CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 1. MRI system overview of the Siemens 7T MRI scanner. In green boxes: minor
changes to standard system. In red circles: major changes needed to adapt to 7 T.
Figure 2. Schematic setup for B1 shimming and TX sense at ultra high fields. Instead of
using one RFPA only, the entire TX line is split into several TX channels, allowing freely
programmable amplitude, phase, and pulse shapes per channel.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 3. Dielectric effects in phantom images at 7 T. TE/TR = 4/100 ms, FL2D = 60º, BW
= 300, FOV = 220 x 220; sagittal slice (top left), coronal slice (top right), axial slice (lower
left).
Figure 4. MR images of head-shaped phantoms acquired with the same type of RF coil.
Different permitivities ε are set for the phantom images. Upper row: 70% isopropanol alcohol, 30% H20, 0.9% NaCl; ε = 49. Middle row: NaCl loaded H20 phantom with sinus and
ear canals; ε = 78; Lower row: coronal and axial slice through a human head. All images
are acquired with TE/TR/flip angle of 5.5/40/10º.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 5. Low flip angle multi-slice GRE sequence in axial, coronal, and sagittal orientations. Data are acquired with a CP TX/RX birdcage coil. Flip angle = 20º, TE/TR = 3.3/113
ms, BW = 530 Hz/pxl, sl = 5 mm, = FOV = 228 x 280 with rectangular FOV.
Figure 6. PD-weighted GRE images: TR = 200, TE = 3.92, flip = 20, 256 x 256 matrix, 200mm FOV, TA = 29 sec. Left: acquired with a TEM-TX/RX coil. Right: same slice, but TEM
TX and 8-channel RX coil.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 7. RF profiles: TEM TX/RX coils vs. array coil. SNR is plotted for a TX/RX TEM
coils and the 8-channel RX coil array.
Figure 8. Gradient coils for 7 T. Left: nested whole-body gradient coil and nested insert
head gradient coil. Middle: with covers.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 9. Acoustic noise spectra of same gradient coil in 1.5 and 7T magnets. Acoustic
noise spectra of a whole body X gradient over a frequency range from 0 to 3.2 kHz. For
comparison the spectra of the same gradient coil is shown for 1.5T (green) measured at 5
mT/m and 7 T (red), measured at 1 mT/m.
Figure 10. Noise level of clinical and technical sequences measured on our 7T whole-body
system using a whole-body gradient coil.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 11. Image of medial temporal lobe (left) acquired at 7 T with 100-?m isotropic voxels (TR = 20, TE = 7.8, flip angle = 5, 10, 15, 20, 25º, FOV = 52 x 52 mm; 512 x 512 matrix,
256 slices). Right: oblique slice through layer II of EC showing the layer II islands as
brighter regions. Layers in the CA fields of the hippocampus and the dentate gyrus are
clearly visible.
Figure 12. Left: coronal cut through a human head, acquired on a whole-body gradient coil
with an end-capped TEM coil (27-cm diameter and 14 cm long, dimensions shown in red).
Although the coil is short, signal is received from the chest and shoulder regions. Right:
sagittal MR image acquired with the same RF coil on a head insert gradient coil. Fold in
artifacts are visible even in the center of the image.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 13. Interaction between magnet homogeneity, linearity volume of a gradient coil
(long and short), and RF TX and RX profiles.
Figure 14. Gradient coil resistance vs. frequency shown for the same type of head insert
design: (a) resistance outside magnetic field, (b) inside 3T magnet, (c) inside 7T magnet.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 15. Sagittal field maps through a mid-sagittal plane and temporal lobe (inner ear
region) of a human brain. Top row shows results taken at 3 T and the bottom row taken at
7 T.
Figure 16. Upper left: axial cut, supine position. Upper right: axial cut, subject lying on left
side. Lower left: prone position. Lower right: axial cut, subject lying on right side.
Obviously, the dark bands (signal voids due to RF eddy currents) (see arrows) are bound
to the human anatomy and not to the B profile of the RF coil. Image provided courtesy of
Melanie Schmitt.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 17. Relation of RF pulse duration to SAR.
Figure 18. Comparison of T1-weighted SE imaging SE at 1.5 and 3.0 T at exactly the same
slice positions using AutoAlign. For further information, please see text.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 19. Reformatted whole-brain MPRAGE data set: FOV = 220 x 220, matrix = 256 x
256 x 160, pixel size = 0.85 x 0.85 x 1 mm3, acquisition time = 9:34 min; adiabatic inversion
pulse, without B1 normalization, acquired with an 8-channel receive array in sagittal orientation. Multiplanar reconstruction in (a) axial, (b) sagittal, and (c) coronal orientations.
(D) Brightening in sinus region (see arrow) is caused by low-bandwidth adiabatic inversion pulse (for further information, please see text).
Figure 20. Comparison of finger-tapping fMRI experiment. ASL is shown on left; BOLD is
shown on the right.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 21. Axial TOF vessel tree image acquired at 7 T. A 3D-GRE sequence with
TE/TR/flip angle of 5.6/49/20º was used. Total scan time 6 minutes and 45 seconds. A
TEM TX/RX coil was used.
Figure 22. Image normalization: TSE, 11 echoes, 7-min exam, 20-cm FOV, 512 x 512 (0.4 x
0.4 mm) 9 slices, 3 mm thick. Left: un-normalized image. Right: Image intensity normalized.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 23. High-resolution T2-weighted TSE images. 512 x 760 matrix, resulting in 0.27 x
0.27 x 2 mm3 pixels size. Overall scan time was 9 minutes. An 8-channel phased array RX
coil was used. The line of Gennari can be resolved. Excellent SNR is achieved. SNR is at
least a factor of 2 above what can be reached at 3 T with a 32-channel head array coil.
Figure 24. Axial T2 hyperecho scan (0.4 x 0.4 x 3 mm); intensity normalized. FOV = 17.3 cm
x 18.5 cm, matrix = 512 x 512, slthk =2 mm, TE = 66 ms, TR = 4500 ms, ETL = 15. Image
courtesy of J. Hennig, University of Freiburg, Germany.
CHAPTER 4: Aspects of Clinical Imaging at 7 T
Figure 25. Gradient echo EPI scan acquired with the head insert gradient coil: 26 slices of
a 1282 matrix, TE of 20 ms; FOV = 20 cm, 1.5 x 1.5 mm pixel size; 2-mm slice thickness.