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Transit-time Ultrasound vs. Doppler Technology for Graft Patency Assessment

transit-time-ultrasoundYou’ve undoubtedly heard of the importance of using technology to determine graft patency during CABG surgery. Objective measurements, instead of subjective methods like palpation, helps ensure a graft is functioning properly, thus improving patient outcomes and reducing the need for repeat surgeries.

When it comes to techniques to evaluate CABG graft patency, options include transit-time ultrasound technology and Doppler technology. This post will take a look at both, allowing you to make an informed decision.

Let’s start at the beginning. How do these technologies work?

Transit-time Ultrasound Technology

Transit-time ultrasound works by producing an electrical excitation that causes the transducer to emit a plane wave of ultrasound. The ultrasonic waves are emitted from two crystals, pass through the vessel in upstream and downstream directions, bounce off the fixed “acoustic reflector,” which redirects the signals back through the vessel to the opposing crystal for detection. From these signals, the measurement device derives the “transit time” it takes for the waves of ultrasound to travel from each transducer to the other (traveling upstream takes more time than traveling downstream), providing an accurate measurement of the total flow passing through the vessel.

Doppler Technology

This technology works by using ultrasound waves to detect the speed of moving particles, such as red blood cells, in a vessel. When the ultrasound reflects off of one or more moving objects it undergoes a phase shift which is detected by the transducer. The amount of phase shift is related to the speed at which the object is moving (faster objects create greater phase shifts).

Transit-time Ultrasound Technology (TTU) vs Doppler:

  • Directly Measures Volume Flow vs.Velocity
    Transit-time ultrasound flowprobes integrate changes in velocity over the entire cross-sectional area of a vessel; Doppler probes measure localized velocity in a vessel. Ultrasonic illumination of transit-time ultrasound probes measures velocity of fluid across the entire bandwidth of graft/vessel and derives volume flow directly without additional measurements. Doppler provides an estimate of average velocity within the vessel. The reflected signal is dependent on the depth of vessel. Thus flow is not able to be determined by Doppler; only relative speed.
  • Measures Flow in Different Fluid Types
    Transit-time ultrasound measurement does not depend on particulate matter in the fluid (for example, RBC) like Doppler does, in order to measure flow. This allows measurement in homogeneous fluids like saline in addition to blood.
  • Shows Insensitivity to Misalignment of Probe on Vessel
    The design of a transit-time ultrasound flowprobe and reflector compensates for misalignment of a vessel within the lumen of the  probe. With Doppler  misalignment, the vessel can produce serious inaccuracy in measurement.

While Doppler technology is widely used to determine graft patency, TTU can provide additional information and improved accuracy that isn’t available with Doppler technology.

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