Sie befinden sich hier
P. M. Heiler1, S. Konstandin1, L. R. Schad1
Multi-Echo FLASH imaging in combination with echo train shifting [1,2] offers BOLD sensitivity for matrix sizes up to 256 x 256 leading to in-plane resolutions of about 1 mm2. Long measurement times and low Signal-to-Noise-Ratio (SNR) preclude BOLDfMRI going to higher resolutions. Hence, only a reduction in the Field-of-View (FOV) further increases spatial resolution. Latter, however, is limited by aliasing artifacts due to signals from outside of the FOV. 2D-selective excitation  in slice- and phase encoding direction, as shown in Fig. 1, allows for small inner FOV sizes resulting in increased spatial resolution. Recently, applications with two-dimensional pulse profiles such as spiral  or blipped planar  excitation trajectories were reported. In this study, latter one was chosen because the limiting side excitations only occur in one single dimension.
The design of the 2D-selective excitation pulse was based on a duration of 27.5 ms, spatial dimensions of 50 mm in read direction and 4 mm in slice direction and a flip angle of 50°. kmax,x was set to 2.5 cm-1, kmax,y to 0.84 cm-1 and Dky to 0.04 cm-1.
For data acquisition, the Double Contrast Echo Train Shifted Multi-Echo FLASH sequence  was chosen with the following sequence parameters: TE=38 ms, TR=126 ms. The matrix size was 192 x 114 leading to an in-plane resolution of 0.5 x 0.5 mm2.
For data acquisition, the Double Contrast Echo Train Shifted Multi-Echo FLASH sequence  was performed on a 3 T (Trio, Siemens, Erlangen, Germany) using a matrix head coil with the following sequence parameters: TE=38 ms, TR=126 ms, flip angle α=50°. The matrix size was set to 192 x 114 leading to an in-plane resolution of 0.5 x 0.5 mm2. To demonstrate BOLD-sensitivity, an initial BOLD-experiment was performed by acquiring 200 images in 2.9 s/image. Functional activation was achieved by a visual stimulation task showing a checkerboard alternating with a gray screen (5 images in 14.5 s each).
Functional results were processed and analyzed with Statistical Parametric Mapping 5 (SPM5). The images were realigned to the average image (Fig. 3) and subsequently smoothed with a 2 mm Gaussian Kernel. The alpha error was set to α=0.01 and minimal shown cluster size was k=10 pixels.
Fig. 3 shows the average image of 200 acquisitions during the fMRI experiment. It should be noted that the image is free of noticeable artifacts at an in-plane resolution of 0.5 x 0.5 mm2. Activation maps of the human brain (Fig. 4) show BOLD-signal only close to the venules.
We presented a high resolution T2*-weighted acquisition sequence in combination with a two-dimensional excitation pulse suitable for functional experiments. Its feasibility includes both, high resolution morphological T2*-weighted imaging with high contrast to the venule system (Fig. 3) and local high resolution functional studies (Fig. 4). Latter showed activation only in areas close to venules instead of a blurred activation-signal in the whole cortex, as achieved with generic EPI-BOLD sequences.
1 Computer Assisted Clinical Medicine, University of Heidelberg, Mannheim, Germany
 Voit D, Frahm J. NMR Biomed 18, 481-488 (2005)
 Heiler P et al, Proceedings of the ISMRM 15, 1940 (2007)
 Haase A et al, J. Magn Reson 67, 258-266 (1986)
 Pauly J et al, J. Magn Reson 81, 43-56 (1989)
 Rieseberg S et al, Magn. Reson. Med. 47, 1186-1193 (2002)