1 / 22

J. Su 1 , M.Saranathan 1 , and B.K .Rutt 1

ISMRM 2012 E-Poster #4275. Accelerated Variable Flip Angle T 1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space . J. Su 1 , M.Saranathan 1 , and B.K .Rutt 1 1 Department of Radiology, Stanford University, Stanford, CA, United States. 2.3x Undersamping. T 1 Maps.

danika
Download Presentation

J. Su 1 , M.Saranathan 1 , and B.K .Rutt 1

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ISMRM 2012 E-Poster #4275 Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space J.Su1, M.Saranathan1, and B.K.Rutt1 1Department of Radiology, Stanford University, Stanford, CA, United States 2.3x Undersamping T1 Maps Fully Sampled Accelerated % Difference

  2. ISMRM 2012 E-Poster #4275 Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space J.Su1, M.Saranathan1, and B.K.Rutt1 1Department of Radiology, Stanford University, Stanford, CA, United States Declaration of Conflict of Interest or Relationship I have no conflicts of interest to disclose with regard to the subject matter of this presentation.

  3. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Background • Variable flip angle T1 mapping (VFA) is a quantitative image method in which a series of scans at different flip angles are collected to extract whole brain relaxation times • The collection of many angles for accuracy across the wide range of T1 values in tissue is time consuming1 1Deoni et al. Magn Reson Med. 2003 Mar;49(3):515-26.

  4. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Purpose • Accelerate VFA by using a view sharing scheme similar to DISCO2 • A novel pseudo-random sampling pattern is used to greatly reduce the appearance of coherent artifacts • Assess the accuracy and variation of the accelerated T1 maps compared to the fully sampled source 2Saranathan et al. J MagnReson Imaging. 2012 Feb 14.

  5. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Scanning Methods • 3T GE Signa MR750, 8-channel head RF coil • VFA: 2mm isotropic covering whole brain, about 15 min., 110x110x80 matrix, fully sampled ellipse • SPGR: TE/TR = 1.2/3.7ms, α = 14 nonlinearly spaced angles, shown below for simulated curves • This is necessary for accurate estimation of T1 across all brain tissues

  6. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Sampling • This is represents the fully sampled ellipse of data points in Cartesian k-space • Define two regions in k-space: • The center region (A) • The remaining outer region (B) B A Phase Encode Slice Encode

  7. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Sampling • The center region (A) • 16% of k-space which is fully sampled • The center data is collected for every flip angle frame A A Phase Encode Slice Encode

  8. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Sampling B1 B2 B3 B • The outer region (B) • Broken down into 3 pseudo-random subsampling patterns, each undersampled by a factor 3 B A Phase Encode Slice Encode

  9. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Sampling B B1+B2+B3 B1 B2 B3 • The outer region (B) • Broken down into 3 pseudo-random subsampling patterns, each undersampled by a factor 3 • The patterns interlace and can be combined to form a complete composite outer region B A Phase Encode Slice Encode

  10. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Sampling A+B3 A+B2 • For each flip angle frame, a different undersampling pattern is used: • A+B1 • A+B2 • A+B3 • This results in a 2.3x acceleration of the acquisition A+B1

  11. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods A+B3 . . . • The sampling patterns are cycled through with each acquired flip angle frame • Sequential sets of 3 frames are used to reconstruct the accelerated composite data like a sliding window A+B1 . . . A+B2

  12. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Reconstruction • A complete composite of the current flip angle is formed by mixing in outer region samples from the previous and next adjacent angles • The fully sampled center region of the current flip angle is retained in its entirely • For example, here the 2nd flip angle is created by combining with the 1st and 3rdangle . . . . . .

  13. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Reconstruction • The first angle is a special edge case, outer samples are instead mixed from the following two angles

  14. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Reconstruction • The last angle is also a special case, samples are borrowed from the preceding two angles

  15. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods: Reconstruction • Borrowed samples are scaled to compensate for the difference between frames caused by the SPGR signal behavior • The average magnitude in the center region is computed for each frame, μi • The scale factor is then μ(target angle)/μ(borrowed angle) • Fixes large discontinuities in k-space

  16. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Reconstruction • Matlab was used to synthesize the accelerated composites from the fully sampled acquired data • The k-space data are then brought into the image domain by standard methods

  17. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Post-Processing • Linearly coregister and brain extract the images with FSL3 • Extract whole-brain T1 values by linearizing the data according to the SPGR signal equation and performing a fit4 • Comparisons are made to the fully sampled images on a voxel-by-voxel level 3FMRIB Software Library 4Fram et al. MagnReson Imaging. 1987;5(3):201-8.

  18. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Results: Worst Case – First Flip Angle • The percent difference between the accelerated and fully sampled volumes is shown • The first and last flip angles are the least faithfully reconstructed since they borrow from non-adjacent angles • Even in the worst case, the reconstructed SPGR images are very similar to the originals • Median percent difference: 0.025% • Interquartile range: 2.235%

  19. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Results: T1 Map – Fully Sampled

  20. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Results: T1 Map – Accelerated

  21. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Results: T1Map • The percent difference between the accelerated and fully sampled T1 maps is shown • Accuracy is excellent, the mean shift in T1 values is << 1% • Median: 0.030% • The variation in the difference map is very low over the whole brain • Standard deviation < 3% • Interquartile range: 1.492%

  22. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 Discussion & Conclusions • 2.3x acceleration of VFA is reliably and simply achieved with negligible loss in accuracy and precision • Reconstruction is fast and possible on the scanner • Compatible with parallel imaging methods like GRAPPA and SPIRiT • Simply combine the 3 sampling patterns by the parallel imaging acquisition pattern • With a modest 2x2 acceleration, a net 6-7x speed up is easily achievable, reducing this 15-minute 2mm isotropic protocol to 2.5 minutes • Useful for high-resolution mcDESPOTacquisitions as well

More Related