MR vascular imaging researchers, interventional radiologists, interventional cardiologists, vascular surgeons

Many of the challenges associated with planning and performing lower-extremity percutaneous vascular interventions (PVI) are associated with limited visualization. X-ray fluoroscopy does not allow the visualization of the vessel wall or plaque components. This information may assist with predicting the success of an intervention, navigation within an occlusion, and wire/device selection.

We hypothesize that MRI can characterize peripheral arterial disease (PAD) plaque components and their associated mechanical properties. Understanding PAD plaque composition may help in planning and performing PVI by identifying soft lesion components to facilitate guide wire passage. We aim to develop and validate MRI methods for characterizing PAD lesions to ultimately use this information to plan PVI for patients.

MRI was performed on 31 excised human peripheral arterial plaques from 6 patients who underwent amputation. Each sample was imaged at 7 Tesla at high resolution (75μm³ voxels) to produce 3D T2-weighted (T2W) and Ultrashort Echo Time (UTE) images. For T2W imaging, a fast spin-echo sequence with an echo-train length of 8 and an effective TE of 47 ms was used. For UTE imaging, a gradient-echo radial sequence with a TE of 20μs was used. 15 samples were studied to validate MR biomarkers for the following components (T2W, UTE): lumen (∅,∅), fat (+, ≈), thrombus (∅, +), loose fibrous tissue (≈, ≈), dense fibrous tissue/collagen (∅,≈), and calcium (∅,-), where ∅ means no signal, and -,≈,+ mean hypo-, iso-, an hyper-intense versus muscle respectively. Tissue classes were validated with microCT and histology.

The remaining 16 lesions were all chronic total occlusions (CTOs) and were used for guide wire force-displacement testing. CTOs with MR imaging signatures corresponding to fat, thrombus or those with microchannels were classified as “soft”. CTOs with loose fibrous tissue imaging signatures were classified as “intermediate”. CTOs with dense fibrous tissue/collagen MR imaging signatures were classified as “hard”. Guide wire crossing studies were performed on each sample. A 2kg load cell advanced the back end of a 0.035” Lunderquist® Extra Stiff (Cook Medical) guide wire at a fixed displacement rate of 0.05mm/s through the CTO, and the forces required to cross the lesion were measured.

MRI biomarkers for “hard”, “intermediate” and “soft” CTOs correlated with guide wire force-displacement testing. Though densely calcified plaques were studied for sequence validation (Figure 1), there were no densely calcified CTOs in the subset of samples that underwent force-displacement testing. Preliminary testing of densely calcified plaques failed mechanical testing likely because they required puncture forces greater than 20N (the upper limit of our current experimental set-up). Non-calcified hard CTOs (n=2) required an average puncture force of 1.92N ± 0.83 (Figures 2 and 3). Intermediate CTOs (n=10) required an average puncture force of 0.36N ± 0.21. Soft CTOs (n=4) required an average puncture force of 0.05N ± 0.02. There was a statistically significant difference between groups as determined by one-way ANOVA (F(2,13) = 30.407, P <.05).PVI offers an attractive, minimally invasive approach to treating PAD, but not all lesions are amenable to PVI. Densely calcified lesions are resistant to dilatation, have higher failure and complication rates¹ with PVI and are typically recommended for bypass surgery². Though current clinical imaging modalities can characterize calcium well, they offer limited visualization of non-calcified hard plaques, which also influence procedural success³. The results of this study demonstrate the potential of using high-resolution MRI to differentiate densely calcified plaques, non-calcified hard plaques and soft plaques to determine ease of guide wire crossing. This study provides a foundation for future in-vivo studies on predicting the lesion crossability of various PAD plaque types from MRI data sets of patients who undergo PVI.