Low mechanical index contrast mode versus high mechanical index contrast mode: which is a more sensitive method for detecting Sonazoid microbubbles in the liver of normal subjects?

Using the low mechanical index (MI) contrast mode and the high MI contrast mode of contrast-enhanced ultrasonography, we evaluated which method is more sensitive for detecting Sonazoid microbubbles in the liver of normal subjects. Thirteen normal subjects received an intravenous bolus injection of 0.2 mL of Sonazoid. We defined the intensity difference as the intensity post-injection minus the intensity pre-injection. We evaluated the intensity difference at the portal vein using both the low MI (0.21–0.23) and the high MI (0.7–1.2) at 1 min, at every 10 min between 10 to 60 min, and at every 30 min between 60 to 300 min post-injection. The intensity difference at the liver parenchyma was also evaluated at eight points (1, 10, 30, 60, 120, 180, 240, and 300 min) using the low MI and at three points (1, 10, and 300 min) using the high MI. The intensity differences at the portal vein measured using high MI were significantly higher than those measured using the low MI at each point between 1 and 240 min (P