How Can Blood Flow Be Measured?
Section snippets
Color Doppler Imaging
Color Doppler imaging (CDI) is an ultrasound technique that combines B-scan images with velocity information obtained from the Doppler shift of the moving erythrocytes.19, 39 Originally developed for monitoring blood flow in the heart, carotid artery, and peripheral vessels, CDI can also be used for the assessment of blood velocity of retrobulbar vessels. In the CDI image the color-coded velocity information is displayed together with the B-scan image. The operator can identify the peak
Angiographic Techniques
Based on clinical angiograms using sodium fluorescein several attempts have been made to quantify retinal blood flow. Angiography allows for the visualization of anatomic structures by the passage of a fluorescent dye. Most approaches are based on the measurement of the time required for the dye to pass through the retinal circulation. The mean retinal circulation time, defined as the difference between venous and arterial times, is used as a measure for retinal blood velocity. Alternatively,
Blue Field Entoptic Technique
The blue field entoptic technique uses the optical blue field entoptic phenomenon in order to investigate the leucocyte dynamics in retinal perifoveal vessels.30 The blue field entoptic phenomenon is best visible when looking into a blue light with a narrow optical spectrum at a wavelength of approximately 430 nm. Under these conditions many tiny corpuscles can be observed by the subject under study around an area of the center of the fovea. The phenomenon is produced by the different
Laser Doppler Velocimetry
An approach to measure blood flow velocities in retinal arterioles and venules is based on laser Doppler velocimetry (LDV). This technique uses the optical Doppler shift of light, which is directly proportional to the blood velocity when the vessel is illuminated with a high coherent laser beam. The power spectrum of the reflected laser light consists of a range of frequency shifts corresponding to the flow velocities within the vessel under study.28 The maximum frequency shift corresponds to
Retinal Vessel Diameters
All the techniques mentioned so far are limited by the fact that only information about blood velocity is available but not blood flow per se. Thus, the interpretation of the results is hampered by the fact that it is difficult to decide whether an increase in blood velocity is caused by an increase of blood flow, or by vasoconstriction within the measured vascular bed. Consequently, to determine blood flow, the exact determination of vessel diameters is crucial. The methods proposed for the
Laser Speckle Technique
Another approach to assess retinal blood flow is the use of the laser speckle phenomenon. The laser speckle phenomenon occurs when the ocular fundus is illuminated by coherent light and is characterized by a rapidly varying pattern, caused by backscattered light from the rough surface of the ocular fundus. The rate of variation gives an estimate of blood cell velocity and can therefore be used for the quantification of retinal blood flow.5 Based on these findings, instruments for the mapping
Laser Doppler Flowmetry
Laser Doppler flowmetry (LDF) is a technique in which the laser light is not directed towards a retinal vessel, but on vascularized tissue with no larger vessels visible. Based on a scattering theory for light in tissue formulated by Bonner and Nossal,4 which assumes a complete randomization of light directions impinging on the erythrocytes, relative measures of the mean velocity of the erythrocytes and the blood volume can be obtained.26 Relative values of blood flow are calculated as the
Pulsatile Ocular Blood Flow
Techniques have been introduced to estimate pulsatile ocular blood flow (POBF) based on the changes in ocular volume and pressure during the cardiac cycle. Basically, two different approaches have been proposed for the assessment of POBF. First, the pneumotonometric approach is based on the observation that intraocular pressure changes during the cardiac cycle. The maximum IOP change during a heart cycle is called pulse amplitude and a theoretical model has been introduced to calculate POBF.16,
Optical Doppler Tomography
Among the most promising new approaches to assess ocular hemodynamics in humans is optical Doppler tomography. This technique combines the principles of laser Doppler and optical coherence tomography. Instruments to measure velocity profiles in larger retinal arteries and veins have been realized using time-domain43, 44 and frequency-domain approaches.17, 18, 40 In addition, the feasibility of optical Doppler tomography for the visualization of the vascular system of the eye has been
Conclusion
The rapid technical progress has provided us a couple of new innovative technologies, which are currently available for the assessment of ocular blood flow. However, only a few of these technologies are currently used in routine patient care and none of these techniques can be regarded as a gold standard for the measurement of ocular blood flow. Nevertheless, the continuous investigation of blood flow parameters has brought us new insights into the physiology of ocular blood flow regulation and
Method of Literature Search
Literature selection for this review was based on a Medline database search using the terms ocular blood flow, retinal blood flow, choroidal blood flow and optic nerve head blood flow, from the period 1966 to 2007. Pertinent articles from the English-language journals only were selected. The articles referenced were primarily from 1990 to 2006 and relevant references contained within those articles were gathered.
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The authors reported no proprietary or commercial interest in any products mentioned or concepts discussed in this article.