What PET technologies are used in psychiatric research and diagnosis today?
A brain positron emission tomography (PET) scan is another imaging test of the brain. It uses a radioactive substance called a tracer to look for disease or injury in the brain. It requires a small amount of radioactive material (a tracer). This tracer is given through a vein (IV), usually on the inside of the elbow. Then, this tracer travels through the patient’s blood and collects in organs and tissues. The tracer helps the doctor see certain brain areas and allows identifying diseases more clearly. That is the case also for brain tumors and anomalies of brain vascularization. The FDG (fluorodeoxyglucose) tracer is successfully used to differentiate Alzheimer’s disease from other dementing processes, and also to make early diagnosis of Alzheimer’s disease. It is also used to differentiate Parkinson’s disease from other movement disorders. Once the tracer is absorbed by the body, this usually takes about one hour, the PET scanner detects signals from the tracer. A computer changes the results into 3-D images. The scans are then displayed on a monitor which allows the specialist in nuclear medicine to identify the nature of the illness.
In psychiatry numerous compounds that bind selectively to neuroreceptors of interest in biological psychiatry have been radiolabeled. Radioligands that bind to dopamine receptors (D1, D2 receptor, reuptake transporter), serotonin receptors (5HT1A, 5HT2A, reuptake transporter) opioid receptors (mu) have been used successfully in studies with human subjects. Studies have been performed examining the state of these receptors in patients compared to healthy controls in different psychiatric disorders such as schizophrenia, substance abuse and mood disorders. But PET Scans are also used to examine links between specific psychological processes or disorders and brain activity.
In OCD, for example, some data showing different abnormal metabolism of glucose in patients compared to healthy subjects (Millet et al. 2013) were replicated recently. Executive functions were found to be slightly impaired. Modifications in glucose metabolism were observed in several frontal regions, namely the OFC (BA 45), the dorsolateral prefrontal cortex (DLPFC (BA 9)) and the ACG (BAs 24 and 31). This publication is a contribution to previous studies which already showed the central role of the pre-frontal cortex in OCD.
But PET Scan is of particular interest when you use therapeutics such as invasive brain stimulation which requires implanting metals within the brain. That is the case in Parkinson’s disease patients treated by high frequency deep brain stimulation as well as psychiatric patients suffering from severe OCD (Mallet et al. 2008), and Treatment Resistant Depression (Millet et al. 2014). Those patients with implanted electrodes cannot be explored using fMRI where there is a risk of burning the brain when electrodes come in contact with the magnetic field of this technique. PET Scans, with their tracer FDG, measure regional glucose use; while the post-op structural scanner represents a essential neuroimaging alternative to visualize the location of electrodes within the brain allowing adjustment to pre-operatory MRI. FDG-PET Scans give important information about the consequences of the Deep Brain Stimulation technique. Concomitantly to the STOC study published in the New England Journal of Medicine (Mallet et al 2008), which showed for the first time the efficacy of Subthalamic Nucleus Deep Brain Stimulation, we did a PET Scan on each patient at the end of every 3-month period allowing a comparison of patients during “on” stimulation versus “off” stimulation. A significant decrease in cerebral metabolism was observed in the left cingulate gyrus and the left frontal medial gyrus in On-Stimulation conditions compared with Off-Stimulation conditions. In addition, the improvement assessed by Y-BOCS scores during the On-Stimulation conditions was positively correlated with PET signal changes at the boundary of the orbitofrontal cortex and the medial prefrontal cortex, between PET signal changes and the Y-BOCS score modifications in On-Stimulation status.This study suggested for the first time also that the therapeutic effect of STN DBS was related to a decrease in prefrontal cortex metabolism.
Today, fMRI is becoming possible in patients who have been implanted. However and without any doubt, the PET Scan is going to still be helpful in discovery of brain stimulation action mechanisms in psychiatric diseases.