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Validation of DTI Analysis

Validation of DTI Analysis. Guido Gerig, Clement Vachet, Isabelle Corouge, Casey Goodlett UNC Chapel Hill. Validation Concepts. Validity: Comparison to truth / gold standard Reliability, Reproducibility: Repeated measures Outcome measures: Sufficient to answer clinical question.

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Validation of DTI Analysis

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  1. Validation of DTI Analysis Guido Gerig, Clement Vachet, Isabelle Corouge, Casey Goodlett UNC Chapel Hill

  2. Validation Concepts • Validity: Comparison to truth / gold standard • Reliability, Reproducibility: Repeated measures • Outcome measures: Sufficient to answer clinical question

  3. Validation: DTI Image Analysis • Validity: • In vivo human imaging: Truth not known • Ex vivo pathological samples: tissue dynamics not preserved • Animal imaging: • Hi-res DT-MRI with animal scanner • Possibility to induce pathology: lesions, de-myelination • Goldish or Silver Standard: Phantom • Nylon fibers embedded in water, interweaved to simulate crossings: Far from simulation of axons and myelination sheaths • Computational phantom: Simulated diffusion imaging? • Others?

  4. Validation: DTI Image Analysis • Reliability, reproducibility: • Reproducibility of scanning: Repeated scans single or multi-site • Reproducibility of image analysis and/or individual steps • Key Problems: • Metric to determine differences • Imaging and image analysis come with large set of parameters • Validation of single processing steps versus validation of whole system • Relation of validation metric to clinical question and group findings

  5. Very preliminary attempts • Repeatability of quantitative tractography-based FA/MD (ISMRM 2005) • Tool for geometric fidelity of fiber tract ROI (Casey Goodlett)

  6. Validation: Fiber Tracking • Goal: Evaluate the reproducibility of quantitative tractography (FiberViewer) • Study: • 6 scans of the same patient with small motion (MacFall, Duke) • 3 callosal fiber tracts • Method: • Fiber Extraction • Selection of a ROI on one scan • Registration for the 5 other scans • Tracking of Fibers • Analysis across the 6 scans • Diffusion properties: FA, ADC, λ1, λ2, λ3 DTI average 6 repeated scans

  7. Validation: 6 repeated scans Registration of ROI Extraction T B01B02 Selection of a ROI Scan 2… Scan2… T B01B06 Extraction Scan1 …Scan 6 … Scan6 Extraction DTI Average DTI Average

  8. The fallacy of #”fibers” Number of “Fibers” (streamlines): Really not meaningful DTI Average

  9. Tract-based diffusion properties Statistics across 6 repeated scans: Curves of MeanFA and MeanMD, with Standard Deviation FA MD FA

  10. Tract-based diffusion properties Statistics across 6 repeated scans: eigenvalues l1 l2 l3 l1 l2 l3

  11. Tract-based diffusion properties Curves of MeanFA and MeanMD in comparison to the Average DTI FA MD FA

  12. Tract-based diffusion properties Curves of mean eigenvalues in comparison to the eigenvalues of the average DTI λ1 λ2 λ3

  13. Quantitative Results Table: Results at the center, i.e. at the position of the midsagittal plane, are listed. The second row lists the number of streamlines obtained for each experiment. Although the number of streamlines varies due to the instability of tractography, the resulting diffusion tensor statistics are approximately within 5%std for FA and MD but only 2%std for the first eigenvalue λ1. The FA of the DTI average (last column) is slightly lower than the 6 case mean. Guido Gerig , Isabelle Corouge, Clement Vachet, Ranga Krishnan and James MacFall, Quantitative Analysis of Diffusion Properties of White Matter Fiber Tracts: A Validation Study, International Society of Magnetic Resonance ISMRM, May 2005 (peer reviewed long abstract)

  14. Geometric reproducibility of tracts • How to compare geometry tractography results • Proposed method (BIRN): Map tractography result to binary voxel map, binary overlap test (Dice, kappa stats)) • very low enthusiasm • Brainstorming with Casey Goodlett: • Streamline tracking unstable, ill-posed • Key question: Correspondence?

  15. Work in progress • Distance measure between sets of bundles • Give up on point-to-point correspondence between streamlines • Solution/Tool: Measure geometric distance of points in bundle A to closest point in bundle B • Measure is non-symmetric

  16. Validation: Tract Comparison Comparison of scan 1 with original and modified tracking method

  17. Validation: Tract Comparison Comparison of scan 1 with scan 4

  18. Conclusions • NAMIC could develop concept, strategy and tools for validation of DTI • Validation of reproducibility of diffusion properties (tensor stats) • Validation of geometry of extracted bundles: Metric for bundle distance • ….

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