Bench-to-bedside review: Contrast enhanced ultrasonography - a promising technique to assess renal perfusion in the ICU

Table 2 CEUS-derived parameters in a 60-year-old patient before and after coronary artery bypass graft surgery complicated by acute kidney injury stage I

The mean transit time (or the 'speed' of replenishment) increased by close to 10% after surgery compared to baseline. In the same time, the relative blood volume (rBV; or echo density) decreased by almost 20%. Altogether, the resulting perfusion index dropped by 26%. This indicates diminished renal perfusion. rBV is a measure of pixel luminance and is proportional to local contrast agent concentration. This value does not have a physical unit (a.u. = arbitrary units) and is thus not comparable from one ultrasound machine to another, but its modifications are proportional to changes in contrast agent concentration. The software applies a linearization of the video data to generate a number proportional to the concentration of contrast. This operation aims to reverse the log compression applied by all ultrasound machines and is based on the pixel luminance. The luminance observed in each pixel is transformed by a program that attributes a value between 0 and 255 to each pixel.
This value is then squared to reflect signal power. Possible values are between 0 and 255^2. These values do not have physical units but are proportional to the local concentration of contrast agent. In terms of absolute number, the values are not comparable between different systems (ultrasound machines) or different settings of a same system, but the kinetics of these values are respected. AOI, area of interest.

ISSN: 1364-8535