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AJNR - American Journal of Neuroradiology

Published ahead of print on November 27, 2008
doi: 10.3174/ajnr.A1400

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Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications, Part 1

E.M. Haacke^a,b, S. Mittal^c,d, Z. Wu^e, J. Neelavalli^b and Y.-C.N. Cheng^a,b

^a Department of Radiology, Wayne State University, Detroit, Mich
^b Department of Biomedical Engineering, Wayne State University, Detroit, Mich
^c Department of Neurosurgery, Wayne State University, Detroit, Mich
^d Karmanos Cancer Institute, Detroit, Mich
^e Department of Biomedical Engineering, McMaster University, Hamilton, Ontario, Canada

View larger version (17K): [in this window] [in a new window]   Fig 1. Gradient-echo sequence design. Ts refers to the sampling time interval. The gradient pulses are designed to give first-order flow compensation. SS indicates the slice-select gradient; PE, the phase-encoding gradient; ADC, analog-to-digital conversion.

View larger version (16K): [in this window] [in a new window]   Fig 2. A, Plots showing the signal-intensity behavior as a function of flip angle for gray matter (GM) and white matter (WM) at field strengths 1.5T and 3T. B, Plots showing the corresponding contrast between GM-WM and GM-CSF. These curves are calculated taking into consideration the higher signal intensity available at 3T and assuming that bandwidth is correspondingly increased at 3T to ensure equivalent geometric distortion. Tissue parameters are given in Table 1. Note that the GM/WM contrast is highest around 20° at 3T and there is a minimum or reversal of contrast around 5° (where GM is now brighter than WM; ie, it is a high-contrast spin echo-weighted image).

View larger version (66K): [in this window] [in a new window]   Fig 3. A, Raw phase image. B, HP-filtered phase image with a central filter size of 32 x 32; C, Filtered-phase image with a central filter size of 64 x 64.

View larger version (65K): [in this window] [in a new window]   Fig 4. Short-echo T1-weighted image (A), compared with the SWI long-echo gradient-echo processed magnitude (B) and HP-filtered phase data (C).

View larger version (111K): [in this window] [in a new window]   Fig 5. A, HP-filtered phase image in the midbrain acquired at 4T with TE = 15 ms. B, India ink-stained cadaver brain section showing a strong correspondence to variations in signal intensity as seen with SWI HP-filtered phase.

View larger version (11K): [in this window] [in a new window]   Fig 6. Pictorial depiction of the phase-masking process. A, Phase profile in a filtered-phase image. B, Corresponding profile of the mask created from A. Once the mask is raised to the fourth power, the vein that has a phase of – / 2 and a mask value of 0.5 will become 1-/16 and, therefore, this vein will be almost as well suppressed as the vein with a phase of – and f(x) = 0. GM indicates gray matter; WM, white matter.

View larger version (103K): [in this window] [in a new window]   Fig 7. A, Unprocessed original SWI magnitude image. B, HP-filtered phase image. C, Processed SWI magnitude image (ie, after phase-mask multiplication with m = 4). D, Unprocessed original SWI magnitude image showing the dark hypointense ring around vein cross-sections (arrows). E, HP-filtered phase image showing the veins have low signal-intensity (arrows). F, Processed SWI magnitude image showing that the veins now appear with uniform low signal intensity (arrows).

View larger version (92K): [in this window] [in a new window]   Fig 8. mIP data over the original magnitude images (A) and over the processed SWI images (B). The mIP is carried out over 7 sections (representing a 14-mm slab thickness). The images were collected at 3T. There is a dropout of signal intensity in the frontal part of the brain, but otherwise the vessel visibility is much improved in B. In the future, this frontal dropout should not be a problem when the air/tissue geometries are corrected (Fig 9).

View larger version (166K): [in this window] [in a new window]   Fig 9. Results of the improved processing methodology. A, Original SWI phase image. B, Simulated phase due to air/tissue geometry at 40 ms. C, Result of subtracting A from B through the complex division. D, Result of HP filtering of A. E, Result of HP filtering of C. F, Corresponding unprocessed magnitude SWI image. G, Processed SWI magnitude image by using a phase mask from D. H, Processed SWI magnitude by using a phase mask from E. I, Result of subtracting G from H. Corresponding magnitude and phase images have been adjusted to the same, but appropriate, contrast levels. The size of the central filter in the HP process is 64 x 64.

View larger version (131K): [in this window] [in a new window]   Fig 10. A and B, Phase images, keeping the product BoTE constant. A phase image acquired at 1.5T (A) and the same subject at 3T (B) show the same overall contrast but with a better SNR. The gray/white matter contrast in these images comes from the increased MR-visible iron content in the gray matter, giving it an appearance similar to a T1-weighted scan.

View larger version (173K): [in this window] [in a new window]   Fig 11. A sample mIP from 7T data, with a resolution of 215 x 215 x 1000 µ, TE = 16 ms, TR = 45 ms, FA = 25°, and mIP over 8 sections. Image courtesy of Ge Y and Barnes S.

View larger version (128K): [in this window] [in a new window]   Fig 12. A zoomed image from the same dataset shown in Fig 11 reveals a dark band between the gray matter and white matter, which we assume represents the white matter arcuate fibers (diagonal arrows). The small vessels joining the pial veins are the venules (vertical arrows). Image courtesy of Ge Y and Barnes S.