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Abstract
The field of biomedical optics has matured rapidly over the last decade and is poised
to make a significant impact on patient care. In particular, wide-field (typically
> 5 cm), planar, near-infrared (NIR) fluorescence imaging has the potential to revolutionize
human surgery by providing real-time image guidance to surgeons for tissue that needs
to be resected, such as tumors, and tissue that needs to be avoided, such as blood
vessels and nerves. However, to become a clinical reality, optimized imaging systems
and NIR fluorescent contrast agents will be needed. In this review, we introduce the
principles of NIR fluorescence imaging, analyze existing NIR fluorescence imaging
systems, and discuss the key parameters that guide contrast agent development. We
also introduce the complexities surrounding clinical translation using our experience
with the Fluorescence-Assisted Resection and Exploration (FLARE™) imaging system as
an example. Finally, we introduce state-of-the-art optical imaging techniques that
might someday improve image-guided surgery even further.
We introduce a methodology, fluorescence lifetime imaging (FLIM), in which the contrast depends on the fluorescence lifetime at each point in a two-dimensional image and not on the local concentration and/or intensity of the fluorophore. We used FLIM to create lifetime images of NADH when free in solution and when bound to malate dehydrogenase. This represents a challenging case for lifetime imaging because the NADH decay times are just 0.4 and 1.0 ns in the free and bound states, respectively. In the present apparatus, lifetime images are created from a series of phase-sensitive images obtained with a gain-modulated image intensifier and recorded with a charge-coupled device (CCD) camera. The intensifier gain is modulated at the light-modulation frequency or a harmonic thereof. A series of stationary phase-sensitive images each obtained with various phase shifts of the gain-modulation signal, is used to determine the phase angle or modulation of the emission at each pixel, which is in essence the lifetime image. We also describe am imaging procedure that allows specific decay times to be suppressed, allowing in this case suppression of the emission from either free or bound NADH. Since the fluorescence lifetimes of probes are known to be sensitive to numerous chemical and physical factors such as pH, oxygen, temperature, cations, polarity, and binding to macromolecules, this method allows imaging of the chemical or property of interest in macroscopic and microscopic samples. The concept of FLIM appears to have numerous potential applications in the biosciences.
Photon penetration into living tissue is highly dependent on the absorption and scattering properties of tissue components. The near-infrared region of the spectrum offers certain advantages for photon penetration, and both organic and inorganic fluorescence contrast agents are now available for chemical conjugation to targeting molecules. This review focuses on those parameters that affect image signal and background during in vivo imaging with near-infrared light and exogenous contrast agents. Recent examples of in vivo near-infrared fluorescence imaging of animals and humans are presented, including imaging of normal and diseased vasculature, tissue perfusion, protease activity, hydroxyapatite and cancer.
Despite technical advances in many areas of diagnostic radiology, the detection and imaging of human cancer remains poor. A meaningful impact on cancer screening, staging, and treatment is unlikely to occur until the tumor-to-background ratio improves by three to four orders of magnitude (ie, 10(3)- to 10(4)-fold), which in turn will require proportional improvements in sensitivity and contrast agent targeting. This review analyzes the physics and chemistry of cancer imaging and highlights the fundamental principles underlying the detection of malignant cells within a background of normal cells. The use of various contrast agents and radiotracers for cancer imaging is reviewed, as are the current limitations of ultrasound, x-ray imaging, magnetic resonance imaging (MRI), single-photon emission computed tomography, positron emission tomography (PET), and optical imaging. Innovative technologies are emerging that hold great promise for patients, such as positron emission mammography of the breast and spectroscopy-enhanced colonoscopy for cancer screening, hyperpolarization MRI and time-of-flight PET for staging, and ion beam-induced PET scanning and near-infrared fluorescence-guided surgery for cancer treatment. This review explores these emerging technologies and considers their potential impact on clinical care. Finally, those cancers that are currently difficult to image and quantify, such as ovarian cancer and acute leukemia, are discussed.
[1
]From the Division of Hematology/Oncology, Department of Medicine, and Department of
Radiology, Beth Israel Deaconess Medical Center, Boston, MA, and CEA-LETI-MINATEC,
Grenoble, France
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