Kevin R. Forrester

Comparison of laser speckle and laser Doppler perfusion imaging: Measurement in human skin and rabbit articular tissue

Laser Doppler perfusion imaging (LDI) is currently used in a variety of clinical applications, however, LDI instruments produce images of low resolution and have long scan times. A new optical perfusion imager using a laser speckle measurement technique and its use for in vivo blood flow measurements are described. Measurements of human skin and surgically exposed rabbit tissue made using this instrument were compared with a commercial laser Doppler perfusion imaging instrument. Results from blood flow measurements showed that the laser speckle imager measured an 11–67% decrease in blood flow under arterial occlusion. Under similar conditions, the laser Doppler imager measured blood flow decreases of 21–63%. In comparison with LDI, it was observed that the higher temporal resolution of the laser speckle imager was more sensitive to measuring the hyperaemic response immediately following occlusion. This in vivo study demonstrated some of the several advantages laser speckle imaging has over conventional LDI, making the new instrument more versatile in a clinical environment.

A laser speckle imaging technique for measuring tissue perfusion

Laser Doppler imaging (LDI) has become a standard method for optical measurement of tissue perfusion, but is limited by low resolution and long measurement times. We have developed an analysis technique based on a laser speckle imaging method that generates rapid, high-resolution perfusion images. We have called it laser speckle perfusion imaging (LSPI). This paper investigates LSPI output and compares it to LDI using blood flow models designed to simulate human skin at various levels of pigmentation. Results show that LSPI parameters can be chosen such that the instrumentation exhibits a similar response to changes in red blood cell concentration (0.1%-5%, 200 /spl mu/L/min) and velocity (0-800 /spl mu/L/min, 1% concentration) and, given its higher resolution and quicker response time, could provide a significant advantage over LDI for some applications. Differences were observed in the LDI and LSPI response to tissue optical properties. LDI perfusion values increased with increasing tissue absorption, while LSPI perfusion values showed a slight decrease. This dependence is predictable, owing to the perfusion algorithms specific to each instrument, and, if properly compensated for, should not influence each instruments ability to measure relative changes in tissue perfusion.

Laser Doppler imaging of burn scars: a comparison of wavelength and scanning methods

UNLABELLED Laser Doppler perfusion imaging (LDI) is a useful tool for the early clinical assessment of burn depth and prognostic evaluation of injuries that may require skin grafting. We have evaluated two commercially available laser Doppler imagers for the perfusion measurement of normal and burn scar tissue. METHODS A single wavelength (635 nm), step-wise scanning LDI and a dual wavelength (633 and 780 nm), continuous scanning LDI were used. Twenty patients with hypertrophic burn scars (time since injury: 1 month-8 years) were recruited and the color and elevation of the scar was clinically assessed using a modified Vancouver Burn Scar Scale. Perfusion of each scar region was measured using both imagers. A symmetric contralateral region of unburned skin was also imaged to record baseline perfusion. RESULTS Comparisons of wavelength and scanning technique were made using perfusion values obtained from 22 burn scars. Highly significant positive correlation was observed in all comparisons. In addition, output from both instruments was strongly and significantly correlated with the clinical grading of the scar. SIGNIFICANCE Both LDI scanners perform similar perfusion measurements. The results also indicate that red and near-infrared (NIR) wavelength photons provide similar blood flow information. The faster, continuous scanning method provides a clinical advantage without a significant loss of blood flow information. However, a critical evaluation of both instruments suggests that caution must be exercised when using these optical diagnostic techniques and that some knowledge of light-tissue interaction is required for the proper analysis and interpretation of clinical data.

Evaluation of laser Doppler imaging to measure blood flow in knee ligaments of adult rabbits

Laser Doppler imaging (LDI) is investigated as a novel method for in vivo ligament tissue blood flow determination. LDI output signal is obtained from surgically exposed rabbit medial collateral ligaments (MCL). The LDI signal, is compared with simultaneously determined, coloured microsphere (CM)-derived standardised MCL blood flow. Correlation of LDI output with the CM flow data and a linear regression of 17 data points in nine rabbits (joint injured to provoke an acute vascular response in the tissues) indicate that LDI provides a reasonable estimate of MCL blood flow, at least over the ranges assessed. If properly calibrated, and given enough tissue-specific data points, LDI may have advantages over conventional, but more invasive, techniques. The potential clinical application of LDI technology to joint injury and arthritis research is discussed.

