The laser speckle perfusion imager is a powerful, economical technique to image dynamic motion with high spatial and temporal resolution. It is a generally accessible vascular imaging apparatus with possible use at the bedside or during surgeries. It gives a measure of blood flow velocity by measuring the reduction in speckle contrast due to ‘blurring’ of dynamic speckles inside a fixed camera exposure time. In addition, LSPI offers ways to infer the clinically important parameters such as blood flow and perfusion from the vessel geometry in microcirculation images.
Blood flow can be calculated by using many methods that rely on direct examination, for example, plethysmography and thermal or radio-isotope clearance strategies. These procedures are time-consuming, inconvenient and invasive and aggravate the tissue’s regular state. Of the less invasive methods, ultrasound can be utilized. However, it is restricted to estimating higher flows at lower resolutions and is unsuccessful for estimating capillary blood flow. Strategies utilizing the Doppler effect or Laser speckle perfusion imaging technique use optical systems to record tissue perfusion, a relative measure of blood flow. These strategies have quick response times, are negligibly invasive and are equipped for generating almost real-time, two-dimensional images of tissue microcirculation.
To date, the examination of microvascular blood flow can be executed with a few optical systems, among which laser Doppler flowmetry (LDF) and laser speckle contrast imaging (LSCI) are presently being utilized. LSPI is an innovative full-field optical strategy and a synchronized technique that does not require any scan and uses an ordinary CCD or complementary metal oxide semiconductor (CMOS) camera.
Laser Speckle Perfusion Imager (LSPI) depends on the following principle: the region of interest is lit up by a laser with an extended beam, the backscattered light builds an interference pattern on the detector (a camera). Owing to the phase dissimilarity associated with the backscattered light, there are constructive and destructive interferences. The latter generates a design made out of light and dark parts on the camera. This design is known as a speckle pattern.
When a tissue is illuminated with laser light from LSPI and is later imaged, the coherent laser light will create a speckle structure inside the image of the illuminated tissue. The laser speckle perfusion imager utilizes full field illumination of the tissue and gives an instantaneous capture of image estimation points. A full-field laser perfusion imager uses a 780 nm laser light to gauge the product of normal blood speed in illuminated tissue and the concentration of moving red cells in a tissue sample volume by performing contrast examination on images procured from a video camera. It essentially measures apparent blood flow in the skin to a limited depth of around 1 mm. The velocity of blood flow is a vital parameter for understanding the physiological function and pathological alterations in microcirculation. The speckle structures inside the tissue image are an arbitrary interference pattern made by anomalies on and close to the surface of the laser-illuminated tissue.
For a stationary object, the speckle pattern shows a sharp speckle difference that stays static in time. If the object has a few individual particles experiencing movement, for example, red blood cells, then the interference pattern is dynamic and will vary in time. A spontaneous capture of a dynamic speckle design will likewise display a sharp speckle contrast, however, when a dynamic speckle structure is imaged over a limited integration period, then multiple speckle patterns become superimposed over each other, and the speckle pattern progresses toward becoming decorrelated or ‘blurred.’ The level of blurring is evaluated by the speckle contrast K that is inversely related to blood flow. The degree of decorrelation relies upon the speed and volume of blood flow inside the tissue. In LSPI, the whole tissue is illuminated with extended laser light, and a CCD camera with imaging lens records an image containing the superimposed speckle design. The decorrelation of the speckled design over the limited integration time of the CCD apparatus is utilized to measure blood flow.
Laser speckle perfusion imager (LSPI) is based on the recent laser speckle contrast imaging technology which is established on the principles of speckle contrast analysis and presents an index of blood flow. The technique is employed for imaging tissue vascular structure. It has been in use since the early 1980s and as of late has been modified to utilize a charge-coupled device (CCD) camera and image processing method. It makes use of the spatial statistics of time-integrated speckle. It was first introduced by Fercher and Briers (Fercher and Briers, 1981).
Although LSPI was first presented as an alternative to Laser Doppler flowmetry for mapping microvascular perfusion in different tissues including the skin and the retina, it has now been extended to other areas and adapted to generate flow maps of the external layers of the cerebral cortex. LSPI proved to be highly suitable for the purpose, and a detailed assessment of LSPI against laser Doppler flowmetry has shown that the two methodologies convey correlating flow data and are likewise applicable and powerful, with LSPI having the benefit of a better spatial resolution. LSPI flow maps are calculated utilizing fluctuating intensity of the arbitrary interference effect called as speckle. Both methods determine flow data based on the same physical phenomenon and yield similar outcomes. Specifically, laser Doppler flowmetry and LSPI were established as equally suitable for the description of CBF changes, CO2 challenge, or after middle cerebral artery occlusion of rodents in animal models.
LSPI has been profoundly adopted for use in neuroscience, for example, blood flow imaging of neurovascular pathologies, functional revival, and even human cortical blood flow imaging throughout neurosurgery. The dynamic imaging capacity of LSPI emerges from connections between coherent photons and tissue. At the point when these photons collaborate with moving elements, the inconsistency of the distinguished intensity variations of the speckle pattern changes, which cause spatial blurring while averaging over a set exposure period. This blurring is specifically connected to the change in the intensity autocorrelation function g2(t), which is consecutively linked to sample movement through the field autocorrelation function g1(t). Nearly all speckle contrast models aim to relate variations in speckle contrast to variations in the autocorrelation decay time, tc. The autocorrelation time is said to be inversely related to the speed of the scatterers, with multiple scattering theories including a weighting term for each added dynamic scattering occurrence. Chronological intravascular scattering has been examined with regards to diffuse correlation spectroscopy; however, it still needs to be analyzed for LSPI.
