Matthew H. Smith

Mueller matrix imaging polarimetry in dermatology

Recent studies have indicated that polarized light may be useful in the discrimination between benign and malignant moles. In fact, imaging polarimetry could provide noninvasive diagnosis of a range of dermatological disease states. However, in order to design an efficacious sensor for clinical use, the complete polarization-altering properties of a particular disease must be well understood. We present Mueller matrix imaging polarimetry as a technique for characterizing various dermatological diseases. Preliminary Mueller matrix imagery at 633 nm suggests that both malignant moles and lupus lesions may be identified through polarimetric measurements. Malignant moles are found to be less depolarizing than the surrounding tissue, and lupus lesions are found to have rapidly varying retardance orientation.

Interpreting Mueller matrix images of tissues

In recent years, several investigators have begun to explore polarized light imagery as a potential diagnostic tool. For example, polarimetric images have shown promise in identifying a variety of dermatological conditions. Because tissues tend to depolarize a large fraction (~85%) of incident light, the Mueller calculus lends itself well to these applications. A particular property of the Mueller matrix, the Depolarization Index, has demonstrated promise in discriminating between cancerous and benign moles. In this paper, we discuss the depolarizing aspects of tissues, however we primarily attempt to analyze the small fraction of light that has retained a polarization state. Analyzing the residual polarizing properties of a sample is challenging, and it requires a polar decomposition of the measured Mueller matrix into the basic properties of diattenuation, retardance, and depolarization. The diattenuation and retardance images contain information about the complex refractive index of the tissue, including any spatial variations in the index. We present measurements of the diattenuation and retardance of laser light reflected from skin as a function if incident angle and scattered angle.

Effect of multiple light paths on retinal vessel oximetry

Techniques for noninvasively measuring the oxygen saturation of blood in retinal arteries and veins are reported in the literature, but none have been sufficiently accurate and reliable for clinical use. Addressing the need for increased accuracy, we present a series of oximetric equations that explicitly consider the effects of backscattering by red blood cells and lateral diffusion of light in the ocular fundus. The equations are derived for the specific geometry of a scanning-beam retinal vessel oximeter; however, the results should also be applicable to photographic oximeters. We present in vitro and in vivo data that suggest the validity of these equations.

OXYGEN SATURATION MEASUREMENTS OF BLOOD IN RETINAL VESSELS DURING BLOOD LOSS

We describe a noninvasive technique and instrumentation for measuring the oxygen saturation of blood in retinal arteries and veins. The measurements are made by shining low-power lasers into the eye, and scanning the beams across a retinal blood vessel. The light reflected and scattered back out of the eye is collected and measured. The oxygen saturation of blood within the vessel is determined by analyzing the vessel absorption profiles at two wavelengths. A complete saturation measurement can be made in less than 1 s, allowing real-time measurement during physiologic changes. The sensitivity of this measurement technique to changes in retinal saturation has been demonstrated through a series of pilot studies in anesthetized swine. We present data indicating that retinal venous oxygen saturation decreases during ongoing blood loss, demonstrating a potential application of an eye oximeter to noninvasively monitor blood loss.

Polarization sensing for target acquisition and mine detection

Previous infrared polarization imaging research has shown manmade objects to be sources of emitted and reflected polarized radiation while natural backgrounds are predominantly unpolarized. The prior work underscored the dramatic improvements in signal to clutter ratio that could be achieved in a typical target acquisition scenario using polarization sensing techniques. Initial investigations into the polarization signatures of surface scattered mines have also shown polarization techniques are able to provide strong cues to mine presence. The US Army Night Vision and Electronics Sensors Directorate has developed complete Stokes imaging polarimeters in the midwave infrared (MWIR, 3-5 µm)1,2, the short wave infrared (SWIR, 1-2 µm) and the visible wavebands in order to investigate polarization phenomenology and to quantify expected improvements to target acquisition and mine detection. This paper will review the designs of the polarization cameras and will summarize efforts to calibrate them. An example phenomenology study of MWIR polarization from surface scattered landmines will be presented.

