Improved continuous light diffusion imaging in single- and multi-target tissue-like phantoms

Design and evaluation of a miniature probe integrating diffuse optical tomography and electroencephalographic source localization

We present a dual-modality three-dimensional imaging approach that integrates diffuse optical tomography (DOT) and electroencephalographic source localization (ESL). This dual-modal DOT/ESL approach is evaluated using solid tissue-like phantoms where targets having both optical and electrical contrasts relative to the background phantom are included. The results obtained from extensive phantom experiments show that this dual-modal approach is suitable for imaging seizure focus in the study of epilepsy.

Handheld miniature probe integrating diffuse optical tomography with photoacoustic imaging through a MEMS scanning mirror

We describe a novel dual-modality imaging approach that integrates diffuse optical tomography (DOT) and photoacoustic imaging (PAI) through a miniaturized handheld probe based on microelectromechanical systems (MEMS) scanning mirror. We validate this dual-modal DOT/PAI approach using extensive phantom experiments, and demonstrate its application for tumor imaging using tumor-bearing mice systematically injected with targeted contrast agents.

A handheld fluorescence molecular tomography system for intraoperative optical imaging of tumor margins

PURPOSE

Accurate identification of tumor margins presents a major challenge in the surgical treatment of human cancers. Inability of complete removal of tumor lesions after surgery causes local recurrence and increases the incidence of developing tumor metastasis. It is clear that novel approaches that allow defining tumor margins intraoperatively for removal of small tumor lesions in the surgical cavity is critical for improving prognosis of cancer patients. To facilitate image-guided surgery using targeted optical imaging probes, we have developed a reflection-mode fluorescence molecular tomography (FMT) system with a handheld probe that is able to provide three-dimensional tumor margin information.

METHODS

The imaging method and system were validated using both simulated and phantom experiments. We further examined the accuracy of the handheld FMT system in an orthotopic mouse mammary tumor model following systemic delivery of near-infrared (NIR) dye-labeled and urokinase plasminogen activator receptor targeted magnet iron oxide nanoparticles.

RESULTS

Our results show that when the targets are located within 5 mm beneath the surface of the media, fluorescent images can be reliably detected and reconstructed with an average positional error of 0.5 mm laterally and 1.5 mm axially. For in vivo imaging in the mouse tumor model, the location and size of the tumor detected by FMT correlated well with that measured by the magnetic resonance imaging (MRI).

CONCLUSIONS

Our system can three-dimensionally image targets located at a depth of up to 7 mm. The in vivo results suggest that in combination with targeted optical imaging probes, this handheld FMT system can be potentially used as an intraoperative tool for the detection of tumor margins and for image-guided surgery.

Integrated diffuse optical tomography and photoacoustic tomography: phantom validations

We designed, fabricated and tested a novel imaging system that fuses diffuse optical tomography (DOT) and photoacoustic tomography (PAT) in a single platform. This platform takes advantages of both DOT and PAT, and can potentially provide dual-modality two dimensional functional and cellular images of the breast quantitatively. Here we describe this integrated platform along with initial tissue phantom validations.

Diffuse Optics for Tissue Monitoring and Tomography

This review describes the diffusion model for light transport in tissues and the medical applications of diffuse light. Diffuse optics is particularly useful for measurement of tissue hemodynamics, wherein quantitative assessment of oxy- and deoxy-hemoglobin concentrations and blood flow are desired. The theoretical basis for near-infrared or diffuse optical spectroscopy (NIRS or DOS, respectively) is developed, and the basic elements of diffuse optical tomography (DOT) are outlined. We also discuss diffuse correlation spectroscopy (DCS), a technique whereby temporal correlation functions of diffusing light are transported through tissue and are used to measure blood flow. Essential instrumentation is described, and representative brain and breast functional imaging and monitoring results illustrate the workings of these new tissue diagnostics.

