Ronny Ziegler

Linear image reconstruction for a diffuse optical mammography system in a noncompressed geometry using scattering fluid

Diffuse optical tomography (DOT) is a potential new imaging modality to detect or monitor breast lesions. Recently, Philips developed a new DOT system capable of transmission and fluorescence imaging, where the investigated breast is hanging freely into the measurement cup containing scattering fluid. We present a fast and robust image reconstruction algorithm that is used for the transmission measurements. The algorithm is based on the Rytov approximation. We show that this algorithm can be used over a wide range of tissue optical properties if the reconstruction is adapted to each patient. We use estimates of the breast shape and average tissue optical properties to initialize the reconstruction, which improves the image quality significantly. We demonstrate the capability of the measurement system and reconstruction to image breast lesions by clinical examples.

Development of a multi-channel time-domain fluorescence mammograph

We developed an eight-channel scanning time-domain fluorescence mammograph capable of imaging the distribution of a non-specific fluorescent contrast agent in the female breast, besides imaging intrinsic absorption and scattering properties of healthy breast tissue and tumors. The apparatus is based on the PTB multi-channel laser pulse mammograph, originally designed for measurements of absorption and scattering coefficients at four selected wavelengths (&lgr; = 652 nm, 684 nm, 797nm, and 830 nm). It was upgraded for time-resolved detection of fluorescence, excited at 735 nm by a ps diode laser with 10 mW output power and detected at wavelengths &lgr; ⩾ 780 nm. Cooled PMTs with GaAs photocathodes are used to detect laser and fluorescence photons at five positions in transmission and three positions in reflection. Measurements are performed with the breast being slightly compressed between two parallel glass plates. The transmitting and receiving fiber bundles are scanned synchronously over the breast in steps of typically 2.5 mm. At each scan position, distributions of times of flight of laser photons are measured by time-correlated single photon counting at eight detector positions, followed by measurements of distributions of times of arrival of fluorescence photons. The performance of the fluorescence mammograph was investigated by using breast-like phantoms with a fluorescent inhomogeneity with dye enrichment varying between 2:1 and 10:1 over background values.

Nonlinear reconstruction of absorption and fluorescence contrast from measured diffuse transmittance and reflectance of a compressed-breast-simulating phantom.

We report on the nonlinear reconstruction of local absorption and fluorescence contrast in tissuelike scattering media from measured time-domain diffuse reflectance and transmittance of laser as well as laser-excited fluorescence radiation. Measurements were taken at selected source-detector offsets using slablike diffusely scattering and fluorescent phantoms containing fluorescent heterogeneities. Such measurements simulate in vivo data that would be obtained employing a scanning, time-domain fluorescence mammograph, where the breast is gently compressed between two parallel glass plates, and source and detector optical fibers scan synchronously at various source-detector offsets, allowing the recording of laser and fluorescence mammograms. The diffusion equations modeling the propagation of the laser and fluorescence radiation were solved in frequency domain by the finite element method simultaneously for several modulation frequencies using Fourier transformation and preprocessed experimental data. To reconstruct the concentration of the fluorescent contrast agent, the Born approximation including higher-order reconstructed photon densities at the excitation wavelength was used. Axial resolution was determined that can be achieved by various detection schemes. We show that remission measurements increase the depth resolution significantly.

Investigation of detection limits for diffuse optical tomography systems: II. Analysis of slab and cup geometry for breast imaging.

Using a statistical (chi-square) test on simulated data and a realistic noise model derived from the systems hardware we study the performance of diffuse optical tomography systems for fluorescence imaging. We compare the predicted smallest size of detectable lesions at various positions in slab and cup geometry and model how detection sensitivity depends on breast compression and lesion fluorescence contrast. Our investigation shows that lesion detection is limited by relative noise in slab geometry and by absolute noise in cup geometry.

Optical Fluorescence Imaging of Breast Cancer

We report on the first results obtained with the Philips diffuse optical tomography system and Schering AGs Omocianine fluorescent dye.

Investigation of detection limits for diffuse optical tomography systems: I. Theory and experiment

We present a statistical test using simulated photon migration data and a noise model derived from the hardware of a particular diffuse optical tomography system to predict its detection limits. Our method allows us to assess the spatial distribution of the detection sensitivity of arbitrary geometries and noise without requiring phantom measurements and reconstructions. We determine the minimal detectable lesion size at selected lesion positions and compare the predicted results with phantom measurements carried out in a cup geometry.

