• A Helpful Overview - A Primer about Imaging in Psychiatry

Modalities

BOLD fMRI utilizes Magnetic Resonance Imaging and the fact that hemoglobin gives off a different magnetic signal when it is carrying oxygen (oxyhemoglobin) compared to when it is not carrying oxygen (deoxyhemoglobin). Thus, brain areas with high demand, or more active, will have a different ratio of oxy-to deoxy-hemoglobin. By taking very fast images (on the order of an image or more per second) one can rapidly image the contrast between activity at rest and during a specific behavior, thus demonstrating function as well as structure. A major benefit of using magnetic based technologies to image as opposed to radioactive based is that there is no limit to the number of scans that can be performed.

• Language mapping in normal volunteers
• Language mapping of a patient with previous meningitis
• Presurgical motor mapping of a patient with tumor
• fMRI studies of auditory cortex
• Investigating anxiety using separation cries of infants
• Using visual cues to induce craving for alcohol in alcoholics and social drinkers

Perfusion fMRI is a new magnetic resonance imaging technique being developed to create maps of brain perfusion, i.e. localized blood exchange in brain tissue. A so-called "spin-labeling" technique, it utilizes the intrinsic protons of blood and brain tissue, labeled by special preparation pulses, rather than exogenous tracers injected into the blood. Typically, the method consists of two parts. First, for reference, all the spins in the brain are "labeled" and a series of images acquired to follow their normal return to equilibrium. Second, only the spins in the section of interest are "labeled", and an identical series of images is acquired, this time to observe the more rapid return to equilibrium as "labeled" spins are replaced by unlabeled spins with the exchange of blood. By quantitative comparison of the two series, a map of brain perfusion can be computed.

•    Repeated perfusion scans in rapid-cycling bipolar affective disorder
• Perfusion imaging to detect brain changes in Alzheimer's disease

Recently, neurologists and neurophysiologists have perfected a way to non-invasively stimulate the brain by applying magnetic stimulation to the scalp. This technique, known as transcranial magnetic stimulation (TMS), can map brain functions as well as possibly treat neuropsychiatric diseases such as Parkinson's disease and depression.

Some recent MRI studies combined with TMS.

• Phase map of magnetic fields
• fMRI interleaved dose
• fMRI TMS vs. volition
• Correlations with distance

SPECT : Single Photon Emission Computed Tomography.

SPECT involves peripheral injection of a radiotracer which settles into the neurons and glia within 2-5 minutes and distributes relative to blood flow. Specific tracers can target a receptor type of interest in the brain and show the receptors location and distribution. The gamma rays these radiotracers emit as they decay are detected by rotating cameras and reconstructed into a three dimensional image. Some commonly used tracers include HMPAO and those tagged to specific ligands which bind receptors like dopamine (D1, D2).

PET: Positron Emisson Tomography

PET involves the peripheral injection of radiotracers which, when they degrade, emit positrons. These are highly unstable particles which travel a short distance and then collide with an electron. This reaction releases two photons traveling in exactly opposite directions (180 degrees apart). These photons are then detected by rotating cameras outside the head and computer reconstructed. In PET, as opposed to SPECT, the cameras are instructed to only include for final analysis those particles that are recorded simultaneously in a camera and in its 180 degree counterpart, thus enabling a more precise reconstruction of exactly where the photon originated. This, in general, gives PET a higher image resolution than SPECT. The majority of PET imaging in normal and pathological mood has been done using labeled glucose (18-FDG) or oxygen (O15) in the water form. These compounds have to be produced in a nearby cyclotron, which adds greatly to the cost as well as limiting the availability of these types of scans. PET O15 image aquisition takes approximately 1-5 minutes, with FDG requiring on the order of a half-hour.

• Transient Sadness O15 PET
• Depression FDG PET