Direct-current-based image reconstruction versus direct-current included or excluded frequency-domain reconstruction in diffuse optical tomography

We study the level of image artifacts in optical tomography associated with measurement uncertainty under three reconstruction configurations, namely, by using only direct-current (DC), DC-excluded frequency-domain, and DC-included frequency-domain data. Analytic and synthetic studies demonstrate that, at the same level of measurement uncertainty typical to optical tomography, the ratio of the standard deviation of mu(a) over mu(a) reconstructed by DC only is at least 1.4 times lower than that by frequency-domain methods. The ratio of standard deviations of D (or mu(s)') over D (or mu(s)') reconstructed by DC only are slightly lower than those by frequency-domain methods. Frequency-domain reconstruction including DC generally outperforms that excluding DC, but as the amount of measurements increases, the difference between the two diminishes. Under the condition of a priori structural information, the performances of three reconstruction configurations are seemingly equivalent.

Multistage inversion algorithm for biological tissue imaging

A new inversion method for diffuse optical tomography is proposed. This is a multistage algorithm, that uses a signal subspace-based method to simplify the inverse problem and proposes a guided iterative inversion process to improve the imaging. First, subspace-based analysis is used to determine the voxels that definitely belong to the background and exclude them from further consideration. Then, the pseudo-inverse technique is applied for reconstruction. In the final stage, the reconstruction is improved iteratively by finding and excluding more voxels belonging to background. The method reduces the ill-posedness of the image reconstruction problem iteratively such that good imaging results are obtained for multiple heterogeneities having complicated geometries even in the presence of 3% additive white noise.

Highly resolved diffuse optical tomography: a systematic approach using high-pass filtering for value-preserved images

We attempt to develop a systematic scheme through adopting high-pass filtering (HPF) to well resolve value-preserved images such as medical images. Our approach is derived from the Poisson maximum a posteriori superresolution algorithm employing the HP filters, where four filters are considered such as two low-pass-filter-combination based filters, wavelet filter, and negative-oriented Laplacian HP filter. The proposed approach is incorporated into the procedure of finite-element-method (FEM)-based image reconstruction for diffuse optical tomography in the direct current domain, posterior to each iteration without altering the original FEM modeling. This approach is justified with various HPF for different cases that breast-like phantoms embedded with two or three inclusions that imitate tumors are employed to examine the resolution performances under certain extreme conditions. The proposed approach to enhancing image resolution is evaluated for all tested cases. A qualitative investigation of reconstruction performance for each case is presented. Following this, we define a set of measures on the quantitative evaluation for a range of resolutions including separation, size, contrast, and location, thereby providing a comparable evaluation to the visual quality. The most satisfactory result is obtained by using the wavelet HP filter, and it successfully justifies our proposed scheme.

Multispectral breast imaging using a ten-wavelength, 64 x 64 source/detector channels silicon photodiode-based diffuse optical tomography system

We describe a compact diffuse optical tomography system specifically designed for breast imaging. The system consists of 64 silicon photodiode detectors, 64 excitation points, and 10 diode lasers in the near-infrared region, allowing multispectral, three-dimensional optical imaging of breast tissue. We also detail the system performance and optimization through a calibration procedure. The system is evaluated using tissue-like phantom experiments and an in vivo clinic experiment. Quantitative two-dimensional (2D) and three-dimensional (3D) images of absorption and reduced scattering coefficients are obtained from these experiments. The ten-wavelength spectra of the extracted reduced scattering coefficient enable quantitative morphological images to be reconstructed with this system. From the in vivo clinic experiment, functional images including deoxyhemoglobin, oxyhemoglobin, and water concentration are recovered and tumors are detected with correct size and position compared with the mammography.

Accuracy of fluorescent tomography in the presence of heterogeneities: study of the normalized Born ratio

We studied the performance of three-dimensional fluorescence tomography of diffuse media in the presence of heterogeneities. Experimental measurements were acquired using an imaging system consisting of a parallel plate-imaging chamber and a lens coupled charge coupled device camera, which enables conventional planar imaging as well as fluorescence tomography. To simulate increasing levels of background heterogeneity, we employed phantoms made of a fluorescent tube surrounded by several absorbers in different combinations of absorption distribution. We also investigated the effect of low absorbing thin layers (such as membranes). We show that the normalized Born approach accurately retrieves the position and shape of the fluorochrome even at high background heterogeneity. We also demonstrate that the quantification is relatively insensitive to a varying degree of heterogeneity and background optical properties. Findings are further contrasted to images obtained with the standard Born expansion and with a normalized approach that divides the fluorescent field with excitation measurements through a homogeneous medium.