Linear Image Reconstruction for a Diffuse Optical Mammography System in a Non-Compressed Geometry Using Scattering Fluid

Diffuse optical tomography (DOT) is a potential new imaging modality to detect or monitor breast lesions. Recently, Philips developed a new DOT system capable of transmission and fluorescence imaging, where the investigated breast is hanging freely into the measurement cup containing scattering fluid. We present a fast and robust image reconstruction algorithm that is used for the transmission measurements. The algorithm is based on the Rytov approximation. We show that this algorithm can be used over a wide range of tissue optical properties if the reconstruction is adapted to each patient. We use estimates of the breast shape and average tissue optical properties to initialize the reconstruction, which improves the image quality significantly. We demonstrate the capability of the measurement system and reconstruction to image breast lesions by clinical examples.

Reconstruction of absorption and fluorescence contrast for scanning time-domain fluorescence mammography

We report on the reconstruction of absorption and fluorescence from measured time-domain diffuse reflectance and transmittance of laser and fluorescence radiation. Measurements were taken on slab-like, diffusely scattering and fluorescent phantoms containing fluorescent inhomogeneities, using fs laser pulses (&lgr; = 730 nm) and time correlated single photon counting. The source was scanned across the entrance face of the phantom, and at each source position data were collected in transmission and reflection at various detector positions. These measurements simulate in vivo data that will be obtained employing a scanning, time-domain fluorescence mammograph, where the breast is gently compressed between two parallel glass plates, and source and detector optical fibers scan synchronously at various source-detector offsets, allowing to record laser and fluorescence mammograms. The diffusion equations for the propagation of the laser and fluorescence radiation were solved in frequency domain by the finite element method. Measured time-resolved phantom data were Fourier-transformed to frequency domain prior to image reconstruction. Signal-to-noise ratios were high enough to use several data sets simultaneously in the reconstruction process belonging to various modulation frequencies up to several hundred MHz. To obtain the spatial distribution of the fluorescent contrast agent the Born approximation of the fluorescence diffusion equation was used.

Image reconstruction and evaluation of system performance for optical fluorescence tomography

Diffuse optical tomography is a non-invasive method aiming at the detection of breast cancer. The sensitivity and specificity of the method can be increased if a fluorescent contrast agent is used that accumulates in malignant lesions. Recently, Philips developed an optical scanner, where the patient is lying on a bed, with one breast hanging freely in a cup containing an optical matching fluid. 507 optical fibers are mounted in the surface of the measurement cup. The breast is illuminated sequentially by half of these fibers while the other half is used to collect the light that is emanating from the breast. The system uses near-infrared light of continuous wave solid-state lasers to illuminate the breast at four different wavelengths. A complete measurement takes less than ten minutes and involves five breast scans: transmission data are collected for four wavelengths, and fluorescence data for excitation at one wavelength. Here, we present the image reconstruction scheme and a novel method to assess the system performance in terms of lesion detectability. This method uses a statistical significance test on simulated data with and without a lesion. It allows the quantification of the detectability of lesions for different size, position, or contrast of the lesion. It also allows to analyze the potential impact of system improvements or to judge the performance of an image reconstruction algorithm.

Algebraic reconstruction techniques for spectral reconstruction in diffuse optical tomography.

Reconstruction in diffuse optical tomography (DOT) necessitates solving the diffusion equation, which is nonlinear with respect to the parameters that have to be reconstructed. Currently applied solving methods are based on the linearization of the equation. For spectral three-dimensional reconstruction, the emerging equation system is too large for direct inversion, but the application of iterative methods is feasible. Computational effort and speed of convergence of these iterative methods are crucial since they determine the computation time of the reconstruction. In this paper, the iterative methods algebraic reconstruction technique (ART) and conjugated gradients (CGs) as well as a new modified ART method are investigated for spectral DOT reconstruction. The aim of the modified ART scheme is to speed up the convergence by considering the specific conditions of spectral reconstruction. As a result, it converges much faster to favorable results than conventional ART and CG methods.