| Zugriffsnummer | 30507 |
| Dokumenttyp | Konferenzartikel |
| Sprache | Englisch |
| Titel | Scattering of laser light in turbid media: optical tomography for medical diagnostics? |
| Autor(in); Institution |
Rinneberg, Herbert; 10.1, Medizinische Messtechnik, PTB-Berlin
|
| Quelle/Jahr | The inverse problem: Symposium ad memoriam Hermann von Helmholtz:(1995), 107 - 141 |
| Herausgeber(in) |
Lübbig, Heinz; 8.1, Physikalische Grundlagen, PTB-Braunschweig ; 8.13, Wissenschaftliche Bibliotheken, PTB-Braunschweig und Berlin
|
| ISBN | 3-05-501689-0 ; 3-527-29330-2 |
| Verlag | Berlin ; Weinheim: Akademie Verl. ; VDH |
| Konferenzangaben | Symposium ad memoriam Hermann von Helmhotz, Berlin, 9, September, 1994, GERMANY |
| Zusammenfassung | The paper discusses the use of red and near infrared laser radiation for non-invasive imaging in medical diagnostics, in particular for optical mammography. Imaging of objects embedded in highly scattering media by diffuse photon density waves is considered. Diffusion of photons in turbid media, propagation of diffuse photon density waves, their excitation using intensity-modulated or pulsed laser radiation as well as experimental techniques for measuring modulated or time-resolved transmittance are briefly reviewed. Spatial resolution of diffusive wave imaging id determined from edge spread functions in the time and frequency domain derived from measured distributions of time of flight of ps-laser pulses or their Fourier transforms. In transmission geometry spatial resolution is shown to be proportional to the thickness d of the turbid medium. Using monochromatic diffuse photon density waves of wavelength Λ and modulation frequency ν, normalized spatial resolution Δxν / d depends on (normalized) wavelength Λ / d and (normalized) penetration depth δ / d. When exciting a spectrum of diffuse photon density waves by a short laser pulse, normalized spatial resolution Δxf / d is determined by the fractional transmittance, used for imaging, i.e. by the selected fraction f of the total number of photons detected with times of flight τ ≤ τf. Normalized spatial resolution Δxf / d is independent of the transport scattering coefficient of the medium, provided absorption can be neglected. Transillumination images of a cube (1 cm3) are shown embedded into a solid, diffusely scattering material with optical properties differing from those of the cube, serving as a phantom of a compressed breast with a tumor inside. A variety of quantities, such as fractional transmittance, Fourier amplitudes and phase shifts as well as effective scattering and absorption coefficients derived from measured distributions of times of flight of ps-laser pulses are used to generate transillumination images. Contrast and signal to noise ratio of transillumination images are discussed based upon fractional transmittance. Transillumination images of a female breast with a tumor are shown, recently reported in the literature. The paper concludes with a brief discussion of the inverse problem to be solved in optical tomography. For illustration, reconstruction of an assumed distribution of absorbers in two dimensions is shown obtained by a least squares fit and numerical integration of the diffusion equation. |