Model-based ultrasound tomography: Tissue phantom experiments

We present a detailed experimental study to evaluate our finite element based nonlinear reconstruction algorithm for recovery of acoustic properties in heterogeneous scattering media. Using a circularly scanning ultrasound system at 500 KHz, tissue phantom experiments were performed to study spatial resolution and contrast issues in model-based ultrasound tomography. Our results show that both acoustic attenuation and speed images can be quantitatively reconstructed in terms of the location, size, shape, and acoustic property value of the target when different contrast levels between the target and background were used. We also demonstrate that a high contrast target as small as 3 mm in diameter can be quantitatively resolved with our acoustic speed and attenuation images.

Mesh-based enhancement schemes in diffuse optical tomography

Two mesh-based methods including dual meshing and adaptive meshing are developed to improve the finite element-based reconstruction of both absorption and scattering images of heterogeneous turbid media. The idea of dual meshing scheme is to use a fine mesh for the solution of photon propagation and a coarse mesh for the inversion of optical property distributions. The adaptive meshing method is accomplished by the automatic mesh refinement in the region of heterogeneity during reconstruction. These schemes are validated using tissue-like phantom measurements. Our results demonstrate the capabilities of the dual meshing and adaptive meshing in both qualitative and quantitative improvement of optical image reconstruction.

Source-detector calibration in three-dimensional Bayesian optical diffusion tomography

Optical diffusion tomography is a method for reconstructing three-dimensional optical properties from light that passes through a highly scattering medium. Computing reconstructions from such data requires the solution of a nonlinear inverse problem. The situation is further complicated by the fact that while reconstruction algorithms typically assume exact knowledge of the optical source and detector coupling coefficients, these coupling coefficients are generally not available in practical measurement systems. A new method for estimating these unknown coupling coefficients in the three-dimensional reconstruction process is described. The joint problem of coefficient estimation and three-dimensional reconstruction is formulated in a Bayesian framework, and the resulting estimates are computed by using a variation of iterative coordinate descent optimization that is adapted for this problem. Simulations show that this approach is an accurate and efficient method for simultaneous reconstruction of absorption and diffusion coefficients as well as the coupling coefficients. A simple experimental result validates the approach.

Absorption and scattering images of heterogeneous scattering media can be simultaneously reconstructed by use of dc data

We present a carefully designed phantom experimental study aimed to provide solid evidence that both absorption and scattering images of heterogeneous scattering media can be reconstructed independently from dc data. We also study the important absorption-scattering cross-talk issue. In this regard, we develop a simple normalizing scheme that is incorporated into our nonlinear finite-element-based reconstruction algorithm. Our results from the controlled phantom experiments show that the cross talk of an absorption object appearing in scattering images can be eliminated and that the cross talk of a scattering object appearing in absorption images can be reduced considerably. In addition, these carefully designed phantom experiments clearly suggest that both absorption and scattering images can be simultaneously recovered and quantitatively separated in highly scattering media by use of dc measurements. Finally, we discuss our results in light of recent theoretical findings on nonuniqueness for dc image reconstruction.

Three-dimensional time-resolved optical tomography of a conical breast phantom

A 32-channel time-resolved imaging device for medical optical tomography has been employed to evaluate a scheme for imaging the human female breast. The fully automated instrument and the reconstruction procedure have been tested on a conical phantom with tissue-equivalent optical properties. The imaging protocol has been designed to obviate compression of the breast and the need for coupling fluids. Images are generated from experimental data with an iterative reconstruction algorithm that employs a three-dimensional (3D) finite-element diffusion-based forward model. Embedded regions with twice the background optical properties are revealed in separate 3D absorption and scattering images of the phantom. The implications for 3D time-resolved optical tomography of the breast are discussed.

