Lars O. Svaasand
Norwegian University of Science and Technology
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Featured researches published by Lars O. Svaasand.
Journal of The Optical Society of America A-optics Image Science and Vision | 1994
Richard C. Haskell; Lars O. Svaasand; Tsong-Tseh Tsay; Ti-Chen Feng; Matthew S. McAdams; Bruce J. Tromberg
Using the method of images, we examine the three boundary conditions commonly applied to the surface of a semi-infinite turbid medium. We find that the image-charge configurations of the partial-current and extrapolated-boundary conditions have the same dipole and quadrupole moments and that the two corresponding solutions to the diffusion equation are approximately equal. In the application of diffusion theory to frequency-domain photon-migration (FDPM) data, these two approaches yield values for the scattering and absorption coefficients that are equal to within 3%. Moreover, the two boundary conditions can be combined to yield a remarkably simple, accurate, and computationally fast method for extracting values for optical parameters from FDPM data. FDPM data were taken both at the surface and deep inside tissue phantoms, and the difference in data between the two geometries is striking. If one analyzes the surface data without accounting for the boundary, values deduced for the optical coefficients are in error by 50% or more. As expected, when aluminum foil was placed on the surface of a tissue phantom, phase and modulation data were closer to the results for an infinite-medium geometry. Raising the reflectivity of a tissue surface can, in principle, eliminate the effect of the boundary. However, we find that phase and modulation data are highly sensitive to the reflectivity in the range of 80-100%, and a minimum value of 98% is needed to mimic an infinite-medium geometry reliably. We conclude that noninvasive measurements of optically thick tissue require a rigorous treatment of the tissue boundary, and we suggest a unified partial-current--extrapolated boundary approach.
Optics Express | 2008
Wim Verkruysse; Lars O. Svaasand; J.S. Nelson
Plethysmographic signals were measured remotely (> 1m) using ambient light and a simple consumer level digital camera in movie mode. Heart and respiration rates could be quantified up to several harmonics. Although the green channel featuring the strongest plethysmographic signal, corresponding to an absorption peak by (oxy-) hemoglobin, the red and blue channels also contained plethysmographic information. The results show that ambient light photo-plethysmography may be useful for medical purposes such as characterization of vascular skin lesions (e.g., port wine stains) and remote sensing of vital signs (e.g., heart and respiration rates) for triage or sports purposes.
Physics in Medicine and Biology | 1995
Bahman Anvari; Thomas E. Milner; B. S. Tanenbaum; Sol Kimel; Lars O. Svaasand; J.S. Nelson
The ability to control the degree and spatial distribution of cooling in biological tissues during a thermally mediated therapeutic procedure would be useful for several biomedical applications of lasers. We present a theory based on the solution of the heat conduction equation that demonstrates the feasibility of selectively cooling biological tissues. Model predictions are compared with infrared thermal measurements of in vivo human skin in response to cooling by a cryogen spurt. The presence of a boundary layer, undergoing a liquid-vapour phase transition, is associated with a relatively large thermal convection coefficient (approximately 40 kW m-2 K-1), which gives rise to the observed surface temperature reductions (30-40 degrees C). The degree and the spatial-temporal distribution of cooling are shown to be directly related to the cryogen spurt duration.
Applied Optics | 1993
Bruce J. Tromberg; Lars O. Svaasand; Tsong-Tseh Tsay; Richard C. Haskell
Amplitude-modulated light launched into multiple-scattering media, e.g., tissue, results in the propagation of density waves of diffuse photons. Photon density wave characteristics in turn depend on modulation frequency (omega) and media optical properties. The damped spherical wave solutions to the homogeneous form of the diffusion equation suggest two distinct regimes of behavior: (1) a high-frequency dispersion regime where density wave phase velocity V(p) has a radicalomega dependence and (2) a low-frequency domain where V(p), is frequency independent. Optical properties are determined for various tissue phantoms by fitting the recorded phase (?) and modulation (m) response to simple relations for theappropriate regime. Our results indicate that reliable estimates of tissue like optical properties can be obtained, particularly when multiple modulation frequencies are employed.
Photochemistry and Photobiology | 1983
Lars O. Svaasand; Reinold Ellingsen
Abstract— Optical properties of the human brain in the wave‐length region from blue to near infrared are presented. There are significant variations in the optical penetration depth from the neonatal and to the adult brain. Typical values for the penetration depth in the adult brain are: 0.5 mm for blue/green, 1.5 mm for red and 3.5 mm for near infrared. The values for the neonatal brain are typically 2–3 times larger.
Physics in Medicine and Biology | 1995
Bahman Anvari; B. S. Tanenbaum; Thomas E. Milner; Sol Kimel; Lars O. Svaasand; J. S. Nelson
The successful treatment of port wine stain (PWS) patients undergoing laser therapy is based on selective thermal coagulation of blood vessels without damaging the normal overlying epidermis. Cryogen spray cooling of skin may offer an effective method for minimizing epidermal thermal injury. Inasmuch as the density of melanosomes and depth of PWS blood vessels can vary considerably, an optimum cooling strategy is required on an individual patient basis. We present a theoretical study of the thermal response of various pigmented PWS lesions to spray cooling in conjunction with flashlamp-pumped pulsed dye laser irradiation (585 nm). Results of our model indicate that precooling of skin using tetrafluoroethane as the cryogen spray is sufficient to eliminate epidermal thermal injury when using incident fluences less than 10 J cm-2 and 8 J cm-2 on patients with intermediate and high epidermal melanin content, respectively. Cryogens that have lower boiling points than tetrafluoroethane may allow successful treatment when using fluences equal to or greater than those indicated.
