Neuroimaging, in its most basic definition, refers to a set of techniques used to create pictures of the structure and functionality of the brain. Functional neuroimaging refers to visualizing functional processes in the brain that derive from the electrochemical activity of the brain’s neurons, synaptic transmission, and the metabolism that supports these processes. Neurons can control the localized flow of blood by molecular signaling mediated by the neurotransmitter glutamate via the surrounding astrocytes1. When blood enters the brain, it doesn’t course indiscriminately through the organ’s vessels; instead it is selectively channeled to specific regions in a need-based fashion. As a result perfusion to cortical regions and even areas as small as a single cortical column adjusts moment-by-moment to meet the constantly changing metabolic demands of the neurons1. Areas of the brain become more active during a task, requiring more energy, which is provided by an almost instantaneous change in local perfusion to provide the necessary glucose. Functional neuroimaging techniques, such as functional magnetic resonance imaging (fMRI) and SPECT, take advantage of this intimate association between neuronal activity, metabolic activity, and local perfusion.
Perfusion SPECT is the oldest and most widely available functional neuroimaging technique in the world2. The number of SPECT facilities in the United States outnumbers PET facilities by a factor of 12:1. Similarly, in Canada there are 11 times as many SPECT cameras as there are PET cameras. Unfortunately SPECT is grossly underutilized in the United States and elsewhere, for reasons which include poor education about neuroimaging and unrealistic expectations for its precision. In addition PET functional neuroimaging and fMRI have largely supplanted SPECT in the United States academic centers. This infatuation with the latest technology has ultimately slowed the progress of applying neuroimaging to clinical applications. The result has been decreased access to care and delays in the diagnosis of disease processes, such as traumatic brain injury and Alzheimer’s disease (AD). While SPECT has been shown to speed the diagnostic process for complex psychiatric disorder and reduce the number of medication trials, it is sparsely utilized.
There is an expectation in the field of neuroimaging, particularly among psychiatrists, that the results of any tests performed should be perfect. That is, they should be 100% sensitive (meaning all people who actually have the disease will test positive for it) and 100% specific (all patients who do not have the disease will test negative). In addition many believe there should be a neuroimaging fingerprint for any and every diagnosis found in the DSM-IV/DSM-V (Diagnostic and Statistical Manual of Mental Disorders, fourth edition/fifth edition)3, the American Psychiatric Association’s “gold standard” book providing standard language and criteria for classifying mental disorders. The problem with this is simple to see: there is no test that gives a 100%-correct result every time. We use tests in medicine every day for many, many different reasons, and while most people assume they are completely accurate, often they are merely indicative—meaning they are enough to help us form an assumption about how the patient’s ailment will progress, but not enough to say for certain that the patient’s issue is this illness or that injury4. Within the DSM system, the diagnoses are largely verbal, descriptive constructs, not etiological diagnoses based on demonstrable disruptions of either structure or function—in other words, diagnoses backed up by neuroimaging techniques such as SPECT. Similarly, it is unlikely that a DSM diagnosis will represent one distinct neurophysiological entity; it will more likely prove to represent a group of neurophysiological processes. This is because psychiatric symptoms do not exist in a vacuum; they likely represent abnormal neurophysiological processes that span across diagnoses5. For example impulsivity is a key component of the diagnosis of impulse control disorder NOS, ADHD, mania, and certain personality disorders, but it is also seen in mild traumatic brain injury (TBI), toxic brain injury, and frontal temporal dementia.
The International Society of Applied Neuroimaging is committed to the education of the medical field on these and other issues involved in functional neuroimaging. The Society is committed to enhancing the research and knowledge base upon which SPECT neuroimaging and other functional neuroimaging can be applied in the clinical setting. We look forward to addressing your needs.
With greatest respect,
Theodore Henderson, MD, PhD
International Society of Applied Neuroimaging
- Cox SB, Woolsey TA, Rovainen CM. Localized dynamic changes in cortical blood flow with whisker stimulation corresponds to matched vascular and neuronal architecture of rat barrels. J Cereb Blood Flow Metab 1993; 13: 899-913. [PMID: 8408316].
- Devous MD. SPECT Functional Brain Imaging. In: Toga AW, Mazziotta JC. Brain Mapping The Methods. 2nd ed. Amsterdam: Academic Press, 2002: 513-536.
- Sorboro, J. Prognosis Negative: psychiatry and the foibles of the Diagnostic and Statistical Manual V (DSM-V). Skeptic Magazine, 2010; 15 (3): 44-49.