Neuroimaging is a generic term for a number of techniques and methods aimed at detecting meaningful information through the acquisition of brain images of different kinds. The presentation of different classification criteria may help the reader to get a sense of this rich but often confounding landscape. A first classification may be technology based:
■ Single photon emission computed tomography (SPECT)
■ Electroencephalography (EEG)
■ Magnetoencephalography (MEG)
Each of the above technologies requires different hardware, and each measures different physical quantities. It should be noted that whereas PET and SPECT employ radioactive tracers that limit their repeated use on the same subject, the other techniques are noninvasive and therefore allow more latitude in regard to experimental design and the subsequent statistical treatment of the data. MRI and PET/SPECT scanners are able to implement different imaging protocols, according to the specific acquisition modality employed (MRI) or the nature of the injected radioisotope (PET/SPECT). More recently, the availability of combined PET-computed tomography (PET-CT) and PET-MRI scanners has yielded an added dimension to our capacity to understand human brain in action (Blodgett et al. 2007).
The neurally related variable that is actually imaged may provide an alternative classification framework:
■ Vascular (or hemodynamic) effects engendered by neural activity: PET (H215O), fMRI (blood oxygenation level-dependent [BOLD] contrast, perfusion imaging), SPECT
■ Metabolic demand: PET (18fluorodeoxyglucose)
■ Receptor density: PET/SPECT (radioligands)
■ Neurochemistry: Magnetic resonance spectroscopy (MRS)
■ Connectivity pathways: MRI (diffusion tensor imaging), fMRI-based functional connectivity analysis
■ Surface electromagnetic effects of brain activity: EEG/MEG
■ Morphometry of brain structures: MRI
We begin this chapter with current thoughts on the physiological basis of functional imaging and the phenomenon of neural activation it attempts to study (referred to as "neurovascular coupling") and then proceed to examine the two major functional imaging modalities of relevance to psychiatry—PET and fMRI. The section on fMRI will also incorporate basic principles of MRI. We end the chapter with a brief review of magnetic resonance (MR)-based structural imaging modalities and how they might confer newer insights into the understanding of mental illness and development of newer treatments. The focus is intended to be on how these imaging methods are used to better understand psychopharmacology. More details on individual disorders will be available in their respective sections.
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