Molecular imaging is a field of medical imaging that focuses on imaging molecules of medical interest within living patients. This is in contrast to conventional methods for obtaining molecular information from preserved tissue samples, such as histology. Molecules of interest may be either ones produced naturally by the body, or synthetic molecules produced in a laboratory and injected into a patient by a doctor. The most common example of molecular imaging used clinically today is to inject a contrast agent (e.g., a microbubble, metal ion, or radioactive isotope) into a patient's bloodstream and to use an imaging modality (e.g., ultrasound, MRI, CT, PET) to track its movement in the body. Molecular imaging originated from the field of radiology from a need to better understand fundamental molecular processes inside organisms in a noninvasive manner. The ultimate goal of molecular imaging is to be able to noninvasively monitor all of the biochemical processes occurring inside an organism in real time. Current research in molecular imaging involves cellular/molecular biology, chemistry, and medical physics, and is focused on: 1) developing imaging methods to detect previously undetectable types of molecules, 2) expanding the number and types of contrast agents available, and 3) developing functional contrast agents that provide information about the various activities that cells and tissues perform in both health and disease.
Overview Molecular imaging emerged in the mid twentieth century as a discipline at the intersection of molecular biology and in vivo imaging. It enables the visualisation of the cellular function and the follow-up of the molecular process in living organisms without perturbing them. The multiple and numerous potentialities of this field are applicable to the diagnosis of diseases such as cancer, and neurological and cardiovascular diseases. This technique also contributes to improving the treatment of these disorders by optimizing the pre-clinical and clinical tests of new medication. They are also expected to have a major economic impact due to earlier and more precise diagnosis. Molecular and Functional Imaging has taken on a new direction since the description of the human genome. New paths in fundamental research, as well as in applied and industrial research, render the task of scientists more complex and increase the demands on them. Therefore, a tailor-made teaching program is in order. Molecular imaging differs from traditional imaging in that probes known as biomarkers are used to help image particular targets or pathways. Biomarkers interact chemically with their surroundings and in turn alter the image according to molecular changes occurring within the area of interest. This process is markedly different from previous methods of imaging which primarily imaged differences in qualities such as density or water content. This ability to image fine molecular changes opens up an incredible number of exciting possibilities for medical application, including early detection and treatment of disease and basic pharmaceutical development. Furthermore, molecular imaging allows for quantitative tests, imparting a greater degree of objectivity to the study of these areas. One emerging technology is MALDI molecular imaging based on mass spectrometry. Many areas of research are being conducted in the field of molecular imaging. Much research is currently centered on detecting what is known as a predisease state or molecular states that occur before typical symptoms of a disease are detected. Other important veins of research are the imaging of gene expression and the development of novel biomarkers. Organizations such as the SNMMI Center for Molecular Imaging Innovation and Translation (CMIIT) have formed to support research in this field. In Europe, other "networks of excellence" such as DiMI (Diagnostics in Molecular Imaging) or EMIL (European Molecular Imaging Laboratories) work on this new science, integrating activities and research in the field. In this way, a European Master Programme "EMMI" is being set up to train a new generation of professionals in molecular imaging. Recently, the term molecular imaging has been applied to a variety of microscopy and nanoscopy techniques including live-cell microscopy, total internal reflection fluorescence microscope (TIRF)-microscopy, stimulated emission depletion (STED)-nanoscopy, and atomic force microscopy (AFM) as here images of molecules are the readout.
Imaging modalities There are many different modalities that can be used for noninvasive molecular imaging. Each have their different strengths and weaknesses and some are more adept at imaging multiple targets than others.
Magnetic resonance imaging
MRI has the advantages of having very high spatial resolution and is very adept at morphological imaging and functional imaging. MRI does have several disadvantages though. First, MRI has a sensitivity of around 10−3 mol/L to 10−5 mol/L which, compared to other types of imaging, can be very limiting. This problem stems from the fact that the difference between atoms in the high energy state and the low energy state is very small. For example, at 1.5 tesla, a typical field strength for clinical MRI, the difference between high and low energy states is approximately 9 molecules per 2 million. Improvements to increase MR sensitivity include increasing magnetic field strength, and hyperpolarization via optical pumping, dynamic nuclear polarization or parahydrogen induced polarization. There are also a variety of signal amplification schemes based on chemical exchange that increase sensitivity. To achieve molecular imaging of disease biomarkers using MRI, targeted MRI contrast agents with high specificity and high relaxivity (sensitivity) are required. To date, many studies have been devoted to developing targeted-MRI contrast agents to achieve molecular imaging by MRI. Commonly, peptides, antibodies, or small ligands, and small protein domains, such as HER-2 affibodies, have been applied to achieve targeting. To enhance the sensitivity of the contrast agents, these targeting moieties are usually linked to high payload MRI contrast agents or MRI contrast agents with high relaxivities. In particular, the recent development of micron-sized particles of iron oxide (MPIO) allowed to reach unprecedented levels of sensitivity to detect proteins expressed by arteries and veins.
Optical imaging
… excerpt ends here. Continue reading the full article.
