Alexander Rauscher

Susceptibility weighted imaging at ultra high magnetic field strengths: Theoretical considerations and experimental results

We present numerical simulations and experimental results for susceptibility weighted imaging (SWI) at 7 T. Magnitude, phase, and SWI contrast were simulated for different voxel geometries and imaging parameters, resulting in an echo time of 14 msec for optimum contrast between veins and surrounding tissue. Slice thickness of twice the in‐plane voxel size or more resulted in optimum vessel visibility. Phantom and in vivo data are in very good agreement with the simulations and the delineation of vessels at 7 T was superior compared to lower field strengths. The phase of the complex data reveals anatomical details that are complementary to the corresponding magnitude images. Susceptibility weighted imaging at very high field strengths is a promising technique because of its high sensitivity to tissue susceptibility, its low specific absorption rate, and the phases negligible sensitivity to B1 inhomogeneities. Magn Reson Med 60:1155–1168, 2008.

Nonnvasive assessment of vascular architecture and function during modulated blood oxygenation using susceptibility weighted magnetic resonance imaging.

Susceptibility weighted imaging (SWI) is a BOLD‐sensitive method for visualizing anatomical features such as small cerebral veins in high detail. The purpose of this study was to evaluate high‐resolution SWI in combination with a modulation of blood oxygenation by breathing of air, carbogen, and oxygen and to directly visualize the effects of changing blood oxygenation on the magnetic field inside and around venous blood vessels. Signal changes associated with the response to carbogen and oxygen breathing were evaluated in different anatomic regions in healthy volunteers and in two patients with brain tumors. In the magnitude images inhalation of carbogen led to significant signal intensity changes ranging from +4.4 ± 1.9% to +9.5 ± 1.4% in gray matter and no significant changes in thalamus, putamen, and white matter. During oxygen breathing mean signal changes were smaller than during carbogen breathing. The method is capable of producing high‐resolution functional maps of BOLD response to carbogen and oxygen breathing as well as high‐resolution images of venous vasculature. Its sensitivity to changes in blood oxygenation was demonstrated by in vivo visualization of the BOLD effect via phase imaging. Magn Reson Med 54:87–95, 2005.

Optimized 3 T EPI of the amygdalae

The optimum parameters for single-shot gradient-recalled (GR) EPI-based fMRI studies of the limbic region are systematically established at 3 T via their ability to mitigate intravoxel dephasing-measured via SNR and T2* in the amygdalae-and their implications for temporal resolution (or brain coverage). Conventional imaging parameters (64 x 64 matrix size and 4-6 mm thick slices) are confirmed to be inadequate for functional studies at 3 T. Measurements of main magnetic field variations across the amygdalae suggest that such variations are equal in the craniocaudal and anterior-posterior directions, and slightly lower in the mediolateral direction, with this and other considerations leading us to conclude an oblique axial orientation to be most suitable. In-plane resolution of approximately 1.7 mm was sufficient to recover signal in the area of the amygdalae. SNR was found to peak at a slice thickness of between 2.0 and 2.5 mm, dependent on the subject. T2* time in the amygdalae was measured with a standard EPI protocol to be 22 +/- 3 ms. Using the optimized (high resolution) EPI protocol proposed here, the measured T2* time increased to 48 +/- 2 ms (compared with 43 +/- 3 ms for a reference FLASH scan), only slightly lower than the cortex (49 +/- 2 ms measured with optimized EPI and 52 +/- 2 ms with FLASH). The FLASH measurement of 43 ms is taken to be a suitable effective echo time (TE(eff)) to achieve maximum BOLD sensitivity in the amygdalae. Time series data acquired with these parameters showed a 60% increase in SNR in the amygdala over that obtained with a standard low-resolution protocol and suggest sufficient SNR and BOLD sensitivity to make functional studies feasible. Arteries, but no substantial draining veins, were found in high-resolution BOLD venograms of the region. Our results indicate that EPI protocols need to be carefully optimized for structures of interest if reliable results from single subjects are to be established in this brain region.

Automated unwrapping of MR phase images applied to BOLD MR-venography at 3 Tesla

To improve the diagnostic value of BOLD MR‐Venography by removing artifacts related to phase wrapping, particularly in regions of large background susceptibilities at high magnetic field strengths.

