Treatment monitoring of Aminolevunilic-acid (ALA) - Photodynamic Therapy (PDT) of basal-cell
carcinoma (BCC) calls for superficial and subsurface imaging techniques. While superficial imagers exist
for this purpose, their ability to assess PpIX levels in thick lesions is poor; additionally few treatment
centers have the capability to measure ALA-induced PpIX production. An area of active research is to
improve treatments to deeper and nodular BCCs, because treatment is least effective in these.
The goal of this work was to understand the logistics and technical capabilities to quantify PpIX at depths
over 1mm, using a novel hybrid ultrasound-guided, fiber-based fluorescence molecular spectroscopictomography
system. This system utilizes a 633nm excitation laser and detection using filtered
spectrometers. Source and detection fibers are collinear so that their imaging plane matches that of
ultrasound transducer. Validation with phantoms and tumor-simulating fluorescent inclusions in mice
showed sensitivity to fluorophore concentrations as low as 0.025μg/ml at 4mm depth from surface, as
presented in previous years.
Image-guided quantification of ALA-induced PpIX production was completed in subcutaneous xenograft
epidermoid cancer tumor model A431 in nude mice. A total of 32 animals were imaged in-vivo, using
several time points, including pre-ALA, 4-hours post-ALA, and 24-hours post-ALA administration. On
average, PpIX production in tumors increased by over 10-fold, 4-hours post-ALA. Statistical analysis of
PpIX fluorescence showed significant difference among all groups; p<0.05. Results were validated by exvivo
imaging of resected tumors. Details of imaging, analysis and results will be presented to illustrate
variability and the potential for imaging these values at depth.
Image-guidance in fluorescence tomography is used to more accurately recover contrast at the cost of performing image segmentation. We propose a method of regularization which uses anatomical priors directly and does not require image segmentation
Subsurface fluorescence imaging is desirable for medical applications, including protoporphyrin-IX (PpIX)-based skin tumor diagnosis, surgical guidance, and dosimetry in photodynamic therapy. While tissue optical properties and heterogeneities make true subsurface fluorescence mapping an ill-posed problem, ultrasound-guided fluorescence-tomography (USFT) provides regional fluorescence mapping. Here USFT is implemented with spectroscopic decoupling of fluorescence signals (auto-fluorescence, PpIX, photoproducts), and white light spectroscopy-determined bulk optical properties. Segmented US images provide a priori spatial information for fluorescence reconstruction using region-based, diffuse FT. The method was tested in simulations, tissue homogeneous and inclusion phantoms, and an injected-inclusion animal model. Reconstructed fluorescence yield was linear with PpIX concentration, including the lowest concentration used, 0.025 μg/ml . White light spectroscopy informed optical properties, which improved fluorescence reconstruction accuracy compared to the use of fixed, literature-based optical properties, reduced reconstruction error and reconstructed fluorescence standard deviation by factors of 8.9 and 2.0, respectively. Recovered contrast-to-background error was 25% and 74% for inclusion phantoms without and with a 2-mm skin-like layer, respectively. Preliminary mouse-model imaging demonstrated system feasibility for subsurface fluorescence measurement in vivo. These data suggest that this implementation of USFT is capable of regional PpIX mapping in human skin tumors during photodynamic therapy, to be used in dosimetric evaluations.
An ultrasound-guided fluorescence molecular tomography system is under development for in vivo quantification of
Protoporphyrin IX (PpIX) during Aminolevulinic Acid - Photodynamic Therapy (ALA-PDT) of Basal Cell Carcinoma.
The system is designed to combine fiber-based spectral sampling of PPIX fluorescence emission with co-registered
ultrasound images to quantify local fluorophore concentration. A single white light source is used to provide an estimate of the bulk optical properties of tissue. Optical data is obtained by sequential illumination of a 633nm laser source at 4 linear locations with parallel detection at 5 locations interspersed between the sources. Tissue regions from segmented ultrasound images, optical boundary data, white light-informed optical properties and diffusion theory are used to estimate the fluorophore concentration in these regions. Our system and methods allow interrogation of both superficial and deep tissue locations up to PpIX concentrations of 0.025ug/ml.
