This technique is designed for measuring pressures non-invasively within the body. It's particularly useful for monitoring pressures in the heart, for monitoring treatment of cancers, and in this particular project for measuring pressures in the liver. To prepare the ultrasound contrast agent, remove the protective cap from the syringe port of the chemo spike and use a chemo spike to perforate the stopper of the first of three vials of the agent.
Add two milliliters of sterile water to the vial and immediately shake the vial for one minute. When a homogenous product has been obtained, withdraw the product into the syringe. The reconstituted ultrasound contrast agent contains microbubbles at a concentration of eight microliters per milliliter.
After repeating the reconstitution for the other two vials in the same manner, use saline to fill up connecting tubes and connect the tubes to a three-way stopcock. Connect the stopcock to the extension tubing leading to the cannula and connect the syringe directly to the stopcock. Then mount the syringe onto a syringe pump at the same level or below the patient.
To acquire an initial set of ultrasound images, power up an ultrasound scanner and select the C1-6-D curvilinear probe. Select an abdominal preset on the scanner. Use a subcostal approach and a curvilinear array to acquire grayscale images of both the portal and hepatic veins in the same imaging plane and at similar depths.
Optimize the images based on good clinical practice, taking care to select the hepatic vein region away from the inferior vena cava to avoid the influence of retrograde flow. After the initial ultrasound imaging, open the stopcock and infuse the saline solution at a rate of 120 milliliters per hour while co-infusing the contrast at a rate of 0.024 microliters per kilogram body rate per minute. For activating subharmonic imaging in dual display mode, use the subharmonic contrast touch panel button to activate contrast mode.
Select subharmonic imaging amplitude modulation on the rotary control. Perform subharmonic imaging at a transmit frequency of 2.5 megahertz and obtain the received signals at 1.25 megahertz. Confirm the patency of the portal and the hepatic veins as well as the presence of microbubbles, which can take up to one to two minutes from the start of the infusion.
Select time intensity curve analysis on the touch panel followed by F6 and K to compensate for varying depths and attenuations to activate the SHAPE automated optimization code to optimize the SHAPE imaging. The SHAPE optimization algorithm will acquire subharmonic data for every acoustic output level. Once the data acquisition is complete, position a region of interest on the portal vein in the contrast sample window.
Plot the average subharmonic data within the region as a function of acoustic output and fit a logistic curve to the data. Then select the inflection point of this curve as the optimized power, as this has been shown to be the point of greatest SHAPE sensitivity. Adjust the acoustic output power to the inflection point value to maximize the sensitivity of the SHAPE and acquire subharmonic data from the microbubbles in five to 15 second segments during the infusion of the UCA suspension.
When imaging for diagnosing portal hypertension in the liver, the key is to visualize both the portal and hepatic veins at the same depth to minimize the impact of attenuation. After a UCA induced equilibrium enhancement, the optimization algorithm should be activated and a region of interest in the portal vein should be selected to produce subharmonic amplitude and output power selection curves. In these SHAPE images from patients with and without clinical significant portal hypertension, the main visual difference is the marked subharmonic signal present in the hepatic vein in the patient with hypertension that is absent in the patient without hypertension.
Note that some patients present with clinical and laboratorial signs of portal hypertension but have HVPG values that are normal or zero. This can be attributed to a number of anatomical and/or vascular variations, resulting in an incorrect SHAPE diagnosis. This technique is non-invasive, safe, and accurate.
It allows the acquisition of relative as well as absolute pressure estimates, and it's something that no other imaging technique can currently do.
