To many young physicians, the picture on the left is a mystery. Is it an example of black and white pointillism art? Could it be a Rorschach test? The picture on the right obviously is a modern CT scan of the head. Some doctors are often surprised to learn that the two pictures are both CT scans of the head, the one on the left was taken in the 1970s when CT was in its infancy, while the scan on the right was taken within the past few years.
In the 1970s, CT scan had a resolution of a mere 6,400 pixels, compared to the modern scan on the right which has 256,000 pixels. The vintage scan on the left took 5-10 minutes to complete, which contributes to significant movement artifact and blurring. The scan on the right was completed in just a few seconds, during a single breath-hold. In order to diagnose abnormalities such as a subdural hematoma or a subarachnoid bleed, nearly every head CT scan in the 1970s had to be performed without contrast and then repeated with IV contrast, a dual study. The resolution of the CT scan on the right is now sufficiently robust that IV contrast is rarely necessary. In fact, for the past few decades, it has rarely been necessary for any CT head to be performed with IV contrast or as a dual study.
Sometimes it is difficult for providers to determine whether a CT scan for their patient should be performed with contrast or without contrast or both. For example, almost all neck CT scans require contrast. CT of the head, bones, and spine are almost always non-contrast. Whether a CT of the chest, abdomen, and pelvis requires contrast or not depends upon the diagnosis. For example, if a kidney stone is suspected, CT abdomen and pelvis without contrast is the usual imaging study of choice. For diverticulitis, appendicitis, peritonitis, and abdominal trauma, a CT performed with IV contrast is preferred. CT of the chest may be non-contrast if performed for lung cancer screening, suspected interstitial lung disease or follow-up of a solitary pulmonary nodules, while many other lung diseases are better diagnosed using contrast. Contrast helps the radiologist visualize blood vessels and lymph nodes. For specific recommendations, see guidelines from a radiology benefits manager or the American College of Radiology appropriateness criteria.
If an ultrasound discovers a mass in the liver, kidney, pancreas, or spleen, guidelines often will recommend a dual study. Some of these masses (such as a suspected liver hemangioma) may even require a triple phase study (that is, a scan before contrast is administered, another immediately after contrast, and a third delayed scan). Progress in technology has resulted in such high resolution CT scans that a dual or triple phase study is seldom needed.
And yet every day providers request dual studies that are not recommended by any guideline. The lesson is clear: before ordering a dual CT scans, think twice and then look it up in the ACR appropriateness criteria or any RBM guideline to see if a single imaging study would be adequate. Modern CT scanners are faster, have better resolution, and often do not require dual imaging.