| Laser Doppler imaging | |
|---|---|
 Microangiography of theoptic disc region of the human retina, by laser Doppler imaging. The image was rendered computationally by optical wave propagation and measurement of optical fluctuations. | |
| Purpose | measure blood flow in eye |
| Based on | Digital holography |
Laser Doppler imaging (LDI) is animaging method that uses a laser beam to image live tissue. When the laser light reaches the tissue, the moving blood cells generateDoppler components in the reflected (backscattered) light. The light that comes back is detected using a photodiode that converts it into an electrical signal. Then the signal is processed to calculate a signal that is proportional to the tissueperfusion in the imaged area. When the process is completed, the signal is processed to generate an image that shows theperfusion on a screen.[1]
The laser Doppler effect was first used to measuremicrocirculation by Stern M.D. in 1975.[2][3] It is used widely inmedicine, some representative research work about it are these:[4][5][6]
Theeye offers a unique opportunity for the non-invasive exploration ofcardiovascular diseases. LDI bydigital holography can measure blood flow in theretina andchoroid.[7] In particular, thechoroid is a highly vascularized tissue supplying the retinal pigment epithelium and photoreceptors. Yet investigating the anatomy and flow of the choroid remains challenging. LDI provides high-contrast visualization of local blood flow in choroidal vessels in humans, with a spatial resolution comparable to state-of-the-art indocyanine green angiography.[8] Differences inblood pressure drive the flow of blood throughout the circulation. The rate of mean blood flow depends on both blood pressure and the hemodynamic resistance to flow presented by the blood vessels. LDI can enable mapping of the localarterial resistivity index, and the possibility to perform unambiguous identification of retinal arteries and veins on the basis of theirsystole-diastole variations, and reveal ocularhemodynamics in human eyes.[9]
The local velocity of blood flow measured by laser Doppler holography in the digit (photoplethysmogram) and the eye fundus has apulse-shaped profile with time. These remote pulse wave measurements can be done clinically to revealhemodynamics in arteries and veins and can be readily measured non-invasively.Principal component analysis of digital holograms[10] is an efficient way of performing temporal demodulation of digital holograms reconstructed from on-axis interferograms and can be used to reveal surface waves on the hand.
LDI provides a direct measure of female sexual response that does not require genital contact; signals are gathered at a depth of two to three millimetres below the skin's surface.[11] Two studies have suggested that LDI is avalid measure of female sexual arousal.[11][12] Waxman and Pukall[11] showed that LDI hasdiscriminant validity; that is, it can differentiate sexual response from neutral, positive, and negative mood induced states. Compared tovaginal photoplethysmography (VPG), LDI is advantageous because it does not require genital contact. Also, LDI provides a direct measure ofvasocongestion and has an absolute unit of measurement, consisting of flux or units of blood flow. The disadvantages of LDI are that it cannot provide a continuous measure of sexual response and the laser Doppler perfusion imager is much more costly that other methods of genital sexual arousal assessment, such as VPG.[11]
