Erwin L. Hahn Institute for MRI

Research

Two outstanding projects performed at the Institute in the last two years by researchers from the UDE demonstrate the broad application of 7T MRI. In the first of these, the radiofrequency techniques originally developed for imaging the torso have been extended to performing angio­graphy of the lower limbs without the application of an endogenous contrast agent, which is contra-indicated in some pathologies, in particular those associated with renal impairment.

All volunteers were examined in feet-first ­supine position on a custom-built AngioSURF ­table, which was manually positioned inside the stationary RF coil system for multi-station ­imaging. In this way a set of angiograms can be combined by keeping the radiofrequency components stationary and moving the volunteer between acquisitions. This means that the lower limbs can be imaged using a series of segments as shown in the figure as a MIP (maximum intensity projection). A custom-built 16-channel transmit/receive coil was utilized, consisting of 16 meander microstrip elements. Five elements were positioned flat under the AngioSURF table; the other 11 elements were arranged in a semi­circle above the table. Time-Interleaved Acquisition of Modes (TIAMO, a technique developed and patented in Essen) was integrated to reduce B1 artifacts and to obtain near homogeneous image quality of the arteries.

In the current study, non-contrast-enhanced magnetic resonance angiography (MRA) imaging at 7T was performed in volunteers with known peripheral arterial occlusive disease (PAOD), as there is a high prevalence of chronic renal impairment and the necessity of dialysis in volunteers with PAOD due to its association with diabetes. Images were compared to contrast-agent-based MRA at 1.5T and the presence of arterial stenosis and occlusions counted for each artery segment in both MRA techniques. Initial results in a small volunteer group demonstrate good performance of non-enhanced 7T MRA.

In a second study, researchers from the ­University examined how the brain performs ­decision making under stress. In some situations people have to make decisions with potentially serious consequences; for example, a doctor in the operating room or a policeman during a street fight. These situations often elicit psychological stress, which could have an influence on the decisions people make. Additionally, these situations often require simultaneous ­actions while making a crucial decision. It is therefore ­important to understand what happens to ­peoples’ decision-making performance in such situations. The combination of stress with an ­additional load seems to preserve ­decision-making performance, probably by a switch from serial to parallel processing. The question remains as to what happens in the brain in such demanding situations and which brain areas are involved in decision making ­under stress in a dual-task situation (action plus decision).

Stress was hypothesized to lead to changes in neural activity in brain areas involved in making decisions under risk and working memory, i. e. dorsolateral prefrontal (dlPFC) areas and parts of the anterior cingulate cortex (ACC). Data were analyzed from 33 right-handed, healthy participants, randomly assigned to the stress group (n = 16) and the control group (n = 17). The interaction between stress (induced by the Trier Social Stress Test), risky decision making (Game of Dice Task, GDT), and a parallel executive/working memory task was investigated. A region of interest (ROI) analysis was conducted in the prefrontal cortex, in particular in the dlPFC, the ACC, and the parietal cortex.

The results show that on a behavioural level, stressed participants did not show significant differences in task performance. Interestingly, when comparing the stress group with the control group, the stress group showed a greater increase in neural activation in the more anterior part of the dorsolateral prefrontal cortex (dlPFC) when performing the working memory task simultaneously with the GDT than when performing each task alone. This brain area is associated with parallel processing. Additionally, in the stress group an increase in the stress level was found to be associated with a decrease in neural activation in the dorsal part of the dlPFC. This brain region is associated with serial processing of information. The combination of these findings may point in the following direction: in stressful dual-tasking situations, where a decision has to be made ­simultaneously with a high working memory ­demand, a stronger activation of a brain area associated with parallel processing (anterior part of the dlPFC) takes place. At the same time, brain areas associated with serial processing (dorsal part of the dlPFC) are related to decreased activation. The findings are in line with the idea that stress seems to trigger a switch from serial to parallel processing in demanding dual-tasking situations.