Robert Trampel

Prioritizing spatial accuracy in high-resolution fMRI data using multivariate feature weight mapping

Although ultra-high-field fMRI at field strengths of 7T or above provides substantial gains in BOLD contrast-to-noise ratio, when very high-resolution fMRI is required such gains are inevitably reduced. The improvement in sensitivity provided by multivariate analysis techniques, as compared with univariate methods, then becomes especially welcome. Information mapping approaches are commonly used, such as the searchlight technique, which take into account the spatially distributed patterns of activation in order to predict stimulus conditions. However, the popular searchlight decoding technique, in particular, has been found to be prone to spatial inaccuracies. For instance, the spatial extent of informative areas is generally exaggerated, and their spatial configuration is distorted. We propose the combination of a non-parametric and permutation-based statistical framework with linear classifiers. We term this new combined method Feature Weight Mapping (FWM). The main goal of the proposed method is to map the specific contribution of each voxel to the classification decision while including a correction for the multiple comparisons problem. Next, we compare this new method to the searchlight approach using a simulation and ultra-high-field 7T experimental data. We found that the searchlight method led to spatial inaccuracies that are especially noticeable in high-resolution fMRI data. In contrast, FWM was more spatially precise, revealing both informative anatomical structures as well as the direction by which voxels contribute to the classification. By maximizing the spatial accuracy of ultra-high-field fMRI results, global multivariate methods provide a substantial improvement for characterizing structure-function relationships.

A computational framework for ultra-high resolution cortical segmentation at 7 Tesla

This paper presents a computational framework for whole brain segmentation of 7Tesla magnetic resonance images able to handle ultra-high resolution data. The approach combines multi-object topology-preserving deformable models with shape and intensity atlases to encode prior anatomical knowledge in a computationally efficient algorithm. Experimental validation on simulated and real brain images shows accuracy and robustness of the method and demonstrates the benefits of an increased processing resolution.

Functional perfusion imaging using continuous arterial spin labeling with separate labeling and imaging coils at 3 T

Functional perfusion imaging with a separate labeling coil located above the common carotid artery was demonstrated in human volunteers at 3 T. A helmet resonator and a spin‐echo echo‐planar imaging (EPI) sequence were used for imaging, and a circular surface coil of 6 cm i.d. was employed for labeling. The subjects performed a finger‐tapping task. Signal differences between the condition of finger tapping and the resting state were between −0.5% and −1.1 % among the subjects. The imaging protocol included a long post‐label delay (PLD) to reduce transit time effects. Labeling was applied for all repetitions of the functional run to reduce the sampling interval. Magn Reson Med 49:791–795, 2003.

Isotropic Submillimeter fMRI in the Human Brain at 7 T: Combining Reduced Field-of-View Imaging and Partially Parallel Acquisitions

Echo‐planar imaging is the most widely used imaging sequence for functional magnetic resonance imaging (fMRI) due to its fast acquisition. However, it is prone to local distortions, image blurring, and signal voids. As these effects scale with echo train length and field strength, it is essential for high‐resolution echo‐planar imaging at ultrahigh field to address these problems. Partially parallel acquisition methods can be used to improve the image quality of echo‐planar imaging. However, partially parallel acquisition can be affected by aliasing artifacts and noise enhancement. Another way to shorten the echo train length is to reduce the field‐of‐view (FOV) while maintaining the same spatial resolution. However, to achieve significant acceleration, the resulting FOV becomes very small. Another problem occurs when FOV selection is incomplete such that there is remaining signal aliased from the region outside the reduced FOV. In this article, a novel approach, a combination of reduced FOV imaging with partially parallel acquisition, is presented. This approach can address the problems described above of each individual method, enabling high‐quality single‐shot echo‐planar imaging acquisition, with submillimeter isotropic resolution and good signal‐to‐noise ratio, for fMRI at ultrahigh field strength. This is demonstrated in fMRI of human brain at 7T with an isotropic resolution of 650 μm. Magn Reson Med, 2012.

Cortical lamina-dependent blood volume changes in human brain at 7 T

Cortical layer-dependent high (sub-millimeter) resolution functional magnetic resonance imaging (fMRI) in human or animal brain can be used to address questions regarding the functioning of cortical circuits, such as the effect of different afferent and efferent connectivities on activity in specific cortical layers. The sensitivity of gradient echo (GE) blood oxygenation level-dependent (BOLD) responses to large draining veins reduces its local specificity and can render the interpretation of the underlying laminar neural activity impossible. The application of the more spatially specific cerebral blood volume (CBV)-based fMRI in humans has been hindered by the low sensitivity of the noninvasive modalities available. Here, a vascular space occupancy (VASO) variant, adapted for use at high field, is further optimized to capture layer-dependent activity changes in human motor cortex at sub-millimeter resolution. Acquired activation maps and cortical profiles show that the VASO signal peaks in gray matter at 0.8-1.6mm depth, and deeper compared to the superficial and vein-dominated GE-BOLD responses. Validation of the VASO signal change versus well-established iron-oxide contrast agent based fMRI methods in animals showed the same cortical profiles of CBV change, after normalization for lamina-dependent baseline CBV. In order to evaluate its potential of revealing small lamina-dependent signal differences due to modulations of the input-output characteristics, layer-dependent VASO responses were investigated in the ipsilateral hemisphere during unilateral finger tapping. Positive activation in ipsilateral primary motor cortex and negative activation in ipsilateral primary sensory cortex were observed. This feature is only visible in high-resolution fMRI where opposing sides of a sulcus can be investigated independently because of a lack of partial volume effects. Based on the results presented here, we conclude that VASO offers good reproducibility, high sensitivity and lower sensitivity than GE-BOLD to changes in larger vessels, making it a valuable tool for layer-dependent fMRI studies in humans.

