Opioids and brain imaging

Since the introduction of the gate-control theory, a plethora of evidence to support the spinal processing of pain signals has come to light. Cognitive and affective aspects of the pain experience indicate the importance of supraspinal structures, but the biological mechanisms have remained inadequately explored. Within the past decade, imaging techniques have emerged that enable in vivo assessment of the central opioidergic system and the central processing of pain. The two most important imaging modalities to this end are functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). This article will describe the underlying principles of these techniques and explain their importance in determining the loci of opioidergic pathways and their neuromodulatory influence on acute and chronic pain conditions, role in placebo effects, implication in drug dependence, and potential role in studying the analgesic efficacy of new drugs.