Quantitative photoacoustic integrating sphere (QPAIS) platform for absorption coefficient and Grüneisen parameter measurements: Demonstration with human blood.

Quantitative photoacoustic imaging in biomedicine relies on accurate measurements of relevant material properties of target absorbers. Here, we present a method for simultaneous measurements of the absorption coefficient and Grüneisen parameter of small volume of liquid scattering and absorbing media using a coupled-integrating sphere system which we refer to as quantitative photoacoustic integrating sphere (QPAIS) platform. The derived equations do not require absolute magnitudes of optical energy and pressure values, only calibration of the setup using aqueous ink dilutions is necessary. As a demonstration, measurements with blood samples from various human donors are done at room and body temperatures using an incubator. Measured absorption coefficient values are consistent with known oxygen saturation dependence of blood absorption at 750 nm, whereas measured Grüneisen parameter values indicate variability among five different donors. An increasing Grüneisen parameter value with both hematocrit and temperature is observed. These observations are consistent with those reported in literature.

Quantitative blood oxygen saturation imaging using combined photoacoustics and acousto-optics.

In photoacoustic spectroscopy (PAS), wavelength dependent optical attenuation of biological tissue presents a challenge to measure the absolute oxygen saturation of hemoglobin (sO2). Here, we employ the combination of photoacoustics and acousto-optics (AO) at two optical wavelengths to achieve quantification, where AO serves as a sensor for the relative local fluence. We demonstrate that our method enables compensation of spatial as well as wavelength dependent fluence variations in PAS without a priori knowledge about the optical properties of the medium. The fluence compensated photoacoustic images at two excitation wavelengths are used to estimate the absolute oxygen saturation of blood in a spatially and spectroscopically heterogeneous phantom.

Perfusion Intensity Correlates with Expression Levels of Psoriasis-Related Genes and Proteins.

BACKGROUND: Previous research revealed heterogeneity in the perfusion intensity within clinically homogenous-appearing plaques, without differences in erythema. In addition, an increased perfusion was found within the perilesional skin. This raises the question whether the heterogeneity in perfusion found both inside and outside a lesion influences the expression levels of genes and proteins involved in the pathogenesis of psoriasis. OBJECTIVES: To correlate the perfusion intensity to mRNA and protein expression of genes associated with the pathogenesis of psoriasis and to visualize the dynamics of the perfusion intensity over time using laser Doppler perfusion imaging. METHODS: Fourteen patients with plaque psoriasis were included. The superficial microcirculation and clinical local scores (single usability metric, SUM, scores) were analysed in one representative lesion every 2 weeks. After 8 weeks 4 biopsies were taken, one from a highly perfused area (hotspot) and one from a low perfusion area (coldspot) of the lesional skin, one biopsy from the highly perfused perilesional skin and one from the distant uninvolved skin. RESULTS: Statistically significant differences in mRNA and protein expression, including IL-17 and TBX21/T-Bet, were found between hotspots and coldspots, and between the highly perfused perilesional and the uninvolved skin. Hotspots tend to remain on the same location during 8 weeks of follow-up. CONCLUSIONS: Within homogenous-appearing psoriatic plaques, there are remarkable differences in mRNA and protein levels, which are correlated with the perfusion intensity and can be detected by using laser Doppler perfusion imaging. In addition, differences in mRNA and protein expression between the highly perfused perilesional skin and the uninvolved skin were found, indicating that several biological changes occur well before clinical changes become manifest.

Solving the speckle decorrelation challenge in acousto-optic sensing using tandem nanosecond pulses within the ultrasound period.