In fluorescence molecular tomography, optical measurements at the surface are used with diffusion theory modeling to reconstruct the maps of the fluorophore distribution in the tissue using an iterative error minimization algorithm. While normalizing the fluorescence signal with the excitation signal has been shown to correct for source and detector inconsistencies somewhat, this approach does not always correct for tissue heterogeneities and inaccuracies that are not matched by the forward diffusion model. Using computer simulations and an ultrasound-guided fluorescence tomography (FT) system designed for spatial mapping of Protoporphyrin IX (PpIX), the errors in fluorophore concentration recovery by assignment of incorrect optical properties are analyzed. Using simulations and experiments, white light spectroscopy was used to obtain more accurate tissue properties for forward diffusion model, prior to FT. Using white light spectroscopy the accuracy in FT values improved by 97% on average and the minimal detectable concentration of PpIX with the system was 0.025μg/ml.
Aminolevulinic acid (ALA)-induced Protoporphyrin IX (PpIX)-based photodynamic therapy (PDT) is an effective treatment for skin cancers including basal cell carcinoma (BCC). Topically applied ALA promotes PpIX production preferentially in tumors, and many strategies have been developed to increase PpIX distribution and PDT treatment efficacy at depths > 1mm is not fully understood. While surface imaging techniques provide useful diagnosis, dosimetry, and efficacy information for superficial tumors, these methods cannot interrogate deeper tumors to provide in situ insight into spatial PpIX distributions. We have developed an ultrasound-guided, white-light-informed, tomographics spectroscopy system for the spatial measurement of subsurface PpIX. Detailed imaging system specifications, methodology, and optical-phantom-based characterization will be presented separately. Here we evaluate preliminary in vivo results using both full tomographic reconstruction and by plotting individual tomographic source-detector pair data against US images.
An ultrasound coupled handheld-probe-based optical fluorescence molecular tomography (FMT) system has been in
development for the purpose of quantifying the production of Protoporphyrin IX (PPIX) in aminolevulinic acid
treated (ALA), Basal Cell Carcinoma (BCC) in vivo. The design couples fiber-based spectral sampling of PPIX
fluorescence emission with a high frequency ultrasound imaging system, allowing regionally localized fluorescence
intensities to be quantified [1]. The optical data are obtained by sequential excitation of the tissue with a 633nm
laser, at four source locations and five parallel detections at each of the five interspersed detection locations. This
method of acquisition permits fluorescence detection for both superficial and deep locations in ultrasound field. The
optical boundary data, tissue layers segmented from ultrasound image and diffusion theory are used to estimate the
fluorescence in tissue layers. To improve the recovery of the fluorescence signal of PPIX, eliminating tissue autofluorescence
is of great importance. Here the approach was to utilize measurements which straddled the steep Qband
excitation peak of PPIX, via the integration of an additional laser source, exciting at 637 nm; a wavelength
with a 2 fold lower PPIX excitation value than 633nm.The auto-fluorescence spectrum acquired from the 637 nm
laser is then used to spectrally decouple the fluorescence data and produce an accurate fluorescence emission signal,
because the two wavelengths have very similar auto-fluorescence but substantially different PPIX excitation levels.
The accuracy of this method, using a single source detector pair setup, is verified through animal tumor model
experiments, and the result is compared to different methods of fluorescence signal recovery.
The in vivo performance of a Fluorescence Molecular Tomography system as a function of pathophysiological
parameters that determine the penetration of nonbinding fluorescent nanoparticle was examined through imaging of
a series of three tumor models. The pathophysiological parameters examined were, vessel density, interstitial fluid
pressure (IFP), and collagen content. Drug delivery and IFP were measured in vivo via fluorescence spectroscopy
and a fiber-optic coupled pressure probe. Vessel density and collagen content were determined ex vivo through
histochemical analysis. The kinetics of the 40 nm,10000 KDa, fluorescent particles, which were injected into the tail
vein of the mice, was monitored by sequential excitation of the tissue on and off the tumor site through employment
of sixteen source detector pairs interspersed linearly in reflectance geometry. Each optical fluorescence data set was
collected at discrete time intervals in order to monitor drug uptake for a period of 45 minutes. The kinetics of the
drug delivery and the average nanoparticle uptake were correlated with the vessel density, interstitial pressure and
collagen content. The results of the correlations were verified to be consistent with the published relationship
between the three pathophysiological parameters and nanoparticle drug delivery.
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