Tiffany C. Kwong

Temperature-modulated fluorescence tomography based on both concentration and lifetime contrast.

It is challenging to image fluorescence objects with high spatial resolution in a highly scattering medium. Recently reported temperature-sensitive indocyanine green-loaded pluronic nanocapsules can potentially alleviate this problem. Here we demonstrate a frequency-domain temperature-modulated fluorescence tomography system that could acquire images at high intensity-focused ultrasound resolution with use of these nanocapsules. The system is experimentally verified with a phantom study, where a 3-mm fluorescence object embedded 2 cm deep in a turbid medium is successfully recovered based on both intensity and lifetime contrast.

Differentiation of tumor vasculature heterogeneity levels in small animals based on total hemoglobin concentration using magnetic resonance-guided diffuse optical tomography in vivo.

Insight into the vasculature of the tumor in small animals has the potential to impact many areas of cancer research. The heterogeneity of the vasculature of a tumor is directly related to tumor stage and disease progression. In this small scale animal study, we investigated the feasibility of differentiating tumors with different levels of vasculature heterogeneity in vivo using a previously developed hybrid magnetic resonance imaging (MRI) and diffuse optical tomography (DOT) system for small animal imaging. Cross-sectional total hemoglobin concentration maps of 10 Fisher rats bearing R3230 breast tumors are reconstructed using multi-wavelength DOT measurements both with and without magnetic resonance (MR) structural a priori information. Simultaneously acquired MR structural images are used to guide and constrain the DOT reconstruction, while dynamic contrast-enhanced MR functional images are used as the gold standard to classify the vasculature of the tumor into two types: high versus low heterogeneity. These preliminary results show that the stand-alone DOT is unable to differentiate tumors with low and high vascular heterogeneity without structural a priori information provided by a high resolution imaging modality. The mean total hemoglobin concentrations comparing the vasculature of the tumors with low and high heterogeneity are significant (p-value 0.02) only when MR structural a priori information is utilized.

Multi-wavelength fluorescence tomography

The strong scattering and absorption of light in biological tissue makes it challenging to model the propagation of light, especially in deep tissue. This is especially true in fluorescent tomography, which aims to recover the internal fluorescence source distribution from the measured light intensities on the surface of the tissue. The inherently ill-posed and underdetermined nature of the inverse problem along with strong tissue scattering makes Fluorescence Tomography (FT) extremely challenging. Previously, multispectral detection fluorescent tomography (FT) has been shown to improve the image quality of FT by incorporating the spectral filtering of biological tissue to provide depth information to overcome the inherent absorption and scattering limitations. We investigate whether multi-wavelength fluorescent tomography can be used to distinguish the signals from multiple fluorophores with overlapping fluorescence spectrums using a unique near-infrared (NIR) swept laser. In this work, a small feasibility study was performed to see whether multi-wavelength FT can be used to detect subtle shifts in the absorption spectrum due to differences in fluorophore microenvironment.

Excitation light leakage suppression using temperature sensitive fluorescent agents

Fluorescence tomography is a non invasive, non ionizing imaging technique able to provide a 3D distribution of fluorescent agents within thick highly scattering mediums, using low cost instrumentation. However, its low spatial resolution due to undetermined and ill-posed nature of its inverse problem has delayed its integration into the clinical settings. In addition, the quality of the fluorescence tomography images is degraded due to the excitation light leakage contaminating the fluorescence measurements. This excitation light leakage results from the excitation photons that cannot be blocked by the fluorescence filters. In this contribution, we present a new method to remove this excitation light leakage noise based on the use of a temperature sensitive fluorescence agents. By performing different sets of measurements using this temperature sensitive agents at multiple temperatures, the excitation light leakage can be estimated and then removed from the measured fluorescence signals . The results obtained using this technique demonstrate its potential for use in in-vivo small animal imaging.

Temperature-modulated fluorescence tomography: modulating tissue temperature using HIFU for high-resolution in vivo fluorescence tomography

Low spatial resolution due to strong tissue scattering is one of the main barriers that prevent the wide-spread use of fluorescence tomography. To overcome this limitation, we previously demonstrated a new technique, temperature modulated fluorescence tomography (TM-FT), which relies on key elements: temperature sensitive ICG loaded pluronic nanocapsules and high intensity focused ultrasound (HIFU), to combine the sensitivity of fluorescence imaging with focused ultrasound resolution. While conventional fluorescence tomography measurements are acquired, the tissue is scanned by a HIFU beam and irradiated to produce a local hot spot, in which the temperature increases nearly 5K. The fluorescence emission signal measured by the optical detectors varies drastically when the hot spot overlays onto the location of the temperature dependent nanocapsules. The small size of the focal spot (~1.4 mm) up to a depth of 6 cm, allows imaging the distribution of these temperature sensitive agents with not only high spatial resolution but also high quantitative accuracy in deep tissue using a proper image reconstruction algorithm. Previously we have demonstrated this technique with a phantom study with nanocapsules sensitive to 20-25°C range. In this work, we will show the first nanocapsules optimized for in vivo animal imaging.

A combined HIFU-Fluorescence Tomography high-resolution imaging technique using temperature-modulated thermodots

We present a high-resolution fluorescence imaging technique based on the use of in-vivo temperature sensitive ICG loaded pluronic nanocapsules and the synergistic combination of two imaging techniques: Fluorescence Tomography (FT) and High Intensity Focused Ultrasound (HIFU).

Modulating tissue temperature for high-resolution, in vivo fluorescence tomography

As a molecular imaging modality, optical fluorescence imaging can provide the distribution of molecular probes in vivo using non-ionizing radiation and low-cost instrumentation.1–4 Although fluorescence imaging technology is slowly moving into the clinical arena, it has already found its niche in small imaging research, not only for a better understanding of the fundamental molecular biologic and biochemical nature of various diseases, but also for development of new contrast agents and pathway-specific imaging probes. Nevertheless, themain barrier preventing the widespread use of 3D fluorescence tomography (FT) has been its low resolution and quantitative accuracy. Small animal fluorescence optical imaging systems became commercially available nearly a decade ago, with their number increasing every year. However, most systems can only provide 2D projection images. Research labs, including ours, have made extensive efforts to develop 3D tomographic, whole-body animal imaging systems. While several commercial FT systems have recently started to appear in the market, high tissue scattering causes blurring in the images and degrades the resolution and quantitative accuracy, especially in 3D tomographic mode.5, 6 Several alternative approaches have been implemented to improve the performance of FT.7–10 We believe our approach is unique in that we use novel temperature-sensitive fluorescent agents together with focused ultrasound. Our technique is called temperature-modulated FT (TM-FT). It uses temperature modulation of the fluorescence quantum efficiency of temperature-sensitive contrast agents, or what we call thermodots. The medium is irradiated with both light and high-intensity, low-power, focused ultrasound (HIFU) waves. Figure 1. (a) Phantom and high-intensity focused ultrasound (HIFU) transducer. (b) During the HIFU scan, a localized temperature increase ( 4ıC) occurs on the focal spot ( 1mm). (c) When the thermodots are present within the HIFU focal zone, the temperature increase alters the quantum efficiency, hence the light intensity of the fluorescence emission.