Video-rate near-infrared optical tomography using spectrally encoded parallel light delivery

Exploring the feasibility of wavelength modulated near-infrared spectroscopy

SIGNIFICANCE

The application of near-infrared spectroscopy (NIRS) to determine the concentrations of tissue chromophores has typically relied on three alternative technological approaches: continuous-wave, frequency-domain, and time-domain. It is often the case that uncertain and variable coupling of light into and out of the skin surface renders absolute measurements unreliable, and NIRS methods are mostly used to measure changes of chromophore concentrations and of physiological parameters such as blood volume and oxygenation.

AIM

The aim has been to investigate whether an approach based on a wavelength-modulated source may enable measurements to be acquired, which are independent of surface coupling and may facilitate derivation of absolute values of tissue parameters.

APPROACH

An analysis is performed using the modified Beer-Lambert law.

RESULTS

It is shown that the relative modulation in detected intensity resulting from a wavelength-modulated source could be used to estimate absolute concentrations of chromophores if unknown surface coupling losses and geometrical factors are insensitive to small changes in wavelength.

CONCLUSIONS

Wavelength modulated NIRS could be an effective tool for quantitative in vivo analysis of tissues, although it may be technically challenging.

Addressing Multipath Signal Corruption in Microwave Tomography and the Influence on System Design and Algorithm Development

In developing a microwave tomography system, we started by examining the fundamental signal measurement challenges-i.e., how to interrogate the target while suppressing unwanted multi-path signals. Beginning with a lossy coupling bath to suppress unwanted surface waves, we have developed a robust and reliable system that is both simple and low profile. However, beyond the basic measurement configuration, the lossy coupling medium concept has also informed our choice of array antenna and imaging algorithms. The synergism of these concepts has produced a novel concept which is embodied in a system that has been successfully translated to the clinic.

Frequency domain near-infrared multiwavelength imager design using high-speed, direct analog-to-digital conversion

Frequency domain near-infrared spectroscopy (FD-NIRS) has proven to be a reliable method for quantification of tissue absolute optical properties. We present a full-sampling direct analog-to-digital conversion FD-NIR imager. While we developed this instrument with a focus on high-speed optical breast tomographic imaging, the proposed design is suitable for a wide-range of biophotonic applications where fast, accurate quantification of absolute optical properties is needed. Simultaneous dual wavelength operation at 685 and 830 nm is achieved by concurrent 67.5 and 75 MHz frequency modulation of each laser source, respectively, followed by digitization using a high-speed (180  MS/s) 16-bit A/D converter and hybrid FPGA-assisted demodulation. The instrument supports 25 source locations and features 20 concurrently operating detectors. The noise floor of the instrument was measured at <1.4  pW/√Hz, and a dynamic range of 115+ dB, corresponding to nearly six orders of magnitude, has been demonstrated. Titration experiments consisting of 200 different absorption and scattering values were conducted to demonstrate accurate optical property quantification over the entire range of physiologically expected values.

Effective contrast recovery in rapid dynamic near-infrared diffuse optical tomography using ℓ(1)-norm-based linear image reconstruction method

Traditional image reconstruction methods in rapid dynamic diffuse optical tomography employ ℓ(2)-norm-based regularization, which is known to remove the high-frequency components in the reconstructed images and make them appear smooth. The contrast recovery in these type of methods is typically dependent on the iterative nature of method employed, where the nonlinear iterative technique is known to perform better in comparison to linear techniques (noniterative) with a caveat that nonlinear techniques are computationally complex. Assuming that there is a linear dependency of solution between successive frames resulted in a linear inverse problem. This new framework with the combination of ℓ(1)-norm-based regularization can provide better robustness to noise and provide better contrast recovery compared to conventional ℓ(2)-based techniques. Moreover, it is shown that the proposed ℓ(1)-based technique is computationally efficient compared to its counterpart (ℓ(2)-based one). The proposed framework requires a reasonably close estimate of the actual solution for the initial frame, and any suboptimal estimate leads to erroneous reconstruction results for the subsequent frames.

Singular value decomposition based computationally efficient algorithm for rapid dynamic near-infrared diffuse optical tomography

PURPOSE

A computationally efficient algorithm (linear iterative type) based on singular value decomposition (SVD) of the Jacobian has been developed that can be used in rapid dynamic near-infrared (NIR) diffuse optical tomography.

METHODS

Numerical and experimental studies have been conducted to prove the computational efficacy of this SVD-based algorithm over conventional optical image reconstruction algorithms.

