Types of Functional Neuroimaging
Functional neuroimaging is based on the use of various techniques applied to measuring the activity level in different brain structures, with the goal of correlating it with specific mental functions and dysfunctions.
Brain SPECT (Single Photon Emission Computed Tomography) enables the evaluation of the level of activity in various gray matter areas by mapping the relative blood flow in the whole brain volume . It is technically the easiest modality for obtaining a functional imaging of the brain. The equipment is, in general, widely available as is the radiopharmaceutical needed and the radionuclide use (Tc-99m) is optimal for detection and safety ( 140 Kev and short half life of 6 hrs). From the protocol point of view it is by far the most practical and well suited for Neuropsychiatry patients . Indeed following the intravenous injection of the radiopharmaceutical there are only 3-4 minutes needed before the maximum accumulation occurs in the gray matter. From that point on the patient is free to do anything and imaging can be started anytime up to 2-3 hrs later without significant changes in the brain distribution. This is extremely important, practical and advantageous because as far as the patient’s condition is concerned the only thing that counts are those 3-4 min post injection,. This short time makes controlling a patient’s behavior and affective status much easier and, if need is, after this brief interval the patient can even be sedated before the scanning starts . For practical and optimization reasons the imaging starts at 30-45 min post injection and the duration of the scan is 15 -30 minutes .
A significant advantage of Brain SPECT is the long , extensive experience and long list of references available for its use in a great variety of Neuropsychiatric disorders .
Brain PET (Positron Emission Tomography) maps the metabolic activity in the brain and is a technically more complex modality due to its use of Fluorodeoxiglucose (FDG) labeled with a very short lived and higher energy radionuclide (F-18). This requires a more complex hardware (coincidence based positron detection ), poses more logistical problems (109 min half life) and requires more precautions due to higher energy ( 511 Kev) More importantly it requires a 45-60 min ramping up time in order to achieve the maximum concentration in the gray matter. Implicitly this means that it is more difficult to assure a constant status of the patient’s mentation during the one hour post injection.
A significant drawback of Brain PET is also the fact that there is a more limited experience available for its use in Neuropsychiatric disorders, with the exception of Degenerative Diseases.
fMRI (functional Magnetic Resonance Imaging). While this has been, and is, a remarkable research tool it
has a limited clinical use because it does not really evaluate the whole brain volume. Indeed at its core , it is based on comparing two consecutive acquisitions : one being the baseline and the other acquired during a specific task which can be of many types (visual, acoustic, tactile , cognitive, affective etc. etc.) This in effect means that it requires “a priori” information i.e. the need to know ahead of time which aspect of brain function has to be evaluated. A more useful variant is the so called “resting state” fMRI (rfMRI) which is based on a single acquisition obtained at rest and then submitted to a complex processing aiming to the detection of functional connectivity between spatially remote brain regions . Such functional connectivity analyses allow the characterization of interregional neural interactions, during rest. There is presently information indicating that there are significant differences in these connectivities in various Neuropsychiatric disorders. The technique is still not used in routine clinical evaluations and remains for the time being a very interesting and important research tool which. The information it provides can also be applied to refine the interpretation of a commonly used technique as is Brain SPECT.
MEG ( Magnetic Encephalography ) is technically very complex both from the point of view of equipment as well as signal processing and is not widely available. It produces a brain map of the magnetic fields induced by the electrical currents generated in turn by neuronal activity . It is used in research and, to a limited extent, in clinical settings for localizing regions affected by pathology , including epilepsy, as well as “eloquent cortex” before neurosurgical procedures. It can be said that overall it is a promising technique and….will (may ?) always be !