The temperature dependence of gradient system response characteristics.

PURPOSE: The gradient system transfer function (GSTF) characterizes the frequency transfer behavior of a dynamic gradient system and can be used to correct non-Cartesian k-space trajectories. This study analyzes the impact of the gradient coil temperature of a 3T scanner on the GSTF. METHODS: GSTF self- and B0 -cross-terms were acquired for a 3T Siemens scanner (Siemens Healthcare, Erlangen, Germany) using a phantom-based measurement technique. The GSTF terms were measured for various temperature states up to 45°C. The gradient coil temperatures were measured continuously utilizing 12 temperature sensors which are integrated by the vendor. Different modeling approaches were applied and compared. RESULTS: The self-terms depend linearly on temperature, whereas the B0 -cross-term does not. Effects induced by thermal variation are negligible for the phase response. The self-terms are best represented by a linear model including the three gradient coil sensors that showed the maximum temperature dependence for the three axes. The use of time derivatives of the temperature did not lead to an improvement of the model. The B0 -cross-terms can be modeled by a convolution model which considers coil-specific heat transportation. CONCLUSION: The temperature dependency of the GSTF was analyzed for a 3T Siemens scanner. The self- and B0 -cross-terms can be modeled using a linear and convolution modeling approach based on the three main temperature sensor elements.

Gradient waveform pre-emphasis based on the gradient system transfer function.

PURPOSE: The gradient system transfer function (GSTF) has been used to describe the distorted k-space trajectory for image reconstruction. The purpose of this work was to use the GSTF to determine the pre-emphasis for an undistorted gradient output and intended k-space trajectory. METHODS: The GSTF of the MR system was determined using only standard MR hardware without special equipment such as field probes or a field camera. The GSTF was used for trajectory prediction in image reconstruction and for a gradient waveform pre-emphasis. As test sequences, a gradient-echo sequence with phase-encoding gradient modulation and a gradient-echo sequence with a spiral read-out trajectory were implemented and subsequently applied on a structural phantom and in vivo head measurements. RESULTS: Image artifacts were successfully suppressed by applying the GSTF-based pre-emphasis. Equivalent results are achieved with images acquired using GSTF-based post-correction of the trajectory as a part of image reconstruction. In contrast, the pre-emphasis approach allows reconstruction using the initially intended trajectory. CONCLUSION: The artifact suppression shown for two sequences demonstrates that the GSTF can serve for a novel pre-emphasis. A pre-emphasis based on the GSTF information can be applied to any arbitrary sequence type.

[A novel transcutaneous vagus nerve stimulation leads to brainstem and cerebral activations measured by functional MRI].

BACKGROUND: Left cervical vagus nerve stimulation (VNS) using the implanted NeuroCybernetic Prosthesis (NCP) can reduce epileptic seizures and has recently been shown to give promising results for treating therapy-resistant depression. To address a disadvantage of this state-of-the-art VNS device, the use of an alternative transcutaneous electrical nerve stimulation technique, designed for muscular stimulation, was studied. Functional magnetic resonance imaging (MRI) has been used to test non-invasively access nerve structures associated with the vagus nerve system. The results and their impact are unsatisfying due to missing brainstem activations. These activations, however, are mandatory for reasoning, higher subcortical and cortical activations of vagus nerve structures. The objective of this study was to test a new parameter setting and a novel device for performing specific (well-controlled) transcutaneous VNS (tVNS) at the inner side of the tragus. This paper shows the feasibility of these and their potential for brainstem and cerebral activations as measured by blood oxygenation level dependent functional MRI (BOLD fMRI). MATERIALS AND METHODS: In total, four healthy male adults were scanned inside a 1.5-Tesla MR scanner while undergoing tVNS at the left tragus. We ensured that our newly developed tVNS stimulator was adapted to be an MR-safe stimulation device. In the experiment, cortical and brainstem representations during tVNS were compared to a baseline. RESULTS: A positive BOLD response was detected during stimulation in brain areas associated with higher order relay nuclei of vagal afferent pathways, respectively the left locus coeruleus, the thalamus (left >> right), the left prefrontal cortex, the right and the left postcentral gyrus, the left posterior cingulated gyrus and the left insula. Deactivations were found in the right nucleus accumbens and the right cerebellar hemisphere. CONCLUSION: The method and device are feasible and appropriate for accessing cerebral vagus nerve structures, respectively. As functional patterns share features with fMRI BOLD, the effects previously studied with the NCP are discussed and new possibilities of tVNS are hypothesised.