Kinetics of blood flow during healing of excisional full‐thickness skin wounds in pigs as monitored by laser speckle perfusion imaging

Background/purpose: The laser speckle perfusion imaging (LSPI) system is a new, non‐invasive technique for rapidly and reproducibly measuring tissue perfusion. The high resolution and frame rate of the LSPI overcome many of the limitations of traditional laser Doppler imaging techniques. Therefore, LSPI is a useful means for evaluating blood flow in a variety of situations. The present study investigates the ability of the LSPI system to detect temporal changes in blood flow during the healing of cutaneous wounds in a well‐characterized animal model.

In vivo comparison of scanning technique and wavelength in laser Doppler perfusion imaging: Measurement in knee ligaments of adult rabbits

At present, there are only two laser Doppler perfusion imaging systems (LDls) manufactured for medical applications: a ‘tepwise’ and a ‘continuous’ scanning LDI. The stepwise scanning LDI has previously been investigated and compared with coloured microsphere determined standardised flow. The continuous scanning LDI is investigated and compared with the stepwise scanning LDI for its ability to measure in vivo, hypoaemic, ligament tissue blood flow changes. The continuous scanning system was supplied with two lasers, red and near infrared (NIR), allowing for additional assessment of the effect of wavelength on imaging ligament perfusion. Perfusion images were obtained from surgically exposed rabbit medial collateral ligaments (MCL). Continuous and stepwise LDI scans were compared using correlation and linear regression analysis of image averages and standard deviations. Using the same method of analysis, LDI measurements using red and NIR lasers indicated a high degree of correlation, at least over the ranges of perfusion assessed, indicating that red and NIR lasers measure similar regions of flow in the rabbit MCL. These experiments confirm that both LDI techniques provide a valid in vivo measure of dynamic changes in connective tissue perfusion and could have significant impact on the understanding and treatment of joint injury and arthritis.

A transmissive laser speckle imaging technique for measuring deep tissue blood flow: an example application in finger joints.

Laser speckle perfusion imaging (LSPI) is a minimally invasive optical measure of relative changes in blood flow, providing real‐time, high resolution, two‐dimensional maps of vascular structure. Standard LSI imaging uses a light‐reflective geometry that limits the measurement to a thin surface layer of 0.2–1 mm. The objective of this study was to test a new LSI instrument geometry with the laser source opposed to the image capture plane (light transmissive). Captured light then travels the entire tissue thickness (10–15 mm), sampling much deeper regions of interest than conventional optical imaging techniques.

Spatially resolved diffuse reflectance with laser Doppler imaging for the simultaneous in-vivo measurement of tissue perfusion and metabolic state

Laser Doppler Imaging (LDI) has become an established technique for the two dimensional measurement of tissue perfusion but the uncertainty of photon penetration depth leads to ambiguous interpretation of what fraction of the tissue microcirculation is being sampled. This study investigates a diffuse reflectance technique for measuring tissue optical properties during LDI perfusion measurement for the simultaneous determination of photon penetration depth and tissue metabolic state. LDI and diffuse reflectance spectroscopy measurements were made on surgically exposed ligaments in pregnant and non-pregnant rabbits. Photon penetration depths are reported. It was observed that anisotropic scattering occurs due to the ordered alignment of collagen fibers within ligament. Tissue perfusion in the ligaments of pregnant animals was significantly lower than in non-pregnant animals. Tissue hemoglobin concentration and oxygenation, and percent vascularization are also reported showing no statistical difference between the ligaments in pregnant and non-pregnant rabbits. A significant difference was observed in the photon scattering coefficient between the pregnant and non-pregnant groups suggesting a change in fibril spacing and/or orientation, most likely caused by an increased laxity in the ligaments of the pregnant animals. These investigations compare well with previous biochemical and biomechanical information obtained on ligaments.

In-vitro measurements of light transmission parallel and perpendicular to the collagen orientation in tendons

Collimated light transmission studies of bovine tendon have been carried out to show the collagen waveguide effect. A monochromator/lamp apparatus was used to irradiate tissue samples and the transmitted light intensity was measured parallel and perpendicular to the collagen fiber orientation. The intensity parallel to the collagen fibers was at least twice that seen with perpendicular propagation. This indicates there is less scattering parallel to the fibers and light waveguiding is present for photon scattering. Also, absorption of light due to hemoglobin around 550 nm and due to water at 980nm was more prominent for parallel than perpendicular propagation. The light may travel either through the collagen fibril or fiber bundles or through the interstitial matrix. Sequential tests during tissue dehydration were performed and it was found that the transmitted light intensity increased with dehydration. This suggest that light may not be traveling through the interstitial matrix where water is the major component. Water may be acting as a reflection boundary for the light that is passing through the fibril or fiber bundles. Collagen waveguiding may be utilized to elucidate the collagen structure. Also, tissue water content could be measured from the transmission profiles. These may be of use in diagnosis and repair of connective tissues.