A novel coil array for combined TMS/fMRI experiments at 3 T

PURPOSE: To overcome current limitations in combined transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) studies by employing a dedicated coil array design for 3 Tesla. METHODS: The state-of-the-art setup for concurrent TMS/fMRI is to use a large birdcage head coil, with the TMS between the subject's head and the MR coil. This setup has drawbacks in sensitivity, positioning, and available imaging techniques. In this study, an ultraslim 7-channel receive-only coil array for 3 T, which can be placed between the subject's head and the TMS, is presented. Interactions between the devices are investigated and the performance of the new setup is evaluated in comparison to the state-of-the-art setup. RESULTS: MR sensitivity obtained at the depth of the TMS stimulation is increased by a factor of five. Parallel imaging with an acceleration factor of two is feasible with low g-factors. Possible interactions between TMS and the novel hardware were investigated and were found negligible. CONCLUSION: The novel coil array is safe, strongly improves signal-to-noise ratio in concurrent TMS/fMRI experiments, enables parallel imaging, and allows for flexible positioning of the TMS on the head while ensuring efficient TMS stimulation due to its ultraslim design

Comparison of Stimulus Types for Retinotopic Cortical Mapping of Macular Disease

Published: March 13, 2023Purpose: Retinotopic maps acquired using functional magnetic resonance imaging (fMRI) provide a valuable adjunct in the assessment of macular function at the level of the visual cortex. The present study quantitatively assessed the performance of different visual stimulation approaches for mapping visual field coverage. Methods: Twelve patients with geographic atrophy (GA) secondary to age-related macular degeneration (AMD)were examined using high-resolution ultra-high field fMRI (Siemens Magnetom 7T) and microperimetry (MP; Nidek MP-3). The population receptive field (pRF)-based coverage maps obtained with two different stimulus techniques (moving bars, and rotating wedges and expanding rings) were compared with the results of MP. Correspondence between MP and pRF mapping was quantified by calculating the simple matching coefficient (SMC). Results: Stimulus choice is shown to bias the spatial distribution of pRF centers and eccentricity values with pRF sizes obtained fromwedge/ring or bar stimulation showing systematic differences. Wedge/ring stimulation results show a higher number of pRF centers in foveal areas and strongly reduced pRF sizes compared to bar stimulation runs. A statistical comparison shows significantly higher pRF center numbers in the foveal 2.5 degrees region of the visual field for wedge/ring compared to bar stimuli. However, these differences do not significantly influence SMC values when compared to MP (bar 2.5 degrees: 0.88±0.11;wedge/ring<2.5 degrees: 0.89 ± 0.12 wedge/ring; >2.5 degrees: 0.86 ± 0.10) for the peripheral visual field. Conclusions: Both visual stimulation designs examined can be applied successfully in patients with GA. Although the two designs show systematic differences in the distribution of pRF center locations, this variability has minimal impact on the SMC when compared to the MP outcome.Supported by the Austrian Science Fund (FWF); KLI 670-B3

The pulvinar nucleus and antidepressant treatment : dynamic modeling of antidepressant response and remission with ultra-high field functional MRI

Functional magnetic resonance imaging (fMRI) successfully disentangled neuronal pathophysiology of major depression (MD), but only a few fMRI studies have investigated correlates and predictors of remission. Moreover, most studies have used clinical outcome parameters from two time points, which do not optimally depict differential response times. Therefore, we aimed to detect neuronal correlates of response and remission in an antidepressant treatment study with 7 T fMRI, potentially harnessing advances in detection power and spatial specificity. Moreover, we modeled outcome parameters from multiple study visits during a 12-week antidepressant fMRI study in 26 acute (aMD) patients compared to 36 stable remitted (rMD) patients and 33 healthy control subjects (HC). During an electrical painful stimulation task, significantly higher baseline activity in aMD compared to HC and rMD in the medial thalamic nuclei of the pulvinar was detected (p = 0.004, FWE-corrected), which was reduced by treatment. Moreover, clinical response followed a sigmoid function with a plateau phase in the beginning, a rapid decline and a further plateau at treatment end. By modeling the dynamic speed of response with fMRI-data, perigenual anterior cingulate activity after treatment was significantly associated with antidepressant response (p < 0.001, FWE-corrected). Temporoparietal junction (TPJ) baseline activity significantly predicted non-remission after 2 antidepressant trials (p = 0.005, FWE-corrected). The results underline the importance of the medial thalamus, attention networks in MD and antidepressant treatment. Moreover, by using a sigmoid model, this study provides a novel method to analyze the dynamic nature of response and remission for future trials

