In vivo subharmonic pressure estimation of portal hypertension in canines

In this work, the development and first-ever application of in vivo subharmonic pressure estimation to detect portal pressures and investigate portal hypertension (portal vein pressures >; 12 mmHg) is presented. A modified Logiq 9 scanner (GE Healthcare, Milwaukee, WI) operating in pulse inversion subharmonic mode (f
_transmit
=2.5 MHz, f
_receive
=1.25 MHz) was used to scan 14 canines. The canines received an intravenous infusion of Sonazoid microbubbles (GE Healthcare, Oslo, Norway) at a dose of 0.015 μl/kg/min. Two models for inducing portal hypertension in the canines were used: low-flow using Gelfoam and high-flow using an arteriovenous shunt. A 5F micro-tip pressure catheter (Millar Instruments, Houston, TX) inserted in the portal vein provided the gold standard. The radiofrequency (RF) data were acquired in synchrony with the pressure transducer for 5 s segments (n = 3) for different transmit parameters, before and after inducing portal hypertension. The RF data were processed to extract the subharmonic components and correlated with the gold standard. A significant decrease (p <; 0.001) in subharmonic amplitude with an increase in portal vein pressures was seen for both models. The overall reduction in absolute subharmonic amplitude showed an r
²
value of 0.92 with an increase in portal vein pressures averaged over canines per treatment for 3 transmit cycles, whereas for 4 transmit cycles this r
²
value was 0.90 at 40% acoustic output from the scanner. A new technique to quantify absolute pressures in the portal vein and to detect portal hypertension using subharmonic ultrasound has been implemented.