Gert W. t'Hooft

First results from the Philips optical mammoscope

The Philips Optical Mammoscope Prototype is a versatile CW optical mammography apparatus with which fully 3D images of breast tissue can be obtained at several wavelengths. We present the properties of the machine, and show the first images obtained with it. We elaborate on the tissue parameter that can be quantitatively reconstructed from the measurements, and show some preliminary research into the natural variation of this parameter.

DETECTION LIMIT IN LOCALIZING OBJECTS HIDDEN IN A TURBID MEDIUM USING AN OPTICALLY SCANNED PHASED ARRAY

In this paper the sensitivity of a phased array and a continuous wave measurement setup are compared. Both techniques are used to image inhomogeneities hidden inside a tissuelike turbid medium. We have found that despite the higher technical complexity of the phased array measurement, its sensitivity in detecting inhomogeneities is virtually the same as the sensitivity of the continuous wave measurement.

Optical image reconstruction with deconvolution in light diffusing media

We describe a simple reconstruction method for diffuse optical imaging based on modified back-projection approach for medical tomography. These modifications are based primarily on the deconvolution of the broadened image by a point spread function which is dependent on the scattering of light in tissue. The nonlinearities in the image formation are handled empirically by coordinate transformations. Although our method is an approximate image reconstruction technique, it may provide a basis for rapid, real time medical monitoring by using optical projections. We have applied this method to experimental projections taken by parallel and fan beam tomography geometries on biomedical phantoms with multiple objects. The results presented in this report indicate considerable improvement in image resolution.

Resolution and sensitivity limits of optical imaging in highly scattering media

Using photon-density-waves (PDW), (multiple) objects embedded in a highly scattering medium with optical properties similar to tissue are detected. The measurements were performed with near-infra-red laser light at 675 nm which was either kept continuous-wave (cw) or amplitude modulated (AM) at 219 MHz or 650 MHz. We find that the spatial resolution of the projection image shows only slight improvements as the frequency is increased. This improvement comes at the expense of signal strength in the modulated part of the light. That is, the PDW shows a much stronger attenuation as compared to the cw light intensity. The implications of the lower signal-to-noise ratio at high modulation frequencies is that the modulated light projections are less suitable for further data processing. For a given data acquisition system, this fact cancels the advantages of the higher raw resolution of PDW as compared to cw light. Therefore, we find no clear advantage of PDW over cw light for obtaining sharper tomographic images of the diffuse media, regardless of the inverse scattering method used.

Dependence of image quality on optical parameters in time-resolved transillumination experiments

Time-resolved transillumination experiments on turbid media have been performed using a streak camera with 6 ps time resolution and a mode-locked Ti:sapphire laser with a pulse width of 160 fs. One dimensional and two dimensional images have been recorded. The resolution of the images is related to the optical properties of the media as obtained from experimental data taken with the double-integrating-sphere method, and from fitting the time- resolved pulse profiles to a diffusion model.

Fully 3D reconstruction of attenuation for diffuse optical tomography using a finite element model

In this study, we present reconstruction results for optical tomography based on measurements with a 3D scanner using a finite element forward model. We show the importance of taking the correct Greens function everywhere, which is possible using the FEM approach. Further, it is shown that for the simple case of a few spheres in a homogeneous liquid, the Rytov approximation suffices.

Silicon-on-insulator resonant cavity photodiode without a slow carrier diffusion tail

We report on a novel silicon-based resonant cavity photodiode with a buried silicon dioxide layer as the bottom reflector. The buried oxide is created by using a separation by implantation of oxygen technique. The device shows large Fabry-Perot oscillations. Resonant peaks and anti-resonant troughs are observed as a function of the wavelength, with a peak responsivity of about 50 mA/W at 650 nm and 709 nm. The leakage current density is 85 pA/mm2 at -5 V, and the average zero-bias capacitance is 12 pF/mm2. We also demonstrate that the buried oxide prevents carriers generated deep within the substrate from reaching the top contacts, thus removing any slow carrier diffusion tail from the impulse response.