Rapid whole cerebrum myelin water imaging using a 3D GRASE sequence.

Myelin water imaging, a magnetic resonance imaging technique capable of resolving the fraction of water molecules which are located between the layers of myelin, is a valuable tool for investigating both normal and pathological brain structure in vivo. There is a strong need for pulse sequences which improve the quality and applicability of myelin water imaging in a clinical setting. In this study, we validated the use of a fast multi echo T(2) relaxation sequence for myelin water imaging. Using a multiple combined gradient and spin echo (GRASE) technique, we attain whole cerebrum myelin water images in under 15 minutes. Region of interest analysis indicates that this fast GRASE imaging sequence produces results which are in good agreement with pure spin echo measurements (R(2)=0.95, p<0.0001). This drastic improvement in speed and brain coverage compared to current spin echo standards will allow increased inclusion of myelin water imaging in neurological research protocols and opens up the possibility of applications in a clinical setting.

The influence of white matter fibre orientation on MR signal phase and decay.

MRI phase images of the brain exhibit excellent contrast and high signal‐to‐noise ratio. It has been shown recently that the phase contrast not only depends on a tissues magnetic susceptibility but also on its architecture, which offers new ways of studying biological tissues in vivo. We combined diffusion tensor imaging and multi‐echo susceptibility‐weighted imaging to investigate the relationship between white matter fibre orientation and gradient‐echo phase and magnitude. The local angle between white matter fibres and the main magnetic field was computed from the principal diffusion direction. The phase and signal decay of the gradient‐echo images revealed a characteristic relationship with fibre orientation. The phase is in agreement with a recently reported model of cerebral white matter phase contrast in MRI. Copyright

Susceptibility weighted imaging with multiple echoes.

To extend susceptibility weighted imaging (SWI) to multiple echoes with an adapted homodyne filtering of phase images for the computation of venograms with improved signal to noise ratio (SNR) and contrast to noise ratio (CNR) and to produce high resolution maps of R2* relaxation.

Phase unwrapping of MR images using ΦUN - A fast and robust region growing algorithm

We present a fully automated phase unwrapping algorithm (Phi UN) which is optimized for high-resolution magnetic resonance imaging data. The algorithm is a region growing method and uses separate quality maps for seed finding and unwrapping which are retrieved from the full complex information of the data. We compared our algorithm with an established method in various phantom and in vivo data and found a very good agreement between the results of both techniques. Phi UN, however, was significantly faster at low signal to noise ratio (SNR) and data with a more complex phase topography, making it particularly suitable for applications with low SNR and high spatial resolution. Phi UN is freely available to the scientific community.

ToF-SWI: simultaneous time of flight and fully flow compensated susceptibility weighted imaging.

To perform systematic investigations on parameter selection of a dual‐echo sequence (ToF‐SWI) for combined 3D time‐of‐flight (ToF) angiography and susceptibility weighted imaging (SWI).

A prospective study of physician-observed concussion during a varsity university hockey season: white matter integrity in ice hockey players. Part 3 of 4

OBJECT The aim of this study was to investigate the effect of repetitive head impacts on white matter integrity that were sustained during 1 Canadian Interuniversity Sports (CIS) ice hockey season, using advanced diffusion tensor imaging (DTI). METHODS Twenty-five male ice hockey players between 20 and 26 years of age (mean age 22.24 ± 1.59 years) participated in this study. Participants underwent pre- and postseason 3-T MRI, including DTI. Group analyses were performed using paired-group tract-based spatial statistics to test for differences between preseason and postseason changes. RESULTS Tract-based spatial statistics revealed an increase in trace, radial diffusivity (RD), and axial diffusivity (AD) over the course of 1 season. Compared with preseason data, postseason images showed higher trace, AD, and RD values in the right precentral region, the right corona radiata, and the anterior and posterior limb of the internal capsule. These regions involve parts of the corticospinal tract, the corpus callosum, and the superior longitudinal fasciculus. No significant differences were observed between preseason and postseason for fractional anisotropy. CONCLUSIONS Diffusion tensor imaging revealed changes in white matter diffusivity in male ice hockey players over the course of 1 season. The origin of these findings needs to be elucidated.