We present a novel acousto-optic (AO) method, based on a nanosecond laser system, which will enable us to obtain AO signals in liquid turbid media. By diverting part of the light in a delay line, we inject tandem pulses with 27 ns separation. The change of the speckle pattern, caused by the ultrasound phase shift, reduces the speckle contrast of the integrated speckle pattern captured in a single camera frame. With these tandem pulses, we were able to perform AO on a 2 cm liquid turbid medium in transmission mode. We show the raw signal and a spatial AO scan of a homogenous water-intralipid sample. This approach is potentially capable of AO probing in vivo, since the acquisition time (of approximately 40 ns) is four orders of magnitude less than the typical time scales of speckle decorrelation found in vivo. The method may eventually enable us to obtain fluence compensated photoacoustic signals generated by the same laser.

Photoacoustic measurement of the Grüneisen parameter using an integrating sphere.

A method that uses an integrating sphere as a platform for photoacoustic measurement of the Grüneisen parameter Γ of absorbing liquids is developed. Derivation of a simple equation for determining Γ is presented. This equation only requires the voltage peak-to-peak value of the photoacoustic signal detected by a flat transducer and the relative energy of the incident light measured by a photodetector. Absolute detector sensitivities are not required. However, a calibration procedure is necessary. An experimental setup is constructed in order to implement and verify the method. Aqueous ink solutions are used as absorbing liquids to determine the calibration (instrument) constants. Validation of the equation is done by determining Γ of ethanol at room temperature. The obtained value of Γ(ethanol) = 0.72 ± 0.06 has a 7% relative difference to the calculated value from known thermal properties reported in literature.

Mapping optical fluence variations in highly scattering media by measuring ultrasonically modulated backscattered light.

Knowledge of the local optical fluence in biological tissue is of fundamental importance for biomedical optical techniques to achieve quantification. We report a method to noninvasively measure the local optical fluence in optically inhomogeneous scattering media. The concept is based on two aspects: the local tagging of light using ultrasonic modulation and the photon path reversibility principle. Our method has advantages over known computational-based fluence mapping techniques, for its purely experimental nature and without the requirement of prior knowledge of the optical properties of the medium. We provide a theoretical formalism and validation of the method with experiments in tissue-like phantoms. Further, we combine our method with photoacoustic imaging and compensate the photoacoustic signals for fluence variations in optically inhomogeneous media.

Whole field laser Doppler imaging of the microcirculation in psoriasis and clinically unaffected skin.

BACKGROUND: Angiogenesis represents a key phenomenon in psoriasis. Insights in the microcirculation within psoriatic lesions in a whole field are lacking. Recently, the Twente Optical Perfusion Camera (TOPCam) was developed, which provides the possibility of evaluating the superficial cutaneous microcirculation in a whole field. OBJECTIVES: This pilot study aims to examine whether the TOPCam can be used to visualize the microcirculation within and around psoriatic lesions, and whether it is capable of revealing vascular changes during topical treatment. METHODS: Five patients with chronic plaque psoriasis were included. The superficial microcirculation and clinical local scores (SUM score) were analyzed in two comparable lesions within one patient. At baseline and after 2, 4, 6, and 8 weeks the disease's natural course was evaluated in one plaque versus topical treatment in the other. RESULTS: The TOPCam was able to visualize an increased microcirculation within psoriatic lesions and perfusion variability due to the heartbeat. Whole field images demonstrated heterogeneity in perfusion intensity (hot and cold spots) within clinically homogeneous-looking plaques. Topical therapy induced a decrease in overall perfusion and a significant decrease in SUM score. CONCLUSION: The TOPCam is the first noninvasive technique to visualize the microcirculation of psoriatic lesions in a whole field, to correct images for the heartbeat, and to reveal heterogeneity in perfusion intensity.

Laser speckle contrast analysis for quantifying the Allen test: a feasibility study.

