Imaging and Hemodynamics in Aneurysm Evolution

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Nancy Washington
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1 Disclosures Imaging and Hemodynamics in Aneurysm Evolution David Saloner, PhD There are no financial conflicts of interest to report Department of Radiology and Biomedical Imaging VA Medical Center San Francisco University of California San Francisco The use of Gd-chelate products for MR angiography is an off-label use Overview Non-invasive imaging provides new capabilities in assessing changes over time in untreated aneurysms, and the possibility to relate those changes to biomechanical forces such as hemdynamics. Non-invasive MRI can be used to: Determine prevalence of volume change over time by location, type, and size of aneurysm Use MRI to define the lumenal contours and the outer-wall of the aneurysm and follow these features over time Correlate hemodynamic descriptors with observed changes in aneurysm morphology Utilize patient-specific Computational Fluid Dynamics based on MRA geometries and MRV flow, to define the velocity fields in these aneurysms Study population Subjects with untreated aneurysms where there was no plan to intervene were recruited with IRB approval for serial MR imaging initially repeated annually, more recently at 6 month intervals 78 subjects had one or more follow up studies (average of 4 follow-up studies) Of these, 11 subjects had intra-lumenal thrombus 1

2 Methods Lumenal contours defined with CE-MRA Contrast-enhanced MRA 2cc at 2cc/s Outer wall with balanced steady state SENSE = 2.6 x.8 x 1. Tacq = 36s Fusiform basilar aneurysm Image outer wall 3D balanced Steady State Imaging Image outer wall 3D balanced Steady State Imaging Balanced SSFP Imaging Strong contrast (particularly with CSF) Visualize thrombus and lumen walls Balanced SSFP Imaging Strong contrast (particularly with CSF) Visualize thrombus and lumen walls 2

Image outer wall using, e.g. 3D balanced SSFP imaging

Use a reference unaffected vessel assume unchanged over

volume is unchanged from baseline Longitudinal MRA

follow-up MRA are co-registered follow-up MRA are

obtained by calculating the aneurysmal volume at each study

3 Image outer wall using, e.g. 3D balanced SSFP imaging Manual segmentation Thrombus deposition Thrombus deposition Use a reference unaffected vessel assume unchanged over time Constrain follow-up threshold so that reference vessel volume is unchanged from baseline Longitudinal MRA co-registration Surfaces obtained from baseline and follow-up MRA are co-registered Use a reference unaffected vessel assume unchanged over time Constrain follow-up threshold so that reference vessel volume is unchanged from baseline Longitudinal MRA co-registration Surfaces obtained from baseline and follow-up MRA are co-registered Quantitative measurement of the changes obtained by calculating the aneurysmal volume at each study 3

Results 74 of 88 aneurysms in 78 patients had more than 1 study and remained thrombus free Number of aneurysms 25 2 15 1 5 2 3 4 5 6 7 8 Number of studies This provides a

all locations, types, and size of aneurysms: average error of measurement of aneurysm volume = 4.

filled, the surface is smoothed and divided into patches to be used as mesh boundaries Representative data from 13 subjects: Aneurysm volume at sequential time points with 5%

4 Results 74 of 88 aneurysms in 78 patients had more than 1 study and remained thrombus free Number of aneurysms Number of studies This provides a total of 252 interval measurements Error of measurement - reproducibility Data was fit with a mixed effects maximum residual likelihood model Averaged over all locations, types, and size of aneurysms: average error of measurement of aneurysm volume = 4.9% anterior posterior middle saccular fusiform large med small Patient-specific model construction Holes are filled, the surface is smoothed and divided into patches to be used as mesh boundaries Representative data from 13 subjects: Aneurysm volume at sequential time points with 5% error bars One clear grower High resolution (.6 x.6 x 1.2 mm) CE- MRA images are used to obtain luminal contours of the aneurysmal arteries including the distal and proximal vessels 3D surface is formed from Dicom data, threshold is set to match the boundaries 4

5 Basilar Aneurysm Flow Determine patient-specific velocity field Stroke Volume = 1.9 mls Stroke Volume = 1.9 mls Stroke Volume =.1 mls Stroke Volume = 2.3 mls In vivo MRA Calculated velocities Calculated based on in-vivo geometry and flow Measured in-vivo Measured in-vitro 5

6 Wax lumen in silastic Calculated based on in-vivo geometry and flow Measured in-vivo AP lateral Measured in-vitro Yr Yr 1 Yr 2 Yr 3 Yr 4 Geometry change over time Wall shear stress red <.3 N/m**2 6

7 Results Results Binary map of wall shear stress red wss <.3 N/m**2 Lumen change from baseline to 6 months later Geometry change over time Flow computed with CFD Aneurysm growth observed with CE- MRA studies over time WSS computed with CFD Aneurysm growth observed with CE- MRA studies over time 7

Growing aneurysm Stable aneurysm Number of nodes / Total Nodes.35.3.25.2.15.1 Stable Fraction of surface area with wss <.3 N/m**2.

8 σ mean +σ Wall Shear Stress (Pa) Histograms of wall shear stress Comparison of wall shear stress 6 subjects with growing aneurysms compared to 6 subjects with stable aneurysms Growing aneurysm Stable aneurysm Number of nodes / Total Nodes Stable Fraction of surface area with wss <.3 N/m**2.5 -σ mean +σ Number of nodes/ Total Nodes Wall Shear Stress (Pa) Conclusions CE-MRA + steady state imaging permits monitoring of progression of inner and outer wall of intracranial aneurysms Reproducibility of volume measurement is ~ 5% 14% of aneurysms show growth Patient-specific Computational Fluid Dynamics based on MRA and MRV indicates that aneurysms showing growth have larger fractions of their surface exposed to low wall shear stress Discussion Imaging with higher resolution (3T) should provide improved reproducibility and can reduce subject numbers needed to define role of different mechanisms in aneurysm evolution Identification of growing versus stable aneurysms will permit surgical staging and determination of response to pharmacologic interventions MR is ideal tool non-invasiveness permits serial monitoring; geometric morphology of lumen, thrombus, and flow determination 8

9 Acknowledgments Radiology Vitaliy Rayz, PhD Alastair Martin, PhD Donne Nieuwoudt, MD Sahand Sohrabi, MD Farshid Faraji Neurology Wade Smith, MD Andy Josephson, MD Anthony Kim, MD Nerissa Ko, MD Heather Fullerton, MD Neurointerventional Radiology Randall Higashida, MD Van Halbach, MD Chris Dowd, MD Steve Hetts, MD Biostatistics Chuck McCulloch, PhD Neurosurgery Michael Lawton, MD Anesthesiology Bill Young, MD Funding NIH (NINDS, NHLBI), VA Merit 9

NIH Public Access Author Manuscript J Vasc Interv Radiol. Author manuscript; available in PMC 2012 July 1.

NIH Public Access Author Manuscript Published in final edited form as: J Vasc Interv Radiol. 2011 July ; 22(7): 1007 1011. doi:10.1016/j.jvir.2011.01.425. Temporal Stability of Dysmorphic, Fusiform Aneurysms