Wolfgang Driesel

Online motion correction for diffusion-weighted imaging using navigator echoes: Application to RARE imaging without sensitivity loss

This article describes the first application of true online motion correction to diffusion‐weighted RARE imaging. Two orthogonal navigator echoes were acquired and zeroth and first‐order phase corrections applied in less than 8 ms between a diffusion‐weighted magnetization preparation and data acquisition using the RARE sequence. The zeroth‐order phase correction was realized by pulsing the systems B0‐coil: the first‐order error corrected with appropriate magnetic field gradient pulses. Online correction ensured that no irreversible signal loss could occur in the imaging experiment. Diffusion‐weighted images of the brain were obtained from healthy volunteers. EGG‐triggered acquisition was applied at 400 ms after the R‐wave. Data were acquired on a matrix of 256 × 256 with a RARE factor of 16 and a b‐value of 804 smm−2. The images obtained with online motion correction showed a remarkably high image quality, while those acquired without motion correction were severely degraded by artifacts. Magn Reson Med 45:729–733, 2001.

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.

Towards quantification of blood-flow changes during cognitive task activation using perfusion-based fMRI.

Multi-slice perfusion-based functional magnetic resonance imaging (p-fMRI) is demonstrated with a color-word Stroop task as an established cognitive paradigm. Continuous arterial spin labeling (CASL) of the blood in the left common carotid artery was applied for all repetitions of the functional run in a quasi-continuous fashion, i.e., it was interrupted only during image acquisition. For comparison, blood oxygen level dependent (BOLD) contrast was detected using conventional gradient-recalled echo (GE) echo planar imaging (EPI). Positive activations in BOLD imaging appeared in p-fMRI as negative signal changes corresponding to an enhanced transport of inverted water spins into the region of interest, i.e., increased cerebral blood flow (CBF). Regional differences between the localization of activations and the sensitivity of p-fMRI and BOLD-fMRI were observed as, for example, in the inferior frontal sulcus and in the intraparietal sulcus. Quantification of CBF changes during cognitive task activation was performed on a multi-subject basis and yielded CBF increases of the order of 20-30%.

Mapping of arterial transit time by intravascular signal selection

The arterial transit time (δa) is a potentially important physiological parameter which may provide valuable information for the characterization of cerebrovascular diseases. The present study shows that δa can be measured by arterial spin labeling (ASL) applied quasi‐continuously in an amplitude‐modulated fashion at the human neck. Imaging was performed using short repetition times and excitation flip angles of 90°, which resulted in the selection of an ASL signal of mostly intravascular origin. Model‐independent estimates of δa were obtained directly from the temporal shift of the ASL time series. An extended two‐compartment perfusion model was developed in order to simulate the basic features of the proposed method and to validate the evaluation procedure. Vascular structures found in human δa maps, such as the circle of Willis or cerebral border zones, hint at the sensitivity of the method to most sizes of arterial vessels. Group‐averaged values of δa measured from the carotid bifurcation to the tissue of interest in selected regions of the human brain ranged from 925 ms in the insular cortex to 2000 ms in the thalamic region. Copyright

Shielded dual-loop resonator for arterial spin labeling at the neck.

To construct a dual‐loop coil for continuous arterial spin labeling (CASL) at the human neck and characterize it using computer simulations and magnetic resonance experiments.