Methodological concerns with laser speckle contrast imaging in clinical evaluation of microcirculation

Abstract

Background: Laser Speckle Contrast Imaging (LSCI) is a non-invasive and fast technique for measuring microvascular blood flow that recently has found clinical use for burn assessment and evaluation of flaps. Tissue motion caused by for example breathing or patient movements may however affect the measurements in these clinical applications, as may distance between the camera and the skin and tissue curvature. Therefore, the aims of this study were to investigate the effect of frame rate, number of frames/image, movement of the tissue, measuring distance and tissue curvature on the measured perfusion.

Methods: Methyl nicotinate-induced vasodilation in the forearm skin was measured using LSCI during controlled motion at different speeds, using different combinations of frame rate and number of frames/image, and at varying camera angles and distances. Experiments were made on healthy volunteers and on a cloth soaked in a colloidal suspension of polystyrene microspheres.

Results: Measured perfusion increased with tissue motion speed. The relation was independent of the absolute perfusion in the skin and of frame rate and number of frames/image. The measured perfusion decreased with increasing angles (16% at 60°, p = 0.01). Measured perfusion did not vary significantly between measurement distances from 15 to 40 cm (p = 0.77, %CV 0.9%).

Conclusion: Tissue motion increases and measurement angles beyond 45° decrease the measured perfusion in LSCI. These findings have to be taken into account when LSCI is used to assess moving or curved tissue surfaces, which is common in clinical applications.

Fig 1. The setup of the experiment in healthy subjects.

When investigating the effect of tissue motion, the forearm was resting on a laboratory shaker that generated motion at controlled speeds. When investigating the effect of camera angle and distance, the forearm was still and the camera head was tilted and moved up and down in relation to the skin surface of the forearm.

Fig 2. Areas in which different concentrations of methyl nicotinate were applied (1: 40 mM, 2: 10 mM, 3: 2.5 mM) on the volar side of the forearm at different speeds (A: 0 mm/s, B: 25 mm/s, C: 41 mm/s, D: 90 mm/s).

Fig 3. Relation between motion and measured perfusion in calibration fluid using different system settings (frame rate and number of frames).

Fig 4

A-C. The relation between speed (mm/s) and mean measured Perfusion (PU) in the skin of the forearm after application of different concentrations of Methyl Nicotinate (MN), depending on the frame rate and the number of frames over which the perfusion is averaged (n = 8).

Fig 5

A-B. Relation between motion and measured perfusion in the skin of the forearm at different baseline perfusion values (A, absolute perfusion; and B, increase in perfusion).

Fig 6. Influence of distance and angle on the measured perfusion in vivo on the dorsal side of the forearm.

Fig 7. Example of images with and without motion artefacts of the hands in 2 burn patients.

Images A and C do not have motion artefacts, while image B and D do. All images were taken of the same patient and injury. Note that measured perfusion is higher in images B and D, as a result of patient motion.

Fig 8. Example of breathing artifact during perfusion measurements in a Deep Inferior Epigastric Perforator (DIEP) flap.

Several images were acquired during the same breathing cycle. The first image (A) is taken at end-expiratory level, when movement of the abdomen is minimal, and has a perfusion in the central region of 75 PU, whereas the second image (B) is taken at mid-expiratory level when the movement is relatively large, and a perfusion of 95 PU was measured in the same region. Thus, measured perfusion varied by 24% during the breathing cycle.