Background: It is challenging for the current Doppler imaging to detect blood flow in small retinal vessels. Power Doppler, with its high sensitivity to detect minimal blood flow, can be used with spectral Doppler to measure blood velocity in small vessels of the eye and orbit.
Methods: Sixteen eyes of twelve normal pigmented rabbits were studied, using a dedicated small animal, high-resolution imaging unit (Vevo 770) and 17.6 MHz ultrasound probe. Spectral Doppler (ISPPA = 67.1 W/cm(2), ISPTA = 483.7 mW/cm(2), MI = 0.5) was combined with power Doppler (ISPPA = 137.7 W/cm(2), ISPTA = 83.1 mW/cm(2), MI = 0.77) to measure the blood velocity over each identified vessel, including the central retinal artery and vein, branch retinal artery and vein, choroidal vein, and the long and short posterior ciliary artery. Three readings from each vessel were averaged to reduce measurement error. Three indices were calculated from the arterial blood velocity readings: the resistive index, the pulsatility index and the A/B ratio.
Results: The highest arterial blood velocity was measured over the long posterior ciliary artery; peak systolic velocity was 18.29 cm/s and end diastolic velocity was 16.63 cm/s, while the lowest arterial blood velocity was measured over the branch retinal artery; peak systolic velocity was 5.08 cm/s and end diastolic velocity was 3.25 cm/s. On the other hand, the highest venous blood velocity was measured over the choroidal veins (7.0 cm/s), and the lowest venous blood velocity was measured over the branch retinal vein (2.88 cm/s). No statistically significant difference was observed between the nasal and temporal retinal arterial blood velocity. Combining power Doppler with spectral Doppler imaging caused no damage and is a safe technique to measure blood velocity.
Conclusion: A combination of spectral Doppler and power Doppler ultrasound can be used as a noninvasive and efficient tool for reproducible measurement of the blood velocity in the posterior segment.