We present a new method to reconstruct arbitrary large volumes in (fluorescence) diffuse optical tomography by
splitting the volume of reconstruction into sub-volumes. This allows to perform nonlinear reconstruction on large
grids with a larger number of measurement data and more grid nodes than conventional reconstruction schemes,
where images are reconstructed on a single grid. We investigate how the reconstructed spatial distributions of
diffusion and absorption coefficients using the new method depend on the size of the sub-volumes, compare the
convergence to the conventional nonlinear approach, and present an error estimation.
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.
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.
We have carried out phantom studies for optimizing the design of a fluorescence mammograph employing time-domain
and cw measurements, for improving data analysis and methods of reconstruction. By scanning pulsed (100 fs) laser
radiation across a fluorescent, rectangular breast-like phantom with a spherical inhomogeneity simulating a tumor
bearing breast slightly compressed between two parallel glass plates, distributions of times of flight of laser and
fluorescence photons were measured in transmission and reflection for various detector arrangements. Absorption
coefficients and dye concentrations were reconstructed using perturbation solutions of the diffusion equation at the laser
and fluorescence wavelengths. We additionally employed a CCD camera to measure time-integrated intensity of
fluorescence and laser radiation transmitted through the phantom. The increased number of projection angles entering the
reconstruction improved spatial resolution. Further improvements were obtained when combined cw data and time-resolved
remission data were used in the reconstruction.
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.
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