Spectral Characterization in Ultrasonic Doppler Velocimetry
11.1. Velocity measurement in medical imaging
This chapter is devoted to velocimetry, i.e., the imaging of the velocity of moving structures. Velocimetry is employed in atmosphere imaging, industrial control, medical imaging, etc. In the medical field, it is essentially used to characterize blood flow and heart movements for the purpose of diagnosing cardiovascular pathologies. These mainly concern stenosis, i.e., narrowing of the arteries connected with the presence of atheromatous plaque, and its repercussions on the organs supplied by the affected vessels (heart, brain, etc.). The resulting pathologies may be chronic (silent ischaemia) or acute (infarction). The information obtained is morphological: reduction of the cross-sectional area of the artery at the stenosis, estimation of the dimensions of the heart (cardiac hypertrophy) and monitoring of its contractions (hypokinesis, dyskinesia). Ultrasound (US) and magnetic resonance imaging (MRI) both allow local velocity images to be acquired and thus give access to specific information that is complementary to that provided by morphological and functional imaging. Although the physical principles underlying these two techniques are radically different, the data measured and the problems posed show strong similarities. This chapter focuses on Doppler ultrasound, which is still the most delicate to interpret. We will see that the problems arising concern spectral characterization: