Strengths and Weaknesses of Different Neuroimaging Techniques

This essay will discuss the different strengths and weaknesses of different neuroimaging techniques that cognitive neuroscientists use to understand different mental processes and different biological bases of behaviour. These examples include fMRI (functional magnetic resonance imaging), PET (positron emission tomography), MEG (magnetoencephalography) and TMS (transcranial magnetic stimulation).

fMRI is a hemodynamic method that measures the amount of deoxygenated blood in parts of the brain (Ward, 2016). The amount of deoxygenated blood gives off a BOLD signal (Blood-Oxygen-Level Dependent) and convert into voxels (form of pixels) which allow us to see a 3D image of the brain and to track the neural activity (Martin & Carlson, 2019). When the amount of neuronal activity increases, so does the amount of blood flow which prompts the increase of glucose levels in the brain. In turn, this causes the dilation of the blood vessels; evidently increasing the volume of deoxygenated blood (Heeger & Ress, 2002). A strength of this technique is that it is non-invasive which means that we don’t have to insert anything into the body or brain to track this neuronal activity. As well as this, fMRI scans have good spatial resolution: meaning the accuracy of which we can see where a certain event is occurring is good (Pinel & Barnes, 2017). It can even reach up to 1 millimetre of spatial resolution if the fMRI scan is extremely high resolution, however the typical BOLD response will have around 3 – 6 millimetres of spatial resolution (XUE et al., 2010).

Despite the positives of the fMRI scan, it also has its disadvantages such as poor temporal resolution – implying that the time in which it takes to take a single image of the brain is slow (Cabeza & Nyberg, 2000). Martin and Carlson (2019) stated that it takes roughly 3 seconds to complete one frame of the brain which is slower than other techniques, such as an MEG scan. As well as having poor temporal resolution, fMRI scans show correlation rather than causation as other physiological activities in the body may cause the influx of deoxygenated blood to flow to a certain area of the brain; therefore making it hard to say if the BOLD signal measures what it is intending to measure. Overall, fMRI is a safe, non-invasive way of measuring neural activity in the brain with amazing spatial resolution. Despite the poor temporal resolution, fMRI is a popular method of measuring neuronal activity in the brain due to it’s lack of invasiveness (XUE et al., 2010).

Another hemodynamic scanning technique is PET. However unlike fMRI scanning, PET scans measures the changes in the blood flow directly in contrast to the bold signals that measure the deoxyhaemoglobin (Ward, 2016). PET scans work by inserting a radioactive tracer into the body – typically, it’s a form of glucose. For example, F-18-2-fluro-2-deoxyglucose (FDG) which travels to the brain to release positron. The positrons proceed to collide with electrons in the brain, releasing gamma rays at 180° to each other and the activity levels of areas are shown by the different concentrations (Martin & Carlson, 2019). For instance, a higher level of FDG uptake is found in places that could be affected due to different infections (e.g. an autoimmune disease) and therefore the concentrations of FDG will be higher in those areas. A strength of this technique is that it has good spatial resolution – roughly 5 – 6 millimetres.

However a major limitation of this scanning technique is that it is invasive as it requires the injection of a tracer into the bloodstream (Martin & Carlson, 2019). As well as this, the temporal resolution is poor as it is around 30 seconds (Cabeza & Nyberg, 2000). Moreover, due to the radioactivity levels and invasiveness of this technique, children aren’t permitted to have these types of scans which therefore means we can’t see how the brain develops through childhood with this technique and limits our knowledge surrounding this area (Martin & Carlson, 2019). To conclude, PET scans are an invasive technique that hinders our knowledge of how the brain develops during the early stages of childhood. However, despite this, PET scans have amazing spatial resolution and they have helped us with our understanding of other mental processes such as speech perception, memory and reading (Martin & Carlson, 2019).

