In advanced stages of Parkinson’s disease, more and more patients are treated with deep brain stimulation, says Birgit Plantinga, who last week obtained her PhD at Biomedical Image Analysis (Department of Biomedical Engineering). “In this process a small nucleus deep inside the brain, the subthalamic nucleus, is stimulated by an electrode. As a result, the symptoms are often reduced significantly, though not to the same degree in each patient.” Also, side effects occur occasionally, such as depression, compulsive gambling or sexual disinhibition.
“The theory is that this is due to the fact that the electrode then does not stimulate precisely the right area in the brain”, as Plantinga explains. “However, if you want to be able to say more about that, you need more accurate MRI scanners than the ones they have in hospitals. That’s why I have used an MRI device with a magnetic field of 7 Tesla. This allows you to make scans with a higher resolution and hence less interference than with the hospital scanners of 1.5 or 3 Tesla.” Such a scanner can be found at Maastricht University, with which Plantinga has cooperated closely for her PhD. “That scanner enabled us to distinguish details measuring 0.5 millimeter.”
Which is important, for the subthalamic nucleus is very small: less than a cubic centimeter. In addition, this cerebral nucleus is divided into three sections, each of which is in contact with different parts of the cortex – the outermost layer of the brain.
The electrode must only make contact with the motor section
“Of the cortex we know fairly well where which functions are located, such as speech, sight and motor skills. By showing the connections between the different sections of the subthalamic nucleus and the cortex, we could make a distinction between the motor, the emotional and the cognitive sections of the nucleus. In theory you want the electrode only to make contact with the motor section.”
Whilst the size of the three aforementioned subsections differs per person, as measurements have borne out, the order is always the same. The PhD candidate also found differences between the subthalamic nucleus of Parkinson patients and those of healthy trial subjects. “We have seen that in patients there are certain sites where more iron accumulates, and also that there are differences in the concentration of myelin, a substance with an important function in the brain.” Plantinga and the Maastricht doctors have not got round to actual surgery on the basis of the detailed MRI scans, she says. “Of course, that is the next step, and my successor will definitely take it from there.”