Quantitative optical image reconstruction of turbid media by use of direct-current measurements

We present a detailed experimental study concerning quantitative optical property reconstruction of heterogeneous turbid media by use of absolute dc data only. We performed experiments by using tissuelike phantoms in both single-target and multitarget configurations in which variations in target size and optical contrast with the background were explored. Our results show that both scattering and absorption images can be reconstructed quantitatively by use of dc data only, whereas it was impossible to obtain such quantitative information in previously reported studies. We believe that this improvement is primarily a result of the realization of a novel data preprocessing/optimization scheme for accurately determining several critical parameters needed for reconstruction. The use of this data preprocessing/optimization scheme also eliminates the calibration reference measurement previously required for reconstruction. Experimental confirmation of this scheme is given in detail.

Multiple-slice imaging of a tissue-equivalent phantom by use of time-resolved optical tomography

Following several years of development the construction of a multichannel time-resolved imaging device for medical optical tomography has been completed. Images are reconstructed from time-resolved measurements by use of a scheme that employs a finite-element diffusion-based forward model and an iterative reconstruction solver. Prior to testing on clinical subjects the fully automated instrument and the reconstruction software are evaluated with tissue-equivalent phantoms. We describe our first attempt to generate multiple-slice images of a phantom without uniform properties along the axial direction, while still using a computationally fast two-dimensional reconstruction algorithm. The image quality is improved by the employment of an approximate correction method that uses scaling factors derived from the ratios of finite-element forward simulations in two and three spatial dimensions. The 32-channel system was employed to generate maps of the internal scattering and the absorption properties at 14 different transverse planes across the phantom. The images clearly reveal the locations of small inhomogeneous regions embedded within the phantom. These results were obtained by use of purely temporal data and without resource to reference measurements.

Three-dimensional diffuse optical mammography with ultrasound localization in a human subject

We describe an approach that combines clinical ultrasound and photon migration techniques to enhance the sensitivity and information content of diffuse optical tomography. Measurements were performed on a postmenopausal woman with a single 1.8 x 0.9 cm malignant ductal carcinoma in situ approximately 7.4 mm beneath the skin surface (UCI IRB protocol 95-563). The ultrasound-derived information about tumor geometry enabled us to segment the breast tissue into tumor and background regions. Optical data was obtained with a multifrequency, multiwavelength hand-held frequency-domain photon migration backscattering probe. The optical properties of the tumor and background were then computed using the ultrasound-derived geometrical constraints. An iterative perturbative approach, using parallel processing, provided quantitative information about scattering and absorption simultaneously with the ability to incorporate and resolve complex boundary conditions and geometries. A three to four fold increase in the tumor absorption coefficient and nearly 50% reduction in scattering coefficient relative to background was observed (lambda = 674, 782, 803, and 849 nm). Calculations of the mean physiological parameters reveal fourfold greater tumor total hemoglobin concentration [Hbtot] than normal breast (67 microM vs 16 microM) and tumor hemoglobin oxygen saturation (SOx) values of 63% (vs 73% and 68% in the region surrounding the tumor and the opposite normal tissue, respectively). Comparison of semi-infinite to heterogeneous models shows superior tumor/background contrast for the latter in both absorption and scattering. Sensitivity studies assessing the impact of tumor size and refractive index assumptions, as well as scan direction, demonstrate modest effects on recovered properties.

Concurrent MRI and diffuse optical tomography of breast after indocyanine green enhancement

We present quantitative optical images of human breast in vivo. The images were obtained by using near-infrared diffuse optical tomography (DOT) after the administration of indocyanine green (ICG) for contrast enhancement. The optical examination was performed concurrently with a magnetic resonance imaging (MRI) exam on patients scheduled for excisional biopsy or surgery so that accurate image coregistration and histopathological information of the suspicious lesions was available. The ICG-enhanced optical images coregistered accurately with Gadolinium-enhanced magnetic resonance images validating the ability of DOT to image breast tissue. In contrast to simple transillumination, we found that DOT provides for localization and quantification of exogenous tissue chromophore concentrations. Additionally our use of ICG, an albumin bound absorbing dye in plasma, demonstrates the potential to differentiate disease based on the quantified enhancement of suspicious lesions.