Automatic full field analysis of perfusion images gained by scanning laser Doppler flowmetry

Abstract

Background-Scanning laser Doppler flowmetry (SLDF) enables the measurement of the laser Doppler frequency shift in retinal tissue. This process allows the quantification of retinal and optic nerve head perfusion in an area of 2.7 mm × 0.7 mm within 2 seconds and with a spatial resolution of 10 µm × 10 µm. Owing to the local heterogeneity of the retinal microcirculation itself and to heart associated pulsation the capillary retinal blood flow depends on location and time. Because of technical limitations measurements of flow are only valid in retinal points with adequate brightness and focus, and away from big vessels. To include the heart beat associated pulsation and the spatial heterogeneity of retinal blood flow into the evaluation of blood flow an algorithm was developed examining automatically the whole SLDF perfusion image. Aim-To report intraobserver reliability and interobserver reliability of a new method for analysing automatically full field perfusion images. Method-The base of blood flow calculation by the automatic full field perfusion image analyser (AFFPIA) was 16 384 intensity time curves of all pixels of the whole perfusion image gained by the SLDF. AFFPIA calculates the Doppler frequency shift and the haemodynamic variables flow, volume, and velocity of each pixel. The resulting perfusion image was processed with respect to (1) underexposed and overexposed pixels, (2) saccades, and (3) the retinal vessel tree. The rim area and the saccades were marked interactively by the operator. The capillaries and vessels of the retinal vessel tree were identified automatically by pattern analysis. Retinal vessels with a diameter greater than 30 µm, underexposed or overexposed areas, and saccades were excluded automatically. Based on the whole perfusion image total mean flow, total mean volume, total mean velocity, standard deviation, cumulative distribution curve of flow, and the capillary pulsation index were calculated automatically. Heart beat associated pulsation of capillary blood flow was estimated by plotting the mean capillary flow of each horizontal line against time. There is clinical need to quantitate noninvasively retinal or optic nerve head blood flow. Many ocular diseases are associated with decreased retinal or optic nerve head blood flow. By scanning laser Doppler flowmetry (Heidelberg retina flowmeter, HRF) it is possible to generate high resolution perfusion maps of the retinal and optic nerve head circulation. The principles of function of scanning laser Doppler flowmetry (SLDF) are described in detail elsewhere. [1][2][3][4] Briefly, SLDF measures the Doppler frequency shift in each of 16 000 retinal points with a size of 10 µm × 10 µm in a retinal area of 2.7 mm × 0.7 mm within 2 seconds. This enables the quantification of the perfusion of the retina and the optic nerve head in any area of the scanned sector. Owing to the spatial heterogeneity of the retinal microcirculation itself and heart associated pulsation the retinal perfusion is not constant within the perfusion map. T...

Reliability of Different Image Analysis Methods for Scanning Laser Doppler Flowmetry

“…37 The AFFPIA full-field option provides the highest reliability as it includes the largest possible image area for analysis, and automatically adjusts for the quality of the topographic image, "auto-cleaning" and masking any extra-wide vessels, optic nerve head, and over-or under-exposed pixels. 12 So, for a given cursor size, we would expect the AFFPIA reliability to be better than for HRF software. All image analysis issues that affect reliability will affect reproducibility as well as imaging-related issues, such as choice of location within the eye itself, distance between camera and eye, room lighting, sharpness of topographic image, and changes within the subject, such as level of relaxation 14 and ability to concentrate and fixate.…”

“…2) in accordance with the methods outlined elsewhere. 12,15,16,23,25,26 Briefly, AFFPIA calculates the Doppler frequency shift of the hemodynamic flow of each pixel from the entire image, while the standard HRF software only analyzes within a given small-framed area (i.e., the pixel cursor). For a valid estimation of retinal blood flow, AFFPIA, unlike the HRF software, adjusts brightness to mask under-or over-exposed pixels.…”

“…Figure 2 shows the steps of imaging processing. 12 Two different analysis options were utilized from each of two different image analysis software programs, resulting in four methods tested:…”

“…22 We aim to assess intra-observer reliability of retinal capillary flow, volume and velocity measurements of the HRF, and intra-subject reproducibility of multiple topographic scans obtained over a short period of time on the same examination day, using four analysis methods: the original default HRF software 23 with cursor options of 10 × 10 pixels (method 1) or 20 × 20 pixels (method 2), and the automatic full-field perfusion image analyzer (AFFPIA) with cursor options of 20 × 20 pixels (method 3) or a full-field option (method 4). 12 …”

Central Pulse Pressure Is an Independent Determinant of Vascular Remodeling in the Retinal Circulation

“…15 Briefly, a retinal sample of 2.56×0.64×0.30 mm was scanned within 2 seconds at a resolution of 256 points × 64 lines × 128 lines. The confocal technique of the device ensured that only the capillary flow of the superficial retinal layer of 300 µm was measured.…”