Flash mode is a technique specific to CEUS-capable ultrasound devices, and in brief is a short ultrasound pulse with a very high mechanical index, resulting in almost complete destruction of the contrast agent microbubbles in the imaging plane.
Physics
The microbubbles utilized as contrast agent for CEUS are inherently sensitive to insonation, thus the mechanical index (MI) of ultrasound pulses has to be kept at the lowest possible level which provides reliable contrast, and is generally much lower than during routine B-mode US. When flash mode is triggered by the user, the probe emits a short US pulse with a very high MI (typically at least a magnitude greater than that of the CEUS preset itself). This pulse instantly and permanently destroys almost all microbubbles within the imaging plane 1. The "flash" is visible in the CEUS-view as a sudden, bright flare - hence the name.
Practical points
By using the flash mode, microbubbles that previously accumulated within the area of interest can be annihilated. This sudden loss of the microbubbles can be of advantageous, e.g. to evaluate the portal venous phase refilling dynamics of a liver lesion, which had previously already filled in completely during the early arterial phase. This approach allows us to interrogate the slower, venous phase refill, thus it is termed flash-replenishment technique 2.
Flash can also be useful for reducing contrast to near background level before a re-injection of a contrast agent (e.g. for the re-evaluation of a lesion whose fill-in dynamics was missed initially). It is however crucial to understand that wash-out should never be evaluated in a lesion/area on which the flash mode was previously used unless a re-injection of contrast is performed. Recent studies point towards further possible uses of the flash mode, such as grading liver fibrosis (the extent of microbubble destruction by a flash correlates inversely with the severity of fibrosis) 3.
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