1. NA-MIC National Alliance for Medical Image Computing http://na-mic.org Validation of DTI Analysis Guido Gerig, Clement Vachet, Isabelle Corouge, Casey Goodlett UNC Chapel Hill

2. National Alliance for Medical Image Computing http://na-mic.org Validation Concepts Validity: Comparison to truth / gold standard Reliability, Reproducibility: Repeated measures Outcome measures: Sufficient to answer clinical question

3. National Alliance for Medical Image Computing http://na-mic.org 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. National Alliance for Medical Image Computing http://na-mic.org 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. National Alliance for Medical Image Computing http://na-mic.org Very preliminary attempts Repeatability of quantitative tractography-based FA/MD (ISMRM 2005) Tool for geometric fidelity of fiber tract ROI (Casey Goodlett)

6. National Alliance for Medical Image Computing http://na-mic.org Quantitative Analysis of DTI data Concept: fiber-tract oriented analysis –Measurements across/along fiber bundles with complex geometry –Modeling of fiber bundles Statistics of shape – Geometric modeling - Statistics of diffusion properties (FA,MD) Statistics of tensors (non-Euclidean metric) Analysis scheme Tracking/ Clustering Selection FA FA along tract

7. National Alliance for Medical Image Computing http://na-mic.org 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 6 repeated scans DTI average

8. National Alliance for Medical Image Computing http://na-mic.org Validation: 6 repeated scans Scan1 Scan2… T B0 1  B0 2 … Scan6 DTI Average T B0 1  B0 6 Extraction Scan 2… …Scan 6 DTI Average Selection of a ROI Registration of ROI

9. National Alliance for Medical Image Computing http://na-mic.org Number of “Fibers” (streamlines): Really not meaningful DTI Average The fallacy of #”fibers”

10. National Alliance for Medical Image Computing http://na-mic.org Tract-based diffusion properties Statistics across 6 repeated scans: Curves of MeanFA and MeanMD, with Standard Deviation FA MD

11. National Alliance for Medical Image Computing http://na-mic.org Tract-based diffusion properties Statistics across 6 repeated scans: eigenvalues 1 2 3   

12. National Alliance for Medical Image Computing http://na-mic.org Tract-based diffusion properties FA MD Curves of MeanFA and MeanMD in comparison to the Average DTI

13. National Alliance for Medical Image Computing http://na-mic.org λ1λ1 λ2λ2λ3λ3 Tract-based diffusion properties Curves of mean eigenvalues in comparison to the eigenvalues of the average DTI

14. National Alliance for Medical Image Computing http://na-mic.org 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)

15. National Alliance for Medical Image Computing http://na-mic.org 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?

16. National Alliance for Medical Image Computing http://na-mic.org 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

17. National Alliance for Medical Image Computing http://na-mic.org Validation: Tract Comparison Comparison of scan 1 with original and modified tracking method

18. National Alliance for Medical Image Computing http://na-mic.org Validation: Tract Comparison Comparison of scan 1 with original and modified tracking method

19. National Alliance for Medical Image Computing http://na-mic.org Validation: Tract Comparison Comparison of scan 1 with scan 4

20. National Alliance for Medical Image Computing http://na-mic.org Validation: Tract Comparison Comparison of averaged data with scan 1

21. National Alliance for Medical Image Computing http://na-mic.org 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 ….