Optimum wavelength combinations for retinal vessel oximetry

Several investigators have demonstrated techniques for noninvasive measurement of the oxygen saturation of blood in retinal arteries and veins. These techniques have been based on measuring the optical density of a retinal vessel at multiple wavelengths and on calculating the oxygen saturation on the basis of the known absorption coefficients of hemoglobin and oxyhemoglobin. A technique is presented for determining the optimum wavelengths for retinal oximetry measurements. What is believed to be a novel wavelength combination of 488, 635, and 905 nm is found to provide excellent oxygen sensitivity across a broad range of typical vessel diameters and saturations. The use of this wavelength combination should allow for the most accurate retinal saturation measurements made to date.

Retinal large vessel oxygen saturations correlate with early blood loss and hypoxia in anesthetized swine

BACKGROUND Noninvasive monitoring would likely improve trauma care. Using laser technology, we monitored the oxygen saturation in retinal vessels during exsanguination and hypoxia. METHODS Seven anesthetized swine were bled at 0.4 mL/kg/min for 40 minutes. During exsanguination, retinal venous saturation (SrvO2) was measured using an eye oximeter, and central venous saturation (SvO2) was measured using a fiber-optic catheter. After the shed blood was reinfused, the FiO2 was progressively decreased from 0.97 to 0.07. Femoral artery oxygen saturation (SaO2) and retinal artery oxygen saturation (SraO2) were measured at each increment. RESULTS During exsanguination, SrvO2 correlated with blood loss (r = -0.93) and SvO2 (r = 0.94). SraO2 correlated with SaO2 during incremental hypoxia (R2 = 0.93 +/- 0.15). CONCLUSIONS In this model of exsanguination, retinal venous oxygen saturation correlates with blood volume and with central venous oxygen saturation. The SraO2 correlates with SaO2 during graded hypoxia. Use of an eye oximeter to noninvasively monitor trauma patients appears promising and warrants further study.

Beam wander considerations in imaging polarimetry

To make accurate imagin polarimetry measurements, it is imperative that the individual polarization images comprising a data set are spatially well registered. Both the Stokes Imaging Polarimeter at the University of Alabama in Huntsville and the Multispectral IR Stokes Imagining Polarimeter at Night Vision Laboratories use a rotating quarter-wave linear retarder followed by a fixed linear polarizer to measure Stokes vector images. The rotating retarder cause small amounts of image wander at the detector array. In this paper, we quantify the effects of this image wander on the calculated polarization parameters, and present data indicating the maximum acceptable image wander for typical scenes.

Retinal Imaging Techniques in Diabetes

Diabetic retinopathy is progressive, and detection early is essential for the prevention of blindness. Doppler flowmetry, retinal photography, scanning laser ophthalmoscopy, and retinal oximetry measurements may identify proliferative disease early. Drawbacks of these methods include lack of compliance, failure to refer, and failure to identify disease early. As a result, diabetic retinopathy is a leading cause of blindness. Our retinal oximeter measures the blood oxygen saturation in the large vessels of the retina near the optic disc. Retinal vessel oxygen saturations measured with our instrument are sensitive indicators of blood loss and hypoxia in swine. We are generating scientific data that suggests that retinal vessel oxygen saturations may be used to identify retinal hypoxia prior to changes in retinal vessel architecture. We expect to study humans within the next two years, and a clinically useful eye oximeter should be available in the near future.

Multispectral infrared Stokes imaging polarimeter

We describe the design and development of an imaging polarimeter that will simultaneously measure Stokes vector images in the mid-wave IR and long-wave IR wavebands. We present an analysis of the expected errors that arise due to spectral variations in the polarization elements of the instrument across each waveband. FInally, instrument calibration and polarization images acquired in the MWIR waveband are presented.