BACKGROUND AND OBJECTIVE: The radial artery has become a routinely used conduit for coronary artery bypass graft surgery (CABG). Prior to surgery the Allen test is performed to test the patency of the ulnar artery. A positive Allen test, reperfusion >5 seconds, suggests an insufficient perfusion of the hand by the ulnar artery. In this study we investigated if laser speckle contrast analysis (LASCA) provides an objective determination of the reperfusion time. MATERIALS AND METHODS: When the hand is illuminated with coherent laser light, the backscattered light will result in an interference pattern consisting of bright and dark areas called speckles. This speckle pattern will change due to movement of red blood cells. LASCA uses these changes to visualize the perfusion during the Allen test. Reperfusion is measured on the palmar side of the hand. The reperfusion time is defined as the time from onset of reperfusion to maximum perfusion calculated by a polynomial curve fit. The reperfusion time of the hand of patients undergoing CABG (n = 30) is measured using LASCA and is compared to the conventional Allen test performed by the nurse practitioner. RESULTS: LASCA measurements showed a negative Allen test of both hands of 16 patients. Fourteen had a borderline reperfusion time of 5-6 seconds and/or a positive Allen test of one or both hands. No statistical significant difference was observed for the LASCA Allen test compared to the conventional Allen test, P = 0.549 for the left hand and P = 0.223 for the right hand. CONCLUSION: LASCA is able to visualize perfusion of the hand and measure a quick, moderate, slow reperfusion response or no reperfusion. It is technically feasible to determine the reperfusion time of the hand. LASCA can be a useful and objective tool to assess ulnar collateral blood supply to the hand prior to harvesting of the radial artery as a bypass graft.

Photoacoustic guided ultrasound wavefront shaping for targeted acousto-optic imaging.

To overcome speed of sound aberrations that negatively impact the acoustic focus in acousto-optic imaging, received photoacoustic signals are used to guide the formation of ultrasound wavefronts to compensate for acoustic inhomogeneities. Photoacoustic point sources composed of gold and superparamagnetic iron oxide nanoparticles are used to generate acoustic waves that acoustically probe the medium as they propagate to the detector. By utilizing cross-correlation techniques with the received photoacoustic signal, transmitted ultrasound wavefronts compensate for the aberration, allowing for optimized and configurable ultrasound transmission to targeted locations. It is demonstrated that utilizing a portable commercially available ultrasound system using customized software, photoacoustic guided ultrasound wavefront shaping for targeted acousto-optic imaging is robust in the presence of large, highly attenuating acoustic aberration.

Relation between the contrast in time integrated dynamic speckle patterns an 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.

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.

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.

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.

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.

Photoacoustic imaging of blood vessels with a double-ring sensor featuring a narrow angular aperture.

A photoacoustic double-ring sensor, featuring a narrow angular aperture, is developed for laser-induced photoacoustic imaging of blood vessels. An integrated optical fiber enables reflection-mode detection of ultrasonic waves. By using the cross-correlation between the signals detected by the two rings, the angular aperture of the sensor is reduced by a factor of 1.9, from 1.5 to 0.8 deg. Consequently, photoacoustic images could be obtained in a manner analogous to the ultrasound B-scan mode. Next, the cross section of artificial blood vessels is visualized by reconstruction of the absorbed energy distribution. Finally, in vivo imaging and the subsequent reconstruction of the absorbed energy distribution is demonstrated for superficial blood vessels in the human wrist.

Photoacoustic determination of blood vessel diameter.

A double-ring sensor was applied in photoacoustic tomographic imaging of artificial blood vessels as well as blood vessels in a rabbit ear. The peak-to-peak time (tau(pp)) of the laser (1064 nm) induced pressure transient was used to estimate the axial vessel diameter. Comparison with the actual vessel diameter showed that the diameter could be approximated by 2ctau(pp), with c the speed of sound in blood. Using this relation, the lateral diameter could also precisely be determined. In vivo imaging and monitoring of changes in vessel diameters was feasible. Finally, acoustic time traces were recorded while flushing a vessel in the rabbit ear with saline, which proved that the main contribution to the laser-induced pressure transient is caused by blood inside the vessel and that the vessel wall gives only a minor contribution.