Fig. 7.12 NeXT computer output of LDF measurements in human optic nerve head (ONH) tissue. Vel (kHz), Vol (AU = arbitrary units), and Flow (AU) are shown as a function of time (21 samples/s). Gaps in data are caused by blinks during which recording is automatically disabled. Waveforms on the right side are the data (within a

user-deÞned time period), averaged according to the heart pulse (i.e., each point is the average of all data recorded at the same delay in the heart cycle), showing the systolic and diastolic values of the ßow parameters (Adapted from Petrig and Riva [71] with permission from the Publisher)

(670 nm) is delivered to a discrete site (about 150 mm at the disk or at the fovea) [8]. The scattered light is collected by an optical Þber with the image of its aperture (approximately 150 mm in diameter) focused onto the illuminated site. This Þber guides the scattered light to a photodetector. An area of the fundus (30¡ in diameter) is illuminated in red-free light for observation of the fundus and positioning of the laser beam. For some applications requiring measurements in darkness, a laser probing beam in near-infrared (between 750 and 805 nm) is used, and the fundus is also illuminated in near-infrared light (826 nm). Observation is achieved by means of a CCD camera and video monitor presentation [33, 70]. In addition, the position of entrance of the laser beam at the pupil can be monitored by means of a CCD Þnger camera [70]. For precise placement of the laser beam at the desired site in ONH blood ßow measurements, a point-like target consisting of the aperture of a 50-mm-diameter optical Þber illuminated by a red or green diode can be focused in the retinal plane of the ophthalmoscopic lens and moved in this plane. For the investigation of the effect of increased retinal activity on ONH blood ßow, a system for delivering visual stimuli (ßicker and contrast reversal pattern) has been incorporated to the fundus camera-based instrument [70].

A NeXT computer system with dedicated software is used for LDF in the human eye [71]. This software allows averaging of the Doppler

signal in phase with the heart cycle so that precise measurements of RBC ßux variations during the systolic and diastolic phases can be obtained. Recordings of Vel, Vol, and Flow from the ONH of a normal volunteer are shown in Fig. 7.12.

More recently, a compact confocal laser Doppler ßowmeter has been developed for the measurement of SFCH blood ßow (Fig. 7.13). This instrument detects the light scattered by the RBCs within an annulus centered on the illumination site of the laser at the fovea [72]. A miniaturized version of this instrument has been mounted on a helmet, thus facilitating SFCH blood ßow measurements during various types of dynamical physiological maneuvers, such as biking [73]. In both instruments, the illuminated laser spot and the detection annulus are selfaligned. The pupil of the optical system is small enough to allow SFCH blood ßow measurement without pupil dilatation.

7.3.4Critical Questions Regarding the Application of LDF to Ocular Blood Flow

7.3.4.1 LDF Sample Volume

A central question in the application of LDF to the ONH is the depth of the sampling volume. This depth determines the relative contribution to the Doppler signal from the superÞcial layers,

those supplied by the central retinal artery, and the deeper layers supplied by the posterior ciliary arteries. These two vascular beds may have different blood ßow regulation properties. Furthermore, the deep layers of the ONH appear to be particularly susceptible to ischemic disorders, including glaucoma.

Investigations on a model system suggests that when the light collecting aperture coincides with the tissue volume illuminated by the probing laser, layers of the ONH tissue as deep as 300 mm contribute to the LDF signal [74]. In man, however, although the depth of tissue sampling in the ONH remains to be experimentally assessed, it appears that the LDF technique detects predominantly the motion of RBCs within the intraocular region of the ONH [75]. A study on monkey eyes concluded that LDF is predominantly sensitive to blood ßow changes in the superÞcial layers of the ONH and

less to those layers of the prelaminar and deeper regions of the ONH, and their relative proportions are still unknown [76]. The weaker signal from the deep layers cannot be separated from the dominant signal from the superÞcial layers to exclusively study the circulation in the deep layers.

7.3.4.2 Linearity of LDF

LDF does not provide absolute measurement of blood ßow. Nevertheless, valid measurements of the changes in blood ßow are obtained if Flow varies linearly with the actual blood ßow. Linearity between Flow and actual blood ßow has been documented for various tissues, such as the skin, skeletal muscle, cerebral cortex, nerves, and others [1].

To test this linearity for the ONH and SFCH, Flow was measured in both tissues of the cat eye while the mean ocular perfusion pressure was