Brain imaging of analgesic and antihyperalgesic effects of cyclooxygenase inhibition in an experimental human pain model: a functional MRI study.

One of the most distressing symptoms of many neuropathic pain syndromes is the enhanced pain sensation to tactile or thermal stimulation (hyperalgesia). In the present study we used functional magnetic resonance imaging (fMRI) and explored brain activation patterns during acute impact pain and mechanical hyperalgesia in the human ultraviolet (UV)-B model. To investigate pharmacological modulation, we examined potential differential fMRI correlates of analgesic and antihyperalgesic effects of two intravenous cyclooxygenase inhibitors, i.e. parecoxib and acetylsalicylic acid (ASA). Fourteen healthy volunteers participated in this double-blinded, randomized and placebo-controlled crossover study. Tactile stimuli and mechanical impact hyperalgesia were tested at the site of a UV-B irradiation and acute mechanical pain was tested at a site distant from the irradiated skin. These measurements were conducted before and 30 min after a 5-min intravenous infusion of either saline (placebo), parecoxib 40 mg or ASA 1000 mg. Acute mechanical pain and mechanical hyperalgesia led to widespread activations of brain areas known to comprise the human pain matrix. Analgesic effects were found in primary (S1) and secondary (S2) somatosensory cortices, parietal association cortex (PA), insula, anterior parts of the cingulate cortex and prefrontal cortices. These brain areas were also modulated under antihyperalgesic conditions. However, we observed a greater drug-induced modulation of mainly PA and inferior frontal cortex during mechanical hyperalgesia; during acute mechanical pain there was a greater modulation of mainly bilateral S2. Therefore, the results of the present study suggest that there is a difference in the brain areas modulated by analgesia and antihyperalgesia.

Brain activation during input from mechanoinsensitive versus polymodal C-nociceptors.

C-nociceptors mediating cutaneous pain in humans can be distinguished in mechano-heat-responsive units (CMH) and mechano-insensitive units (CMi). However, if sensitized in damaged tissue, CMi play an important role in inflammatory pain. CMi differ from CMH by higher electrical thresholds and by mediating the axon reflex. Using these properties, we established two stimulation paradigms: (1) transcutaneous stimulation (TCS) of low current density below the CMi threshold and (2) intracutaneous stimulation (ICS) of high current density that excites CMi. This was proven by the quantification of the axon-reflex flare. Applying these stimulation paradigms during functional magnetic resonance imaging, we investigated whether nociceptor stimulation that recruits CMi leads to different cerebral activation than stimuli that do not recruit CMi. Brain activation by CMi was inferred by subtraction. Both stimuli recruited multiple afferents other than CMi, and we expected a common network of regions involved in different aspects of pain perception and motor nocifensive reactions in both stimuli. ICS that additionally recruited CMi should activate regions with low acuity that are involved in pain memory and emotional attribution. Besides a common network of pain in both stimuli, TCS activated the supplementary motor area, motor thalamic nuclei, the ipsilateral insula, and the medial cingulate cortex. These regions contribute to a pain processing loop that coordinates the nocifensive motor reaction. CMi nociceptor activation did not cause relevant activation in this loop and does not seem to play a role in withdrawal. The posterior cingulate cortex was selectively activated by ICS and is apparently important for the processing of inflammatory pain.