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Radial Wall Stress Is the Primary Biomechanical Force Determining Wall Thickness in Human Lower Extremity Vein Grafts: Assessment of Wall Characteristics and Remodeling Patterns Using Ultra-High Resolution Magnetic Resonance Imaging (MRI)

Christopher D. Owens, Amanda G. Whitmore, Dimitrios Mitsouras, Ji Min Kim, Frank J. Rybicki, Michael S. Conte.
Brigham and Women's Hospital, Boston, Mass.

OBJECTIVES: Characterization of remodeling of lower extremity vein bypass grafts (LEVBG) has been limited to lumen caliber due to poor discrimination of wall structure by ultrasound (US). We sought to describe structural changes in vein grafts using ultra-high resolution MRI, and to determine their relationship to mechanical forces.

METHODS: Patients (n=28) who underwent LEVBG were assessed by a combination of US (1 and 6 months) and MRI (1 and 6 mos.; T1- and T2-weighted), focused on a spatially registered index segment of the graft.

RESULTS: T1 and T2 lumen diameters (LD) were highly correlated (r=.93, p<.0001) while T1 LD best correlated with M-mode ultrasound (r=.79, p<.0001). T1 wall thickness (WT) was greater than T2 at both 1 and 6 months (.85 vs. .76, .89 vs. .74 mm respectively; p<.05). Ex vivo validation (n=3) was consistent with the hypothesis that T2 signal intensity reflects a predominantly cellular matrix (intima-media), while T1 was reflective of fibrous components (T1-T2=adventitia). Although pooled mean values suggest little overall change in LD or WT between 1 and 6 mos in this cohort (Table), considerable variability in remodeling (both magnitude and direction) was apparent. The change in LD over this interval correlated strongly with changes in overall vessel size (r=.88, p<.001; Figure A), and was not correlated with changes in WT. Radial wall stress at 1 month correlated positively with the 1-6 months changes in WT (r=.78, p=.006; Figure B) and outer vessel diameter (r=.61, p=.045). Wall stress at 1 month was inversely correlated with 1-6 months change in the lumen radius to WT ratio (r=-.61, p=.044).

Conclusion: Combined, high-resolution US and MRI are capable of defining the structural remodeling of human LEVBG. T1 and T2 signal frequency characteristics may differentiate between intima-media and adventitia. In normally functioning LEVBG, changes in lumen caliber between 1 and 6 months are primarily a function of changes in overall vessel size, as opposed to wall thickness. Radial wall stress is the primary hemodynamic force that drives adaptive wall thickening of the vein graft over this interval.

AUTHOR DISCLOSURES: C.D. Owens, None; A.G. Whitmore, None; D. Mitsouras, None; J. Kim, None; F.J. Rybicki, None; M.S. Conte, None.