important for patients with a chronic condition like inflammatory bowel disease because of the high likelihood of excess radiation when CT is used for multiple imaging studies.

Its only negatives are operator dependency and a small number of technically inadequate examinations, she added.

For confirmation of appendicitis, the American College of Radiology (ACR) appropriateness rating for CT of the abdomen (intravenous contrast) for acute right lower quadrant pain is 8, compared with 6 for ultrasound (graded compression), Dr. Singh pointed out.

A properly trained physician can confirm suspected appendicitis with ultrasound, Dr. Wilson countered.

The main reason that reports of accuracy vary in the literature is that ultrasound is an operator-dependent modality, she said.

Ultrasound has a sensitivity of 76%-89% and a specificity of 85%-92% for diagnosis of appendicitis.

For a differential diagnosis of acute appendicitis, the physician should image the area that hurts, she said, calling this "the sonographic McBurney's sign."

Compared with ultrasound, CT for appendicitis has a higher sensitivity (90%-100%) and specificity (95%-97%) (Radiology 2000;215:337-48). CT is widely available and does not depend on operator experience, Dr. Singh said; its drawbacks are ionizing radiation and high cost.

The risks of radiation in inflammatory bowel disease should not be disregarded, Dr. Wilson emphasized. The onset of Crohn's disease is generally in youth or young adulthood, and the lifelong chronic disease has frequent exacerbations with corresponding demands for imaging.

Diagnosis, surveillance, monitoring disease response to therapy, and evaluation at the time of exacerbation are better managed with ultrasound, she said, especially as a patient with the disease may require as many as 50 scans over a period of a lifetime.

Ileoscopy is the standard for Crohn's disease, Dr. Singh said. Fluoroscopic barium studies have traditionally been the mainstay of small intestine imaging, he said, and the advantages of CT are the ease of evaluating superimposed small intestine loops and extraintestinal disease.

The ACR appropriateness rating for CT enterography at initial presentation is 9, compared with 5 for ultrasound of the abdomen and pelvis, Dr. Singh pointed out.

"For initial assessment of Crohn's disease in adult patients, one should perform CT enterography. For assessment of Crohn's disease in pediatric patients, one can perform CT enterography or MR enterography," Dr. Singh said.

Ultrasound "shows bowel wall layers and perienteric tissues with really exquisite detail," Dr. Wilson said. It also shows peristalsis, and the extent and scope of disease, she added.

"Operator dependency - this is definitely true. But everyone with effort can be proficient at it. The deterioration of the practice of ultrasound in North America is related to lack of attention to this modality, rather than to a failure of the technique," Dr. Wilson said.

Dr. Singh agreed that "in the United States, ultrasounds are done by residents, fellows, ultrasound technologists. Not everybody is able to achieve the sensitivities that are achieved in some of the European papers."

Dr. Singh reported no relevant financial disclosures. Dr. Wilson disclosed receiving a research grant from Lantheus Medical Imaging and being on the advisory board of Koninklijke Philips Electronics.

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