Efforts to build up a more methodical, neurovascular-specific comprehension of speckle imaging depend on revisiting hypotheses utilized in dynamic light scattering models and including practical data about the complex spatial structure of the vascular system. Latest researches have demonstrated that the level of intravascular multiple amid speckle imaging is lower than the photon dispersion limit and is considerably different when imaging surface vessels and parenchyma. Moreover, speckle visibility models that integrate multiple scattering as an element of vessel quality have been shown to consistently predict the association amongst tc and red blood cell (RBC) speed fluctuations in surface vessels in vivo. None of these techniques have studied the sensitivity of the LSPI signal to variations in flow in particular areas of the vascular bed. The speckle contrast signal emerges from a collective average of all dynamic scattering occurrences experienced by detected photons. Deciding the sensitivity of speckle contrast imaging to changes in flow, thus, needs an account of intravascular scattering not just directly under the detector, but also in each vessel that a detected photon may have traveled through.
Humeau-Heurtier et al. evaluated the microvascular blood flow using the generalized differences algorithm. LSPI technique has a drawback of leading to a huge quantity of data. Efforts to perform a spatial averaging of blood flow by clinicians only lead to a reduced spatial resolution for the analyzed data. To overcome the problem of poor spatial resolution, a new post-acquisition visual representation for LSPI perfusion data by means of the generalized differences (GD) algorithm was proposed. For the experiment, LSPI produced 15 simulated images, each one 30 × 30 pixels2. This technique produced a new single perfusion image which in itself presented the changes of the blood flow on the complete images of the stack. Furthermore, this latest image had the benefit of having a similar spatial resolution as the original images. The data confirmed that the generalized differences algorithm presented a new method of visualizing LSPI perfusion data.
Laser speckle imaging has become a pervasive technique for imaging blood flow in various tissues. However, because of its wide field imaging characteristic, the measured speckle contrast is a depth-integrated measure and understanding of baseline values. The depth-dependent sensitivity of those values to alterations in basic flow has not been comprehensively assessed. Davis et al. presented a newly developed procedure for measuring the autocorrelation function for ordered flow in 3D geometries. Laser speckle contrast imaging was used to determine the sensitivity of LSCI to variations in underlying velocity utilizing Monte Carlo simulations of light scattering in the cortical vasculature and a relative account of blood flow in all vessels. It was established that the regularly used type of g1(t), depends on assumptions concerning the quantity and form of scattering that are not correct. In addition, it was demonstrated that using the generally used speckle contrast models lead to almost the same sensitivity to underlying flow.
Blood flow and perfusion are essential experimental microcirculation factors. Laser speckle flowmetry has suffered from some speculation regarding the estimation of the inverse relation involving decorrelation time (τc) and blood flow velocity (V) i.e., 1/τc = αV. Nadort et al. utilized a microcirculation imager, i.e. integrated sidestream dark field – laser speckle contrast imaging (SDF-LSCI) device. The SDF – LSCI empirically investigated the effect of the optical properties of scatterers on α in vitro and in vivo. The imaging tip of the integrated SDF-LSCI appliance was lightly set in contact with the chorioallantoic membrane tissue of the chick embryo to avoid disruption of blood flow and in vivo image frames were recorded. The apparatus was secured by hand for sublingual microcirculation imaging while it was fixed to a stand for chick embryo microcirculation imaging. The data concluded that SDF-LSCI provided a quantifiable estimate of flow velocity as well as vessel morphology, allowing the quantification of the clinically significant blood flow, velocity, and tissue perfusion.
Numerous studies have revealed that the LSCI has great potential to be an important cerebral blood flow measuring procedure for neurosurgery. Yet, the quantitative precision and sensitivity of LSCI are inadequate and vastly reliant on the exposure duration. An addition to LSCI called multi-exposure speckle imaging (MESI) has overcome these restrictions. Richards et al. used the LSCI to evaluate patients going through brain tumor resection intraoperatively. This experimental research measured several exposure times from the same cortical tissue area and assessed images separately as single-exposure LSCI and combined using the MESI model. The results revealed that the MESI measurements presented the widespread flow sensitivity for sampling the extent of flow in the brain, narrowly followed by the shorter exposure times. In conclusion, intraoperative MESI can be conducted with high quantitative precision and sensitivity for cerebral blood flow monitoring.
LSPI has also been used to evaluate the chronic wide-field imaging of brain hemodynamics. Chronically assessing brain activity throughout active and social behavioral situations has provided significantly relevant physiological information on pathological conditions. Miao et al. presented a new standalone micro-imager for examining the cerebral blood flow (CBF) and total hemoglobin (HbT) behavior in the freely moving status of animals utilizing the laser speckle contrast imaging (LSCI) and optical intrinsic signal (OIS) techniques. Moreover, a novel cranial window technique, utilizing contact lens and wide field optics, was presented to attain the chronic and wide-field imaging of rat’s cerebral cortex. Chronic imaging revealed enhanced CBF and HbT in motor cortex while the rats were going up the cage wall. Additionally, after the climb, CBF completely returned to the baseline while HbT demonstrated a late recovery. The micro-imager equipment offers the new potential for brain imaging in cognitive neuroscience experiments like analysis of brain activities in social activities and social impulses.