Multimodality Imaging Introduction

Noninvasive “multimodal” in vivo imaging is not just becoming standard practice in the clinic, but is rapidly changing the evolving field of experimental imaging of genetic expression (“molecular imaging”). The development of multimodality methodology based on nuclear medicine (NM), positron emission tomography (PET) imaging, magnetic resonance imaging (MRI), and optical imaging is the single biggest focus in many imaging and cancer centers worldwide and is bringing together researchers and engineers from the far-ranging fields of molecular pharmacology to nanotechnology engineering. The rapid growth of in vivo multimodality imaging arises from the convergence of established fields of in vivo imaging technologies with molecular and cell biology. The cross-pollination of these disciplines has been accelerated in part by the establishment of the National Institutes of Health NCI P20 and P50 awards, for example, and by the sheer potential of the technology. Multimodality imaging is widely considered to involve the incorporation of two or more imaging modalities, usually within the setting of a single examination using, for example, dualor triple-labeled optical or nuclear medicine “reporter” agents or by performing ultrasound or optical studies within the MR, single-photon emission computed tomography (SPECT), or x-ray computed tomography (CT) environment. Clinically, the best example of multimodality imaging is seen in the rapid evolution of PET-SPECT and PET-CT scanner hybrids. The PET modality has developed into perhaps the most used “multimodal” imaging method. The incorporation of PET into single, hybrid, and multimodality units to provide functional (typically from injected F-18DG studies) and anatomic information is becoming extremely popular,1–2 so much so that, for example, PET/CT hybrids can be found in outpatient screening centers located in shopping malls. The role of any multimodal imaging approach ideally should provide the exact localization, extent, and metabolic activity of the target tissue, yield the tissue flow and function or functional changes within the surrounding tissues, and in the topic of imaging or screening, highlight any pathognomonic changes leading to eventual disease. Multimodal clinical NM, PET, and MRI techniques have to date fallen into the growing fields of molecular and functional imaging for primary-to-metastatic cancer screening of the body,3– 4 neuroassessment of gliomas,5 integrated stroke imaging exams,6 and functional neuroimaging exams.7 Newer tools seen in integrated neuroimaging methodologies that possess potential multimodal utility include near-infrared spectroscopy (NIRS) or optical imaging for real-time assessment of wavelength-specific absorption of photons by oxygenated and deoxygenated tissues.8 –9 NIRS is promising in that the contrast mechanism for the signals is closely related to that of intrinsic optical imaging of exposed cortex using visible light. Because of this, NIRS is an inexpensive means of assessing the newborn or the ischemic brain oxygenation. As such, it is ideal for combination studies in the MR scanner, for example. NIRS has only recently been used to investigate functional activation of the human cerebral cortex, although effort has begun to use imaging systems that allow the generation of images of a larger area of the subject’s head and, thereby, the production of maps of cortical oxygenation changes.10 A second new field with multimodal potential is the incorporation of two-dimensional electroencephalography (EEG) and magnetoencephalography (MEG) techniques. The clear utility of EEG/MEG lies in the fact that the observed signals are directly coupled to neuronal electrical activity. That is, EEG and MEG reflect the electric potential and magnetic fields resulting from synaptic transmembrane currents in neurons. Importantly, the EEG and MEG scans can be rapidly and noninvasively acquired as the neuronal activity is being monitored, as in epilepsy, for example. Because of this, many new reports of the untidy of acquiring the EEG signal together with and within the functional MRI (fMRI) examination have appeared to shed new light on the high-speed dynamics of neuronal activation and activation center communication and processing.11–12 In contrast to many of the imaging modalities mentioned, fMRI provides good spatial localization (0.5 to 2 mm, limited only by the signal-to-noise ratio) of an increased blood oxygenation activity with a relatively good temporal resolution (0.1 to 10 seconds) by measuring the blood oxygenation level– dependent change in image