Alois K. Popp

Synthesis of voltage-sensitive fluorescence signals from three-dimensional myocardial activation patterns.

Voltage-sensitive fluorescent dyes are commonly used to measure cardiac electrical activity. Recent studies indicate, however, that optical action potentials (OAPs) recorded from the myocardial surface originate from a widely distributed volume beneath the surface and may contain useful information regarding intramural activation. The first step toward obtaining this information is to predict OAPs from known patterns of three-dimensional (3-D) electrical activity. To achieve this goal, we developed a two-stage model in which the output of a 3-D ionic model of electrical excitation serves as the input to an optical model of light scattering and absorption inside heart tissue. The two-stage model permits unique optical signatures to be obtained for given 3-D patterns of electrical activity for direct comparison with experimental data, thus yielding information about intramural electrical activity. To illustrate applications of the model, we simulated surface fluorescence signals produced by 3-D electrical activity during epicardial and endocardial pacing. We discovered that OAP upstroke morphology was highly sensitive to the transmural component of wave front velocity and could be used to predict wave front orientation with respect to the surface. These findings demonstrate the potential of the model for obtaining useful 3-D information about intramural propagation.

Microscope-based static light-scattering instrument

We describe a new design for a microscope-based static light-scattering instrument that provides simultaneous high-resolution images and static light-scattering data. By correlating real space images with scattering patterns, we can interpret measurements from heterogeneous samples, which we illustrate by using biological tissue.

Microscopic origin of light scattering in tissue.

A newly designed instrument, the static light-scattering (SLS) microscope, which combines light microscopy with SLS, enables us to characterize local light-scattering patterns of thin tissue sections. Each measurement is performed with an illumination beam of 70-microm diameter. On these length scales, tissue is not homogeneous. Both structural ordering and small heterogeneities contribute to the scattering signal. Raw SLS data consist of a two-dimensional intensity distribution map I(theta, phi), showing the dependence of the scattered intensity I on the scattering angle theta and the azimuthal angle phi. In contrast to the majority of experiments and to simulations that consider only the scattering angle, we additionally perform an analysis of the azimuthal dependence I(phi). We estimate different contributions to the azimuthal scattering variation and show that a significant fraction of the azimuthal amplitude is the result of tissue structure. As a demonstration of the importance of the structure-dependent part of the azimuthal signal, we show that this function of the scattered light alone can be used to classify tissue types with surprisingly high specificity and sensitivity.

Internal point spread imaging of cardiac tissue to provide depth resolution for bulk tissue imaging experiments

In order to provide depth resolution for bulk tissue imaging experiments using absorption signals, we have designed an internal laser point spread technique. A laser light source has been imbedded in different depths into cardiac tissue and tissue phantoms, the signal on the tissue surface detected by a CCD detector. These measurements in combination with an analytic solution of the diffusion equation allow us to estimate optical properties of the investigated tissue. We show how this information provides the core of depth quantification of fluorescence and absorption measurements in bulk tissue and investigate experimentally the transition from single scattering to diffuse photon transport in cardiac tissue and suspensions of microscopic spherical particles that serve as model systems.

Light-scattering microscope as a tool to investigate scattering heterogeneity in tissue

Rayleigh light scattering has not yet been used for quantitative investigations of heterogeneous systems. Preconditions such an experiment are a well defined scattering geometry and independent information about the local state of the sample. We have designed a new instrument that meets these criteria: a light-scattering microscope with simultaneous imaging. We demonstrate the ability to characterize local differences within one tissue type as well as global differences between tissue types. Real space images of the sample are taken by normal video microscopy techniques. The light scattering pattern in analyzed by the evaluation of wave-vector dependence and scattering direction of the scattered intensity. Statistical analysis of scattering patterns show what is important for the characterization and classification of tissues and heterogeneous structures. Real space images provide context for scattering analysis. The light scattering microscope is a powerful tool for characterization of local structural order in inhomogeneous structures like tissues.

Voltage-sensitive fluorescence imaging on the heart surface

Optical methods provide a rather precise insight into cardiac electrical activity. Voltage-sensitive dyes like di 4-ANEPPS convert the electric signal into a fluorescent signal that can be measured by standard optical methods. A realistic picture of the dynamic patterns that govern electrical activity in the human heart can be obtained only with thick tissue preparations, from large animals. We measure the fluorescence signal of an approximately 2.5 x 2.5 cm area on the surface of 8 mm thick porcine right ventricle preparations with a fast CCD camera at low magnification, and perform advanced simulations of the macroscopic dynamic features involved. To extract meaningful qualitative and quantitative data from these signals, details of the conversion from electrical to optical signal have to be known, and the problem of the 2D surface signal originating from a 3D distribution below has to be addressed. We compare experiment to simulation results applying a composite model based on both electrical and optical tissue properties. The model predicts optical action potential upstroke morphology, involving optical point spread functions and simplified Beeler-Reuter kinetics for the electrical wave propagation. Optical point spread functions have been calculated from scattering and absorption properties applying diffusion models and Monte-Carlo simulations. First of all, the forward problem has been solved for uniform light illumination and simulations have been compared to experiments. Furthermore, we also address the question of the inverse problem and provide an analysis of the limitations for this approach.

Quantification of local optical properties of tissue

Optical imaging of ex vivo tissue models to study heart fibrillation is normally performed using voltage-sensitive dyes. Upon stimulation by an electrode, time-dependent fluorescence or absorption signals are recorded, often in trans-illumination geometry. In order to provide quantification of the origins of these signals inside the tissue, the locally varying optical properties of the tissue have to be known and their change due to the presence of the dyes. To provide experimental input for further modeling efforts, we have performed depth dependent measurements with a fiber optic laser source inside the tissue, recording light profiles on the tissue surface, mainly in transmission geometry. From these measurements, optical properties have been extracted and the obtained profiles have been used as input into a preliminary image reconstruction scheme, together with Monte Carlo simulations. Experiments at different locations in the same sample show the variation of optical properties. Additionally, effects from the presence of heterogeneities on the signal have been investigated.