Synthetic exposure with a CMOS camera for multiple exposure speckle imaging of blood flow

Abstract

Speckle contrast imaging is an established technique to obtain relative blood flow maps over wide fields of view. A major improvement of the method relies on the acquisition of raw speckle images at different exposure times but requires simultaneous modulation of a laser pulse in duration and intensity and precise synchronization with a camera. This complex instrumentation has limited the use of multiple exposure speckle imaging. We evaluate here the use of a CMOS camera for a simplified approach based on synthetic exposure images created from the sum of successive frames acquired at a 1 ms exposure time. Both methods have been applied to evaluate controlled flows in micro-channels. The contribution of noises to the speckle contrast have been quantified and compared. Dark, readout and shot noise contributions to the total contrast remain constant for modulated exposure, while all these contributions decrease with increasing exposure time for synthetic exposure. The relative contribution of noises to speckle contrast depends on the level of illumination and the exposure time. Guidelines for flow measurements and limitations of the use of a CMOS camera with a limited frame rate for synthetic exposure acquisition scheme are discussed. The synthetic exposure method is simple to implement and should facilitate the translation of multiple exposure speckle imaging to clinical set-ups.

Wide dynamic range measurement of blood flow in vivo using laser speckle contrast imaging

“…The results indicate that β and ρ are critical parameters for deducing the BFI, and their accurate determination can greatly improve both the linearity and WDR 13 , 19 , 25 , 26 . Conventionally, when it comes to realizing a WDR for a flow rate with LSCI, the multi-exposure technique in which the camera exposure time range has been clearly given has been mentioned 13 , 14 . However, for obtaining the blood flow rate, compared with 15 exposure times from 50 μs to 80 ms with a standard multi-exposure speckle imaging (MESI) and 6 exposure times from 1 to 20 ms with a synthetic MESI implementation, in our LSCI setup, we only used 50 ms (for 1000 speckle images with 50 μs exposure time).…”

“… – 9 The research work currently focuses on imaging blood flow in the retina, skin, and cerebral cortex and has been reported in Refs. 10–14. In addition, a handheld LSCI device was developed, bringing convenience for both patients and clinical staff 15 …”

Standardizing Laser Speckle Orthogonal Contrast Imaging: Achieving Reproducible Measurements across Instruments

“…Qualitative approaches aim to visualize the morphology of blood flow without quantifying a physical parameter. Semi-quantitative methods, such as multiple exposure speckle imaging (MESI) [29,30], attempt to model and distinguish the contributions of static and dynamic layers to speckle contrast. They are generally more complex to implement than simple qualitative methods based on a single exposure time.…”

Fast analysis of multiple exposure speckle data to provide relative blood flow maps using convolutional neural networks

“…The MESI imaging system is composed of a 634 nm laser diode (Shangai laser, China) with a maximum output power of 400 mW and sCMOS camera (Orca Flash Hamamatsu, Japan). MESI data are acquired in the synthetic exposure approach with careful subtraction of noise contributions 6 . The microfluidic channels and set-up were described in details previously 4 .…”

“…On the instrumental side, FPGA processing was used to acquire multiple exposure data with high temporal resolution 5 . Implementation of MESI set-up based on synthetic exposure without laser modulation was further demonstrated using a sCMOS camera and careful noise corrections 6 . Once the speckle contrast K images are obtained at several exposures T, the flow-related decorrelation time tc of the scatterers is obtained by fitting a mathematical model of K (T, tc).…”