Reports on Progress in Physics | 2008
Qian Peng; Asta Juzeniene; Jiyao Chen; Lars O. Svaasand; Trond Warloe; Karl Erik Giercksky; Johan Moan
It is hard to imagine that a narrow, one-way, coherent, moving, amplified beam of light fired by excited atoms is powerful enough to slice through steel. In 1917, Albert Einstein speculated that under certain conditions atoms could absorb light and be stimulated to shed their borrowed energy. Charles Townes coined the term laser (light amplification by stimulated emission of radiation) in 1951. Theodore Maiman investigated the glare of a flash lamp in a rod of synthetic ruby, creating the first human-made laser in 1960. The laser involves exciting atoms and passing them through a medium such as crystal, gas or liquid. As the cascade of photon energy sweeps through the medium, bouncing off mirrors, it is reflected back and forth, and gains energy to produce a high wattage beam of light. Although lasers are today used by a large variety of professions, one of the most meaningful applications of laser technology has been through its use in medicine. Being faster and less invasive with a high precision, lasers have penetrated into most medical disciplines during the last half century including dermatology, ophthalmology, dentistry, otolaryngology, gastroenterology, urology, gynaecology, cardiology, neurosurgery and orthopaedics. In many ways the laser has revolutionized the diagnosis and treatment of a disease. As a surgical tool the laser is capable of three basic functions. When focused on a point it can cauterize deeply as it cuts, reducing the surgical trauma caused by a knife. It can vaporize the surface of a tissue. Or, through optical fibres, it can permit a doctor to see inside the body. Lasers have also become an indispensable tool in biological applications from high-resolution microscopy to subcellular nanosurgery. Indeed, medical lasers are a prime example of how the movement of an idea can truly change the medical world. This review will survey various applications of lasers in medicine including four major categories: types of lasers, laser-tissue interactions, therapeutics and diagnostics.
Advances in Experimental Medicine and Biology | 1983
Daniel R. Doiron; Lars O. Svaasand; A. Edward Profio
Dosimetry in photoradiation therapy (PRT) is an area in need of research and development. If PRT is to be optimized and perfected for routine clinical use, methodology and techniques for predicting and monitoring the therapeutic reaction, e.g., dose, need to be developed.
Journal of Biomedical Optics | 2005
Rong Zhang; Wim Verkruysse; Bernard Choi; John A. Viator; Byungjo Jung; Lars O. Svaasand; Guillermo Aguilar; J.S. Nelson
We present an initial study on applying genetic algorithms (GA) to retrieve human skin optical properties using visual reflectance spectroscopy (VRS). A three-layered skin model consisting of 13 parameters is first used to simulate skin and, through an analytical model based on optical diffusion theory, we study their independent effects on the reflectance spectra. Based on a preliminary analysis, nine skin parameters are chosen to be fitted by GA. The fitting procedure is applied first on simulated reflectance spectra with added white noise, and then on measured spectra from normal and port wine stain (PWS) human skin. A normalized residue of less than 0.005 is achieved for simulated spectra. In the case of measured spectra from human skin, the normalized residue is less than 0.01. Comparisons between applying GA and manual iteration (MI) fitting show that GA performed much better than the MI fitting method and can easily distinguish melanin concentrations for different skin types. Furthermore, the GA approach can lead to a reasonable understanding of the blood volume fraction and other skin properties, provided that the applicability of the diffusion approximation is satisfied.
Lasers in Medical Science | 1990
Lars O. Svaasand; Charles J. Gomer; Elisa N. Morinelli
The rationale of hyperthermic treatment of tumours is based on the phenomenon that several tumours have a reduced tolerance for exposure to temperatures in the region 42–47 °C for 20–30 min. Hyperthermia may also be used as an adjunct to other modalities such as chemotherapy, conventional radiation therapy or photodynamic therapy.Methods for introducing hyperthermia include electromagnetic radiation in the radiofrequency and microwave region as well as ultrasonic waves. The use of lasers emitting within the visible and near infrared part of the spectrum has the potential to initiate a local elevation of the temperature. The extension of the heated region will depend on optical wavelength, tissue composition and blood perfusion. Typical linear dimensions of the 42–47 °C temperature region will range from 2–3 mm and up to about 10 mm. Superficial tumours may be heated by direct irradiation whereas deeper lying tumours or lesions of large extent may require light delivered by one or several inserted optical fibres. Laser-induced hyperthermia may be of particular relevance in the treatment of retinal or choroidal tumours. Visible and near infrared radiation can be transmitted through the cornea, the lens and the vitreous with negligible loss. The absorption in the tumour is, however, significantly larger and the main part of the optical power will be absorbed within 0.5–5 mm into the neoplastic tissue. This paper emphasizes a discussion of the basic principles.