Perfusion estimation using subharmonic contrast microbubble signals

Contrast enhanced subharmonic imaging (SHI) may facilitate estimation of quantifiable flow parameters such as perfusion. A modified Logiq 9 scanner operating in grayscale SHI mode was used to measure SHI time intensity curves in a flow phantom. Different concentrations (1 and 2% by volume) of the contrast agent Optison and flow rates (9.8 and 19.6 ml/min) were evaluated. In vivo, 4 dogs received intravenous contrast injections (dose: 0.1 ml/kg) and renal SHI was performed. Low perfusion states were induced by ligating surgically exposed segmental renal arteries. Following 3 contrast injections a microvascular staining technique based on stable (non-radioactive), isotope labeled microspheres was employed to quantify the degree of perfusion in 8 sections of each kidney. Digital clips were transferred to a PC: and SHI time intensity curves acquired in each section using Image-Pro Plus software. SHI fractional blood volumes (FBV) were calculated and the perfusion estimated from the initial slope of the FBV uptake. In vitro, SHI intensity increased approximately linearly with time as the bubbles flowed in. The slopes of the uptake curves were estimated to 0.0074, 0.016 and 0.015 s^-1, respectively. The slope approximately doubled when either concentration or flow rate doubled. In vivo, 270 SHI time intensity curves were acquired. SHI perfusion estimates correlated significantly with microsphere results (r = 0.57; p < 0.0001). The best SHI perfusion estimates occurred for high perfusion states in the anterior of the kidneys (r = 0.73; p = 0.0001). In conclusion, SHI perfusion estimates have been obtained in vitro and in vivo. The perfusion estimates were in reasonable agreement with a microvascular staining technique.