A simulation environment for validating ultrasonic blood flow and vessel wall imaging based on fluid-structure interaction simulations: ultrasonic assessment of arterial distension and wall shear rate.

PURPOSE: Ultrasound (US) is a commonly used vascular imaging tool when screening for patients at high cardiovascular risk. However, current blood flow and vessel wall imaging methods are hampered by several limitations. When optimizing and developing new ultrasound modalities, proper validation is required before clinical implementation. Therefore, the authors present a simulation environment integrating ultrasound and fluid-structure interaction (FSI) simulations, allowing construction of synthetic ultrasound images based on physiologically realistic behavior of an artery. To demonstrate the potential of the model for vascular ultrasound research, the authors studied clinically relevant imaging modalities of arterial function related to both vessel wall deformation and arterial hemodynamics: Arterial distension (related to arterial stiffness) and wall shear rate (related to the development of atherosclerosis) imaging.
METHODS: An in-house code ("TANGO") was developed to strongly couple the flow solver FLUENT and structural solver ABAQUS using an interface quasi-Newton technique. FIELD II was used to model realistic transducer and scan settings. The input to the FSI-US model is a scatterer phantom on which the US waves reflect, with the scatterer displacement derived from the FSI flow and displacement fields. The authors applied the simulation tool to a 3D straight tube, representative of the common carotid artery (length: 5 cm; and inner and outer radius: 3 and 4 mm). A mass flow inlet boundary condition, based on flow measured in a healthy subject, was applied. A downstream pressure condition, based on a noninvasively measured pressure waveform, was chosen and scaled to simulate three different degrees of arterial distension (1%, 4%, and 9%). The RF data from the FSI-US coupling were further processed for arterial wall and flow imaging. Using an available wall tracking algorithm, arterial distensibility was assessed. Using an autocorrelation estimator, blood velocity and shear rate were obtained along a scanline.
RESULTS: The authors obtained a very good agreement between the flow and the distension as obtained from the FSI-US model and the reference FSI values. The wall application showed a high sensitivity of distension measurements to the measurement location, previously reported based on in vivo data. Interestingly, the model indicated that strong reflections between tissue transitions can potentially cloud a correct measurement. The flow imaging application demonstrated that maximum shear rate was underestimated for a relevant simulation setup. Moreover, given the difficulty of measuring near-wall velocities with ultrasound, maximal shear rate was obtained at a distance from the wall [0.812 mm for the anterior and 0.689 mm for the posterior side (9% distension case)]. However, ultrasound shear rates correlated well with the FSI ground truth for all distension degrees, suggesting that correction of the severe underestimation by ultrasound might be feasible in certain flow conditions.
CONCLUSIONS: The authors demonstrated a simulation environment to validate and develop ultrasonic vascular imaging. An elaborate technique to integrate FSI and FIELD II ultrasound simulations was presented. This multiphysics simulation tool was applied to two imaging applications where distensible ultrasound phantoms are indispensable: Wall distension and shear rate measurement. Results showed that the method to couple fluid-structure interaction and ultrasound simulations provides realistic RF signals from the tissue and the blood pool.