Matthijs Draijer

Review of laser speckle contrast techniques for visualizing tissue perfusion

When a diffuse object is illuminated with coherent laser light, the backscattered light will form an interference pattern on the detector. This pattern of bright and dark areas is called a speckle pattern. When there is movement in the object, the speckle pattern will change over time. Laser speckle contrast techniques use this change in speckle pattern to visualize tissue perfusion. We present and review the contribution of laser speckle contrast techniques to the field of perfusion visualization and discuss the development of the techniques.

Twente Optical Perfusion Camera: system overview and performance for video rate laser Doppler perfusion imaging.

We present the Twente Optical Perfusion Camera (TOPCam), a novel laser Doppler Perfusion Imager based on CMOS technology. The tissue under investigation is illuminated and the resulting dynamic speckle pattern is recorded with a high speed CMOS camera. Based on an overall analysis of the signal-to-noise ratio of CMOS cameras, we have selected the camera which best fits our requirements. We applied a pixel-by-pixel noise correction to minimize the influence of noise in the perfusion images. We can achieve a frame rate of 0.2 fps for a perfusion image of 128x128 pixels (imaged tissue area of 7x7 cm2) if the data is analyzed online. If the analysis of the data is performed offline, we can achieve a frame rate of 26 fps for a duration of 3.9 seconds. By reducing the imaging size to 128x16 pixels, this frame rate can be achieved for up to half a minute. We show the fast imaging capabilities of the system in order of increasing perfusion frame rate. First the increase of skin perfusion after application of capsicum cream, and the perfusion during an occlusion-reperfusion procedure at the fastest frame rate allowed with online analysis is shown. With the highest frame rate allowed with offline analysis, the skin perfusion revealing the heart beat and the perfusion during an occlusion-reperfusion procedure is presented. Hence we have achieved video rate laser Doppler perfusion imaging.

Burn imaging with a whole field laser Doppler perfusion imager based on a CMOS imaging array

Laser Doppler perfusion imaging (LDPI) has been proven to be a useful tool in predicting the burn wound outcome in an early stage. A major disadvantage of scanning beam LDPI devices is their slow scanning speed, leading to patient discomfort and imaging artifacts. We have developed the Twente Optical Perfusion Camera (TOPCam), a whole field laser Doppler perfusion imager based on a CMOS imaging array, which is two orders of magnitude faster than scanning beam LDPI systems. In this paper the first clinical results of the TOPCam in the setting of a burn centre are presented. The paper shows perfusion images of burns of various degrees. While our system encounters problems caused by blisters, tissue necrosis, surface reflection and curvature in a manner similar to scanning beam imagers, it poses a clear advantage in terms of procedure time. Image quality in terms of dynamic range and resolution appears to be sufficient for burn diagnosis. Hence, we made important steps in overcoming the limitations of LDPI in burn diagnosis imposed by the measurement speed.

Relation between the contrast in time integrated dynamic speckle patterns and the power spectral density of their temporal intensity fluctuations

Scattering fluid flux can be quantified with coherent light, either from the contrast of speckle patterns, or from the moments of the power spectrum of intensity fluctuations. We present a theory connecting these approaches for the general case of mixed static-dynamic patterns of boiling speckles without prior assumptions regarding the particle dynamics. An expression is derived and tested relating the speckle contrast to the intensity power spectrum. Our theory demonstrates that in speckle contrast the concentration of moving particles dominates over the contribution of speed to the particle flux. Our theory provides a basis for comparison of both approaches when used for studying tissue perfusion.

