CHAPTER 9: Magnetic Susceptibility Effects in High Field MRI

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1 Figure 1. In the brain, the gray matter has substantially more blood vessels and capillaries than white matter. The magnified image on the right displays the rich vasculature in gray matter forming porous, sponge-like structures. The high concentration of blood in the gray matter vasculature makes the gray matter highly sensitive to the blood oxygen leveldependent (BOLD) effect.

2 Figure 2. The major air-containing structures included in the model are the frontal sinus (turquoise), the sphenoid sinus (pink), the maxillary sinuses (blue), mastoid air cells (red), nasal and oral cavities, and trachea (yellow). Calculated (top) and measured (bottom) B 0 distributions demonstrate local gradients in the sagittal (a), coronal (b), and axial (c) planes. Magnetic field deviations are color coded in parts per million (ppm). Reprinted with permission from [30].

3 Figure 3. Calculated B 0 distributions inside the human head model demonstrate the remarkable distortion by the air-containing structures inside the human head. Illustration courtesy of Drs. Shizhe Li, C.M. Collins, Q.X. Yang, and M.B. Smith. Figure 4. Axial plane view of numerically calculated B 0 field magnitudes in ppm deviation from applied field. The gray scale is set to reveal the B 0 deviation due to the susceptibility difference between gray and white matter in the brain. Reprinted with permission from [35].

4 Figure 5. Timing diagram of 2D gradient echo sequence for a voxel. A, slice selection gradient; B, slice rewinding gradient. During TE the local gradient in the slice selection dimension, G z l, causes intravoxel phase dispersion, which results in signal loss in GE images. A compensation gradient, G c, or a tailored RF pulse, α[φ 0 (z)] can be applied to cancel the phase dispersion by G z l. Figure 6. The GESEPI image sets of the human brain obtained at 3.0 Tesla before (2D-FT data set) and after (3D-FT) a Fourier transform along the slice direction with respect to k z by the compensation gradient. The 2D-FT image set was obtained by sequentially incrementing G c to obtain N = 16 frames. Note the removal of the dark artifacts in the images from 4 6 sub-slices (ca. 1 mm). Reprinted with permission from [46]).

5 Figure 7. A coronal view of the phantom and images of the axial slice along the midline between the two spheres with the GE and GESEPI methods. The phantom consists of two air-filled spheres positioned inside a gelatin-filled cylindrical container with its longitudinal axis perpendicular to B 0. The voxels P, Q, and R indicated in the GE image are labeled P', Q', and R' in the corresponding GESEPI images. The phantom images were obtained with a flip angle of 25º, TR/TE = 80 ms/25 ms, FOV = cm 2, image matrix = , slice thickness = 5 mm. Adapted with permission from [46]. Figure 8. Serial signal intensity plots from the representative voxels P, Q, and R (see Figure 7) in the 2D-FT data set, and from the corresponding voxels P', Q', and R' in the 3D-FT data set. The image number axis is shown above each plot, corresponding to either k z (2D- FT, top row) or position z along the slice direction (3D-FT, bottom row). The dashed lines in the top row plots indicate k = 0, where 2D images were acquired. Reprinted with permission from [46].

6 Figure 9. GESEPI effectively removed signal-loss artifacts in heavily T 2 *-weighted images at 3.0 and 7.0 Tesla. All the images have the same slice thickness 5 mm. The GESEPI images were obtained with 8 G c steps. The intensities of the globus pallidus (GP) and substantia nigra (SN) are obscured in the GE images and enhanced in the GESEPI images. Note that the signal nonuniformity in the GESEPI image at 7.0 Tesla is caused by the B 1 field distortion due to the wave effect. Figure 10. T 2 *-weighted images of a mouse brain at 14.0 Tesla. By removing the artifacts in the GE image, the GESEPI image improves contrast resolution and displays anatomic details not observable in spin-echo images. Reprinted with permission from [50].

7 Figure 11. GE (top row) and GESEPI (bottom row) images of an immature rat brain as a function of TE taken on a 9.4T system. TR = 80 ms, matrix = , FOV = mm 2, slice thickness = 1 mm. Reprinted with permission from [ 46]. Figure 12. Average R 2 * map of the human brain with GESEPI.

8 Figure 13. The voxel signal intensity from the excited slice of the GESEPI data is plotted as a function of k c and acquisition time t along the trajectory (a) and its Fourier transform (b). The plots were calculated using Eqs. (13) and (14) with T 2 * = 30 ms, G z l = 0.5 ppm/cm, z 0 = 5 mm, and B 0 = 3.0 Tesla. The plots in (c) are standard 2D voxel signal intensity along the trajectory with k l = 0 (Plot I), k l 0 (Plot II), and GESEPI intensity with k l 0 (Plot III), respectively. The Plot II in (c), obtained by intersecting the two-dimensional plot in (a) with k z = 0, shows a sinc-function modulation of T 2 * decay. Plot III, obtained by intersecting the plot in (b) with z = 0, coincides with Plot I of normal T 2 * relaxation. Figure 14. Top row: the signal intensities acquired without phase-encoding in the left right direction. Center row: image intensity plot, representing point-spread function (PSF) profile in phase-encoding direction. Bottom row: images acquired with phaseencoding, demonstrating the reduction of blurring artifacts in EPI using the GESEPI method. The quasi-point source phantom for axial images consists of two 5-mm diameter tubes filled with gelatin, with a 1.8-mm distance between the tubes. The tubes are aligned with the B 0 field. The image intensities are adjusted to facilitate visualization of PSF. Adapted with permission from [71].

9 Figure 15. Spin-echo EPI images acquired without (a) and with SENSE with a reduction factor r = 3 (b) from an inferior brain slice. In spin echo images, the signal-loss artifact is minimal so that the geometric distortion due to the in-plane gradient can be demonstrated clearly. In order to highlight the brain geometry, a diffusion gradient was applied to reduce the CSF signals. Both images were acquired from a Philips Intera 3.0T MRI system with the same acquisition parameters. Reprinted with permission from [75].

10 Figure 16. The EPI images were acquired from the same human brain volume with SENSE (r = 3) using the GE (a), GESEPI (b), and SE (c) methods. The typical signal-loss artifact in the GE images (a) in the inferior frontal and temporal brain regions indicated by the arrows is reduced significantly with GESEPI (b). Reprinted with permission from [71].

11 Figure 17. Relationship of T 2 * decay with Forward and Reversed spiral acquisition. Illustration provided courtesy of Dr. V. Andrew Stenger. Figure 18. Simulated k-space signals of a point source with reversed and forward spiral acquisitions (a) and the corresponding image profiles (b). Illustration provided courtesy of Dr. V. Andrew Stenger.

12 Figure 19. Simulated (a) and experimental (b) image profiles with different data acquisition methods in the absence and presence of local gradients. Illustration provided courtesy of Dr. V. Andrew Stenger. Figure 20. Spiral images taken from the inferior brain region show remarkable signal-loss artifact. As indicated by the arrows, the artifact is mitigated by reversed spiral and removed when GESEPI is incorporated into the reversed spiral technique.

13 Figure 21. Olfactory fmri activation at orbitofrontal and olfactory tract with GESEPIreversed-spiral imaging.