RESULTS

These studies indicate that the performance of linear iterative algorithms in terms of contrast recovery (quantitation of optical images) is better compared to nonlinear iterative (conventional) algorithms, provided the initial guess is close to the actual solution. The nonlinear algorithms can provide better quality images compared to the linear iterative type algorithms. Moreover, the analytical and numerical equivalence of the SVD-based algorithm to linear iterative algorithms was also established as a part of this work. It is also demonstrated that the SVD-based image reconstruction typically requires O(NN2) operations per iteration, as contrasted with linear and nonlinear iterative methods that, respectively, require O(NN3) and O(NN6) operations, with "NN" being the number of unknown parameters in the optical image reconstruction procedure.

CONCLUSIONS

This SVD-based computationally efficient algorithm can make the integration of image reconstruction procedure with the data acquisition feasible, in turn making the rapid dynamic NIR tomography viable in the clinic to continuously monitor hemodynamic changes in the tissue pathophysiology.

Broadband frequency-domain near-infrared spectral tomography using a mode-locked Ti:sapphire laser

Frequency-domain near-infrared (NIR) diffuse spectral tomography with a mode-locked Ti:sapphire laser is presented, providing tunable multiwavelength quantitative spectroscopy with maximal power for thick tissue imaging. The system was developed to show that intrinsically high stability can be achieved with many wavelengths in the NIR range, using a mode-locked signal of 80 MHz with heterodyned lock-in detection. The effect of cumulative noise from multiple wavelengths of data on the reconstruction process was studied, and it was shown that inclusion of more wavelengths can reduce skew in the noise distribution. This normalization of the data variance then minimizes errors in estimation of chromophore concentrations. Simulations and tissue phantom experiments were used to quantify this improvement in image accuracy for recovery of tissue hemoglobin and oxygen saturation.

Optimal probing of optical contrast of breast lesions of different size located at different depths by US localization

We report a frequency domain optical tomography system utilizing three RF modulation frequencies, which are optimized for probing breast lesions of different size located at different depths. A real-time co-registered ultrasound scanner is used to provide on-site estimation of lesion size and location. Based on the lesion information, an optimal light modulation frequency can be selected, which may yield more accurate estimates of lesion angiogenesis and hypoxia. Phantom experiments have demonstrated that a high modulation frequency, such as 350Mhz, is preferable for probing small lesions closer to the surface while a low modulation frequency, such as 50Mhz, is desirable for imaging deeper and larger lesions. A clinical example of a large invasive carcinoma is presented to demonstrate the application of this novel technique.

Rapid near-infrared diffuse tomography for hemodynamic imaging using a low-coherence wideband light source

Rapid near-infrared (NIR) diffuse optical tomography is implemented using a low-coherence source. The spectral bandwidth of the low-coherence source is dispersed and coupled to linearly bundled fibers, such that "spread"-spectral encoding among the bundled fibers is formed, and could be used for parallel source illumination onto tissue. In comparison with a previous spectral-encoding technique that employed multiple laser diodes, the use of a low-coherence source for spread-spectral encoding presents a few unique characteristics: (1) it provides shift-free spectral encoding; (2) it reduces the reconstruction uncertainty significantly owing to the minimization of spontaneous channel-to-channel intensity fluctuation; and (3) it enables the implementation of NIR tomography into an endoscopic imaging mode. A 20-mW superluminescent diode centered at 840 nm with a 40-nm bandwidth is used as the source, and a sampling speed of 5 Hz is obtained in a 27-mm imaging array consisting of eight sources and eight detection channels. The principles of using a low-coherence source for spread-spectral encoding are elaborated, the characteristic performances are demonstrated, and the preliminary results of imaging hemoglobin absorption variations during 10 s of voluntary breath-holding are presented.

Endoscopic, rapid near-infrared optical tomography

This is believed to be the first demonstration of near-infrared (NIR) optical tomography employed at the endoscope scale and at a rapid sampling speed that allows translation to in vivo use. A spread-spectral-encoding technique based on a broadband light source and linear-to-circular fiber bundling was used to provide endoscopic probing of many source-detector fibers for tomography as well as parallel sampling of all source-detector pairs for rapid imaging. Endoscopic NIR tomography at an 8 Hz frame rate was achieved in phantoms and tissue specimens with a 12 mm probe housing eight sources and eight detectors. This novel approach provides the key feasibility studies to allow this blood-based contrast imaging technology to be attempted in detection of cancer in internal organs via endoscopic interrogation.