Defining and evaluating a sham condition for interleaved TMS/fMRI experiments using a dedicated MR head coil array

Purpose/Introduction: Experiments combining transcranial magnetic stimulation (TMS) and fMRI have recently received increased attention [1]. However, the standard set-up used for these measurements presents several limitations. To overcome them, we developed a thin 7-channel MR head coil array (see Fig. 1a) designed to be placed between the TMS system and the subject’s head [2]. Another critical issue for these experiments is the definition of a sham condition [3]. This is required to have control over confounding TMS effects, like the ones produced on the auditory or somatosensory system. In this work, a viable set-up for a sham condition is proposed, and its influence on MR data quality is investigated. Subjects and Methods: The sham condition was defined placing a 3 cm thick hollow block (an empty TMS stimulator housing) between TMS and MR coil (see Fig. 1b), which should prevent stimulation on the target while producing the same surrounding conditions such as vibrations or sounds. FLASH images of a spherical phantom were acquired at three different echo times (TE = 2.45/12.5/32.5 ms, TR = 400 ms, FoV = 181 9 223 mm2, resolution = 0.7 9 0.7 9 3 mm3, 10 slices, FA = 41, BW= 332 Hz/pixel) in the normal and the sham condition in a 3T MR Scanner (Tim Trio, Siemens, Erlangen, Germany). SNR maps were calculated for the three echo times and the two measurement conditions. SNR maps were co-registered to a coordinate system relative to the imaging coil, smoothed using a 3 mm Gaussian kernel, and masked to the phantom area. Relative SNR loss in the sham condition versus the normal condition was calculated. Results: The increased distance to the TMS coil changes the tuning and matching conditions of the MR coil. Figure 2 and Table 1 show the relative SNR loss when using the sham condition. Results are shown for 3, 4.2 and 5 cm depth, and for the three echo times. A test participant reported subjectively similar noise and vibration levels for real and sham stimulation. Discussion/Discussion/Conclusion: The presented sham condition is suitable to be used in future interleaved TMS/fMRI experiments. For the typical application in fMRI, where echo times are around 30 ms at 3T, the mean SNR losses are lower than 10 . The gain in SNR for the sham condition can be explained by decreased susceptibility effects caused by the TMS coil, when using the block between TMS and the MR head coil. SNR losses are attributed to MR coil detuning when removing the TMS. The MR head coil array could be tuned and matched to a more general condition in order to minimize them

Artificial scotoma estimation based on population receptive field mapping

Population receptive field (pRF) mapping based on functional magnetic resonance imaging (fMRI) is an ideal method for obtaining detailed retinotopic information. One particularly promising application of pRF mapping is the estimation and quantification of visual field effects, for example scotomata in patients suffering from macular dysfunction or degeneration (MD) or hemianopic defects in patients with intracranial dysfunction. However, pRF mapping performance is influenced by a number of factors including spatial and temporal resolution, distribution of dural venous sinuses and patient performance. This study addresses the ability of current pRF methodology to assess the size of simulated scotomata in healthy individuals. The data demonstrate that central scotomata down to a radius of 2.35° (4.7° diameter) visual angle can be reliably estimated in single subjects using high spatial resolution protocols and multi-channel receive array coils