Imaging Behind the Plaque: Improved Blood Flow Quantification Using an Iterative Scheme for Active Attenuation Correction

OBJECTIVE: Blood flow quantification using high frame-rate (HFR), contrast-enhanced ultrasound followed by particle image velocimetry (PIV), termed echoPIV, allows the study of blood flow phenomena in diseased arteries before and after treatment. However, acoustic shadows caused by atherosclerotic plaques may lead to incomplete flow quantification. As a global increase in transmit pressure to compensate for the attenuation would lead to contrast agent destruction in unattenuated areas, this article proposes a method to locally enhance the signal amplitude, thereby improving flow quantification accuracy. METHODS: The acoustic pressure was locally increased by adjusting the transmit apodization of the transducer elements using a proportional integral controller coupled to an acoustic model based on the Rayleigh integral. These iterative adjustments were performed prior to the HFR recording. This iterative scheme for active attenuation correction (ISAAC) was applied in vitro on phantoms with different levels of attenuation. A PIV analysis was then performed on each of the recorded HFR datasets. RESULTS: Without ISAAC, using a driving voltage of 11.2V, the mean errors in velocity estimates were below 20% for attenuation values up to 6.4 dB. Using ISAAC, the errors were reduced to less than 10% for attenuation values up to 8.5 dB and to less than 20% for attenuation up to 10.6 dB. CONCLUSION: The proposed iterative scheme for attenuation correction was shown to compensate effectively for acoustic signal loss in acoustic shadows. ISAAC led to an improved accuracy in echoPIV-derived flow velocities.