The first brain imaging studies in OCD focused on brain morphometry and glucose metabolism, using computed tomography (CT) (e.g., Behar et al., 1984), single photon emission computed tomography (SPECT) and positron emission tomography (PET) (e.g., Rubin, Anath, Villanueva‐Meyer, Trajmar, & Mena, 1995). In the 1990s, magnetic resonance imaging (MRI) began to dominate the field. High resolution structural MRI enabled morphometric analyses of subcortical volume and shape, cortical thickness and surface area, as well as cortical gyrification. Functional MRI (fMRI) began to be used to visualize brain activation patterns during specific states relevant to the disorder, using a range of emotional and cognitive paradigms that may be crucial in the disease, for example, symptom provocation (Jaspers‐Fayer et al., 2019; Thorsen et al., 2018), fear and extinction learning (Milad et al., 2013), response inhibition (Norman et al., 2019), planning (e.g., Van den Heuvel et al., 2005), working memory (e.g., De Vries et al., 2014), and reversal learning (e.g., Remijnse et al., 2006).
