Molecular Imaging of the Lungs PDF

Preface

A suite of emerging techniques, known collectively as "molecular imaging," now offer scientists an unprecedented opportunity to identify, follow, and quantify biologic processes at the cellular level with molecular specificity in intact organisms. For instance, it is now possible to evaluate, with imaging, the distribution, magnitude, and timing of gene expression in genetically altered animals (1-3). Even though nonimaging methods are also available to monitor gene expression, such methods-usually based on tissue sampling-are invasive and unattractive as routine procedures for clinical investigations. In contrast, molecular imaging can provide a seamless translation from studies in animals to later studies in humans.

While most studies that employ molecular imaging have so far focused on non-organ specific applications (such as cancer detection or treatment monitoring, and gene therapeutics), the ability to image fundamental processes (such as gene expression, inflammation, cell trafficking, apoptosis), provides ample reason to employ these methods in studies of lung biology. This rapidly developing, multidisciplinary field capitalizes on recent advances in the techniques of molecular and cell biology, on new highly specific probes that serve as sources for the imaging signal, and on dramatic improvements in imaging instrumentation (especially for small animals).

The number of options already available is dazzling. For instance, noninvasive real-time analysis of gene expression is possible using reporter genes with optical signatures [e.g., green fluorescent protein (gfp), firefly luciferase (3,4), and bacterial luciferase]. Key advances in detector technology for imaging low levels of light now enable imaging of these probes in living animals with charge coupled device (CCD) cameras. Alternatively, gene expression imaging can be accomplished with radionuclide-based techniques such as positron emission tomography (PET) or single photon emission computed tomography (SPECT) (5). In these cases, gene expression is monitored by measuring the accumulation of highly specific radiotracers in tissues expressing the target gene. Gene expression can also be followed by magnetic resonance imaging (MRI) using probes that, again, only accumulate in tissues expressing a target gene capable of activating a specific nanomagnetic probe (5).

Even within one type of imaging modality, choices are rapidly increasing in number. For instance, three general strategies have already evolved for optical molecular imaging in vivo: use of endogenous fluorochromes; use of reporter genes that generate internal light from specific biochemical reactions or external illumination (bioluminescence and fluorescent proteins); and use of injected optical contrast agents incorporating visible light fluorophores, near-infrared fluorophores, or activatable fluorophores (1,4,6).

Added to this array of new techniques for molecular imaging is a growing capability to perform anatomic or structural imaging in small animals. So-called "micro" computed tomography (CT), MRI, and ultrasound scanners and instruments now allow one to measure morphologic properties such as airway caliber or right ventricular dimensions, even in mice or other rodents. In many cases, these structural measurements can be coupled with various physiologic measurements relevant to lung physiology and pathophysiology, such as ventilation, perfusion, lung water, and pulmonary arterial pressure, among others. Furthermore, since each of the modern molecular imaging technologies like PET, MRI, and optical imaging has specific advantages and weaknesses, it makes sense to identify ways to benefit from each through so-called "fusion imaging," allowing detailed mapping of structure to function (e.g., with X-ray CT and PET). The use of multifunctional reporter genes that link two or more modalities is another approach that should be highly informative (4). Fusion genes that encode bioluminescent and fluorescent reporter proteins effectively couple the powerful in vivo capabilities of bioluminescence with the subset-discriminating capabilities of fluorescence-activated cell sorting (4,7). Similarly, dual reporters that combine nuclear imaging techniques (e.g., PET) with fluorescence and/or bioluminescence for gene expression studies have also been developed (8-11).

These exciting developments will allow pulmonary scientists to study in vivo lung biology at an unprecedented cellular and molecular level. New and established basic scientists, as well as physician-scientists interested in emerging techniques for noninvasive imaging of molecular events in the lungs of intact animals and in humans, need to become aware of these approaches to studying lung biology, both for their promise and limitations. Thus, this volume is divided into two main sections. In the first, "Methodologies," topics are presented and reviewed relevant to the underlying techniques themselves, including instrumentation, radionuclide-based methods, optical imaging approaches, magnetic resonance methods, and ultrasonography. In the second, "Applications," the focus turns to how these methods have already been-and will be in the future-applied to studies involving the lungs, including such diverse topics as gene expression imaging, inflammation imaging, imaging pulmonary cytokine regulation, molecular imaging of lung cancer, and imaging cell death, among others.

Taking advantage of these new capabilities should allow pulmonary scientists a "view" on lung biology that could not be imaged-or even imagined-a few years ago.