Navigation concepts for MR image-guided interventions.

The ongoing development of powerful magnetic resonance imaging techniques also allows for advanced possibilities to guide and control minimally invasive interventions. Various navigation concepts have been described for practically all regions of the body. The specific advantages and limitations of these concepts largely depend on the magnet design of the MR scanner and the interventional environment. Open MR scanners involve minimal patient transfer, which improves the interventional workflow and reduces the need for coregistration, ie, the mapping of spatial coordinates between imaging and intervention position. Most diagnostic scanners, in contrast, do not allow the physician to guide his instrument inside the magnet and, consequently, the patient needs to be moved out of the bore. Although adequate coregistration and navigation concepts for closed‐bore scanners are technically more challenging, many developments are driven by the well‐known capabilities of high‐field systems and their better economic value. Advanced concepts such as multimodal overlays, augmented reality displays, and robotic assistance devices are still in their infancy but might propel the use of intraoperative navigation. The goal of this work is to give an update on MRI‐based navigation and related techniques and to briefly discuss the clinical experience and limitations of some selected systems. J. Magn. Reson. Imaging 2008;27:276–291.

Do the Congenitally Blind Have a Stria of Gennari? First Intracortical Insights In Vivo

The primary visual cortex V1, when dissected, is characterized by an easily identifiable anatomical landmark: the stria of Gennari or Gennari stripe. However, the origin and function of the Gennari stripe is so far unknown. In order to shed some light on this question, we acquired 7-T magnetic resonance imaging (MRI) brain scans of congenitally blind (CB) people, who have never had visual experience. If the stria of Gennari requires visual input to develop or to maintain its homeostasis, such subjects should lack this structure. If it is reliably detectable in the CB, it must form and persist independently of visual sensation. This question has never previously been explored in living subjects. For the first time, the use of 7-T high-resolution MRI enables such investigations because of the excellent signal-to-noise ratio at this magnetic field strength. For comparison, we scanned sighted subjects using the same experimental parameters. We detected the stria of Gennari reliably in both sighted and blind subjects, showing that this anatomical feature is not a developmental result of visual input, and it does not degenerate in the absence of visual input.

Continuous arterial spin labeling using a local magnetic field gradient coil.

Continuous arterial spin labeling (ASL) using a locally induced magnetic field gradient for adiabatic inversion of spins in the common carotid artery of human volunteers is demonstrated. The experimental setup consisted of a helmet resonator for imaging, a circular RF surface coil for labeling, and gradient loops to produce a magnetic field gradient. A spin‐echo (SE) echo‐planar imaging (EPI) sequence was used for imaging. The approach is independent of the gradients of the MR scanner. This technology may be used if the imaging gradient system does not produce an appropriate magnetic field gradient at the location of the carotid artery—for example, in a head‐only scanner—and is a prerequisite for the development of a system that allows continuous labeling during the imaging experiment. Magn Reson Med 48:543–546, 2002.

High-Resolution MR Imaging of the Human Brainstem In vivo at 7 Tesla

The human brainstem, which comprises a multitude of axonal nerve fibers and nuclei, plays an important functional role in the human brain. Depicting its anatomy non-invasively with high spatial resolution may thus in turn help to better relate normal and pathological anatomical variations to medical conditions as well as neurological and peripheral functions. We explored the potential of high-resolution magnetic resonance imaging (MRI) at 7 T for depicting the intricate anatomy of the human brainstem in vivo by acquiring and generating images with multiple contrasts: T 2-weighted images, quantitative maps of longitudinal relaxation rate (R 1 maps) and effective transverse relaxation rate ([Formula: see text] maps), magnetic susceptibility maps, and direction-encoded track-density images. Images and quantitative maps were compared with histological stains and anatomical atlases to identify nerve nuclei and nerve fibers. Among the investigated contrasts, susceptibility maps displayed the largest number of brainstem structures. Contrary to R 1 maps and T 2-weighted images, which showed rather homogeneous contrast, [Formula: see text] maps, magnetic susceptibility maps, and track-density images clearly displayed a multitude of smaller and larger fiber bundles. Several brainstem nuclei were identifiable in sections covering the pons and medulla oblongata, including the spinal trigeminal nucleus and the reticulotegmental nucleus on magnetic susceptibility maps as well as the inferior olive on R 1, [Formula: see text], and susceptibility maps. The substantia nigra and red nuclei were visible in all contrasts. In conclusion, high-resolution, multi-contrast MR imaging at 7 T is a versatile tool to non-invasively assess the individual anatomy and tissue composition of the human brainstem.

Recent applications of UHF-MRI in the study of human brain function and structure : a review

The increased availability of ultra‐high‐field (UHF) MRI has led to its application in a wide range of neuroimaging studies, which are showing promise in transforming fundamental approaches to human neuroscience. This review presents recent work on structural and functional brain imaging, at 7 T and higher field strengths. After a short outline of the effects of high field strength on MR images, the rapidly expanding literature on UHF applications of blood‐oxygenation‐level‐dependent‐based functional MRI is reviewed. Structural imaging is then discussed, divided into sections on imaging weighted by relaxation time, including quantitative relaxation time mapping, phase imaging and quantitative susceptibility mapping, angiography, diffusion‐weighted imaging, and finally magnetization‐transfer imaging. The final section discusses studies using the high spatial resolution available at UHF to identify explicit links between structure and function. Copyright