Background as published in Eur J Nucl Med 29:1-2, 2002
Molecular imaging has its roots in nuclear medicine and in many ways is a direct extension of this discipline. Nuclear Medicine has always focused on patient management through the use of injected radiolabeled tracers in conjunction with imaging technology.

Definition as published in Genes & Dev, 2003, 17: 545-580
Molecular imaging is the visual representation, characterization, and quantification of biological processes at the cellular and subcellular levels within intact living organisms. The images obtained reflect cellular and molecular pathways and in vivo mechanisms of disease present within the natural physiological environment.

Definition according to the American College of Radiology
Molecular imaging is the spatially localized and/or temporally resolved sensing of molecular and cellular processes in vivo.

Overview of Molecular Imaging
Development of the first microscope in the late sixteenth century led to a keen interest in observations of structure of tissues and organs. This interest in organ structure and function has driven advances in biology ever since.

Molecular imaging originated from the field of radiopharmacology due to the need to better understand the fundamental molecular pathways inside organisms in a noninvasive manner. Molecular imaging unites molecular biology and in vivo imaging and enables the visualization of cellular function and the follow-up of the molecular process in living organisms without perturbing them.

Molecular Imaging is multidisciplinary and involves utilization of a variety of imaging techniques. One must have an in-depth knowledge of cellular and molecular biology, chemistry and radiochemistry, pharmacology, physiology and medicine, medical physics, mathematics, computer science, and related topics. It extends observations in living subjects to a more meaningful dimension.

There are very few studies performed in Nuclear Medicine which are not examples of molecular imaging The underlying principles of molecular imaging have been extended to other imaging modalities such as optical imaging and magnetic resonance imaging (MRI). Molecular imaging probes can now also be developed by taking advantage of the rapidly increasing knowledge of available cellular/molecular targets.

Molecular imaging has evolved very rapidly since 1995 through the integration of cell biology, molecular biology and diagnostic imaging. The present rapid pace of advancements in biotechnology and functional genomics is resulting in parallel progress in molecular imaging innovations and applications. The development, validation, and application of these novel imaging techniques in living subjects should further enhance our understanding of disease mechanisms and go hand in hand with the development of molecular medicine

Molecular Imaging: Goals

• to develop noninvasive in vivo imaging methods that reflect specific cellular and molecular processes, for example, gene expression, or more complex molecular interactions such as protein–protein interactions
• to monitor multiple molecular events near-simultaneously
• to follow trafficking and targeting of cells
• to optimize drug and gene therapy
• to image drug effects at a molecular and cellular level
• to assess disease progression at a molecular pathological level; and
• to create the possibility of achieving all of the above goals of imaging in a rapid, reproducible, and quantitative manner, resulting in the ability to monitor time-dependent experimental, developmental, environmental, and therapeutic influences on gene products in the same animal or patient.