A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE J.Su 1, M.Saranathan 1, and B.K.Rutt 1 1 Department of Radiology, Stanford University, Stanford, CA, United States ISMRM 2012 E-P OSTER #4275 Fully Sampled 2.3x Undersamping T 1 Maps Accelerated% Difference

Declaration of Conflict of Interest or Relationship I have no conflicts of interest to disclose with regard to the subject matter of this presentation. A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE J.Su 1, M.Saranathan 1, and B.K.Rutt 1 1 Department of Radiology, Stanford University, Stanford, CA, United States ISMRM 2012 E-P OSTER #4275

Background Variable flip angle T 1 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 T 1 values in tissue is time consuming 1 A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 # Deoni et al. Magn Reson Med Mar;49(3):

Purpose Accelerate VFA by using a view sharing scheme similar to DISCO 2 – A novel pseudo-random sampling pattern is used to greatly reduce the appearance of coherent artifacts Assess the accuracy and variation of the accelerated T 1 maps compared to the fully sampled source 2 Saranathan et al. J Magn Reson Imaging Feb 14. A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE 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 T 1 across all brain tissues A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE 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) A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275 Phase Encode Slice Encode B B A A

A A 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 CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275 Phase Encode Slice Encode A A

B1 B2 B3 B B View Sharing Methods: Sampling The outer region (B) – Broken down into 3 pseudo-random subsampling patterns, each undersampled by a factor 3 A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275 Phase Encode Slice Encode A A B B

B1 B2 B3 B B View Sharing Methods: Sampling 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 A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275 Phase Encode Slice Encode A A B B B1+B2+B3

View Sharing Methods Define two regions in k-space: – The center region (A), 16% of k-space which is fully sampled and collected for every flip angle frame – The remaining outer region (B1-3) which is subsampled by a factor of 3 – This results in an overall acceleration of 2.3x There are 3 interlaced pseudorandom sampling patterns for the outer region that combine to form a complete composite data set B2A+B1B3 A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275 Phase Encode Slice Encode A+B1+B2+B3

View Sharing Methods: Sampling 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 CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275 A+B1 A+B2 A+B3

View Sharing Methods 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+B3 A+B2 A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE 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 2 nd flip angle is created by combining with the 1 st and 3 rd angle... A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE 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 A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275

View Sharing Methods: Reconstruction The last angle is also a special case, samples are borrowed from the preceding two angles A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE 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, μ i – The scale factor is then μ (target angle) /μ (borrowed angle) – Fixes large discontinuities in k-space A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE 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 A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275

Post-Processing Linearly coregister and brain extract the images with FSL 3 Extract whole-brain T 1 values by linearizing the data according to the SPGR signal equation and performing a fit 4 Comparisons are made to the fully sampled images on a voxel-by-voxel level 3 FMRIB Software Library 4 Fram et al. Magn Reson Imaging. 1987;5(3): A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE 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 A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275 Even in the worst case, the reconstructed SPGR images are very similar to the originals – Median percent difference: 0.025% – Interquartile range: 2.235%

Results: T 1 Map – Fully Sampled A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275

Results: T 1 Map – Accelerated A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275

Results: T 1 Map The percent difference between the accelerated and fully sampled T 1 maps is shown Accuracy is excellent, the mean shift in T 1 values is << 1% – Median: 0.030% A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275 The variation in the difference map is very low over the whole brain – Standard deviation < 3% – Interquartile range: 1.492%

Results: T 1 Map The percent difference between the accelerated and fully sampled T 1 maps is shown Accuracy is excellent, the mean shift in T 1 values is << 1% – Median: 0.030% Precision is good, the differences have a standard deviation < 3% – Interquartile range: 1.489% A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE 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 mcDESPOT acquisitions as well A CCELERATED V ARIABLE F LIP A NGLE T 1 M APPING VIA V IEW S HARING OF P SEUDO -R ANDOM S AMPLED H IGHER O RDER K-S PACE ISMRM 2012 #4275