A Molecular Imaging Primer: Modalities, Imaging Agents, and Applications

Abstract

<jats:p>Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.</jats:p>

In vivo pentamodal tomographic imaging for small animals

“…High tissue penetrating ability of high-energy gamma photons allows us to collect functional information by locating radiopharmaceuticals in the body with single-photon emission computed tomography (SPECT). Further utilizing coincidences of paired gamma photons from annihilations of positrons, positron emission tomography (PET) significantly enhances the imaging sensitivity to nano-/pico-molar magnitude [5]. As attenuation of X-ray is generally related to tissue density and thickness, X-ray computed tomography (CT) has good resolution on bones, but bad on soft tissues compared with magnetic resonance imaging (MRI) [6].…”

“…As attenuation of X-ray is generally related to tissue density and thickness, X-ray computed tomography (CT) has good resolution on bones, but bad on soft tissues compared with magnetic resonance imaging (MRI) [6]. Optical imaging, including bioluminescence imaging/tomography (BLI/BLT), fluorescence molecular imaging/tomography (FMI/FMT), Cerenkov luminescence imaging/tomography (CLI/CLT), radioluminescence imaging (RLI) and so on, provides highly sensitive, highthroughput, non-invasive targeting imaging in vivo [5,7]. Optical instrument costs much lower and has higher superficial resolutions than others while penetration depth of visual/near infrared photons is limited [5].…”

“…Optical imaging, including bioluminescence imaging/tomography (BLI/BLT), fluorescence molecular imaging/tomography (FMI/FMT), Cerenkov luminescence imaging/tomography (CLI/CLT), radioluminescence imaging (RLI) and so on, provides highly sensitive, highthroughput, non-invasive targeting imaging in vivo [5,7]. Optical instrument costs much lower and has higher superficial resolutions than others while penetration depth of visual/near infrared photons is limited [5]. MRI provides high spatial resolution, excellent soft tissue contrast, qualitative and partial quantitative functional measurements, while the instrument costs much higher and safety of metal implants must be evaluated [8].…”

Molecular, Functional, and Structural Imaging of Major Depressive Disorder

“…Molecular imaging is defined as the noninvasive, real-time visualization of biological processes at the cellular and molecular levels in vivo [7,8]. SPECT and PET provide information on disease-specific molecular changes in the brain of MDD patients [9], and MRI provides information on brain structure, function and connectivity by using techniques of structural MRI and functional MRI (fMRI).…”

Life is not black and white, nor just Shades of Gray

“…A ll of these im aging modal it ies are excit ing instrumentational and methodological approaches to diagnosing and characterizing diseases [4]. Anatomical and functional imaging coexisted for decades until, in the 1990s, the use of combined Banato-metabolic^imaging [5] was proposed and subsequently resulted in the introduction of combined SPECT/CT [6], PET/CT [7] and then PET/MR systems [8] in the first decade of the twenty-first century.…”