In the medical field, the assessment of blood flow through a tubular cardiovascular structure frequently plays an important role, for example, in order to assess the severity of stenosis of the tubular cardiovascular structure. An example of such a tubular cardiovascular structure is the aortic valve (AV). Degenerative aortic valve stenosis (AS) is the second most common cardiovascular disease, having an incidence rate of 2-7% in the Western European and North American populations aged beyond 65 years. Management of patients with degenerative AS typically depends on the severity of the disease.
For ca. 60-70% of patients, Ultrasound (US) may be used to image the aortic valve and to measure blood velocities via Doppler measurements. In case of a severe enough stenosis of the aortic valve, the effective opening area of the aortic valve is reduced, resulting in the blood flowing at higher velocities. Such higher blood flow velocities show up in the Doppler measurements and are associated with an increased pressure drop across the aortic valve and, accordingly, considered as indicator of aortic valve stenosis.
Alternatively, the degree of stenosis may be assessed from image data obtained by, example, Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) As a result, a three-dimensional image of the cardiovascular structure may be acquired. In case of an aortic valve, Electrocardiography (ECG) gating may be employed to reconstruct or acquire the image(s) from a selected narrow cardiac phase interval, thereby obtaining a three-dimensional image that shows the aortic valve in its relatively short open state. Having obtained image data showing the valve opening, the valve opening may be measured by obtaining angulated cut planes through the three-dimensional image and delineating the apparent valve opening in said cut planes. This technique is referred to AV area planimetry, as described by, e.g., G. M. Feuchtner et al. in “Multislice Computed Tomography for Detection of Patients With Aortic Valve Stenosis and Quantification of Severity”, Journal of the American College of Cardiology 2006, 47 (7), 1410-1417, as well as by Y. Westermann et al. in “Planimetry of the aortic valve orifice area: Comparison of multi-slice spiral CT and MRI”, European Journal of Radiology 2011, 77, 426-435.
The measured area of the apparent valve opening may be used to assess the degree of stenosis, namely by applying the measured area in Bernoulli's equation to compute the pressure drop across a narrowing of the tubular cardiovascular structure in a steady-state flow scenario. Here, an area may be deemed the ‘effective’ cross-sectional area of the tubular cardiovascular structure if its use within Bernoulli's equation yields a similar pressure drop as that obtained by so-termed Computational Fluid Dynamics (CFD)-based blood flow simulations. Besides assessing the degree of stenosis in the aortic valve, the effective cross-sectional area of a tubular cardiovascular structure also plays a role in other medical applications, such as in aortic coarctation where the (thoracic) aorta shows a narrowing.
A problem of the use of area planimetry is that it is insufficiently accurate in estimating the effective cross-sectional area of a tubular cardiovascular structure.