B0 shimming of the human heart at 7T.

PURPOSE: Inhomogeneities of the static magnetic B0 field are a major limiting factor in cardiac MRI at ultrahigh field (≥ 7T), as they result in signal loss and image distortions. Different magnetic susceptibilities of the myocardium and surrounding tissue in combination with cardiac motion lead to strong spatio-temporal B0 -field inhomogeneities, and their homogenization (B0 shimming) is a prerequisite. Limitations of state-of-the-art shimming are described, regional B0 variations are measured, and a methodology for spherical harmonics shimming of the B0 field within the human myocardium is proposed. METHODS: The spatial B0 -field distribution in the heart was analyzed as well as temporal B0 -field variations in the myocardium over the cardiac cycle. Different shim region-of-interest selections were compared, and hardware limitations of spherical harmonics B0 shimming were evaluated by calibration-based B0 -field modeling. The role of third-order spherical harmonics terms was analyzed as well as potential benefits from cardiac phase-specific shimming. RESULTS: The strongest B0 -field inhomogeneities were observed in localized spots within the left-ventricular and right-ventricular myocardium and varied between systolic and diastolic cardiac phases. An anatomy-driven shim region-of-interest selection allowed for improved B0 -field homogeneity compared with a standard shim region-of-interest cuboid. Third-order spherical harmonics terms were demonstrated to be beneficial for shimming of these myocardial B0 -field inhomogeneities. Initial results from the in vivo implementation of a potential shim strategy were obtained. Simulated cardiac phase-specific shimming was performed, and a shim term-by-term analysis revealed periodic variations of required currents. CONCLUSION: Challenges in state-of-the-art B0 shimming of the human heart at 7 T were described. Cardiac phase-specific shimming strategies were found to be superior to vendor-supplied shimming.

On the way to routine cardiac MRI at 7 Tesla - a pilot study on consecutive 84 examinations.

INTRODUCTION: Cardiac magnetic resonance (CMR) at ultrahigh field (UHF) offers the potential of high resolution and fast image acquisition. Both technical and physiological challenges associated with CMR at 7T require specific hardware and pulse sequences. This study aimed to assess the current status and existing, publicly available technology regarding the potential of a clinical application of 7T CMR. METHODS: Using a 7T MRI scanner and a commercially available radiofrequency coil, a total of 84 CMR examinations on 72 healthy volunteers (32 males, age 19-70 years, weight 50-103 kg) were obtained. Both electrocardiographic and acoustic triggering were employed. The data were analyzed regarding the diagnostic image quality and the influence of patient and hardware dependent factors. 50 complete short axis stacks and 35 four chamber CINE views were used for left ventricular (LV) and right ventricular (RV), mono-planar LV function, and RV fractional area change (FAC). Twenty-seven data sets included aortic flow measurements that were used to calculate stroke volumes. Subjective acceptance was obtained from all volunteers with a standardized questionnaire. RESULTS: Functional analysis showed good functions of LV (mean EF 56%), RV (mean EF 59%) and RV FAC (mean FAC 52%). Flow measurements showed congruent results with both ECG and ACT triggering. No significant influence of experimental parameters on the image quality of the LV was detected. Small fractions of 5.4% of LV and 2.5% of RV segments showed a non-diagnostic image quality. The nominal flip angle significantly influenced the RV image quality. CONCLUSION: The results demonstrate that already now a commercially available 7T MRI system, without major methods developments, allows for a solid morphological and functional analysis similar to the clinically established CMR routine approach. This opens the door towards combing routine CMR in patients with development of advanced 7T technology.

Cerebellar transcranial direct current stimulation modulates the fMRI signal in the cerebellar nuclei in a simple motor task.

BACKGROUND: In a seminal paper, Galea et al. (Modulation of cerebellar excitability by polarity-specific noninvasive direct current stimulation. 2009. J Neurosci 29, 9115-9122) showed that cerebellar transcranial direct current stimulation (ctDCS) alters cerebellar-M1 connectivity. This effect has been explained by ctDCS-related changes of excitability of the cerebellar cortex with consecutive modulation of its main output, the dentate-thalamo-cortical pathway. OBJECTIVES: The aim of this functional magnetic resonance imaging (fMRI) study was to provide evidence that cathodal ctDCS decreases the activity of the cerebellar cortex, resulting in increased activity of the cerebellar nuclei, whereas anodal ctDCS has the opposite effect. METHODS: A total of 48 participants (female/male: 23/25, age: 23.8 ±â€¯4.1yrs., mean ±â€¯standard deviation) performed a finger tapping task with the right hand in a 3T MRI scanner. Functional MR images were acquired prior, during and after tDCS of the right cerebellum. Participants were assigned randomly to anodal, cathodal or sham ctDCS. RESULTS: No significant difference of cerebellar cortical activation was found after comparing the three modes of stimulation. On the level of the dentate nuclei, however, a significant increase of activation was detected during and after cathodal stimulation. Furthermore, dentate nuclei activation was suppressed on a trend level following anodal stimulation. CONCLUSIONS: The present findings support the hypothesis that cathodal ctDCS leads to a disinhibition of the dentate nucleus, whereas anodal ctDCS may have the opposite effect.