Time domain algorithm for accelerated determination of the first order moment of photo current fluctuations in high speed laser Doppler perfusion imaging

Advances in optical array sensor technology allow for the real time acquisition of dynamic laser speckle patterns generated by tissue perfusion, which, in principle, allows for real time laser Doppler perfusion imaging (LDPI). Exploitation of these developments is enhanced with the introduction of faster algorithms to transform photo currents into perfusion estimates using the first moment of the power spectrum. A time domain (TD) algorithm is presented for determining the first-order spectral moment. Experiments are performed to compare this algorithm with the widely used Fast Fourier Transform (FFT). This study shows that the TD-algorithm is twice as fast as the FFT-algorithm without loss of accuracy. Compared to FFT, the TD-algorithm is efficient in terms of processor time, memory usage and data transport.

Connecting Laser Doppler Perfusion Imaging and Laser Speckle Contrast Analysis

Laser Speckle Contrast Analysis (LASCA) and Laser Doppler Perfusion Imaging (LDPI) are techniques widely used for determining cerebral blood flow, the skin perfusion in burns and during drug uptake, and cerebral blood flow. Both techniques are based on the dynamic speckle pattern on the detector generated by the sample under investigation. In LASCA the speckle pattern is recorded using a long exposure time (i.e. milliseconds) resulting in a blurred image, the perfusion map is obtained by calculating the contrast in the blurred image over small areas (e.g. 5x5 or 7x7 pixels). In LDPI a series of speckle patterns are recorded using a short integration time (i.e. microseconds). By determining the power spectrum of the intensity fluctuations per pixel and calculating the first moment, the perfusion map is obtained. Because both techniques are based on the same phenomenon we show it is possible to relate the outputs of LASCA and LDPI. Such a connection is important because of the growing interest in LASCA techniques. Here we perform the first steps in the comparison of both techniques, using both simulated signals and signals measured with a high speed camera which can perform LDPI as well as LASCA.

Time domain algorithm for whole field laser Doppler perfusion imaging

Recently, various groups have developed wide field laser Doppler perfusion imaging systems based on high speed cameras. The limiting factor for the frame rate and measurement duration in whole field laser Doppler perfusion imaging is the speed of transfer and analysis of data. We present an algorithm for calculating perfusion estimations with much lower demands for data storage and computational effort than the conventional FFT-based method. Our algorithm works in the time domain and estimates perfusion through simple time differentiations and multiplications of speckle image values. The algorithm is partly based on mathematical reasoning, and partly on a hypothesis that cannot be proven with rigorous mathematics. We will compare our algorithm with the frequency-domain counterpart for phantom studies involving static and dynamic media, and in vivo experiments on human skin. It is found that both algorithms, applied on the same dataset, approximately give the same perfusion estimations. The random differences are similar to the random variations found in tissue perfusion. Systematic differences between the algorithms smaller than 15% are found. The algorithm is currently twice as fast as the FFT-counterpart. Another advantage is that our algorithm can be included in a moving average scheme, where a new perfusion value can be determined based on the previous value and a small number of new raw speckle images.

Comparison of scanning beam and whole field laser Doppler perfusion imaging

Currently, laser perfusion imaging (LDPI) is undergoing a technology shift from scanning beam perfusion imagers to whole field systems. The latter can be subdivided in laser Doppler methods systems based on high speed CMOS cameras, and laser speckle contrast analysis (LASCA) technologies using slow imaging arrays, mostly CCD-based. In scanning beam systems, a collimated laser beam scans the tissue with diffusely back reflected light being captured with a single detector. In whole field systems a large tissue area is illuminated, and the reflected light is imaged onto an array and captured at once. Unlike scanning beam systems, both whole field methods enable perfusion imaging at video rate. In this study we experimentally compare the scanning beam LDPI principle with whole field LDPI, using Intralipid phantoms. For the tissue phantoms, the Monte Carlo simulation technique will be used as a reference. From measurements on Intralipid phantoms compared to Monte Carlo, we conclude that in whole field LDPI the flux image, representing the first order moment of the power spectrum of photocurrent fluctuations is much closer related to real perfusion than for scanning beam systems. This difference can be explained in terms of the different behaviour of dynamic speckle patterns generated in both methods, in response to varying tissue optical properties.