The Use of Magnetoencephalography in Studying the Effects of Trauma
July 1, 2008
Researchers examining the neural underpinnings of Post-traumatic Stress Disorder (PTSD) are faced with a variety of options including Functional Magnetic Resonance Imaging (fMRI), Positron Emission Topography (PET), and Magnetoencephalography (MEG). Much of the decision regarding which imaging technique to use is guided by the research question, access to available equipment, and knowledge of existing imaging techniques. This summary will briefly compare and contrast the imaging techniques used to examine the neurological substrates of PTSD with a specific emphasis on the role of MEG imaging techniques in this line of research.
Numerous studies have examined the neurological substrates of PTSD using traditional fMRI techniques. fMRI affords researchers the opportunity to observe structures deep within the brain with relatively high resolution (Papanicolaou, 1998). As such researchers can identify brain structures activated within a temporal range following the presentation of a particular stimulus. For example, if a researcher was interested in whether or not the amygdala was active during the presentation of a trauma-relevant stimulus, fMRI would be an ideal choice. However, despite excellent spatial resolution, fMRI is limited with regard to temporal specificity. This limitation is due in part to delays associated with the metabolation rates of hemoglobin. fMRI determines activity patterns based on the ratio of hemoglobin to deoxyhemoglobin (Papanicolaou, 1998). That is, fMRI produces images of activated brain regions by detecting the indirect effects of neural activity on local blood volume, flow, and oxygen saturation. Thus, as activity increases, metabolism of oxygen in the blood increases, and the ratio of deoxyhemoglobin to hemoglobin increases. As such, a blood oxygen level dependent (BOLD) signal is acquired. However, measuring this process results in a 5-8 second temporal delay. As such, the temporal specificity of this instrument is of limited accuracy and time-locked responding of structures is problematic.
An alternative imaging technique is the PET scan, in which a radioactive tracer isotope is injected in subjects after which they can engage in a variety of tasks. Tracer concentration in various brain regions corresponds with cortical activity and can be mapped in 3-dimensional space. Fewer studies of PTSD have used PET scanning perhaps owing to the high cost, and the more invasive nature of this imaging procedure. Despite these impediments, PET offers some distinct advantages. PET offers the opportunity to examine energy metabolism and substrate supply through measurements of: 1) regional cerebral blood flow, 2) regional cerebral metabolic rate, and 3) oxygen utilization. It’s unique in its ability to quantify receptor density. For example, using particular radioactive isotopes that bind to particular neurotransmitter receptors in the brain allows researchers to determine which neurotransmitter systems are being actively bound by ligands during the performance of specific tasks. Essentially, temporal resolution is achieved when radioactive tracers that have been introduced into the blood stream are metabolized as a result of the brain’s activity (Solso, 2001). Despite these advantages, PET is limited with regard to spatial and temporal resolution when compared with fMRI. However, the development of more specific isotopes allows researchers to examine structures and neurotransmitter systems that may provide unique insight into function.
Magnetoencephalography (MEG) is a device used in research and clinical settings to produce a functional image of the human cortex (Cohen, 1972). To do this, magnetic fields resulting from the brain’s electrical activity are measured using superconducting quantum interference devices (SQUIDs). The MEG signal can be thought of as an estimate of the ionic flow of currents in the dendrites of cortical neurons. The more ionic flow, the stronger the MEG signal detected. In order for the MEG signal to be distinguishable from noise approximately 50,000 neurons need to be simultaneously active (Okada, 1983).
MEG provides unique advantages with regard to imaging. One specific advantage of MEG is temporal specificity (Papanicolaou, 1998). MEG can detect nearly instantaneous alterations in the pattern of neuronal activity. For example, using MEG it is possible to detect rapid activation and deactivation of multiple brain regions during the performance of a complex cognitive task. Such temporal specificity cannot be achieved using other imaging techniques. In addition, MEG reduces the impact of motion artifact, making it an ideal choice for imaging children and clinical populations.
Despite the unparalleled temporal specificity offered by MEG, this imaging technique has limited spatial resolution (Papanicolaou, 1998). This is because MEG does not always give a clear outline of the specific source of activity. This is particularly true if two adjacent structures are simultaneously active. In this case, a complex activity distribution pattern is produced and it is difficult to calculate a particular source. Furthermore, the activation pattern detected by MEG is considered incomplete with regard to the source of emanation. For instance, the signal from structures located deep within the brain falls below threshold by the time it reaches the head surface. Consequently, deep brain structures relevant for the performance of certain tasks may be overlooked when using this imaging technique.
Though spatial resolution is limited under conditions of simultaneous, adjacent activation and deep structure activation, there are situations in which MEG’s spatial resolution exceeds that of any other imaging technique (Papanicolaou, 1998). In particular, when activation is circumscribed to single cortical columns, the spatial resolution of MEG is considered ideal. Furthermore, new techniques such as multi-resolution FOCUSS (MR-FOCUSS), a current density imaging method, which use complex mathematical algorithms have been developed to enhance the spatial resolution of MEG when multiple structures are simultaneously active (Moran, Bowyer, & Tepley, 2001;Bowyer et al., 2004).
To date there has been limited use of MEG to study the neural substrates of PTSD. Moreover, those studies which have used MEG to examine neuronal activation patterns in PTSD can be classified as exploratory. In one study, MEG was used to assess the presence of abnormal slow waves in the insula and the right frontal areas while at rest in individuals diagnosed with PTSD (Kolassa et al., 2007). The authors concluded that the abnormal slow wave pattern in these structures may account for many of the symptoms of PTSD including alexithymia, reduced inhibition, and the diminished extinction of the conditioned fear response. Thus, MEG procedures have been used to demonstrate the neural substrates of PTSD symptomatology.
To conclude there are multiple advantages and disadvantage to each of the aforementioned imaging techniques. With regard to PTSD, the research question informs which imaging technique will be most informative. That said, the MEG imaging system may provide unique insight to the field of PTSD research by allowing for the analysis of cortical activation in real-time.
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