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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.

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

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  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 acceleration is achieved with high fidelity • Percent errors in T1 havemean << 1% and standard deviation < 3% across the whole brain 2.3x Undersamping T1 Maps Fully Sampled Accelerated % Difference Computer No. 55

  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, DESPOT1) 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 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 • The sampling patterns are cycled through in order with each acquired flip angle frame

  12. Accelerated Variable Flip Angle T1 Mapping via View Sharing of Pseudo-Random Sampled Higher Order K-Space ISMRM 2012 #4275 View Sharing Methods A+B1 • Sequential sets of 3 frames are used to reconstruct the accelerated composite data like a sliding window • In other words, a mixture of “R, G, and B” is always needed to construct a composite A+B2 A+B3

  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 = + + • 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 3rd angle

  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 first angle is a special edge case, outer samples are instead mixed from the following 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 = + + • The last angle is also a special case, samples are borrowed from the preceding two angles

  16. 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

  17. 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

  18. 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.

  19. 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%

  20. 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

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

  22. 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%

  23. 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 low variation and negligible loss in accuracy • 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

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