## 2 Materials and Methods  
### Experimental Setup  
A patient-specific vessel wall geometry of a Carotid Artery (CA), including the common carotid, the internal and external carotid, and the bifurcation [23], was used to build an ultrasound-compatible (i.e., matching the acoustical properties of soft tissue) phantom made of PVA-cryogel (Mw 85.0-124.0 99+% hydrolyzed, Sigma-Aldrich, Zwijndrecht, The Netherlands). Two molds were created using CAD (Autodesk Inventor 2015, Student version, San Rafael, CA, USA) to convert the CA-geometry (Figure 1a) and the surrounding body into 3D printable casts. Subsequently, these molds were filled with a pre-heated \(\sim\)85\({}^{\circ}\) homogeneous mixture of 54% by weight (wt. %) distilled water, 36 wt. % anti-freezing agent (ethylene glycol, Sigma-Aldrich, Zwijndrecht, The Netherlands), and 10 wt. % PVA including either 2.0 wt. % or 0.5 wt. % Silica gel particles (Merck Kiesel 60, 0.063-0.100 mm, Boom B.V., Meppel, The Netherlands) for the CA-geometry and the surrounding tissue, respectively. After injection and degassing for 1 h, the solutions in the molds were subjected to 4 cycles of freezing (\(-\)25 \({}^{\circ}\)C) and thawing (21 \({}^{\circ}\)C) of 16 and 8 h, respectively. Prior to the last cycle, the surrounding tissue was casted around the pre-fixated carotid artery bifurcation, resulting in the final composed phantom (Figure 1b). This yielded a higher elastic modulus for the CA-geometry compared to the surrounding tissue. The Young's modulus of the CA embedded in surrounding tissue was experimentally estimated to be 225 kPa, which is within range of values corresponding to intima tissue from atherosclerotic plaques [39]. The phantom was connected to an inlet flow tube at the common carotid artery side and to two tubes (internal and external carotid artery) at the outlet side. The connections were supported by a mounting bracket also allowing pressure measurements (TruWave pressure transducer, Edwards Lifesciences Corp., Irvine, CA, USA), see Figure 2b. Fluid was circulated in a closed loop circuit by a gear pump driven by in-house software written in LabView (National Instruments, Austin, TX, USA). The CA was pressurized using a realistic pulsatile flow waveform at 60 bpm with a mean flow of 0.62 L min\({}^{-1}\) and a peak flow of 1.39 L/min. At both outlet tubes, adjustable flow resistors were set such that the dynamic pressure, measured at the inlet of the CA, was 150 over 100 mmHg, mimicking the pressures encountered in a stenotic carotid artery. With these pressure profiles, strains of up to 10% over the pressure cycle were induced. As can be observed in Figure 2a, the surrounding body contained two orthogonal surfaces for ultrasound imaging, from which the averaged depth to the bifurcation was \(\sim\)20 mm. Two L12-5 ATL linear array probes were positioned in a probe holder, which allowed fine-tuning to ensure orthogonal fixation and coinciding transversal fields of view. Both probes were closely hovering above the phantom imaging surfaces, while ultrasound gel established the acoustic coupling and simultaneously prevented physical deformation of the phantom due to direct phantom-probe contact. The probe holder was connected to a translation stage enabling elevational movement for 120 equally spaced (0.1 mm) positions while maintaining a constant in plane orientation of the probes. Each transducer was connected to a Verasonics ultrasound system (a Vantage 256 and a V1, Verasonics, Kirkland, WA, USA). Figure 1: (**a**) 3D CAD-based mold of a realistic patient-specific carotid artery and (**b**) Polyvinyl alcohol phantom casted in a surrounding body. Figure 2: (**a**) A schematic drawing of the image acquisition setup in which two probes were fixated orthogonally. The position of the probes was set such that the carotid bifurcation was at a realistic depth and all steered and non-steered insonified regions by the plane wave transmissions encompassed the carotid artery.