Colour Doppler Imaging of Ocular and Orbital Blood Vessels in Retinal Diseases

Abstract

ImagingColour Doppler Imaging (CDI) is an established method for investigation of the ocular and orbital blood flow characteristics. It is an ultrasonographic method that has been employed for the evaluation of the circulatory status in much ocular pathology, but is most used for the investigation of the circulatory status in retinal vascular disorders and in glaucoma. [1][2][3] Ultrasound imaging (sonography) is accomplished with a pulse-echo technique. Pulses of ultrasound (generated by a transducer) are sent into the patient where they produce echoes at organ boundaries and within tissues. These echoes then return to the transducer, they are being detected and then displayed. Colour-flow imaging extends the use of the pulse-echo imaging principle. Echoes returning from stationary tissues are detected and presented in greyscale in appropriate locations along the scan line. The depth is determined by the echo's arrival time and the brightness is determined by the echo's amplitude. If a returning echo has a different frequency than what is emitted a frequency change has occurred because the echo-generating object was moving. If the motion is towards or away from the transducer, the Doppler shift is positive or negative, respectively. Along with the transducer, the colour-flow instrument consists of a pulser, a beam-former, a receiver, a memory and a display (see Figure 1).In ophthalmology, CDI is most often used to study the circulatory parameters in the retrobulbar blood vessels -the central retinal artery, central retinal vein, ophthalmic artery and the short and long posterior ciliary arteries. However, intraocular blood vessels have also been studied, such as vortex veins, blood vessels in the intraocular tumours, in detached retina, etc. The measurement is usually obtained through closed eyelids in a supine or in a seated position. Nagahara et al.patented a device for CDI measurement in seated subjects that increased the reproducibility of the measurement by 50% (see Figure 2). 4 The reflected ultrasound from the moving cells in the measured blood vessels is recorded by the CDI device and represented as a velocity wave. Two such velocity waves from the central retinal artery and the central retinal vein are presented in Figure 3.Several blood flow parameters can be obtained from the velocity wave, such as peak systolic blood velocity (PSV), end-diastolic blood velocity (EDV), mean blood velocity (MV), resistivity index (RI) and pulsatility index (PI): RI is calculated as (PSV-EDV)/PSV; PI is calculated as (PSV-EDV)/MV.Although the CDI device is equipped for measurement of blood vessel diameter, the dimension of the retrobulbar blood vessels is too small to obtain a reliable estimation of their diameter. Therefore, total blood flow cannot be evaluated in the retrobulbar blood vessels using CDI.The recording of the blood flow parameters of the central retinal artery and vein is obtained inside the optic nerve, a few millimetres away from the lamina cribrosa (see Figure 3). A specific finding of the blood flo...

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