A technique unlike both fMRI and PET scans is TMS which is an electromagnetic technique that, rather than recording the neuronal activity of the brain, stimulates it instead (Ward, 2016). TMS works by placing a magnetic coil next to the head and inside the magnetic coil, there is a changing electrical current that creates an electromagnetic field (Kobayashi & Pascual-Leone, 2003; Pashut et al., 2011). The magnetic field of the coil causes the neurons inside the brain to either depolarise (more likely to fire an action potential) or hyperpolarise (less likely to fire an action potential) (Kobayashi & Pascual-Leone, 2003). A big advantage of this technique is that it is one of the only ways of measuring causation rather than correlation (Walsh & Cowey, 2000). This means that, unlike fMRI or PET scans, the magnetic coil besides the skull directly affects the neuronal activity which then induces a change in behaviour, for example, the movement of the hand (Pashut et al., 2011). Similarly to fMRI, this is a non-invasive technique as there is no requirement for a tracer to be injected into the body (Hallett, 2007; Kobayashi & Pascual-Leone, 2003).

A limitation of this technique is that if the magnetic coil increases in temperature, it could potentially harm the patient and/or damage the machine (Wassermann, 1998). However this problem could be solved by the production of a water-cooled coil that can prevent this problem from occurring (Wassermann, 1998). Despite single-pulse TMS being relatively safe, repeated-pulse techniques could induce a seizure onto patients who have had a history with seizures (Sack & Linden, 2003; Ward, 2016). Patients who have been diagnosed with epilepsy (or have a family history with epilepsy) aren’t permitted to use TMS due to the danger of inducing a possible epileptic fit. As well as this, patients should be informed before having TMS that it may cause some discomfort as it can induce involuntary twitching of the facial muscles, giving them the right to withdraw at any time if the discomfort becomes too extreme (Ward, 2016). In summary, TMS has it’s dangers however if participants are informed beforehand and have no history of epilepsy, this technique is effective as it establishes a direct cause and is being used to treat several disorders such as depression and anxiety (Walsh & Cowey, 2000).

The final scanning technique in this essay is MEG which measures the electrical brain activity with a superconducting quantum interference device (SQUID) (Martin & Carlson, 2019). The SQUID device has to be submerged in liquid helium but they are sensors that are placed on the patient’s head to measure the magnetic activity of neurons inside the brain. After the SQUID is placed onto the skull, the head has to be scanned with small magnetic coils to ensure that the person’s skull is relative to the SQUID device (Proudfoot et al., 2014). A major strength of this technique is that is a good localising technique as when the neuronal activity is stimulated in a certain area by a specific stimulus, SQUID detects the magnetic field and then it shows the area that has been used (Martin & Martin, 2003). Moreover, MEG is a non-invasive technique as it only measures the magnetic field of the electrical activity of the neurons; therefore not requiring a tracer to be inserted.

However during a MEG scan, if there is any movement of the head, it immediately reduces the quality of the data and therefore the patient must remain extremely still when the scan is taking place (Boto et al., 2018). As well as this, MEG’s are extremely expensive and as they have to be conducted behind a magnetically shielded room, they are extremely difficult to move around (Stam, 2010). Overall, MEG’s are a non-invasive scan that has an amazing localising technique, and it is safe for participants to use, unlike TMS which has its dangers of inducing epileptic seizures. However due to the SQUID needing to be immersed in liquid helium and the technique needing to take place in a magnetically shielded room, MEG is an expensive technique and it’s impossible to move around.

In summary, all of the neuroimaging techniques have all of their strengths and weaknesses; for example, fMRI, MEG and TMS are all non-invasive techniques whereas PET scans are invasive as they require the injection of a tracer which enters the body. The hemodynamic techniques have better spatial resolution compared to their temporal resolution, whereas the opposite could be said for the electromagnetic and magnetic techniques. PET, fMRI and MEG also allow us to see a correlation between brain activity and cognitive activity whereas TMS is one of the only techniques that can be used to establish a clear cause and effect.

References

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